principles and applications of organic light emitting diodes (oleds)

PRINCIPLES AND APPLICATIONS OF ORGANIC LIGHT EMITTING DIODES (OLEDs)

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Woodhead Publishing Series in Electronic and Optical Materials

PRINCIPLES AND APPLICATIONS OF ORGANIC LIGHT EMITTING DIODES (OLEDs)

N. THEJO KALYANI

HENDRIK SWART

S. J. DHOBLE

Woodhead Publishing is an imprint of ElsevierThe Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United StatesThe Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom

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v

CONTENTS

1. Luminescence: Basic Principles and Applications 11.1 Introduction 1

1.2 Light 1

1.3 Mechanism of Light Emission 3

1.4 Terminology Associated with Luminescence 24

1.5 Realm of Luminescent Materials 28

1.6 Conclusion 34

References 35

2. Luminescence in Organic Semiconductors 392.1 Introduction 39

2.2 Organic Compounds 40

2.3 Organic Semiconductors 45

2.4 HOMO and LUMO in Organic Semiconductors 52

2.5 Charge Transport in Organic Materials and Devices 54

2.6 Luminescent Organic Materials: An Overview 59

2.7 Organic verses Inorganic Luminescent Materials 62

2.8 Conclusions 62

References 63

3. Evolution of Luminescent Materials for Organic Light-Emitting Diodes 653.1 Introduction 65

3.2 Red-Light-Emitting Materials for OLEDs 66

3.3 Green-Light Emitting Materials for OLEDs 73

3.4 Blue-Light-Emitting Materials and OLEDs 76

3.5 White-Light-Emitting Materials and OLEDs 80

3.6 Conclusions 81

References 81

4. Artificial Lighting: Origin—Impact and Future Perspectives 874.1 Introduction 87

4.2 Light 88

4.3 Lighting 89

4.4 Classification of Lighting 89

4.5 Artificial Lighting: Origin and Impact 90

4.6 Lighting Terminology 91

Contentsvi

4.7 Light Sources 92

4.8 Evaluating Quality of White Light 94

4.9 Spectral Distribution of Different Light Sources 97

4.10 Electrically Powered Incandescent Lamps 97

4.11 Electrically Powered Luminescent Lamps 99

4.12 Solid-State Lighting 103

4.13 Future Perspectives 109

References 112

5. Solid-State Lighting 1155.1 Introduction 115

5.2 Solid-State Lighting: A Brief History 116

5.3 Requisite of Solid-State Lighting 116

5.4 Solid-State Lighting With LEDs 117

5.5 CSL With OLEDs: Future Lighting Sources 130

5.6 Advantages of Organic Over Inorganic 136

5.7 LEDs Versus OLEDs 137

5.8 Conclusions 138

References 138

6. Organic Light-Emitting Diodes: The Future of Lighting Sources 1416.1 Introduction 141

6.2 Organic Light-Emitting Diodes 141

6.3 Structure of OLEDs 142

6.4 Light-Emitting Mechanism of OLEDs 145

6.5 Materials for OLEDs 146

6.6 Efficiency of OLEDs 155

6.7 Device Architectures 160

6.8 Advantages of OLEDs 161

6.9 OLED Research Hurdles and Challenges 162

6.10 OLED Applications 164

6.11 Conclusions 167

References 168

7. Review of Literature on Organic Light-Emitting Diode Devices 1717.1 Introduction 171

7.2 Device Architecture 171

7.3 Review of Literature on Red OLEDs 174

7.4 Review of Literature on Green OLEDs 182

Contents vii

7.5 Review of Literature on Blue OLEDs 188

7.6 Review of Literature on White OLEDs 192

7.7 Conclusions 198

References 200

8. History of Organic Light-Emitting Diode Displays 2058.1 Introduction 205

8.2 Displays 206

8.3 Display Device 207

8.4 Display Terminology 208

8.5 Display Categorization 211

8.6 History of Display Technology 211

8.7 Plasma Display Panels 219

8.8 Light-Emitting Diode Displays 220

8.9 Organic Light-Emitting Diode Displays 220

8.10 Future Outlook 223


References 224

9. Organic Light-Emitting Diode Fabrication and Characterization Techniques 2279.1 Introduction 227

9.2 OLED Fabrication 227

9.3 Fabrication Technologies 231

9.4 Characterization of OLEDs 240

9.5 Conclusions 251

References 251

10. Photo-Physical Properties of Some RGB Emissive Materials 25310.1 Introduction 253

10.2 Experimental Details 255


References 284

11. Future Prospects of Organic Light-Emitting Diodes 28711.1 Introduction 287

11.2 Current Status of OLEDs 287

11.3 Future Prospects of OLEDs 288

Contentsviii

11.4 OLEDs Research Trends in Past, Present, and Future 290

11.5 OLEDs: Future Perspectives 292

11.6 OLEDs in the Overall Lighting Sector 294

11.7 Industrial Challenges 296


References 306

Index 309

1Principles and Applications of Organic Light Emitting Diodes (OLEDs).DOI:

© Elsevier Ltd.All rights reserved.2017

http://dx.doi.org/10.1016/B978-0-08-101213-0.00001-1

CHAPTER 1

Luminescence: Basic Principles and Applications

1.1 INTRODUCTION

Since time immemorial, light emissions from glowworms, sea creatures, and the extravagant light shows of the aurora borealis have been fascinat-ing, and a great deal of work has been done to comprehend their origins. Until the advent of quantum mechanics, the basic origins of these emis-sions could not be adequately understood. Later, after numerous resource-ful attempts to determine the origin behind these emissions, it was finally concluded that they are due to the phenomena of luminescence, which involves the absorption of suitable energy and subsequent emission of light as ultraviolet (UV), visible light, or infrared (IR) radiation from materials. Over time, major breakthroughs in luminescence studies made this field of research a major focus of innovation.

1.2 LIGHT

Light has fascinated mankind since ancient times through its diverse shades and colors as it plays a vital role in almost all spheres of mod-ern life. Various natural wonders such as the shades of sunrises and sun-sets, rainbows, the blues of ocean and sky, etc., involve light. But what is light? In simple terms, it is a physical quantity that is emitted by a lumi-nous body and when incident on the eye causes the sensation of sight through nerves. It constitutes a tiny proportion of the whole electromag-netic spectrum that is visible to the human eyes. Though our capabili-ties for perception of light are highly elevated, only a very narrow range of the electromagnetic spectrum, which extends from the deepest violet (400 nm) to the deepest red (750 nm), can be seen by us. According to the wavelength and frequency, the color of light also changes and hence a spectrum of VIBGYOR can be observed. In VIBGYOR, red occupies more space and hence reaches our eyes first. Red, green, and blue (RGB)

Principles and Applications of Organic Light Emitting Diodes (OLEDs)2

occupies two-thirds of the spectrum and a specific combination thereof creates white light. The visible (VIS) spectrum and its wavelength range and bandwidth of the different colors of the VIS spectrum are shown in Fig. 1.1 and Table 1.1, respectively.

The emission of wavelengths corresponding to the visible region requires a minimum excitation energy ranging between 1.8 and 3.1 eV as calculated by Einstein’s law. This law states that the excitation energy (E) is equal to the ratio of Planck’s constant (h) times the velocity of light (c) in vacuum to its wavelength (λ), which is given by

E h

hc= =υ

λ

Figure 1.1 Visible spectrum [1].

Table 1.1 Wavelength range and band width of different colors of the visible spectrum [1]Color Wavelength

(nm)Bandwidth (nm)

Frequency (THz)

Photon energy (eV)

Violet 380–450 70 668–789 2.75–3.26Blue 450–495 45 606–668 2.50–2.75Green 495–570 75 526–606 2.17–2.50Yellow 570–590 20 508–526 2.10–2.17Orange 590–620 30 484–508 2.00–2.10Red 620–750 130 400–484 1.65–2.00

Luminescence: Basic Principles and Applications 3

1.3 MECHANISM OF LIGHT EMISSION

Light is a form of energy and hence another form of energy is needed to create light. This is practically possible by two phenomenon, namely incandescence and luminescence. Light is electromagnetic radiation gener-ated by changes in vibration of electrically charged particles from heated molecules or by the downward transition of electrons in atoms. The first phenomenon is known as incandescence while the latter is known in luminescence.

1.3.1 IncandescenceIf a material is heated to a high enough temperature, it starts glowing. This process in which light is emitted from heat energy is known as incandes-cence. When atoms are heated, they release some of their thermal vibra-tions as electromagnetic radiation in the form of incandescent light. This is the most common type of light obtained from the sun, stars, a burning piece of coal, and a piece of iron heated to very high temperature. The sun provides almost all of the heat, light, and other forms of energy that are necessary for life on our planet by the process of incandescence. Stars twinkle red if their temperature is low and glow blue if the temperature is high because different temperatures result in different colors. Similarly, a piece of iron appears dark at room temperature and when heated, it appears faint crimson at 500°C, then turns red, orange, gradually yellow at 800°C, and finally emits white light above 1000°C due to incandescence. This phenomenon of light emission is well explained by Planck’s black-body emission theory.

1.3.1.1 Incandescence SourcesAmong many sources of light available, the most common light sources are thermal sources, which emit light in the form of hot emission, e.g., when the tungsten filament or ordinary incandescent lightbulb is heated, it glows brightly white hot due to incandescence, hence the popularity of incandescent lamps. These incandescent sources consist of a filament made of tungsten, a special metal that can stay at a high temperature for more than 100 hours without burning (oxidizing). The sun and the fila-ment of an electric bulb are shown in Fig. 1.2A. When electrical cur-rent runs through a thin wire, the resistance creates heat. When the wire reaches a high temperature, the atoms in the material absorb energy and the electrons are excited to the higher energy states. After their lifetime,


they return to the lower energy state along with the emission of light and heat. In these lamps only 15% of light is emitted in the visible range and the rest is released in the form of heat [2] as shown in Fig. 1.2B. According to the inverse square law, the intensity of light per unit area varies in inverse proportion to the square of the distance between the source and targeted area. The distance is measured to the first luminating surface—the filament of a clear bulb, or the glass envelope of a frosted bulb. If I denotes the intensity of light per unit area and d is the distance between the source and targeted area, then the flux density E is given by the fol-lowing relation:

E

I

d= >

2)(For d 5 times the diameter of the source

Figure 1.2 Demonstration of incandescence in (A) (i) sun and (ii) filament of an electric bulb and (B) intensity versus wavelength of thermal radiation at different temperatures.


For example, if an incandescent lamp emits 40 lm/m2 at a distance of 0.5 m, it can emit only 10 lm/m2 if the distance is increased to 1 m as shown in Fig. 1.3.

As the intensity of light drastically varies with distance they are not effective sources of lighting. Filament break is the usual end of the lamp life and hence they are least expensive to purchase and most expensive to operate. Light-conversion efficiency can be represented mathematically as:

Eciency

Useful energy at theoutput

Total energy== = =

30

2000 15 1. 55%

Thus the efficiency of an incandescent bulb is only 15% and the remaining energy is lost in the form of heat.

1.3.2 LuminescenceGerman physicist and science historian Eilhard Wiedemann was the first to introduce the term luminescence, which comes from the Latin root Lumin, meaning light [4]. In general, luminescence is the study of the laws of absorption and emission of radiation by matter [5]. Luminescence is cool emission caused by the movement of electrons within a substance from more energetic states to less energetic states and hence it is a process of giving off light at normal or cool temperatures without generating heat. This can be caused by absorption of photons, chemical or biochemical reactions, activity of subatomic particles, radiation, or stress on a crystal.

Figure 1.3 Incandescent lamp—demonstration of inverse square law [3].


The wavelength of light emitted is characteristic of luminescent substance and not of the incident radiation [6,7]. The law of luminescence states that the wavelength of emitted radiation is always greater than the excit-ing radiation (λemi > λexc). The materials emitting luminescence are called luminophors or phosphors [8]. Fig. 1.4 shows the luminescence process.

Luminescence is an interdisciplinary subject as it is applicable to various fields such as physics, chemistry, biological science, medical sci-ence, forensic science, geology, material science, engineering technology, etc. Current research is characterized by strong interaction among other branches of solid state and between different areas of luminescence using inorganic and organic materials.

1.3.2.1 Luminescence in Transition Metal IonsTransition metals are those elements in which atoms have a partially filled d subshell or an incomplete d subshell that can give rise to cations [9,10]. f-Block lanthanide and actinide series are also considered as transition met-als, and are generally called inner-transition metals. Color in transition-series metal compounds is generally due to electronic transitions of two major types, namely (1) charge transfer transitions, where an electron can jump from a ligand orbital to metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition; and (2) d–d transitions, where an elec-tron jumps from one d-orbital to another d-orbital. Such transitions are more likely to occur when the metal is in a low-oxidation state. Vibrant col-ors can be obtained from dichromate and permanganate ions and aqueous solutions of transition metal ions [Co (NO3)2 (red); K2Cr2O7 (orange)].

1.3.2.2 Luminescence in Rare Earth Metal ComplexesLuminescence is the distinguishing and appealing feature of lanthanide tri-valent ions. Rare earth metals are a family of 17 elements with atomic

Figure 1.4 Schematic diagram of the luminescence process: (A) electron in lowest energy state, (B) excited state, and (C) light emission.


numbers 21, 39, and 57–71. Of these, the element with atomic no. 57, Lanthanum (La), has no free electron in the 4f shell, while the element with atomic no. 71, Lutetium (Lu), has a completely filled 4f shell with 14 electrons. One peculiar characteristic of all 13 elements among the rare earths, starting from Cerium (Ce) to Ytterbium (Yb), is that the 4f shell is incompletely filled, but is completely screened by the outer 5s and 5p sub-shells, which are completely filled. The optical and electromagnetic prop-erties of these 13 rare-earth elements are essentially due to the screening of this incompletely filled 4f shell. Luminescence in tripositive rare earth ions arises mainly due to energy-level transition within the 4f shell, which is generally forbidden by quantum mechanical spin and parity prohibition rules. Efficient luminescence can still occur in these ions under conditions where such ions do not occupy a position having a center of symmetry in a crystalline lattice. Some of the tripositive ions such as Europium (Eu3+), Terbium (Tb3+), and Dysprosium (Dy3+) are good luminescent emit-ters, a factor depending on the number of electrons in its 4f shell. The ions, which are inert to luminescent emission, are those of Yb, La, Ga, and Lu. Table 1.2 describes and classifies rare earth metals.

Origin of Luminescence in Lanthanides: Screening EffectLanthanides from Ce3+ to Lu3+ have 1 to 14 4f electrons added to their inner-shell configuration, which is equivalent to Xe. Ions with no 4f elec-trons, i.e., Sc3+, Y3+, La3+, and Lu3+, have no electronic energy levels that can induce excitation and luminescence processes in or near the visible region. In contrast, the ions from Ce3+ to Yb3+, which have partially filled 4f orbitals, have energy levels characteristic of each ion and show a variety of luminescence properties around the visible region. Many of these ions can be used as luminescent ions in phosphors, mostly by replacing Y3+, Gd3+, La3+, and Lu3+ in various compounds.

The luminescence from the lanthanide ions is the result of competi-tion between radiative and nonradiative pathways in the relaxation of an electronically excited species. By the selection rule ΔJ = 0, ±1 (ΔJ = 0 is forbidden), hypothetically in lanthanide ions only magnetic dipole transitions are permissible [12]. In the coordinating sphere of lanthanide, electric-dipole transitions are also preferential as the ligand field mixes slightly odd parity configurations into the [Xe] 4fn 5d° configuration. As coordinating chromophores absorb energy, most of the lines of absorption and emission come out due to electric-dipole transition. Both magnetic-dipole and electric-dipole transitions of lanthanide ions are quite weak as


compared to fully allowed transitions in organic chromophores separately. The excited state of lanthanides is not solely relaxed by a radiative process but also by nonradiative processes. The emissive properties of lanthanides can be enhanced by increasing the excited state population and mini-mizing nonradiative pathways. In the case of lanthanides, the emission is due to transitions inside the 4f shell, i.e., these transitions are intracon-figurational f–f transitions. The deep-lying partially filled 4f shell, which is not completely filled with electrons, is well shielded by those outer 5s25p6 shells (except in La3+ and Lu3+), which gives rise to the number of

Table 1.2 Descriptive classification of rare earth metals [11]Light rare earth elements

Common uses Heavy rare earth elements

Common uses

Lanthanum—57 Camera lenses, catalytic cracking catalyst for refining oil, high refractive index glass, battery electrodes

Europium—63 Lasers, mercury vapor lamps

Cerium—58 Glass and ceramics, polishing powder, chemical oxidizing agent

Gadolinium—64 Rare earth magnets, lasers, X-ray tubes, MRI, computer memory

Praseodymium—59 Rare earth magnets, lasers, carbon arc lighting

Terbium—65 Lasers, fluorescent lamps

Neodymium—60 Rare earth magnets, lasers

Dysprosium—66 Rare earth magnets, lasers

Promethium—61 Nuclear batteries Holmium—67 LasersSamarium—62 Rare earth

magnets, lasers, masers

Erbium—68 Lasers, vanadium steel

Thulium—69 X-ray machinesYtterbium—70 LasersLutetium—71 PET scanners, high

refractive glass


discrete energy levels and the ligands in the first and second coordination spheres perturbing the electronic configurations of the trivalent lanthanide ions only to a very limited extent.

This shielding, also known as the screening effect, is responsible for the specific properties of lanthanide luminescence, specifically for the narrow-band emission and for the long lifetimes of the excited states. Most of the lanthanide ions show luminescence in the visible region of the optical spectrum [13,14]. The energy-level diagram for the Ln(III) ions showing the main emissive levels and the possible transitions to the ground-state levels is well illustrated in Fig. 1.5 through the Dieke diagram.

The Antenna Effect: Sensitized EmissionIn the early 1990s, Lehn [16] coined the term antenna to denote the absorption, energy transfer, and emission sequence involving distinct absorb-ing (the ligand) and emitting (the lanthanide ion) components in lumines-cent lanthanide complexes that work as light-conversion molecular devices

Figure 1.5 Dieke diagram: Energy-level diagram for the Ln(III) ions showing the main emissive levels and the possible transitions to the ground state levels [15].


(LMCDs). The antenna effect in lanthanides and organic chelate is well illustrated in Fig. 1.6. The introduction of an antenna in lanthanide com-plexes provides an alternate pathway for energy transfer and enriches the lanthanide-emitting levels, which then relax to ground state by emitting light [17–19]. For an effective sensitization process in sensitizer–function-alized–lanthanide complexes for various applications generally the chro-mophore should fulfill some requirements: (1) The antenna chromophore should possess a high molar extinction coefficient to obtain high pho-toluminescence quantum yield in the process of absorption-energy trans-fer-emission. (2) The antenna chromophore should match the triplet state energy levels for effective energy transfer to the lanthanide luminescent states.

If the energy transfer between donor and acceptor is too large, it may lead to slower energy-transfer rates, whereas a thermally activated back energy transfer can occur in a small energy gap. (3) The antenna chro-mophore should be in close proximity to the lanthanides ion for effective energy transfer. (4) The intersystem crossing yield of the antenna chromo-phore should be high. (5) To get rid of the quenching problem by water or solvent molecules, the antenna chromophore should saturate the inner coordination sphere of lanthanide metal ions with a coordination number of at least 8.

Ligand and Lanthanide Ion Excitation: Jablonski DiagramThe ligand-enhanced lanthanide luminescence mechanism is basically a three-step process: (1) The ligand absorbs the excitation light; (2) the absorbed energy is transferred to the lanthanide ions; and (3) finally, the ions emit light. In addition to the previously introduced central ion energy levels, there are several ligand energy levels involved in the process [20]. These steps related to lanthanide luminescence are well illustrated by

Figure 1.6 Illustration of antenna effect.


the Jablonski diagram. The energy levels in this diagram are arranged ver-tically by energy and grouped horizontally by spin multiplicity as depicted in Fig. 1.7.

In the ligand–lanthanide complex the absorption of a photon is a very fast process (~10−15 s) that occurs from the energetically lowest ground state, since in a nonexcited molecule electrons tend to occupy these ener-getically lowest lying levels. Most lanthanide complexes are excited at the near-UV range (the wavelengths rarely exceeding 350 nm). From the original excited singlet energy level of the ligand, the electrons may decay nonradiatively by means of internal conversion (within 10−12 s) to some excited vibrational level, or to the lowest excited electronic level. The sen-sitization process may involve several ligand singlet and triplet states and also intraligand charge-transfer (ILCT) states. Traditionally the energy flow is considered to depart from the ligand singlet state to the ligand trip-let state by intersystem crossing, and from the (lowest) ligand triplet state through intramolecular energy transfer to the excited energy levels of the central ion [22–24]. In some cases, the singlet state may directly transfer energy to the central ion. This is, however, not common, since the singlet state is short lived and thus the process is not efficient [25].

There are two main mechanisms for the intramolecular energy trans-fer from the triplet state of the ligand to the central ion: (1) the Dexter

Figure 1.7 Illustration of lanthanide luminescence by the Jablonski diagram [21], where IC, internal conversion; ISC, intersystem crossing; ILCT, intraligand charge-trans-fer; LMCT, ligand-to-metal charge transfer; IET, intramolecular energy transfer; RET, rare earth transition.


(electron exchange) mechanism and (2) the Förster (dipole–dipole) mech-anism. The Dexter mechanism involves a mutual electronic exchange between the ligand and the central ion [26] requiring physical contact between the two components. On the other hand, in the Förster mecha-nism, the triplet-state transition dipole moment associates with the dipole moment of the 4f orbitals. For this reason, the Förster mechanism does not require physical contact between the components and therefore functions at longer distances compared to the Dexter mechanism [27,28]. In addi-tion to these main mechanisms, other mechanisms for exciting the central ion, e.g., the metal-to-ligand charge transfer (MLCT) from chromophore containing d-transition metal ions [29] and the ligand-to-metal charge transfer (LMCT) [30], are also possible. Schematic diagrams of the Förster and Dexter mechanisms are shown in Fig. 1.8.

The charge-transfer transitions are allowed, but they require high ener-gies, which are most prominent with Sm3+, Eu3+, and Yb3+, as these are the most easily reduced ions. When utilizing the LMCT states to transfer energy to the excited 4f states of the lanthanide ion, the energy of the LMCT state should be high enough compared to the emitting energy level of the ion to minimize quenching of the luminescence. In addition to the f–f tran-sitions and charge-transfer transitions, lanthanide ions also display a third

Forster energy transfer

S1 S1 S1

S0S0S0S0

S0S0S0S0

S0S0S0S0

Host

Dexter energy transfer (singlet to singlet)

Dexter energy transfer (triplet to triplet)

Guest Host Guest

Host Guest Host Guest

Host Guest Host Guest

S1

S1 S1 S1S1

T1 T1 T1T1

Figure 1.8 Schematics of the Förster and Dexter mechanisms [31].


type of electronic transitions: the f–d transitions, i.e., the promotion of a 4f electrons into the 5d subshell. The f–d transitions are allowed, are broader than f–f transitions, and (contradictory to f–f transitions) their spectral posi-tion largely depends on the ligand field. The physical, chemical, thermal, and optical properties of lanthanides are given in Table 1.3.

1.3.2.3 Luminescence in ActinidesThe actinide series includes 15 chemical elements, actinium (Z = 89) to lawrencium (Z = 103). They are popularly known as f-block elements and exhibit much more variable valence than the lanthanides. All actinides are radioactive and release energy upon radioactive decay. Within actinides, uranium, thorium, and plutonium are the most abundant actinides on earth. There are two overlapping groups: transuranium elements, which follow uranium in the periodic table and transplutonium, which follow plutonium. Actinides have similar properties to lanthanides; the 6d and 7s electronic shells are completed in actinium and thorium, and the 5f shell is filled with further increase in atomic number; the 4f shell is filled in the lanthanides. The characteristics of emission spectra are often very sen-sitive to the energetic position of these states. Even more drastic is their influence on the temperature quenching of these emissions. Some hexava-lent uranium exhibits luminescence properties. This emission is due to an octahedral ( )UO6

6− group and not to the well-known uranyl ( )UO22−

group. Charge-transfer states involving 5f and possibly 6d levels determine the dependence of the emission characteristics on the host lattice.

1.3.2.4 Luminescence in Heavy MetalsA heavy metal is a member of a loosely defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Luminescence detection of transition and heavy metals by inversion of excited states, synthesis, spectroscopy, and X-ray crystallography of Ca, Mn, Pb, and Zn complexes of 1,8-anthra-quinone-18-crown-5 was carried out by Kadarkaraisamy and Sykes [41]. They achieved optimum fluorescence enhancement using cations of high charge, large cations that form long bonds within the host, and cations that do not coordinate solvent or the counter anion, all of which are necessary for inversion of excited states to occur.

1.3.2.5 Luminescence in Electron–Hole CentersIn the crystalline structure of certain types of matter such as quartz, feld-spar, and aluminum oxide, the electrons trapped between the valence band

Table 1.3 Physical, chemical, thermal, and optical properties of lanthanides [32–40]Element Symbol Color Z Electronic

configurationAt. wt. Abundancy

(ppm)M.P. (°C)

B.P. (°C)

Density (g/cm3)

Crystal structure

Hypersensitive transitions

Emissive energy level

λemi Atomic radii

Ionic radii (3+ ions)Excited

stateGround state

Lanthanum La White 57 [Xe]5d16s2 139.91 35 918 3464 6.145 dhcp – – – 187.7 103.2Cerium Ce White 58 [Xe]4f15d16s2 140.12 66 798 3433 6.77 fcc – – – – 182.5 101.0Praseodymium Pr Bluish

green59 [Xe]4f36s2 140.9 9.1 931 3520 6.773 dhcp 3H5

3F2

3H4 – – 182.8 99.0

Neodymium Nd Rose violet

60 [Xe]4f46s2 144.24 40 1021 3074 7.007 dhcp 4G5/22G7/24G7/2

4I9/24F3/2 – 182.1 98.3

Promethium Pm – 61 [Xe]4f56s2 147 0.0 1042 3000 7.26 dhcp 5G125G3

5I4 – 88010601330

181.0 97.0

Samarium Sm – 62 [Xe]4f66s2 150.35 7 1074 1794 7.52 rhomb 4H7/26F1/26F3/2

6H5/24G5/2 – 180.2 95.8

Europium Eu Colorless 63 [Xe]4f76s2 151.96 2.1 822 1529 5.243 bcc 7F27F1

7F05D0 – 204.2 94.7

Gadolinium Gd Colorless 64 [Xe]4f75d16s2 157.25 6.1 1313 3273 7.9 hep – – 6P1/2 580590613650690710

180.2 93.8

Terbium Tb Colorless 65 [Xe]4f96s2 158.92 1.2 1356 3230 8.229 hep 7F5490

7F65D4 – 178.2 92.8

Dysprosium Dy Shinning yellow

66 [Xe]4f106s2 162.5 4.5 1412 2567 8.55 hep 6F11/26H13/2

6

H11/2

6H15/24F9/2 545

590620650

177.3 91.2

Holmium Ho Light yellow

67 [Xe]4f116s2 164.93 1.3 1474 2700 8.755 hep 5G63H6

5I8 – – 176.6 90.1

Erbium Er Pink 68 [Xe]4f126s2 167.26 3.5 1529 2868 9.066 hep 2H11/2 4G11/2

4I 15/24I13/2 – 175.7 89.0

Thulium Tm – 69 [Xe]4f136s2 168.93 0.5 1545 1950 9.321 hep 3F43H43H5

3H6 – 1550 174.6 88.0

Ytterbium Yb – 70 [Xe]4f146s2 173.04 3.1 819 1196 6.965 fcc – – 2F5/2 – 194.0 86.8Lutetium Lu Colorless 71 [Xe]4f145d16s2 174.97 0.8 1663 3402 9.84 hep – – 2F5/2 980 173.4 86.1

Hecp, hexagonal close packed; dhcp, double hexagonal close packed; rhomb, rhombohedral; fcc, face-centered cubic; bcc, body-centered cubic.


and conduction band can also cause luminescence. The trapping sites are imperfections of the lattice impurities or defects. The ionizing radiation produces electron–hole pairs; electrons are in the conduction band and holes are in the valence band. The electrons that have been excited due the conduction band may become entrapped in the electron or hole traps [42]. Under stimulation of light, the electrons may free themselves from the trap and get into the conduction band, then recombine with holes trapped in hole traps. Emission of light occurs if the center with the hole is a luminescence center (radiative recombination center).

1.3.2.6 Luminescence in Extended DefectsExtended defects can also generate luminescence. Lee and Choi [43] stud-ied the temperature and power dependence of the photoluminescence spectra that arose from dislocations at the hetero-interface of very thin and partially strained Si0.6 Ge0.4 alloys grown on silicon substrates.

1.3.3 Classification of Luminescence Based on Time LagBased on time lag, luminescence can be formally divided into two catego-ries as (1) fluorescence, where luminescence lifetime is less than 10−8 s, and (2) phosphorescence, where luminescence lifetime is greater than 10−8 s.

1.3.3.1 FluorescenceThe term fluorescence was coined in 1852 when it was experimentally demonstrated that certain substances absorb the light of a narrow spectral region (e.g., blue light) and instantaneously emit light in another spectral region not present in the incident light (e.g., yellow light); this emission ceases when the irradiation of the material comes to an end. Thus fluo-rescence is the property of some atoms and molecules to absorb light at a particular wavelength and to subsequently emit light of longer wavelength after a brief interval, which is called the fluorescence lifetime [44]. When the atoms in the ground state are exposed to UV or visible radiations, they absorb the photons and thus by acquiring the energy make a transition to the higher energy states known as an excited singlet state, a state with lifetime of about 10−8 s. The excited electrons after some time release this excess energy in the form of photons, thereby making a back transition to the ground state (singlet state), emitting excitation energy as fluorescence. During this electronic transition the spin of the electron is not altered; the singlet ground state and the excited singlet state have like multiplicity. Thus fluorescence is an almost instantaneous effect, where light emission


is connected with electronic transitions between levels of like multiplic-ity ending within about 10−8 s after excitation. It refers to the light emis-sion of relatively short persistence about 10−6 to 10−12 s. Fluorescent lamps works on the same phenomenon. They contain mercury vapor at very low pressure. When current is passed through mercury vapor, it gets excited and emits UV light. The walls of fluorescent light, coated with phosphor, absorb UV light and transmit it into visible light.

1.3.3.2 PhosphorescencePhosphorescence is delayed luminescence or afterglow [45]. Unlike flu-orescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower timescale associated with the re-emission of light is due to forbidden energy state transitions and defect levels in the material. The absorbed radiation is re-emitted at a lower intensity that can last for several hours. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released slowly in the form of light. Phosphorescence is the mechanism by which glow-in-the-dark materials emit light when exposed to primary excitation. In phosphores-cence an electron may be excited, under reversal of its spin, to a higher energy level, called an excited triplet state. Singlet ground states and excited triplet states have different multiplicity. For quantum mechanical reasons, transitions from triplet states to singlet states are forbidden and therefore the lifetime of triplet states is considerably longer than that of singlet states, i.e., the luminescence originating in triplet states has a far longer duration than that originating in singlet states. A triplet-singlet transition is much less probable than a single-singlet transition. The lifetime of an excited triplet state can be up to 10 seconds, in compar-ison with the 10−5 to 10−8 s average lifetime of an excited singlet state. Emission from triplet-singlet transitions can continue after initial irradi-ation. It persists longer (>10−8 s), sometimes even seconds, minutes, and hours. Many glow-in-the-dark products, especially toys, paints, etc., for children, involve substances that receive energy from light, and emit the energy again as light later. These slower time scales of re-emission are associated with forbidden energy state transitions and hence these tran-sitions occur less often in materials in which absorbed radiation may be re-emitted at a lower intensity for up to several hours. Phosphorescence triggered by visible light or infrared light is known as optically stimulated luminescence. The Jablonski diagram illustrating fluorescence and phos-phorescence is shown in Fig. 1.9.


According to Reinhoudt’s empirical rule [20] the intersystem process will be effective when ΔE (S1−T1) is at least 5000 cm−1 for all type of ligands. It has been reported that transfer of energy from singlet excited state of ligand to lanthanide emissive levels is not very important when considering examples within a theoretical model [47]. The triplet state thus plays an important role in the energy-transfer process as confirmed by experimental evidence, as back-energy transfer has been reported in many cases if there is low energy gap between the lowest triplet state of chromophore and lanthanide emissive levels. An empirical rule given by Latwa states that ligand-to-metal energy transfer takes place only when ΔE (T1−5D4) is greater than 2500 cm−1 in the case of Eu(III) complexes, which results in higher photoluminescence quantum yield [48]. This phe-nomenon can be observed in organic as well as inorganic complexes. For example, let us consider an inorganic phosphor SrAl2O4:Eu2+, Dy3+, which is a well-studied phosphorescent (persistent) phosphor [49–53]. The Sr2+ and Eu2+ ions are very similar in their ionic size (i.e., 1.21 and 1.20 Å, respectively), suggesting that Eu2+ are likely to occupy Sr2+ positions in the crystal structure. In a given host, the emission of light by Eu2+ is influ-enced by the covalency, size of the cation, strength of the crystal field as well as the alignment. The crystal field strength determines the splitting of the energy level [54] as illustrated for MAl2O4:Eu2+ (M = Ca, Ba, and Sr) phosphor in Fig. 1.10A [55]. The PL spectra of combustion synthesized and snapshots of (A) CaAl2O4:Eu2+, Dy3+, (B) BaAl2O4:Eu2+, Dy3+, and (C) SrAl2O4:Eu2+, Dy3+ phosphors after excitation with a UV source are given in Fig. 1.10B.

In BaAl2O4 and SrAl2O4, the Sr and Ba ions form linear chains in the lattice [57]. A divalent europium ion in these chains experiences in

Figure 1.9 Illustration of fluorescence and phosphorescence. Solid and dotted lines represent radiative and nonradiative transitions, respectively [46].


addition to the negative charges of the nearest anion neighbors posi-tive charges due to cation neighbors in the chain direction. The positive charges can orient one d-orbital preferentially. This will lower its energy and therefore result in Eu2+ emitting at longer wavelengths. Eu2+ prefer-entially oriented 5d-orbital results emission at 570 nm, while the intense 510 nm peak results from the rest of the orbitals. Some electrons promoted to the 5d levels may get trapped at oxygen-defect levels. These electrons are then released to the conduction band at a later stage depending on the temperature (energy) and may in turn be captured by the Eu2+ ions and

Figure 1.10 (A) Schematic energy level diagram of Eu2+ ions versus the crystal field Δ in MAl2O4 (M=Ca, Sr, and Ba) [55]; (B) PL spectra [56] of (a) CaAl2O4:Eu2+, Dy3+, (b) BaAl2O4:Eu2+, Dy3+; and (c) SrAl2O4:Eu2+, Dy3+.


the energy is released to the 4f level and a subsequent emission of light with a long afterglow. The afterglow time can be increased further by the addition of trivalent ions such as Dy3+. Several authors have proposed and published different mechanisms for the effect of the Dy3+ on the after-glow time, but basically it increases the defect-level depth of the captured electrons. An example of the application of a long afterglow phosphor is shown in Fig. 1.11, where SrAl2O4:Eu2+, Dy3+ was mixed with a com-mercial polymer to illustrate the glow-in-the-dark effect of the phosphor.

1.3.4 Classification of Luminescence Based on the Source of ExcitationThe emission of light via the luminescence process can be classified on the basis of the source of excitation used for triggering the electrons. Classification of the luminescence phenomenon based on the source of excitation and its applications is given in Table 1.4.

1.3.5 Luminescent SourcesLuminescent sources work on the principle of luminescence and hence emit cool light. These sources include (1) linear fluorescent lamps that employs the phenomenon of fluorescence to generate white light, which can last about 10–20 longer than standard incandescent lamps with life-times around 7000–10,000 hours of usage. (2) A compact fluorescent lamp (CFL) employs a curved or folded fluorescent tube to fit into the space of an incandescent bulb and a compact electronic ballast in the base of the lamp as shown in Fig. 1.12A. They are commonly used in commercial projects and in residential applications. However, they contain mercury (hazardous waste) [59], which complicates their disposal. Light-emitting

Figure 1.11 Schematic example of part of the phosphorescent mechanism of SrAl2O4:Eu2+, Dy3+ [57].

Table 1.4 Classification of luminescence and their applications [58]Phenomenon Definition Energy source Types Applications

Photoluminescence It is the phenomenon in which emission of light takes place from any form of matter after the absorption of photons.

Absorption of electromagnetic radiation (photons)

Fluorescence: It is a type of luminescence characterized by very short lifetimes; typically a spin-allowed process.

Phosphorescence: It is a type of luminescence characterized by a long lifetime; frequently a spin-forbidden process.

Fluorescent lamps, phototherapy lamps, highlighting paints and inks, secret inks, image intensifier, display devices, optically pumped solid state lasers, up-conversion lasers, luminescent solar concentrators, diagnosis.

Radioluminescence It is the phenomenon in which light is created in the material after bombarding with ionizing radiation.

Bombardment by ionizing radiation such as X-rays or γ-rays or beta particles.

– X-ray imaging, X-ray scintillators, scintillation detectors and dosimetry.

Electroluminescence It is the phenomenon in which, typically a semiconductor emit light in response to an electrical current or a strong electric field.

Electric current passing through a substance (electric field)

Cathodoluminescence: The phenomenon in which electrons impacting on a luminescent material cause the emission of photons, which may have wavelengths in the visible spectrum.

LEDs, laser diodes, thin film electroluminescent lamps and displays, TV screens.

(Continued)


Chemiluminescence It is the phenomenon in which two chemicals react to form an excited intermediate, breaks down releasing some of its energy as photons of light to reach its ground state.

Chemical reactions Bioluminescence: The phenomenon in which light emission is by a living organism when energy is released from bio chemical reactions occurring inside the organism.

Electrochemiluminescence: The phenomenon in which light emission results from electrochemical reactions.

In analytical chemistry for chemical analysis

Thermoluminescence It is the luminescence, displayed by certain crystalline materials, where the previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light when the material is triggered by temperatures above a certain threshold.

Ionizing radiation – Dosimetry of ionizing radiation, geological and archeological dating, environmental monitoring.

(Continued)


Mechanoluminescence It is a type of luminescence induced by any mechanical action in solids by elastic deformation, plastic deformation, and fracture of solids.

Mechanical energy Triboluminescence: The luminescence generated through the breaking of chemical bonds in a material when it is pulled, scratched, crushed or rubbed.

Fractoluminescence: The luminescence created from fractured crystals.

Piezoluminescence: The luminescence created by pressure upon certain solids.

Sonoluminescence: The emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

Crystalloluminescence It is defined as the emission of light during the crystallization of certain salts from liquid solution or from the fused phase.

Crystallization –


diodes (LEDs) and organic light-emitting diodes (OLEDs) create vis-ible light by means of electroluminescence as shown in Fig. 1.12B and C, respectively. They have the potential to surpass the energy efficiencies of traditional lighting lamps and transfigure the versatility in the field of lighting.

1.4 TERMINOLOGY ASSOCIATED WITH LUMINESCENCE

Some of the terminology associated with luminescence is defined below:Phosphor: A solid material in natural or synthesized form that exhibits

luminescent properties when exposed to radiation, UV light, or an elec-tron beam is called luminescent material, which is also known as phos-phor. Phosphors are usually microcrystalline powders or thin films designed to provide visible color emission. In a broader sense, the word phosphor is equivalent to solid luminescent material and dates back to the early seventeenth century. They generally consist of a host lattice in which activator ions are incorporated. The activator reacts with a center, which absorbs excitation energy and converts it into visible radiation.

Luminescence center: The addition of impurities disturbs the perfect peri-odicity of the crystal and thereby introduces additional localized levels in the forbidden gap. Luminescence occurs efficiently in such materials at molecular sites at which absorbed energy can be reemitted optically by electron transitions. In solids, such states are known as luminescence cen-ters. They absorbs the exciting radiation and rises to the excited state. They come back to the ground state by radiative emission or nonradiative decay.

Color centers: Color centers are the absorbing sites in solids caused by lattice defects, trapped electrons or holes, or by the formation of new chemical species.

Figure 1.12 (A) Fluorescent flower lamp, (B) LED lamps, and (C) OLED lamps.


Chromophore: A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others.

Dopant: A dopant, also called a doping agent, is a trace impurity ele-ment that is inserted into a substance in very low concentrations in order to alter the electrical or optical properties of the substance.

Activator: A type of dopant used in phosphors and scintillators in a very minute quantity in order to enhance the luminescence process.

Sensitizer: In some cases the excitation radiation is not absorbed by the activators but the other ion may absorb the exciting radiation and then transferred to the activator. In this case the absorber is known as sensitizer.

Quenching: Deactivation of an excited state by a nonemissive pathway is known as quenching.

Concentration quenching: If the activator concentration in the host exceeds a specific value known as the critical value, the efficiency decreases. This effect is known as concentration quenching. If the con-centration of the activator becomes so high that the probability of energy transfer exceeds that for emission then the excitation energy repeatedly goes from the one activator to the other and is eventually lost at the sur-face, dislocations or impurities. Thus it makes no contribution to the luminescence. The efficiency then decreases in spite of the increase of the activator concentration. Fig. 1.13 illustrates concentration quenching.

Thermal quenching: At lower temperatures the host lattice offers favorable conditions for luminescence while at high temperatures the nonradiative processes become dominant. This is known as thermal quenching.

Killer impurities: The impurities that reduce the intensity of phosphors even when in very small amounts are known as killer impurities.

Singlet state: A singlet state is a molecular electronic state in which all electron spins are paired, i.e., the spin of the excited electron is still paired with the ground state electron.

Triplet state: A triplet is a quantum state of a system with a spin of 1, such that there are three allowed values of the spin component, −1, 0 and +1. An energy-level diagram showing the spin in singlet and triplet states is shown in Fig. 1.14.

Excimer: An excited complex that does not exist in the ground state but is formed between one excited and one ground-state molecule of the same type is known as an excimer.

Exciplex: An excimer formed between two molecules of different types is known as an exciplex.


Figure 1.13 Concentration quenching.

Exciton: An electron and hole pair capable of emitting light after recombination is known as an exciton.

Hole: In solids, an electron-deficient center that can move frequently through the lattice is called a hole.

Trap: A lattice defect or chemical center in solids that can lock in an electron or a hole is known as a trap.

Intersystem crossing: Intersystem crossing is a process where there is a crossover between electronic states of different multiplicity as demon-strated in the singlet state to a triplet state, i.e., conversion of a system from a state of one spin multiplicity to another is known as intersystem crossing.

Internal conversion: Internal conversion is an intermolecular process of molecule that passes to a lower electronic state without the emission of radiation. It is a crossover of two states with the same multiplicity, mean-ing singlet-to-singlet or triplet-to-triplet states, i.e., relaxation of a system

Figure 1.14 Energy-level diagram showing singlet and triplet states.


from an upper state to a lower one of the same spin multiplicity is known as internal conversion. The internal conversion is more efficient when two electronic energy levels are close enough that two vibrational energy levels can overlap.

Stokes shift: Stokes shift, named after Irish physicist George G. Stokes, is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra of the same electronic transition as shown in Fig. 1.15. Stokes shift occurs due to the difference in interatomic separation in the ground state and excited state. Phosphors with large Stokes shift exhibit low-temperature quenching.

Quantum yield: The quantum yield of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.

Quantum efficiency: It is defined as the ratio of the number of photons emitted to the number of photons absorbed, i.e.,

Quantum eciency

No.of photonsemitted

No.of photons absorbed=

Phosphors with a quantum efficiency of 80% or greater are considered as efficient phosphors.

Hysochromic shift: Hypsochromic shift is a change of spectral band posi-tion in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a shorter wavelength (higher frequency). Because the blue color in the visible spectrum has a shorter wavelength than most other colors, this effect is also commonly called a blue shift.

Stoke’s shift

Absorption

Emission

Wavelength (nm)

Nor

mal

ized

inte

nsity

Figure 1.15 Absorption and emission spectra showing Stokes shift.


Bathochromic shift: Bathochromic shift is a change of spectral band posi-tion in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a longer wavelength (lower frequency). Because the red color in the visible spectrum has a longer wavelength than most other col-ors, this effect is also commonly called a red shift. These shifts in spectral band due to absorption and change in frequency are shown in Fig. 1.16.

Hyperchromic shift: Hyperchromic shift is the increase in the absorption wavelength in optical UV-Vis absorption spectra.

Hypochromic shift: Hypochromic shift is the decrease in the absorption wavelength in optical UV-Vis absorption spectra.

1.5 REALM OF LUMINESCENT MATERIALS

Man has really learned to make life comfortable for humanity. And it is rightly said that necessity is the mother of invention. Innovation in the field of luminescence technology has become popular and is particu-larly relevant to environmental and energy conservation problems. The advances in understanding luminescent phenomena and the discoveries of unusual luminescent processes, e.g., up-conversion and quantum splitting, provide unusual opportunities for the applications of luminescence. After decades of research and development, thousands of ecofriendly phosphors have been designed and are widely used in many areas today. Lasers, paints, and inks, lamp phosphor TV screens, cathode ray tubes, LEDs, OLEDs, SSL, and flat-panel displays are just some examples of luminescence innovation.

Hypsochromicshift

Hyperchromic shift

Bathochromic shift

Hypochromicshift

Wavelength (nm)

Nor

mal

ized

inte

nsity

Figure 1.16 Illustration of shift in spectral band due to absorption and change in frequency [60].


1.5.1 Lamp PhosphorLamps are substitutes of natural sunlight, and satisfy different socioeco-nomic needs of humanity. Hence, scientific research on lamp phosphors has a long history beginning more than a century ago. Researchers, scien-tists, and industrialists have investigated novel lamp phosphors with emi-nent characteristics such as good color rendition, stability under industrial handling, high quantum efficiency, high quenching temperature, energy efficient, toxic free, good lifetime, etc., in an effort to create energy-saving and ecofriendly lighting technology. As a result, in recent years there have been dramatic changes in the field of solid-state lighting (SSL) with LED lighting sources as well as the emergence of OLED lighting sources.

1.5.2 LasersThe process of luminescence is applicable to lasers and optical amplifiers, where lanthanide ions are employed as light-generating and amplifying constituents. For example, Nd3+: YAG laser employs the lanthanide triva-lent ion Nd3+, while Er3+ is used in optical fiber amplifiers, where light is used as the vehicle to carry the information.

1.5.3 NanophosphorsA new dimension was added to the field of luminescence with the discov-ery of nanophosphors. Nanotechnology is expected to facilitate the pro-duction of smaller and cheaper devices with increasing efficiency. In this regard, luminescent nanophosphors have gained a great deal of interest in the fields of analytical chemistry, bioengineering, and electroluminescent devices [61,62] due to their tailored properties [63–68]. They offer reduced electron-penetration depths along with the possibility of films with higher packing densities, resulting in better performance than their conventional micron-sized counterparts. The utility of nanoparticles for application as a luminescent material depends strongly on surface properties [69]. Hence, researchers in the field of light technology are focusing on various meth-ods to synthesize novel nanoparticles for different applications such as luminescent nanophosphors for RGB and white LEDs, displays, and other optoelectronic devices. The resolution of a display greatly improves with a reduction in the size of the pixels or the phosphors. Nanocrystalline zinc selenide, zinc sulfide, cadmium sulfide, and lead telluride are suitable can-didates for improving the resolution of monitors. The use of nanophos-phors is envisioned to reduce the cost of these displays and also to render


high-definition TVs and personal computers affordable. The production of displays with low energy consumption could be accomplished with car-bon nanotubes (CNTs). Due to their smaller diameter (a few nanometer), they can be used as field emitters with extremely high efficiency for field emission displays (FEDs). Nanotechnological approaches to organic/inor-ganic LEDs or quantum-caged atoms (QCAs) could apply to maximum light conversion as well as strong reduction of energy consumption for light. Novel applications and highly improved performance of existing devices has made research and development activities in the field of nanophosphors very important nationally and internationally.

1.5.4 Super Luminescent DiodesA super luminescent diode (SLD) is a periphery-emitting light source made of semiconductor. It is capable of emitting light with a wide emis-sion band, ranging between 5 and 100 nm, by employing electrolumi-nescence. SLDs combine the properties of high power and brightness of laser diodes with the low coherence of usual LEDs. They find application in fiberoptic gyroscopes, white light interferometry, optical coherence tomography, and optical sensing.

1.5.5 Light-Emitting DevicesLight-emitting devices (LEDs) have been around for nearly 50 years, but until a decade ago, they were used only in electronic devices as indica-tor lamps. This technology flourished due to its high efficiency, high reli-ability, rugged construction, durability, and the fact that it is mercury free. Technological development in achieving brighter LEDs resulted in applica-tions in small-area lighting, traffic lighting, indicators, electronic billboards, and headlamps for motor vehicles, flashlights, searchlights, cameras, store signs, destination signs on vehicles, general illumination, visual display, deco-rative purposes, etc. Light-emitting devices offer flexibility in their design, from zero to three-dimensional lighting, and are also used as seven-segment LED displays, in optical switching applications, visual signals, illumina-tion where light is reflected from objects to give visual response of these objects, as aviation lighting, automotive lighting, advertising, traffic signals, etc. Infrared LEDs can be used as a source in optical fiber communications and in the remote-control units of many commercial products including televisions, DVD players, and other domestic appliances. They can also be employed in communication devices as they exhibit faster response times. Some of the applications of LEDs are shown in Fig. 1.17.


1.5.6 Organic Light-Emitting DiodesOne of the newest kinds of LED displays is the organic LED display com-monly known as OLEDs, which use organic material that lights up when pro-vided with a current. These displays are flexible or conformal displays with striking visual appeal. OLEDs are a promising technology because they can be printed on any medium and offer large viewing angle, high resolution, high speed, and good color, although lifetime issues still need to be addressed. Some RGB OLED devices are shown in Fig. 1.18. The advantages of organic over inorganic LEDs include low-cost synthesis, good chemical compatibility, and relative ease of handling. They possess the properties of plastics as well as semiconductors and offer a simpler manufacturing process. Hence, there are a number of applications for organic semiconductors. In fact, most of the copy machines and laser printers in use today already use organic photoconduc-tors. In the future, a number of exciting developments like organic solar cells, organic field emission transistors, etc., are expected.

1.5.7 Solid-State LightingSSL is the alternative lighting achieved by an ecofriendly, energy-efficient, new green technology, where illumination is obtained through semiconductor devices like LEDs, OLEDs, and LEPs as sources of illumination, where light emis-sion is due to recombination of electron–hole pairs. This technology has the potential to exceed the energy efficiencies of incandescent and fluorescent lighting.

Figure 1.17 LED applications. (A) LED rope lights, (B) lantern string light, (C) signages, (D) traffic signals, (E) TV, (F) torch, (G) car bulbs, and (H) SSL.


In fact, SSL as the next generation of light sources for general illumination is expected to be used in homes to commercial applications offering low energy consumption and reduced maintenance. Novel research has been carried out by many researchers globally to stimulate the development of the science and technology. In practice, there are many challenges, and efficiently creating white light from semiconductor materials with band-gaps that span the visible spectrum is extremely challenging.

1.5.8 DisplaysIn today’s world of information technology, the display is an important inter-face of communication. It is the visual representation of texts and graphic images using display devices such as cathode ray tubes, liquid crystal displays, FEDs, plasma displays, organic/inorganic LEDs, or other image projection technology. The display generally consists of a projection screen and a device that produces the information on the screen. Displays can use analog signals as input to the display image-creation mechanism. With the emergence of semi-conductor technology in the 1950s, scientists recreated all the existing tech-nologies including displays with the help of semiconductors, which led to the invention of LED displays. LEDs have grown in popularity because (1) their size is compact, (2) they can be squeezed into tiny display units with minimal circuit parts, and (3) they consume much lesser power because the glow is due to emission of energy by electrons. They work on the principle of electrolu-minescence—the phenomenon in which electrical energy is converted into light energy by the recombination of electron–hole pairs.

1.5.8.1 LED DisplaysLight-emitting displays were introduced around 1967 but were very expensive at that time. Calculators used LEDs that were arranged to form

Figure 1.18 RGB OLED devices.


either a seven-segment display or a dot-matrix display. An LED display is a flat panel display that uses an array of LEDs as pixels as shown in Fig. 1.19. A cluster of red, green, and blue diodes can be driven together to form a full-color pixel. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. As they are smaller in size, they can be easily attached to printed circuit boards. Hence, the essential requirements of the next generation of displays are reproduction of light quality, brightness, contrast, improved color variation, resolution, low weight, reduction in thickness, reduction in cost, and low power con-sumption. The only way to achieve all these is by OLED displays, which are discussed in the following.

1.5.8.2 Organic Light-Emitting Diode DisplaysOLED display technology is an exciting and viable new innovation in dis-play monitors and lighting, and it is the next generation of display technol-ogy, which combines great colors and contrast with low power. The screens of OLED displays is much thinner and brighter than their predecessors including CRTs, plasmas, LCDs, etc. Human hair is 200X the thickness of OLED layers. The OLED display is created by arranging several OLEDs in a pattern with alternating compounds to provide full color. OLED dis-plays use one of two modes of operation to control all of the OLED pixels, either passive matrix (PMOLED) or active matrix (AMOLED) addressing schemes. Displays made of OLEDs are flexible or conformal with striking visual appeal that can be printed on to any medium.

One of the major advantages of OLEDs is that they are self-emissive, i.e., they provide their own light and don’t need any backlight. This makes them very thin and light, consuming less power than LCD screens. They offer large viewing angle, high resolution, high speed, and good color. Furthermore, OLED displays do not suffer from motion lag or motion blur as LCD displays do since they the fastest response rate time of any

Figure 1.19 LED displays [8].


type of display due to the fact that they utilize AMOLED. They have near perfect viewing angle, create light (are emissive) rather than block light, and have superior technological efficiencies in manipulating lighter, sim-pler carbon-based material, generating deeper blacks, brighter whites, and all the gray scales in between. Microsoft recently launched a fitness tracker made of a curved flexible OLED display with GPS, heart rate monitor, accelerometer, gyrometer, sleep and calorie tracking, ambient light sen-sor, skin temperature sensor, UV sensor, capacitive sensor, barometer, and Cortana integration. Some OLED applications are shown in Fig. 1.20.

1.6 CONCLUSION

The area of application of luminescent materials is vast and varied. Since time immemorial, incredible changes have evolved in the field of lighting, but these developed technologies have faced many challenges in an effort to offer eco-friendly and energy-efficient SSL. The history of lighting can be viewed as the expansion of increasingly efficient technologies for generating visible light in the desired spectral region. With the development of electricity the luminosity of artificial lighting improved and incandescent light bulbs became popular for indoor use. However, only about 15% of the consumed energy is emitted in

Figure 1.20 OLED applications. (A) Watch, (B) SSL, (C) curved TV, (D) slim TV, (E) rollable displays, and (F) fitness tracker display.


the form of light and the rest as heat. Incandescent lamps are the least expen-sive to buy but the most expensive to operate. Later with the invention of fluorescent lamps and CFLs, incandescent lamps lost their popularity. Mercury, contained in compact fluorescent bulbs, is more dangerous than lead or arse-nic. Warnings about the dangers of CFL bulbs have been prevalent for many years now. When these bulbs end up in landfills, waterways, oceans, and the ground it affects not only humans, but animals and the environment. The truly devastating consequences of this will mostly be seen by future generations, so it is important to understand the dangers now and take appropriate action to help mitigate further damage. With their good energy efficiency and charac-teristics that allow the adjustment of light intensity and spectral composition, LEDs have already opened up new research prospects for energy conversion and conservation and OLEDs are expected to emerge as a highly competent and viable alternative to existing lighting technologies.

REFERENCES [1] N. Thejokalyani, S.J. Dhoble, Defect Diffus. Forum 357 (2014) 1–27. [2] M. Yamad, Y. Narukawa, T. Makai, Jpn. J. Appl. Phys. 41 (2002) L246. [3] L.J. Davis, Light Measurement Handbook, International Light Arcade Publishing,

New York, 2003, https://www.intl-lighttech.com/services/ilt-light-measurement- handbook.

[4] B. Valeur, M.N. Berberan Santos, J. Chem. Edu. 88 (2011) 731. [5] D.R. Vij, Luminescence of Solids, Plenum Press, New York, 1998. [6] C. Fourassier, Luminescence Encyclopedia of Inorganic Chemistry, Academic Press,

New York, 1984. [7] S.W. Mc Keever, Thermoluminescence in Solids, Cambridge University Press,

Cambridge, 1985. [8] C. Ronda, Luminescence: From Theory to Applications, Wiley-VCH Verlag GmbH &

Co. KgaA, Weinheim, 2008. [9] R.H. Petrucci, W.S. Harwood, F.G. Herring, General Chemistry, eighth ed.,

Prentice-Hall, New York, 2002, pp. 341–342. [10] C.E. Housecroft, A.G. Sharpe, Inorganic Chemistry, second ed., Pearson Prentice-Hall,

Harlow, 2005, pp. 20–21. [11] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 32 (2014) 448–467. [12] M.J. Weber, T.E. Varitimos, B.H. Matsinger, Phys. Rev. B 8 (1973) 47–53. [15] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience

Publishers, New York, 1968. [13] Y. Haas, G. Stein, J. Phys. Chem. 75 (1971) 3677. [14] W. De, W. Horrocks, D.R. Sudnick, J. Am. Chem. Soc. 101 (1979) 334. [16] J.M. Lehn, Angew. Chem. Int. Ed. Engl. 29 (1990) 1304. [17] N. Sabbatini, M. Guardigli, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201. [18] G.F. Sa, O.L. Malta, C. Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, et al.,

Coord. Chem. Rev. 196 (2000) 165. [19] S. Tobita, M. Arakawa, I.J. Tanaka, Phys. Chem. 88 (1984) 2697. [20] G.F. deSá, O.L. Malta, C. de Mello Donegá, A.M. Simas, R.L. Longo, P.A. Santa-Cruz,

et al., Coord. Chem. Rev. 196 (2000) 165–195.

http://refhub.elsevier.com/B978-0-08-101213-0.00001-1/sbref1


https://www.intl-lighttech.com/services/ilt-light-measurement-handbook

https://www.intl-lighttech.com/services/ilt-light-measurement-handbook



























[21] J. Vuojola, M. Syrjänpää, U. Lamminmäki, T. Soukka, Anal. Chem. 85 (2013) 1367–1373. [22] S.I. Weissman, J. Chem. Phys. 10 (1942) 214–217. [23] G.A. Crosby, R.E. Whan, R.M. Alire, J. Chem. Phys. 34 (1961) 743–748. [24] M.H. Werts, M.A. Duin, J.W. Hofstraat, J.W. Verhoeven, Chem. Commun. (1999)

799–800. [25] C. Yang, L.M. Fu, Y. Wang, J.P. Zhang, W.T. Wong, X.C. Ai, et al., Angew. Chem. Int. Ed.

Engl. 43 (2004) 5010–5013. [26] D.L. Dexter, J. Chem. Phys. 21 (1953) 836–850. [27] T. Förster, Ann. Phys. 437 (1948) 55–75. [28] J.P. Leonard, C.B. Nolan, F. Stomeo, T. Gunnlaugsson, Topics in Current Chemistry;

Photochemistry and Photophysics of Coordination Compounds, vol. 281, Springer-Verlag, Berlin, 2007, pp. 1–43.

[29] S. Faulkner, L.S. Natrajan, W.S. Perry, D. Sykes, Dalton Trans. (2009) 3890–3899. [30] G. Blasse, The Influence of Charge-Transfer and Rydberg States on the Luminescence

Properties of Lanthanides and Actinides, vol. 26, Springer-Verlag, Berlin, 1976, pp. 43–79.

[31] G. Dipti, M. Katiyar, Deepak, Opt. Mater. 28 (2006) 295–301. [32] P. Belli, Nucl. Phys. A 789 (2007) 15–29. [33] W.W. Meinke, R.E. Anderson, Anal. Chem. 26 (5) (1954) 907–909. [34] C.K. Gupta, N. Krishnamurthy, Int. Mater. Rev. 37 (1992) 197–248. [35] N.H. Mc Coy, J. Am. Chem. Soc. 58 (9) (1936) 1577–1580. [36] M.E. Weeks, J. Chem. Educ. 9 (10) (1932) 1751. [37] A.M. Srivastava, C.R. Ronda, Electrochem. Soc. Interface 12 (2003) 48–51. [38] P. Caro, Rare Earths (1998) 323–325. [39] S. Cotton, Lanthanide and Actinide Chemistry, John Wiley & Sons, Ltd., Chichester,

2006, p. 77. [40] K.A. Gschneidner, J.-C. Bünzli, V.K. Pecharsky, Handbook on the Physics and

Chemistry of Rare Earths, Elsevier, Burlington, 2005. [41] M. Kadarkaraisamy, A.G. Sykes, Inorg. Chem. 45 (2) (2006) 779–786. [42] H. Lee, S.-H. Choi, J. Appl. Phys. 85 (3) (1999) 1771–1774. [43] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 16 (2012) 2696–2723. [44] J.S. Sparks, R.C. Schelly, W.L. Smith, M.P. Davis, D. Tchernov, V.A. Pieribone, et al.,

PLoS One, 9 (2014) p. e83259. [45] A. Köhler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701. [46] F.J. Steemers, W. Verboom, D.N. Reinhoudt, E.B. Van der Tol, J.W. Verhoeven, J. Am.

Chem. Soc. 117 (1995) 9408. [47] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.C. Rodriguez-Ubis, J. Kanakare,

J. Lumin. 75 (1997) 149. [48] A. Kohler, H. Bassler, Mater. Sci. Eng. R 66 (2009) 71–109. [49] G.R. Hayes, B. Deveaud, Phys. Status Solidi A 190 (3) (2002) 637–640. [50] P. Sharma, D. Haranath, H. Chander, S. Singh, Appl. Surf. Sci. 254 (2008) 4052. [51] P.D. Nsimama, O.M. Ntwaeaborwa, H.C. Swart, J. Lumin. 131 (2001) 119–125. [52] Y.L. Chang, H.I. Hsiang, M.T. Liang, J. Alloys Compd. 461 (2008) 598. [53] P.D. Sarkisov, N.V. Popovich, A.G. Zhelnin, Glass Ceram. 60 (2003) 9. [54] Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zhang, B. Lu, Mater. Chem. Phys. 70 (2001)

156–159. [55] H.C. Swart, O.M. Ntwaeaborwa, Comprehensive Inorganic Chemistry II, vol. 4,

Elsevier, Amsterdam, 2013, pp. 73–86. [56] B.M. Mothudi, O.M. Ntwaeaborwa, J.R. Botha, H.C. Swart, Physica B Condens.

Matter 404 (22) (2009) 4440–4444. [57] M. Kamada, J. Murakami, N.J. Ohno, J. Lumin. 87–89 (2000) 1042. [58] N.A. Atari, Phys. Lett. A 90 (1–2) (1982) 93–96.






















































[59] G.E. Buono-core, H. Li, Coord. Chem. Rev. 99 (1990) 55. [60] R. Soniya, N. Thejokalyani, S.J. Dhoble, Organic Light Emitting Diodes(OLED)

Materials, Technology and Advantages, NOVA Science Publishers, New York, 2016, pp. 41–92.

[61] B.M. Tissue, Chem. Mater. 10 (1998) 2837. [62] J.A. Nelson, E.L. Brant, M.J. Wagner, Chem. Mater. 15 (2003) 688. [63] C.H. You, K.Y. Lee, R.Y. Chey, R. Menguy, Gastroenterology 79 (2) (1980) 311–314. [64] X. Fang, T. Zhai, U.K. Gautam, L. Li, L. Wu, Y. Bando, et al., Prog. Mater. Sci. 56 (2011)

175–287. [65] M. Bredol, M. Kaczmarek, J. Phys. Chem. A 114 (2010) 950–955. [66] X. Fang, Y. Bando, M.Y. Liao, T.Y. Zhai, U.K. Gautam, L. Li, et al., Adv. Funct. Mater.

20 (2010) 500–508. [67] E. Chang, J.S. Miller, J.T. Sun, W.W. Yu, V.L. Colvin, R. Drezek, et al., Biochem.

Biophys. Res. Commun. 334 (2005) 1317–1321. [68] H. Chander, S. Chawla, Bull. Mater. Sci. 31 (2008) 401–407. [69] P. Ashutosh, P. Anjana, R. Mukesh Kumar, H.C. Verma, Mater. Chem. Phys. 96 (2006)

466–470.


















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http://dx.doi.org/10.1016/B978-0-08-101213-0.00002-3

CHAPTER 2

Luminescence in Organic Semiconductors

2.1 INTRODUCTION

Since times immemorial, humans have been inspired by nature, from mac-roscale to microscale and now to nanoscale. Animals have been inspir-ing designs of cars, robotics, and even computational algorithms on the basis of their behavior. Some of today’s toughests materials were inspired by deer antlers, and chemical products like waterproofing sprays were inspired by specific nanostructures of lotus leaves. In particular, scientists and researchers have taken advantage of the fact that nature can adapt itself to better respond to environmental changes and can provide feasible solutions to problems like better configurations of structure of matter. In fact, nature tends to optimality in all different ways. For instance, consider atom structures that tend to minimize energy in bonds and at the same time preserve particular characteristics depending on atom relationships [1]. Similarly, chemical organic compounds inspire artificial organic net-works that may lead to technological revolutions in the field of organic electronics. Today’s technology cannot be imagined without the opto-electronic devices resulting from inorganic/organic semiconductors such as light-emitting diodes/organic light-emitting diodes, transistors/organic field effect transistors, solar cells/organic solar cells, etc. Organic materi-als illustrate mechanical and chemical properties that drastically distinguish them from inorganic ones. They have many advantages; first and foremost, they employ lower-technology processing with less sensitivity to the pro-cessing environment, flexibility, and the ability to tailor the properties of the materials to specific applications. Furthermore, they are hydrocarbon molecules that mingle semiconducting properties with mechanical prop-erties such as straightforward process ability and flexibility. The weak Van der Waals forces that unite the molecules to create a solid entail a low dielectric constant and thus Coulomb and exchange interactions between electrons are significant. As a result, photo excitation or electrical excita-tion create strongly bounded excitons (electron–hole pairs). Based on the


relative orientation of the electron and hole spin, the exciton may be a singlet or triplet spin state. Organics in particular can emit light directly as conventional cathode-ray-tube and plasma display panels, rather than rely-ing on backlighting systems such as liquid-crystal displays. Consequently, they have led to exciting opportunities in the field of optoelectronic devices based on organic materials. It’s easy to imagine these technolo-gies leading to poster-size televisions that can be rolled up and stored in mailing tubes, or unrolled and thumbtacked to a wall. These materials are already being applied in compact, lightweight, power-efficient light-emitting devices such as cell-phone displays. The primary problem with all organic devices is stability and hence significant efforts are still needed to improve device efficiency by developing higher efficiency materials or optimizing device structures. The already established commercialization of organic semiconductor includes display applications, lighting applica-tions, and photocopier machines [2]. Today, the large-scale exploitation of organic semiconductor materials is in the xerographic process of any common photocopier machine. As in inorganic materials, organic mate-rials also exhibit insulating, semiconducting, and conducting properties. The energy-gap differences between valence band or highest occupied molecular orbital (HOMO) and conduction band or lowest unoccupied molecular orbital (LUMO) are moderate, typically in the range of 1–4 eV [1]. Due to these fascinating properties, the technological exploitation of organic semiconductors is constant.

2.2 ORGANIC COMPOUNDS

Organic compounds are those compounds that consist of carbon and a few other elements like hydrogen, oxygen, nitrogen, sulfur, phosphorus, and halogens like chlorine, bromine, fluorine, iodine, etc. However, some carbon-containing compounds, such as carbonates, carbides, cyanides, and simple oxides of carbon (CO and CO2) are not considered to be organic. Organic compounds have important physical and chemical prop-erties. Generally, physical properties are associated with the structure of the organic compounds while chemical properties relate to their behav-ior. Organic compounds are comprised of basic units called atoms. When atoms interact among them, they form molecules and compounds. Thus the structure of organic compounds includes the set of atoms and the ways they are bonded, but energy minimization and geometric configura-tion also play an important role in the structure. Fig. 2.1 shows a simple carbon atom model and its energy-level diagram.

Luminescence in Organic Semiconductors 41

The discovery of electrical conductivity in organic materials, a cat-egory of materials that previously was thought to be exclusively isolat-ing, opened a new and fascinating research field. In 2000 the Nobel Prize in Chemistry was awarded to Alan J. Heeger, Alan G. McDiarmid, and Hideki Shirakawa for “The discovery and development of conductive polymers.” Their experiments performed in the 1970s with trans-poly-acetylene revealed that it was possible to determine the conductivity of this covalent organic material by exposing it to vapors of chlorine, bro-mine, or iodine [4]. Nevertheless, previous studies conducted by Weiss and coworkers in the early 1960s had already demonstrated electrical conduc-tivity in iodine-doped oxidized polypirrole [5–7].

2.2.1 Classification of Organic CompoundsOrganic compounds can also be classified on the basis of the parameters discussed in the following.

2.2.1.1 Based on the Presence of HeteroatomsBased on the presence of hetero atoms, organic compounds are classified as (1) organo-metallic compounds, which feature bonds between carbon and a metal [8] and (2) organo-phosphorous compounds, which feature bonds between carbon and phosphorous compounds [9].

2.2.1.2 Based on SizeBased on their size, organic compounds are mainly divided into two classes: small molecules (short chain) and polymers (long chain) mole-cules. These materials have been studied for organic electroluminescence because they have an extended region of alternating single and double bonds in a chain of carbon atoms in common. An important difference between the two classes of materials lies in how they are processed to form thin films.

Figure 2.1 Simple carbon atom model of and its energy-level diagram [3].


Small MoleculesThese are generally materials with low molecular weight and hence gen-erally named as small molecules or organic compounds. They have a molar mass approximately <1000 g/mol. An advantage of small molecules is more facile control of charge transport by modification of various molec-ular parameters. For example, the ability of these molecules to pack into well-organized polycrystalline films leads to higher mobility as compared to polymeric semiconductors. The structure of some popular small mol-ecules is depicted in Fig. 2.2.

PolymersThe peculiar property of carbon is that it can readily form chains or net-works that are linked by carbon–carbon bonds. The linking process is called polymerization, while the chains or networks are called polymers. Polymers have a molecular weight greater than 1000. Just by tailoring the conditions of polymerization, the chemical composition of the product and its properties such as chain length, branching, and tacticity can be eas-ily altered. The source compound is basically known as a monomer. With a single monomer as a start, the product is a homopolymer. A second-ary component(s) may be added to produce a heteropolymer (copolymer), and the degree of clustering of the different components can also be con-trolled. Physical characteristics, such as density, tensile strength, mechanical strength, hardness, abrasion resistance, heat resistance, transparency, color, etc., depend on the final composition. In organic compounds, polymers are molecules of long chains formed by monomer bonds [10] with rel-atively higher molecular weights. In broad spectrum, monomers used in polymers are iteratively repeated through the whole network and relations are done by covalent bonds. They have different structures; the most com-mon are linear, branched, comb-shaped, ladder, star, cross-linked network, and dendritic. Polymers have high structural stability, good solubility, and tunable electrical properties, and the physical properties of polymers are related to resistance, elasticity, etc. The semiconducting behavior of a poly-mer can be determined by conjugation length of the alternate single and double bonds and rotational freedom of constituent chains. The solubility can also be attributed to the alkyl chains of polymers. Polymers can only be deposited by solution-processable techniques. They exist in two main groups: (1) synthetic or industrial polymers, which are artificially manufac-tured, and (2) biopolymers, which occur within the natural environment without any human intervention. Many complex multifunctional group


Figure 2.2 Structure of some popular small molecules. (A) 1,3,5-Tris(carbazol-9-yl)benzene; (B) 9,10-Bis(2-naphthyl)anthraces; (C) Alq3-Tris(8-hydroxyquinoline)alumi-num; (D) Bathocuproine; (E) Cobalt phthalocyanine (CoPc); (F) Copper(II) phthalo-cyanine (G) Ir(ppy)3-Tris[2-phenylpyridine]iridium; (H) m-MTDATA—4,4′,4″-Tris[phenyl (m-tolyl)amino] triphenylamine; (I) N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine; (J) 3-(Biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole; (K) 2-TNATA— 4,4′,4″-Tris[2-naphthyl(phenyl)amino] triphenylamine; (L) 4,4′-Bis(N-carbazolyl)-1,1′- biphenyl.


molecules are important in living organisms. Some are long-chain bio-polymers, which include peptides, DNA, RNA, and polysaccharides such as starches in animals and celluloses in plants. The other main classes are amino acids, carbohydrates, the nucleic acids, and the lipids. The chemical structures of some of the most-studied organic polymeric semiconductors are shown in Fig. 2.3.

2.2.1.3 Based on Functional GroupCarbon has the unique property of uniting with other carbon atoms through strong covalent bonds to form long chains and rings. These com-pounds may be classified into different functional groups depending on the type of bonding between carbon and oxygen atoms. The functional group can be defined as an atom or a group of atoms that are joined together in a specific manner, which is responsible for the characteris-tic chemical properties of organic compounds. It is a molecular module, which can have decisive influence on the chemical and physical properties of organic compounds. The concept of functional groups plays a vital role in classifying structures and also for predicting properties. Molecules are generally classified on the basis of their functional groups. For example, structural features such as C = C, C ≡C, C = O, OH, NH2, and C ≡N are the functional groups of alkenes, alkynes, carbonyl compounds, alcohols, amines, and nitriles, respectively.

2.2.2 CharacterizationOrganic compounds can be characterized by the analytical methods listed in Table 2.1.

2.2.3 PropertiesThe physical properties of organic compounds include both quantita-tive features such as melting point, boiling point, and refractive index and qualitative features such as odor, consistency, solubility, and color [8,12–14]. Organic compounds typically melt and many boil. Some, especially sym-metrical ones, sublime. They are not very stable at temperatures above 300°C, although some exceptions exist. The melting and boiling points correlate with the polarity of the molecules and their molecular weight. Neutral organic compounds are less soluble in water than in organic sol-vents (hydrophobic). Exceptions include organic compounds that contain ionizable groups as well as low-molecular-weight alcohols, amines, and car-boxylic acids where hydrogen bonding occurs. Solubility in the different solvents depends on the solvent type and on the functional groups if present.


Figure 2.3 Chemical structures of some of the most-studied organic polymeric semi-conductors: polyparaphenylenevinylene (PPV), polyparaphenylene (PPP), polythio-phene (PT), polyfluorene (PF), and polyfluorene copolymers (where X stands for various (hetero)cycles); polyfluorene (PFO); poly(3-alkylthiophene) (P3AT); polymethyl methacrylate (PMMA); polyvinylcarbozole (PVK); polystyrene (PS) [11].

2.3 ORGANIC SEMICONDUCTORS

Today, the most novel applications of electronics are based on semicon-ductors. When we think of semiconductors, we generally think of silicon, germanium, gallium arsenide, gallium nitride, etc., which are all inorganic crystalline materials. In contrast, organic evokes the thought of plastics,


Table 2.1 Organic compounds: Characterization techniques and targeted outcomesS.No. Characterization technique Targeted outcome

1. Elemental analysis To determine the elemental composition of a molecule

2. Mass spectrometry To indicate the molecular weight of a compound and to identify the exact formula of a compound

3. Crystallography To determine molecular geometry, crystalline or amorphous nature

4. Nuclear magnetic resonance (NMR) spectroscopy

To determine the physical and chemical properties of atoms or the molecules in which they are contained

5. Fourier transform infrared (FTIR) spectroscopy

To confirm the formation of the synthesized complex

6. Thermo gravimetric analysis/differential thermal analysis (TGA/DTA)

To determine the thermal properties such as decomposition temperature, melting point

7. UV/VIS spectroscopy To probe absorption wavelengths and to determine stoke’s shift and energy gap

8. Photoluminescence spectra To determine the emission wavelength

9. Scanning electron/tunneling electron microscopy

To study the morphology and to determine the particle size

which are generally electrical insulators. But with the emergence of organic electronics, semiconducting organic materials in solid forms are now possible. The term organic semiconductors generally refers to materi-als made up by carbon and hydrogen atoms, with a few heteroatoms such as sulfur, oxygen, and nitrogen incorporated. They belong to a class of materials that possess the electronic advantages of semiconducting mate-rials with the chemical and mechanical benefits of organic compounds such as plastics. They exhibit properties such as absorption and emission of light in the visible spectral range and a degree of conductivity that is sufficient for the operation of traditional semiconductor devices such as LEDs, solar cells, and field-effect-transistors (FETs). The ability to absorb light, conduct electricity, and emit light is united with a material structure that can easily be modified by chemical synthesis. These properties pro-pel them for semiconductor applications such as displays, lighting panels,


or solar cells, which may be fabricated with a wide variety of solution-processing techniques or vacuum-deposition techniques. But how do organic semiconductors differ from inorganic semiconductors in the pro-cess of conduction? Traditional inorganic semiconductors have low band-gaps, e.g., germanium (0.67 eV), silicon (1.12 eV), and GaAs (1.4 eV). Their typical intrinsic conductivities are in the range of about 10−8 to 10−2 Ω−1/cm. Their dielectric constant is large (εr = 11), so that Coulomb effects between electrons and holes are unimportant due to dielectric screening, and light absorption at room temperature creates free electrons and holes, leading to current and hence conductivity. In contrast, the conductivity of organic semiconductors is extrinsic, which results from the injection of charges at electrodes, from doping, and from the dissociation of photo-generated electron–hole pairs that are bound by their mutual Coulomb attraction. The characteristic features of organic materials are (1) absorp-tion as well as emission in the range of 2–3 eV, which precludes creating any significant charge carrier concentration by thermal excitation at room temperature; and (2) low dielectric constant (εr = 3.5), which results in the Coulomb interactions being significant. Hence, electron–hole pairs are created by optical or thermal excitation, bound by a Coulomb energy of about 0.5–1.0 eV. Traditional semiconductors with polymeric semiconduc-tors and crystalline/inorganic and molecular/organic solids are compared in Tables 2.2 and 2.3, respectively.

For electronic applications, the organic materials that contain π elec-trons are generally preferred. For example, ethylene molecule (C2H4) is sp

2 hybridized; carbon forms three coplanar σ-bonds, one with carbon atom and two with hydrogen atoms as shown in Fig. 2.4.

The fourth orbital (pZ) is perpendicular to the sp2 hybridized orbital plane and leads to additional π bonding between two carbon atoms. Thus the molecular orbits split into bonding (occupied) and antibond-ing (empty) states, which are generally known as HOMO and LUMO as shown in Fig. 2.5. These molecules form solids due to intermolecular bonding (Van der Waals) and take the shape of a narrow energy band. Due to these bonds, the electronic properties of organic solids are determined by the molecules.

The role of weak forces is to hold the organic molecules in a solid together. Some organic molecules, even unpaired electrons, can remain stable for a long time. In such cases, unpaired electrons will be the car-riers. This type of semiconductor is also obtained by pairing an electron donor molecule and an electron acceptor molecule and hence is known


Table 2.2 Comparison of traditional semiconductors with polymeric semiconductors [15]S. no. Parameter Traditional

semiconductorsPolymeric semiconductors

1. Bandgap (Eg) Si: 1.1 eV Poly (acetylene): 1.4 eVGe: 0.67 eV PPV: 2.2 eVGaAs: 1.35 eV MEH-PPV: 2.1 eV

2. Doping <l020 cm−3 incorporated in crystal structure.

up to 1022 cm−3 in interchain sites

3. Conductivity (σ) Doped silicon 10−l cm−1

PPV 10−14 Ω−l/cm

Intrinsic Si 0.0001 Ω−l/cm

Doped poly (acetylene) max. 104 Ω−1/cm

4. Purity Very high Polymer: lowSublimed molecules: high

5. Transport Three-dimensional Quasi-one-dimensional with 3D hopping to neighboring chains

6. Charge carriers Electrons and holes Electrons and holes localized as polarons or bipolarons

7. Refractive index (n) Si: 3.4 PPV is anisotropic: n = 2.2 and 1.7GaAs: 3.6

InGaAs: 3.58. Morphology Single crystal Disordered but tendency

for molecules to lie parallel to substrate

9. Stability Good PoorerDiffusion of dopant

requiresProne to photo-oxidation

High temperatures Organics are permeable to gases

10. Bonding Covalent bonding Molecules held by weak Van der Waals forces; atoms in moleculesheld covalently

11. Preparation Grown from melt, sawn, and polished

Deposited from solution as thin film (polymer) or vacuum sublimed (small molecule)


as a complex. Light emission from these materials is particularly promising. When voltage is applied to a thin film of semiconducting polymer it gives out light, providing the basis of new display technologies. Versatile exam-ples of organic semiconductor applications are listed in Table 2.4.

2.3.1 Charge Transport in Organic SemiconductorsOrganic materials are generally considered as electrical insulators, e.g., plastics. However, emergent research proves that organic materials pos-ses interesting conducting and semiconducting properties [24]. As stated earlier, the organic conducting polymers or conjugated organic semi-conductors have carbon backbones as their main constituent. In conju-gated materials along the carbon skeleton, the σ bond is localized, whereas

Table 2.3 Comparison of crystalline/inorganic and molecular/organic solids [16]S. no. Parameter Crystalline/inorganic

solidMolecular/organic solid

1. Bonding Metallic, ionic, covalent Ionic or covalent2. Charge carriers Electrons, holes, ions Polarons, excitons3. Transport Band Hopping4. Conductivity mode Intrinsic Extrinsic

10−8 to 10−2 Ω−1/cm5. Dielectric constant (εr) 11 3.56. Mobility 102 to 104 cm2/V-s 10−6 to 1 cm2/V-s7. Exciton Wannier-Mott Frenkel, charge

transfer8. Effective mass me or less (102 to 103) me9. Mechanism of light

emissionBand to band

recombinationExciton

recombination

Figure 2.4 Ethylene molecule. (A) Structure and (B) σ and π bonds.


the π bond is delocalized. The conjugation occurs due to sp2 hybridiza-tion of carbon atoms in the pz-orbital. By considering the atomic orbital, the bonding (π) orbital is formed by constructive interference of its wave function; otherwise, antibonding (π*) is formed.

Usually, the highest filled molecular orbit (HOMO) corresponds to a filled π-type orbital or a lone pair (nonbonding electrons), and the lowest unoccupied molecular orbit (LUMO) corresponds to an empty π*-type orbital or (if there is no π system) to an empty σ* orbital. As the pz-orbital is partially filled, there is an energy gap between the HOMO and LUMO. In conjugated polymers, the σ orbitals are tightly bonded and electroni-cally transition from σ to σ* as shown in Fig. 2.6. Accordingly, transition of electrons from the π to π* orbital requires the lowest energy and thus the conduction between the π–π* orbital occurs easily. Hence, the exis-tence of delocalized π electrons plays a vital role in electrical conduction. It is clear that they are wide-bandgap and small-bandwidth semiconduc-tors with a HOMO–LUMO gap in the range of 1–4 eV [26]. With such a large gap, the organic materials might be expected to be insulators (an electron would have to acquire a large thermal energy to make the jump from the valence band to the conduction band), but they are not. The fol-lowing are some effective methods that can generate charge carriers in organic semiconductors: Injection of carriers from metallic electrodes Optical excitation (creation of electron–hole pairs) Electrostatic or chemical doping

Figure 2.5 Formation of HOMO and LUMO states in ethylene molecule [16].


Table 2.4 Versatile examples of organic semiconductor applications [17]S. no. Organic semiconductor Structure Application Reference

1. 8-Hydroxy-quinoline aluminum (Alq3)

OLED [18]

2. Azomethin-zinc complex N,N1-disalicylidene-triethylenetetramine zinc(II) (1AZM-TEEA)

Display [19]

3. Polypyrrole Battery [20]

4. Poly(p-methoxytri phenylamine-9,9-octylfluorene)

Transistor [21]

5. Poly(3-octylthiophene) Solar cell [22]

6. 3,6-Bis(thianthrene)-N-hexadecylcarbazole

Biosensor [23]


The chemical structure of some p-type and n-type semiconductors is shown in Fig. 2.7.

2.4 HOMO AND LUMO IN ORGANIC SEMICONDUCTORS

In an organic semiconductor, the HOMO and LUMO play a vital role as the difference of their energy levels serves as a measure of the excitability of the molecule. The energy difference between the HOMO and LUMO level is regarded as the bandgap energy as shown in Fig. 2.8. The HOMO level in organic semiconductors is the valence band in inorganic semi-conductors. Similarly, the LUMO level in organic semiconductors is the conduction band in inorganic semiconductors. The basis for organic semi-conductors as a whole revolves around the ability to develop the required material parameters, e.g., the desired HOMO and LUMO energy bands.

Figure 2.6 Schematic view of bonding and antibonding orbitals [25].

Figure 2.7 Chemical structure of some p-type and n-type semiconductors; C60, fuller-ene spherical buckyball; TCNQ, 7,7,8,8-tetracyanoquinodimethane.


The HOMO is the orbital that can act as an electron donor, since it is the outermost (highest energy) orbital containing electrons. The LUMO is the orbital that can act as the electron acceptor, since it is the inner most-orbital that can accept electrons. Hence, a single orbital may be both the LUMO and the HOMO. All the electrons in all molecular orbits deter-mine the structure of the molecule, but the HOMO and LUMO are the most important from the point of view of reactivity. In some chemical reactions such as electron-transfer reactions the interactions between the HOMO and LUMO lead to reorganization of bonding of both react-ing partners. The HOMO and LUMO are called the frontier orbitals or boundary orbitals; they determine the way in which the molecule inter-acts with other species [27]. The electron-rich HOMO will interact strongly with the electron-deficient LUMO. The difference in energy between these orbitals and the overlap between orbital lobe sizes will par-tially determine the facility of the reaction and the site of bond formation. Unoccupied orbitals will provide the initial impetus for the reorganization of existing bonding.

The energy of the specific molecular structure depends on the energy of its electrons in the occupied molecular orbitals. All the electrons will influence the structure because different molecular geometries will have the different energies of their molecular orbitals. But from reactivity point of view some electrons and some orbitals are more important than others.

Figure 2.8 Molecular orbitals and energy level in organic semiconductors.


The electrons of the highest energy are the ones that the molecule would like to dump and the empty orbitals of the lowest energy in the reaction partner are the best dumping grounds. When the molecule forms a dim-mer or an aggregate, the proximity of the orbitals of the different mol-ecules induce a splitting of the HOMO and LUMO energy levels. This splitting produces vibrational sublevels in which each has its own energy, slightly different from one another. There are as many vibrational sublev-els as there are molecules that interact together. When there are enough molecules influencing each other, for example in an aggregate, there are so many sublevels that their discrete nature can no longer be perceived: they form a continuum and hence energy levels are considered as energy bands. Orbital states can be described with terms such as filled, empty, occupied, and unoccupied. An orbital that contains the maximum number of elec-trons it can hold is filled, an orbital that contains no electrons is empty, and an orbital that contains at least one electron is occupied while an orbital that contains at least one open space for an electron is unoccupied. A filled orbital is occupied, but an occupied orbital is not necessarily filled. Also, an orbital can be both occupied and unoccupied, i.e., occupied means that one space is occupied by an electron and unoccupied means at least one space is free to accept an electron. When biased, charge is injected into the highest HOMO at the anode and the LUMO at the cathode. These injected charges migrate through the applied field until two charges of opposite polarity encounter each other and at that point they annihilate to produce light.

2.5 CHARGE TRANSPORT IN ORGANIC MATERIALS AND DEVICES

Organic semiconductors must consist of π structure of molecules. In conju-gated polymers (intramolecular interaction) the π-bonded structure estab-lishes the delocalized state, allowing electrons to conduct easily along the conjugation length of the molecule. However, in organic films the inter-molecular overlap of the π orbital is much smaller than the intramolecular overlap. Hence, the mobility is lower due to weaker interaction between molecules rather than within a molecule. Consequently, most of the elec-trons become localized over the whole film. As a result, charge transport is not only within a molecule, but rather by the charge transfer from one molecule to another. Hence, to achieve good conductivity the conjugation length (intermolecular interaction) must be taken into consideration [28].


The charge transport properties of both polymers and small-molecule organic semiconductors can be modified by adding chemical constituents. Hence, conduction in organic devices can be modified by employing het-erojunction structures. A heterojunction device can be formed by mixing two or more than two intrinsic semiconductor materials (with different fermi energies and bandgaps). The band bending and alignment of the junction can be determined by the work function and electron affinity of each constituent [29]. Hence, polymer bulk heterojunction can be formed by blending organic semiconductors with two or more than two organic or inorganic semiconductor materials. In bulk heterojunction systems, charge carriers are dissociated throughout the active region of the device. The donor and acceptors are closely blended in the whole bulk, mostly in a disordered manner. The main advantage of bulk heterojunction structure is the increase in interface area between donor and acceptor entities.

In conjugated polymers charge carriers can be produced by chemical doping, light-induced photo carrier generation, and injection of carriers by electrodes in an electric field [30]. In general, charge transport depends on the width of the energy bands in which it moves. The transport mechanism in organic semiconductors (conjugated polymers) depends on conjugation length Lcon (extent of the delocalized π electrons in the polymer back-bone), interchain interaction between molecules, concentration of carrier density, and width of the energy band. In organic semiconductors, mobility (µ) is lower due to the localization of states. At room temperature, hopping-transport mobilities are much lower than the band-transport mobilities.

2.5.1 Band-Transport MechanismBand transport of charges is typically observed in crystalline structures. In this mechanism, the width of the energy bands is wide and charge carri-ers are freely transported. Electronic states are delocalized (extended) and charge transport is restricted only by scattering of the lattice vibration. The lattice vibrations are the lowest at low temperatures and hence the mobil-ity of the charge carriers can be enhanced by declining the temperature. Since inorganic materials are highly ordered, the band-transport mecha-nism is mostly observed in these materials. Accordingly, this mechanism cannot be considered in broad spectrum for organic materials (disordered structure). However, for organic materials (with highly ordered structure) whose mobility is large (>1 cm2/V-s), the band-transport mechanism is observed as shown in Fig. 2.9. The variation in mobility with temperature depends on the origin of scattering of the crystal lattice [31]. However, in


highly ordered and crystalline structure it obeys a power law, i.e., µ = T−n, where µ is mobility, T is temperature, and n is constant, which is always greater than 0. This law clearly establishes that mobility decreases with increase in temperature.

2.5.2 Hopping-Transport MechanismHopping transport of charges is often observed in amorphous (disordered) structures because they have weak intermolecular bonding. Due to this, dislocations in crystalline structure are formed in organic semiconductors (mainly in polymers). The disorder in organic polymers is mainly attributed to the dislocations formed within the system. Charge localization is due to the randomness of disorder potential. The dislocated segments result in potential wells, in which charge carriers are trapped as shown in Fig. 2.10. Consequently, the mobility of the material is reduced. In order to take part in conductivity, these trapped charges must hop out from these potential wells. The process in which charge carriers hop out from one site to another is called hopping. These charge carriers polarize from one site to the other and hence are called polarons.

In hopping transport, the width of the energy bands is narrow and the electronic states are considered as localized. Due to localization the mobility of organic semiconductors is low (<1 cm2/V-s). In crystalline structures the transport is hindered by vibration in lattice structure (phonon scattering), whereas for amorphous structure it is phonon-assisted. Accordingly, the mobility increases with temperature for disordered organic semiconductors [32]. The relationship between mobility and temperature is given as:

µ µ

α= −

0

0

1

expT

T

Figure 2.9 Band-transport mechanism.


where T is temperature, µ is mobility, µ0 and T0 are material property parameters, and α is a positive integer [33]. Generally, charge transport in disordered organic semiconductors is delineated by phonon-assisted hop-ping of charges from one localized site to the next site. Hopping transport of charges is mostly observed in disordered and multicrystalline organic thin films. The mobility of a material around 1 cm2/V-s can be considered as a boundary line between the hopping and band-transport mechanisms [34,35]. As the randomness in disorder potential increases, the overlap of the wave functions of the localized state exponentially decreases [36].

2.5.3 Tunneling-Transport MechanismThe tunneling effect states that if a particle is impinging on the barrier with energy less than the height of a potential barrier, it is not necessary for it to be totally reflected by the barrier but there is always probability that it may cross the barrier and continue its forward motion. This phe-nomenon is known as tunneling in quantum mechanics or more popu-larly known as the tunnel effect. In other words, the movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling.

Let us consider a particle of energy E that is less than the height of the potential barrier “V0” is approaching the barrier of width “L” from region I as shown in Fig. 2.11. Quantum mechanically, a fraction of the particles entering from region I have some probability to penetrate into region II, cross the barrier, and appear in region III. Thus there is a small but final probability of the subatomic particles such as electrons of energy E penetrating through the potential barrier W, even when they do not possess sufficient energy to overcome the potential barrier. The probability

Figure 2.10 Localized states in energy bands. (A) Hopping and (B) tunneling mechanisms.


of penetrating through the potential barrier decreases exponentially with the width of the barrier and also with the barrier height. But, according to classical theory, as the energy of the particle is less than the height of the potential barrier, the particle can never penetrate into region II and reach region III.

But, according to classical mechanics, a particle must have energy at least equal to the height of the potential barrier if it has to move from one side of the barrier to another. Thus the particle with energy E on the left-hand side of the potential barrier of height W cannot pass to the right-hand side of the potential barrier if E is less than W as shown in Fig. 2.12.

This tunneling mechanism of charge transport is observed in pure and crystalline structure, where the distance between the molecules is very small. In this mechanism, charges tunnel through the barrier at very low temperature if the barrier width is small and band and hopping mechanisms have less contribution, as shown in Fig. 2.10. It is consid-ered that mobilities are very low in the tunneling transport mechanism (µ ≪ 1 cm2/V-s) and depend less on temperature. The tunneling transport of charges is observed less in organic polymers.

Figure 2.11 Quantum illustration of the tunneling effect.

Figure 2.12 Classical illustration of tunneling phenomena.


2.6 LUMINESCENT ORGANIC MATERIALS: AN OVERVIEW

In today’s digital era, luminescent organic compounds, which contain a sig-nificant amount of carbon, have created a technological revolution in the field of light sources and flat-panel displays [37,38]. A wide variety of sub-strates can be used to deposit organic thin films on plastic so that flexible, transparent, and unbreakable displays can be fabricated. Electroluminescence (EL) in organic materials was first reported in 1965 in anthracene single crystals while studying charge injection and luminescence properties [39]. It remained an academic interest for the next two decades due to the dif-ficulty of growing a large crystal and the high voltage required (about 1000 V) to produce luminescence. The pitiable performance of these pre-mature devices can be accredited to three key factors: (1) disparity between the electroluminescence of organic materials and contact electrodes, (2) inconsistency between the mobility of the electrons and holes, and (3) lack of film-forming properties of these materials. However, extensive studies on optical and electrical properties of low molecular organic light emit-ting diodes (OLEDs), and polymer light emitting diodes (PLEDs) were reported after demonstration of the first efficient hetero structure device by Tang and Vanslyke [40] in 1987 on small molecules and by Burroughs in 1990 on conjugated polymers [41,42]. This significant breakthrough in OLED technology addressed all three issues and laid the foundation for the realization of full-color flat-panel display technologies based on organic devices (commonly known as OLEDs) [43]. Currently there is consider-able interest in the integration of electronic and photonic devices based on organic materials, which can be incorporated as active layers in elec-tronic thin-film devices such as OLEDs, organic solar cells, electrochemi-cal cells, and organic field effect transistors (OFETs) [44–48]. Progress in the field of OLEDs for display applications was recently highlighted by the demonstration of 55-inch full-color active matrix OLED display drivers by amorphous Si thin-film transistor [49]. The need for new lightweight, low-power, wide-viewing-angled, and handheld portable communication devices has pushed the display industry to revisit the current flat-panel digi-tal display technology used for mobile applications. Struggling to meet the needs of demanding applications such as e-books, smart networked house-hold applications, identity management cards, and display-centric hand-held mobile imaging devices, the flat-panel industry is now looking at new luminescent material-based devices with following challenges: Better electricity-to-light conversion efficiency Improvised device stability and lifetime


Material selection and optimization Encapsulation Uniformity over large areas Lowering the manufacturing cost Fine patterning Contrast Improvised pixel-switching rate

Based on the mechanism of luminescence in organic luminescent materials, they can be divided into two main categories: fluorescent and phosphorescent materials.

2.6.1 Fluorescent MaterialsThere are two branches of fluorescent materials, small molecules and poly-mers, which are based on molecular weight. Besides Alq3, coumarin, and rubrene, some metal chelates, such as zinc and beryllium, copper and bar-ium chelates, and conjugated small molecules have been widely used as small molecular emitters in OLEDs as shown in Fig. 2.13.

Fluorescent small molecules can be easily synthesized and purified. Until now, the three primary colors, red, green, and blue (RGB) light emis-sion, all can be obtained from small molecular materials with high bright-ness and efficiency in multilayer devices. However, the poor solubility of

Figure 2.13 Structures of small molecular fluorescent materials for OLEDs.


small molecules will not allow them to be solution processed. Furthermore, small molecules tend to crystallize readily and hence usually exist as crys-tals below their melting points, which will shorten the lifetime of devices. Thus the solubility of small molecules must be improved by introducing substituents to create solution processibility and at the same time suppress their crystallization in solid state by designing three-dimensional structural molecules. The development of fluorescent polymers is in line with small molecules. The semiconducting properties of conjugated polymers result from their extensively delocalized p-orbitals along the polymer chains. In conjugated polymers, the emission wavelength and solubility considerably depend on the nature and regularity of their side chains. Alternation of substituents has a crucial effect on the properties of polymers, which offers a flexible and versatile approach to achieve the desired properties. Polymers are advantageous in processibility over small molecules and the efficiency of materials is mainly determined by the polymer structures. However, the purity of polymers is normally poorer than that of small molecules, which results in relatively low device efficiency and lifetime. The device perfor-mance is also affected by two other factors: one is the excimer and the other is the quenching center in the materials. Thus improving the purity, reducing defects in polymers, and suppressing their close packing are effec-tive ways to enhance the efficiency of PLEDs.

2.6.2 Phosphorescent MaterialsTransition heavy metal (Pt, Ru, Ir, Re) complexes and rare earth metal complexes (Eu, Tb) containing suitable ligands have been real-ized as highly efficient phosphorescent materials at room temperature. Their luminescence originates from the lowest triplet metal to ligand charge transfer excited state (MLCT). In the process of MLCT, an elec-tron located in a metal-based d-orbital is transferred to the ligands. Phosphorescent materials can harvest both singlet and triplet excitons, and triplet harvesting allows all the excited states to contribute to light emis-sion. Thus, in theory, the internal quantum efficiency of phosphorescent materials can reach 100%. Compared with the lifetime of singlet exci-tons, the lifetime of triplet excitons is in microseconds. In the last decade, research on OLEDs has been directed at phosphorescent materials, which have demonstrated high external quantum efficiency (EQE). As a matter of fact, phosphorescent materials have broken through the EQE upper limit for fluorescent materials, which is around 5%. The EQEs of phos-phorescent materials can be as high as 20%. Three primary color polymer


electrophosphorescent light-emitting diodes have already been demon-strated and the efficiencies of the devices are improving steadily.

2.7 ORGANIC VERSES INORGANIC LUMINESCENT MATERIALS

In a crystal-like silicon or germanium (inorganic), the strong compiling between the constituting atoms and the long-range order lead to the delo-calization of the electronic states and the formation of allowed valence and conduction bands, separated by a forbidden energy gap. By thermal activation or photoexcitation, free electrons are generated in the conduc-tion band, leaving behind positively charged holes in the valence band. The transport of these free carriers is described in terms of Bloch func-tion, K-Space, and dispersion relation, all familiar to solid-state physicists. While in organic solids, intermolecular interactions are mainly covalent, intermolecular interactions are due to much weaker Van der Waals and London forces. As a result, the transport bands in organic crystals are much narrower than those of their inorganic counterparts and the band struc-ture is easily disrupted by introducing disorder in the system. Thus, even in molecular crystals, the concept of allowed energy bands is of limited valid-ity and excitations and interactions localized on individual molecules play a predominant role. The common electronic feature of many organic pig-ments is the π-conjugated system, which is formed by the overlap of car-bon pz orbitals. Due to the orbital overlap, the π electrons are delocalized within a molecule and the energy gap between the HOMO and LUMO is relatively small with transition frequencies within the visible range.

2.8 CONCLUSIONS

In this digital smart era, most people expect improved energy efficiency and reduced energy demand to tackle global climate change. But at the global level, the correlation between increased wealth and increased energy consumption is very strong and the impact of policies to reduce energy demand is both limited and contested. One of the key enablers in the history of OLED advancement can be attributed to the continuing discovery of new and improved electroluminescent materials, which was made possible by the dedication and ingenuity of many organic chem-ists who provide the design and skilled synthesis. Indeed, from small mol-ecules and oligomers to conjugated polymers, intense research in both


academia and industry has yielded OLEDs with remarkable color fidel-ity, device efficiencies, and operational stability. Nanostructures of organic and complex materials have also attracted a great deal of excitement due to noncovalent intermolecular interactions such as hydrogen bonding, Van der Waals force, and π–π stacking, which have allowed them to be used for cheap and novel optoelectronic nanodevices. White OLEDs made of organic materials have drawn particular interest because of their potential applications in full-color displays, backlights, as well as for general illumi-nation purposes. OLEDs also have the potential to produce white color as well as various individual colors in visible region by using a wide selection of organic fluorescent materials, proving that the development of organic materials for optoelectronic applications is significant.

REFERENCES [1] H.E.P. Espinosa, P. Ponce-Cruz, A. Molina, Artificial Organic Networks: Artificial

Intelligence Based on Carbon Networks, vol. 1, Springer International Publishing, Cham, 2014, pp. 31–52.

[2] N. Thejokalyani, S.J. Dhoble, R.B. Pode, Indian J. Phys. 86 (7) (2012) 613–618. [3] A. Virkar, Investigating the Nucleation, Growth, and Energy Levels of Organic

Semiconductors for High Performance Plastic Electronics. PhD Thesis, Stanford University, Stanford, CA, 2010.

[4] H. Shirakawa, J.E. Lewis, G.A. MacDiarmid, K.C. Chang, J.A. Heeger, J. Chem. Soc. Chem. Commun. 7 (1977) 578–580.

[5] R. McNeill, R. Siudak, J.H. Wardlaw, D.E. Weiss, Aust. J. Chem. 16 (1963) 1056–1075. [6] B.A. Bolto, D.E. Weiss, Aust. J. Chem. 16 (1963) 1076–1089. [7] B.A. Bolto, R. McNeill, D.E. Weiss, Aust. J. Chem. 16 (1963) 1090–1094. [8] S.L. Seager, M.R. Slabaugh, Chemistry for Today: General, Organic, and Biochemistry,

Thomson Brooks/Cole, Belmont, CA, 2004, p. 342. [9] H.M. Leicester, H.S. Klickstein, A Source Book in Chemistry, vol. 309, Harvard

University Press, Cambridge, MA, 1951, pp. 1400–1900. [10] R.W. Gymer, Endeavour 20 (3) (1996) 115–120. [11] V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, J.-L. Brédas, Chem.

Rev. 107 (2007) 926–952. [12] N. Thejokalyani, S.J. Dhoble, Defect Diffus. Forum 357 (2014) 1–27. [13] S.A. Benner, K.G. Devine, L.N. Matveeva, D.H. Powell, Proc. Natl. Acad. Sci. 97 (6)

(2000) 2425–2430. [14] E. Pretsch, P. Bühlmann, M. Badertscher, Structure Determination of Organic

Compounds, fourth ed., Springer-Verlag, Berlin Heidelberg, 2009. [15] W.H. Brown, C.S. Foote, B.L. Iverson, Anslyn E.V. Organic Chemistry, sixth ed.,

Cengage Learning, Belmont, CA, 2011, p. 1254. [16] G. Deepak, Directions 6 (1) (2007) 111–117. [17] A. Świst, J. Sołoducho, CHEMIK 66 (4) (2012) 289–296. [18] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [19] G. Yu, Y. Liu, Y. Song, X. Wu, D. Zhu, Synth. Met. 117 (2001) 211–214. [20] J.-Z. Wang, S.-L. Chou, H. Liu, G.X. Wang, C. Zhong, Mater. Lett. 63 (2009)

2352–2354.































[21] M.T. Bernius, E.P. Woo, Semiconducting polymer field effect transistor. US Patent 6204515, 2001.

[22] A. Hagfeldt, M. Grätzel, Acc. Chem. Res. 33 (2000) 269–277. [23] J. Sołoducho, A. Świst, Protein sensor for glucose detection. P 394553, 2011. [24] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W.

Langeveld-Voss, et al., Nature 401 (1999) 685. [25] A. Dodabalapur, H.E. Katz, L. Torsi, R.C. Haddon, Science 269 (1995) 1560. [26] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers,

second ed., Oxford University Press, New York, 1999. [27] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 44 (2015) 319–347. [28] S. Schols, Device Architecture and Materials for Organic Light-Emitting Devices,

Springer, New York, 2011. [29] Y. Sun, S.E. Thompson, T. Nishida, Strain Effect in Semiconductors: Theory and

Device Applications, Springer, New York, 2010. [30] C.J. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci, Organic Photovoltaics:Concepts and

Realization, Springer-Verlag, Berlin Heidelberg, 2003. [31] G. Meller, T. Grasser, Organic Electronics, Springer-Verlag, Berlin Heidelberg, 2010. [32] N. Karl, Syn. Met. 133–134 (2002) 649–657. [33] C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res. Dev 45 (1) (2001) 11. [34] F. Gutman, L.E. Lyons, Organic Semiconductors, Part A, Robert E. Krieger Publishing

Company, Malabar, FL, 1980, p. 251. [35] F. Gutman, H. Keyzer, L.E. Lyons, R.B. Somoano, Organic Semiconductors, Part B,

Robert E. Krieger Publishing Company, Malabar, FL, 1983, p. 122. [36] N.F. Mott, Metal-Insulator Transition, second ed., Taylor & Francis, London, 1990. [37] J.R. Sheats, H. Antoniadis, M.R. Hueschen, W. Leonard, J.N. Hiller, R.L. Moon, et al.,

Science 273 (1996) 884. [38] V. Bulovic, A. Shoustickov, M.A. Maldo, E. Bose, V.G. Kozlov, M.E. Thompson, et al.,

Chem. Phys. Lett. 287 (1998) 455. [39] W. Helfrich, W.G. Schneider, Phys. Rev. Lett. 14 (1965) 229. [40] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 12 (1996) 225. [41] P.E. Burrows, S.R. Forrest, M.E. Thompson, Curr. Opin. Solid State Mater. Sci.

2 (1997) 236. K.M. Vaeth, Inf. Disp. 19 (2003) 13. [42] J.H. Burrows, D.D.C. Bradley, A.R. Brown, R.N. Marks, R.H. Friend, P.L. Burn, et al.,

Nature 347 (1998) 539. [43] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughs, R.N. Marks, C. Taliani, et al.,

Nature (London) 397 (1999) 121. [44] P. Peumans, S. Uchida, S.R. Forrest, Nature (London) 425 (2003) 158. [45] L. Edman, M. Panchard, B. Lui, G. Bazan, D. Moses, J. Heeger, Appl. Phys. Lett.

82 (2003) 3961. [46] K. Kido, M. Yamashina, T. Moriizuni, Jpn. J. Appl. Phys., Part 1 23 (1984) 130. [47] T. Tsujimura, Y. Kobayashi, K. Murayama, A. Tanaka, M. Morooka, E. Fukumoto,

et al., SID 2003 Technical Digest, vol. 34, Society for Information Display, San Jose, CA, 2003, pp. 6–9.

[48] J.H. Burroughs, A.C. Jones, R.H. Friend, Nature (London) 335 (1988) 137. [49] J. Kido, M. Nagai, Y. Ohashi, Chem. Lett. 657 (1990) 13.













































http://dx.doi.org/10.1016/B978-0-08-101213-0.00003-5

CHAPTER 3

Evolution of Luminescent Materials for Organic Light-Emitting Diodes

3.1 INTRODUCTION

Cities all around the world are experiencing increase in population and hence increased energy consumption, a global problem. This chap-ter examines the feasibility of adopting energy-efficient materials for light sources, which can minimize energy consumption and reduce the use of conventional fluorescent lamps. Solid-state lighting (SSL) technol-ogy, based on solid-state devices such as light-emitting diodes (LEDs), is a green approach to illuminate the whole world with ecofriendly and energy-efficient lighting [1]. Light-emitting diodes are more economi-cal and versatile in appearance than traditional lighting, and LEDs made out of organic light-emitting diodes (OLEDs) have added one more quill to the crown of SSL since emission of light here is brought about by the organic emissive layer, making this technology more ecofriendly. Hence, OLEDs have attracted significant scientific and commercial inter-est due to their potential application as displays. White light from these sources can be achieved by the use of combined light-emissive mate-rials such as blue, green, and red [2]. The combination of these sources offers much more freedom in designing lighting at very low voltages [3]. In the last two decades, europium and samarium complexes have proved to be the best luminescent materials for red electroluminescent devices since they emit highly monochromatic red light at around 613 nm while other red organic materials give a broad emission spectra, which results in dull colors [4]. Likewise, tris(8-hydroxyquinoline)-aluminum (Alq3) and its derivatives is currently one of the most commonly used electron- transporting and green luminescent materials for molecular-based OLEDs [5]. π-Conjugated rigid poly(quinoline)s have been extensively investi-gated as thermally stable, photoconductive, photoluminescent, and nonlin-ear blue-light-emitting polymeric materials [6,7]. Of the hetero aromatic


polymers, quinolone-based polymers are of interest as these are reported to exhibit n-type conductivity upon doping and possess excellent thermal as well as oxidative stability [8,9]. Out of these possibilities, OLED mate-rials have the potential to open a gateway for an ecofriendly and power-saving green technology in the field of SSL and displays, which is essential today [10].

3.2 RED-LIGHT-EMITTING MATERIALS FOR OLEDs

In the past three decades, rare earth β-diketonates, namely europium Eu(III) and samarium Sm(III) β-diketonates, have been intensively stud-ied with respect to their applications for luminescence and laser mate-rials, especially since the first successful laser action from europium β-diketonate, reported by Lempicki and Samelson in 1963 [11]. These rare earth β-diketonates are complexes of β-diketones (1,3-diketones) with rare earth ions and are the most popular and most intensively investigated. This popularity is partially due to the fact that many differ-ent β-diketonesare are commercially available and the fact that synthesis of the corresponding rare earth complexes is relatively easy. Europium is best known for the production of red phosphors, which are widely used in color TV screens. Recently, Eu(III) complexes have attracted exten-sive interest for use in OLED devices due to their extremely sharp emis-sion bands and potentially high internal quantum efficiency [12–13]. These characteristics make Eu(III) complexes promising candidates for the next generation of flat-panel displays with a low-voltage drive [14–16]. β-Diketonates represent one of the oldest classes of chelating ligands, and have gained momentum because of the recent industrial applications of several of their metal complexes. β-diketonates are made by meth-ods similar to those for alkoxides [17–21]. They form anions as a result of enolization and ionization after α-proton extraction by base. These β-ketonate ions form very stable chelate complexes with most metal ions. They are the most studied and widely used organic ligands to coordinate to RE(III) ions. Three main types of rare earth β-diketonate complexes have to be considered: (1) tris complexes: The neutral tris complexes or tris(β-diketonates) have three β-diketonate ligands for each rare earth ion and can be represented by the general formula [R(β-diketonate)3]; (2) Lewis base adducts of the tris complexes (ternary rare earth β-diketonates); and (3) tetrakis(β-diketonates) complexes: arrangement of four β-diketonate ligands around a single rare earth ion with general

Evolution of Luminescent Materials for Organic Light-Emitting Diodes 67

formula [R(β-diketonate)4]. Bases that form adducts with rare earth tris β-diketonates are shown in Fig. 3.1. In 1904, β-diketonates, their hydrates, and hydroxobis (β-diketonate) were synthesized by Biltz [23], Jantzch and Meyer [24], and Van Uitert [25], respectively. These ketonates had oxygen donors that included the six-coordinates hexaderivatives. In 1959, Moeller and Horwitz demonstrated lanthanide complexes containing nitrogen donor groups [26]. Popular β-diketonate complexes with nitrogen and oxygen donors are shown in Fig. 3.2. In 1963, Hart and Laming reported new 1,10-phenanthroline chelates [27]. In 1964, Melby and his coworkers broadened the area of rare earth coordination chemistry and accordingly

Figure 3.1 Bases that form adducts with rare earth tris β-diketonates. bipy, 2,2′-bipyri-dine; phen, 1,10-phenanthroline; terpy, 2,2′,6′,2″-terpyridyl; bath, bathophenanthroline or 4,7-diphenyl-1,10-phenanthroline; Hpbm, 2-(2 pyridyl)benzimidazole; tppo, triphe-nylphosphine oxide; tbpo, tri-n-butylphosphineoxide; topo, tri-n-octylphosphineoxide; tbp, tributylphosphate; dmso, dimethylsulfoxide [22].


prepared a variety of new crystalline complexes based on anionic tetra-kis (β-diketone) derivatives with coordination number 8 [28]. In 1965, Eu(dbm)3(py)2, Eu(dbm)3(pyO), Eu(dbm)3(quinoline)2, and europium (III) tris-(β-diketonates) complexed with monocyclic nitrogen donors like pyr-idine or piperidine were described by Ohlmann and Charles [29].

In 1969, Dresner first considered organic material for fabrication of practical electroluminescent (EL) devices [30]. Gold in 1971 [31] and Drexhage in 1977 [32] prepared a large number of organic materi-als with high fluorescence quantum efficiency in the visible spectrum. In 1985 Mattson et al. synthesized red-light-emissive Eu(fod)3(phen) and Eu(fod)3(dmso) (fod: conjugate base of 6,6,7,7,8,8,8-heptafluoro-2,2-di-methyl-3,5-octanedione, dmso: dimethylsulfoxide) complexes [33] for OLEDs. Kido et al. in 1991 investigated the suitability of Eu(III) com-plex, Eu(ttfa)3, as a red-light emitter, commonly used as a red phosphors in cathode ray tubes (CRTs) [34].

Figure 3.2 β-Diketonates complexes with nitrogen and oxygen donors: (A) Tris (β-diketone) Eu(III) monophenanthroline; (B) Tris (substituted β-diketone) Eu(III) tripyri-dyl chelate; (C) Tris (aromatic or fluorine substituted β-diketone) Eu(III) phenanthroline; (D) Tris (β-diketone) Eu(III) distyrylphenanthroline; (E) Tris (β-diketone) bis (picoline N-oxide) Eu(III); (F) Tris (β-diketone) bis (triplenyl phosphine oxide) Eu(III); and (G) Bis (diketonate) bis (triphenylphospline oxide) Eu(III) mononitrate.


Again in 1993 Kido et al. synthesized red electroluminescent tris (theotyltrifluoroacetonate) Eu3+ (Eu(TTA)3) complex, which is nonvola-tile [35]. Such volatile complexes can be made volatile by incorporating ligands. In 1994, a brighter red electroluminescence was observed using tris(dibenzoylmethanato) phenanthroline Eu3+, Eu((DBM)3(Phen)), as red-emitting material. The second added ligand, phenanthroline, acts to saturate the coordination number of Eu ion and also to improve the fluo-rescence intensity, volatility, and stability of the Eu complex [36]. Uekawa et al. [37] examined the photoluminescence (PL) excitation of Eu com-plexes with different ligands like TTA: thenoyltrifluro acetone; FIHA: 1-(2-fluorenyl)-4,4,5,5,6,6,6-heptafluoro-1,3, hexanedione; and DNM: Di (2-naphthoyl) methane TPD (N,N′-biphenyl-N,N′-(3-methyl phenyl)-1,10-biphenyl-4,4′-diamine) and obtained superior results.

In 1998, Rodriguez-Ubis et al. discovered a simple ligand based on acetophenone bearing excellent quantum yield for the excitation of Eu3+ and Tb3+ [38]. Tetra-acid ligand derived from acetophenone was synthe-sized and the luminescence properties of their chelates with Eu(III) and Tb(III) were evaluated in aqueous and methanol solutions. These com-plexes showed excellent quantum yields of triplet sensitization of lantha-nide luminescence. In subsequent years, poly(3-alkylthiophene)s (PATs) and other polythiophene (PTs) became popular red electroluminescent materials because of their ease in tunability [39,40], good solubility, and chemical stability. The properties of these materials can be changed by structural modification, which allows to control the torsion of the main chain and thus the adjustment of the effective conjugation length [41–46]. In 1999 Hao et al. studied the luminescence behavior of Eu(TTA)3 doped in sol-gel films [47], formed by dip-coating of the EuCl3 and TTFA (Thenoyltrifluoro acetone) codoped solution. The luminescence intensity of the Eu(TTA)3-doped sol-gel films was significantly increased with the increasing film thickness. Miyamoto et al. synthesized Eu(III) β-diketonate complex, Eu(DBM)3Phen [48], when doped with phospho-rescent material in OLED, which showed excellent EL spectra at room temperature. This depends on the host materials, energy transfer from triplet states of the phosphorescent materials to the ligand triplet state of the Eu complex. Fu et al. [49] synthesized and studied the luminescent properties of the ternary europium complexes with ligands thenoyltri-fluroacetone (TTA) and phenanthroline (Phen) incorporated into SiO2 polymer matrix by a sol-gel method. The lifetime of rare earth ion Eu3+ in SiO2/PVB gel matrix doped with Eu(TTA)3Phen was found to be


longer than in pure Eu(TTA)3Phen powder and solvated Eu(TTA)3Phen in ethanol solution. The efficient energy transfer from the polymer to the Eu complex resulted in complete quenching of the broad emission of the polymer [50,51]. In 2000, Adachi et al. investigated the mecha-nism for energy transfer leading to electroluminescence of lanthanide complex Eu(TTA)3Phen doped into 4-4′-N,N′-dicarbozole-biphenyl (CPB) host [52]. In 2001 Yang et al. designed a red-light-emitting copo-lymer containing carbazole, Eu complex, and methyl methacrylate [53]. In 2001 Chen et al. [54] used a series of tris-(8-hydroxyquinoline) metal chelates with central metal ions of Al3+, Ga3+, In3+ as host materials. A red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(8-methoxy-1,1,7,7-tetra methyljulolidyl-9-enyl)4H-pyran (DCJMTB), was used as the emitter or guest dopant material. In 2002 Ma et al. [55] synthesized a novel red luminescent material N,N-bis4-[2-(4-dicyanomethylene-6-methyl-4H-pyran-2-yl)ethylene]phenyl aniline (BDCM) with two (4-dicyanomethylene)-4Hpyran electron-acceptor moieties and a triphe-nylamine electron-donor moiety for application in OLEDs.

In 2003, Hu et al. [56] adopted emulsifier-free emulsion polymeriza-tion to synthesize rare earth (RE) containing submicron polymer particles under microwave irradiation. To control the size and distribution of the particle, the relationship between reaction time, monomer content, and particle radius was studied for the polymerization of methyl methacry-late (MMA) in the absence and presence of rare earth ions, with water as solvent. In 2004, Liu et al. [57] synthesized europium-doped ternary complexes, namely Eu(DBM)3phen, Eu(DBM)3(DB-bpy), Eu(DBM)3 (DN-bpy), and Eu(DBM)3biq (DBM, Phen, DB-bpy, DN-bpy and biq refer to dibenzoylmethane, 1,10-phenanthroline, 4,4′-di-tertbutyl-2,2′-dipyridyl, 4,4′-dinonyl-2,2′-dipyridyl, and 2,2′-biquinoline, respectively), in polymethylmetacrylate (PMMA) matrix. Investigations on Eu-doped β-diketonatesEu(III) ions in the doped Eu(DBM)3/PMMA matrix revealed two distinct symmetric sites, and the emission band changes greatly with the compositions. Ohmori et al. [58] employed two kinds of material systems utilizing energy transfer and energy confinement: (1) a codoped OLED, which consisted of two different kinds of materials doped in the emissive layer; and (2) a europium (Eu) complex, Eu(TTA)3phen doped in poly(N-vinylcarbazole) (PVK). Red-light emission at 614 nm was obtained with efficient emission relaxation in Eu3+ sites.

In 2006, Lee et al. [59] synthesized Eu complex containing nanopar-ticles using the ultra-dilute solution method. The size of the nanoparticles


were found to be in the range of 30–150 nm. Jiu et al. [60] studied the fluo-rescence enhancement of Eu(DBM)3Phen and Tb(DBM)3Phen in PMMA. A combinatorial methodology was adopted to allow rapid optimization of the fluorescence enhancement conditions of thin-film samples in arrays of microwells. Based on Eu(DBM)3Phen-doped PMMA, three material libraries were generated in order to compare the effects of species iden-tity and Tb(DBM)3Phen content on the effect of other complexes con-taining enhancing ions (La3+, Gd3+, Dy3+, Y3+, Ce3+) on the luminescence efficiency of the Eu3+ complex in PMMA. The fluorescence enhancement of Eu(DBM)3Phen in PMMA is considered to originate by the process of intramolecular and intermolecular energy transfer. In 2006 Qu et al. [61] synthesized two novel polymers, PQP (poly(3,7-N-octyl phenothiozinyl-cyanoterephthalylidene)) and PQM (poly(3,7-N-octyl phenothiozinyl-cyanoisophthalylidene)), containing phenothiazine for application in red/orange OLEDs. Europium complexes featuring fluorinated β-diketonate ligands (thenoyltrifluoroacetone (tta), 4,4,4-trifluoro-1-phenyl-1,3-bu-tanedione (btfac), and 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac)) and nitrogen p,p′-disubstituted bipyridine and phenanthroline ligands (4,4′-dimethoxy-2,2′-bipyridine (dmbipy) and 4,7-dimethyl-1,10-phen-anthroline (dmphen)) were synthesized. Ultraviolet excitation of the com-plexes led to a red luminescence characteristic of trivalent europium ion. The high overall quantum yields observed for the europium complexes bearing hfac and dmbipy or dmphen ligands were rationalized in terms of the relatively high ligand-to-metal energy transfer efficiencies [62]. In 2008 a binuclear Eu(III) complex Eu2(btbt)3 4H2O. CH3CH2OH. N(CH2CH3)3 was synthesized by Park et al. [63] (btbt is 4,4-bis(4″,4″,4″,-trifluoro-1″,3″-dioxobutyl)-o-terphenyl). The complex emits the characteristic red lumi-nescence of Eu3+ ion due to the 5D0–

7Fj (j = 0, 1, 2, 3, 4) transitions under 395 nm light excitation with good luminescent quantum efficiency around 32% and exhibits high thermal stability until 337°C. A novel europium (III) ternary complex, Eu(TPBDTFA)3Phen, was designed and synthesized by Xiang et al. [64], which exhibits strong red emission due to the 5D0–

7Fj transitions of Eu3+ ions with appropriate Commission International de lÉclairage (CIE) chromaticity coordinates (0.66, 0.33) under 310–420 nm light excitation. The luminescence quantum yield for the Eu3+ complex was 0.18. Thermogravimetric analysis (TGA) confirmed high thermal sta-bility of the complex with a decomposition temperature of 341°C.

In 2009 Lyu et al. designed highly efficient phosphorescent silicon-cored spirobifluorene derivative (SBP-TS-PSB) as a host material for


the red phosphorescent Ir(III) complexes [65]. Seo et al. in 2009 synthe-sized novel red phosphorescent heteroleptic-tris-cyclometalated-iridium complex, bis(2-phenylpyridine) iridium(III)[2(5′-methylphenyl)-4-di-phenylquinoline] [Ir(ppy)2(dpq-5CH3)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH3) ligand. ppy ligand in heteroleptic iridium complexes plays an important role as sensitizer in the efficient energy transfer from the host (CBP;4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH3), demonstrat-ing high efficiency through the sensitizing ligand [66]. In 2010 Lu et al. [67] reported the synthesis of the rare earth ternary complex of Eu3+ with thenoyltrifluoroacetone and 4,7-2NH2 phenanthroline complex Eu(TTA)3(2NH2-Phen) with emission peaking att 614 nm.

In 2010, Xu et al. found that the insertion of a π-spacer between the donor and the acceptor group of a ligand effectively diminishes the inductive effect of Eu(III) ions on frontier molecular orbital electron cloud distributions of the ligand [68]. The use of rigid bidentate ligands enhanced the thermal and morphological stability of the complexes and also reduced nonradiative transitions due to structural relaxation. In 2011 Kalyani et al. [69] developed red-light-emitting with Eu(TTA)3Phen organic complex as emissive layer, which peaked the maximum intensity at 611 nm in the orange/red region. They also studied the solvent effect on the optical properties of the synthesized complex. Novel trinuclear europium complexes with two tris-β-diketones ligands have been syn-thesized, and the chemical structures of ligands and complexes were characterized by Yang in 2012 [70]. Europium β-diketonate complexes EuxRe(1−x)TTA3Phen (Re = Y/Tb Y: Yttrium, Tb: Terbium TTA x = 0.5) were dispersed in polystyrene (PS) matrix by Phatak et al. [71] in 2012 to study the compatibility of these complexes with the polymer interms of PL. Absorption spectra of all these complexes have two absorption peaks at 280 and 360 nm, attributed to n − π* and π − π* transition of TTA. In 2014, red-light-emitting lanthanide β-diketonate complex of molecu-lar formula [Eu0.45Tb0.55(btfa)3(4,4′-bpy)(EtOH)] was synthesized by Lima et al. [72]. In the same year, Yawalkar et al. [73] synthesized volatile Eu(acac)3phen as a red-light-emitting complex for OLEDs showing high intensity of emission peak centered at 612 nm with spectral bandwidth 5 nm when excited at a wavelength of 323 nm.

Wang Dong Mei et al. [74] successfully prepared the nanometer-sized (200 nm) quaternary rare earth complex Eu(BA)(TTA)2phen with the emission centered at 615 nm. Myungkwan Song et al. [75] in 2015


synthesized two highly efficient red phosphorescent Ir(III) complexes, bis[2,3-diphenylquinoxalinato-N,C2′]iridium(III) pyrazinate (dpq)2Ir(prz) and bis[2,3-diphenylquinoxalinato-N,C2′]iridium(III) 5-methylpyrazinate (dpq)2Ir(mprz), which are based on the use of 2,3-diphenylquinoxa-line as the ligand, and investigated their photo physical, electrochemical, and electroluminescence properties. Both Ir(III) complexes are soluble in common organic solvents, and uniform thin films can be readily spin-coated onto substrates. In 2016, a novel europium complex, formulated as [Eu4(H3Pimda)4 (Himba)2·4H2O]·4H2On (H3Pimda= 2-propyl-1H-imidazole-4,5-dicarboxy acid, and Himba= 4-(1H-imidazole-1-ly) benzoic acid), was obtained by the hydrothermal reaction by Sun et al. [76]. This complex exhibits efficiently sensitized red luminescence in the vis-ible region in N,N-dimethyl formamide (DMF) based on characterization emission of europium(III) ion. The emission spectrum of complex contains a series of emission peaks at 363, 595, 618, and 704 nm. Thus a wide variety of red-light-emitting materials have been synthesized with sharp emission peaks in the ideal red region of electromagnetic spectrum with higher sta-bility and improved efficiency. The structures of some popular luminescent europium (III) β-diketonate complexes are shown in Fig. 3.3.

3.3 GREEN-LIGHT EMITTING MATERIALS FOR OLEDs

Interest in green-light-emitting devices based on 8-hydroxyquinolinate stemmed from the demonstration of bright-green emission from alu-minum tris(8-hydroxyquinolinate) demonstrated by Tang and Vanslyke in 1987 [83]. Consequently, in 1990 Kido et al. reported Tb-tris-(acetylacetonato), Tb(acac)3 as green-light-emitting material. A strong emission peak at 544 nm peak, corresponding to the 5D4→7F5 transition of the Tb3+ ion, was observed [84]. Kasim et al. [85] synthesized new conjugated polymer, poly(2, bquinolinevinylene) (PQV), which exhib-ited maximum fluorescence at 515 nm when excited at 430 nm. A ther-mally stable terbium-tris(tetradecylphethalate) phenanthroline complex Tb(MTP)3Phen was prepared and used as green-emitting layer by Ma et al. in 1999 [86]. Phosphorescent dendrimers with fac-tris(2-phenyl-pyridyl)iridium(III) cores, biphenyl-based dendrons, and 2-ethylhexyloxy surface groups, which emit green light, were reported by Markham et al. [87]. In 2002, Zheng synthesizeda terbium complex, Tb(acac)3bath (acac: acetylacetone, bath: 4,7-diphenyl-1,10-phenanthroline), and its lumines-cent properties were investigated and compared with the reported terbium


Figure 3.3 Structure of some popular luminescent europium (III) β-diketonate com-plexes [77–82]. TTA, thenoyltrifluoro acetone; Phen, 1,10-phenanthroline; bath, 4,7-diphenyl-1,10-phenanthroline; DBM, dibenzoylmethanato; N-HPA, N-pheny-lanthranilic acid; NIP, 2-(naphthalen-1-yl)-1H-imidazo[4,5-f ] [1,10] phenanthroline; ENIP, 1-ethyl-2-(naphthalen-1-yl)-1H-imidazo [4,5-f ] [1,10] phenanthroline; pyphen, 1,4,8,9-tetraaza-triphenylene; TFA, trifluoroaceticacid; bipy, 2,2′-bipyridine; DPDBM, 1-(4-diphenylamino-phenyl)-3-phenylpropane-1,3-dione; DPPZ, 4,5,9,14-tetraazaben-zotriphenylene; TPPHZ, 4,5,9,13,14,18-hexaaza-phenanthro-9,10-triphenylene; DPBT, 2-(N,N-diethylanilin-4-yl)-4, 6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine; DSACAC, 1,5-styrylacetylacetonel.


complex, Tb(acac)3phen (phen: phenanthroline) [88]. In 2003, Palilis et al. [89] designed novel green/blue emissive fluorescent silole deriva-tives, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenyl silacyclo pentadiene (PPSPP), 9-silafluorene-9-spiro-1′-(2′,3′,4′,5′-tetraphenyl)-1′-H-silacyclopentadiene (ASP), and 1,2-bis(1-methyl-2,3,4,5,-tetraphe-nylsilacyclopentadienyl)ethane (2PSP), with high solid-state PL quantum yields of 0.85, 0.87, and 0.94, respectively. Mishra et al. synthesized a bluish-green aluminum complex, bis(2-methyl 8-hydroxiquinoline) alumi-num hydroxide (Almq2OH), as an emissive material [90] for OLEDs. In 2009, Liu et al. investigated highly efficient [91] green phosphorescent irid-ium complexes as the host material. Wang reported the synthesis of three chemical bonded hybrid materials based on silica matrices composed of terbium ions and aromatic acid derivatives [92]. In 2009, Limaain designed organic–inorganic hybrids incorporating Tb(acac)3·3H2O (where acac is acetylacetonate) via conventional hydrolysis sol-gel reaction in the presence and absence of an acid catalyst (hydrochloric acid, HCl). The host frame-work of these materials, called di-ureasils, is formed by polyether-based chains grafted to both ends of a siliceous backbone through urea cross-linkages . Four different concentrations of HCl (0.5, 1.0, 1.5, and 2.0 mol/L) were used as catalyst for hydrolysis reactions [93].

In 2010, Min Ju et al. synthesized 9-((6-Phenylpyridin-3-yl) methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazoleas ligands by attaching a carbazolyl group to the pyridine in 2-phenylpyridine and 2-(4-fluorophenyl)pyridine, respectively. Four different Ir(III) complexes were prepared using a simple procedure, the solubility of which was signifi-cantly greater than that of conventional green-emitting Ir(ppy)3 [94]. A series of phosphorescent Ir(III) complexes were synthesized based on aryl(6-aryl-pyridin-3-yl) methanone ligands, and their photophysical and electrolumines-cent properties were characterized by Ju Kang [95]. In 2013, Yawalkar et al. synthesized a series of Alq3 and TbxAl(1 − x)q3 (where x = 0.1, 0.3, 0.5, 0.7, and 0.9) metal complexes, which demonstrated bright emission of green light with blue-light excitation (440 nm) [96]. Pure and Ba2+ doped Alq3 complexes were synthesized by Bhagat et al. It was observed that barium-doped complexes exhibit the highest intensity in comparison to Alq3 phos-phor. The excitation spectra of the synthesized complexes are in the range of 300–480 nm with a broad peak in the range of 429–440 nm and shoulder at 380 nm, but with varying intensity. The emission wavelength lies in the range of 501–506 nm [97]. In the preceding year, the same group synthesized Na+-doped Alq3 complexes synthesized by a simple precipitation method at room


temperature, maintaining a stoichiometric ratio. The excitation spectra of the synthesized complexes were in the range 242–457 nm when weak shoulders are also considered. The emission wavelength of all the synthesized complexes were in the bluish green/green region range and 531 nm with elevation in the emission intensity as compared to pure Alq3 [98]. Although Alq3 is used as a green emissive layer in OLEDs, it tends to degrade over time leading to a decrease in device performance and efficiency. Alq3 is very sensitive to the atmospheric environment, and the performance of Alq3 is effected by oxygen, moisture, and light exposure. Duvenhage et al. [99] showed that the lumi-nescence intensity will decrease by 50% within the first 24 hours of expo-sure to UV light. A decrease of 90% in luminescence intensity was observed after ~300 hours of irradiation. This decrease was ascribed to the rupturing of the phenoxide ring due to oxidation. The oxygen and moisture in the atmo-sphere reacts with the phenoxide ring by breaking the bond at the C-7 position. and bonds will form at this position. The highest occupied molecular orbit (HOMO) is mainly situated on the phenoxide ring and the lowest unoccupied molecular orbit (LUMO) on the pyridyl ring. The emis-sion of metal quinolates originates from the ligand’s electronic π − π* transi-tions. If the ring is broken these transitions can’t take place and the molecule is rendered nonluminescent. Znq2 powder was successfully synthesized using the coprecipitation method [100]. X-ray diffraction (XRD) measurements confirmed that the Znq2.2H2O crystal had formed during the synthesis. The PL data also confirmed that the Znq2.2H2O crystal form was present with a broad emission peak centered at 506 nm. Under prolonged UV exposure it was observed that the luminescence intensity decreased with time. Only a 30% decrease in intensity was observed compared to an 80% decrease for Alq3 powder under the same conditions. X-ray photo electron spectroscopy (XPS) studies done on the degraded powder confirmed the presence of and bonds after prolonged exposure to UV. This is an indication that the phenox-ide ring had ruptured during UV exposure due to oxidation. There was no change to the bond, confirming that the pyridyl ring stayed intact. The structures of some popular green-light-emitting materials synthesized so far are shown in Fig. 3.4.

3.4 BLUE-LIGHT-EMITTING MATERIALS AND OLEDs

Among the three primary RGB colors, green and red phosphors meet the necessary requirements in terms of efficiency and lifetime. Conversely, the blue phosphors still are in a state of improvization. In 1992, Grem


et al. for the first time reported on poly(p-phenylene) (PPP) [105]. Later, many materials such as distyrylarylene derivatives (DSA) [106], hydroxy-phenyl–pyridine beryllium complex [107], unidentateorganolithium complex [108], bistriphenylenyl [109], spirobifluorene-cored conjugated compounds [110], and anthracene derivatives [111–113] were synthesized and used in blue OLEDs. Among these anthracene derivatives, 9,10-di-(2-naphthyl) anthracene (ADN) is one of the most promising blue fluo-rescent materials because of its color purity and thermal stability [114]. Basically there are four approaches to obtain blue-light emission: (1) to synthesize organic dyes and polymers that are able to emit blue light, e.g., oxadiazole [115], distylylpyrazine [116], cyclopentadiene derivative

Figure 3.4 Stuctures of some popular green light-emitting materials. (A) Tb-tris (tetradecylphethalate) phenanthroline complex Tb(MTP)3Phen [86]; (B) Alq2–Ncd = 6,11-dihydoxy-5,12-naphtacene-dione·Alq3 complex [101]; (C) 1,4-dihdoxy-5,8- naphtquinone·Alq3 complex (Alq2–Nq) [102]; (D) Ir(Cz-ppy)3 [101]; (E) Ir(Cz-ppy)2(Cz-Fppy)1 [94]; (F) Ir(Cz-ppy)1(Cz-Fppy)2 [94]; (G) (Cz-Fppy)2 [101], where Ir = Irridium, Cz-ppy 9-((6-phenylpyridin-3-yl)methyl)-9H-carbazole, Cz-Fppy = 9-((6-(4-fluorophe-nyl)pyridin-3-yl)methyl)-9H-carbazole [94]; (H) methylene diphosphonic acid (MPD) [103]; (I) Alq3; and (J) Na-doped Alq3 [104].


[117], poly(alkyl-uorene) [118], poly(p-phenylene) [105], and poly-(pyridine) [119]; (2) to add blue-light-emitting dyes into a polymer such as oligomeric PPV doped in polymer mixture [120]; (3) to make blue-light-emitting bilayer or trilayer films from light-emitting materials with electron-transporting and hole-transporting materials like bilayer films of poly(benzoyl-1,4-phenylene) (PBP) with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [26]; and (4) to synthesize chelate metal complexes such as azomethin zinc complex, etc., [121–123]. A wide range of oligo(p-phenylenevinylene)s with alkyl or alkoxy substituents were synthesized during 1996–98 by Leising et al. They were used as blue emitters in electroluminescent devices, which exhibited high-fluorescence quantum yields [124–126].

In 1999, Tao and Suzuki [127] reported the blue emitter LiB(qm)4. In 2000 Liu and his coworker designed M(acea)3(Phen) (M = Y, La, and Gd ion) complexes [128]. In the same year, Hong and his coworker were the first to use Tm3+ ion to design and synthesize blue-emissive material tris(acetylacetonato)-monophenanthroline Tm complex [129]. Liu and Wang in 2001 [130] reported the blue emitter Bepp2. In 2005, Kumar et al. used lithium tetra-(8-hydroxy-quinolinato) boron complex as blue light emitter for full color displays [131]. A new series of blue fluores-cent emitters based on t-butylatedbis(diarylaminoaryl) anthracenes were synthesized by Lee et al. [132] in 2010. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching.

Pu et al. [133] synthesized hole-transporting arylamino-9,10-di-phenylanthracene derivatives by cross-coupling with palladium catalyst. These materials showed higher glass transition temperatures (135–177°C). Alq3-based green-light-emitting devices containing the arylamino-9,10-diphenylanthracene derivatives as hole-transport layers were fabricated. A novel blue-light-emitting polyfluorene-based copo-lymer triphenylamine (PTHD) containing electron-richtriphenylamine and electron-poor phenylquinoline side chains in the C-9 position of fluorineunit was described by Lin et al. [134]. In the same year, Wettach reported low-molecular-weight triphenylene derivatives in a one-step pro-cedure. The corresponding photoluminescence emission spectra displays an intense blue peak at around 400 nm [135]. Tyagi et al. demonstrated blue Zn(hpb)2 and yellow Zn(hpb)mq-emitting materials. It was observed that when the thickness of Zn(hpb)mq layer was increased, the domi-nant wavelength shifts from the bluish to yellowish region [136]. Chen et al. [137] synthesized three anthracene derivatives featuring carbazole


moieties as side groups, -tert-butyl-9,10-bis[4-(9-carbazolyl) phenyl] anthracene(Cz9PhAnt), 2-tert-butyl-9,10-bis4-[3,6-di-tert-butyl-(9 carba-zolyl)] phenyl anthracene(tCz9PhAnt), and 2-tert-butyl-9,10-bis4′-[3,6-di-tert-butyl-(9-carbazolyl)] biphenyl-4-yl anthracene (tCz9Ph2Ant) for use in blue OLEDs with high glass-transition temperature of 220°C.

A new series of blue-light-emitting 2,4-diphenylquinoline (DPQ) substi-tuting blue-light-emitting organic phosphors, namely, 2-(4-methoxy-phenyl)-4-phenyl-quinoline (OMe–DPQ), 2-(4-methyl-phenyl)-4-phenylquinoline (M-DPQ), and 2-(4-bromophenyl)-4-phenylquinoline (Br-DPQ), were syn-thesized by substituting methoxy, methyl, and bromine at the 2-para position of DPQ, respectively, by Friedländer condensation of 2-aminobenzophenone and corresponding acetophenone. The synthesized polymeric compounds demonstrate bright emission in the blue region in the wavelength range of 405–450 nm in solid state [138,139]. The chemical structures of some popular blue-light-emitting materials are illustrated in Fig. 3.5.

Figure 3.5 Structures of some popular blue-light-emissive materials. (A) Anthracene derivatives featuring carbazole moieties as side groups -tert-butyl-9,10-bis[4- (9-carbazolyl)phenyl]anthracene (Cz9PhAnt), 2-tert-butyl-9,10-bis4-[3,6-di-tert-butyl- (9-carbazolyl)]phenylanthracene(tCz9PhAnt), and 2-tert-butyl-9,10-bis4′-[3,6-di-tert- butyl-(9-carbazolyl)]biphenyl-4-ylanthracene (tCz9Ph2Ant) [140]; (B) azomethin-zinc complex N,N1-disalicylidene-triethylenetetramine zinc(II) (1AZM-TEEA) [123]; (C) tetra-(8-hydroxy-quinolinato) boron complex [129]; (D) 4,41-di-(1-pyrenyl)-411-[2-(9,91-dimethylfluorene)]triphenylamine (DPFA) [141]; (E) oligo(p-phenylene) [107]; (F) BePP2 [130]; (G) methoxy substituted 2,4-diphenylquinoline (DPQ) [142].


3.5 WHITE-LIGHT-EMITTING MATERIALS AND OLEDs

Differing from the other colors the primary advantage of white emission from OLEDs is its use as backlight in LCDs to produce full-color displays using micro-pattered color filter [143]. Globally, researchers and industrial-ists have designed materials for these devices, which include conjugated polymers, metal complexes [144], and organic dyes from which a broad band of white light, covering the whole visible (VIS) region, has been observed [145,146]. In order to achieve maximum efficiency and high color purity, white light should be composed of three discrete peaks in the blue, green, and red region or two discrete peaks in the yellow and blue region. The first white OLED was fabricated by Kido et al. in 1994 [36]. This device contained red-, green-, and blue-light-emitting compounds that together produced white light with efficiency less than 1 lm/W. Use of phosphorescent dopants as white-light emitters was first demonstrated by D’Andrade et al. in 2001 [147]. Kido et al. [144] designed Tb3+ and Eu3+ complexes to achieve white emission with more uniform and more energy efficiency than that of fluorescent lights.

Bright-white emission from blue-light-emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)2] doped with orange luminescent 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) dye was used by Kumar Rai et al. to achieve white light from Zn(hpb)2 by adjusting the concentration of DCM dye [148]. Chang et al. reported iridium complex Ir(dfbppy) (fbppz)2 and a wide-band-width yellow-emitting osmium complex Os(bptz)2(dppee) for white OLED [149]. Chen et al. in 2010 [150] achieved white-light byutilizingru-brene (Rb)-doped N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) yellow-light-emitting layer. In 2011 Hu et al. [151] reported a theoretical investigation of the white-light emission from a single polymer system with simultaneous blue polyfluorene as host and orange 2,1,3-benzothiadiazole (BTD)-based derivative as dopant emission. They employed quantum chemical approaches to study the variations in electronic and optical properties as a function of the chemical composition of the backbone in BTD-based derivatives. Tm3+/Dy3+ codoped LiYF4 single crystals were synthesized by using a vertical Bridgman method in sealed Pt crucibles for the UV light-excited white-light-emitting diodes were designed by Tang [152]. Recent advances in white polymer light-emitting devices (WPLEDs) attracted special attention to design novel luminescent dopants so as to minimize the gap (for both efficiency and


stability) from other lighting sources such as fluorescent lamps, LEDs based on inorganic semiconductors, and vacuum-deposited small-molecular devices, thus rendering WPLEDs equally competitive as these counter-parts currently in use. Highly efficient sky-blue phosphorescent bipolar host material, diphenyl (10-phenyl-10H-spiro[acridine-9,9′-fluoren]-2′-yl)phosphine oxide (POSTF), was designed by Ding et al. [153]. Recently, the development of organic printed electronics [154,155] has been expanding to a variety of applications and is expected to achieve high-performance organic materials with cost-efficient fabrication processes.

3.6 CONCLUSIONS

Substantial efforts were undertaken by researchers and industrialists glob-ally in order to develop novel red, green, blue, and white emissive materi-als for energy efficient and ecofriendly lighting, popularly known as SSL and flat-panel technology. Various possible combinations of red, blue, and green/yellow and red complexes that can offer intense white light with enhanced efficiency and lifetime have been explored. If we succeed in further improving the efficiency, lifetime, and fabrication processes, our present lighting systems could be replaced by ecofriendly, energy-efficient green technology called SSL, which would play a significant role in reduc-ing global energy consumption in the near future.

REFERENCES [1] E.F. Schubert, Light-Emitting Diodes, Cambridge University Press, Cambridge and

New York, 2006. [2] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. Cryst. Growth. 268 (2004)

449–456. [3] D. Evans, High-luminance LEDs replace incandescent lamps in new applications, Proc.

SPIE 3002 (1997) 142–153. [4] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [5] F. Salata, A. de LietoVollaro, A. Ferraro, Energy Convers. Manage. 84 (2014) 623–632. [6] F. Papadimitrakopoulos, X. Zhang, K.A. Higginson, IEEE J. Sel. Top. Quantum

Electron. 4 (1998) 49. [7] J.Y. Hung, Y.C. Chiu, Y. Chi, Org. Electron. 11 (2010) 412. [8] T.W. Kwon, A.P. Kulkarni, S.A. Jenekhe, Synth. Met. 158 (2008) 292. [9] B. Huang, J. Li, Z. Jiang, J. Qin, G. Yu, Y. Liu, Macromolecules 38 (2005) 6915. [10] C.K. Gan, A.F. Sapar, Y.C. Mun, K.E. Chong, Procedia Eng. 53 (2013) 208–216. [11] A. Lempicki, H. Samelson, Phys. Lett. 4 (1963) 133. [12] L.H. Wang, W. Wang, W.G. Zhang, E.T. Kang, W. Huang, Chem. Mater. 12 (2000) 2212. [13] (a)L. Huang, K.Z. Wang, C.H. Huang, F.Y. Li, Y.Y. Huang, J. Mater. Chem. 11 (2001)

1790.(b)X.C. Gao, H. Cao, C.H. Huang, S. Umitani, G.Q. Chen, P. Jiang, Synth. Met. 19 (1999) 127.(c)G. Yu, Y.Q. Liu, X. Wu, D.B. Zhou, H.Y. Li, L.P. Jin, et al., Chem.























Mater. 12 (2000) 2537.(d)W.P. Hu, M. Matsumura, M.Z. Wang, L.P. Jin, Appl. Phys. Lett. 77 (2000) 4271.

[14] M.R. Robinson, M.B. O’Regan, G.C. Bazan, Chem. Commun. 7 (2000) 1645. [15] J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 30 (1993) 192. [16] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994)

2124. [17] V.C. Arunasalam, I. Baxter, S.R. Drake, M.B. Hursthouse, K.M.A. Malik, D. Otway, J.

Inorg. Chem. 34 (1995) 5295. [18] S.R. Drake, M.B. Hursthouse, K.M.A. Malik, S.A.S. Miller, D.J. Otway, Inorg. Chem.

32 (1993) 4464. [19] S.R. Drake, M.B. Hursthouse, K.M.A. Malik, D.J.J. Otway, Chem. Soc., Dalton Trans.

(19) (1993) 2883. [20] L. Huang, S.B. Turnipseed, R.C. Haltiwanger, R.M. Barkley, R.E. Sievers, Inorg. Chem.

33 (1994) 798. [21] S.R. Drake, S. Miller, D. Williams, J. Inorg. Chem. 32 (15) (1993) 3233. [22] N.S. Koen Binnema, , in: K.A. GschneidnerJr., , J.-C.G. Bünzli, V.K. Pecharsky(Eds.),

Handbook on the Physics and Chemistry of Rare Earths, vol. 35, Elsevier North Holland, Amsterdam and Boston, MA, 2005 <http://dx.doi.org/10.1016/S0168-1273(05)35003-3>.

[23] W. Biltz, Liebigs Ann. 331 (1904) 334. [24] G. Jantzch, E. Meyer, Ber. Dtsch. Chem. Ges. A/B 53 (1920) 1577. [25] L.G. Van Uitert, R.R. Soden, , in: S. Kirseliner (Ed.), Advances in the Chemistry of

Coordination Compounds, The Macmillan Co., New York, 1961, pp. 613. [26] T. Moeller, E.P. Horwitz, J. Inorg. Nucl. Chem. 12 (1959) 49. [27] F.A. Hart, F.P. Laming, Proc. Chem. Soc. 107 (1963) 49. [28] L.R. Melby, N.R. Roze, E. Abramson, J.C. Caris, Am. J. Chem. Soc. 86 (1964) 5117. [29] R.C. Ohlmann, R.G. Charles, J. Chem. Phys. 40 (1964) 3131. [30] J. Dresner, RCA Rev. 30 (1969) 322. [31] H. Gold, , in: K. Venkataraman(Ed.), The Chemistry of Synthetic Dyes, vol. 5,

Academic, New York, 1971, pp. 535. [32] K.H. Drexhage, , in: F.P. Schafer (Ed.), Topics in Applied Physics. Vol. 1; Dye Lasers,

Springer, New York, 1977, pp. 144. [33] S.M. Mattson, E.J. Abramson, L.C. Thompson, J. Less, Common Met. 112 (1985) 373. [34] J. Kido, K. Nagai, Y.O. Kamoto, T. Skotheim, Appl. Phys. Lett. 59 (1991) 2760. [35] J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 192 (1993) 30. [36] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994) 17. [37] M. Uekawa, Y. Miyamoto, H. Ikeda, K. Kaifu, T. Nakaya, Synth. Met. 91 (1997) 259. [38] J.C. Rodritguez, M.T. Ubis, O.J. Alonso, R. Sedano, E. Brunet, J. Lumin. 79 (1998) 121. [39] P. Barta, J. Sanetra, M. Zagórska, Synth. Met. 4 (1998) 119. [40] P. Barta, F. Cacialli, R.H. Friend, M. Zagorska, J. Appl. Phys. 84 (1998) 6279. [41] M. Pomerantz, Y. Cheng, R.K. Kasim, R.L. Elsenbaumer, J. Mater. Chem. 9 (1999)

2155. [42] S.D. Jung, D.H. Hwang, T. Zyung, W.H. Kim, K.G. Chittibabu, S.K. Tripathy, Synth.

Met. 98 (1998) 107. [43] S.D. Jung, T. Zyung, W.H. Kim, C.J. Lee, S.K. Tripathy, Synth. Met. 100 (1999) 223. [44] M.R. Andersson, O. Thomas, W. Mammo, M. Svensson, M. Theander, O. Inganäs, J.

Mater. Chem. 9 (1999) 1933. [45] A. Bolognesi, C. Botta, L. Cecchinato, Synth. Met. 187 (2000) 111–112. [46] S.H. Ahn, M.Z. Czae, E.R. Kim, H. Lee, S.H. Han, J. Noh, Macromolecules 34 (2001)

2522. [47] X. Hao, X. Fan, M. Wang, Thin. Solid. Films. 353 (1999) 223. [48] Y. Miyamoto, M. Uekawa, H. Ikeda, K. Kaifu, J. Lumin. 81 (1999) )159.

















http://dx.doi.org/10.1016/S0168-1273(05)35003-3

http://dx.doi.org/10.1016/S0168-1273(05)35003-3



































[49] L. Fu, H. Zhang, S. Wang, Q. Meng, K. Yang, J. Ni, J. Sol Gel Sci. Technol. 15 (1999) 49. [50] M.D. McGeHee, T. Bergstedt, C. Zhang, A.P. Staab, M.B. Regan, G.C. Bazan, J. Adv.

Mater. 11 (1999) 1349. [51] M.R. Robinson, M.B. O’Regan, G.C. Bazan, Chem. Commun. 4 (2000) 1645–1646. [52] M.A. Baldo Adachi, S.R. Forest, J. Appl. Phys. 87 (2000) 11. [53] M.J. Yang, Q.D. Ling, W.Q. Li, Y. Wang, R.G. Sun, Q.B. Zheng, J. Mater. Sci. Eng. B 85

(2001) 100. [54] X. Chen Lin, L. Cheng, C. Lee, W.A. Gambling, S. Lee, J. Phys. D: Appl. Phys. 34 (2001)

30–35. [55] B. Ma, Z. Zhang, P. Liang, X. Xie, B. Wang, Y. Zhang, J. Mater. Chem. 12 (2002)

1671–1675. [56] J. Hu, H. Zhao, Q. Zhang, W. He, J. Appl. Polym. Sci. 82 (2003) 1124–1133. [57] H.G. Liu, Y. Lee, S. Park, K. Jang, S.S. Kim, J. Lumin. 110 (2) (2004) 11–116. [58] Y. Ohmari, H. Kajii, T. Sawatani, H. Ueta, K. Yoshino, J. Curr. Appl. Phys. 5 (4) (2005)

345–347. [59] C. Lee, J.S. Lim, S.H. Kim, D.H. Suh, J. Polym. 47 (15) (2006) 5253–5258. [60] H. Jiu, J. Ding, Y. Sun, J. Bao, C. Gao, Q. Zhang, J. Non Cryst. Solids 352 (3) (2006)

197–202. [61] B. Qu, Z. Chen, Y. Liu, H. Cao, S. Xu, S. Cao, J. Phys. D: Appl. Phys. 39 (2006)

2680–2683. [62] C.R. De Silva, J.R. Maeyer, R. Wang, S.G. Nichol, Z. Zheng, Inorg. Chim. Acta 360

(11) (2007) 3543–3552. [63] G.Y. Park, Y. Ha, Synth. Met. 158 (2008) 120–124. [64] N. Xiang, Y. Xu, Z. Wang, X. Wang, L.M. Leung, J. Wang, et al., Spectrochim. Acta, Part

A 69 (4) (2008) 1150–1153. [65] L. Yi-Yeol, K. Jeonghun, S.J. Woo, B. Younghun, S.L. Hyo, K. Doseok, et al., Adv. Funct.

Mater. 19 (2009) 420–427. [66] H.S. Ji, C.L. Seung, K.K. Young, S.K. Young, Thin. Solid. Films. 517 (2009) 4119. [67] Y.-G. Lu, G. Li, C.-H. Shi, L.-S. Yu, F. Luan, F.-J. Zhang, Trans. Nonferrous Met. Soc.

China 20 (12) (2010) 2336–2339. [68] H. Xu, Y. Wei, B. Zhao, W. Huang, J. Rare Earths 28 (2010) 666–670. [69] N. Thejokalyani, S.J. Dhoble, R.B. Pode, Luminescence 28 (2) (2013) 183–187. [70] C. Yang, J. Luo, J. Ma, M. Lu, L. Liang, B. Tong, Dyes Pigm. 92 (1) (2012) 696–704. [71] V.S. Pathak, N. Thejokalyani, S.J. Dhoble, Int. J. Knowl. Eng. 3 (1) (2012) 124–126. [72] P.P. Lima, F.A.A. Paz, C.D.S. Brites, W.G. Quirino, C. Legnani, M. Costa e Silva, et al.,

Org. Electron. 15 (3) (2014) 798–808. [73] P.W. Yawalkar, S.J. Dhoble, Adv. Mater. Lett. 5 (11) (2014) 678–681. [74] D. Mei, W. Bo, J. Fan, Sci. China Chem. 57 (6) (2014) 791–796. [75] M. Song, S.-J. Yun, K.-S. Nam, H. Liu, Y.-S. Gal, J.W. Lee, et al., J. Organomet. Chem.

794 (2015) 197–205. [76] Y.-L. Sun, X. Feng, N. Guo, L.-Y. Wang, R.-F. Li, R.-F. Bai, Inorg. Chem. Commun.

67 (2016) 90–94. [77] A.H. Nigel Male, O.V. Salata, V. Christou, Synth. Met. 12 (2002) 7–10. [78] H. Liang, F. Xie, Spectrochim. Acta, Part A 77 (2010) 348–350. [79] L. Zhang, B. Li, J. Lumin. 129 (2009) 1304–1308. [80] H. Liang, F. Xie, X. Ren, Y. Chen, B. Chen, F. Guo, Spectrochim. Acta, Part A 116

(2013) 317–320. [81] S. Linfang, L. Yanwei, C. Huaru, G. Ying, M. Yongchun, J. Rare Earths 30 (1) (2012)

17–20. [82] M. Fernandes, V. de Zea Bermudez, R.A. Sá Ferreira, L.D. Carlos, A. Charas, J.

Morgado, et al., Chem. Mater. 19 (2007) 3892–3901. [83] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (12) (1987) 913.






















































[84] J. Kido, M. Nagai, Y. Ohashi, Chem. Lett. 657 (1990) 13. [85] R.K. Kasim, S. Satyanarayand, R.L. Elsenbaumer, Synth. Met. 02 (1999) 1059. [86] D. Ma, D. Wang, B. Li, Z. Hong, S. Lu, L. Wang, et al., Synth. Met. 102 (1999) 1136. [87] J.P.J. Markham, S.C. Lo, S.W. Magennis, P.L. Burn, I.D.W. Samuel, Appl. Phys. Lett. 80

(2002) 2645. [88] Y. Zheng, J. Lin, Y. Liang, Q. Lin, Y. Yu, S. Wang, et al., Opt. Mater.; 20 (2002) 273–278. [89] L.C. Palilis, H. Murata, M. Uchida, Z.H. Kafafi, Org. Electron. 4 (1) (2003) 113–121. [90] A. Mishra, P. Kumar, L. Kumar, Indian J. Eng. Mater. Sci. 12 (2005) 321–324. [91] Z. Liu, Z. Bian, F. Hao, D. Nie, F. Ding, Z. Chen, et al., Org. Electron. 12 (2) (2009)

247–255. [92] Q.M. Wang, Solid State Sci. 11 (2009) 1617–1620. [93] P.P. Limaa, R.A.S. Ferreiraa, S.A. Júniorb, O.L. Maltab, L.D. Carlos, J. Photochem.

Photobiol. A 201 (2009) 214–222. [94] M.J. Cho, J.-I. Jin, D.H. Choi, J.H. Yoon, C.S. Hong, Y.M. Kim, et al., Dyes Pigm. 85

(2010) 143–151. [95] Y.J. Kang, K.H. Lee, S.J. Lee, J.H. Seo, Y.K. Kim, S.S. Yoon, Thin. Solid. Films. 519

(2011) 6544–6549. [96] P.W. Yawalkar, S.J. Dhoble, N. Thejokalyani, R.G. Atram, N.S. Kokode, Luminescence

28 (2013) 63–68. [97] S.A. Bhagat, S.V. Borghate, N.S. Koche, N. Thejokalyani, S.J. Dhoble, Optik 125

(2014) 3813–3817. [98] S.A. Bhagat, S.V. Borghate, N. Thejokalyani, S.J. Dhoble, Luminescence 30 (2015)

251–256. [99] M.M. Duvenhage, O.M. Ntwaeaborwa, H.C. Swart, Phys. B 407 (2012) 1521–1524. [100] M.-M. Duvenhage, J.J. Terblans, M. Ntwaeaborwa, H. Swart, Phys. B 480 (2016)

141–146. [101] H.C. Yoon, K.S. Yang, H.K. Shin, B.S. Kim, C. Kim, Y.S. Kwon, Mol. Cryst. Liq. Cryst.

405 (2003) 97–103. [102] C. Kim, Korea Patent, 10-2002-0047187, 2002. [103] Q. Lin, C.Y. Shi, Y.J. Liang, Y.X. Zheng, S.B. Wang, H.J. Zhang, Synth. Met. 114 (2000)

373. [104] S.A. Bhagat, S.V. Borghate, N. Thejokalyani, S.J. Dhoble, Luminescence 30 (2015)

251–256. [105] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992) 36. [106] C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett. 67 (1995)

3853. [107] Y. Liu, J.H. Guo, J. Feng, H.D. Zhang, Y.Q. Li, Y. Wang, Appl. Phys. Lett. 78 (2001)

2300. [108] W.B. Im, H.K. Hwang, J.G. Lee, K. Han, Y. Kim, Appl. Phys. Lett. 79 (2001) 1387. [109] H.T. Shih, C.H. Lin, H.H. Shih, C.H. Cheng, Adv. Mater. 14 (2002) 1409. [110] C.C. Wu, Y.T. Lin, H.H. Chiang, T.Y. Cho, C.W. Chen, K.T. Wong, et al., Appl. Phys.

Lett. 81 (2002) 577. [111] Y.H. Kim, D.C. Shin, S.H. Kim, C.H. Ko, H.S. Yu, Y.S. Chae, et al., Adv. Mater. 13

(2001) 1690. [112] J.M. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2002) 3201. [113] Y. Kan, L.D. Wang, L. Duan, Y.C. Hu, G.S. Wu, Y. Qiu, Appl. Phys. Lett. 84 (2004) 1513. [114] J. Xie, C. Chen, S. Chen, Y. Yang, M. Shao, X. Guo, et al., Org. Electron. 12 (2011)

322–328. [115] M. Nohara, M. Hasegawa, C. Hosokawa, H. Tokailin, T. Kusumoto, Chem. Lett.

(1990) 189. [116] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 56 (1990) 799. [117] M. Uchida, Y. Ohmori, C. Morishima, K. Yoshino, Synth. Met. 55 (1993) 4168.




















































[118] D.D. Gebler, Y.Z. Wang, J.W. Blstchford, S.W. Jessen, L.B. Lin, T.L. Gustafson, et al., J. Appl. Phys. 78 (1995) 4264.

[119] J. Kito, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 63 (1993) 2627. [120] W. Tachelet, S. Jacobs, H. Ndayikengurukiye, H.J. Geise, Appl. Phys. Lett. 64 (1994)

2364. [121] A. Edwards, S. Blumstengel, I. Sokolik, R. Dorsinville, H. Yun, T.K. Kwei, et al., Appl.

Phys. Lett. 70 (1997) 298. [122] Y. Hamada, T. Sato, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32

(1993) 511. [123] G. Yu, Y. Liu, Y. Song, X. Wu, D. Zhu, Synth. Met. 117 (2001) 211–214. [124] G. Leising, E.J.W. List, S. Tasch, C. Brandstätter, W. Graupner, P. Markart, et al., Proc.

SPIE 3476 (1998) 76. [125] G. Leising, S. Tasch, F. Meghdadi, L. Athouel, G. Froyer, U. Scherf, Synth. Met. 81

(1996) 185. [126] N. Koch, A. Pogantsch, E.J.W. List, G. Leising, R.I.R. Blyth, M.G. Ramsey, et al., Appl.

Phys. Lett. 74 (1999) 2909. [127] X.T. Tao, H. Suzuki, T. Wada, S. Miyata, H. Sasabe, J. Am. Chem. Soc. 121 (1999) 9447. [128] W.L. Li, Z.Q. Gao, Z.Y. Hong, C.S. Lee, S.T. Lee, Synth. Met. 111 (2000) 53. [129] Z. Hong, W. Li, D. Zhao, C. Liang, X. Liu, J. Peng, et al., Synth. Met. 104 (2000) 165. [130] Y. Liu, J. Guo, J. Feng, Y. Wang, Appl. Phys. Lett. 78 (2001) 2300. [131] L. Kumar, R.R. Koireng, A. Mishra, P. Kumar, S.K. Dhawan, M.N. Kamalasanan,

et al., Indian J. Pure Appl. Sci. 43 (2005) 56–59. [132] K.H. Lee, J.N. You, S. Kang, J.Y. Lee, H.J. Kwon, Y.K. Kim, et al., Thin. Solid. Films.

518 (22) (2010) 6253–6258. [133] Y.-J. Pu, A. Kamiya, K.-I. Nakayama, J. Kido, Org. Electron. 11 (2010) 479–485. [134] Y. Lin, Y. Chen, Z. Chen, D. Ma, B. Zhang, T. Ye, et al., Eur. Polym. J. 46 (2010)

997–1003. [135] H. Wettach, S.S. Jester, A. Colsmann, U. Lemmer, N. Rehmann, K. Meerholz, et al.,

Synth. Met. 160 (2010) 691–700. [136] P. Tyagi, R. Srivastava, A. Kumar, V.K. Rai, R. Grover, M.N. Kamalasanan, Synth. Met.

160 (2010) 756–761. [137] Y.-H. Chen, S.-L. Lin, Y.-C. Chang, Y.-C. Chen, J.-T. Lin, R.-H. Lee, et al., Org.

Electron. 13 (1) (2012) 43–52. [138] H.K. Dahule, N. Thejokalyani, S.J. Dhoble, Luminescence 30 (2015) 405–410. [139] S. Raut, N. Thejokalyani, S.J. Dhoble, , in: D. Rivera (Ed.), Organic Light Emitting

Diodes(OLED) Materials, Technology and Advantages, NOVA Science Publishers, New York, 2016, pp. 41–92.

[140] Y.-H. Chen, S.-L. Lin, Y.-C. Chang, Y.-C. Chen, J.-T. Lin, R.-H. Lee, et al., Org. Electron. 13 (1) (2016) 43–52.

[141] S. Tao, Y. Jiang, S.-L. Lai, M.-K. Fung, Y. Zhou, X. Zhang, et al., Org. Electron. 12 (2011) 358–363.

[142] B.M. Bahirwar, R.G. Atram, R.B. Pode, S.V. Moharil, Mater. Chem. Phys. 106 (2007) 364–368.

[143] J. Kido, W. Ikeda, M. Kaimura, K. Nagai, Jpn. J. Appl. Phys. 35 (1996) 394. [144] J. Kido, M. Kaimura, K. Nagai, Science 267 (1995) 1332. [145] S. Tasch, E.J.W. List, O. Ekstrom, W. Graupner, G. Leising, Appl. Phys. Lett. 71 (1997)

2883. [146] X. Zhang, R. Sun, Q. Zheng, T. Kobayashi, W. Li, Appl. Phys. Lett. 71 (1997) 2596. [147] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Adv. Mater. 14 (2002) 147. [148] V.K. Rai, R. Srivastava, M.N. Kamalasanan, Synth. Met. 159 (2009) 234–237. [149] C.-H. Chang, C.-C. Chen, C.-C. Wu, S.-Y. Chang, J.-Y. Hung, Y. Chi, Org. Electron.

11 (2010) 266.






















































[150] W. Chen, L. Lu, J. Cheng, Optik 121 (2010) 107–112. [151] B. Hu, J. Zhang, Y. Chen, Eur. Polym. J. 47 (2011) 208–224. [152] L. Tang, H.-P. Xia, P.-Y. Wang, J.-T. Peng, Y.-P. Zhang, H.-C. Jiang, J. Mater. Sci. 48

(2013) 7518–7522. [153] L. Ding, S.-C. Dong, Z.-Q. Jian, H. Chen, L.S. Liao, Adv. Funct. Mater. 25 (4) (2015)

645–650. [154] L. Ying, C.-L. Ho, H. Wu, Y. Cao, W.-Y. Wong, Adv. Mater. 26 (16) (2014) 2459–2473. [155] C. Sekine, Y. Tsubata, T. Yamada, Adv. Mater. 4 (2016) 184.











http://dx.doi.org/10.1016/B978-0-08-101213-0.00004-7

CHAPTER 4

Artificial Lighting: Origin—Impact and Future Perspectives

4.1 INTRODUCTION

Lighting plays a major role in human life, as it directly affects day-to-day life [1–4]. Almost 33% of electricity generated on the planet is employed for lighting applications and thus current research is focused on develop-ing new and more ecofriendly lighting solutions [5–6]. With our reduced fossil fuels, lighting is expected to have a greater impact on the worldwide energy problem with possible catastrophic consequences. Hence, ineffi-cient lighting sources must be replaced with efficient ones capable of uti-lizing our available power more effectively.

Early artificial light sources included fire, candles, and kerosene-oil lan-terns. Later, gas lamps, halogen lamps, and hot filament bulbs were com-mon. Today, we have fluorescent lamps, compact fluorescent lamps (CFLs), and now solid-state lighting (SSL) with light-emitting diode (LED) sources, and hopefully organic light-emitting diode (OLED) lighting in the near future. From commercially practical tungsten filament incandes-cent lamps invented by Thomas Edison in 1879 to SSL with LED lamps, technology has revolutionized artificial lighting sources. In order to reduce energy consumption and generate pollution-free lighting, development of energy-efficient and ecofriendly materials has been a priority in the field of materials research. This focus led to the development of solid-state miniaturized light-emitting sources and fabrication of compact optoelec-tronic and photonic devices. SSL has many advantages over conventional light sources such as monochromaticity, higher energy efficiency, higher brightness, lower power consumption, space savings, good reliability, lon-ger service life ranging between 50,000 and 100,000 hours [7–9], and eco-friendliness [10]. These advantages have created applications for SSL for use in high-power applications, such as in display backlighting, communi-cations, medical services, signages, automotives, outdoor displays, and gen-eral illumination [11–12]. They are designed for use as point-light sources or as lamps with LED clusters.


There is major concern that blue LEDs and cool-white LEDs exceed safe limits of the blue-light hazard as defined in eye safety specifications [13]. They also cause more light pollution than other light sources. Hence, the future perspective of artificial lighting is safe, mercury free, and bio-degradable lighting with OLEDs- the future light sources with no light pollution. White light from these sources can be achieved by the use of combined light-emissive materials such as blue, green, and red [14–16]. They offer much more freedom in designing lighting at very low voltages [8,17–20]. Slim panels, just 1 mm thick, can be directly placed on ceilings rather than using fixtures that are suspended, leading to novel and versatile SSL.

4.2 LIGHT

Light has been mesmerizing mankind since the beginning of time. Without light, life on earth would not be possible.

Light plays a significant role in day-to-day life. In simple terms, light is a physical quantity emitted by a luminous body and when incident on the eye causes the sensation of sight through nerves. Light entering the eye is focused onto sensitive cells called rods and cones on the retina. These cells are responsible for human perception of color. The human eye can per-ceive around 10 million different colors. The difference in color of differ-ent objects is a function of the interaction between light and the eye. The perception of different colors by the human eye is depicted in Fig. 4.1.

Figure 4.1 Perception of different colors by human eye.

Artificial Lighting: Origin—Impact and Future Perspectives 89

Light was the foundation of several applications like lamp phosphors, lasers, LEDs, and SSL that had a profound impact on technology.

4.3 LIGHTING

Lighting includes both natural daylight emitted by the sun and arti-ficial light sources like electrically powered lamps. Light emitted by the sun is considered as natural lighting and its effective use can decrease the cost and energy consumption during the day. Day lighting includes (1) active day lighting, where sunlight is collected using a mechanical device to increase the efficiency of light collection, and (2) passive day lighting, where sunlight is collected using nonmoving and nontracking systems such as windows, clerestory windows, glass doors, skylights, roof lanterns, and tubular day-lighting devices. In today’s world, artificial lighting signifi-cantly contributes to the quality and productivity of human life. But even today, one-third of the world’s population still has no access to electric-ity, which is a significant barrier to human development. Most govern-ments today emphasize the development of new energy-efficient sources and energy-saving technologies, which offer advantages such as low power consumption, long lifetime (>100,000 hours), and environmental friendli-ness, in order to better meet the needs of these underserved areas as well as the world’s population as a whole.

4.4 CLASSIFICATION OF LIGHTING

Based on the distribution of light produced by the fixture, purpose, and application, lighting can be classified as ambient lighting, task lighting, and accent lighting.

4.4.1 Ambient LightingThis type of lighting is mainly used for general illumination of an area. It radiates comfortable levels of brightness. It is also known as general light-ing. It can be accomplished with ceiling- or wall-mounted fixtures, track lights, and lanterns.

4.4.2 Task LightingThis type of lighting is generally used to perform specific tasks such as reading, sewing, cooking, homework, hobbies, games, surgical proce-dures, etc., with lighting levels up to 1500 lux. Such lighting is provided by


lower-level track lighting, pendant lighting, and portable lamps. Task light-ing should be bright enough to prevent eyestrain.

4.4.3 Accent LightingThis type of lighting is mainly used for decorative purposes, interior design, and landscaping. As a part of a decorating design, it is used in spot-light paintings, house plants, sculpture, to highlight the texture of a wall, for outdoor landscaping, etc.

4.5 ARTIFICIAL LIGHTING: ORIGIN AND IMPACT

Artificial lighting started with the accidental discovery of fire by Homo erectus. Though fire was employed by our primate ancestors 2–6 million years ago, it is still considered as a creditable human innovation [21]. It was believed that fire was given to mankind as a divine gift, impulsively created when a bolt of lightning struck a tree. One of the earliest develop-ments was the discovery of a flaming torch, an iron basket supported with burning wood. It was the first portable lamp, representing early man’s first use of artificial lighting. Primitive lamps to illuminate caves were made from naturally occurring materials, such as hollow rocks, shells, horns and stones, filled with animal or vegetable fat as fuel. Man finally learned to control fire and the rate of burning by adding wicks. Olive oil was prob-ably the principal fuel employed in the Mediterranean countries and oils like sesame oil, nut oil, fish oil, castor oil, and plant oils in other countries. With this, man gained freedom from the blindness of night to a smaller degree and was on the road to civilization. In the West Indian Islands, fire-flies were imprisoned in primitive cages with a source of convenient light illumination by the process of bioluminescence [22]. After the natural oil lamp, lamps were also manufactured by pottery wheel and molding tech-niques. Pottery lamps were a cheap and practical means of illumination. They were easy to produce and easy to use, but rather messy to handle. The oil would often ooze from the wick hole and run down from the lamp [23]. Later coal, natural gas, and kerosene lamps grew popular [24]. The first commercial use of gas lighting began in 1792, and the electric carbon arc lamp was invented later in 1801.

In the early nineteenth century, with the discovery of electromagne-tism and electric current, numerous scientists attempted to devise simple and affordable electrically powered home lighting. These efforts led to the invention of the incandescent lightbulb by Humphry Davy in 1802. But it


didn’t even illuminate a small room, and its lifetime was less than 15 hours of use. The credit for inventing the first long-lasting filament goes to Edison and Joseph Wilson Swan who created the carbon filament bulb, which burnt brilliantly for 600 hours when introduced in 1879. This is probably one of the greatest inventions of mankind and remains an important land-mark in the field of lighting. Edison received US Patent 223,898 for his incandescent lamp in 1880. Contrary to popular belief, Thomas Alva Edison did not invent the first lightbulb, but rather he improved upon a more than 50-year-old idea. Later, scientists tried to improve the filament until they found that tungsten filaments outlasted all the types and thus were installed in billions of homes and became immensely popular across the globe. Over time, fluorescent lamps were invented and became the new trend. However, they contain mercury (hazardous waste) and hence alternative, more eco-friendly lighting has emerged recently, such as SSL and OLEDs.

4.6 LIGHTING TERMINOLOGY

Luminaire: A complete lighting unit, consisting of the lamp housing, ballast, sockets, and other necessary components placed together.

Luminaire efficiency: The ratio of lumens emitted by a luminaire to the total lumens emitted from the light source within the luminaire.

Luminous energy: The energy emitted or propagated in the form of light. Its units are lumen-seconds (lm-s).

Luminous intensity: The amount of light power emanating from a point source with a solid angle of 1 steradian (sr). Its standard unit is candela (cd).

Luminous flux: Luminous energy per unit time. It is the measure of the perceived power of light. In other words, it is the quantity of useful light emitted by a light source, measured in lumen (lm).

Luminous efficacy: The ratio of luminous flux to radiant flux. It measures the amount of usable light emanating from the fixture per used energy, i.e., it measures the conversion efficiency (electricity into visible light) of the source, expressed in lumen/watt (lm/W).

Lumen: An international unit of luminous flux used to measure total amount of visible light emitted by a light source. This unit only quanti-fies the visible radiation and excludes invisible infrared and ultraviolet light [25]. One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd.


Luminosity: The comparative degree to which light of a given wave-length induces the sensation of brightness when perceived by our eyes.

Illuminance: The amount of light energy reaching a given point on a defined surface area. The SI unit of illuminance is lux (lx).

Luminous exposure: Illuminance at the given point, over the given dura-tion, measured in lux second (lx-s).

Intensity of illumination: Luminous power incident on a surface. Its unit is lux (lx).

Lux: The quantity of light falling on a unit area of a surface measured per lumens per square meter area. 1lux 1lumen m2= / .

Luminous density: Luminous energy per unit volume. Its unit is lumen second per cubic meter (lm-s/m3).

Luminance: Luminous power per unit solid angle per projected surface area is known as luminance. In other words, luminance is the light output from the light source, defined as the ratio of luminous intensity per unit area. It is a key component for assessing lighting quality; generally, a higher bright-ness level is preferred. Its unit is candela per square meter (cd/m2).

Lifetime: The time required for the emission to be reduced by half its initial value. The device lifetime is severely limited if exposed to humidity and oxygen. It is generally measured in hours.

Light pollution: Also known as photo pollution or luminous pollution, this is a broad term that refers to numerous problems caused by exces-sive, inefficient, obtrusive artificial light that may lead to adverse health effects. This type of pollution involves the emission of carbon dioxide from some artificial lamps [26,27].

4.7 LIGHT SOURCES

A source that emits light is called a light source. The sun, stars, fire, can-dles, and electric bulbs are all the examples of light sources. As they radiate light, they are said to be luminous. Light sources can be natural or artifi-cial. The major milestones in the evolution of light sources are depicted in Fig. 4.2.

4.7.1 Natural Light SourcesAmong the many light sources, the most common and natural light source is a simple thermal source—the sun—which emits light by the pro-cess of incandescence. Energy from the sun in the form of sunlight sup-ports almost all life on earth and drives the earth’s climate and weather.


The radiation emitted by the chromosphere of the sun at nearly about 6000K in the visible region of the electromagnetic spectrum when plotted in wavelength units. Approximately 44% of sunlight energy that reaches the ground is visible. Its lifetime is 5 billion years with a CRI equal to 100 and a CCT ranging from 2400K to 7500K. It is energy efficient as it is free of cost. However, this energy is available only during the day, it con-tains UV radiation, and it is not dimmable. Hence, artificial light sources are the requisite on a cloudy day and after every sunset.

4.7.2 Artificial Light SourcesArtificial light sources extend the day and allow us to lengthen our working time after sunset [28]. Hence, lot of research is undertaken globally in order to improve lighting in all aspects of life as well as to develop ecofriendly and energy-efficient technology. Artificial light sources are electrically powered lamps that work either on the phenomena of incandescence or lumines-cence. These lamps can be broadly classified into three categories, namely lamps that operate on the principle of generation of light by (1) filament heating (incandescent and halogen lamps); (2) gas discharge (high-intensity discharge (HID) lamps, and fluorescent lamps); and (3) recombination of electrons and holes (LEDs and OLEDs), as shown in Fig. 4.3.

Different bulbs produce different lighting effects with varying perfor-mance. In today’s world, efficient lightbulbs that have the ability to emit quality white light and save energy are highly essential. Colors and light sources that emit from the violet/blue end of the spectrum are referred

Figure 4.2 Major milestones in evolution of light sources: (A) sun, (B) candle, (C) incan-descent bulb, (D) halogen lamp, (E) linear fluorescent lamp (LFL), (F) CFL, (G) spiral CFL, (H) LED, (I) LED lamp, (J) organic LED (OLED) lamp.


to as cool colors and sources, while those toward red/orange/yellow are referred to as warm colors and sources, respectively.

4.8 EVALUATING QUALITY OF WHITE LIGHT

The quality of white light generated by light sources is evaluated by three parameters: Commission Internationale de L’Eclairage (CIE) coordinates, color rendering index (CRI), and correlated color temperature (CCT).

4.8.1 CIE CoordinatesThe color of a light source is typically characterized in terms of the CIE system. The chromaticity diagram, representing the CIE coordinates of different colors, is shown in Fig. 4.4. These coordinates describe how the human eyes perceive the emission color of a light source with a pair of two numbers enclosed in parentheses and separated by commas (e.g., x, y) in the CIE chromaticity diagram [30–32]. These coordinates (x, y) can be obtained from tristimulas values X, Y, and Z and can be calculated by color-matching functions and spectral power distribution by the formula:

xX

X Y Zy

Y

X Y Z=

+ +=

+ +

For lighting application, light sources should give CIE coordinates similar to that of a blackbody radiator and at the same time must be closer to the ideal white point at (0.33, 0.33) for better color purity. The CIE

Figure 4.3 Flowchart of electrically powered lamps.


coordinates of the main light source should only drift in the range of 0.005–0.01 or less for both the x- and y-values in the whole brightness range and the entire operation process.

4.8.2 Color Rendering IndexThe emission spectrum of lighting sources should be broad and continu-ous, covering the entire visible-light spectral region so that objects with any color can be illuminated vividly by the white light produced. The parameter employed to judge the ability of a lamp to accurately render all colors in a lighted space is called the CRI. Generally, the CRI of a light source is measured in a scale ranging between 0 and 100. The higher the CRI value, the stronger the ability to reproduce the true color of illumi-nated objects as shown in Fig. 4.5.

The highest possible CRI value occurs when there is no difference in color rendering between the light source and the standard illumi-nant. Generally, lighting sources with a CRI of 80 or above have good

Figure 4.4 Chromaticity diagram, representing the CIE coordinates of different colors [29].


color properties. Such sources would not significantly distort or dimin-ish the color of the object being illuminated and hence are preferred for indoor-lighting applications [33]. The color-rendering property of lamps is extremely important in many applications, especially in retail stores, muse-ums, galleries, etc., so as to have a realistic visual effect.

4.8.3 Color Correlated TemperatureBesides the CIE and CRI, another parameter to characterize the quality of white light is color correlated temperature (CCT), which is defined as the absolute temperature that describes the overall color appearance of a lamp, measured in kelvin (K). This scale was named after Lord Kelvin, a British inventor. The correct unit is kelvin (lowercase), not degrees Kelvin. Light sources such as fluorescent lamps and LEDs emit light by ways other than thermal radiation. Color temperature is just a way of correlating the source’s appearance to that of a heated object. Hence, the color tempera-ture values associated with these light sources are CCT and not true color temperatures. For high-quality, white-light illumination the CCT should lie between 2500K and 6500K. Lighting-source categorization and appli-cations based on CCT values are summarized in Table 4.1.

Figure 4.5 Demonstration of the realistic visual effect for different values of CRI at CCT = 2700K.

Table 4.1 Lighting-source categorization and applications based on CCT valuesCCT (K) Source categorization and applications

2700–3000 Warm light source, incandescent color, used for task lighting3000–3200 Warm white-light source, used for interior applications3700–4000 A neutral white source, used for general lighting applications,

factories, parking lots, warehouses5000–6500 A moderately high CCT daylight source, used for general retail

lighting applications8000–10,000 Very high CCT daylight source, used for horticulture and

aquarium applications


In addition to these three parameters, color stability also plays a signifi-cant role in evaluating the potential of new lighting sources [34]. However, it is still a great challenge to establish a good compromise among all three parameters in artificial lighting.

4.9 SPECTRAL DISTRIBUTION OF DIFFERENT LIGHT SOURCES

Light is essential for vision, but at the same time excessive exposure of light may damage our eyes and skin through absorption of its energy [35]. Light has a dual nature (wave or particle), and the wavelength of light determines the color perceived by our eye. In the visible region, short wavelengths between 380 and 500 nm are considered as violet and blue light. In this region of visible light, photons possess higher energy and hence this range of visible light is known as high-energy visible (HEV) light. There is a unique nerve cell in the retina that is most sensitive to blue light with a wavelength around 480 nm. The sun is the primary natu-ral source of blue light; every moment that we are outside we are exposed to certain amount of blue and ultraviolet light. Blue light (λ ≈ 480 nm) has many important functions in the human body, including melatonin regulation, pupillary light reaction, cognitive performance, mood, loco-motor activity, memory, and body temperature. Blue light is also vital for life because generation of white light from any source involves blue light. Among all the available natural and artificial sources, the cool white LED actually produce significantly higher amount of HEV light.

4.10 ELECTRICALLY POWERED INCANDESCENT LAMPS

An electrically powered lamp is a device that produces light by the flow of electrical current. These lamps work on the principle of incandescence—the phenomena in which light is emitted by heat energy. These lamps can be classified as (1) incandescent lamps and (2) halogen lamps.

4.10.1 Incandescent LampsAn incandescent lamp, also known as a general lighting lamp, is 125 years old, and is still enjoying popularity as a major source of domestic light across the globe. It bestows light by the process of incandescence. These lamps create warm, yellow-white light that contains less red and blue. It consists of a filament made of tungsten, which would quickly burn away


if exposed to oxygen. Hence, it is placed in a sealed evacuated glass or quartz bulb, which is filled with inert gas inorder to prevent oxidation. When a wire filament is heated to a high temperature by passing an elec-tric current through it, light is generated at all frequencies from infrared to ultraviolet. These lamps are manufactured in a wide array of sizes, light output, and voltage ratings and are common in household and commercial lighting applications. For a 100 W/230 V lamp, around 100 cm long tung-sten wire is required. The wire is coiled to form a spring-like structure, which is known as a single-coil filament. When this coil is coiled again, it is known as coil-coil filament. This type of filament is smaller in size as compared to single-coil filament and hence the efficacy of coil-coil fil-ament is higher. The luminous efficacy of a typical incandescent bulb is 12–18 lm/W with CIE coordinates 0.44, 0.40. They are best suitable for task lighting that demands high levels of brightness, and their lifetime is more than 1000 hours of use. However, during operation, the evaporated tungsten eventually condenses on the interior surface of the glass enve-lope, thereby darkening it. For bulbs that enclose vacuum, the darkening is uniform across the entire surface of the envelope, while for those filled with inert gas, the evaporated tungsten is carried in the thermal convec-tion currents of the gas, depositing preferentially on the uppermost part of the envelope and blackening just that portion of the envelope. Also, a very small amount of water vapor inside a lightbulb can significantly affect lamp darkening. Hence, a huge amount of light loss occurs due to filament evaporation and bulb blackening. Also, they are inefficient as they are capa-ble of converting less than 10% of their energy as visible light [36] and the remainder in infrared as heat. Hence, they are not very efficient in today’s energy-conscious world.

4.10.2 Halogen LampsA halogen lamp is the ultimate evolution of the incandescent lamp. It consists of a tungsten filament sealed with a compact transparent envelop filled with an inert gas and high-pressure halogen gas like bromine or iodine in a small amount in order to increase the lifetime and brightness. They offer longer lifetime, more light per watt, and also produce brighter and whiter light than other incandescent bulbs. The luminous efficiency of a halogen lamp lies in the range of 16–29 lm/W with (0.44, 0.40) as its CIE coordinates. US Patent 2,883,571 was granted to Elmer Fridrich and Emmett Wiley for a tungsten halogen lamp—an improved type of incan-descent lamp—in 1959. A better halogen lamp socket that could fit into a


standard lightbulb was later invented in 1960 by General Electric engineer Fredrick Moby, for which he was granted US Patent 3,243,634. As these bulbs are filled with halogen gas at low pressure, rather than only inert gas, uneven evaporation of the filament is reduced and darkening of the envelope is eliminated. These lamps give light at higher temperatures than nonhalogen incandescent lamps, and are widely used as lighting sources, e.g., on lawns for video recording and photography.

4.11 ELECTRICALLY POWERED LUMINESCENT LAMPS

These lamps work on the principle of luminescence, a general term applied to all forms of cool light from sources such as electron stimulated luminescence (ESL) lamps, HID lamps, and SSL lamps. Luminescence is caused by the movement of electrons within a substance from higher energetic states to lower energetic states.

4.11.1 Electron-Stimulated Luminescence LampsThese lamps generate light when a beam of electron is allowed to hit flu-orescent light-emitting phosphor coated on a transparent glass envelope. Light produced so far have a CRI of 90 with energy consumption 70% less than that of a standard incandescent lightbulb. The claimed lifetime is about 10,000 hours, which is more than 10 times that of a standard incandescent lightbulb. Their disadvantages include heavy weight, slight delay before illumination begins, and a static charge that attracts dust to the bulb face.

4.11.2 Discharge LampsThese lamps generate light by gas discharge. Atoms of gas or metal vapors are excited by the fast-moving electrons. When these atoms come back to their original state, they emit light, which may be in ultraviolet or vis-ible or infrared region. As UV light is invisible to human eye, it is con-verted to visible light by using appropriate fluorescent material. These lamps are divided into two categories: (1) high-pressure discharge lamps or HID lamps, which include high-pressure mercury vapor lamps, high- pressure sodium lamps, and metal halide lamps, with a working gas pres-sure of between 2 and 3 Torr; and (2) low-pressure discharge lamps, which include low-pressure mercury vapor lamps and low-pressure sodium lamps, with a working gas pressure of a few mm of mercury.


4.11.2.1 High-Intensity Discharge LampsA HID lamp gives higher lumen output from a compact source, which is made possible for generating the required arc discharge at high vapor pressure of the metal components, used inside the discharge tube. These lamps are specially designed with inner glass tubes that include tungsten electrodes with electrical arc. They produce light by striking an electri-cal arc across tungsten electrodes inside a specially designed quartz dis-charge tube mounted on a nickel plate steel-wire frame, which is filled with both gas and metal. The gas aids in the starting of the lamp, while the metal produces the light once it is heated to a point of evaporation, forming plasma. Auxiliary equipment like starters and ballasts must be pro-vided for proper operation of the bulb. These lamps have long lives and are extremely energy efficient, with the exception of metal halides they do not produce pleasing light colors. They are generally used where high levels of light are required over large areas, which include outdoor activity areas, gymnasiums, large public areas, pathways, roadways, and parking lots.

4.11.2.2 Metal-Halide LampsA metal-halide lamp is a type of high-intensity gas discharge electric lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides. These lamps consist of a small fused quartz or ceramic arc tube that contains the gases and the arc, enclosed inside a larger glass bulb, that has a coating to filter out the ultraviolet light produced. They operate at a pressure between 4 and 20 atm, and require special fixtures and electric ballast to operate safely. When a metal halide lamp is started, the mercury vapors are excited and produce radiation of mercury. As the temperature of the discharge tube increases, the metal halides, which were in solid state, now change into vapor phase and are carried into the hot region of the arc by diffusion and convection. Due to high temperature, these molecules break up into metal and halogen atoms. The metal atoms get excited and produce their characteristic radia-tion. When these atoms reach the cooler zone near the walls, they again recombine to form metal halide molecules. The spectrum of mercury lies in the blue (436 nm), green (546 nm), and yellow (579 nm) regions, and is deficit of red and other parts of the visible spectrum. To modify the spectrum of mercury, it is necessary to introduce metals so as to generate sufficient vapors. Metals are selected such that they are easily excited and their resonance lines fall in the required region. Also, these vapors should not react chemically with silica tube at the operating temperature of the


discharge tube. Initially, metals like sodium, lithium, tin, gallium, etc., were used, but no single metal gave the desired result. Moreover, the vapors of alkaline elements are highly reactive with silica tube, which results in early lamp failure. But halides of these elements were found to be much less reactive because of the halogen cycle, i.e., halogen breaks into metal and hydrogen ions at high temperature in the plasma of discharge tube [37,38]. As the discharge tubes use these metal halides, they are known as metal halide lamps. The halide group consists of fluorine, chlorine, bro-mine, and iodine, among which the reactivity of fluorine is the best and is the worst for iodine. Thus, iodines are preferred over other halides such as sodium iodide. These lamps have a high luminous efficacy of around 75–100 lm/W, which is about twice that of mercury vapor lights and 3–5 times that of incandescent lights, and produce an intense white light. They require a warm-up period of several minutes to reach full light output. Lamp life lies between 6000 and 15,000 hours. These lamps are most com-monly used for wide-area overhead lighting in halls, public spaces such as parking lots, sports arenas, factories, and retail stores, traffic lights, on stages, and for industrial and commercial purposes [39–42].

4.11.2.3 Mercury Vapor LampsThe mercury vapor lamp is a high-intensity gas discharge lamp that uses an electric arc through vaporized mercury in a high-pressure tube to cre-ate very bright light directly. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger glass bulb made of borosilicate. They are of two types: high-pressure and low-pressure mer-cury vapor lamps. In a low-pressure mercury vapor lamp, glow is observed when electricity is passed through liquid mercury. Later, the lamp heats fast and mercury changes its state into vapor and the light intensifies as the arc becomes stronger in the tube. Today, high-pressure lamps with a fused quartz inner discharge tube are used as they offer high efficiency than low-pressure lamps. This type of lamp starts with a small arc between the starting electrode and the main electrode. This arc goes through argon gas and heats the tube. When the tube gets hot enough to vapor-ize, the solid mercury sticks to the sides. The vaporized mercury creates a strong light between the two main electrodes. They are more energy efficient than incandescent lamp with luminous efficacies of 35–65 lm/W [43] and (0.31, 0.32) as CIE coordinates. Their lifetime is in the range of 24,000 hours with high-intensity and clear white-light output. However, some UV radiation can still pass through the outer bulb of the lamp,


which is harmful [44]. They find applications in lighting large areas like parks, high ceiling buildings, gyms, and in streets.

4.11.2.4 Sodium LampsSodium lamps are among the most efficient lamps in the world because they use all the current to create monochromatic yellow light, the most sensitive color to the human eye. They are generally classified as (1) low-pressure sodium lamps (invented in 1920 by Arthur H. Compton), which emanate yellow light by creating an electric arc through vaporized sodium metal when heated and operate much like a fluorescent lamp and require a short heat-up time to reach full brightness. As sodium is highly corrosive, it blackens ordinary silica glass when it hits the surface of the bulb. The difficulty with the Compton models is their poor CRI. Hence they are commonly used in places like roads, pathways, outdoor areas, and park-ing where color is not important. (2) High-pressure sodium lamps, the most omnipresent lamps for street lighting on the planet with better CRI, consist of a narrow arc made of aluminum oxide ceramic that is resis-tant to the corrosive effects of alkalis like sodium. The arc tube is held at high pressure so as to achieve higher efficiency, and sodium, mercury, and xenon are generally used inside it. Mercury helps to add a blue spectrum of light to the pure yellow of sodium. The light emitted by these lamps has CIE coordinates (0.51, 0.41). These lamps find applications in streets, foot-paths, home yards, and industries.

4.11.2.5 Linear Fluorescent LampsA linear fluorescent tube, commonly known as a tube light, is operated with electromagnetic ballast or high-frequency electronic ballast (20–26 kHz) on 220 V, 50 Hz supply. It is a low-pressure, mercury-vapor gas-discharge lamp that utilizes the phenomenon of fluorescence to generate white light. It is made up of a narrow glass tube with two electrical con-nections on each side of the metal cap that seal the ends of the tube. It contains inert gas such as argon, neon, krypton, and mercury vapor. When the tube light is off, the mixture of mercury and gas is not conductive. A high voltage discharge starts the flow of current through the gases and excites the electrons in the mercury atoms, so as to emit shortwave ultra-violet radiation. The tube emits light when this UV strikes the phosphor coated on the walls of the tube. These lamps are about 3–5 times as effi-cient and can last for about 10–20 times as compared to standard incan-descent lamps. Their lifetime is around 7000–10,000 hours. Fluorescent


lamps don’t require high temperatures to produce light like incandescent bulbs do, and the luminous efficiency is about 80–100 lm/W. However, a single fluorescent lamp contains 5 mg of mercury (hazardous waste), and their efficiency is only ~50%. They also occupy a lot of space as they can be fixed only to walls. Furthermore, the light emission from these bulbs is time consuming and they are easily breakable, but they are widely used for domestic applications.

4.11.2.6 Compact Fluorescent LampsA CFL, also called an energy-efficient lamp or energy-saving light, is a fluorescent lamp designed to replace linear fluorescent lamps. These lamps use a tube that is curved or folded to fit into the space of an incandescent bulb and a compact electronic ballast in the base of the lamp as shown in Fig. 4.5. The principle of operation in a CFL bulb remains the same as in other fluorescent lighting. These lamps consist of two main parts: (1) spiral glass tube filled with inert gas and mercury at low-pressure (2) magnetic or electronic ballast. When electric current from the ballast flows through the gas, it creates UV light. This UV light later excites phosphor coated on the inner walls of the tube, thereby emitting white light [44], which complicates their disposal. As compared to general-service incandescent lamps giving the same amount of visible light, CFLs use one-fifth to one-third of the electric power and last 8–15 times longer. CFLs cost more than incandescent lamps, but can save over five times the purchase price in electricity costs over the lamp’s lifetime. These lamps cannot be used with dimmers and usually can last for 10,000 hours. The luminous efficiency of a CFL is about 60 lm/W, and they are commonly used in commercial projects and in residential applications. As these mercury lamps pose envi-ronmental concerns, alternative green lighting technology such as SSL is preferred.

4.12 SOLID-STATE LIGHTING

General lighting applications that make use of LEDs, OLEDs, or light-emitting polymers (LEPs) are universally referred to as SSL. These devices create visible light by means of electro-luminescence, a phenomenon in which electric current passing through a specially formulated P-N junc-tion diode causes the semiconducting material to glow. The revolutionary idea of SSL came from Dr. Dave Irvine Halliday in 1997 [45]. The prin-cipal advantage of SSL lamps over incandescent and fluorescent lamps lies


in their superior energy-conversion efficiency and average lifetime. For example, a typical incandescent bulb converts about 10% of the supplied electrical energy into visible light, while the rest is emitted as heat (infra-red radiation), which is invisible. In contrast, SSL converts about 90% of the supplied energy into visible light, and only 10% into infrared radiation. A typical SSL lamp lasts for 35,000–50,000 hours, which is more than 20 times the average life of an incandescent bulb and roughly 6 times the life of a CFL. A high-quality SSL device can also function at a lower tempera-ture, thereby offering color shades ranging from cool blue to warm yellow. Hence, this technology has the potential to replace conventional incan-descent and fluorescent lamps for general-lighting purposes. We will look closer at SSL in the next chapter.

4.12.1 LED LampsLED lamps are semiconducting devices capable of emitting a fairly narrow bandwidth of visible or invisible light with suitable forward electric cur-rent or voltage. These lamps are capable of emitting wide range of colors in visible regions like orange, red, yellow, blue, green, and white, as well as invisible light that includes infrared light. The prevalent advantage of these devices is their high power-to-light conversion efficiency. Light from an individual LED is small as compared to an incandescent or fluorescent lamp. Hence, multiple diodes are often used as surface-mount devices arranged on a piece of printed circuit board to form the LED panel, which is the heart of the LED lamp. The anatomy of an LED lamp, which is generally used for domestic lighting, is shown in Fig. 4.6.

Figure 4.6 Anatomy of an LED lamp.


An LED lamp consists of six main components: (1) the lower por-tion of the bulb that screws into an existing socket called the base; (2) the electrical connector, which connects the electrical contacts on the base to the 12-V driver that powers the LEDs; (3) the driver, which converts the AC voltage to the DC voltage used by LED bulbs and generates a large amount of heat and hence has to be driven off, accomplished with (4) the heat sink, which can disperse heat from electrical components, and is generally made of aluminum; (5) the LED panel, which comprises a num-ber of surface-mount devices (SMD) LED, arranged on a piece of printed circuit board to form the LED array that provides the light from the bulb with the application of voltage; standard household LED bulbs contain a single LED or an array of 5–10 SMDs; and (6) A frosted globe, made of plastic is used in order to disperse the light produced by SMD LEDs evenly. This gives the LED bulbs, the look and feel of more traditional frosted white incandescent bulbs. The efficiency of these bulbs is almost 50 times greater than incandescent lamp.

4.12.1.1 Various Approaches to Obtaining White Light from LEDsThe basic requirement of any light source for domestic application is to emit bright and intense white light. Various approaches for obtaining white light from LEDs include (1) stacking of red, green, and blue (RGB) light-emitting materials, generally in ratios of 60%, 30%, and 10%, respec-tively; (2) stacking of blue and yellow phosphor between anode and cath-ode; and (3) by a combination of RGB phosphor and UV LED lamp as shown in Fig. 4.7A, B, and C, respectively. LED lamps last from 40,000 to 100,000 hours depending on the color, and have made their way into numerous lighting applications including traffic signals, exit signs, under-cabinet lights, and various decorative applications. LEDs have been around for nearly 50 years, but until a decade ago, were used only in electronic devices as indicator lamps to indicate power (ON/OFF) and in seven-segment LED displays. The development of brighter LEDs has resulted in applications in small-area lighting, traffic signal lighting, advertis-ing, automotive lighting, electronic billboards, and headlamps for motor vehicles, flashlights, searchlights, cameras, store signs, destination signs on vehicles, general illumination, optical switching applications, visual dis-play, decorative purposes, etc. They offer flexibility in their design, from zero to three-dimensional lighting. Infrared LEDs can be used in opti-cal fiber communications and in the remote control units of many com-mercial products including televisions, DVD players, and other domestic


appliances. They can also be employed in communication devices as they exhibit faster response times. The response time of the LED is in the range of 0.1 µs while for an incandescent lamp it is 100 ms. Hence, these devices are widely used as visual indicators and dancing light displays.

The advantages of LED include long-lasting lifetime ranging between 40,000 and 100,000 hours depending on the color, they consume much less power than earlier lamps, emit a very narrow spectral band, do not have mercury or any other toxic heavy metals like lead or cadmium and hence are toxic free and can be safely disposed of easily, and are lighter in weight. However, one of the biggest challenges of these LED devices is the issue of self-heating, which is a significant factor in the design of SSL equipment and negatively impacts luminous efficiency. Their perfor-mance largely depends on the ambient temperature of the operating envi-ronment, with optimal efficiency between −40°C to +50°C [46]. Though LEDs have many advantages, there are still some challenges that must be addressed to bring them into the realm of ecofriendly lighting.

4.12.1.2 LED Filament BulbA very popular bulb that is in use now is the LED filament bulb, shown along with one of the filaments and an electrically powered bulb in Fig. 4.8A, B, and C, respectively. This is an innovative and stylish series of

Figure 4.7 Illustration of various approaches to obtain white light from OLEDs; Stacking of (A) red, green, and blue (RGB) light-emitting materials; (B) blue and yellow phosphor between anode and cathode; and (C) RGB phosphor and UV LED lamp.


the COC (chips on cord) LED bulbs, which are designed to have a classic lightbulb look.

The lightbulb looks like an old incandescent lamp but uses a lot less energy. With the special filament structure, it delivers lighting in all directions. The distribution is more similar to a normal incandescent lamp than other LEDs and offers high luminous efficiency (class A++ in the European stan-dard). These bulbs are ideal replacements for decorative incandescent bulbs.

The basic construction of the LED filament is a glass substrate with a series of different LED chips in series bonded on top of the glass as shown in Fig. 4.9. The emitted color can be obtained by a combination of the different color LED chips, which are covered by a phosphor/SiO2 coating. These energy-efficient bulbs may be one of our future lighting sources.

4.12.2 Organic Light-Emitting Diode LampsOLED lamps are thin-film optoelectronic devices in which organic mate-rials are sandwiched between the anode and cathode. A thin layer of either small molecule, dendrimers, or polymeric substance is deposited as an

Figure 4.8 (A) LED filament bulb, (B) one of the filaments, and (C) an electrically pow-ered LED.


emissive electroluminescent layer. These lamps operate on the principle of electroluminescence. With the application of voltage across an OLED, electrical current flows from cathode to anode through organic layers. The cathode provides electrons, while the anode provides holes to the emissive layer of organic molecules. When the charges in exciton pairs are com-bined, they emit light depending on the type of organic molecule in the emissive layer. The emission color is basically determined by the energy difference of the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) of the emitting organic material. The intensity or brightness of the light depends on the amount of elec-trical current applied. They are mercury free, emit light across the visible, ultraviolet, and infrared wavelengths with very high brightness, and have the potential for use as energy-efficient solutions [47–49]. They also have revolutionary lamp properties, which include excellent transparency, color tunability, and flexible lighting sources [50,51]. OLED technology offers a slim (~1 mm) organic panel that can be placed directly on ceilings rather than using fixtures that are suspended. These OLED devices are able to produce all emission colors in accordance with a wide selection of organic emitting materials. OLEDs can also be used in light sources for general space illumination as they are area lighting sources that can be used at low

Figure 4.9 Construction of one of the LED filaments.


voltages. They are flexible and can be bended or rolled. They offer excep-tional color reproduction, with outstanding contrast levels and brightness. They can be operated effectively within the temperature range −20°C to +70°C and can be printed onto any medium, hence creating light-weight designs and flexible screens. With all of their advantages, OLEDs are still more expensive than existing displays, and also have operational lifetime challenges and color-balance issues. Blue OLED compounds are the most susceptible to aging and have a 50% drop in brightness in about 14,000 hours. The drop in brightness of blue OLEDs happens much faster than in red and green OLEDs, which causes color-balance issues in OLEDs [52,53]. Other key challenges include electricity-to-light conver-sion efficiency, device stability and lifetime (especially blue color), material selection and optimization, encapsulation manufacturing cost, fine pat-terning, contrast, pixel switching, and color saturation. Some of the param-eters of electronically powered lamps are compared in Table 4.2.

4.13 FUTURE PERSPECTIVES

Energy saving and ecofriendly SSL with LEDs and OLEDs is emerging as a highly competent and viable alternative to existing lighting tech-nologies. As compared to present lighting technologies—e.g., incandes-cent, fluorescent, and HID lamps—LED and OLED technology has the potential to achieve significant energy and Co2 savings, without compro-mise in color rendering or switching speed. Hence, the contemporary objective in the field of optoelectronic engineering is to replace these conventional lighting sources with more power-efficient semiconducting light sources in order to make them more ecofriendly and self-sustaining. It is estimated that the use of SSL could reduce electricity used for light-ing 62%. However, there are some challenges that must be addressed.

Fabrication of consistent, efficient, and long-life blue OLEDs is a chal-lenge due to the complications in aligning energy levels at the layer inter-faces. Novel energy-efficient materials have to be synthesized. The operating lifetime may be improved by using materials with higher glass transition temperature, adopting better encapsulation techniques, and optimizing device process and structure. If and when these hurdles are overcome, SSL may be of vital importance for existing and future lighting technology.

Table 4.2 Comparison of different parameters of electronically powered lamps [30,54–56]Parameter Incandescent

lampsHalogen lamps Sodium

lampsMercury lamps

Linear fluorescent lamps

Compact fluorescent lamps

SSL LEDs SSL OLEDs

Power (W) 60 40–100 200 40–1000 40–60 13–15 6–8 001–3Luminous

efficacy (lm/W)

12–18 16–29 100–190 35–65 80–100 70 28–150 64

Luminous flux (lm)

1200 – – – 3400 – 1000 –

CIE (x,y) (044,040) (044,040) (051,041) (031,032) (037,036) (037,036) (034,035) (033,036)CRI 100 99 24 20–60 74–90 89 70 92CCT (K) 2700–5000 2800–3400 2100 6800 3600 4080 2540–

10,0005410

Lifetime (h) 750–1200 1700–2500 18,000 24,000 7000–10,000 8000 35,000–50,000

10,000

Advantages Least expensive, operated at normal voltages, do not have any toxic materials to dispose

Offer warm, brilliant accented light with highest color reproduction, pleasant CCT and luminous efficiency

Good efficiency, smaller in size, better bulb life

Good efficiency, high CRI, good lifetime

Highly efficient, offer appreciable lifetime

Consume less power, very little warm-up time, lower greenhouse gas emissions, have longer lifetime

Very narrow spectral band, longer lifetime, consume much less power than the earlier lamps

Fast response time, wide viewing angle, outstanding contrast and brightness, extremely thin and lightweight, flexible

Disadvantages Poor conversion efficiency, not suitable for large lighting areas, are easily breakable

Expensive, emit lots of heat, short lifespan

Poor CRI, Sodium is highly volatile and hazardous materials that can combust when exposed to air

Requires warm up time, contain mercury (hazardous waste)

Contain mercury and are time consuming

Costly, they contain toxic mercury

Point sourceNot flexible

They are costly, needs encapsulation, degrade fast, poor lifetime (especially blue)

Applications Great for small-area lighting, task lighting

Lighting in lawns for video recordings and photography

Lighting in streets, footpaths, home yards, and in industries

For lighting large areas like parks, high ceiling buildings, gyms, and in streets

For household lighting

For domestic lighting

For ambient lighting in hotels, drawing room as well as lights of cars and bikes, traffic signals

SSL, in mobile, watch displays


REFERENCES [1] G.Y. Yun, H.J. Kong, J.T. Kim, Energy Build. 46 (2012) 146–151. [2] D.H.W. Li, E.K.W. Tsang, I.R. Edmonds, Indoor Built Environ. 20 (6) (2011) 638–648. [3] G.Y. Yun, T. Hwang, J.T. Kim, Indoor Built Environ. 19 (1) (2010) 137–144. [4] F.-K. Wang, Y.-C. Lu, Microelectron. Reliab. 54 (2014) 1307–1315. [5] S. Sinha, S.S. Chandel, Renew. Sustain. Energy Rev. 32 (2014) 192–205. [6] P. Hawken, A. Lovins, L. Hunter, Natural Capitalism, Back Bay Press, Time Warner

Book Group, New York, 2000. [7] E.F. Schubert, Light-Emitting Diodes, Cambridge University Press, Cambridge, 2006. [8] M.R. Krames, O.B. Shchekin, R. Mueller-Mach, G.O. Mueller, L. Zhou, G. Harbers,

et al., J. Disp. Technol. 3 (2) (2007) 160–175. [9] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. Cryst. Growth. 268 (2004)

449–456. [10] D. Evans, Proc. SPIE 3002 (1997) 142–153. [11] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [12] F. Salata, A. de Lieto Vollaro, A. Ferraro, Energy Convers. Manage. 84 (2014) 623–632. [13] A.A. Efremov, N.I. Bochkareva, R.I. Gorbunov, D.A. Lavrinovich, Y.T. Rebane, D.V.

Tarkhin, et al., Semiconductors 40 (5) (2006) 605. [14] E. Hecht, Optics, fourth ed., Addison Wesley (2002), p. 591. ISBN 0-19-510818-3. [15] Z.K. Chen, H.S.L. Nancy, H. Wei, Y.S. Xu, C. Yong, Macromolecules 36 (2003)

1009–1020. [16] W.K. Seung, J.J. Byung, A. Taek, K.S. Hong, Macromolecules 35 (2002) 6217–6223. [17] C.K. Gan, A.F. Sapar, Y.C. Mun, K.E. Chong, Procedia Eng. 53 (2013) 208–216. [18] V. Van Elsbergen, H. Boerner, H.-P. Lobl, C. Goldmann, S.P. Grabowski, E. Young, et al.,

Proc. SPIE 7051 (2008) 70511A-1. [19] Q. Huang, K. Walzer, M. Pfeiffer, V. Lyssenko, G. He, K. Leo, Appl. Phys. Lett. 88 (2006)

113515. [20] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 32 (2014) 448–467. [21] N. Thejokalyani, S.J. Dhoble, Defect Diffus. Forum 357 (2014) 1–27 Trans Tech

Publications, Switzerland. [22] Lighting 1, Early oil lamps, British Science Museum, 1966. [23] D.M. Bailey, Greek and roman pottery lamps, The British Museum, 1972. [24] Z. Bilkadi, The Oil Weapons, Saudi Aramco World, University of California, Berkeley,

CA, 1995, pp. 20–27. [25] R. Fouquet, Heat, Power and Light: Revolutions in Energy Services, Edward Elgar

Publishing, Cheltenham and Northampton, MA, 2008, p. 411. ISBN 1-84542-660-6. [26] M. Velds, Sol. Energy 73 (2) (2002) 95–103. [27] C. Israel, N. Bleeker, Electr. Wholesaling 89 (9) (2008) 38–41. [28] B. Bowers, Lengthening the Day: A History of Lighting Technology, Oxford University

Press, Oxford, 1998. [29] J.E. Kaufman, J.F. Christensen, Lighting Handbook, Waverly Press, Baltimore, MD,

1972. [30] Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett. 351 (2002) 163. [31] Q.Y. Zhang, K. Pita, S. Buddhudu, C.H. Kam, J. Phys. D 35 (2002) 3085–3090. [32] W.R. Stevens, Building Physics: Lighting, Pergamon Press, London, 1969. [33] C. Adachi, M.A. Baldo, S.R. Forest, M.E. Thompson, Appl. Phys. Lett. 77 (2000)

904–906. [34] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 44 (2015) 319–347. [35] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Mater. Sci. Eng. R 71 (2010) 1–34. [36] T.J. Keefe, The nature of light, 2007. Archived from the original on July 24, 2012.

Retrieved November 5, 2007.
















































[37] K.V.R. Murthy, J.K. Kapoor, S. Shah, Glimpses of 125 years of lamps and technology (present and past); Hikey media Ch.10, 2014, pp. 125–127; ISBN 978-93-82570-69-1.

[38] D.E. Work, Light Res. Technol. 13 (1987) 143–152. [39] M.F. Hordeski, Dictionary of Energy Efficiency Technologies, CRC Press, Boca

Raton, FL, 2005, pp. 175–176. ISBN 0-8247-4810-7. [40] P. Flesch, Light and Light Sources: High-Intensity Discharge Lamps, Springer, Berlin,

2006, pp. 45–46. ISBN 3-540-32684-7. [41] D. Fromm, J. Seehawer, W.-J. Wagner, E. Winzerling, High pressure electric discharge

lamp containing metal halides. US Patent 4171498, 1979. [42] G.H. Reiling, Metallic halide electric discharge lamps. US Patent 3234421, 1966. [43] R.J. Williams, Phys. Chem. 39 (1963) 382–388. [44] G. Heilmeier, J. Castellano, L. Zanoni, Mol. Cryst. Liq. Cryst. 8 (1969) 293. [45] R. Peon, G. Doluweerra, I. Platonova, D. Irvine Halliday, G. Irvine Halliday, Proc. SPIE

5941 (2005) 109–123. [46] L.H. da Cunha Andrade, S.M. Lima, R.V. da Silva, M.L. Baesso, Y. Guyot, J. Mater.

Chem. C 2 (2014) 10149. [47] J. Iveland, Phys. Rev. Lett. 24 (2013) 428. [48] D.Y. Kim, H.N. Cho, C.Y. Kim, Prog. Polym. Sci. 25 (2000) 1089–1139. [49] T.C. Tsai, W.Y. Hung, L.C. Chi, K.T. Wong, C.C. Hsieh, P.T. Chou, Org. Electron. 10

(2009) 158–162. [50] S. Tao, Y. Jiang, S.-L. Lai, M.-K. Fung, Y. Zhou, X. Zhang, et al., Org. Electron. 12

(2011) 358–363. [51] N. Ide, H. Tsuji, N. Ito, H. Sasaki, T. Nishimori, Y. Kuzuoka, et al., Proc. SPIE 7051

(2008) 705119-1. [52] J. Zhao, S. Xie, S. Han, Z. Yang, L. Ye, T. Yang, Synth. Met. 114 (2000) 251. [53] J. Kido, K. Nagai, Y. Okamoto, Chem. Lett. 13 (1990) 657. [54] D. MacIsaac, G. Kanner, G. Anderson, Phys. Teach. 37 (1999) 520–525. [55] D. Chitnis, N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 64 (2015)

727–748. [56] N. Thejokalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 16 (2012) 2696–2723.




























http://dx.doi.org/10.1016/B978-0-08-101213-0.00005-9

CHAPTER 5

Solid-State Lighting

5.1 INTRODUCTION

In today’s digital world, smart lighting is a requisite, due to which, this area of research is playing an increasingly important role in the electrical energy sector. A great deal of attention has been focused on energy efficiency with the common goal of reducing greenhouse gas emissions and developing smarter ways to use electrical energy due to growing concerns over the environmental impact of conventional energy sources. Decreased use of energy means lower energy bills, reduced load on the grid, and less envi-ronmental impact. Lighting represents a significant portion of the world’s total electricity consumption, which has led to the development of energy-efficient technologies such as solar photovoltaic (PV) [1] and solid-state lighting (SSL) referred to as climate-smart lighting (CSL), an integrated and sustainable approach to creating lighting with ecofriendly resources. While the upfront costs of green technology are higher, the associated costs sav-ings and environmental benefits over the longer-term period may be sig-nificant. Since the invention of visible light-emitting diodes (LEDs) based on III–V semiconductor P-N junction materials, SSL has been developed extensively and is now challenging Edison-style incandescent lamp and flu-orescent lamps due to their high efficiency, lower power consumption, and exceptional reliability. First, red LEDs were developed, followed by amber and green LEDs [2]. However, extending their operation to even shorter wavelengths was limited to the widest direct bandgap energy of III-AsP materials, roughly corresponding to a yellow/green color. It took another 30 years for LEDs to cover the whole visible spectrum. In 1991, Shuji Nakamura demonstrated viable blue LEDs, based on new wide-bandgap materials [3]. This milestone made LEDs a serious competitor in general lighting, rather than mainly a technology for small indicator lights. Today, organic light-emitting diodes (OLEDs) are in a race with LEDs as light-ing and display devices. The ability of LEDs and OLEDs to emit photons in all three primary colors and to pump phosphors to produce white light marked the beginning of the era of smart lighting, which has great poten-tial to reduce energy costs and to change the lighting industry as a whole.


5.2 SOLID-STATE LIGHTING: A BRIEF HISTORY

Electricity generation is the main source of energy-related greenhouse gas emissions. On average, thousands of units of electricity are con-sumed for household lighting. It was estimated that generating one unit of electric power uses 400 gm of coal and 7.5 L of water (approximately). For every one unit of electricity generated 1 kg of CO2 gas is emit-ted (approximately) and a huge amount of fly ash is produced, which is a serious environmental concern. Many researchers have focused on these specific aspects [1,4]. There is a window of opportunity for environmen-tally benevolent technology to flourish with SSL, which is intended to be compact, cost-effective, energy efficient, UV free and environmentally friendly with wide variety of designable features [5–7] and eventually a high-quality replacement for incandescent and fluorescent lamps. In SSL, illumination is obtained through semiconductor devices like LEDs and (OLEDs), as well as light-emitting polymers (LEPs). SSL using OLEDs is poised to reduce electricity consumption by at least 50%, so that lighting will then use less than one-tenth of all electricity generated.

5.3 REQUISITE OF SOLID-STATE LIGHTING

Globally, the lighting industry faces many challenges in the development of CSL. Billions of people around the world still depend on fuel-based lighting, which is climate hazardous, and thus many governments are in the process of planning and developing policies in an effort to acceler-ate the developmentof green lighting to ensure environmentally friendly lighting for all. New CSL technology such as SSL is well-suited for all climatic conditions, offering the same level of illumination without being hazardous to the environment. Climate change and globalization are the major concerns that will drive future lighting technology [8,9]. The effects of climate change on lighting systems is more pronounced in develop-ing countries and thus requires national lighting planning, investments, and programs to ensure the development of CSL. Superior qualities of SSL include their extensive lifetime, their ability to create visible light with virtually no heat or energy dissipation, as well as the fact that they consume less power and hence are energy efficient. They are also shock and vibration resistant and do not have any toxic elements like mer-cury or lead. They are also available as flexible light fixtures, which can emit cool light as shown in Fig. 5.1. A robust CSL strategy addresses

Solid-State Lighting 117

lighting for better livelihoods and climate change challenges in both the short and long run.

5.4 SOLID-STATE LIGHTING WITH LEDs

Henry Joseph Round first observed the phenomenon of electrolumines-cence in 1907 and later invented the first LED. With the materialization of semiconductor technology in the 1950s, scientists put their efforts into restructuring existing lighting and display technologies with these semi-conductors. In 1962, when Nick Holonyak Jr. invented the first visible red LED, it was not bright enough to illuminate the environment and were only used as indicators and seven-segment displays. By the mid-1960s, more colors like green and yellow were manufactured with improved effi-ciency. By the early 1980s LEDs were being used for outdoor applications. They used less power and were ten times brighter than the previous ones. In the early 1990s high brightness LED packages were developed and were widely used as traffic lights. In the mid-1990s Dr. Shuji Nakamura of Nichia Chemical Corporation invented high brightness GaN blue LED [10], which paved the way for the development of white LEDs, created by coating the lights with phosphor, which eventually led to high-power

Figure 5.1 Solid-state lighting.


white-light LEDs. Today billions of LEDs are manufactured every year. LEDs are generally made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce distinct color. Different LED chip technologies emit light in specific wavelengths of the visible light spectrum and produce different intensity levels. Direct band-gap semiconductors (vertical transitions) emit more light while indirect bandgap semiconductors (indirect transitions) emit less light.

5.4.1 Strategies for Solid-State Lighting With LEDsWhat humans perceive to be white light is actually the perception of the combination of two or more colors of light. When white light passes through a prism, it is split into component colors, making a version of a rainbow. Mixing two colors that are opposite to each other in a color space, such as blue and yellow, creates binary white light. Red, green, and blue (RGB) can be mixed together to make white light. A common appli-ance that uses an RGB-like technique is the cathode ray tube monitor, where three beams of colored light are used to create a myriad of col-ors and white. Regardless of the technique used, the relative amount of energy of each color must be balanced to create the perception of a neu-tral white; otherwise the result appears cool, closer to the blue end of the spectrum or warm, closer to the red end of the spectrum. Since the first manifestation of the ultrahigh-brightness blue LEDs and subsequently white LEDs, SSL sources for general illumination have gained momen-tum [11–13]. Some of the techniques used by the manufacturers around the world today to create white light from LED-based SSL include the following. Selection of right fraction of tricolors (RBG)

The traditional way of generating white light is mixing red, green, and blue in the right proportions as shown in Fig. 5.2. However, this approach has two basic problems: (1) the efficiency of green LEDs is less than that of red and blue (generally known as the green-gap prob-lem), which leads to poor overall efficiency; and (2) the efficiencies of red, blue, and green change over time at different rates, leading to deg-radation in the quality of white light. However, these problems can be minimized by careful choice of LED emission wavelengths.

Blue LED covered with yellow-emitting phosphorIn this approach, a blue LED chip is covered with a thin layer of

phosphor that can emit yellow light when excited by blue light. The phosphor layer is selected such that it is sufficiently thin so that some


blue light is transmitted through it. The combination of blue and yel-low produces a cool white light as shown in Fig. 5.3.

Combination of RBG and yellow creates white lightThe problem of poor CRI can be overcome with a combination of

quadracolor LEDs—red, green, blue, and yellow—as shown in Fig. 5.4. Near UV or blue LED and RGB phosphors

A better route to achieve higher quality white light could be the com-bination of near UV LED and RGB phosphors. A thick phosphor layer would be used so that no near UV light would be transmitted, similar to the way phosphor coatings on CFLs prevent the transmission of UV light [14].

Apart from these mechanisms, the other strategies for solid state lighting with LEDs include: Coat blue LEDs with quantum dots, which absorb the blue light

and emit a warm white light. Coat near UV with europium-based red- and blue-emitting phos-

phors so as to achieve white light.

Figure 5.2 Demonstration: Tricolors generating white light.

Figure 5.3 Demonstration: Blue LED covered with yellow-emitting phosphor to create white light.


Transfer near UV radiation to visible light through the process of photoluminescence in phosphor materials to obtain white light.

RGB quantum dots in a single LED generate white light.

5.4.2 Overview of LED TypesLEDs are available as discrete components in (1) single color (red, orange, amber. green, blue, yellow and white) (2) bi-color (3) tri-color (4) clus-ters (5) seven segment displays and (6) high intensity light source products, they can be chosen on the basis of their applications.

5.4.2.1 Discrete LEDsLEDs when used in isolation as a single LED are known as discrete LEDs. They can be single-color, bicolor, or tricolor LEDs.

5.4.2.2 Single-Color LEDsThe standard type of LED is in the form of a small round dome epoxy encapsulation. LEDs are also available in variety of other shapes and sizes as shown in Fig. 5.5.

Rectangular shape and triangular LEDs are available for applications like panel indicators. LED components like axial-leaded LEDs, bargraph displays, tricolor RGB LEDs, and surface-mount technology components

Figure 5.4 Demonstration: Right proportion of RGBY generating white light.


are also available. The sailent features of these LEDs include high bright-ness, low driving voltage, flashing, and multicolor varients.

5.4.2.3 Bicolor LEDsThere are several occasions where an indicator capable of displaying more than one color is required. This can be achieved by packing two LEDs into a single package with a (1) common anode, where the positive con-nection is shared, and (2) with a common cathode, where the negative connection is shared, as shown in Fig. 5.6A and B, respectively.

A bicolor LED has three terminals either with a common anode or cathode, wired in inverse parallel (one forward and other backward), com-bined in one package, one of each color with two leads. The two LEDs are often green and red, and are wired back to back, so one glows for cur-rent in one direction, and the other glows for current in the other direc-tion, i.e., if current flows one way through the device the LED is green, and if current flows the other way the LED is red. Only one of the LEDs can be lit at a time. Electrically, they still require a current-limiting resistor.

Figure 5.5 Array of LED shapes and sizes [15].

Figure 5.6 Schematic diagram bicolour LED package types with common (A) Anode and (B) Cathode.


5.4.2.4 Tricolor LEDsThe most popular type of tricolor LED has red, green, and blue LEDs combined into one package with four leads of different lengths as shown in Fig. 5.7. They share a common cathode, but three separate anodes; one for red, one for green, and the other for blue. Depending on the require-ment, the desired sequence of color emissions can be achieved.

5.4.2.5 LED Clusters and LightsIn order to produce brighter and high-power lights, groups or clusters of LEDs are used as shown in Fig. 5.8.

The total brightness of the LED cluster decreases if there is a break down of any of the individual LEDs within the group. However, applica-tions of this technology is rapidly increasing in the market due to high-density light yield.

5.4.2.6 Alphanumeric DisplaysAround 1967 LED alphanumeric displays were introduced. Calculators used LEDs that were arranged to create either seven-segment displays or dot-matrix displays as shown in Fig. 5.9.

Figure 5.7 Schematic diagram of tricolor LEDs.

Figure 5.8 LED clusters and lights.


The seven-segment display is energized by applying potential rang-ing from 1.5 to 3.3 V with a constant current of 50–100 mA. These dis-plays can produce different colors such as red, blue, green, and orange. In a dot-matrix LED display array, the illuminated points appear bright, while nonilluminated points appear dark. In most display applications seven- segment displays are used.

5.4.3 Physical FunctionAn LED is an optoelectronic device, usually made with crystalline

compounds. It is a stack of N-type layer grown on a P-type substrate using diffusion techniques. Metal contacts are refined at the external edge of the P-layer so that most of the upper surface is left free for light to escape. Metal film coated at the bottom of the substrate acts as cathode, which reflects a large amount light to the surface of the device. It is surrounded by a transparent, hard-plastic epoxy resin shell or body that protects it from both vibration and shock. Light emission from an LED occurs due to electroluminescence—the process of converting electrical energy into light energy. When the diode is forward biased, the majority of the charge carri-ers start crossing the junction by the process of diffusion, thereby reducing the potential barrier. Hence, electrons from the N-side and holes from the P-side diffuse toward the P-side and N-side, respectively. When an electron congregates a hole, it hops into a lower energy level and liberates energy in the form of photons, which emit monochromatic light as shown in Fig. 5.10. If Eg represents the energy gap of a semiconductor, then the wave-length of emitted light can be calculated by the formula

E hhc c

hc

E

g

g

= = =

⇒ =

νλ

νλ

λ

∵

Figure 5.9 (A) Seven-segment, (B) 14-segment, (C) 16-segment, and (D) 5× 7 dot-matrix LED displays.


where h is the Planck’s constant, c is the velocity of light, and λ is the wavelength of emitted radiation. The color of the emitted light depends on the type of semiconductor used. Semiconductors whose energy gap lies between 1.7 and 3 eV emit light in the visible region.

If voltage across the leads is measured, the corresponding energy required to light the LED can be calculated. For example, if the voltage measured across the red LED is 1.71 V, then the corresponding energy is calculated as follows:

E qV=

= × ×

= ×

−

−

1.6 10 1.71 Joule

2.74 10 Joule

19

19

Generally, when an LED emits light, there is a loss of light energy due to the reflection losses that arise in LEDs. These losses can be reduced by encapsulating the semiconductor junction with a dome-shaped transpar-ent plastic medium (an epoxy) that has a refractive index greater than air and lower than that of the semiconductor as shown in Fig. 5.11. The shape of the surface of the semiconductor is selected to be a dome or hemi-sphere so that light rays strike the surface angles less than the critical angle θC so as to reduce total internal reflection (TIR):

where

θ

µC =

−Sin 1 1

In this method, P-type material is used to obtain a hemispherical dome-like structure. This dome forms a semiconductor–air interference. When light rays strike the semiconductor–air interface at an angle less

Figure 5.10 Generation of light from LEDs [16].


than the critical angle, they emerge without any loss. Replacement of the semiconductor–air interface in a hemispherical dome by plastic–air inter-face further reduces loss of energy due to reflection. In most commer-cial LEDs, plastic encapsulation is used in order to improve the brightness of the light energy. The essential requirements of an ideal LED are: (1) it should be capable of emitting significant light output from the device, (2) it must emit light from one side and almost all light should emerge out of the device, and (3) it must be encapsulated so as to enhance its lifetime.

5.4.4 LED Configurations and ManufacturingLEDs are uniquely fabricated P-N junction diodes that have been designed to optimize their electroluminescence. The P-N junction can be created by various techniques such as impurity diffusion or ion implanta-tion or it can be incorporated during the epitaxial growth phase. Based on the direction of light emitted by the LED, two basic configurations are possible: (1) Surface-emitting LED structure, which emits light per-pendicular to the plane of the P-N junction; and (2) edge-emitting LED structure, which emits light in a plane parallel to the P-N junction. In this configuration the light can be confined to a narrow angle. The active films of the LED structure are normally grown epitaxially, often by a liq-uid phase or vapor phase. The substrates are chosen to have a close lat-tice match to the active layers. Common substrates used for fabrication of LEDs include GaAs, InP, and GaP. Some of the steps used in manufac-turing LEDs include (1) creating semiconductor wafers, (2) accumulating epitaxial layers, (3) adding metal contacts, and (4) mounting and packag-ing. A cluster of red, green, and blue diodes are driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution.

Figure 5.11 Encapsulation of LEDs by a hemispherical dome.


5.4.5 LED MaterialsThe material selection for the fabrication of LEDs is essential due to their ability to emit different colors in the wide visible spectrum as well as the infrared and UV regions, depending on the semiconducting materi-als employed. For example, infrared and far-infrared light are emitted by lower-bandgap materials, while visible light is emitted by large-bandgap materials. Due to the high melting point, high resistivity, and low struc-tural stability, high- bandgap materials have more advantages in LED applications than low-bandgap materials. Essential factors such as energy gap and P- and N-type semiconductors with efficient pathways should be considered for the fabrication of LEDs.

The semiconducting materials used for LEDs, with their energy-gap and light-emission parameters, are listed in Table 5.1.

5.4.6 AdvantagesLED lighting is a modern and superior CSL system in the visible range with very narrow spectral band and extremely long working lifetime (around 35,000–50,000 hours of usage) depending on the color, a key fea-ture that has attracted the lighting community. LED lamps are currently more expensive than conventional incandescent lamps, but when their longer service life is taken into account, it is easy to see the cost advan-tage. People are fascinated with LEDs because they are small in size, light in weight, less fragile than glass and are not affected by mechanical vibra-tions, and are available in an array of shapes and sizes. They can be squeezed into tiny display units with minimal circuit parts and can be easily attached to printed circuitboards. They do not require heating or warming up time and offer ultrahigh-speed response time in milliseconds [20,21] and hence are used in communications devices. They reach their full brightness when turned on. They generate their own light and have long been considered as the way to better lighting. They spread light typically in one direction at a narrow angle compared to an incandescent or fluorescent lamp of the same lumen level. They are point-light sources and emit targeted light of specific wavelength with low thermal output. LEDs have neither UV nor infrared components in their light, which makes them more flexible, particularly in heat-sensitive areas, such as lighting for food or cosmetics, or in applica-tions with limited space. LEDs are energy efficient because they operate on low driving voltages and currents and hence cost less. They offer flexibility in their design, from zero to three-dimensional lighting, and the efficiency

Table 5.1 Light-emission parameters of various semiconductors used for LEDs [17–19]Semiconducting material

Bandgap type Energy gap Eg (eV) Emission wavelength (Å)

Emission color Emission region

GaAs Direct 1.45 8550 – InfraredInP – 1.27 9763 – InfraredGaP:ZnO Indirect 1.78 7000 Red VisibleGaAsP Direct 1.99 6500 Red VisibleGaAsP:N Indirect 1.95 6300 Red VisibleAlGaAs Direct 1.91 6500 Red VisibleAlGaP Direct 2.00 6200 Red VisibleGaAsP Direct 1.9 6530 Red VisibleGaP Indirect 2.26 5486 Red VisibleAlGaInP – 2.03−1.63 6100–7600 Red VisibleAlAsP:N Indirect 2.10 5850 Yellow VisibleGaP:N Indirect 2.20 5650 Yellow-Green VisibleGaP Indirect 2.26 5550 Green VisibleAlGaP/InGaN – 2.48−2.17 5000–5700 Green VisibleSiC Indirect 2.9−3.05 4800 Blue VisibleGaN Direct 3.5 4500 Blue VisibleZnSe – 2.75−2.48 4500–5000 Blue VisibleGaN – 3.4 3647 Blue VisibleAlN – 5.9 2100 – Ultraviolet


of fixtures is not affected by shape and size. LEDs are superior to conven-tional light sources in monochromaticity, have more light per wattspace, show good reliability, and have long service life [22,23]. LEDs are green products: they do not contain any environmentally harmful substances such as mercury or lead and offer huge potential for energy savings—with-out any reductions in light quality. The light intensity and quality can be adjusted by controlling the flow of current, and they offer high photo-electric conversion efficiency, which results in superior controllability. The semiconductors at the core of LED lamps also allow for remote control as well as the ability to change lighting settings to meet specific needs. LEDs can also emit colored light without using color filters as traditional lighting methods need, which lowers costs. While LEDs are the right candidates for SSL, there are key challenges that must be addressed.

5.4.7 Key ChallengesAlthough LEDs have penetrated the market and have grown in popularity, there are still many research challenges to overcome including: Operating environment

One of the biggest challenges in these inorganic SSL devices is the issue of self-heating since it negatively impacts luminous efficiency. The performance of LEDs largely depends on the ambient temperature of the operating environment. Over-driving the LEDs or not engineer-ing the device to manage heat in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Hence, adequate heat-sinking is required to maintain long life. This is especially important in automotive, outdoor, medical, and military appli-cations where the device must operate over a large range of tempera-tures (LEDs can function at optimal efficiency from −40°C to +50°C).

Color shiftLEDs can shift color due to age and temperature, and two differ-

ent white LEDs will have two different color characteristics, thereby affecting the perceivence of light. They require complex power supply setups to be efficiently driven.

PolarityUnlike incandescent lightbulbs, which illuminate regardless of the

electrical polarity, LEDs will only light with correct electrical polarity. Hence, special care has to be taken.


Light pollutionThere is great concern that blue LEDs and cool-white LEDs

exceed safe limits of blue-light hazards as defined by eye safety specifi-cations. Cool-white light LEDs with high color temperature emit blue light proportional to conventional outdoor light sources such as high-pressure sodium lamps. The strong wavelength dependence of Raleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. Also, LEDs cannot provide divergence below a few degrees. LEDs with a CRI below 75 are not recommended for use in indoor lighting [24].

Correct voltage and currentLEDs must be supplied with the appropriate voltage and current at a

constant flow, which requires good electronic driver design and a resistor used in series to protect it from overloading. Their power consumption is 1.5–4 V and 10 mA current to power them on. They must be supplied with the voltage above the threshold and a current below the rating, which involves series resistors or current-regulated power supplies.

5.4.8 ApplicationsLEDs have been around for nearly 50 years, but until a decade ago, they were only used in electronic devices as indicator lamps. LED technol-ogy flourished due to its high efficiency, high reliability, rugged con-struction, durability, and the fact that this technology is mercury-free. The development of brighter LEDs resulted in applications in small-area lighting, traffic lighting, indicators, electronic billboards, and headlamps for motor vehicles, flashlights, searchlights, cameras, store signs, destination signs on vehicles, general illumination, visual display, decorative purposes, etc. They are also used as seven-segment LED displays, in optical switch-ing applications, as visual signals, as illumination where light is reflected from objects to give visual response of these objects, as aviation lighting, automotive lighting, in advertising, as traffic signals, etc. Infrared LEDs can be used as a source in optical fiber communications and in the remote-control units of many commercial products including TVs, DVD players, and other domestic appliances. Hence, LED light covers a wide spectrum, from the infrared LED in remote controls and the visible LEDs for dis-play applications to the sterilizing ultraviolet LED light in medical field applications.


5.5 CSL WITH OLEDs: FUTURE LIGHTING SOURCES

OLEDs are a novel and attractive class of SSL that generate diffuse, non-glaring illumination with high color rendering. They are semiconductors that emit light when electricity is applied, but unlike LEDs, OLEDs are made from organic, carbon-based materials. They have the potential for use in next-generation lighting technologies. OLEDs are comprised of a complicated system of several very thin layers of various materials situ-ated between electrode layers; this system emits light when electric cur-rent is applied. They use a series of light-emitting thin films that enable the OLED to give out brighter light than LEDs, yet they use less energy. Since the films that emit light are made up of hydrocarbons (organic semi-conductors) and not semiconductors, they are called “organic” (hence the “O” before LED). OLEDs are made up of four layers: an anode that attracts electrons, a cathode that gives electrons, substrate that forms the framework, and organic layers in between [25–27]. These organic layers are divided into hole-transport layer (HTL), hole-injection layer (HIL), electron-transport layer (ETL), and electron-injection layer (EIL). OLEDs generate their light using wafer-thin layers of semiconducting organic materials. The type of material used as the light emitter determines the specific characteristics of these devices. Furthermore, LEDs made of organic substances are spreaded sources, which are chemically compat-ible and possess the properties of plastics and semiconductors. These sources are versatile and innovative, with low cost and driving voltage and improved color performance, fast, self-emitting with superior features of high luminescence, high electroluminescence efficiency, high visibility, resistant to temperature, environmentally friendly, harmless [28–30], and thinner than earlier sources and displays. One major advantage of organic molecules is that their properties can be tailored and tuned by changing the molecular structure just by adding specific functional groups. OLEDs reveal massive superiorities as light source, due to the fact that different red, green and blue combination of organic and doping emitting materials can generate every color of the visible spectrum [31–33].

5.5.1 Anatomy of OLEDsA typical OLED is comprised of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a sub-strate. An OLED can be single-layer, two-layer, triple, or multilayer as shown in Fig. 5.12. A single-layer OLED is made up of a single organic layer


sandwiched between the cathode and the anode that encompasses hole-transport, electron-transport, and emission capabilities. In this case the injec-tion of both the carriers should be the same; otherwise the device will result in low efficiency because the excess of electrons or holes cannot combine. In a two-layer OLED, one organic layer is explicitly chosen to transport holes and the other layer is chosen to transport electrons, namely hole transport layer (HTL) and electron transport layer (ETL), respectively. Recombination of the hole–electron pair takes place at the interface between the two layers, generating electroluminescence. In a triple-layer OLED, the functions of the individual organic layers are distinct and can therefore be optimized inde-pendently. Hence, the luminescence or recombination layer can be chosen to the desired emission color with high luminescence efficiency.

Multilayer OLEDs consist of different layers: (1) the anode, generally a glass substrate coated with ITO, which has high transparency, chemi-cal stability, low roughness, high surface finish, high flatness, high glass-transition-temperature, high work function (4.5–4.7 eV) dimensionally stable at processing temperatures, and high resistance against HNO3, HF, and NaOH [35]. Transparent plastic sheets, metal foil, which is, can also be used as flexible substrate. (2) HIL, which injects holes from the anode to the emissive layer. Materials with high mobility, electron-blocking capac-ity, and high glass transition temperature can be used as HIL. (3) The HTL made of materials with low ionization potential together with low elec-tron affinities and high hole mobility usually function as hole-transport-ing materials by accepting and transporting hole carriers with a positive charge. (4) The emissive layer is the layer in between the HTL and the ETL, which is a good emitter of visible photons, and can be fabricated with a material made of organic molecules or polymers or dendrimers

Figure 5.12 Anatomy of an OLED [34].


with high efficiency, lifetime, and color purity. Depending on the color required, materials can be selected to obtain the desired wavelength. Organic molecules or polymers with high efficiency, lifetime, and color purity are used as emissive materials. (5) The hole-blocking layer blocks the electrons and injects the holes. (6) The ETL, made of materials with good electron-transporting and hole-blocking properties, and high elec-tron affinities together with high ionization potentials usually function as electron-transporting materials by accepting negative charges and allow-ing them to move through the molecules. This layer provides an electron- conductive pathway for negative charge carriers to migrate from the cathode to the emission layer. It also prevents charge leakage and the accu-mulation of charges at the cathode and ETL interface. (7) The cathode is made of material with low work function metal alloy (φw ≈ 2.9 to 4.0 eV). The role of the cathode is to inject electrons into emitting layers. Alloy of magnesium with silver (Mg–Ag) with a work function in the range of 3.7 eV and aluminum with alkali metal compounds are generally used for cathode. Device performance can be enhanced by using highly purified organic complexes, implementing the method of codoping, controlling the thickness of each layer, proper selection of HIL, HTL, and ETL, and device anatomy. Multilayer architecture generally eliminates charge carrier leakage as well as exciton quenching at the interface of the organic layer and the metal. The anatomy of an OLED is shown in Fig. 5.12. The vari-ous requirements of different layers in OLEDs are listed in Table 5.2.

5.5.2 Light-Emitting MechanismOLEDs operate on the principle of electroluminescence. Voltage is applied across the diode such that the anode is positive with respect to the cath-ode. Under the influence of this voltage, electrons from the cathode are pushed toward the emissive layer of organic molecules. Similarly, holes from the cathode are pushed toward the conductive layer of organic mol-ecules. Electrostatic forces bring the electrons and holes toward each other and they recombine forming an exciton. When the charges in the exciton pairs are combined, they give rise to light emission, and the color of the light depends on the type of organic molecule chosen as emissive layer as shown in Fig. 5.13. The color of the emitted radiation depends on the energy gap of the material and the separation in energy between the high-est occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO). Consequently by tailoring these active materials the emis-sion color can be varied across the entire visible spectrum.

Table 5.2 Purpose and basic requirements of different layers in OLEDs [16]Layer of OLED Work function

(eV)Purpose Requirement Materials used

Substrate 4.7–4.9 Serves as base for deposition of all layers

Transparent, high work function, flexible

Glass, plastics, metal foils

Anode 4.5–5.1 Serves as a positive electrode

Low roughness and high work function

ITO, graphene

HIL – Blocks the electrons for recombination

High mobility, electron-blocking capacity, high glass-transition temperature

CuPC, PtPC, MeO-TPD, m-MTDATA

HTL – Transport holes and blocks electrons

Should have good hole-transporting capacity

NPB, TPD, α-NPD

Emissive layer – Emission of light due to recombination of holes and electrons

High efficiency, lifetime, color purity

Organic complexes, Alq3

ETL – Transport electrons and block holes

Should have good electron-transporting capacity

Liq, TPBi, PBD

Cathode 29–40 Serves as a negative electrode

Low work function, transparent Alloy of magnesium with silver, LiF/Al, etc.


5.5.3 White-Light GenerationOrganic LEDs, being composed of LEPs, can emit their own light and offer thin and power-saving displays. SSL creates white light by combin-ing the light of separate RGB LEDs or OLEDs. In SSL, low-wavelength and phosphor-conversion materials play a vital role. Since it is difficult to obtain all primary emissions from a single molecule, excitation of more than one organic species is often necessary [37]. The following techniques are typically employed to obtain white light: Host–guest systems

In this approach, a higher energy-emitting host (donor) material is doped with a lower energy-emitting guest (dye, dopant, or acceptor) material to cause energy transfer from the host to the guest. The dop-ant site can be excited directly by capturing the charge carriers or by energy transfer from the host to guest. As a result, light emission can be obtained from both the host and guests, the combined effect of which produces bright-white light [28,38].

Single-molecule structuresThe fabrication process and device operation of white OLEDs is

very complex and several parameters need to be optimized to increase

Figure 5.13 Light-emitting mechanism of multilayer OLEDs [36].


the luminescence efficiency. All these complexities can be reduced if a single component is used as the emissive layer that can emit in the entire visible range.

Multilayer structuresIn this approach, the concept of multilayer structure allows control

of spectral characteristics by adjusting the exciton recombination zone spatially. In these structures, simultaneous mixing of emission from different layers in primary or complementary colors produces white light [39–41].

Exciplex emission structuresAn exciplex/excimer is a transient charge transfer complex formed

due to the interaction between the excited state of one molecule with the ground state of another molecule. The resulting electron–hole pair complex decays radiatively, the emission of which is considerably red-shifted and broadened as compared to the individual molecules [42].

Down-conversion phosphor systemIn this technique a single-layer blue-emitting OLED is cou-

pled with down-converting orange and red organic phosphors [43]. The phosphor layers absorb emission from the blue OLED and emit according to their intrinsic property. The mixing of unabsorbed emis-sion from the blue OLED and emission from the phosphors produces white light. The emission spectrum can be adjusted by varying the concentration and thickness of the phosphor layers.

Microcavity structuresWhite emission from microcavity structures is obtained by simple

modification of the Fabry–Perot resonant cavity in which the emis-sive material is sandwiched between two metallic mirrors or a metallic mirror and semitransparent distributed Bragg reflector. The structure has two cavities of different lengths and the combined emission from the cavities produces white light. During operation of the device, standing waves are generated, the wavelength of which depends on the length and refractive index of the cavity. The main drawback of micro-cavity structures is the angular dependence of the emission, which is bound to limit its application in white OLEDs.

5.5.4 AdvantagesAdvanced technological efficiencies resulting in lighter, simpler carbon-based material, creating deeper blacks, brighter whites as well as all the gray scales in between are the greatest strengths of OLEDs. They are very thin and light since they are self-emissive and hence energy efficient,offer


exceptional color reproduction, with outstanding contrast levels and brightness, and have high speed, good color gamut, and large viewing angle (to about 170 degrees due to the fact that OLED pixels emit light directly and independently). They project high resolution with blazing fast-est response, and can be operated effectively within the temperature range −20°C to +70°C. They can be printed onto any medium, allowing light-weight designs and flexible screens that they can be bended or rolled [44].

5.5.5 ChallengesTo make OLED devices and displays applicable for SSL some challenges have to be addressed, including (1) inadequate lifetime of the organic materials [45,46], (2) limitations in temperature operating range, (3) lack of availability of stable blue phosphorescent materials [47], (4) degradation of OLEDs [48,49], and (5) inadequate extraction efficiency from OLED devices [46,50]. These challenges are discussed in detail in the next chapter.

5.6 ADVANTAGES OF ORGANIC OVER INORGANIC

The advantages of organic over inorganic include low cost synthesis, good chemical compatibility, and relative ease of handling. They possess the properties of plastics as well as semiconductors and have a simple manufac-turing process. Hence, there are numerous applications for organic semi-conductors. Most copy machines and laser printers already use organic photoconductors. In the future, a number of new exciting developments such as organic solar cells, organic field emission transistors, etc., and low-cost organic lighting technology and electronics are expected. Different parameters in inorganic and organic materials are compared in Table 5.3.

Table 5.3 Comparison of different parameters in inorganic and organic materials [51]S. no. Parameter Inorganic Organic

1. Morphology Amorphous/single crystalline/polycrystalline

Amorphous/crystalline/polycrystalline

2. Charge carrier properties

Lattice Molecular

3. Mobility 1–1500 cm2/V-s 10−3 to 10−5 cm2/V-s4. Processing High temperature Low temperature5. Flexibility Low High6. Stability Excellent Currently an issue7. Source Point source Spreaded source


Figure 5.14 Comparison of organic and inorganic LEDs.

5.7 LEDs VERSUS OLEDs

In spite of employing the same light-emitting mechanism, LEDs and OLEDs still differ in various aspects as illustrated in Fig. 5.14.


5.8 CONCLUSIONS

In spite of current worldwide economic conditions, SSL is creating a vibrant industry led by new and expanding markets based on high-bright-ness LEDs. By considering the growing importance of energy saving and environmental friendliness, ecofriendly SSL is emerging as CSL, a highly competent and viable alternative to existing lighting technologies. Various research organizations and government labs are working toward finding the ideal white light, which would usher in a new era of lighting. Recent developments indicate that key OLED lighting players are ready to take their products to consumers, and are trying to improve the technology and make it more affordable and adoptable. Surface modifications in the hole-blocking and EILs, high mobility materials for hole-blocking and ETLs, and use of high efficiency emitter dopants as emissive layer are the three important processes that govern the effectiveness of an OLED device, but the formation of quenching centers in the emissive zone by rapid dopant diffusion is a prime concern. OLEDs seem to be the per-fect technology for all types of displays but challenges still remain includ-ing high production costs and longevity issues for blue organics. They are also sensitive to water vapor, and thus perfect sealing of the display is required. Many research institutes and researchers are also searching for suitable materials for OLEDs. If and when the technological hurdles are overcome, this technology has the potential to transform the way we light our world.

REFERENCES [1] C.K. Gan, A.F. Sapar, Y.C. Muna, K.E. Chong, Procedia Eng. 53 (2013) 208–216. [2] J.-H. Ryou, R.D. Dupuis, Opt. Express 19 (S4) (2011) A897. [3] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [4] S. Petcharat, S. Chungpaibulpatana, P. Rakkwamsuk, Energy Build. 52 (2012) 145–152. [5] W. Ki, J. Li, J. Am. Chem. Soc. 130 (2008) 8114. [6] H. Shono, T. Ohkawa, H. Tomoda, T. Mutai, K. Araki, ACS Appl. Mater. Interfaces 3

(2011) 654. [7] A. Rizzo, N. Solin, L.J. Lindgren, M.R. Andersson, O. Inganas, Nano Lett. 10 (2010)

2225. [8] A. De Almeida, B. Santos, B. Paolo, M. Quicheron, Renew. Sustain. Energy Rev. 34

(2014) 30–48. [9] M. Asif ul Haq, M.Y. Hassan, H. Abdullah, H.A. Rahman, M.P. Abdullah, F. Hussin,

et al., Renew. Sustain. Energy Rev. 33 (2014) 268–279. [10] S. Nakamura, G. Fasol, The Blue Laser Diode, GaN Based Light Emitters and Lasers,

Springer, Berlin, 1997. [11] N.A.A. Howarth, J. Rosenow, Energy Policy 67 (2014) 737–746.


















[12] T. De Graaf, M. Dessouky, H.F.O. Müller, Renew. Energy 67 (2014) 30–34. [13] D. Radulovic, S. Skok, V. Kirincic, Energy 36 (2011) 1908–1915. [14] C.J. Humphreys, MRS Bull. 33 (2008) )459–471. [15] N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 32 (2014) 448–467. [16] D. Chitnis, N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 64 (2015)

727–748. [17] R.F. Service, Science 310 (2005) 1762. [18] B.W. D’Andrade, M.E. Thompson, S.R. Forest, Adv. Mater 14 (2002) 147. [19] D. Chitnis, N. Thejo Kalyani, S.J. Dhoble, Mater. Sci. 2 (2012) 346–348. [20] E. Hecht, Optics, fourth ed., Addison Wesley (2002), p. 591. ISBN 0-19-510818-3. [21] M.R. Krames, O.B. Shchekin, R. Mueller-Mach, G.O. Mueller, L. Zhou, G. Harbers,

et al., J. Disp. Technol. 3 (2) (2007) 160–175. [22] S. Wang, K. Wang, F. Chen, S. Liu, Opt. Express 19 (4) (2011) A724. [23] P. Narra, D.S. Zinger, An effective LED dimming approach, in: Industry Applications

Conference, 2004. 39th IAS Annual Meeting. Conference Record of the 2004 IEEE 3, 2004, pp. 1671–1676.

[24] A.A. Efremov, N.I. Bochkareva, R.I. Gorbunov, D.A. Lavrinovich, Y.T. Rebane, D.V. Tarkhin, et al., Semiconductors 40 (5) (2006) 605.

[25] C.H. Chen, J. Shi, C.W. Tang, Coord. Chem. Rev. 171 (1998) 161. [26] W.R. Salaneck, K. Seki, A. Kahn, J.J. Pireaux, Conjugated Polymer and Molecular

Interfaces, Marcel Dekker Inc., New York, 2002. [27] J. Wanga, F. Zhanga, J. Zhangb, W. Tangc, A. Tanga, H. Penga, et al., J. Photochem.

Photobiol. C 17 (2013) 69–104. [28] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 (1994) 815–817. [29] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H. Hong, et al., Nat. Photonics

69 (2012) 105–110. [30] R. Schlaf, B.A. Parkinson, P.A. Lee, K.W. Nebesny, G. Jabbour, B. Kippelen, et al., J.

Appl. Phys. 84 (1998) 6729. [31] J.-H. Jou, Y.-C. Chou, C.-W. Wang, J. Mater. Chem. 21 (2011) 18523. [32] J.-H. Jou, S.-M. Shen, J. Yung-Cheng, Org. Electron. 12 (2011) 865. [33] Y. Gao, Acc. Chem. Res. 32 (1999) 247. [34] N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 44 (2015) 319–347. [35] R. Wyckhoff, Crystal Structures, vol. 1, Wiley, New York, 1963, p. 28. [36] P. Guaino, F. Maseri, R. Schutz, M. Hofmann, J. Birnstock, L. Avril, et al., J. Photonics

Energy 1 (1) (2011) 11015. [37] M. Dipti Gupta, D. Katiyar, Opt. Mater. 28 (2006) 295–301. [38] B. Hu, F.E. Karasz, J. Appl. Phys. 93 (2003) 1995. [39] G.W. Kang, Y.J. Ahn, J.T. Lim, C.H. Lee, Synth. Met. 137 (2003) 1029. [40] N.H. Lee, M.J. Lee, J.H. Song, C. Lee, D.H. Hwang, Mater. Sci. Eng. C. 24 (2004) 233. [41] M. Mazzeo, J. Thompson, R.I.R. Blyth, M. Anni, G. Gigli, R. Cingolani, Phys. E 13

(2002) 1243. [42] J.R. Lakowicz, Principles of Florescence Spectroscopy, second ed., Kluwer Academic

and Plenum Publishing, New York, 1999. [43] A.R. Duggal, J.J. Shiang, C.M. Heller, D.F. Foust, Appl. Phys. Lett. 80 (2002) 3470. [44] D.A. Pardo, G.E. Jabbour, N. Peyghambarian, Adv. Mater. 12 (17) (2000) 1249. [45] J.Y. Shen, C.Y. Lee, T.-H. Huang, J.T. Lin, Y.-T. Tao, C.-H. Chien, et al., J. Mater. Chem.

15 (2005) 2455. [46] S.O. Kim, K.H. Lee, G.Y. Kim, J.H. Seo, Y.K. Kim, S.S. Yoon, Synth. Met. 160 (11–12)

(2010) 1259–1265. [47] J. McElvain, H. Antoniadis, M.R. Hueschen, J.N. Miller, D.M. Roitman, J.R. Sheats,

et al., J. Appl. Phys. 80 (1996) 6002.


















































[48] Y.F. Liew, H. Aziz, N.-X. Hu, S.-O. Chan, G. Xu, Z.D. Popovic, Appl. Phys. Lett. 77 (2000) 2650.

[49] C.M. Tan, B.K. Eric Chen, G. Xu, Y. Liu, Microelectron. Reliab. 49 (2009) 1226–1230. [50] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, et al., Appl. Phys.

Lett. 85 (2004) 3911. [51] S. Raut, N. Thejo Kalyani, S.J. Dhoble, Organic Light Emitting Diodes(OLED);

Materials, Technology and Advantages, NOVA Science Publishers, New York, 2016, pp. 41–92.











http://dx.doi.org/10.1016/B978-0-08-101213-0.00006-0

CHAPTER 6

Organic Light-Emitting Diodes: The Future of Lighting Sources

6.1 INTRODUCTION

Organic light-emitting diodes (OLEDs) are an offshoot of existing con-ventional LEDs, and work on the basic principle of electroluminescence, the phenomenon that produce photons by plopping electrons into minia-ture electron holes within the emissive layer of the device. Fundamentally, when electricity is given as input, light emerges as output in such devices. This technology employs a series of thin, light-emitting films composed of hydrocarbon chains rather than semiconductors laden with heavy metals like gallium arsenide phosphide as used in existing LED technology. These organic arrays produce brighter light and utilize less energy [1–3] and can provide soft light that is largely glare-free without harsh shadows. OLEDs are also design elements. Even when switched off they look very differ-ent from conventional light sources, and can be transparent, diffused, or mirrored. White OLEDs with their broadband emission spectrum produce high-quality lighting with color rendering index (CRI) greater than 80. In contrast to tubular and compact fluorescent lamps, they emanate their full output as soon as they are switched on, and can be easily dimmed to any level just by changing the operating current. Thus OLEDs have the potential to enhance our visual environment and satisfy our psychological appetite for brightness [4,5].

6.2 ORGANIC LIGHT-EMITTING DIODES

Following the Hallmark efforts of C.W. Tang in the field of electrolumi-nescence [6], OLEDs are attracting significant research interest from both scientific and industrial communities. They are regarded as one of the best candidates for large-area lighting and flat-panel display technolo-gies and are capable of meeting the most stringent demands [7–9] due to their ability to form thin films and the fact that they are lightweight. They have the potential to offer high-contrast, fast-response, wide-view-angle,


and low-power attributes [10–13]. To become next-generation solid-state lighting sources with energy-saving features, white organic light-emitting diodes (WOLEDs) are preferred since they can be fabricated at low cost. Furthermore, most WOLEDs have shown brightness-dependent color shifts that significantly exceed tolerable margins, which are typically defined as a CIE (Commision Internationale de l’Éclairage) coordinates of 0.005–0.01 or less for both the x- and the y-value over orders of magnitude in bright-ness and over the entire operational lifetime [14]. Although different expla-nations for the tarnished color shift, observed in most devices have been proposed, there is still no consistent quantitative explanation [15]. The color shift observed in WOLEDs based on a single emissive layer of a white- emitting copolymer originates from the competition between electron trapping on red-emitting sites and unperturbed charge transport through the organic layer. However, nearly all highly efficient WOLEDs are con-structed by the expensive vacuum deposition method involving com-plicated doping level control for the doped layers in the devices. This procedure has greatly hindered their practical application. Fortunately, white polymer light-emitting devices (WPLEDs) provide a suitable approach to cope with some of these issues. WPLEDs can be fabricated through cheap solution processes, such as spin-coating, ink-jet printing, roll-to-roll strate-gies, etc. Furthermore, the doped layers in WPLEDs can be easily deposited with a precise doping level. Despite the unique merits of WPLEDs, their disadvantages, such as low EL efficiency and poor white quality, etc., keep WPLEDs from being next-generation solid-state lighting sources. However, researchers have made great progress and WPLEDs are as competitive as other lighting sources currently in use. This chapter illustrates the tech-niques that can be used to improve the performance of OLEDs.

6.3 STRUCTURE OF OLEDs

In 1960, Martin Pope and coworkers at New York University developed an injecting ohmic electrode contacts to organic crystals [16]. In 1963, they first observed electroluminescence under vacuum on (1) a pure single crystal of anthracene and (2) on anthracene crystals doped with tet-racene using a small-area silver electrode at 400 V [17]. However, it had poor device performance and ultrahigh voltage constraints. Later, in 1987, the first double-layer diode device with separate hole-transporting and electron-transporting layers (ETLs) was reported by Ching W. Tang and Steven Van Slyke [18].

Organic Light-Emitting Diodes: The Future of Lighting Sources 143

In this device framework, recombination and light emission occured in the middle of the organic layer, which resulted in reduction of operating voltage and efficiency. This research later led to the current era of OLED research and device production. Luminance exceeding 1000 cd/m2 below 10 V with a quantum efficiency of 1% photon/electron has been achieved. Fig. 6.1 shows the first single-layer and double-layer OLEDs and the structure of the organic materials used. In a three-layer OLED an addi-tional layer is placed between the hole-transporting layer (HTL) and ETL [19]. The emissive layer is primarily the site of electron–hole recombina-tion and hence electroluminescence. Emissive materials with low carrier-transport properties can employ this type of cell structure.

As illustrated in Fig. 6.2, the recombination site of electrons and holes in a single-layer device is haphazardous. The thickness of each organic layer is in the order of 100–200 nm. In such devices both the charge car-riers must be injected at an equal rate in order to achieve high efficiency. In an OLED, with the application of an electric field, electrons are trans-ported through the lowest unoccupied molecular orbit (LUMO), while holes are transported through the highest occupied molecular orbit (HOMO) toward the emissive layer and recombine on the emitter mole-cules so as to form triplet or singlet excitons.

A three-layer framework confines the recombination site of electrons and holes in a emissive layer of the device, leading to improved efficiency.

A multilayer OLED device is fabricated with different layers, each per-forming a different function. The very first layer is a glass substrate that can be replaced by plastic foil in flexible applications, the second layer

Figure 6.1 Frame work of the primary (A) single layer and (B) double-layer OLEDs [16–18].


is the transparent anode, and you have hole transport layer (HTL), elec-tron blocking layer (EBL), emissive layer, hole-blocking layer (HBL), ETL and cathode as shown in Fig. 6.3. In OLEDs, the role of the transport-ing layer is to reduce power consumption, while the role of the block-ing layer is to achieve long life [22]. Moreover, either of the electrodes must be transparent to emit light from the device. The overall thickness of active OLED layers is less than 500 nm, which is 100 times thinner than a human hair. The total thickness of the component is typically 1.8 mm, comprising the glass substrate and the encapsulation. And even this can be reduced considerably by using thinner and more flexible substrates and thin-film encapsulation. This makes the complete OLED itself extremely

Single-layer type Three-layer type

Cathode

100 to200 nm

Emitter layer

Anode (ITO)

Glass substrate

Cathode

Electrontransport layer

Holetransport layer

Anode (ITO)

Glass substrate

–+

–+

–+

–+

–+

–+ –

+–+

–+

–+

–+

–+

Figure 6.2 Configurations of typical electroluminescence cells [20].

HTL

EMLETL

EILMetalHIL

ITO+

+

+

+ –

– –

–

+

Figure 6.3 Working principle of OLED.Reproduced from N.T. Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 16 (2012) 2696–2723 [21].


flat, lightweight, and potentially flexible. As the materials used for different layers are sensitive to high temperatures and dust, due care has to be taken during the process of fabrication.

6.4 LIGHT-EMITTING MECHANISM OF OLEDs

OLEDs operate on the principle of electroluminescence. When volt-age is applied across an OLED electrical current flows from the cathode to the anode through organic layers. The cathode gives electrons to the emissive layer of organic molecules, while the anode removes electrons from the conductive layer of the organic molecules. When the charges in the exciton pairs are combined, they give rise to light emission. The color of the light depends on the type of organic molecule in the emis-sive layer. Emission color is basically determined by the energy difference of the HOMO and LUMO of the emitting organic material. The light-emitting mechanism from an OLED device is shown in Fig. 6.4A. The intensity or brightness of the light depends on the amount of electrical current applied. Another method for light emission is to take the advan-tage of formation of excited state complex known as exciplex/excimer. An exciplex/excimer is a transient charge transfer complex formed due to the interaction between the excited states of one molecule and the ground state of another molecule. The resulting electron–hole pair com-plex decays radiatively, the emission of which is considerably red shifted and broadened as compared to the individual molecules. If the two mol-ecules are different, the transient complex is called an exciplex; on the

Figure 6.4 Schematic representation of electron-hole recombination in an exciton localized on one molecule and in exciplex/excimer is shown in Fig. 6.4 A and B, respec-tively [24,25].


other hand, if two molecules are the same, the transient complex is called an excimer. Schematic representation of electron–hole recombination in an exciton localized on one molecule and in exciplex/excimer is shown in Fig. 6.4A and B, respectively. The exciplex emission is more favorable if the difference between the HOMOs and LUMOs of the two molecules is large. This will tend to accumulate the charge carriers at the interface, causing an increase in the probability of recombination near the interface. The emission color of these devices is highly dependent on the thickness of the layers and the applied electric field [23].

6.5 MATERIALS FOR OLEDs

The materials used in OLEDs should have high conductivity, high electro-luminescence efficiency, good thermal stability, low power-on voltage, etc. Different layers of OLED employ different materials, depending on their requirements.

6.5.1 Substrate MaterialsSubstrate is an indispensable part of a device or display. Substrate require-ments include high transparency, low roughness, high surface finish, high flatness, scratch proof, and dimensional stability at processing temperatures [26]. Glass is generally used as substrate as it is rigid and possesses high glass-transition-temperature. Clear plastic sheet or a metal foil, which is transparent, can also be used as substrate.

6.5.2 AnodeThe anode of the OLED must be transparent in order to inject holes into organic layers and highly conductive in order to achieve a device with high performance and efficiency. The conductivity, transparency, and work function can be varied by the deposition technique and surface treatment, which includes the use of ozone to interact with the surface in order to increase the work function [27–29]. The material of choice for anodes has customarily been the transparent conductor; indium tin oxide (ITO) due to its low roughness and high work function (ΦW = 4.5 to 5.1 eV), which is high enough to inject holes into the HOMOs of organic materials. It has good conductivity, high chemical stability, high work function, good transparency (90%) to visible range, and excellent adhesion to substrate. However, both indium and oxygen can migrate into the organic semicon-ductors, affecting the overall device performance over time. In addition,


it is not the most effective injector of holes and its thin-film resistance is high. It has high refractive index as compared with that of organic materi-als, resulting in only 25% of generated photons ever making it out of the device. Several materials were recently examined as replacements for exist-ing anode ITO with non-ITO anodes [1,30–32].

6.5.2.1 Fluorine-Doped Tin OxideAs an alternative to ITO, fluorine-doped tin oxide (FTO) has been used as an anode to inject holes in polymer light emitting diodes (PLEDs) [33] due to the fact that it is inexpensive and less sensitive to surface cleaning methods than ITO. However, its leakage current is high.

6.5.2.2 Al-Doped Zinc OxideZinc oxide is an N-type semiconductor that is nontoxic and inexpensive with a bandgap of about 3.3 eV. When group III elements such as alu-minum are doped into zinc oxide, it exhibits stable electrical and opti-cal properties. Aluminum doped zinc oxide (AZO) films offer excellent transmission in the visible region. Thin-film deposition techniques such as chemical vapor deposition, magnetron sputtering, and pulsed-laser deposi-tion can be used to grow AZO thin films [34].

6.5.2.3 Transparent Conductive OxidesCui et al. reported highly transparent, high work function materials called transparent conductive oxides (TCO), i.e., Ga0.12In1.88O3(GIO), Ga0.08In1.28Sn0.64O3 (GITO), Zn0.5In1.5O3 (ZIO), and Zn0.46In0.88Sn0.66O3 (ZITO), as OLED anodes [35]. Organic light-emitting diode devices fab-ricated out of these anode materials have shown higher luminance effi-ciency than ITO-based OLED devices. However, the power-on voltage of these fabricated devices is high.

6.5.3 Hole-Transport MaterialsHole-transport materials (HTL) transport holes within the HOMO level and electrons within the LUMO level to a lesser degree. In order to com-pensate for this, HTL materials should possess low ionization potential and high hole mobility [36–38]. The structures of some popular HTL materi-als along with their physical, chemical, thermal, and optical properties are listed in Table 6.1. Materials with low ionization potential together with low electron affinities and high hole mobility usually function as hole-transporting materials by accepting and transporting hole carriers with

Table 6.1 Structure, physical, chemical, thermal, and optical properties of some HTL materials [28,39–43]Abbreviation Full form Formula Molecular

weight (g/mol)

Absorption wavelength (nm)

Emission wavelength (nm)

Structure

NPB N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine

C44H32N2 588.74 339 nm (in THF) 450 nm (in THF)

TAPC Di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane


α-NPD N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine

C46H36N2 616.79 307 nm (in CH2Cl2)

447 nm (in CH2Cl2)

TPD N,N′-bis(3-methyl phenyl)-N,N′-bis(phenyl)-benzidine


TTP N1,N4-diphenyl-N1,N4-dim-tolylbenzene-1,4-diamine

C32H28N2 440.58 314 nm (in CH2Cl2)

415, 439 nm (in CH2Cl2)

Spiro-NPB N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-9,9-spirobifluorene


Spiro-TAD 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene


Spiro-TPD N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene



positive charge. Development efforts of HTLs include improvement of thermal and electrochemical stability, mobility, glass-transition temperature (Tg), and reduction in the energy barrier interface between the anode (ITO) and ETL, and crystallization behavior.

6.5.4 Hole-Injection MaterialsSuitable hole-injecting materials with high mobility, high glass-transition temperature, and electron-blocking capacity are generally preferred for the ITO glass substrate. This layer further helps to reduce the barrier for hole injection into the OLED device. CuPc and m-MTDATA are widely used materials for HIL in OLEDs because their HOMO is comparable to the work function of ITO. The structures of some popular HIL materi-als along with their physical, chemical, thermal, and optical properties are listed in Table 6.2. This layer injects holes from the anode (ITO) to the emissive layer. The materials have high mobility, electron-blocking capac-ity, and high glass-transition temperature.

6.5.5 Emissive MaterialsThe layer in between the HTL and ETL, generally known as the emis-sive layer (EML), must be a good emitter of visible photons. This layer can be a material made of organic molecules or polymers or dendrimers with high efficiency, lifetime, and color purity. The EML material should have a high glass-transition temperature to obtain devices with long lifetime. Depending on the color required, we can select materials such that the energy gap, i.e., the distance between the HOMO and LUMO, is such that the energy released during the recombination will be within the desired wavelength. In OLEDs the EML consists of the host and dop-ants. Host materials transport charge, allow excitons to form, and facili-tate radiative recombination. The dopants are dispersed in the host layer by using coevaporation. There are three main classes of emissive materials for OLEDs: Small molecules Conjugated polymers Conjugated dendrimers

6.5.5.1 Small MoleculesThese are low-molecular-weight molecules, with molecular weight less than 900 Da. They are deposited by either vacuum sublimation (or) ther-mal vacuum evaporation techniques. OLEDs made of small molecules

Table 6.2 Physical, chemical, thermal, and optical properties of some HIL materials [44–46]Abbreviation Full form Formula Molecular

weight (g/mol)



Structure

CuPC Phthalocyanine, copper complex

C32H16N8Cu 576.07 345, 631 nm (in CH2Cl2)

404 nm (film)

H2PC Phthalocyanine C32H18N8 514.54 – –

PtPC Phthalocyanine, platinum complex

C32H16N8Pt 707.60 – 372.5 nm (in CH2Cl2)

PPDN Pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile

C16H6N6 282.26 307 nm (in CH2Cl2)

487 nm (in CH2Cl2)

MeO-TPD N,N,N′,N′-tetrakis(4-methoxyphenyl) benzidine

C40H36N2O4 608.72 302, 351 nm (in THF)

429 nm (in THF)

DBTPB N4,N4′-bis (dibenzo[b,d] thiophen-4-yl)-N4,N4′-diphenyl biphenyl-4,4′-diamine

C48H32N2S2 700.91 279, 338 nm (in CH2Cl2)

407 nm (in CH2Cl2)


are called small-molecule OLED, or SMOLED. Their total thickness is less than 1 μm. They are semicrystalline or crystalline materials with high aqueous solubility. The structures of some small molecular organic com-pounds are shown in Fig. 6.5. Efficient OLEDs using small molecules were first developed by Ching W. Tang et al. [6]. The first observations of electroluminescence in organic materials were in the early 1950s by A. Bernanose and coworkers [47,48].

6.5.5.2 PolymersResearch into polymer electroluminescence culminated in 1990 when Burroughes et al. reported a high-efficiency green-light-emit-ting polymer-based device using poly(p-phenylene vinylene) [49]. Electroluminescence from polymer films was first observed by Roger Partridge [50]. The device consisted of a film of poly(n-vinylcarbazole) up to 2.2 μm thick located between two charge injecting electrodes. The light-emission color of the polymers strongly depends on its chemi-cal composition and the nature of the side groups. Hence, by modulating the polymer structure, a range of soluble light-emitting polymers (LEPs) that can create light in the entire visible spectrum, ranging from 400 to 800 nm, can be made available. The use of emissive additives, also known as dyes, is an interesting aspect of color LEPs. By adding a small amount of a suitable dye to a polymer, energy can be transferred from the poly-mer to the dye, resulting in light emission from the dye. The color from the device can be finetuned using different dyes. The glass-transition tem-perature Tg of the polymer material plays a vital role and must be con-sidered when selecting the material for the device. The study of organic materials as active components has resulted from the necessity to optimize

Figure 6.5 Popular small-molecule organic compounds.


the characteristics of these devices. Conjugated polymers with long-range p-electron delocalization behave as light-emitting semiconductor materi-als in their neutral undoped state. They exhibit strong photoluminescence (PL) in the visible as well as in the near infrared range. In their doped state, they behave as processable organic metals [51]. Switching between doped and undoped states of LEPs leads to number of changes in polymer vol-ume, absorption color, and reversible PL quenching, leading to wide vari-ety of applications [52]. This combination of semiconductivity properties and intense PL results in LEP electroluminescence and its use in polymer light-emitting diodes (PLEDs). LEPs include both polymers containing fully conjugated main chain and those with conjugated segments in the main chain or side groups. LEPs must have high PL quantum efficiency, chemical and thermal stability, color purity (determined by the polymer bandgap and film morphology), matching of ionization potentials and electron affinities between LEP and the different electrode materials, good processability (which involves solubility), solution viscosity, and solvent-substrate compatibility. These requirements can be achieved by adjusting or by changing the chemical structure of the conjugated polymer chains, side groups, incorporation of hetero atoms, molecular weight, structural regularity, and/or copolymerization. Polymer LEDs are very flexible, inex-pensive, and they can be applied on the substrate by solution techniques such as ink-jet printing, casting where no vacuum is required, resulting in an easier production technique. The glass-transition temperature (Tg) of the polymer materials is also an important issue in the choice of material. At temperatures over Tg the display does not obtain proper functionality and lifetime is reduced. The upper limit of the temperature range can be improved by choosing polymer materials with higher Tg [53]. The basic structure of common conducting polymers is shown in Fig. 6.6.

6.5.5.3 Conjugated DendrimersLight-emitting dendrimers are branched molecules consisting of light emitting of cores to which one or more branches (dendrons) are attached as shown in Fig. 6.7. A dendron usually contains a single chemically addressable group called the focal point, which is typically symmetric around the core, and often adopts a spherical three-dimensional morphol-ogy. Surface groups are attached to the distal end of the dendrons in order to provide solubility, which is necessary for solution processing. The den-dritic structure allows independent modification of the core (light emis-sion), branching groups (charge transport), and surface groups (processing


properties). The color of the light emission is defined by the key electronic properties of the core. As a result, the electronic and processing properties can be adjusted independently, giving remarkable scope to the properties of these materials.

Dendrimer LEDs have maximum efficiency of 40 lm/W with an external quantum efficiency of 16%. The properties of small molecules, dendrimers, and linear polymers are listed in Table 6.3.

Figure 6.6 Polymers (A) poly (9,9′-dioctlyfluorene), (B) poly(p-phenylene), (C) poly(p-phenylenevinylene), (D) poly(pyrrole), and (E) polyannaline [53,54].

Figure 6.7 Chemical structure of (A) stilbenoid dendrimer (R˭OC6H13) and (B) poly(propyleneamine) dendrimer [55].


6.5.6 Electron-Transport MaterialsIdeally, the mobility of the ETL and HTL should match each other in order to have the recombination zone in the emissive layer. However, research has shown that the mobility of most HTLs is higher than the mobility of holes in HBLs in organic materials. Thus the function of the HBL is to block the injection of holes through the ETL in OLEDs. Because of high HOMO, few electron-transport materials can be also used as hole blocker energy level. This layer should possess good hole-block-ing and electron-transporting properties. Electron-transport materials enhance the transportation of electrons from cathode to emissive layer, and have conjugated systems that can deliver the electrons more effectively. Materials with good electron-transporting and hole-blocking properties

Table 6.3 Properties of small molecules, dendrimers, and linear polymers [56]S. No. Property Small molecules Dendrimers Linear polymers

1. Structure Compact Compact, globular

Not compact

2. Synthesis Solution technique

Careful and stepwise growth

Single-step poly condensation

3. Structural control

High Very high Low

4. Architecture Regular Regular Irregular5. Shape Fixed Spherical Random coil6. Crystallanity Semicrystalline

or crystallineNoncrystalline,

amorphous materials

Semicrystalline/crystalline materials

7. Glass-transition temperature

– Low High

8. Aqueous solubility

High High Low

9. Nonpolar solubility

High High Low

10. Viscosity Linear relation with molecular weight

Nonlinear relationship with molecular weight

Linear relation with molecular weight

11. Reactivity Moderate High Low12. Compressibility – Low High13. Polydispersity Monodisperse Monodisperse Polydisperse


and high electron affinities together with high ionization potentials usually function as electron-transporting materials by accepting negative charges and allowing them to move through the molecules. This layer provides an electron-conductive pathway for negative charge carriers to migrate from the cathode into the emissive layer. It also prevents the charge leakage and accumulation of charges at the interface of cathode and ETL. The struc-tures of the most common ETL materials along with their physical, chem-ical, thermal, and optical properties are listed in Table 6.4.

6.5.7 Electron-Injection MaterialsElectron-injecting materials with high mobility, high glass-transition tem-perature, and hole-blocking capacity are highly essential to inject as many electrons as possible from the cathode to the emissive layer.

6.5.8 Cathode MaterialsCathode materials used for OLEDs should inject electrons efficiently and possess low work function (φw≈2.9 to 4.0 eV) in order to minimize the barrier of electron injection and ensure low threshold voltage. Alloy of magnesium with silver (Mg–Ag) with a work function in the range of 3.7 eV and aluminum with alkali metal compounds are generally used as cathode materials. Popular cathode materials include Mg:Ag (10:1), LiF, and Mg:Al. An aluminum (Al) layer is widely used as the cathode, and many other insulating layers such as MgO, CsF, Al2O3, and NaCl have been studied in order to enhance electron injection [62–65]. The HOMO, LUMO, and energy gap of some materials used for OLED device fabrica-tion are listed in Table 6.5.

Deposition of all these layers on ITO glass substrate itself is too critical because of the sensitivity of the material to different factors such as high temperature and incorporation of dust during fabrication.

6.6 EFFICIENCY OF OLEDs

The efficiency of an OLED is a function of numerous factors such as the degree of balance of charge carrier injection, light-extraction effi-ciency, recombination efficiency, power efficiency luminescence quantum yield, efficiency to which the excited state emits light, and quantum effi-ciency to name a few. Some of these factors are explained in detail in the following.

Table 6.4 Structure, physical, chemical, thermal, and optical properties of some electron-transporting materials [57–61]Abbrevation Full form Formula Molecular

weight (g/mol)



Structure

Liq 8-Hydroxyquinolinolato-lithium C9H6LiNO 151.09 261 nm (in THF) 331 nm (in THF)

PBD 2-(4-Biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole

C24H22N2O 354.44 305 nm (in THF) 364, 380 nm (in THF)

TPBi 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)

C45H30N6 654.76 305 nm (in THF) 359, 370 nm (in THF)

BCP 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline


TAZ 3-(4-Biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole


Bphen 4,7-Diphenyl-1,10-phenanthroline C24H16N2 332.4 272 nm (in THF) 379 nm (in THF)

Table 6.5 HOMO, LUMO, and energy gap of some materials used for OLED device fabricationS. no Material LUMO (eV) HOMO (eV) Energy gap

(eV)Application Reference

1. ITO 4.7 – Anode [36]2. 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline

(BCP)2.9 6.4 3.5 ETL

3. N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) -2,2′-dimethylbenzidine (α-NPD)

2.3 5.4 3.1 HTL

4. Tris(8-hydroxyquinoline)aluminum (Alq3) 3.1 5.8 2.7 ETL5. Mg:Ag 3.7 – – Cathode6. Phthalocyanine, copper complex (CuPC) 3.6 5.3 1.7 HIL [37]7. Poly(vinylcarbozole) (PVK) 2.3 5.8 3.5 Conducting

polymer8. N,N′-bis(3-methyl phenyl)-N,N′-bis(phenyl)-

benzidine (TPD)2.3 5.5 3.2 HTL

9. Al 4.3 – – Cathode10. N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-

benzidine (NPB)2.3 5.3 3.0 HTL [38]

11. LiF/Al 2.6 – – Cathode12. PEDOT:PSS 3.5 5.2 1.7 Anode [66]13. LiF 4.1 – – Cathode14. Ba/Al 2.8 – – Cathode15. 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-

benzimidazole) (TPBi)2.8 6.3 3.5 Host

16. 3-(4-Biphenyl)-4-phenyl-5-tert- butylphenyl-1,2,4-triazole (TAZ)

2.7 6.3 3.6 ETL


6.6.1 Light-Extraction Efficiency in OLEDsPoor light extraction is one of the most imperative factors that confines the external quantum efficiency of devices and hence enhanced out cou-pling methods are to be build up to get higher efficiencies. Most of the light engendered by an OLED is confined within the glass or plastic substrate. Hence, research activity is directed at the development of surface treatments to increase the extraction efficiency, which is possible if the photons created in the active region can escape out of the epitaxial layers. To haul out the trapped and wave-guided light into external modes, various approaches to diminish the total internal reflection (TIR) at the interfaces have been used. They include the use of a high refractive index substrate, microlenses on the backside of the substrate surface, and formation of monolayer of silica microspheres as scattering medium. Another technique is to supplement an immensely low refractive index silica aerogel layer sandwiched between the ITO and the glass substrate.

6.6.2 Power EfficiencyPower efficiency determines how long the battery lasts in a portable device, and is measured in lumens per watt. Power efficiency is affected by the quantum efficiency of the device as well as by the operating voltage, which is related to the charge injection barrier. Improvement in power efficiency and concomitantly lowering the operating voltage can be achieved by electrical doping [67]. It has been shown that OLEDs with p-doped hole-transport layer and/or n-doped electron-transport layer can deliver efficient carrier injection and transportation [68–71], and also have the ability to regulate the position of the recombination and emission region.

6.6.3 Recombination EfficiencyIf two charges come into Van der Waal’s proximity, then they are cer-tain to annihilate. Although emission from triplet state is spin forbidden in organic molecules, recent progress in developing triplet state emitters containing at least one atom of higher atomic weight has been made. Even if it is spin allowed, the excited state may still decay nonradiatively. Recombination efficiency is generally near to unity.

6.6.4 Luminescence Quantum YieldThe luminescence quantum yield ϕ is defined as the ratio of the number of emitted photons to the number of absorbed photons per unit time.

φ =

Number of photons emitted

Number of photons absorbed


It is also an important parameter for evaluation of efficiency in lumi-nescent materials.

6.6.5 Quantum EfficiencyIt is a well-known fact that the proper choice of host and activator

is essential to obtain an efficient phosphor. When photons are incident upon the phosphor, a few of them are reflected, some are transmitted, and if the phosphor is an efficient combination of host and activator, most of the quanta are absorbed. But not all the absorbed quanta result in an activated center, and once these centers become activated not all will emit a subsequent photon. Some become deactivated via a relax-ation process. The efficiency of a phosphor is generally expressed in terms of quantum efficiency, which is defined as the ratio of the number of photons emitted to the number of photons injected. For example, if only ten photons is emitted out of 1000 incident photons in the emis-sive layer, the quantum efficiency is 1%.

η = = =

No.of photons emitted

No,of photons injected

10

10001%

Quantum efficiency depends on the photon-to-electron ratio. The total external quantum efficiency, (ηex), of any light-emitting device is the product of internal quantum efficiency (ηint) and the extraction efficiency (ηext).

η η ηex int ext= ×

While ηint is the product of current injection efficiency (ηinj) and radioactive recombination efficiency (ηrad), i.e.,

η η ηint inj rad= ×

Hence, the equation for external quantum efficiency reduces to

η η η ηex inj rad ext= × ×

External quantum efficiency values of 20% (red), 19% (green), and 4–6% (blue) have been reported. The quantum efficiency of blue is much less primarily due to (1) its shorter wavelength and hence larger bandgap (around 2.9 eV). (2) As compared to green or red, the blue wavelength is less sensitive to the human eye, which leads to higher barriers and less effi-ciency [72,73]. Hence, improvement in quantum efficiency is one of the challenges ahead.


6.6.6 Techniques to Improve the Efficiency of OLED DevicesDevice performance and efficiency of an OLED can be improved by the following: Codoping: With highly purified organic complexes, implementing

the method of codoping, a method in which suitable host material is codeposited into organic emitting material, the device efficiency can be enhanced.

Layer thickness: The efficiency of the device can be enhanced by con-trolling the thickness of each organic layer.

Appropriate organic materials: Proper selection of HIL, HTL, and ETL materials can result in improved the device performance and efficiency.

Device structure: The device design not only can improve the carrier balance in the emission region, but also plays an important role in improving the color stability and efficiency of the devices.

Harvesting triplet states: The efficiency of OLEDs is limited because only singlet states are responsible for light emission in undoped devices. Recent developments in harvesting triplet states, using phosphorescent materials, has led to an increase in the efficiency and selectivity of pos-sible colors [74,75].

Endothermic energy transfer: An endothermic energy transfer from a molecular organic donor host to an organometallic phosphor (trap) can lead to highly efficient electroluminescence [76].

6.7 DEVICE ARCHITECTURES

Today, many corporations and academic institutions are investing enor-mous raw materials in the quest for OLED technology to create wall-mount lighting sources and advanced displays. Due to their high power efficiency, outstanding contrast, low cost of manufacturing, durabil-ity, lighter weight, and fast response times, OLEDs represent the future of visual displays for portable electronic devices. They are optoelectronic devices finished by placing a layer of organic material between two elec-trodes. When a voltage potential is applied to these electrodes and current is injected through the organic material, visible light is emitted. There are several different types [77–79] of OLEDs, discussed as follows.

6.7.1 Top-Emitting OLEDsTop light emitting OLED’s consists of a conducting layer of anode at the bottom, cathode at the top with emissive layer sandwiched in between


them. When it is turned on, light passes through the top, hence the name. Top-emission devices use a transparent or semitransparent top electrode that emit light directly.

6.7.2 Bottom-Emitting OLEDsBottom light emitting OLED’s consists of a transparent or semi-transpar-ent conducting anode layer at the top, cathode at the bottom with emis-sive layer sandwiched in between them. When turned on, light passes through the bottom. They are better suited for active-matrix applications.

6.7.3 Transparent OLEDsThese OLEDs use transparent or semitransparent electrode contacts on both sides of the device to create displays. They can greatly improve con-trast, making it much easier to view displays in bright sunlight.

6.7.4 Stacked OLEDsThese OLEDs use a pixel design that stacks the red, green, and blue sub-pixels on top of one another instead of next to one another. This leads to substantial increase in gamut and color depth, thereby significantly reduc-ing pixel gap. Currently, other display technologies have the RGB pixels mapped next to each other, decreasing potential resolution. The structures of (A) top-emitting, (B) bottom-emitting, (C) transparent, and (D) stacked OLEDs are shown in Fig. 6.8.

6.8 ADVANTAGES OF OLEDs

The diverse manufacturing techniques of OLEDs have led to numerous improvements over conventional lighting sources and flat-panel displays. They can be printed onto any substrate by employing traditional ink-jet technology, which can significantly lower the cost. Organic light-emitting diode screens can be viewed from almost any angle up to 160 degrees. Their displays generate good brightness, clarity, and consistent image qual-ity with good contrast and high luminescence efficiency. Unlike LCDs, they have neither backlights nor chemical shutters. Instead, every pixel illuminates like a lightbulb. These pixels turn on and off and hence have fast response time. The thin screens of OLEDs are free from backlighting, and they are lighter in weight and faster in action, providing full range of colors with high resolution. They can be shaped out of plastics, offer-ing the choice of flexible OLEDs and displays, and can be operated at


3–4 V and at wider temperatures. They are also more impact resistant and durable than their glass-based counterparts. OLEDs are anticipated to have full fabrication level price tag benefit over most flat-panel displays. These remarkable characteristics can be ascribed to advances in key areas such as phosphor materials, doped guest-host emitters, multilayer anatomy, and a better understanding of the electroluminescence process.

6.9 OLED RESEARCH HURDLES AND CHALLENGES

While OLEDs clearly have many advantages, there are also some chal-lenges that must be overcome.

Degradation: When an OLED is operated under ambient atmosphere, dark-spot formation leads to device degradation within a few hours as shown in Fig. 6.9. This can be attributed to external causes such as accu-mulation of dust particles during the fabrication process, pollution by

Figure 6.8 Structures of (A) top-emitting, (B) bottom-emitting, (C) transparent, and (D) stacked OLEDs [80].


water vapour, diffusion of oxygen, leading to oxidation which delaminates the electrodes. Cathode oxidation and cathode delamination are mainly responsible for the growth of nonemissive spots [81,82]. When an OLED is operated under ambient atmosphere dark-spot formation leads to device degradation within a few hours. Various mechanisms responsible for deg-radation of the device include crystallization of organic solids [76], elec-trochemical reactions at the electrode/organic interface, and migration of ionic species.

Based on the time scale of device degradation, the stability of an OLED can be divided into two types: (1) Short-term degradation, which occurs in the preliminary stage of operation, and recoverable degrada-tion, which occurs at an early stage of the degradation process without the appearance of chemical and physical changes such as dark spots, crys-tallization, etc., and unrecoverable degradation. Recovery phenomena related to degradation mechanisms in OLEDs have been reported [83]. (2) Long-term degradation gradually occurs during subsequent operation. The stability of deposition on film morphology may also affect the device performance. Organic light-emitting diode devices can be improved by protecting them from atmosphere as it plays a major role in device performance.

Duvenhage et al. [84] studied the photoluminescence properties of synthesized mer-[In(qn)3]. H2O. 0.5 CH3OH that can be used in OLEDs as an emission and electron-transport layer. The main absorption peaks

Figure 6.9 Dark spots on an OLED.


were assigned to ligand-centered electronic transitions, while the solid-state photoluminescence excitation peak at 440 nm was assigned to the 0–0 vibronic state of In(qn)3. Broad emission at 510 nm was observed and was ascribed to the relaxation of an excited electron from the S1–S0 level. To study the photon degradation, the sample was irradiated with an UV lamp for approximately 15 hours. The emission data was collected and the change in photoluminescence intensity with time was monitored, as shown in Fig. 6.10A–C. A decoloring of the surface occurred during deg-radation, as shown in Fig. 6.10C. X-ray photoelectron spectroscopy scans revealed that for the degraded sample a change in chemical composition occurred with the possible formation of C O (~532.5 eV), C O H, and O C O H (~530.5 eV) on the phenoxide ring.

Lifetime: The other difficult technical problem to be overcome is the lim-ited lifetime of organic materials. The lifetime, defined as the time required for the emission to be reduced by half its initial value, of such encapsulated devices is more than 5000 hours at 25°C and 1000 hours at 40°C. These values are due to fact that no encapsulation is perfectly hermetic, and the atmospheric environment has a major impact on the device performance. Water also damages displays and destroys the organics; hence improved seal-ing processes are important for practical manufacturing. Table 6.6 clearly shows that red and green OLEDs have longer lifetimes than blue organics.

Thus efficient and stable blue phosphorescent materials remain a challenge for researchers [85]. They must be operated at moderate temperatures or the lifetime drops dramatically with increasing temperature and the organic layer will peel off from the substrate if the passivation film stress is high.

6.10 OLED APPLICATIONS

Novel applications of OLEDs include solid-state wall-mounting light sources, small displays, including microdisplay applications, digital cameras, wall decorators, back-of-seat screens in automobiles and airplanes, OLED drivers, mobile phones, wearable electronic displays such as display sleeves, high-contrast automotive instrument panels, windshield displays, etc., as well as larger displays, such as smart light-emitting windows/shades, etc., flat-panel displays, flexible displays, computer displays, and many more.

6.10.1 OLEDs as Versatile Light SourcesOrganic light-emitting diode lighting has the potential to efficiently emit cool light across large surfaces and to bring new and novel lighting


choices. Due to their exceptional properties such as surface emission, transparency, flatness, and flexibility, OLEDs have potential as wall-mount light sources with light tiles, or as light partitions or transparent light sources that emit light at night and serve as windows during the day.

0

1000

2000

3000

0 10,000 20,000 30,000 40,000 50,000Time (s)

0

1000

2000

3000(A)

(B)

(C)

Inte

nsity

(ar

b un

its)

Inte

nsity

(ar

b un

its)

400 500 600 700Wavelength (nm)

Before degradationAfter 1 hAfter 5 hAfter 15 h

Figure 6.10 Photodegradation spectra of compound mer-[In(qn)3]. H2O. 0.5 CH3OH under UV exposure (λ = 385 nm). (A) Evolution of the emission band with time, (B) quenching of luminescence with time [84], and (C) a color image of the excited compound before and after degradation.


OLEDs can also be integrated into installations in kitchens and bath-rooms, allowing revolutionary design for general-lighting applications that also emit white light at higher brightness. Excellent OLED energy sav-ing and ecofriendly designs for lighting are depicted in Fig. 6.11. Organic light-emitting diode lighting is still emerging, and has the potential to penetrate into the large growing global lighting market by 2025.

6.10.2 OLED Small DisplaysSmall-area OLED displays are tough enough to use in portable devices such as cell phones, digital video cameras, DVD players, car audio

Table 6.6 Typical characteristics of RGB OLEDsColor Emission

wavelength (Å)

External quantum efficiency (%)

Lifetime (h) Current efficiency (cd/A)

Red 610–760 20 22,000 (at 500 cd/m2)

15 (at 500 cd/m2)

Green 500–570 19 20,000 (at 1000 cd/m2)

29 (at 1000 cd/m2)

Blue 450–500 4–6 <1,000 (at 200 cd/m2)

19 (at 100 cd/m2)

Figure 6.11 Vibrant and versatile OLED energy-saving and ecofriendly models for lighting.


equipment, and tablets. Some small-area full-color OLED products are shown in Fig. 6.12.

6.10.3 OLED Large DisplaysOrganic light-emitting diode technology has the fastest response rate time due to the fact that it uses TFT active matrix (AMOLED) technol-ogy. Each diode has three subpixels—red, green, and blue—that make up a single pixel. An OLED TV screen uses 2 million pixels to create the pic-ture, with every pixel lit independently so the light can be seen from off-axis viewing angles easily and accurately. Curved and flat OLED TVs have already been introduced by Samsung and LG. OLEDs TVs have one solid layer of plexiglass-like material that contains all of the color compounds and TFT needed. Thus the flat panel is flexible and nonbreakable with the advantage of no screen burn-in as well. They are flexible, cardboard-thin, and large enough to cover a 9′ × 9′ wall TV. Due to superior techno-logical efficiencies in manipulating lighter, simpler carbon-based mate-rial, resulting in deeper blacks, brighter whites, and all the gray scales in between is a significant advantage of OLEDs. Flexible substrate and sol-uble printing technologies will help the OLED TV market grow signifi-cantly. LG’s unique OLED screen produces astoundingly vivid and realistic images that are vibrant, natural, and enjoyable to view. It makes use of car-bon fiber-reinforced plastic on the exterior, creating an ultrathin OLED TV that weighs just 17 kg. Unlike current displays that require backlights, OLED displays generate their own light, eliminating the need for back-lighting and greatly reducing the thickness of the screen, which results in an infinite contrast ratio, with absolute blacks and brilliant whites.

6.11 CONCLUSIONS

OLEDs are an extension of existing conventional LED technology. They are complementary to most existing light sources and will open up entirely

Figure 6.12 Small area full color OLED products (A) wearable display (B) flexible white OLED (C) rollable display (D) small screen displays.


new areas of application and growth in the lighting market. In fact, they are already inspiring designers to create novel designs unheard of previ-ously. However, there is room for improvement in OLED technology and hence researchers should concentrate on improving factors such as elec-tricity-to-light conversion efficiency, device stability and lifetime, mate-rial selection, proper encapsulation, maintaing uniformity over large areas, novel fabrication technologies, manufacturing cost, etc. If we succeed in improving efficiency, performance, and lifetime, current lighting systems can be replaced by ecofriendly, energy-efficient green technology called solid-state lighting, which would play a significant role in reducing global energy consumption. Fabrication of highly reliable, efficient, and long-life OLEDs are still challenging, due to the difficulty in aligning the energy levels at the layer interfaces. Manufacturing processes are expensive and hence simpler and cheaper technology has to be developed in order for the technology to reach mainstream. In order to develop durable and flexible OLEDs efficient materials have to be synthesized and further development of manufacturing tools is essential. The operating lifetime may be improved by using materials with higher glass-transition temperature, adopting bet-ter encapsulation techniques, and optimizing device process and structure. OLEDs could pave the way for a new era of large-area lighting.

REFERENCES [1] Y.F. Liu, J. Feng, Y.-F. Zhang, H.F. Cui, D. Yin, Y.-G. Bi, et al., Org. Electron. 15 (2014)

478–483. [2] Q.Y. Zhang, K. Pita, S. Buddhudu, C.H. Kam, J. Phys. D 35 (2002) 3085–3090. [3] W. Xiaoxiao, L. Fushan, W. Wei, G. Tailiang, Appl. Surf. Sci. 295 (2014) 214–218. [4] M. Asif ul Haq, M.Y. Hassan, H. Abdullah, H.A. Rahman, M.P. Abdullah, F. Hussin,

et al., Renew. Sustain. Energy Rev. 33 (2014) 268–279. [5] L.H.C. Andrade, S.M. Lima, R.V. Silva, M.L. Baesso, Y. Guyot, J. Mater. Chem. C 2

(2014) 10149. [6] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [7] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend,

et al., Nature 347 (1990) 539. [8] P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. [9] Y. Chi, P.T. Chou, Chem. Soc. Rev. 39 (2010) 638. [10] S.R. Forrest, Nature 428 (2004) 911. [11] N. Matsusue, Y. Suzuki, H. Naito, Jpn. J. Appl. Phys. 44 (2005) 3691. [12] S.M. King, I.I. Perepichka, I.F. Perepichka, F.B. Dias, M.R. Bryce, A.P. Monkman, Adv.

Funct. Mater. 19 (2009) 586. [13] S.L. Lin, L.H. Chan, R.H. Lee, M.Y. Yen, W.J. Kuo, C.T. Chen, et al., Adv. Mater. 20

(2008) 3947. [14] M.C. Gather, R. Alle, H. Becker, Adv. Mater. 19 (2007) 4460–4465. [15] M. Berggren, O. Inganas, G. Gustafsson, Nature 372 (1994) 444–446. [16] H. Kallmann, M. Pope, Nature 186 (1960) 31.
























[17] M. Pope, H.P. llmann, P. Magnante, J. Chem. Phys. 38 (1963) 2042. [18] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [19] J.L. Segura, Acta Polym. 49 (1988) 319. [20] J. Kido, Y. Okamoto, Chem. Rev. 102 (6) (2002) 2357–2368. [21] N.T. Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 16 (2012) 2696–2723. [22] Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing, et al., Adv. Funct. Mater. 19 (1)

(2009) 84–95. [23] D. Gupta, M. Katiyar, Deepak, Opt. Mater. 28 (2006) 295–301. [24] S.W. Cha, J.I. Jin, J. Mater. Chem. 13 (2003) 479. [25] J. Feng, Y. Liu, Y. Wang, S. Liu, Synth. Met. 137 (2003) 1101. [26] R. Wyckhoff, Crystal Structures, vol. 1, Wiley, New York, 1963, p. 28. [27] T. Osada, T. Kugler, P. Broms, W. Salaneck, Synth. Met. 96 (1998) 77. [28] L.S. Hung, C.H. Chen, Mater. Sci. Eng. R 39 (2002) 143–222. [29] J. Kim, M. Granstrom, R. Friend, N. Johansson, W. Salaneck, J. Appl. Phys. 84 (1998)

6859. [30] M. Schwoerer, H.C. Wolf, Org. Mol. Solids 91 (2007) 243502. [31] T.H. Han, Y. Lee, M.R. Choi, S.H. Woo, S.H. Bae, B.H. Hong, et al., Nat. Photonics 6

(2012) 105–110. [32] X. Wu, F. Li, W. Wu, T. Guo, Appl. Surf. Sci. 295 (2014) 214–218. [33] A. Andersson, N. Johansson, P. Broms, N. Yu, D. Lupo, W.R. Salaneck, Adv. Mater. 10

(1998) 859. [34] J. Zhao, S. Xie, S. Han, Z. Yang, L. Ye, T. Yang, Synth. Met. 114 (2000) 251. [35] J. Cui, A. Wang, N.L. Edleman, J. Ni, P. Lee, N.R. Armstrong, et al., Adv. Mater. 13

(2001) 1476. [36] M.A. Baldo, M.E. Thompson, S.R. Forest, Pure Appl. Chem. 71 (11) (1999) 2095–2106. [37] S.K. Kim, T.G. Chung, D.H. Chung, D.H. Chung, H.S. Lee, M.J. Song, et al., Opt.

Mater. 21 (2002) 159–164. [38] S.L. Lai, S.L. Tao, M.Y. Chan, T.W. Ng, M.F. Lo, C.S. Zee, et al., Org. Electron. 11

(2010) 1511–1515. [39] J. Meyer, Appl. Phys. Lett. 91 (11) (2007) 113506–113509. [40] N. Thejo Kalyani, S.J. Dhoble, R.B. Pode, Adv. Mater. Lett. 2 (1) (2011) 65–70. [41] L.C. Palilis, H. Murata, M. Uchida, Z.H. Kafafi, Org. Electron. 4 (2) (2003) 113–121. [42] M. Kimura, S.I. Inoue, K. Shimada, S. Tokido, K. Noda, Y. Taga, et al., Chem. Lett. 5

(2002) 192. [43] S. Tokito, K. Noda, K. Shimada, S.I. Inoue, M. Kimura, Y. Sawaki, et al., Thin Solid Films

363 (2000) 290. [44] Y. Zou, Z. Deng, Z. Lv, Z. Chen, D. Xu, Y. Chen, et al., J. Lumin. 130 (2010) 959–962. [45] T.C. Rosenow, J. Appl. Phys. 103 (4) (2002) 043105–043109. [46] T. Mori, Appl. Phys. Lett. 80 (21) (2002) 3895–3897. [47] M. Bernanose, P. Comte, J. Vouaux, Chim. Phys. 50 (1953) 64. [48] P. Bernanose, J. Vouaux, Chim. Phys. 52 (1955) 509. [49] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. MacKay, R.H. Friend,

et al., Nature 347 (1990) 539. [50] R. Partridge, Polymer 24 (1983) 733. [51] D.T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev. 100 (2000) 2537. [52] Q. Pei, O. Inganas, Adv. Mater. (4) (1992) 277. [53] T.P. Nguyen, P. Molinie, P. Destruel, in: H. Sing Nalwa (Ed.), Organic and Polymer-

Based Light Emitting Diodes. Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 10, Academic Press, San Diego, DA, 2001, pp. 3.

[54] B.K. Nanjwade, H.M. Bechraa, G.K. Derkara, F.V. Manvia, V.K. Nanjwade, Eur. J. Pharm. Sci. 38 (3) (2009) 185–196.

[55] D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev. 110 (4) (2010) 1857–1959.






















































[56] N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 44 (2015) 319–347. [57] J. Bernanose, Chim. Phys. 52 (1955) 396. [58] N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 16 (2012) 2696–2723. [59] Z. Liu, Z. Bian, F. Hao, D. Nie, F. Ding, Z. Chen, et al., Org. Electron. 10 (2) (2009)

247–255. [60] T.Y. Kim, D.G. Moon, Trans. Electr. Electron. Mater. 12 (2) (2011) 84–87. [61] B. Ding, W. Zhu, X. Jiang, Z. Zhang, Curr. Appl. Phys. 8 (2008) 523–526. [62] T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada, M. Tsuchida, IEEE Trans.

Electron. Devices 44 (1997) 1245. [63] G.E. Jabbour, B. Kippelen, N.R. Armstrong, N. Peyghambarian, Appl. Phys. Lett. 73

(1998) 1185. [64] S.J. Kang, D.S. Park, S.Y. Kim, C.N. Whang, K. Jeong, S. Im, Appl. Phys. Lett. 81 (2002)

2581. [65] Z.Y. Zhong, Y.D. Jiang, Proc. SPIE 6030 (2009) 60300. [66] Y. Ling, W. Jing, Z. Ronger, M. Wu, Organic electronics 28 (1) (2004) 68–73. [67] B.X. Mi, Z.Q. Gao, K.W. Cheah, Appl. Phys. Lett. 94 (2009) 073507-1–3. [68] C.W. Chu, C.W. Chen, S.H. Li, Appl. Phys. Lett. 86 (2005) 253503. [69] S.Y. Chen, T.Y. Chu, J.F. Chen, Appl. Phys. Lett. 89 (2006) 053518-1–3. [70] D.S. Leem, H.D. Park, J.W. Kang, Appl. Phys. Lett. 91 (2007) 011113. [71] Y. Lv, P. Zhou, N. Wei, Org. Electron. 14 (2012) 124–130. [72] J.Y. Shen, C.Y. Lee, T.-H. Huang, J.T. Lin, Y.-T. Tao, C.-H. Chien, et al., J. Mater. Chem.

15 (2005) 2455. [73] S.O. Kim, K.H. Lee, G.Y. Kim, J.H. Seo, Y.K. Kim, S.S. Yoon, Synth. Met. 160 (2010)

1259. [74] R.J. Holmes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon, et al., Appl.

Phys. Lett. 82 (2003) 2422. [75] G. Gu, D.Z. Garbuzov, P.E. Burrows, S. Vankatsh, S.R. Forrest, M.E. Thompson, Opt.

Lett. 22 (1997) 396. [76] L. Do, E. Han, N. Yamamoto, M. Fujihira, Mol. Cryst. Liq. Cryst. 280 (1996) 373. [77] J. Kido, W. Ikeda, M. Kaimura, K. Nagai, Jpn. J. Appl. Phys. 35 (1996) L394. [78] J. Kido, M. Kaimura, K. Nagai, Science 267 (1995) 1332–1334. [79] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuana, Appl. Phys. Lett. 65 (1994)

2124–2128. [80] N. Thejo Kalyani, S.J. Dhoble, Defect Diffus. Forum, 357 (2014) pp. 1–27. Trans Tech

Publications, Switzerland. [81] L.M. Do, E. Han, Y. Niidome, M. Fujihira, J. Appl. Phys. 76 (1994) 5118. [82] J. Mc Elvain, H. Antoniadis, M.R. Hueschen, J.N. Miller, D.M. Roitman, J.R. Sheats,

et al., J. Appl. Phys. 80 (1996) 6003. [83] D.C. Zou, M. Yahiro, T. Tsutsui, Synth. Met. 91 (1997) 1911. [84] M.-M. Duvenhage, O.M. Ntwaeaborwa, H.C. Swart, H.G. Visser, Opt. Mater. 35 (4)

(2013) 2366–2371. [85] J.-H. Ryou, R.D. Dupuis, Opt. Express 19 (S4) (2011) A897.













































http://dx.doi.org/10.1016/B978-0-08-101213-0.00007-2

CHAPTER 7

Review of Literature on Organic Light-Emitting Diode Devices

7.1 INTRODUCTION

Attracting interest around the globe, organic light-emitting diodes (OLEDs) have become the most promising displays and power-saving solid-state lighting (SSL) sources available today. This energy-efficient lighting technology plays an important role in reducing global consump-tion of electricity by almost 50%. OLEDs are semiconductors made from organic carbon-based materials that emit light when electricity is applied. They have the potential to far exceed the energy efficiencies of exist-ing displays and incandescent, fluorescent lighting. The vision of SSL has largely been driven by the desire to reduce energy consumption and to create pollution-free lighting. Novel research is being carried out to stim-ulate the development of the science and technology needed to enable the potential of SSL through OLED devices. This chapter reviews the litera-ture on red, blue, and green (RBG) and white OLED devices since the very first device architectures to the progress on the fabrication of differ-ent layers of ecofriendly and energy-efficient OLEDs.

7.2 DEVICE ARCHITECTURE

The history of OLED device architectures reveals an increase in the com-plexity of the devices as shown in Fig. 7.1. Earlier devices employed a simple monolayer structure and subsequently more and more layers have been employed, which perform specialized functions as specified in Table 7.1. The first report of EL in anthracene in monolayer devices was given by Pope et al. in 1963 [2] and later by Helfrich and Schneider in 1965 [3]. However, the electroluminensce (EL) phenomenon from organic materials remained a pure academic interest for almost two decades due to the difficulty of growing large-size single crystals and the require-ment of very high voltage (~1000 V) to produce the luminance. VanSlyke and Tang in 1985 [4] and Tang et al. in 1988 [5] demonstrated that the


poor performance of the early monolayer device could be dramatically improved in a two-layer device, just by the addition of a hole-transport layer (HTL) with thin, amorphous film stacking in the device structure. Subsequently, the Kodak group improved the power-conversion efficien-cies of organic EL devices by doping the emitting layer. Heterostructure configurations were implemented by inserting several layers such as a buffer layer between the anode and HTL [6–8], electron-transport layer (ETL), and hole-blocking layer (HBL) [9] or an interlayer between the cathode and ETL [10,11] in the device structure to improve the device performance. It has been observed that the EL efficiency of OLEDs can be increased by carrier or exciton confinement within a multilayer device. This multilayer device structure often enhances the drive voltage of OLEDs. Chemical doping with either electron donors (for electron-transport materials) or electron acceptors (for hole-transport materials) can significantly reduce the voltage drop across these films. These devices with either a HTL or ETL- doped layer show improved performance, but the operating voltages are still high. Huang and his group proposed the con-cept of p-type doped HTL and n-type doped ETL [12]. P-i-N (P-doped-intrinsic-N-doped) OLED structure devices show high luminance and efficiency at extremely low operating voltages. However, the narrow

Figure 7.1 History of OLED architectures [1].

Table 7.1 Requirements of different layers in OLEDsS. no. OLED layer Work function

(ϕ) in eVPurpose Requirement Materials

1. Substrate 4.7–4.9 Serves as base for deposition of all layers

Transparent, high work function

Glass, plastics, metal foils

2. Anode 4.5–5.1 Serves as a positive electrode

Low roughness and high work function

ITO, graphene

3. HIL – Blocks the electrons for recombination

High mobility, electron blocking capacity, high glass-transition temperature

CuPC, PtPC, MeO-TPD

4. HTL – Transport holes and blocks electrons

Good hole-transporting capacity

NPB, TPD, α-NPD

5. Emissive layer

– Site for emission of light due to recombination of holes and electrons

High efficiency, lifetime, color purity

Organic complexes like Alq3

6. ETL – Transport electrons and block holes

Good electron-transporting capacity

Liq, TPBi, PBD

7. Cathode 2.9–4.0 Serves as a negative electrode

Low work function, high transparency

Alloys of magnesium with silver LiF/Al, etc.


thickness of the emitting layer in P-i-N OLEDs and the complex design architecture of phosphorescent OLEDs are not desirable from a manu-facturing perspective. Hence, researchers are now concentrating on over-coming these challenges and improving the electricity-to-light conversion efficiency, device stability, lifetime, material selection, proper encapsulation, and novel fabrication technologies, with reduced manufacturing cost.

7.3 REVIEW OF LITERATURE ON RED OLEDs

Ample research is being conducted in an effort to develop ecofriendly mate-rials for emission of light in the red region of the visible spectrum. Dresner was the first to consider organic material for the fabrication of practical red electroluminescent devices [13] in 1969. Soon after, organic thin film for EL studies was reported by Kampas and his coworker in 1977 [14] and Kalinowski et al. in 1985 [15]; they employed these thin films for multicolor display applications. However, the work was not successful due to low stabil-ity factor of the thin-film organic EL and broad nature of the luminescent spectra. Hybrid organic materials based on Eu3+ are better known to exhibit photoluminescence with a very sharp spectral band [16]. Kido et al. in 1991 investigated the suitability of Eu(ttfa)3 complex as a red-light emitter in cath-ode ray tubes [17]. In 1993, organic EL devices using lanthanide complexes such as Tb (acac)3 and Eu (ttfa)3 (Tb: terbium, acac: acetylacetonato, Eu: europium, and ttfa: thenoyltrifluorouetonato) were reported by Kido et al. [18]. In 1994, a bright-red EL was observed from tris(dibenzoylmethanato) phenanthroline Eu3+[Eu(DBM)3Phen] as red light-emitting material. The second added ligand, phenanthroline, acts to saturate the coordination num-ber of Eu ions and to improve the fluorescence intensity, volatility, and sta-bility of the Eu complex [19]. Again in the same year, Kido et al. developed bright-red light-emitting EL devices with highly monochromatic light using trivalent europium complexes as the emitter lanthanide complex. However, they exhibited a poor carrier transport property, which can be improved by codeposition with the carrier transport material [20]. In 1995, Sano et al. [21] presented a report on the fabrication of multilayer EL cells with the emission of an Eu complex formed by a vacuum-vapor deposition tech-nique. A volatile Eu-complex Eu(TTA)3(phen) was synthesized and applied to EL cells, which employed 1AZM-HEX (host material) as a emitting layer and Eu(TTA)3phen as a dopant [21].

In 1999, Miyamoto et al. [22] synthesized a Eu(III) β-diketonate complex, Eu(DBM)3Phen. Thin film of Eu(DBM)3Phen doped with

Review of Literature on Organic Light-Emitting Diode Devices 175

phosphorescent material in OLED showed excellent EL spectra at room temperature, which mainly depends on the host materials and the extent of energy transfer from the triplet states of the phosphorescent materials to the ligand triplet state of the Eu complex [22]. With two novel sec-ond ligands, 2-(2-pyridyl)benzimidazole (HPBM) and 1-ethyl-2-(2-pyri-dyl)benzimidazole (EPBM), two europium complexes, Eu(DBM)3HPBM and Eu(DBM)3EPBM (DBM: dibenzoylmethanato), were synthesized and used as emitting materials in organic EL devices by Huang et al. in 2001. The anatomy of the fabricated red-light-originating device was ITO/TPD/Eu(DBM)3HPBM (or) Eu(DBM)3EPBM/Al and ITO/TPD/Eu(DBM)3EPBM/AlQ/Al. The EL of Eu(DBM)3EPBM was found to be much higher than that of Eu(DBM)3HPBM. A maximum lumi-nance of 180 cd/m2 in the triple-layered device of Eu(DBM)3EPBM was achieved at 18 V [23]. Chen et al. [24] used a series of tris-(8-hydroxy-quinoline) metal chelates with central metal ions (Al3+, Ga3+, In3+) as the host materials. A red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(8-methoxy-1,1,7,7-tetra methyljulolidyl-9-enyl) 4H-pyran (DCJMTB), was used as the emitter/guest dopant material. The doped devices with Gaq3 as the host materials produced high efficiencies and saturated red-color chromaticity. The device with 1% DCJMTB doped in Gaq3 showed a current efficiency of 2.64 cd/A. The color coordinates of the Gaq3:1% DCJMTB device were found to be 0.63, 0.36 [24]. In the same year, Yang et al. fabricated a red single-layer type of EL device based on a copoly-mer containing carbazole, Eu complex, and methyl methacrylate [25]. In 2002 Ma et al. [26] employed N,N-bis4-[2-(4-dicyanomethylene-6-methyl-4H-pyran-2-yl) ethylene] phenyl aniline(BDCM) with two (4-dicyanomethylene)-4H-pyran electron-acceptor moieties and a triphe-nylamine electron-donor moiety for application in OLEDs. The three-layered EL device with the structure ITO/CuPc/DPPhP/BDCM/Mg:Ag had a power-on voltage of <4 V, with bright luminance of 582 cd/m2 at 19 V suggesting the excellent electron-injection property of BDCM.

Several devices using a europium complex Eu(TTA)3(DPPz) (TTA = 2-thenoyltrifluoroacetonate; DPPz = dipyrido[3,2-a:2′,3′-c] phenazine) as dopant emitter were fabricated [27]. The device architecture and the chemical structure of the europium complex are shown in Fig. 7.2. With the device structure TPD (50 nm)/Eu:CBP (4.5%, 30 nm)/BCP (30 nm)/Alq (25 nm), they obtained external quantum efficiency of 2.1%, cur-rent efficiency of 4.4 cd/A, power efficiency of 2.1 lm/W, and brightness of 1670 cd/m2. In order to develop new materials for red EL devices, a


novel ligand, 2-phenyl-imidazo[4,5-f]1,10-phenanthroline (L), and an europium (III) complex with dibenzoylmethanate (DBM), Eu(DBM) L, was synthesized by Gao et al. [28]. Single-crystal X-ray diffraction showed that Eu(DBM)3L belongs to the orthorhombic, space group Pbca with cell dimensions of a = 52.1655(6) nm, b = 52.0834(5) nm, c = 52.6469(7) nm, V = 511.942(5) nm, and Z = 58, D = 51.307 g/cm. Each europium atom six coordinated with six oxygen atoms from three biden-tate DBM anions and two nitrogen atoms from one bidentate L, form-ing a distorted square antiprism. The complex can be easily evaporated and can be used as a red-light-emitting material. Upon improvement, the device with of ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,19-biphenyl-4,49-diamine (TPD) (40 nm)/Eu(DBM)L(50 nm)/2,9-di-3-4, 7-diphenyl-1,10-phenanthroline (bathocuproine or BCP) (20 nm)/tris(8-hydroxyquinoline) aluminum (Alq3) (30 nm)/cathode gave off pure red light with luminance of 42 cd/m2 at 16 V. Duan et al. in 2005 successfully prepared two substituted phenanthrolines (L = DEP: 5-diethylamino-1,10-phenanthroline and PiPhen: 5-Piperidine-1,10-phenanthroline) and europium complexes based on these ligands Eu(TTA)3(L) (Eu-L) were synthesized from EuCl3, 2-thenoyltrifluoroacetone (TTA) and L in good yields [29]. These complexes emit a strong sharp red band at ~612 nm in solution and also in solid state. The HOMO levels of these complexes were at 5.6 eV, and EL devices using these two europium complexes as dopant emitters successfully emit saturated red light.

In 2006 Qu et al. [30] synthesized two novel polymers, PQP (poly(3,7-N-octyl phenothiozinylcyanoterephthalylidene)) and PQM

Figure 7.2 Device architecture and chemical structure of the europium complex employed by Ma et al. [26].


(poly(3,7-N-octyl phenothiozinylcyanoisophthalylidene)), containing phe-nothiazine for application in red- and orange-light-emitting diodes. The sin-gle-layer EL devices of ITO/PQP (or PQM)/Mg: Ag and multilayer devices of ITO/PQP (or PQM): N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,10-biphenyl]-4,40-diamine (TPD) (44 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 5 nm)/tris(8 hydroxyquinolinato) aluminum (Alq3, 20 nm)/Mg:Ag were fabricated. The EL spectra from the devices based on PQP, PQM peaked at a wavelength of 664 nm, 608 nm with maxi-mum brightness of 60 cd/m2, 150 cd/m2, at an applied voltage of 17 V and 14 V, respectively. In 2007, novel rare earth complex Eu (TTA)2 (N-HPA) Phen(TTA = thenoyltrifluoroacetone, N-HPA = N-phenylanthranilic acid, and phen = 1,l0-phenathroline), which contains three different ligands, was synthesized by Yanfei et al. [31].

The Eu complex was blended with poly N-vinylcarbazole (PVK) in different weight ratios and spin-coated into films. The chemical structure of Eu(TTA)2(N-PHA) and device architecture of the OLED device fabri-cated by Yanfei et al. are shown in Fig. 7.3A and B, respectively. Multilayer structural devices consisting of ITO/PVK: Eu (TTA)2(N-HPA) phen/BCP/Alq3/Al were fabricated with PVK: Eu (TTA)2(N-HPA) as light-emitting layer. Increasing the concentration of Eu in the PVK thin film would inhibit the emission of PVK to different degrees. Pure-red lumi-nescence of europium (III) was observed when the doping weight ratio was approximately 1:5, indicating an effective energy transfer from PVK to

Figure 7.3 (A) Chemical structure of Eu(TTA)2(N-PHA) and (B) device architecture of OLED device fabricated by Yanfei et al. [31].


rare earth complex at this ratio. For application in OLEDs, efficient new red phosphorescent iridium (III) complexes, bis[2,3-diphenyl-4- methyl-quinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq)2(acac)] and bis[2,3-di(4-methoxy-phenyl)-4-methyl-quinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq(OMe)2)2(acac)], were synthesized from the two-step reactions of IrCl3·xH2O with a corresponding ligand by Park et al. [32] in 2008. Electroluminescent devices with a configuration of ITO/2-TNATA/NPB/CBP:dopant/BCP/Alq3/Liq/Al were fabricated. Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2(acac) showed a luminous efficiency of 8.10 and 9.81 cd/A at a current density of 20 mA/cm2, respectively. The CIE coordinates of Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2 (acac) were 0.644, 0.352, and 0.615, 0.375, respectively [32].

In 2009, Haq et al. [33] fabricated efficient red organic light-emit-ting material with a wide bandgap, i.e., 9,10-bis(2-naphthyl) anthracene (ADN) doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tet-ramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10(2-benzothiazolyl) quinolizine-[9,9a,1gh] coumarin (C545T) as an assistant dopant. The C545T dopant did not emit by itself but did assist the energy trans-fer from the host (ADN) to the red-emitting dopant through a cascade energy-transfer mechanism. This approach significantly improved the EL efficiency of OLEDs. In the same year, Lyu et al. [34] fabricated a highly efficient phosphorescent silicon-cored spiro-bifluorene derivative (SBP-TS-PSB) as a host material for red phosphorescent Ir(III) complexes. Three phosphorescent guests, (piq)3Ir, (piq)2Ir(acac), and (btp)2Ir(acac), were doped in the SBP-TS-PSB host and the device performance was investigated. The external quantum efficiency, power efficiency, and CIE color coordinates obtained from (piq)2Ir(acac), (piq)3Ir, and (btp)2Ir(acac)-based devices were 14.6%, 10.3 lm/W (0.68, 0.32) at current density of 1.5 mA/cm2, 13.5%, 7.8 lm/W (0.66, 0.32) at current density of 1.3 mA/cm2, and 9.9%, 7.0 lm/W (0.66, 0.31) at J = 0.5 mA/cm2, respectively [34]. In 2010, the performance of a red OLED device was improved by codoping 2-formyl-5,6,11,12-tetraphenylnaphthacene(2FRb) and 4-(di-cyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran(DCJTB) in tris-(8-hydroxyquinoline) aluminum (Alq3) host as the emitting layer by Li et al. [35]. The device with 1 wt% DCJTB and 2.4 wt% 2FRb in Alq3 host gave a saturated red emission with CIE chro-maticity coordinates 0.65, 0.35 and maximum current efficiency as high


as 6.45 cd/A, which were 2- and 2.4-fold larger than that of the device with 1 wt% DCJTB (3.28 cd/A) in Alq3 host and the device with 2.4 wt% 2FRb (2.72 cd/A) in Alq3 host at the current density of 20 mA/cm2, respectively. The improvement could be attributed to the effective utiliza-tion of host energy by both energy transfer, trapping in the EL process and the depression of concentration quenching between the dopant molecules.

In 2011 Kalyani et al. [36] designed multilayer OLED devices with the configuration of ITO/m-MTDATA (1000 Å)/α-NPD(200 Å)/TPBi: Eu0.4Y0.6(TTA)3Phen(250 Å)/Alq3(250 Å)/LiF:Al (10:1200 Å) and ITO/m- MTDATA(1000 Å)/α-NPD(200 Å)/TPBi:Eu0.5Y0.5(TTA)3Phen(250 Å)/Alq3(250 Å)/LiF:Al(10:1200 Å). Different characterization techniques such as I-V, J-V-L, V-L characteristics, CIE coordinates, and EL spec-tra were carried out for the fabricated devices at room temperature in ambient atmosphere. Bright and efficient EL devices with narrow lumi-nescent emission and narrow bandwidth were obtained. Full-width at half-maximum (FWFM) was found to be <5 nm for both devices. The power-on voltage of device I and device II was found to be 13 V and 17 V, respectively, with λemi centered at 612 nm. With the host/dopant combi-nation, maximum brightness of 185.6 and 44.72 cd/m2 was observed for devices I and II, respectively. The device anatomy and molecular structures of the materials used are shown in Figs. 7.4 and 7.5, respectively.

Highly efficient single-layer OLEDs based on blended cationic irid-ium (Ir) complexes as emitting layer have been demonstrated using nar-row bandgap Ir complex [Ir(Meppy)2(pybm)](PF6) (C1) as guest and wide bandgap cationic Ir complex [Ir(dfppy)2(tzpy-cn)](PF6) (C2) as host. As

Figure 7.4 Device stack of fabricated OLED devices. (A) Device I and (B) device II.


compared with single cationic Ir complex-emitting layer, these host–guest systems exhibit highly enhanced efficiencies, with maximum luminous efficiency of 25.7 cd/A and external quantum efficiency of 8.6%, which are nearly three-fold of those of pure C1-based devices. Compared with a multilayer host-free device containing C1 as emitting layer and TPBI as ETL, the above single-layer devices also demonstrated two-fold enhance-ment efficiencies. The high efficiencies achieved in these host–guest

Figure 7.5 Molecular structure of the materials used in different layers of OLED devices.


systems are among the highest values reported for ionic Ir complex-based SSL-emitting devices. The results demonstrate that the ionic Ir complex-based host–guest system provides a new approach to achieving highly effi-cient OLEDs with single-layer device structure and solution-processing technique [37]. Two heteroleptic iridium (III) complexes, Ir(piq)2(dbm) and Ir(btp)2(acac), have been tested as emitters for phosphorescent OLEDs (PhOLEDs). The structure of these EL devices, the energy diagram, and the molecular structures of the different materials employed in these devices are shown in Fig. 7.6. Interestingly, device performance exhibited a marked insensitivity in the dopant concentration. In this study, a diben-zoylmethane (dbm)-based complex was also tested for OLEDs. To evalu-ate the emissive properties of this new emitter belonging to a family of complexes that has not been investigated yet, identical devices were pre-pared with the well-known red dopant Ir(btp)2(acac) for comparison. The new complex Ir(piq)2(dbm) exhibited comparable performance to that obtained with Ir(btp)2(acac) [38].

Song [39] synthesized two highly efficient red phosphorescent Ir (III) complexes, bis[2,3-diphenylquinoxalinato-N,C2′]iridium(III) pyra-zinate (dpq)2Ir(prz) (1) and bis[2,3-iphenylquinoxalinato-N,C2′]iridium (III) 5-methylpyrazinate (dpq)2Ir (mprz) (2), which were both based on

Figure 7.6 Structure of EL devices, energy diagram, and molecular structures of the different materials [38].


the use of 2,3-diphenylquinoxaline as the ligand. Both Ir (III) complexes were found to be soluble in common organic solvents, and uniform thin films were readily spin-coated onto substrates. Phosphorescent OLEDs produced using a ITO/PEDOT:PSS/TCTA:TPBi:Ir (III) complex/cath-ode configuration had a maximum external quantum efficiency of 7.32% and a luminance efficiency of 7.15 cd/A with CIE coordinates of 0.70, 0.30 for compound 2, which remained stable on increasing current den-sity. The efficiencies of these PhOLEDs were higher than those previ-ously reported for solution-processed pure red PhOLEDs. Recently, Zhou et al. attempted to enhance the EL performance of trivalent europium complex Eu(TTA)3phen (TTA: thenoyltrifluoroacetone and phen:1,10-phenanthroline) by designing the device structure with stepwise energy levels. The widely used bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phe-nyl)pyridine (26DCzPPy) was chosen as host material, while the doping concentration of Eu(TTA)3phen was optimized to be 4%. To facilitate the injection and transport of holes, the MoO3 anode modification layer and 4,4′,4″-tris(carbazole-9-yl) triphenylamine (TcTa) HTL were inserted in sequence. Efficient pure-red emission with suppressed efficiency roll-off was obtained, which can be attributed to the reduction of accumulation of holes, the broadening of the recombination zone, and the improved balance of holes and electrons on Eu(TTA)3phen molecules. Finally, the device with 3 nm MoO3 and 5 nm TcTa obtained the highest bright-ness of 3278 cd/m2, current efficiency of 12.45 cd/A, power efficiency of 11.50 lm/W, and external quantum efficiency of 6.60%. Such a device design strategy helps to improve the EL performance of emitters with low-lying energy levels and provides a chance to simplify device-fabrica-tion processes [40].

7.4 REVIEW OF LITERATURE ON GREEN OLEDs

Bright-green emission from aluminum tris(8-hydroxyquinolinate) (Alq3) thin-film organic layers was first demonstrated by Tang and VanSlyke in 1987 [41]. Later, in 1990, Kido et al. [20] reported a double-layer OLED containing Tb-tris-(acetylacetonato), Tb(acac)3 as green-light-emitting material, N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′-biphenyl-4,4′ diamine(TPD) as HTL with device structure, ITO/TPD/Tb(acac)3/Al. Strong emission peak at 544 nm peak, corresponding to the 5D4→7F5 tran-sition of the Tb3+ ion, was observed [42]. Kasim et al. [43] synthesized a new conjugated polymer, poly(2,bquinoline vinylene) (PQV), which


exhibited maximum fluorescence at 515 nm when excited at 430 nm. Polymer light emitting diodes (PLEDs) constructed with ITO/PQV/Al configuration displayed broad emission peak at 530 nm. A thermally stable Tb-tris(tetradecylphethalate) phenanthroline complex Tb(MTP)3Phen was prepared and used as green-emitting layer by Ma et al. in 1999 [44]. The chemical structure of Tb-tris(tetradecylphethalate) phenanthroline com-plex Tb(MTP)3(Phen) is shown in Fig. 7.7.

Multilayered EL device consisting of ITO/poly (p-phenylenevi-nylene) (PPV)/PVK: Tb (MTP)3(Phen)/Alq3/Al has been fabricated. In 2000, Lin et al. designed a double-device structure, ITO/PVK: PBD: Tb (MDP)3/Alq3/Al. They achieved sharp-green emission with lumi-nance of 152 cd/m2 at 24 V and poor external quantum efficiency of 0.017% [45]. Phosphorescent dendrimers with fac-tris(2-phenyl-pyridyl) iridium (III) cores, biphenyl-based dendrons, and 2-ethylhexyloxy sur-face groups, which emit green light, were reported by Markham et al. The solution-processable green phosphorescent dendrimers were used to fabricate highly efficient single-layer devices as well as bilayer OLEDs, giving efficiencies of up to 16% with 40 lm/W at 400 cd/m2 [46–48]. In 2003 Palilis et al. [49] reported the performance of molec-ular organic light-emitting diodes (MOLEDs) using novel fluorescent silole derivatives as highly efficient blue- and green-emitting organic materials. Three silole derivatives, namely 2,5-di-(3-biphenyl)-1,1-di-methyl-3,4-diphenyl silacyclopentadiene (PPSPP), 9-silafluorene-9- spiro-1′-(2′,3′,4′,5′-tetraphenyl)-1′H-silacyclopentadiene(ASP), and 1,2-bis (1-methyl-2,3,4,5, tetraphenylsilacyclopentadienyl)ethane (2PSP), with high solid-state PL quantum yields of 0.85, 0.87, and 0.94, respectively, were used as emissive materials. The structures of synthesized com-plexes and the device structure of the fabricated OLEDs by Palilis et al. are shown in Fig. 7.8. The high electron mobility silole derivative,

Figure 7.7 Chemical structure of Tb(MTP)3Phen [44].


2,5-bis(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene(PyPySPyPy), was also used as the electron-transport material. MOLEDs using these siloles as emitters and N,N′-diphenyl-N,N′-(2-napthyl)-(1,1′-phenyl)-4,4′-diamine (NPB) or N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-diamine (TPD) as the hole-transport material showed low operating voltages of 4–4.5 V at a luminance of

N

N N

N

H3C

PyPySPyPy

2PSP

H3CCH3

CH3

Si

Si

ASP

H3C CH3

PPSPP

Mg:Ag

PyPySPyPy EML/ETL

Mg:Ag

PyPySPyPy ETL

Silole EML

NPB (TPD) HTL

ITO

NPB HTL

ITO

Si

SiN

N N N

CH3 H3CTPDNPB

Si

Figure 7.8 Structures of synthesized complexes and device structure of the fabricated OLEDs by Palilis et al. [49].


100 cd/m2 and high external EL quantum efficiencies of 3.4–4.1% at 100 A/m2.

Mishra et al. [50] reported a bluish-green emission from OLEDs based on aluminum complex, bis(2-methyl 8-hydroxiquinoline) alumi-num hydroxide (Almq2OH), as emissive material [50]. In 2005 Hwang et al. developed a stable green OLED using an Al–Cu alloy as a cathode material. The device structure is shown in Fig. 7.9, where CFx is N,N′-bis-s1-naphthyld-N,N′-diphenyl,1,1′-biphenyl-4,4′-diamine(NPB), tris(8-quinolinolato)aluminum (Alq3), and lithium acetate (CH3COOLi) were used as the hole-injection material, hole-transport material, light-emitting material, and electron-injection material, respectively [51].

Ku et al. [52] reported highly efficient undoped green OLEDs by incorporating a novel 9,9-diarylfluorene-terminated 2,1,3-benzothiadia-zole (DFBTA), which exhibited an excellent solid-state photolumines-cence quantum yield of about 81%. The optimal device ITO/DPAInT2/DPAInF/TCTA/DFBTA/Alq3/LiF/Al displayed impressive device char-acteristics, with maximum external quantum efficiency of 12.9 cd/A. Liu et al. in 2009 investigated highly efficient phosphorescent OLEDs [53] based on an orange/red emission iridium complex as the guest and five green emission iridium complexes as the host material, respectively. In 2010 Cho et al. [54] synthesized Ir(Cz-ppy)2(Cz-Fppy)1, Ir(Cz-ppy)1 (Cz-Fppy)2, Ir(Cz-Fppy)3, and Ir(Cz-ppy)3, which exhibited green emis-sion at 515, 511, 496, and 520 nm, respectively. With cationic iridium complexes as dopants and poly (N-vinylcarbazole) as host He et al. [55]

Figure 7.9 Structures of Alq3 and NPB, and structure of the fabricated device by Hwang et al. [51].


fabricated highly efficient blue-green to red and white OLEDs by solu-tion processes. Complexes with cyclometalated 2-phenylpyridine ligands showed better device performance than those containing cyclometalated 2-(2,4-difluorophenyl)pyridine ligands. With the addition of an electron-transporting and exciton-blocking layer, the devices showed improved performance, achieving peak current efficiencies of 24.3, 25.3, 20.5, and 4.2 cd/A for the blue-green, green, yellow, and red EL, respectively.

In 2011, Kim et al. fabricated high-efficiency OLEDs, in which solu-tion-processed ambipolar blends of hole- and electron-transport polymer hosts doped with a green-emitting iridium complex were sandwiched between a photo-crosslinked HTL and a vacuum-deposited ETL. The ambipolar host blends consisted of blends of bis-oxadiazole-functionalized poly(norbornene) electron-transport materials and poly(N-vinylcarbazole). For the best device examined, an external quantum efficiency of 13.6% and a maximum luminous efficiency of 44.6 cd/A at 1000 cd/m2 with a power-on voltage of 5.9 V was obtained [56]. The energy-level diagram showing the estimated ionization potential (HOMO) and electron affinity (LUMO) levels for the materials investigated by Kim et al. are shown in Fig. 7.10.

Two heteroleptic iridium (III) complexes using carbene as cyclometalated ligands and pyridine-triazole as ancillary ligands, namely (fpmi)2Ir(mtzpy) (1) and (fpmi)2Ir(phtzpy) (2) (fpmi:1-(4-fluorophenyl)-3-methylimdazo-lin-2-ylidene-C,C20, mtzpy: 2-(5-methyl-2H-1,2,4-triazol-3-yl) pyridine,

Figure 7.10 Energy-level diagram showing estimated Ionization energy (IE) and elec-tron affinity (EA) levels for materials investigated by Kim et al. [56].


phtzpy: 2-(5-phenyl-2H-1,2,4-triazol-3-yl)pyridine), were synthesized by Li et al. in 2014. Both complexes exhibited bright greenish-blue phosphores-cence (λmax = 490 nm) with quantum yields of about 0.50. The comprehen-sive density functional theory (DFT) approach was then performed to gain insight into their photophysical and electrochemical nature. The fabrication of OLEDs, employing complexes 1 and 2 as phosphorescent dopants, was successfully achieved. Among them, the device based on complex 1 exhibited considerable power efficiency (ηp) of 11.43 lm/W and current efficiency (ηc) of 11.78 cd/A. With the merit of intrinsic characteristic of complex 1, a white OLED comprised of 1 and one orange phosphor (pbi)2Ir(biq) achieved a peak hp of 9.95 lm/W and ηc of 10.81 cd/A, together with CIE coordinates 0.34, 0.40. The results indicate that the two iridium (III) complexes reported here are promising phosphorescent dyes for OLEDs [57]. The device con-figuration and energy-band diagram of the greenish-blue OLEDs and the molecular structures of compounds used by Li et al. are shown in Fig. 7.11.

A green OLED with an extremely high power efficiency of over 100 lm/W was realized through energy transfer from an exciplex. An opti-mized OLED showed a maximum external efficiency of 25.7% and a power efficiency of 79.4 lm/W at 1000 cd/m2, which is 1.6-times higher than that of state-of-the-art green thermally activated delayed fluorescence (TADF) OLEDs [58]. A new class of highly phosphorescent Pt(II) com-plexes (Pt1–Pt3) based on rigid symmetric tetra dentate ligands (L1–L3) were designed and synthesized by Zhang et al. recently. L1–L3 ligands are analogous to N,N-di(2-phenylpyrid-6-yl)aniline (L) except that one coordination phenyl group in L is replaced by other motifs with different

Figure 7.11 Device configuration and energy-band diagram of the greenish-blue OLEDs, and the molecular structures of the compounds used by Li et al. [57].


electron donating/accepting capabilities. The effect associated with the modulation of a single coordination group within each ligand on the photophysical and EL properties of Pt1–Pt3 was investigated systemati-cally. Among Pt1-Pt3, Pt1 had the highest HOMO due to the presence of a strong electron-donating group (3-methylindole), and exhibited the narrowest bandgap; Pt2 had the lowest HOMO due to the lack of strong donor group within the structure, and showed the widest bandgap. The OLEDs based on these three complexes showed yellowish-green to green-ish-yellow EL with high efficiency. Notably, the device based on Pt1 at the doping level of 10 wt% achieved a maximum efficiency of 53.0 cd/A, 35.9 lm/W, and 16.3% with CIE coordinates 0.44, 0.53 [59].

7.5 REVIEW OF LITERATURE ON BLUE OLEDs

Among the three primary RGB colors, the synthetic protocols and fabri-cation methods of green and red phosphors meet the necessary require-ments. Conversely, the design and fabrication of blue phosphors and consequent devices is still an ongoing challenge. In 1992, Grem et al. was the first to report blue EL from OLEDs containing poly(p-phenyl-ene) (PPP) [60]. Organic EL devices with multilayer structures were fab-ricated using a 1,2,4-triazole derivative as the carrier transport layer. 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) was found to be electron-transporting, and a cell with a structure of glass substrate/ITO/triphenylamine derivative (TPD)/TAZ/Alq/Mg:Ag exhib-ited bright-blue EL from the TPD layer. A luminance of 3700 cd/m2 with an emission peak at 464 nm was achieved at a drive voltage of 16 V [61].

A wide range of oligo(p-phenylenevinylene)s with alkyl [62–64] or alkoxy [65,66] substituents have been synthesized. Oligo(p-phe-nylene)s have been used as blue emitters [67,68] in EL devices and exhibit high-fluorescence quantum yields. Tao and Suzuki [69] reported the blue emitter LiB(qm)4 in the device structure of ITO/PVK:NPB (50 nm)/LiB(qm)4 (60 nm)/Mg:Ag. Power efficiency and luminance of the device were 1.3 lm/W and 6900 cd/m2, respectively. Li and coworker prepared Y, La, and Gd ion complexes with M(acea)3(Phen) [70]. However, these complexes were used as electron-transporting materials for the organic emitter, N,N′-bis(1-naphthyl-1,1′-biphenyl-4,4′-diamine) (NPB) with OLED structure ITO/NPB/M(acea)3 (Phen)/Mg:Ag. Hong and his coworker were the first to use Tm3+ ion in OLEDs [71]. They prepared a tris(acetylacetonato)-monophenanthroline Tm complex and double-layer


cell with anatomy ITO/PVK/Tm complex/Al. Liu and Wang in 2001 [72] reported the blue emitter Bepp2. The Bepp2 was put in two differ-ently structured devices: (1) ITO/NPB (60 nm)/Bepp2 (50 nm)/LiF (1 nm)/Al (200 nm) and (2) ITO/CuPc (15 nm)/NPB (60 nm)/Bepp2 (50 nm)/LiG (1 nm)/Al (200 nm). Fluorene-based blue EL polymers poly[9,9′-bis(2-ethylhexyl)fluorene-2,7-diyl] end-capped with N,N′-bis(4-methylphenyl)-N-phenylamine [73] and poly(9,9′-dioctylfluorene-2,7-diyl) (PF8, PFO) [74] also showed blue emission. The OLEDs fabricated with FIrN4 (iridium (III) bis(4,6 difluorophenylpyridinato) (5-(pyridin-2-yl)-tetrazolate)) as dopant in mCP (1,3-bis(9-carbazolyl)benzene) exhibited near-saturated blue electro phosphorescence with CIE coordinates 0.15, 0.24 [75]. Cheng et al. [76] fabricated pure-blue OLEDs with CIE coordinates 0.1638, 0.094 at 16 V using a derivative of oligo(phenylenvinylene), 2,5-diphenyl-1,4-distyrylbenzene with two trans-double bonds, as a light-emitting layer. By introducing perylene as a dopant in the light-emitting layer, luminance and luminous efficiency dramatically improved from 1400 cd/m2 to 5500 cd/m2 and 1.18 cd/A to 3.18 cd/A, respectively. Ding et al. [77] fabricated blue OLEDs using undoped 9,10-di(2-naphthyl)anthracene (ADN) as the emitting layer, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,10-biphenyl-4,40-amine (TPD) as HTL, and one of tris-(8-hydroxy-quinolinato) aluminum (Alq3), 4,7-diphenyl-1,10-phenanthroline (Bphen) and 2-tert-butylphenyl-5-bi-phenyl-1,3,4-oxadiazole (PBD) as ETL. In 2010, Zheng developed effi-cient OLEDs by doping fluorescent- and phosphorescent-type emitters individually into two different hosts separated by an interlayer to form a fluorescence–interlayer–phosphorescence (FIP) emission architecture. Blue OLED with FIP emission structure comprising p-bis(p-N,N-diphe-nylaminostyryl) benzene (DSA-Ph) and bis[(4,6-di-fluorophenyl)-pyr-idinate-N,C2′]picolinate (FIrpic) exhibited a peak luminance efficiency of 15.8 cd/A at 1.54 mA/cm2 and a power efficiency of 10.2 lm/W at 0.1 mA/cm2 [78].

Haq et al. [79] synthesized blue OLEDs with 9,10-bis(2-naphthyl)-2-t-butylanthracene (TBADN) doped with (3 wt%) p-bis(p-N,N-diphenyl-aminostyryl) benzene (DSA-Ph) as an emitting layer; the typical device structure was ITO/HIL (5 nm)/NPB (25 nm)/EML (35 nm)/ETL(15 nm)/LiF (0.8 nm)/Al (100 nm). At the current den-sity of 20 mA/cm2, its driving voltage and power efficiency was 5.2 V and 4.2 lm/W, which was independently reduced by 48% and improved by 44% as compared with those of the m-MTDATA/Alq3-based one,


respectively. In 2010, Pu et al. [80] synthesized hole-transporting aryl-amino-9,10-diphenyl anthracene derivatives by C–N cross-coupling with palladium catalyst. These materials showed higher glass-transition tempera-tures (135–177°C). Alq3-based green-light-emitting devices containing the arylamino-9,10-diphenylanthracene derivatives as HTL were fab-ricated. A new series of blue fluorescent emitters based on t-butylatedbis(diarylaminoaryl) anthracenes were synthesized by Lee et al. [81]. Into these blue materials, t-butyl groups were introduced to prevent molecu-lar aggregation between the blue emitters through steric hindrance and to reduce self-quenching. To improve efficiency, multilayered OLEDs were fabricated into a device structure of ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq3(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. Highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m2 at 12 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm2 were accomplished. The peak wavelength of the EL was observed at 468 nm with CIE coordinates 0.159, 0.198 at 12.0 V. In addition, a deep-blue device with CIE coordinates of (0.159, 0.151) at 12 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm2. RGB phosphorescent P-i-N homojunction devices by using a series of bipolar host materials including 2,6-bis(3-(carbazol-9-yl)phenyl) pyridine (2,6DCzPPy), 3,5-bis(3-(carbazol-9-yl)phenyl) pyridine (3,5DCzPPy), and 4,6-bis(3-(carbazol-9-yl)phenyl) pyrimidine (4,6DCzPPm) were demonstrated by Cai et al. [82] in 2011. Chen et al. [83] synthesized three anthracene derivatives featuring carbazole moieties as side groups –2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz9PhAnt), 2-ter t-butyl-9,10-bis4-[3,6-di-ter t-butyl-(9-carbazolyl)]phenylanthracene(tCz9PhAnt), and 2-tert-butyl-9,10-bis4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-ylanthracene (tCz9Ph2Ant) for use in blue OLEDs with high glass-transition temperature of 220°C as shown in Fig. 7.12. They exhibited strong blue emissions in solution, with high quantum efficiency of 91%.

Blue-light-emitting host materials with a spiro[benzo[de]anthra-cene-7,90-fluorene] core, 3-[10-(naphthalene-1-yl)anthracene-9-yl]spiro[benzo[de]anthracene-7,90-fluorene] (NA-SBAF), and 3-[10-(naph-thalene-1-yl)anthracene-9-yl]-1-methylspiro[benzo[de]anthracene-7,90-fluorene] (NA-MSBAF) were designed and synthesized via coupling reactions. Introduction of a spiro group into the anthracene moieties led


to reduction in crystallization tendency and a high glass-transition tem-perature. Typical blue fluorescent OLEDs with the configuration of ITO/N,N′-di(1-naphthyl)-N,N′-bis[(4-diphenylamino)phenyl]-biphe-nyl-4,40-diamie (60 nm)/N,N,N′,N′-tetra(1-biphenyl)-biphenyl-4,40-diamine(30 nm)/host: dopant (30 nm, 5%)/LG201 (ETL, 20 nm)/LiF/Al were developed using SBAF-type anthracene derivatives as host material and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as sky-blue dopant material. A device obtained from NA-SBAF doped with DSA-Ph was compared with that of 9,10-dinaphthalene-2-yl-anthracene and showed blue color purity of 0.150 and 0.217, a luminance efficiency of 7.57 cd/A, and an external quantum efficiency >5.15% at 5.0 V [84]. In 2015 Chang [85] demonstrated fully solution-processed blue OLEDs with n-type doped multilayer graphene as the top electrode. The work func-tion and sheet resistance of the graphene were modified by an aqueous process that can also transfer graphene on organic devices as the top elec-trodes. With n-doped graphene layers used as the top cathode, all-solution processed transparent OLEDs can be fabricated without any vacuum pro-cess. Recently, Zhu et al. synthesized two blue emitters, 2,7-bis(9-benzyl-9H-carbazol-2-yl)pyrene and 2,7-bis(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)pyrene by a Suzuki coupling reaction. The emission peaks of the two emitters were found to be 430 and 439 nm with 87.5 and 68.6% flu-orescence quantum yields in chloroform, respectively. The emitters both had good thermal stability (Tg > 160°C). Nondoped blue OLEDs with these emitters were achieved with x, y coordinates 0.17, 0.11 and 0.16,

Figure 7.12 Chemical structures of anthracene derivatives and device structure of blue OLEDs fabricated by Chen et al. [83].


0.15, respectively, which are very close to the National Television System Committee standard blue [86].

7.6 REVIEW OF LITERATURE ON WHITE OLEDs

White organic light-emitting diodes (WOLEDs) are becoming more popular as third-generation (3G) lighting sources due to their outstand-ing features such as low energy consumption, low cost, high efficiency, and flexible properties, extremely long life, high durability, and the fact that they are pollution-free. They are used in full-color displays and as back-lights for LCD displays and energy-efficient traditional lighting sources [87]. Several approaches have been proposed to engender white-light with an assortment of OLED device configurations and diverse emissive mate-rials [88–90]. As a promising candidate for lighting, white OLEDs should generate light with spectral distribution similar to that of natural sunlight covering the full visible range as much as possible. To obtain high bright-ness WOLEDs, blue-emitting OLEDs are mainly used in combination with yellow phosphor to partially downconvert the blue emission into light with longer wavelengths or by a combination of RGB phosphor. With intrin-sically high quantum efficiency, WOLEDs incorporating phosphorescent emitters have become the most promising candidates for meeting the strin-gent efficiency requirements for lighting applications today [91,92]. For lighting purposes, light sources with CIE coordinates closer to the ideal white point (0.33, 0.33), CRI above 80, and CCT similar to those of the blackbody radiation between 2500K and 6500K are required for better color purity [93]. Research on WOLEDs has rapidly increased since the first demonstration of WOLED by Kido et al. by dispersing blue, red, and green fluorescent dye in polyvinylcarbazole (PVK) that together produce white light with efficiency <1 lm/W [94]. White-light emission is usually observed by a set of different luminophores with distinct emission colors, typically two (blue and orange/yellow) or three (blue, green, and red). Fig. 7.13 represents the general strategies used to develop devices that combine multiple emitters in the EML to produce white light [95]. Kido et al. [96] used Tb3+ and Eu3+ complexes to achieve multilayer white OLEDs with Eu (aca)3Phen binuclear complexes as the emitting layer. These devices emit bright-white light with more uniformity and energy efficiency than that of fluorescent lights. In 1999, Deshpande et al. obtained white-light emission by the sequential energy transfer between different layers. The device configuration was ITO/α-NPD/α-NPD: DCM2(0.6–8 wt%)/BCP/


Alq3/Mg:Ag (20:1)/Ag. 4,4′-Bis (N-(1-napthyl-N phenylamino)) biphenyl (α-NPD) was used as a hole-injection layer, α-NPD: DCM2 (2,4-(dicyanomethylene)-2-methyl-6-(2-(2,3,6,7-tetrahydro-1H,5H benzo(i,j)quinolizin-8-yl) vinyl)-4H-pyran) was used as HTL and as well as an emitting layer, 2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline (BCP) was used to block holes, and Alq3 was used as ETL and Mg:Ag alloy followed by a thick layer of Ag deposited as the cathode [97].

Use of phosphorescent dopants as emitters in a segregated-layer WOLED was first demonstrated by D’Andrade et al. in 2001 [98] with the device structure ITO/poly(ethylene-dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS)/4,4′-bis[N-(1-napthyl)-N-phenylamino] biphenyl (α-NPD) 30 nm/4,4′-N,N′-dicarbazolebiphenyl (CBP) 20 nm:6 wt% iridium(III) (bis(4,6-di-fluorophenyl)-pyridinato-N,C2) picolinate (FIrpic)/CBP layer 8 wt%: bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3) iridium (acetylaceto-nate) (Btp2Ir(acac)), 2 nm/CBP 8 wt% bis(2-phenyl benzothiozolato-N,C2′) iridium (acetylacetonate) (Bt2Ir(acac))2 nm/2,9-dimethyl-4au,7-diphenyl-1,10-phenanthroline (BCP) as the final organic layer, which served as both a blocking and ETL. Efficiency of about 5.2% external quantum efficiency, CIE coordinates 0.35, 0.36, CRI of 83, and peak brightness over 30,000 cd/m2 were achieved. A white-light-emitting device was fabricated by Xiao et al.

Figure 7.13 General approaches to generating white light from OLEDs using mul-tiple emitters: (A) single-EML structure, (B) multilayer EML structure, (C) stacking and tandem structure, and (D) striped structure [95].


[99] in 2005, with a structure of ITO/NPB/BCP/Alq3/LiF/Al as shown in Fig. 7.14.

The HBL (BCP) results in a mixture of lights from NPB molecules (blue light) and Alq3 molecules (olivine light), thereby emitting white light. A maximum brightness of 5740 cd/m2 with EL efficiency of 2.12 cd/A at the applied voltage of 18 V was achieved. The schematic cross-section of the fabricated OLED structure is shown in Fig. 7.15. In 2005 Tsou et al. fabricated white OLEDs with CIE coordinates 0.32, 0.32 by doping 1% DCM2 in the BCP layer [101]. In the same year, Gong et al. reported high-performance multilayer white light-emitting PLEDs fabricated by using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer and water- or ethanol-soluble PVKSO3Li as the hole-injection/transport layer and t-Bu-PBD-SO3Na as the electron-injection/ETL [102]. Yu et al. in 2009 reported white LEDs using a blue InGaN LED precoated conjugated copolymer/quantum dots (QDs) composite (green-emitting poly(9,9-dioctyl-2,7-divinylenefluorenylene)-alto-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)/red-emitting CdSe QDs) as a hybrid phosphor. The white LED of the hybrid phosphor containing 20 wt% QDs had a luminous efficiency of 44.2 lm/W at 20 mA with CIE coordinates 0.3297, 0.3332, CCT 5620K, and CRI 75.3, respec-tively [103]. Bright WOLEDs with a single active layer were demonstrated from blue-emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)2] doped with orange luminescent 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) dye. White EL spectrum from Zn(hpb)2 was achieved by adjusting the concentration of DCM dye. Additionally, WOLEDs with a structure of ITO/α-NPD/Zn(hpb)2:DCM (x%)/BCP/Alq3/LiF/Al have been fabricated. The EL spectra with two peaks at 446 and 555 nm were obtained. The device

Figure 7.14 Schematic cross-section of fabricated OLED structure by Xiao et al. [99].


emitted white light at 10 V with CIE coordinates 0.27, 0.31 and brightness 1083 cd/m2. The maximum current efficiency of the device was 1.23 cd/A at 9.5 V and maximum luminance reached 2210 cd/m2 at 12 V [100]. The configuration of the WOLEDs and the molecular structures of DCM dye and Zn(hpb)2 are shown in Fig. 7.15.

Chang et al. reported high color-rendering pure-white phosphorescent OLEDs by iridium complex Ir(dfbppy)(fbppz)2 and a wide-band-width yellow-emitting osmium complex Os(bptz)2(dppee). They achieved a CRI of 81 and CIE coordinates 0.33, 0.33, which is close to the ideal white emission [104]. Tyagi et al. in 2010 demonstrated a WOLED by double layers of blue Zn(hpb)2 and yellow Zn(hpb)mq emitting materials. It was

Figure 7.15 (A) White organic light-emitting diodes configuration, (B) WOLED device, and molecular structures of (C) DCM dye and (D) Zn(hpb)2 [100].


observed that when the thickness of Zn(hpb)mq layer was increased the dominant wavelength shifted from bluish to yellowish region, and CIE coordinates 0.29, 0.38 with a low power-on voltage (5 V) was achieved [105]. Chen et al. in 2010 [106] fabricated WOLEDs utilizing two pri-mary color emitters without any additional blocking layer. Anthracene was deposited directly above the rubrene (Rb)-doped NPB yellow light-emitting layer with a structure of ITO/2TNATA(20 nm)/NPB(20 nm)/NPB: rubrene (2%)(10 nm)/ADN(30 nm)/Alq3(20 nm)/LiF(1 nm)/Al(100 nm), a white light with CIE coordinates 0.344,0.372 at a current density of 30 mA/cm2 was generated. In 2011 Hu et al. [107] reported a theoretical investigation of the white-light emission from a single polymer system with simultaneous blue polyfluorene as host and orange 2,1,3-ben-zothiadiazole (BTD)-based derivative as dopant emission. They employed quantum chemical approaches to study variations in electronic and opti-cal properties as a function of the chemical composition of the backbone in BTD-based derivatives. The chemical structure of model polymers is shown in Fig. 7.16.

In the same year, Seo et al. [108] demonstrated hybrid white organic light-emitting diodes (HWOLEDs) on EL characteristics for codoped spacer ratio effect using N,N′-dicarbazolyl-3,5-benzene (mCP) and 4,7-diphenyl-1,10-phenanthroline (BPhen). They achieved external quan-tum efficiency of 6.01%, power efficiency of 8.12 lm/W, and CIE coordi-nates of 0.37, 0.41 at 1000 cd/m2. In 2011, Wang explored the relationship between the electronic properties of a host/dopant system and obtained a high-efficiency single-dopant white polymer light-emitting device with two novel blue-emitting cyclometalated iridium (III) complexes of (dfppy)2Ir(Tfl-pic) and (dfppy)2Ir(Brfl-pic) where dfppy is 2-(2,4-difluo-rophenyl)pyridine, Tfl-pic, and Brfl-pic are picolinic acid derivatives con-taining trialkylfluorene and dibromoalkylfluorene units bridged with an alkoxy chain, respectively. Both iridium (III) complexes exhibited blue emission in dichloromethane solution and their neat films and possessed good dispersibility and thermal properties. Two different devices using (dfppy)2Ir(Tfl-pic) as a single-component emitter and a blend of poly(N-vinylcarbazole) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadia-zole as the host matrix were fabricated [109]. Han et al. fabricated flexible OLEDs by modifying the graphene anode to have high work function and low sheet resistance. They achieved extremely high luminous efficien-cies of 37.2 lm/W in fluorescent OLEDs and 102.7 lm/W in phosphores-cent OLEDs [110]. Semiconductor-based light sources with high energy


efficiencies are critical technologies for reducing the global carbon foot-print. In particular, white OLEDs have received huge worldwide attention in recent years, partially due to their success in the flat-panel display mar-ket and features such as a unique thin, flat, foldable form factor. An over-view on the current status of OLEDs for lighting applications is very well depicted by Chang et al. in 2013. Furthermore, a detailed overview of the state-of-the-art on white OLED design concepts including their working principles was presented [111]. A double-layered graphene/poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conduc-tive film was prepared, in which the PEDOT:PSS layer was obtained by using a spray-coating technique. Flexible white phosphorescent OLEDs based on the graphene/PEDOT:PSS conductive film was fabricated.

Figure 7.16 (A) Chemical structure of model polymer (PF)n–(OMC-CH3)m and (B) investigated BTD-based derivatives (OMC and its derivatives) [107].


Phosphorescent material tris(2-phenylpyridine) iridium (Ir(ppy)3) and the fluorescent dye 5,6,11,12-tetraphenylnapthacene (Rubrene) were codoped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine(NPB) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as HTL and ETL, respectively, and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) was used as blue-light-emitting layer.

The device presented pure white-light emission with CIE coordi-nates 0.31, 0.33 and exhibited excellent light-emitting stability during the bending cycle test with a radius of curvature of 10 mm [112]. The device configuration of the flexible phosphorescent WOLEDs based on grapheme/PEDOT:PSS conductive film and the chemical structures of the organic materials used in this work are shown in Fig. 7.17A and B, respectively. In 2015, Sakumaet et al. proposed a self-layered technique to form an emitting layer with a pseudo-multilayered structure by one-step coating and demonstrated the feasibility of the concept [113]. They also fabricated a highly efficient WOLED with the proposed technique. A maximum power efficiency of 70 lm/W was obtained by improving the effective radiation efficiency, carrier balance efficiency, and light-extraction efficiency. White-emissive devices with a dual-emitting layer based on the orange and blue (FIrPic) phosphor were recently fabricated by Zhang et al. They showed CIE coordinates 0.33, 0.41 and 0.31, 0.40 and maxi-mum current efficiencies of 8.9 and 13.8 cd/A [114]. Current develop-ment of the synthesis of single-layer graphene and its future prospects has been reported in many reviews [115].

7.7 CONCLUSIONS

Organic light-emitting materials have been attracting the attention of researchers from industry and academic institutions due to their appli-cations in OLED devices, flat-panel displays, and solid-state lighting. Although OLED has the potential to redefine many present-day lighting and display solutions, there are a few hurdles to overcome. Fabrication of highly reliable, efficient, and long-life blue OLEDs is still challenging, due to the difficulty in aligning the energy levels at the layer interfaces. Thus manufacturing processes are expensive and simpler and cheaper technol-ogy has to be developed in order to commercialize them. Reliable testing standards are also needed to establish consistency and to reduce uncer-tainty. Researchers should concentrate on improving various factors such


as electricity-to-light conversion efficiency, device stability and lifetime, material selection, proper encapsulation, methods to maintain unifor-mity over large areas, novel fabrication technologies, and reducing man-ufacturing costs. The literature on RBG and white light-emitting device

Figure 7.17 (A) Device configuration of flexible phosphorescent WOLEDs based on grapheme/PEDOT:PSS conductive film. (B) Chemical structures of the organic materials used in this work [112].


architectures reveals various approaches to enhancing the efficiency and lifetime of OLEDs and handling the degradation issues of the organic materials for OLEDs. If we succeed in improving the efficiency, perfor-mance, and lifetime, current lighting systems can be replaced by eco-friendly, energy-efficient green technology called solid-state lighting, which would play a significant role in reducing global energy consump-tion. Cutting-edge research predicts a bright future for display devices as the next generation of light sources for general illumination, from homes to commercial applications, offering low energy consumption and reduced maintenance. As discussed in this chapter, OLEDs can pave the way for a new era of large-area lighting, which is transparent, flexible, and environ-mentally friendly.

REFERENCES [1] J.-H. Kwon, R. Pode, Organic Light Emitting Diode: Material Process and Devices,

InTech, Rijeka, Croatia, 2011, pp. 101–146. ISBN 978-953-307-273-9. [2] M. Pope, H.P. Kallman, P. Magnante, J. Chem. Phys. 38 (1963) 2042–2043. [3] W. Helfrich, W.G. Schneider, Phys. Rev. Lett. 14 (1965) 229–231. [4] S.A. VanSlyke, C.W. Tang, Organic electroluminescent devices having improved power

conversion efficiencies. US Patent 4539507, 1985. [5] C.W. Tang, C.H. Chen, R. Goswami, Electroluminescent device with modified thin

film luminescent zone. US Patent 4769292, 1988. [6] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160–2162. [7] Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yonemoto, et al., Appl.

Phys. Lett. 65 (1994) 807–809. [8] Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999)

2227–2279. [9] V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W.

D’Andrade, et al., Org. Electron. 4 (2–3) (2003) 77–87. [10] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 151–153. [11] J. Kido, Y. Lizumi, Appl. Phys. Lett. 73 (1998) 2721–2723. [12] J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, S. Liu, Appl. Phys. Lett. 80

(2002) 139–141. [13] J. Dresner, RCA Rev. 30 (1969) 322. [14] F.J. Kampas, M. Gouterman, Chem. Phys. Lett. 48 (1977) 233. [15] J. Kalinowski, J. Godlewski, Z. Dreger, Appl. Phys. A 37 (1985) 179. [16] F.J. Bryant, A. Krier, Phys. Status Solidi A 81 (1984) 681. [17] J. Kido, K. Nagai, Y.O. Kamoto, T. Skotheim, Appl. Phys. Lett. 59 (1991) 2760. [18] J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 192 (1993) 30. [19] J.C.G. Bunzli, E. Moret, V. Foiret, K.J. Schenk, M.Z. Wang, L.P. Jin, J. Alloys Compd.

107 (1994) 207–208. [20] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994) 17. [21] T. Sano, M. Fujita, T. Fujii, Y. Hamada, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34

(1995) 1883. [22] Y. Miyamoto, M. Uekawa, H. Ikeda, K. Kaifu, J. Lumin. 81 (1999) 159. [23] L. Huang, K.-Z. Wang, C.-H. Huang, F.-Y. Li, Y.-Y. Huang, J. Mater. Chem. 11 (2001)

790–793.































[24] X. Chen Lin, L. Cheng, C. Lee, W.A. Gambling, S. Lee, J. Phys. D 34 (2001) 30–35. [25] M.J. Yang, Q.D. Ling, W.Q. Li, Y. Wang, R.G. Sun, Q.B. Zheng, et al., J. Mater. Sci. Eng.

B B85 (2001) 100. [26] B. Ma, Z. Zhang, P. Liang, X. Xie, B. Wang, Y. Zhang, J. Mater. Chem. 12 (2002)

1671–1675. [27] P.-P. Sun, J.-P. Duan, H.-T. Shih, C.-H. Chenga, Appl. Phys. Lett. 81 (2002) 792. [28] D. Gao, Z. Biana, K. Wang, L. Jin, C.-H. Huang, J. Alloys Compd. 358 (2003) 188–192. [29] J.-P. Duan, P.-P. Sun, C.-H. Cheng, J. Mater. 1 (2005) 1–11. [30] B. Qu, Z. Chen, Y. Liu, H. Cao, S. Xu, S. Cao, J. Phys. D 39 (2006) 2680–2683. [31] Z. Yanfei, X. Zheng, L. Yuguang, Z. Fujun, W. Yong, Z. Jingchang, J. Rare Earths 25

(2007) 143–147. [32] G.Y. Park, Y. Ha, Synth. Met. 158 (2008) 120–124. [33] K.-U. Haq, L. Shan-Peng, M.A. Khan, X.Y. Jiang, Z.L. Zhang, J. Cao, et al., Curr. Appl.

Phys. 9 (2009) 257–262. [34] Y.-Y. Lyu, J. Kwak, W.S. Jeon, Y. Byun, H.S. Lee, D. Kim, Adv. Funct. Mater. 19 (2009)

420–427. [35] T. Li, W. Li, X. Li, B. Chu, Z. Su, L. Han, et al., J. Lumin. 130 (2010) 1676–1679. [36] N.T. Kalyani, S.J. Dhoble, R.B. Pode, Adv. Mater. Lett. 2 (1) (2011) 65–70. [37] B. Chen, Y. Li, Y. Chu, A. Zheng, J. Feng, Z. Liu, et al., Org. Electron. 14 (2013)

744–753. [38] F. Dumura, M. Lepeltier, H.Z. Sibonic, P. Xiao, B. Graff, J. Lalevée, et al., J. Lumin. 151

(2014) 34–40. [39] M. Song, S.-J. Yun, K.-S. Nam, H. Liu, Y.-S. Gal, J.W. Lee, et al., J. Organomet. Chem.

794 (2015) 197–205. [40] L. Zhou, Y. Jiang, R. Cui, Y. Li, X. Zhao, R. Deng, et al., J. Lumin. 170 (2016) 692–696. [41] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [42] J. Kido, K. Nagai, Y. Okamoto, Chem. Lett. 220 (1990) 657. [43] R.K. Kasim, S. Satyanarayand, R.L. Elsenbaumer, Synth. Met. 102 (1999) 1059. [44] D. Ma, D. Wang, B. Li, Z. Hong, S. Lu, L. Wang, et al., Synth. Met. 102 (1999) 1136. [45] Q. Lin, C.Y. Shi, Y.J. Liang, Y.X. Zheng, S.B. Wang, H. Zhang, Synth. Met. 114

(2000) 373. [46] J.P.J. Markham., S.-C. Lo, S.W. Magennis, P.L. Burn, I.D.W. Samuel, Appl. Phys. Lett.

80 (2002) 2645. [47] S.C. Lo, N.A.H. Male, J.P.J. Markham, S.W. Magennis, P.L. Burn, O.V. Salata, et al., Adv.

Mater. 14 (2002) 975. [48] T.D. Anthopoulos, J.P.J. Markham, E.B. Namdas, I.D.W. Samuel, S.-C. Lo, P.L. Burn,

Appl. Phys. Lett. 82 (2003) 4824. [49] L.C. Palilis, H. Murata, M. Uchida, Z.H. Kafafi, Org. Electron. 4 (2) (2003) 113–121. [50] A. Mishra, P. Kumar, L. Kumar, Indian J. Eng. Mater. Sci. 12 (2005) 321–324. [51] S.-W. Hwang, C.H. Chen, Appl. Phys. Lett. 86 (2005) 093508. [52] S.-Y. Ku, L.-C. Chi, W.-Y. Hung, S.-W. Yang, T.-C. Tsai, K.-T. Wong, et al., J. Mater.

Chem. 19 (2009) 773–780. [53] Z. Liu, Z. Bian, F. Hao, D. Nie, F. Ding, Z. Chen, et al., Org. Electron. 10 (2) (2009)

247–255. [54] M.J. Cho, J.-I. Jin, D.H. Choi, J.H. Yoon, C.S. Hong, Y.M. Kim, et al., Dyes Pigm. 85

(2010) 143–151. [55] L. He, L. Duan, J. Qiao, D. Zhang, L. Wang, Y. Qiu, Org. Electron. 11 (7) (2010)

1185–1191. [56] S.-J. Kim, Y. Zhang, C. Zuniga, S. Barlow, S.R. Marder, B. Kippelen, Org. Electron. 12

(2011) 492–496. [57] H. Li, Y.-M. Yin, H.-T. Cao, H.-Z. Sun, L. Wang, G.-G. Shan, et al., J. Organomet.

Chem. 753 (2014) 55–62. [58] Y. Seino, S. Inomata, H. Sasabe, Y.-J. Pu, J. Kido, Adv. Mater. 28 (13) (2016) 2638–2643.























































[59] X.-Q. Zhang, Y.-M. Xie, Y. Zheng, F. Liang, B. Wang, J. Fan, et al., Org. Electron. 32 (2016) 120–125.

[60] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992) 36. [61] J. Kido, C. Ohtaki, K. Hongawa, K. Okuyama, K. Nagai, Jpn. J. Appl. Phys. 32 (2) (1993)

L917. [62] D. Oelkrug, A. Tompert, H.-J. Egelhaaf, M. Hanack, E. Steinhuber, M. Hohloch, et al.,

Synth. Met. 83 (1996) 231. [63] E. Thorn-Csányi, P. Kraxner, Macromol. Chem. Phys. 198 (1997) 3827. [64] M. Halim, J.N.G. Pillow, I.D.W. Samual, P.L. Burn, Adv. Mater. 11 (1999) 371. [65] V. Gebhardt, A. Bacher, M. Thelakkat, U. Stalmach, H. Meier, H.-W. Schmidt, et al.,

Synth. Met. 90 (1997) 123. [66] J.L. Segura, N. Martin, M. Hanack, Eur. J. Org. Chem. 3 (1999) 643–651. [67] G. Leising, E.J.W. List, S. Tasch, C. Brandstätter, W. Graupner, P. Markart, et al.,

Proc. SPIE 3476 (1998) 76. [68] K.H. Weinfurtner, F. Weissörtel, G. Harmgarth, J. Salbeck, Proc. SPIE 3476 (1998) 40. [69] X.T. Tao, H. Suzuki, T. Wada, S. Miyata, H. Sasabe, J. Am. Chem. Soc. 121 (1999) 9447. [70] W.L. Li, Z.Q. Gao, Z.Y. Hong, C.S. Lee, S.T. Lee, Synth. Met. 111 (2000) 53. [71] Z. Hong, W. Li, D. Zhao, C. Liang, X. Liu, J. Peng, et al., Synth. Met. 104 (2000) 165. [72] Y. Liu, J. Guo, J. Feng, Y. Wang, Appl. Phys. Lett. 78 (2001) 2300. [73] T. Miteva, A. Meisel, W. Knoll, H.G. Nothofer, U. Scherf, D.C. Muller, et al., Adv. Mater.

13 (2001) 565. [74] K.H. Weinfurtner, H. Fujikawa, S. Tokito, Y. Taga, Appl. Phys. Lett. 76 (2000) 2502. [75] S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, et al., Adv. Mater. 17

(2005) 285–289. [76] G. Cheng, Z. Xie, Y. Zhang, H. Xia, Y. Ma, S. Liu, Semicond. Sci. Technol. 20

(2005) 23–25. [77] B. Ding, Curr. Appl. Phys. 8 (2008) 523–526. [78] T. Zheng, C.H.W. Choy, Adv. Funct. Mater. 20 (2010) 648–655. [79] K.-U. Haq, M.A. Khan, X.W. Zhang, L. Zhang, X.Y. Jiang, Z.L. Zhang, J. Non-Cryst.

Solids 356 (2010) 1012–1015. [80] Y.-J. Pu, A. Kamiya, K.-I. Nakayama, J. Kido, Org. Electron. 11 (2010) 479–485. [81] K.H. Lee, J.N. You, S. Kang, J.Y. Lee, H.J. Kwon, Y.K. Kim, et al., Thin Solid Films 518

(22) (2010) 6253–6258. [82] C. Caia, S.-J. Su, T. Chiba, H. Sasabe, Y.-I. Pu, K. Nakayamaa, et al., Org. Electron. 12

(5) (2011) 843–850. [83] Y.-H. Chen, S.-L. Lin, Y.-C. Chang, Y.-C. Chen, J.-T. Lin, R.-H. Lee, et al., Org.

Electron. 13 (1) (2012) 43–52. [84] M.-J. Kim, C.-W. Lee, M.-S. Gong, Dyes Pigm. 105 (2014) 202–207. [85] J.-H. Chang, W.-H. Lin, P.-C. Wang, J.-I. Taur, T.-A. Ku, W.-T. Chen, et al., Sci. Rep.

5 (2015) 1–6. [86] J. Zhu, N. Yin, L. Feng, Dyes Pigm. 132 (2016) 121–127. [87] W. Brian, D. Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585. [88] G. Cheng, F. Li, Y. Duan, J. Feng, S. Liu, S. Qiu, et al., Appl. Phys. Lett. 82 (2003) 4224. [89] Y. Zhang, G. Cheng, Y. Zhao, J. Hou, S. Liu, Appl. Phys. Lett. 86 (2005) 011112. [90] S. Tokito, T. Iijima, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 2459. [91] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503. [92] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, et al., Nature 459

(2009) 234. [93] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585. [94] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 (1994) 815. [95] S. Tao, Y. Jiang, S.L. Lai, M.K. Fung, Y. Zhou, X. Zhang, Org. Electron. 12 (2011)

358–363.






















































[96] J. Kido, M. Kaimura, K. Nagai, Science 267 (1995) 1332. [97] R.S. Deshpande, V. Buloviæ, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 888–890. [98] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Adv. Mater. 14 (2002) 147–151. [99] J. Xiao, Z.-B. Deng, C.-J. Liang, D.-H. Xu, Y. Xu, Displays 26 (2005) 129–132. [100] V.K. Rai, R. Srivastava, M.N. Kamalasanan, Synth. Met. 159 (2009) 234–237. [101] C.-C. Tsou, H.-T. Lu, M. Yokoyama, J. Cryst. Growth 280 (2005) 201–205. [102] X. Gong, S. Wang, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Mater. 17 (2005)

2053–2058. [103] H.J. Yu, K. Park, W. Chung, J. Kim, S.H. Kim, Synth. Met. 159 (2009) 2474–2477. [104] C.-H. Chang, C.-C. Chen, C.-C. Wu, S.-Y. Chang, J.-Y. Hung, Y. Chi, Org. Electron.

11 (2010) 266–272. [105] P. Tyagi, R. Srivastava, A. Kumar, V.K. Rai, R. Grover, M.N. Kamalasanan, Synth. Met.

160 (7-8) (2010) 756–761. [106] W. Chen, L. Lu, J. Cheng, Optik 121 (2010) 107–112. [107] B. Hu, J. Zhang, Y. Chen, Eur. Polym. J. 47 (2011) 208–224. [108] J.H. Seo, J.S. Park, S.J. Lee, B.M. Seo, K.H. Lee, J.K. Park, et al., Curr. Appl. Phys. 11

(3) (2011) 564–567. [109] N. Wang, Y. Liu, Z. Zhang, J. Luo, D. Shi, H. Tan, et al., Dyes Pigm. 91 (3) (2011)

495–500. [110] T.H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.H. Bae, B.H. Hong, et al., Nat. Photonics

6 (2012) 105–110. [111] Y.-L. Chang, Z.-H. Lu, J. Disp. Technol. 9 (6) (2013) 459–468. [112] X. Wu, F. Li, W. Wu, T. Guo, Appl. Surf. Sci. 295 (2014) 214–218. [113] H. Sakuma, M. Harada, H. Wakana, S. Nobuki, M. Kawasaki, S. Aratani, J. Soc. Inf.

Disp. 23 (12) (2015) 587–592. [114] W. Zhang, C. Liang, Z. He, H. Pang, Y. Wanga, S. Zhao, Synth. Met. 215 (2016)

95–103. [115] H.C. Lee, W.-W. Liu, S.-P. Chai, A.R. Mohamed, C.W. Lai, C.-S. Khe, et al., Procedia

Chem. 19 (2016) 916–921.
































http://dx.doi.org/10.1016/B978-0-08-101213-0.00008-4

CHAPTER 8

History of Organic Light-Emitting Diode Displays

8.1 INTRODUCTION

Displays are rooted deeply in history. Since the Indus-valley civilization in 3000 BC, displays such as portraits, picturesque, engravings, rock paint-ings, scroll paintings, wall paintings, etc., have been developed. Studying these artforms help us understand the past, which has resulted in sweeping changes in the field of display device technology. Today, consumers require ecofriendly displays that can be operated at any place and any time. From the cathode ray tubes (CRTs) of a few decades ago to today’s flat-panel displays (FPDs), our increasing ability to visualize more information, more quickly, and over longer distances has expanded the boundaries of tech-nological development. FPD technology derived from unique thin-film architecture, which requires different specifications for layer properties and manufacturing process steps, took the place of CRT technology and has literally revolutionized the display industry by providing higher per-formance and more reliable display devices with ever-decreasing cost. The 21st century is known as the digital age because access to information and communication is available to almost everyone, and digital devices with high performance have been created and have flourished. Today, these dis-play devices have become an integral part of modern-day communica-tion infrastructure and can be found along roads, in buildings, hospitals, and machinery. Technological breakthroughs in the field of displays started with CRTs and later lead to the development of vacuum fluorescent dis-plays (VFDs), incandescent filament displays (IFDs), field emission displays (FEDs), surface-conduction electron-emitter displays (SEDs), liquid crystal displays (LCDs), and plasma display panels (PDPs) [1]. The attributes of any display device include: High resolution (5 μm pixels) High visibility under any brightness level and viewing angle High luminance (300 cd/m2) High-quality and refined visual image


Large color gamut Fast switching response (~nano seconds) Tunable colors (millions) High brightness and contrast Very slender in thickness (<1 μm) Light in weight Low driving voltage, which is indispensable for battery-driven portable

equipment Lost cost Appreciable lifetime (kilo hours)

Displays differ from one another based on the structure and working mechanism involved in their operation (either photoluminescence or elec-troluminescence). Current display technologies like CRT, LCD, and SED have their own characteristics and none have been able to meet all the above features. As the demand for small display devices used in portable equipment is high, development of superior display devices is crucial. Today, research on display technologies is focused on two initiatives: (1) Flexible displays built on plastic or metal foils rather than on rigid glass or silicon wafers and (2) organic devices that feature polymeric or small-molecule compounds with innovative conducting or insulating properties [2]. While LED display tech-nology has already been integrated into everyday life, these displays remain economically inefficient due to their high process costs and due to the fact that they are also limited in shape flexibility because of their brittle struc-tures. They are also tiny light points that have restrictions in large-area dis-play applications. The newest lighting technology, OLED displays, are thin, lightweight with high design flexibility as well as energy efficient and eco-friendly. But further improvements in efficiency, level of luminance, driving voltage, operation costs, lifespan, and light-extraction efficiency are still seen as major challenges in this field of research.

8.2 DISPLAYS

Displays are an important part of communication today [3]. Displays gen-erally consist of a projection screen and a device that produces the infor-mation on the screen. Displays can use analog signals as input. The basic parameters of displays include the following. Screen size

The width of the display screen relative to its height is known as the aspect ratio. Screen sizes are measured in either millimeters or inches diagonally from one corner to the opposite corner.

History of Organic Light-Emitting Diode Displays 207

Color capabilityColor capability includes screen size and the number of colors it

can display. Displays can be operated in one of several display modes that determine the number of bits used to describe the color as well as the number of colors that can be displayed on the screen.

SharpnessScreen resolution and the physical screen size determine the actual

sharpness of a display image, which is measured in dots-per-inch. The absolute physical limitation on the potential image sharpness is the dot pitch, which is the size of an individual beam that gets through, to light up the phosphor on the screen. Displays typically come with a dot pitch of 0.28 mm or smaller. The smaller the dot pitch, the greater the sharpness of the image.

Viewing angleThe viewing angle is a measure of the maximum angle at which a

display can be viewed with acceptable visual performance. When dis-plays are viewed off-axis, colors begin to appear washed out, inverted, or plunge in brightness. Displays with wide viewing angles of 170–178 degrees are desirable as they sustain high image quality. The viewing angle includes the ability to see the screen image well from different angles. FPDs, including those using light-emitting diode (LED) and LCD technology, are often harder to see at angles other than directly.

Projection technologyThe projection surface or screen is the most important part of a

display device. In CRT technology a certain distance from the beam-projection device to the screen is required for its smooth function. The screen of a CRT is a cylindrical curve, while modern LCDs, PDPs, LEDs, and OLEDs employ flat screens and are called FPDs.

8.3 DISPLAY DEVICE

Although we are surrounded by the wonders of nature, it’s impossible for humans to see everything. Display devices allow us to experience the won-ders of our planet no matter where we are and to have visual representa-tions of data. They bridge the gap between electronic devices and humans. Display devices simplify information sharing. There are various approaches to capturing an image signal from an imaging device and displaying the pixel data. When the input information is supplied as an electrical signal, the display is called an electronic display. Display devices are generaly classified into two different categories: active-display and passive-display devices.


8.3.1 Active-Display DevicesA device that emits light on its own when power is supplied is known as active-display device. For example, CRT displays, vacuum fluorescent displays, surface conduction electron emitter displays, LED displays, and OLED displays come under this class.

8.3.2 Passive-Display DevicesPassive-display devices are nonemissive display devices that can modify or modulate the parameters of incident light generated by the external source to display information. Optical parameters such as optical path, path length, absorption, reflection, scattering, or a combination of these parameters can be modified to achieve the desired results. Each modula-tion has its own effects. For example, if the wavelength is modulated, then the display color alters. LCDs fall under this class. Active display and pas-sive display devices are compared in Table 8.1.

8.4 DISPLAY TERMINOLOGY

Some of the important terminology used in this field includes the following. Aspect ratio

The width of a display screen relative to its height is known as the aspect ratio. It is an image projection attribute that portrays the pro-portional relationship between the width and height of an image. It is expressed as two numbers separated by a colon (x:y). Traditional televi-sion and computer displays are designed for an aspect ratio of 1.33:1, which means that the width of the display area is only 1.33 times the height. High-definition television (HDTV) displays have a widescreen format with an aspect ratio of 16:9. Most displays have an aspect ratio of 5:4, 4:3, 16:10, or 16:9.

Table 8.1 Active vs. passive display devicesS. no. Active-display device Passive-display device

1. Are emissive devices Are nonemissive devices2. Do not require any backlight Require backlight3. Do not have any light loss Have light losses and are backlit by a

light source4. Operating voltage is lower Operating voltage is higher5. Higher lifetime Limited lifetime


PixelA pixel, also known as a picture element, is the smallest address-

able and controllable element of a picture represented on the screen of a display device. The address of a pixel corresponds to its physical coordinates. Generally pixels are manufactured in a 2D grid and rep-resented as dots or squares. Each pixel is a sample of an original image [4] and the intensity of each pixel is variable. Fig. 8.1 demonstrates a single pixel and pixels arranged in a display panel.

Color gamutThe color gamut is a measure of how broad a range of colors a

display can express, and is typically measured as a percentage of the NTSC color space. A typical color gamut ranges from 72% to 105%. Gamut refers the complete range; hence the color gamut of a dis-play is the complete range of colors available on the display device. Color gamut is often given by the x, y chromaticity diagram of the XYZ color system established by the International Commission on Illumination (CIE diagram).

Contrast ratioThis is the measure of the luminance of the brightest color (white)

to that of the darkest color (black) a display can produce. Contrast ratio is an important image-quality attribute and affects our ability to per-ceive image brightness and image detail.

Display resolutionDisplay resolution is the measure of the horizontal and vertical pix-

els on a display. The density of these pixels determines the perceived image quality of the display.

Figure 8.1 (A) Single pixel. (B) Pixels arranged in a display panel.


LuminanceLuminance is a measure of the amount of light a monitor produces.

It is a key component for perceived image quality. Displays with lower brightness can look soft and washed out especially at a distance; hence a higher brightness level is preferred.

Refresh rateThe refresh rate of a display is a measure of the number of times

the display presents the data per second, given by the video source. Typical refresh rates for LCDs are 60, 120, or 240 Hz. The faster the refresh rate, the less flicker is perceived, this provides smoother, more fluid motion.

Response timeThe response time of a display is the time taken to respond or to

complete a full black to white transition. Typical response times for LCDs are 8–16 ms for black-white-black and 2–6 ms for gray-to-gray. The faster the response time the less blur there is through video transitions.

Display portThe display port is a digital display interface used to connect a

video source to a display device such as a computer monitor. It can also be used to transmit audio, USB, and other forms of data.

Display colorsThe total number of display colors is a measure of how finely levels

of color can be expressed by a display. This depends on the bit depth of each pixel. Virtually all displays form images by varying the strength of the three primary colors: red, green, and blue. For example, bright yel-low is produced by approximately equal red and green contributions, with modest or no blue contribution.

LifetimeLifetime is defined as the time required for the emission to be

reduced by half its initial value. Lifetime is generally severely limited if the device is exposed to humidity and oxygen [5,6].

Screen burn-inScreen burn-in, also known as image burn-in, is a permanent dis-

coloration of areas on an electronic display due to cumulative nonuni-form use of the pixels.

BacklightDisplay devices such as LCDs are known as passive devices because

they are not capable of emitting light on their own; hence they need


backlight in order to project the image onto the screen. Devices such as LEDs and OLEDs are capable of emitting their own light.

EncapsulationAs OLEDs are extremely sensitive to moisture, nonemissive dark

spots develop initially and continuously grow with time, leading to deg-radation in devices and displays. In order to avoid this, these devices and displays must be encapsulated. Encapsulation is the process of bonding a metal sheet onto the substrate glass using UV-cured epoxy [7,8].

8.5 DISPLAY CATEGORIZATION

Based on the size of the screen, displays are generally categorized as micro, small, medium, large, and superlarge screen displays. Microdisplays

These displays are very small, usually <1 inch diagonal. They cannot be viewed directly and hence need optical magnification, high resolu-tion, and brightness. These displays find applications in camera view-finders and as light valves for projection systems.

Small displaysThese displays are common as they are 1–8 inch diagonally, and are

used in digital cameras, pagers, cell phones, and video-phone applica-tions. They are expensive and thus simple manufacturing methods are needed for cost reduction.

Medium displaysThese displays are 8–30 inch diagonally, and are used for desktop

and laptop computers. Large displays

These displays are 30–50 inch diagonally, and are widely used for high-end televisions, home theaters, large displays, for education and advertisements, etc.

Superlarge displaysThese displays are >50 inch diagonally. They are used in high-defi-

nition TVs, video walls, etc.

8.6 HISTORY OF DISPLAY TECHNOLOGY

Since the commercialization of the CRT in 1922, this technology has domi-nated the display industry [9]. However, new trends such as mobile electron-ics have increased demand for displays that rival and surpass CRTs in areas


such as picture quality, size, and power consumption. Today, screens play a vital role in everyday life, in the form of calculators, video games, wristwatches, mobile phones, cameras, televisions, computers, laptops, and many other small screened electronics. Hence, there is need for devices capable of offering high brightness, contrast, high-quality video images, improved color variation and resolution, high performance, low power consumption, and lighter in weight. The history of display devices is discussed in the following sections.

8.6.1 Cathode Ray TubesElectronic display devices began with the CRT, the oldest and cheapest electronic display technology, which is a specially constructed vacuum tube mainly used for television. It operates at any resolution, geometry, and aspect ratio without the need to rescale the image. It works on the principle of cathodo-luminescence, and consists of three main parts: an electron gun to emit a beam of electrons, a deflection system to deflect the beam of electrons along the length of the CRT, and a fluorescent screen to display the image. In principle, CRTs use a beam of electrons to scan rows of phosphors line-by-line in a sequential manner to produce an image, and are also known as beam scan devices. The interior surface of a CRT is coated with a thin translucent layer of phosphors, which absorbs the energy of the incident electrons and causes a glow at the point where the high-energy electron beam strikes. Generally, phosphors with lifetime >20,000 hours operating lifetime are preferred. At that point, the coating continues to glow for a short period even after the electron beam moves away. The phosphors coating is made thin to allow the light to pass through the screen material and glass shell so that it can be viewed from outside the CRT. In a color TV the colored images are produced by elec-tron beams from three separate electron guns impinginging from slightly different angles onto a triad arrangement of phosphor stripes, each of which emits one of the primary colors, red, green, and blue, as shown in Fig. 8.2. The width of the stripes is so small (<300 µm) that they cannot be resolved at normal viewing distances. A CRT can be an electrostatic type or a magnetostatic type. In an electrostatic type CRT, the electron beam is deflected by the application of a positive voltage on the plates as the beam passes through and bends toward the positive plate. It is quite effective only for smaller display devices like oscilloscope displays, while in magnetostic type CRT, electron beam is deflected by the application of a magnetic field and bends towards the resultant Lorentz force. It is bulky


as the magnets are placed outside CRT and quite effective for larger dis-play devices like TV and computer screens. This technology offers large viewing angle, high brightness, high resolution, and good color gamut. Furthermore, CRTs are self-emissive, robust, cheap, and offer high-quality picture, wide viewing angle, and display sizes between 20 and 40 inch But they consume more power and are not portable. They maintained 70% of the display market until 2000.

8.6.2 Vacuum Fluorescent DisplaysVacuum fluorescent displays (VFDs) are the most primitive flat technol-ogy displays and use matrix addressing; they can display seven-segment numerals and multisegment alpha-numeric characters. They can also use dot matrix to exhibit different alphanumeric characters and symbols. They work on the principle of cathodo-luminescence. They consists of a vacuum tube with three basic types of electrodes namely the anode, and grid. When filament is heated to a temperature just below incandescence, it emits electrons but remains virtually invisible. When a positive voltage

Figure 8.2 Schematic of cathode ray tube.


is applied to the grid and the anodes, the electrons emitted by the cath-ode filament are accelerated and attracted to the positive anode segments. A transparent metal mesh grid covers each digit and controls the elec-trons emitted from the filament toward that digit. Seven phosphor-coated anodes, arranged in the seven-segment design, glow when struck by the electrons [10]. If the grid has a negative potential then it will block the electrons from passing regardless of the potential of the anodes under the grid. In spite of good luminance, excellent viewing angle, low cost, and long life, they suffer from mechanical complexity, low resolution, and con-sume high filament power. The display solely depends on the shape of phosphor resting on the anode.

8.6.3 Incandescent Filament DisplaysAn incandescent filament display (IFD) is typically a seven-segment dis-play enclosed in a vacuum tube, where each display segment is formed with a conductive anode tungsten filament. When suitable voltage is applied between the electrodes, the electrons are emitted by the cathode filament and are attracted to the positive anode. They emit a yellowish-white light that can be filtered to any desired color. The filament voltage (3–5 V DC) can also be varied to change the brightness level of the display. The major disadvantages of these displays include high current consump-tion and slow response time.

8.6.4 Field Emission DisplaysField emission displays (FEDs) have attracted significant attention. They use large-area field-electron emission sources to provide electrons that strike colored phosphor to produce a color image as a display. In this type of display electrons from millions of tiny microtips pass through gates and light up pixels on a screen. Cathode – a negative electrode is a matrix of row and column traces. Each crossover has up to 4500 emit-ters, 150 nm in diameter; this lays the foundation for addressable cathode emitters. With the application of voltage to both rows and columns, elec-trons are generated by the emitters. Pixels are formed by depositing and patterning a black matrix, standard red, green, and blue TV phosphors, and a thin aluminum layer to reflect light toward the viewer. As the dis-tance between the cathode and screen is quite small (around 0.2–5 mm), the screen should be mechanically reinforced by placing spacer strips or posts between the front and back face of the tube [11]. FEDs are a type


of matrix display with very fast response times, wide viewing angle, high brightness, high resolution, high contrast levels, high luminous efficiency, and excellent color gamut. They are self-emissive, distortion-free dis-plays and hence backlighting system and thin-film transistor active matrix are not needed, which greatly reduces the complexity of the display as a whole [12]. This technology offers cold cathode type surface emission and consumes low power. It combines the advantages of CRTs with the packaging advantages of LCD and other flat-panel technologies. However, FEDs require high vacuum levels to operate, which is time consuming as well as difficult to attain and hence requires sealing. Another serious obstacle to the development of full-color FEDs is the lack of suitable blue luminescent materials with high luminance, high efficiency, good chroma-ticity, low saturation, and good aging properties at low electron accelera-tion voltages ≤10 kV and high current densities 10–100 A/cm2. Intense electron bombardment of the phosphor layer will also cause gas release during use, which is harmful [13].

8.6.5 Surface-Conduction Electron-Emitter DisplaysThis is a display technology that uses nanoscopic-scale electron emit-ters to energize colored phosphors and produce an image. It consists of a matrix of tiny CRTs, with each tube forming a single subpixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. Surface-conduction electron-emitter display (SEDs) are very similar to FEDs. The primary difference between the two technologies is that SEDs use a sin-gle emitter for each column instead of individual spots, whereas FEDs use electrons emitted directly toward the front of the screen [14]. They proffer advantages like swift response time, high contrast ratio, and wide view-ing angle, and they consume much less power than an LCD display of the same size. The newly developed surface-emitting type has organic multi-layer structures with many features including: High visibility (self-emitting type) High level of brightness (surface-emitting type), thin body (~4 mm

thickness) No requirement of back up light, quick response time (>1000 times

faster than LCD) Can be used in tough temperature conditions Display size: 5.2 inch Pixel pitch: 0.33 mm


Driving method: Simple matrix Driving duty ratio: 1/120 Emitting colors: 260,000 (64 color tones for each color) Luminance: 150 cd/m2

Contrast: More than 100:1 (with filter) Power consumption: 6 W (100% emitting) Operating life: about 2000 hours (after which brightness reduces by half)

8.6.6 Liquid Crystal DisplaysLiquid crystal is a kind of substance that lies in an intermediate state between a conventional liquid and solid state. Liquid crystals are light-polarizing substances that appear as gel and can be modified by the application of an electric field [15]. They are crystalline solids at low tem-peratures and clear isotropic liquids at high temperatures. Liquid crystals exhibit three different types of states: (1) nematic state, in which molecules are arranged parallel to each other, and are free to move due to the liq-uid properties; (2) cholesteric state, in which the molecules are optically active and hence, have the ability to rotate the plane of polarization of light; and (3) smectic state, in which the molecules are arranged parallel to each other with irregular spacing. In most optic devices the first two states are used for practical display applications. A LCD is based on two principles: (1) polarization—the process of filtering certain unwanted light wavelengths by using glass with parallel thin impenetrable black lines and (2) twisted nematic property—a positive nematic with parallel alignment of molecules with respect to the field direction.

LCDs are constructed in the form of sandwich cells, which con-sist of two conducting glass plates made of SnO2. Both sides of the plate are coated with In2O3, each attached with a crossed polaroid as shown in Fig. 8.3. The distance of separation between the coating surfaces is typi-cally 10 μm. A thin layer of liquid crystal of the order of 3–50 μm is sealed

Figure 8.3 Liquid crystal display.


between the two glass-conducting plates. Thus the liquid molecule has a transition in between the two glass plates. The cell is perfectly sealed to avoid chemical reactions by interacting with the environment.

The liquid crystal can be operated in two modes: (1) reflective mode, the mode of operation in which the incident light gets reflected back from the surface of the reflective aluminum film, which is placed near the glass plate; and (2) transmissive mode, the mode of operation in which light is transmitted through the cell. LCDs are passive devices as they are nonemissive. They simply block/pass light reflected from an external source or provided by a backlighting system, which accounts for about half of the power consumption.

When AC voltage is applied, the crystals within this field align so that the polarized light is not twisted. This allows the light to be blocked by the crossed polarizers, thus making the activated segment or symbol appear dark. LCDs offer superior performance in terms of maximum luminance and intensity and are the most common displays for televisions and computer screens.

The advantages of LCD displays include the projection of perfectly sharp images, zero or negligible geometric distortion, intensely bright images on perfectly flat screens, low cost, small size, and low power con-sumption. Their primary disadvantage is that brightness, contrast, gamma, and color mixtures vary with the viewing angle. They are also nonemissive and require backlight. Fine text and graphics at more than one resolution are difficult to obtain on LCD displays. They also have difficulty producing black and dark grays and hence offer lower contrast and color saturation; hence the readability is poor under less than ideal light conditions. The pixels of an LCD can be turned on/off at a specific refresh rate, depending on the requirements of the image. If the pixels are off, they don’t let the backlight through; when they are on, they let the backlight through. They are suitable for portable devices and flat screens, are lightweight, have high DPI (dots per inch), and low electrical power with a lifetime of 50–80 years.

The advantages and disadvantages of LCDs are listed in Table 8.2.

8.6.6.1 LCD Display CategoriesBased on the type of information displayed, LCDs are classified as mono-chrome character/segment LCD displays, graphical LCD displays, and color LCD displays.


Monochrome character/segment LCD displaysThese LCDs display all alphanumeric values and some special

characters in monochrome but are not capable of projecting graphic animations or images. They are widely used in calculators, remote con-trollers, wristwatches, etc.

Graphical LCD displaysThese displays are capable of producing monochrome graphi-

cal images and animations along with characters, numbers, and special symbols, just by energizing the set of pixels in the display.

Color LCD displaysThese displays enclose three primary color subpixels. By varying the

voltage applied, each subpixel can produce a range of 256 shades. These color displays require more transistors and are widely used for monitors and display panels.

8.6.6.2 LCD TechnologiesBased on the technology used, LCD monitors and LCD screens are classi-fied into two types: active-matrix and passive-matrix technology.

Passive-Matrix LCD DisplaysThese passive LCD displays don’t produce and use a grid of conducting materials to activate each pixel. The types of passive-matrix display tech-nologies include twisted nematic (TN), super twist nematic (STN), film compensated super twist nematic (FSTN), and color super twist nem-atic (CSTN). These displays are less expensive than active-matrix displays

Table 8.2 Advantages and disadvantages of LCDsS. no. Advantages Disadvantages

1. Project perfectly sharp images Nonemissive2. Zero or negligible geometric

distortionRequire backlight to illuminate the

display3. Suitable for portable devices Poor readability under less than

ideal light conditions4. Flat screen Low viewing angle5. Lighter in weight Brightness, contrast, gamma, and

color mixtures vary with the viewing angle

6. Lifetime 50–80 years7. Intensely bright images on

perfectly flat screens


and can be implemented easily, but their response time is very slow and the display contrast is poor. The color on a passive-matrix display is not as bright as on an active-matrix display. Users view images on a passive-matrix display best when working directly in front of it.

Active-Matrix LCD DisplaysThese displays are similar to passive-active matrix displays, but an active device (transistor) and a capacitor is connected to each pixel to precisely control the voltage. The capacitor for each pixel holds the charge for a complete refresh cycle. The pixel response rate is faster than for passive-matrix displays, and they also project higher contrast and resolution with better viewing angle than passive-matrix displays. But these displays are expensive because each pixel requires a transistor, capacitor, and complex hardware. An active-matrix display, also known as a TFT (thin-film transis-tor) display, uses a separate transistor to apply charges to each liquid crystal cell and thus displays high-quality color that is viewable from all angles.

8.7 PLASMA DISPLAY PANELS

A plasma display panel (PDP) is a type of FPD >32 in. diagonally. It uses plasma—electrically charged ionized gas (mixture of electrons and ions)—and has tiny cells with noble gas (generally a mixture of neon/helium and xenon) and mercury inside it. When a voltage is applied, a plasma that emits ultraviolet light is created, which later strikes a phosphor in order to glow a particular color. In plasma televisions, phosphors are lit by plasma instead of scanning electron beam. Unlike in CRTs, pixels can be scanned and lit all at once. The plasma allows the current flow between conductive electrodes and addressing electrodes (gas discharge). Sealing and vacuum-pressure support problems apply to PDPs as well, requiring thicker glass as the screen is enlarged. In addition, the discharge chambers have pixel pitches of more than 1 mm, which makes it difficult to construct HDTV and workstation monitors. Power consumption varies greatly with picture content [15-16], with bright scenes drawing significantly more power than darker ones. This technology also does not work as well at high altitudes above 2 km due to pressure differential between the gases inside the screen and the air pressure at altitude. Plasma displays are generally heavier than LCD, and require more careful handling. They offer poor intrinsic view-ing angle, and performance is effected by temperature and sunlight. They also require backlight, and are slow and inefficient display devices. They


project high resolution, high brightness, high refresh rates, faster response time, low volume, good contrast, good color gamut, large viewing angle, and are available in a thin cabinet profile but are expensive [15]. They are capable of producing deeper blacks, offering superior contrast ratios. They also offer wider viewing angles than LCDs, and thus images do not suffer from degradation at high angles.

8.8 LIGHT-EMITTING DIODE DISPLAYS

With the materialization of semiconductor technology in the 1950s, selec-tive conduction of semiconductors through doping helped to revolution-ize the field of electronics by developing solid-state devices and helping to replace bulky CRTs. They work on the principle of electrolumines-cence, the phenomenon in which electricity is converted into light by the recombination of electron–hole pairs. A LED display can be (1) a seven-segment display or a dot-matrix display that is capable of displaying alphanumeric characters, or (2) a LED outdoor video display or LED TV display, a FPD that uses an array of LEDs as pixels. A cluster of red, green, and blue diodes is driven together to form a full-color pixel. These pixels are spaced evenly apart and are measured from center to center for abso-lute pixel resolution. The performance of LED displays largely depends on the operating environment temperature, which may shift their color with age. They require complex power supply setups to be efficiently driven and are currently expensive [17].

8.9 ORGANIC LIGHT-EMITTING DIODE DISPLAYS

One of the newest kinds of LED displays is the organic LED display, com-monly known as organic light-emitting diode (OLED). This technol-ogy is slowly replacing LCD displays in several area applications such as small displays for mobile applications, TVs, and microdisplays, and has the potential to reshape the industry. All the predecessors to these OLED dis-plays are limited to glass, but OLEDs could enable the development of new display applications including flexible plastic display devices, displays embedded into clothes or wall hangings and head mounted displays due to the fact that they can be printed on to any medium. Rapid advances in novel materials and manufacturing technologies are allowing the fab-rication of thinner, lighter, higher-resolution displays for small handheld devices, computers, laptops, televisions, etc. OLED displays are an exciting


and viable technology that is environmentally friendly and energy effi-cient [18]. It combines great colors and contrast with low power, and the screens of OLED displays are made of pixel-sized organically based ele-ments, which are much thinner and brighter than their predecessors. In order to make flexible OLEDs, an engineered substrate or flexible glass can be used as substrate, although encapsulation is required to protect the display from air and moisture. Light emission from an OLED device occurs due to fluorescence (emission from a singlet excited state of an organic molecule) or by phosphorescence (emission from a triplet excited state of an organic molecule). The key advantages of OLED display design include reliability, lifetime, brightness, operating lifetime, usage mode, power consumption, contrast, luminance color gamut, viewing angle, reso-lution, and design options. Superior technological efficiencies in creating lighter, simpler carbon-based material, generating deeper blacks, brighter whites, and all the gray scales in between are great strengths of OLEDs. Currently, OLED displays are restricted to 370 × 470 mm [10].

8.9.1 OLED Display TypesOLED displays can be either passive-matrix (PMOLED) or active-matrix (AMOLED). In passive-matrix display panels, the electrode mate-rial is deposited in the matrix of rows and columns. Display electronics can illuminate the desired pixel in the array. An image is created by scan-ning through rows and columns sequentially. The complete display screen is refreshed within 1/60th of a second. However, because of the high power consumption, passive-matrix displays can be made with only a lim-ited number of pixels. Active-matrix OLED displays are rapidly replac-ing passive-matrix OLED displays for full-color applications. In these displays, the array is further divided into a series of rows and columns, with a pixel formed at the intersection of each row and column. Active-matrix addressing requires a backplane with transistors (TFT) to control each pixel; the TFT is low-temperature poly crystalline silicon that con-trols the amount of current flowing through the OLED. These transistors have high current-carrying capacity and switching speed, and the TFT in each pixel controls the brightness and current flowing through the OLED. The continuous operation eliminates the need for high current as required in passive-matrix OLED displays. The main disadvantage of these displays is that they are more expensive than passive-matrix displays. The technol-ogy landscape of various display devices is depicted in Table 8.3.

Table 8.3 Technology landscape of LCDs, LEDs, PDPs, FEDs, and OLEDs [19]Technology feature FEDs Active-matrix

LCDsPassive-matrix LCDs

PDPs LEDs OLEDs

Brightness Good Good Good Good Very good Very goodResolution High High High Medium Low HighVoltage High Low Low High High LowViewing angle Excellent Medium Poor Excellent Excellent ExcellentContrast ratio Good Good Fair Good Good ExcellentResponse time Very fast Good Poor Very fast Fast Very fastPower efficiency Very good Good Good Medium Fair-good Very goodTemperature range Very good Poor poor Very good Very good Very goodForm factor Thin Thin Thin Wide Wide Very thinWeight Light Light Light Heavy Moderate LightScreen size Medium Small–large Small–medium Large Small–large Small–largeCost Average Average Low High High Below averageApplications Multiple Laptops, desktop Small displays Large screen Signs, indicators Multiple new/existing


8.10 FUTURE OUTLOOK

There are challenges to the commercialization of OLEDs for large-screen applications, such as electricity-to-light conversion efficiency, device sta-bility and lifetime (especially blue color), material selection and optimi-zation, encapsulation, manufacturing cost, fine patterning, contrast, pixel switching, and color saturation. Once we succeed in overcoming these hurdles, OLEDs may become the most energy-efficient and ecofriendly displays in use in the form of technologies such as quantum dot displays (QD-LEDs), ferro liquid displays (FLDs), thick-film dielectric electrolu-minescent technology (TDEL), telescopic pixel displays (TPDs), and laser-powered phosphor displays (LPDs).

8.10.1 Ultrathin DisplaysThe advent of mobile phones has changed the way we communicate and have even added a new dimension to communication itself. These com-munication tools are getting smaller and smaller, but they are still discrete devices we have to carry with us. Ultrathin mobile phones, laptops, and tablets are the latest developments in the field of OLED displays. Apart from these applications, integrating electronic devices with the human body to enhance or restore body function for biomedical applications is one of the goals of researchers around the world. Researchers at University of Tokyo have developed an ultrathin, ultraflexible, protective layer and demonstrated its use by creating an air-stable (OLED) display. This tech-nology will enable the creation of electronic skin (e-skin) displays of blood oxygen level, e-skin heart rate sensors for athletes, and many other applica-tions. In particular, wearable electronics need to be thin (mm) and flexible to minimize impact at the place, where they are attached to the body.

8.10.2 e-Papere-paper is a portable, reusable storage, and display medium, which is typi-cally thin and flexible. It is literally the electronic substitution for the printed page and typically reproduces mainly static text, usually monochrome, with high flexibility of the whole screen so ultimately it may even be folded or rolled like traditional paper. There are several technologies that offer e-paper properties. In fact, OLEDs are really a family of technologies, and e-paper is an application that can be produced using a number of different tech-nologies [20]. e-paper is based on an active-matrix display using “electronic ink,” i.e., an electrically controlled pigment resembling the ink used in


traditional printing. Thus it may eventually even replace paper. By using a suitable technology (typically a reflective type) an e-paper’s display content can be viewed in full daylight, anywhere that ordinary print on paper can be viewed, using a simple bistable (on/off) mode without refresh.

8.11 CONCLUSIONS

Most popular display systems consume a significant amount of generated electricity. Hence, the research focus on energy-saving display devices. As noted in the research, FPDs with LEDs and OLEDs as emissive devices are intended to be a safe and non-toxic and may eventually replace existing displays. They create white light with virtually no heat or energy dissipa-tion, no CO2 emanations, and also provide great resistance to shock, vibra-tion, and wear, thereby increasing lifespan significantly. However, LCD displays remain economically inefficient due to their high process costs and limited flexibility. Hence, OLEDs have been intensively investigated as next-generation energy-efficient display applications because they are thin, lightweight, and have high design flexibility. Improvements in efficiency, level of luminance, driving voltage, operation costs, lifespan, and light-extraction efficiency are major challenges in this field of research. Once these challenges are addressed, this ecofriendly and power-saving display technology has the potential to change the world of displays.

REFERENCES [1] B.S. Chuah, D.-H. Hwang, S.T. Kim, S.C. Moratti, A.B. Holmes, J.C. De Mello, et al.,

Synth. Met. 91 (1997) 279–282. [2] G.E. Jabbour, IEEE J. Sel. Top. Quantum Electron. 7 (2001) 769–773. [3] K.S. Chen, C.H. Wang, H.T. Chen, Microelectron. Reliab. 46 (2006) 1189–1198. [4] R.F. Graf, Modern Dictionary of Electronics, Newnes, Oxford, 1999, p. 569.

ISBN 0-7506-4331-5. [5] Y. Sato, H. Kanai, Mol. Cryst. Liq. Cryst. 253 (1994) 143. [6] M. Kawaharada, M. Ooishi, T. Saito, E. Hasegawa, Synth. Met 91 (1997) 113. [7] H. Aziz, Z. Popovic, C.P. Tripp, N.X. Hu, A.M. Hor, G. Xu, Appl. Phys. Lett. 72 (1998)

2642. [8] H. Aziz, Z. Popovic, S. Xie, A.M. Hor, N.X. Hu, C. Tripp, Appl. Phys. Lett. 72 (1998)

756. [9] S. Kappaun, Int. J. Mol. Sci. 9 (8) (2008) 1527–1547. [10] J.A. Castellano (Ed.), Handbook of Display Technology, Academic Press, San Diego,

CA, 1992. ISBN 0-12-163420-5. [11] R. Fink, A closer look at SED, FED technologies, in: EE Tines-Asia, 2007, pp. 1–4. [12] S. Itoh, M. Tanaka, T. Tonegawa, J. Vac. Sci. Technol. B22 (2004) 1362. [13] N. Hirosaki, R.-J. Xie, K. Inoue, T. Sekiguchi, B. Dierre, K. Tamura, Appl. Phys. Lett. 91

(2007) 061101.




















[14] M. Williams, Canon signals end of the road for SED TV dreams, in: IDG News Service, August 19, 2010.

[15] R. Williams, J. Phys. Chem. 39 (1963) 382–388. [16] H.G. Lee, D.G. Lee, Mech. Mach. Theory 41 (2006) 790–806. [17] A.A. Efremov, N.I. Bochkareva, R.I. Gorbunov, D.A. Lavrinovich, Y.T. Rebane,

D.V. Tarkhin, et al., Semiconductors 40 (5) (2006) 605. [18] P.P. Lima, F.A.A. Paz, C.D.S. Brites, W.G. Quirino, C. Legnani, M. Costae Silva, Org.

Electron. 15 (2014) 798–808. [19] N. Thejo Kalyani, S.J. Dhoble, Renew. Sustain. Energy Rev. 44 (2015) 319–347. [20] S. Forge, C. Blackman, OLEDs and E-Paper: Their Disruptive Potential for the

European Display Industry, Office for Official Publications of the European Communities, Luxembourg, 2009, pp. 15–30. ISBN 978-92-79-13421-0.













http://dx.doi.org/10.1016/B978-0-08-101213-0.00009-6

CHAPTER 9

Organic Light-Emitting Diode Fabrication and Characterization Techniques

9.1 INTRODUCTION

Organic light-emitting diodes (OLEDs) and polymer light-emitting diodes (PLEDs) are the cutting edge of lighting and display technology. Today, most commercially available small-screen OLED displays are fab-ricated by vacuum sublimation. However, this method of fabrication is expensive and time consuming, and controlling uniformity and dop-ing concentration over large areas is very difficult with this technique. Furthermore, due to evaporation, evaporant condensed on cold walls can flake off, contaminating the system and substrate. Thus solution techniques such as spin-coating, ink-jet printing, and screenprinting have gained momentum as they do not require vacuum, consume less time, and allow deposit of thin layer over a large area at low cost. As organic materials are soluble in different solvents, the desired thickness can be deposited on the substrate by spraying these solvated organic complexes using solution techniques. Whatever the technique, during deposition, uniform thickness of each layer is necessary for device fabrication in order to ensure ade-quate lifetime of the device. The complexity in arraying organic molecules and fabricating OLED and PLED devices is still challenging.

9.2 OLED FABRICATION

OLED fabrication is the process of designing and arraying different organic layers of desired thickness on a suitable substrate. During the design of the electroluminescence (EL) of the organic thin-film device five processes [1] need to be taken into account: (1) Injection: Electrons are injected from a low-work-function metal contact, e.g., Ca or Mg, which is usually chosen for reasons of stability. A wide-gap transparent


indium-tin-oxide (ITO) thin film is normally used for hole injec-tion. In addition, the efficiency of carrier injection can be improved by choosing organic hole and electron-injection layers with a low HOMO or high LUMO level, respectively. (2) Transport: In contrast to inorganic semiconductors, high p- or n-conducting organic thin films can only rarely be obtained by doping. Therefore hole- or electron-transporting organic compounds with sufficient mobility have to be used to transport the charge carriers to the recombination site. Since carriers of opposite polarity also migrate to some extent, a minimum thicknesses is neces-sary to prevent nonradiative recombination at the opposite contact. Thin electron- or hole-blocking layers can be inserted to improve the selec-tive carrier transport. (3) Recombination: The efficiency of electron–hole recombination leading to the creation of singlet excitons is mainly influ-enced by the overlap of electron and hole densities that originate from carrier injection into the emitter layer. Recombination of filled traps and free carriers may also be attributed to the formation of excited states. Energy barriers for electrons and holes to both sides of the emitter layer allow to spatially confine and improve the recombination process. (4) Exciton diffusion and (5) decay: Singlet excitons will migrate with an aver-age diffusion length of about 20 nm followed by radiative or nonradiative decay. Embedding the emitter layer into transport layers with higher sin-glet excitation energies leads to confinement of the singlet excitons and avoids nonradiative decay paths, e.g., quenching at the contacts. Doping of the emitter layer with organic dye molecules allows to transfer energy from the host to the guest molecule in order to tune the emission wave-length or increase the luminous efficiency.

9.2.1 Doosan DND Fabrication SystemIn this section the general specifications and procedure of an OLED fabri-cation system for 150 × 150 R and D line (Doosan DND), mainly used to deposit organic layers by vacuum deposition, is described [2].

9.2.1.1 General SpecificationsThis is an instrument to deposit different organic layers for fabrication and encapsulation of OLED devices consisting of evaporation chamber, trans-fer chamber, glove box, and encapsulation system such as encapsulation chamber, dispenser chamber, etc. System organization type: Cluster type Substrate specification: ITO/glass or flexible substrate

Organic Light-Emitting Diode Fabrication and Characterization Techniques 229

An ITO glass plate generally consists of eight ITO pads on a plain glass substrate. On these eight ITO pads two series of pixels (altogether eight pixels, four having a device area of 4 mm × 4 mm (called the A series) and the other with 2 mm × 2 mm device area (called the B series)) are opened by photolithography on which the organic layers will be deposited as shown in Fig. 9.1.1. ITO glass size (mm): 150W × 150L ± 0.2 mm (W = width, L = length)2. ITO glass thickness: 0.7 mm3. Encapsulation glass: 150W × 150L (tolerance ± 0.2 mm)4. Sheet resistance: <10 ohm/square5. Transparency: 85–90%

Effective area (mm): 130W × 130L Mask changing method: Manually by operator Substrate handling type: Holder less type Substrate traveling type: Horizontal moving type Pass line height: 1100 nm + F.L. Ceiling height: ≥2500 nm on floor System size (mm): About 3156L × 2.950W × 2.488H Clean room space: 6980 (L) mm × 5170 (W) mm × 2500 (H), or

2900 (H) mm

9.2.1.2 OLED Fabrication ProcedureThe ITO glass plate is pretreated at a pressure of 80 milli torr with the flow of argon and oxygen at a rate of 50 SCCM (standard cubic meter) for 420 seconds. This pretreated glass plate is placed on a holder in the glove box and later transferred to the plasma chamber for plasma treat-ment. After plasma treatment this glass plate is transferred to organic chamber #1 with the help of a robot. The ITO glass plate is used as the base and vacuum deposition of several layers is carried out in this chamber.

Figure 9.1 Anatomy of ITO glass substrate.


An organic chamber consists of eight cells (C1, C2, …, C8), where the organic layers are loaded into them. Nitrogen gas is connected to this chamber through a valve. A dry pump is also connected to organic cham-ber #1 through speed rough valve and roughing valve, and a cryo pump is used to create the desire vacuum. The nozzle of the pump is partially shut. For OLEDs around five cells are sufficient. If the hole-transport layer (say the material is in cell 1) is the one to be deposited first on the ITO substrate, the required parameters such as power, rate of deposition, thickness, and temperature are to be entered manually according to the requirements. The shutter of those particular cells remains closed until the required parameters are achieved. As soon as the requirements are fulfilled the shutter automatically opens and the organic material in that particu-lar cell evaporates and gets deposited uniformly according to the specified thickness. Once the desired thickness is reached, the shutter is automati-cally closed. Another cell consisting of a hole-transporting material is selected for deposition on the ITO substrate as the second layer. Next, comes the emission layer (dopant). If the host material is to be added to the emissive organic layer, we need to operate two cells at the same time by feeding specifications such as layer thickness and rate of deposition of host and the dopant simultaneously.

Once all the organic layers are deposited the ITO substrate is trans-ferred to the metal chamber to deposit the electrodes (cathode). For metal deposition we have two boats namely B1, B2 instead of C1, C2, C3…. According to the requirements, cathode materials are deposited at a pres-sure of 6 × 10−7 torr. The sample transfer from one chamber to the other chamber has to be done without breaking the vacuum. As both low-work-function cathode and organic materials are sensitive to oxygen and moisture, the devices are encapsulated using a glass lid sealed to the sub-strate with a bead of UV-cured epoxy in a nitrogen-filled glove box (dew point ≈ 75°C) for 3 minutes after fabrication, which is needed to ensure good lifetime.

9.2.2 Fabrication on Prepatterned ITO SubstrateOssila’s prepatterned ITO substrates are used for a wide variety of teach-ing and research devices, where a high-quality ITO surface is required. Fabricating devices using these substrates is done quickly and easily.

Ossila’s prepatterned ITO substrates are used for a wide variety of teaching and research devices, where a high-quality ITO surface is required. Fabricating devices using these substrates is done quickly and easily.


There are seven basic steps that shape an substrate into a device, as shown in Fig. 9.2: (A) substrate cleaning, which requires sodium hydroxide etch to reveal a perfect and hydrophilic ITO surface (each substrate has six individ-ual pixels and a cathode connection strip); (B) depositing a hole-transport layer (e.g., PEDOT:PSS) and then wiping free the cathode strip after-wards; (C) depositing an active layer on top of the hole-transport layer; (D) depositing cathode strips by thermal evaporation or any other techniques; (E) encapsulation, so that the devices can be stored for extended durations with minimum degradation in performance, which is done by dropping UV-cured epoxy on the device and then placing the glass coverslip on top before curing in a light box; and (F) leg connections with a standard 0.1-inch (2.54 mm) pitch. Devices can be inserted into most prototyping boards allowing electrical connection to the six individual pixels as shown in Fig. 9.2. The device is now ready to test for efficiency and performance.

9.3 FABRICATION TECHNOLOGIES

OLEDs are fabricated by depositing very thin films of organic materials at temperatures less than 100°C to form bright, vivid, power-efficient, self-emissive light-producing elements with fast response times. They can be grown on a wide variety of large-area substrates such as glass, plastic, metal foil, etc. When OLEDs are fabricated on plastics, they are ideally suit-able for high information-content and flexible displays. They can also be fabricated by depositing very thin film of organic materials by vacuum

Figure 9.2 Fabrication process: (A) Clean, (B) deposit hole-transport layer, (C) deposit active layer (D), deposit cathode, (E) encapsulate, and (F) add connection legs [3]. From Ossila.com.


deposition techniques such as thermal vacuum evaporation and physical vacuum deposition (PVD) and solution techniques such as spin-coating, ink-jet printing, screenprinting, etc. The structures of organic materials and of OLEDs fabricated by solution and vacuum-processed techniques by Wang et al. are shown in Fig. 9.3.

There is a big difference between the two methods used for fabricat-ing, and both come with specific advantages and disadvantages. Solution-based processes can damage or degrade previously applied layers, thereby limiting the complexity of the device structure that can be achieved [5]. Vapor deposition of small molecules is free of the damage problem, enabling multilayer stacking for efficient balancing of hole and electron transport and for confining of carriers and excitons. Wang et al. demon-strated that solution-processed OLED devices exceed device performance of vacuum-processed OLED devices of same materials and anatomy. But this statement is not universal. It may be different for a set of layers consid-ered in the anatomy of OLED devices.

9.3.1 Vacuum Deposition TechniquesVacuum deposition techniques allow the fabrication of more advanced multilayer device architectures in order to match the energy levels of compounds. Furthermore, effective purification of the corresponding

Figure 9.3 Structures of organic materials and OLEDs fabricated by solution and vac-uum-processed techniques [4].


materials can be achieved by gradient vacuum sublimation prior to the deposition process, which has a significant impact on device performance and operational lifetime [6]. This technique is limited to vaporizable low-molecular-weight materials and can only be applied to compounds that can endure thermal stress without decomposition. One of the advantages of this technique is that even a complicated multilayer device architec-ture can be constructed without any serious problems. These technologies are relatively expensive and have limited use for large-area devices, but are suited for smaller devices with high quality [7]. A vacuum deposition unit for OLED fabrication is shown in Fig. 9.4.

9.3.1.1 Vacuum Thermal EvaporationVacuum-vapor deposition, a conventional and popular technique for fab-ricating OLED cells, is depicted in Fig. 9.5. This technique is widely used for depositing various active layers for fabricating smaller devices and dis-plays using small molecular organic light-emitting devices (SMOLEDs). High flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high effi-ciencies of small-molecule OLEDs.

In this technique, the organic molecules are gently heated in a vacuum chamber under a vacuum of about 10−5 to 10−7 torr in order to avoid

Figure 9.4 Vacuum deposition unit for OLED fabrication.


interaction between the vapor and atmospheric molecules. In vacuum ther-mal evaporation (VTE) the material is allowed to evaporate until it is depos-ited on the substrate. This material vapor condenses in the form of thin film. At low pressures these particles travel in a straight line from the source of evaporation toward the substrate as the mean-free path of the vapor atoms is comparable with the vacuum chamber dimensions. The average energy of vapor atoms reaching the substrate surface is of the order of kT, which seriously affects the morphology of the films [8]. An organic evaporation chamber, substrate holder, crucible of organic chemicals, and evaporator with glove box is shown in Fig. 9.6.

Advantages of VTE Two-dimensional combitorial arrays of OLEDs can be easily fabricated

by single deposition. The thickness of each layer can be precisely controlled.

Disadvantages of VTE Limited to low-molecular-weight materials. Very low pressure (10−6 or 10−5 torr) is required.

Figure 9.5 Fabrication of an OLED device by VTE.


Polymers cannot be deposited by vacuum-vapor deposition because the structure of conjugated polymers leads to high conductance.

Evaporant condensed on cold walls can flake off, contaminating the system and substrate.

Controlling uniformity and doping concentration over large areas is very difficult, hence the technique is limited to small-area devices.

Flexible displays cannot be fabricated. No way to direct the deposition materials onto the desired areas. Not cost effective.

Can only be applied for smaller-area devices with high quality.

9.3.1.2 Physical Vacuum DepositionVacuum deposition is extensively used for thin-film preparation of inorganic as well as organic materials, due to its higher output and better cost perfor-mance. Recent progress in OLEDs has shown promise with this method.

Figure 9.6 (A) Organic evaporation chamber, (B) substrate holder, (C) crucible of organic chemicals, and (D) evaporator with glove box [9].


The steps involved in the PVD process include: (1) thermally evaporat-ing the organic compound from the source material in vacuum; (2) trans-portation of the evaporant into a diluting, nonreactive gas stream (such as nitrogen) from the source to the substrate; and (3) condensation of the evaporant onto a cool substrate to form the thin-film deposit of desired thickness (even in the nm range) in the clean environment of high vac-uum. The steps involved in the PVD process are shown in Fig. 9.7. The deposition rate diminishes significantly due to the existence of (1) oxygen, (2) reactive metal, and (3) collisions between evaporant from source and gas molecules during their transport toward the substrate.

This process requires a significant amount of heat, preventing the growth of dense films. Good adhesion between the thin film and substrate, control over mechanical stress in the film, high chemical purity, deposi-tion of very thin layers, and multiple layers of different materials can be achieved by this method. Parameters such as deposition rate of the thin film, kinetic energy of the atoms, gas scattering during transport of the evaporant and energy applied to the film during growth, and substrate temperature can be controlled in order to create materials with varying density, optical reflectivity, mechanical strength, adhesion, magnetic prop-erties, and electrical resistivity. The PVD process is illustrated in Fig. 9.8.

Advantages of PVD Improves control over doping Controlled by both temperature and carrier gas-flow rate Better for large-area substrates

Figure 9.7 Steps involved in the process of PVD [9].


Disadvantages of PVD Involves a significant amount of heat, preventing the growth of dense

films

9.3.2 Solution TechniquesSolution techniques provide a means to calculate the energy gap of the synthesized complex [10,11], which plays a vital role in the architecture of the OLED device to be fabricated. As polymers have large molecular weight, PLEDs cannot be fabricated by vacuum deposition methods. Thus polymers are generally deposited by dissolving in an organic solvent, fol-lowed by spin-coating or ink-jet printing, drop casting, roll-to-roll web coating, etc. These processing methods can significantly reduce the cost of fabrication and have the potential to escort to a large-area reel-to-reel production. Scheming multilayers from these techniques is critical because the previously deposited layers are enormously resistant to the solvent used for deposition of the subsequent layers. These techniques are pre-ferred for large-scale production with less material usage. However, solu-bility and coatability of the materials, the formation of homogeneous thin films, and the dissolution between the organic layers in multilayer devices are some of the disadvantages.

Figure 9.8 PVD process [9].


9.3.2.1 Spin-CoatingSpin-coating is a process used to deposit uniform thin films on flat sub-strates using a spinner or spin coater. This technique is commonly used for the deposition of soluble conjugated polymers of desired thickness onto a surface, and is widely used to fabricate monochrome displays. The thick-ness of the layers depends on the concentration of the polymer solution and the composition of the polymers. The production process is com-paratively cheaper than vacuum evaporation, and also enables large-area devices with homogeneity throughout the device/display area. In this technique, the emissive material is deposited on the center of the flat sub-strate, which is uniform across its surface.

The substrate is rotated at high speed until it spreads to the desired thickness by centrifugal force as shown in Fig. 9.9. Rotation is contin-ued while the fluid spins off the edges of the substrate until the desired thickness of the film is achieved. The applied solvent is usually volatile and hence evaporates. Thus the higher the angular speed of spinning, the thin-ner the film. The thickness of the film depends on various parameters such as viscosity, concentration of the solution and solvent, rotational speed of the spin coater, fluid volatility, surface wetting on substrate, fume extrac-tion, temperature, etc. [12]. This technique is also used in microfabrication of oxide layers using sol–gel precursors to create uniform thin films with nanoscale thickness <10 nm [13].

Figure 9.9 Spin-coating technique [9].


9.3.2.2 Ink-Jet PrintingThis technique is used for manufacturing full-color polymer LED dis-plays as well as patterning of pixels. However, pixel pitch of about 28 µm is obtained by this method and hence it is not suitable for microdisplays. During the process of printing, organic materials are diluted into a liq-uid and then sprayed onto substrates like ink sprayed onto paper dur-ing printing as shown in Fig. 9.10. Multilayer preparation from solution is of crucial importance because previously deposited layers are totally resistant to the solvent used for deposition of the subsequent layers. This technique drastically decreases manufacturing costs and paves the way for flexible large-screen displays like billboards and big TV screens with high-resolution and low-information-content displays. The ease of depositing many layers in a display is one of its greatest strength. However, the lay-ers shift and hence dimensional changes take place during the drying and evaporation processes in PLEDs. Over time, moisture can react with the organic layers and cause degradation and defects and thus proper sealing techniques must be used [10]. Organic light-emitting diode fabrication by ink-jet printing is illustrated in Fig. 9.10.

Current OLED production methods rely on evaporation processes, in which the organic materials are deposited onto a glass sheet through a thin metal stencil, also known as a “shadow mask.” This process is prob-lematic, as a significant amount of the material is wasted because it dis-perses all over the mask, in addition to inherent mask changes that expose the sheet to dust and compromise yields (OLEDs are by nature sensitive to contamination). Ink-jet OLED printing has the desirable ability to allow precision deposits without the use of a mask. It also produces less stray particles, thus boosting yields. These significant advantages make this technology interesting to many companies and virtually all OLED makers have active ink-jet printing development projects [14].

Figure 9.10 Demonstration of ink-jet printing [9].


9.3.2.3 ScreenprintingIn this method of printing, ink is squeezed through a distinct screen mask to create print patterns. This printed pattern is subsequently transferred on to the substrate [15]. This method is widely used in commercially printed circuitboards and by many research groups to print active polymer lay-ers as well as electrodes for organic transistors and simple circuits. For materials with high viscosity, the resolution of screenprinting is limited to ≥75 µm. Also, particle size must be sufficiently large so that they will not block the screen. This technique is very simple, cheap, versatile, and reduces the use of material because materials are directed onto the printed areas faster than ink-jet printing.

9.3.2.4 Laser-Induced Thermal ImageThis method is used to manufacture high-resolution full-color displays with OLEDs and PLEDs. As OLEDs are extremely sensitive to moisture, the device fabricated by any one of these methods needs protection by hermetic encapsulation, which is carried out in a glove box that is free from oxygen and water. These coatings must have extremely low water and oxygen permeability, and are applied at temperature <100°C [16].

The different fabrication techniques are compared in Table 9.1.

9.4 CHARACTERIZATION OF OLEDs

In order to demonstrate various characterization techniques of OLEDs, the work of Ritu Srivastava et al. [17] is used as an example. They fab-ricated devices with the configuration ITO/α-N,N´-Di(1-naphthyl)-N,N´-diphenyl-(1,1´-biphenyl)-4,4´ diamine (NPD) (300 Å) (hole

Table 9.1 Exhaustive comparison of fabrication techniquesS. no. Fabrication technique Materials employed Outcome

1. VTE Small molecules SMOLEDs2. PVD Organic/inorganic

moleculesOLEDs and LEDs

3. Spin-coating Soluble conjugated polymers

Monochrome displays with PLEDs

4. Ink-jet printing Polymers Full-color displays with PLEDs

5. Screenprinting Polymers PLEDs6. Laser-induced

thermal imageOrganic molecules/

polymersOLEDs and PLEDs


transport layer (HTL))/4,4´-Bis(N-carbazolyl)-1,1´-biphenyl (CBP): FlrPic (5%): Iridium doped biphenyltetracarboxylic acid (x%) (250 Å)(multi-emitting layers, EML)/Bathocuproine (BCP) (60 Å) (hole blocking layer (HBL))/Alq3(250 Å) (ETLelectron transport layer (ETL))/LiF(10 Å), where x = 0.25% and 0.50% as shown in Fig. 9.11. Performance of the fabri-cated OLED devices was tested by current–voltage (I–V), current density– voltage–luminance (J–V–L) characteristics, electroluminescence (EL) spectra, and Commission International de l’Eclairage (CIE) coordinates.

9.4.1 V–I CharacteristicsThe V–I characteristics of an OLED create a curve between the applied voltage of the anode and cathode (taken along the x-axis) and the current going through the device (taken along the y-axis). These characteristics are created by increasing the voltage and noting the corresponding cur-rent at a sweep voltage of 1 V at a time interval of 1000 milliseconds (may vary). Practically no current flows until the barrier voltage is overcome. The voltage up to which no current flows through the diode is known as the barrier voltage. With further increase in voltage, current starts flowing and the curve has a linear rise like an ordinary conductor.

The voltage at which the current starts flowing through the diode is known as the cut-in voltage. The V–I characteristics of an OLED device reveal the power-on voltage of the device, which is generally defined as the voltage necessary to have a luminance of 1 Cd/m2. Ideally, this value should be as low as possible, e.g., less than 5 V. Usually, with the increase in voltage the current increases exponentially as shown in Fig. 9.12.

LiF/cathode

Alq3BCP

EmitterNPD

ITOGlass substrate

ITO

4.9

5.55.1

5.6

5.9 5.8

6.4Ir-BTPA

Al

(B)

(A)

2.42.5

3.2

CBP Alq3

BC

P2.8 2.9 3.0

2.9

FlrPic

αNPD

Light output

Figure 9.11 Device structure of the OLED device fabricated by Ritu Srivastava et al. [17].


9.4.1.1 J–V–L CharacteristicsThe J–V–L curve characterizes the properties of the current density (J), bias voltage (V), and the electroluminescence output (L). Current den-sity (J) is the ratio of current on the area of the device considered. While creating J–V–L characteristics, the device fabricated by Ritu et al. [17] is focused to spectro radiometer in order to record current and lumi-nance at a particular voltage. It was observed that current density as well as luminance increases with the drive voltage. The power-on voltage of the device was found to be 10 V. The luminescence increased linearly with increase in voltage as shown in Fig. 9.13.

9.4.2 EL SpectraAn EL spectrum is a plot between the wavelength on the x-axis and the intensity along the y-axis. This spectrum shows a peak of the characteristic wavelength of the emissive material with the application of voltage.

Fig. 9.14 shows the normalized EL spectra of the devices with doping concentrations of 5 wt% FIrPic in CBP (device 1), 0.5 wt% Ir-BTPA in CBP (device 2), 5 wt% FIrPic and 0.5 wt% Ir-BTPA in CBP (device 3), and 5 wt% FIrPic and 0.25 wt% Ir-BTPA in CBP (device 4) at 11 V. The EL spectra of all these devices display three emission peaks at 469, 500, and 611 nm) (devices 3 and 4), which create the bluish-white color from the fabricated devices.

Figure 9.12 Ideal V–I characteristics of an OLED device [17].


9.4.3 CIE CoordinatesThe color of a light source is typically characterized in terms of the CIE system, which describes how the human eyes perceive the emission color of any light source. Any color can be expressed by the chromaticity coor-dinates x and y on the CIE chromaticity diagram. Using this method the composition of any color in terms of three primaries can be described [18–20]. The x, y coordinates are usually used to represent the color. If the CIE coordinates of the main source drift only in the range of 0.005–0.01

Figure 9.13 J–V–L characteristics of white OLEDs fabricated by Ritu Srivastava et al. [17].

Figure 9.14 Normalized EL spectra of the devices fabricated by Ritu Srivastava et al. [17].


or less for both the x- and y-values in the whole brightness range and the entire operation process, then it is said to be an efficient device.

A chromaticity diagram representing the CIE coordinates of differ-ent colors is shown in Fig. 9.15 and the variation of the CIE chromatic-ity at different voltages for device 4 fabricated by Ritu et al. is shown in Table 9.2.

Figure 9.15 Chromaticity diagram, representing the CIE coordinates of different colors [21].

Table 9.2 Variation of CIE chromaticity at different voltages for device 4Voltage (V) x y

14 0.279 0.30415 0.277 0.30816 0.277 0.31017 0.274 0.31418 0.273 0.318


9.4.4 Color Rendering IndexThe color rendering index (CRI) enumerates how different a set of test colors appears when illuminated by the source as compared to the same test colors illuminated by a standard illuminant with the same CCT. In simple terms: The property of the light source that influences the appearance of objects in terms of color is called “Color Rendering.” The emission spectrum of any light source should be broad and continu-ous covering the entire visible-light spectral region so that objects with any color can be illuminated vividly by the white-light created. The CRI measures this quality, and is a unit-less quantity, measured on a 0–100 scale. The highest possible CRI value is 100, which occurs when there is no difference in color rendering between the light source and the stan-dard illuminant. The higher the CRI value, the stronger the ability of the lighting source to reproduce the true color of the illuminated objects. Generally, lighting sources with a CRI above 80 are required for indoor-lighting applications [23].

9.4.5 Correlated Color TemperatureThe correlated color temperature (CCT) is the absolute temperature (kel-vin) at which a blackbody radiator must be operated in order to have a chromaticity equal to that of the light source. The light emitted by an incandescent bulb is very close to an ideal blackbody radiator (hence its CCT is the temperature of the filament). For high-quality white-light illumination the CCT should lie between 2500 K to 6500 K. However, it is still a great challenge to establish a good compromise among all these three parameters (CIE, CRI, and CCT) for exploiting artificial light-ing. In addition to these parameters, color stability is another key fea-ture used to evaluate lighting sources [24]. The challenge is to fabricate OLED devices capable of emitting daylight with tuneable CCT that mimics sunlight. Jwo-Huei Jou et al. [25,26] fabricated and demon-strated a simple layered structure that enables OLEDs with daylight colors and a wide color-temperature span. The effects of the hole-modu-lating layer (HML), the electrontransport host material TPBi—1,3,5-tris- (N-phenylbenzimidazole-2-yl)-benzene—and sequence of the EML on the CCT and CCT span of a sunlight style, and CCT-tuneable OLED are demonstrated. The effects are indicated by the emission track, against the daylight locus, on the CIE chromaticity space. Also shown are the energy-level diagrams of their device I and counterpart devices II–V for


comparison. ETL is the electron-transport layer, and HTL is the hole-transport layer [25], Fig. 9.16. The design is a simple OLED structure capable of yielding tunable daylight color with CCT ranging from 2300 to 9700 K, matching that of sunlight, Fig. 9.17.

The CIEs, CRIs, and CCTs for common white-light sources are given in Table 9.3 for comparison.

9.4.6 Color CharacteristicsThe spectral power distribution signature for OLEDs is different from that of LEDs due to the fact that the light is created by a mixture of

Figure 9.16 Effects of the HML, the electrontransport host material TPBi—1,3,5-tris-(N-phenylbenzimidazole-2-yl)-benzene—and sequence of the EML on the CT and CT span of a sunlight-style, CT-tunable OLED. The effects are indicated by the emission track, against the daylight locus, on the CIE chromaticity space. Also shown are the energy-level diagrams of our device I and counterpart devices II–V for comparison. ETL is the electron-transport layer, and HTL is the hole-transport layer [25].


emitting layers rather than by a phosphor-converted energy from a blue- or violet-pump LED, or than from combining multiple LED pri-maries to create white or colored light [24]. The color metrics for two 3000 K OLED products tested for the DOE CALiPER Program, using the IES TM-30–15 calculator and visualization graphics, are shown in

Table 9.3 Comparison of CIEs, CRIs, and CCTs for various white-light sources [27]Light source CIE coordinates CRI CCT (K)

Incandescent lamp (0.44, 0.40) 100 2854Tungsten halogen lamp (0.44, 0.40) 100 2856Sodium lamp (0.51, 0.41) 24 2100Fluorescent white lamp (0.37, 0.36) 89 4080WOLED (0.33, 0.36) 92 5410Daylight (0.31, 0.32) 90 6500

Figure 9.17 Sunlight-style OLED with CT span from 2300 to 9700 K, compared to the CT and/or CT span exhibited by candles as well as typical electricity-driven lighting devices. The OLED device exhibits, at 3.0 V, an orange emission of 2400 K; at 5.5 V, a pure-white emission of 6500 K; and at 8.0 V, a bluish-white emission of 9700 K [25].


Figure 9.18 TM-30–15 color metrics and graphics for the LG 3000 K panel used by Acuity Brands, Inc. [28].

Figs. 9.18 and 9.19 [28]. The color metrics and graphics indicate that the LG display and OLEDWorks 2900 K panels are quite different, even though they are within 50 K CCT of each other.

Some of the commercially available LG panels are produced in the 4000, 3500, and 3000 K (Fig. 9.20) range, with the 3000 K pan-els being slightly more efficacious. A US report [28] suggests that aside from increasing the red content of the OLED light sources, the OLED industry would do well to consider a dim-to-warm option, where the light source grows warmer at lower output, similar to the way incandes-cent lamps behave. Also, OLEDs must have tunable white or tunable color options, which would be a desirable feature for the hospitality, retail, and


commercial markets, especially if they can closely replicate the “dim-to-warm” characteristic of incandescent lamps, or any other desired dim-ming trend. Both of these capabilities are now readily available in LED luminaires.

9.4.7 Lifetime MeasurementsLuminescence lifetime plays a crucial role in the performance of a device and strongly depends on the vibrations of the nearby ligands used in the synthesized complex. Hence, due care has to be taken when selecting the ligands. When these complexes are excited by light energy, the sensitizing ions may absorb light by the vibration of the ligand, which may decrease

Figure 9.19 TM-30–15 color metrics and graphics for the OLEDWorks FL300 panel.From CALiPER Test ITL86057 [28].


the lifetime of the complex and hence the device. For solid-state lighting, all the primary colors, i.e., red, blue, and green, should have good lifetime to ensure good performance. The life of red- and green-light-emitting materials have already reached to their appreciable lifetime, while blue light-emitting materials have wider band gap that affects their lifetime and also require high energy for effective light emission [29,30].

Germany-based blue-thermally activated delayed fluorescence (TADF) OLED emitter developer CYNORA recently announced that it has developed a new blue-emitting material that combines high efficiency with long lifetime (Fig. 9.21). CYNORA’s new material offers an external

Figure 9.20 Three color options for OLED panels available from LG display. Note that the y-axis scale is not identical for all figures [28].

Figure 9.21 A new blue-emitting material that combines high efficiency with long lifetime [31]. With permission from CYNORA <http://www.oled-info.com/cynora-latest-tadf-blue-emitters-feature-higher-efficiency-and-lifetime> (accessed 18.10.2016).


quantum efficiency of 14% and a lifetime of 420 hours (LT80, at 500 cd/m2) [31]. In May 2016 CYNORA announced two blue emitter sys-tems, with one featuring a high efficiency and the other a long lifetime. This time CYNORA managed to create a single system with improved efficiency and lifetime. The company says that they are optimistic that they will reach a commercial TADF blue emitter by the end of 2017.

Researchers from Kyushu University managed to drastically increase the lifetime of TADF OLED emitters by more than eight times [32]. This was achieved by simple modification to the structure of the device, i.e., putting two thin (1–3 nm) layers of Liq (a lithium-containing molecule) on each side of the hold blocking layer. The researchers started with a TADF device, in which the lifetime is only about 85 hours (LT95) under “extreme brightness” to accelerate testing. With the new design and some extra modifications, the device’s lifetime increased to 1300 hours—over 16 times better than the initial device—but this is not yet enough for commercialization.

9.5 CONCLUSIONS

Despite the success of OLEDs in small-screen applications, they still have several disadvantages such as limited lifetime, expensive fabrication tech-niques, and limited efficiency. They have the potential to shape future lighting sources and displays through suitable fabrication technology, but highly stable and efficient materials with smart designs and optimized properties are needed to fabricate OLEDs or PLEDs. If we can address these challenges, OLEDs and PLEDs will probably be the dominant form of lighting within 5–10 years’ time.

REFERENCES [1] <https://www.tu-braunschweig.de/Medien-DB/ihf/ar96da.pdf> (accessed 19.10.2016). [2] D.N.D. Doosan, OLED System Instruction Manual. [3] Organic-photovoltaic-opv-and-organic-light-emitting-diode-oled-fabrication-

manual. [4] Z. Wang, Y. Lou, S. Naka, O. Hiroyuki, AIP Adv. 1 (2011) 032130. [5] S.R. Forrest, Nature (London) 428 (2004) 911. [6] D. Hertel, C.D. Müller, K. Meerholz, Chem. UnsererZeit 39 (2005) 336. [7] A.M. Diaz-Garcia, S.F. Da Avila, Appl. Phys. Lett. 81 (21) (2002) 3924–3926. [8] P. Peumans, Organic thin-film photodiodes, Thesis, University of Princeton. [9] N. Thejo Kalyani, S.J. Dhoble, Ren. Sustain. Energy Rev 44 (2015) 319–347. [10] T. Morita, A. Akashi, M. Fujji, Y. Yoshida, K. Ohmori, T. Yoshimato, et al., Synth. Met.

69 (1995) 433. [11] R. Chesterfield, A. Johnson, C. Lang, M. Stainer, J. Ziebarth, Inform. Display (2011)

124–130.

https://www.tu-braunschweig.de/Medien-DB/ihf/ar96da.pdf











[12] L.E. Scriven, Physics and applications of dip coating and spin coating, MRS proceed-ings, 1988.

[13] D.A.H. Hanaor, G. Triani, C.C. Sorrell, Surf. Coat. Technol. 205 (12) (2011) 3658–3664. [14] <http://www.oled-info.com/oled-inkjet-printing?page=0%2C1> (accessed

18.10.2016). [15] E.G. Jabbour, Screen printing for the fabrication of organic light emitting devices,

IEEE 7 (2001) 5. [16] L.L. Moro, T.A. Krajewski, N.M. Rutherford, O. Philips, R.J. Visser, M.E. Gross,

W.D. Bennett, et al., SPIE 5214, Organic light-emitting materials and devices VII, 83 Conference 5214 (2004).

[17] R. Srivastava, G. Chauhan, K. Saxena, S.S. Bawa, P.C. Srivastava, M.N. Kamalasanan, Indian J. Pure Appl. Phys 47 (2009) 19–23.

[18] Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett. 351 (2002) 163. [19] Q.Y. Zhang, K. Pita, S. Buddhudu, C.H. Kam, J. Phys. D 35 (2002) 3085. [20] W.R. Stevens, Building Physics: Lighting, 1, Pergamon Press, London, 1969, p. 66. [21] J.E. Kaufman, J.F. Christensen, Lighting Handbook, Waverly Press, Maryland, 1972, p. 48. [22] <https://www.konicaminolta.eu/fileadmin/content/eu/Measuring_Instruments/4_

Learning_Centre/C_A/What_is_Colour_Rendering_Index/Colour_Rendering_Index_EN.pdf> (accessed 18.10.2016).

[23] C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 77 (2000) 904–906.

[24] G. Zhou, W.-Y. Wong, S. Suo, J. Photochem. Photobiol. C: Photochem. Rev. 11 (2010) 133–156.

[25] <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.570.7473&rep=rep1&type=pdf> (accessed 18.10.2016).

[26] J.H. Jou, M.H. Wu, S.M. Shen, H.C. Wang, S.Z. Chen, S.H. Chen, et al., Appl. Phys. Lett 95 (2009) 013307.

[27] N. Thejo Kalyani, S.J. Dhoble, Ren. Sustain. Energy Rev 16 (2012) 2696–2723. [28] D.O.E. Solid, State lighting program, “R&D Plan,” edited by James Brodrick, Ph.D.,

June 2016. [29] L.S. Hung, C.H. Chen, Mater. Sci. Eng. R 39 (2002) 143. [30] D. Chitnis, N. Thejo Kalyani, S.J. Dhoble, BioNano Frontier: Mater. Sci. 2 (2012)

346–348. [31] <http://www.oled-info.com/cynora-latest-tadf-blue-emitters-feature-higher-effi-

ciency-and-lifetime> (accessed 18.10.2016). [32] <http://www.oled-info.com/new-method-extend-lifetime-tadf-emitters-may-also-

enhance-phosphorescent-oleds> (accessed 18.10.2016).


http://www.oled-info.com/oled-inkjet-printing?page=0%2C1









https://www.konicaminolta.eu/fileadmin/content/eu/Measuring_Instruments/4_Learning_Centre/C_A/What_is_Colour_Rendering_Index/Colour_Rendering_Index_EN.pdf







http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.570.7473&rep=rep1&type=pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.570.7473&rep=rep1&type=pdf







http://www.oled-info.com/cynora-latest-tadf-blue-emitters-feature-higher-efficiency-and-lifetime

http://www.oled-info.com/cynora-latest-tadf-blue-emitters-feature-higher-efficiency-and-lifetime

http://www.oled-info.com/new-method-extend-lifetime-tadf-emitters-may-also-enhance-phosphorescent-oleds

http://www.oled-info.com/new-method-extend-lifetime-tadf-emitters-may-also-enhance-phosphorescent-oleds



http://dx.doi.org/10.1016/B978-0-08-101213-0.00010-2

CHAPTER 10

Photo-Physical Properties of Some RGB Emissive Materials

10.1 INTRODUCTION

Many organic materials have been synthesized to create high-performance electroluminescent devices. In spite of the remarkable achievements in recent years, finding effective luminescent materials with different emis-sion colors is still a challenge. Current interest in photonic technologies has encouraged research on high-performance materials such as organics and polymers that can be prepared inexpensively. One of the most commonly studied devices is light-emitting diode whose light emissions originate in small organic molecules (OLEDs) or in semiconducting polymers (PLEDs). Emission color (RGB) is generally determined by the energy difference between the Highest Occupied Molecular Orbit (HOMO) and Lowest Unoccupied Molecular Orbit (LUMO) of the emitting organic material. By changing these active materials the emission color can be varied across the entire visible spectrum. To generate white light, the most common and simple technique is to combine tricolor RGB phosphor or bicolor yellow–blue phosphor as luminescent materials for applications in flat-panel dis-plays [1–3], Hg-free lamps, and solid-state lighting (SSL). The bandwidths of the emission spectra of OLEDs and polymers are generally broad with full-width at half-maximum (FWHM ≈ 550–200 nm). Thus obtaining pure emission colors from these materials is difficult. However, rare-earth ion (Eu3+) is known for furnishing very sharp red emission spectra (with FWHM < 5 nm) proving to be ideal for systems with high efficiency, good lifetime, and narrow emission spectrum. Luminescence from europium III (Eu3+) has attracted significant interest due to its fascinating optical proper-ties and potential use in a variety of applications. Europium β-diketonates in particular have good luminescent properties due to the unique elec-tronic structure of Eu3+, the antenna effect of the ligands [4,5], and the 4 f→4 f electron transitions of Eu3+. Potential applications include threshold lasers, electroluminescent displays, high-density optical devices, and high-density frequency-domain optical data-storage materials [6–8].


Metal 8-hydroxyquinoline (Mqn) chelates, especially tris(8-hydroxy-quinoline) aluminum (Alq3), have been extensively investigated for their high stability, good emission, and electron-transporting properties [9–12]. Aluminummetal-chelate compounds are known to yield broad light emis-sion and to provide design freedom needed to control photo-physical processes in green light-emitting devices. Among these chelates, Alq3 metal complexes are especially important because of their synthesis proce-dures and wide spectral response [13,14]. Technical applications began to emerge after a report on an efficient electroluminescent device using Alq3 as the active medium [15]. It is used as a electro-transporting layer and as an emissive layer where green light emission is generated by electron-hole recombination. It also serves as host material for various dyes to tune emission color from green to red. Even today, research and development of OLEDs based on Alq3 are the focus for low-molecular-weight materi-als for these devices. According to geometry analysis, an Alq3 molecule has two possible geometric isomers with facial (fac-) and meridianal (mer-) forms [16]. Early theoretical calculations showed that mer-Alq3 isomer is energetically more stable than fac-Alq3 [17,18]. Two geometric isomers of Alq3 exist, mer (C1 symmetry) and fac (C3 symmetry), and thermal inter-conversion between the two has previously been suggested [19]. Evidence suggests that such interconversion is not a prerequisite for obtaining amor-phous films [20]. However, it is thought that the isomers coexist in the amorphous state, a result of which may be an increase in the stability of this state [21]. Identification of the two isomers has been attempted using IR absorption spectroscopy and peaks at 442, 456, and 472 cm−1 have been assigned to the mer isomer and at 398 and 419 cm−1 to the fac isomer [22,23]. In particular, the Alq3 molecule can have two differ-ent geometrical isomers fac-Alq3 and mer-Alq3 with C3 and C1 symme-try, respectively. Fac-Alq3 has three equivalent bidentate quinoline ligands around the central Al (III) atom; three oxygen and three nitrogen atoms are located on the opposite faces of the distorted octahedron. Mer-Alq3 contains three oxygen and three nitrogen atoms in perpendicular planes of the octahedron and its three quinoline ligands are not in equivalent posi-tions as shown in Fig. 10.1.

The blue-doped emitter in white OLEDs often necessitates the judi-cious selection or design of an appropriate blue host material that has wide enough band-gap energy and matching LUMO/HOMO levels to effect the sensitization. The chemical structures of some blue-light-emit-ting materials are shown in Fig. 10.2.

Photo-Physical Properties of Some RGB Emissive Materials 255

Apart from these blue host and doping materials, pyrazoloquino-line derivatives, distyrylenes, anthracene derivative, and spirofluoreres have shown to be the best blue light-emissive materials for fabricat-ing blue OLEDs. The synthesis of quinolines and their derivatives has drawn considerable interest from many research groups because they are nitrogen-containing heterocyclic aromatic systems with easy synthesis and purification [28,29]. The chemical alteration of the phenyl quino-line can persuade the band gap and accordingly change the color of the emitted light. π-conjugated rigid poly(quinoline)s have also been exten-sively investigated as thermally stable, photo-conductive, photo-lumines-cent, and nonlinear polymeric materials. Of the hetero aromatic polymers, quinolone-based polymers are known to exhibit n-type conductivity upon doping and possess excellent thermal as well as oxidative stability [30,31]. This chapter mainly focuses on the study of the photo-physical properties of RGB light-emitting phosphors and explores their suitability and applications in OLED displays and SSL.

10.2 EXPERIMENTAL DETAILS

The photo-physical properties of red-Eu(TTA)3Phen,Eu(x)Y(1−x)(TTA)3Phen and Eu(x)Tb(1−x)(TTA)3Phen (europium (Eu), Thenoyltrifluoroacetone (TTA), 1-10 phenanthroline (Phen), Yttrium (Y), Terbium (Tb)), green-tris(8-hydroxyquinoline) aluminum (Alq3), and blue—2-([1, 1′-biphenyl]-4-yl)-6-chloro-4-phenylquinoline (P-Acetyl biphenyl Cl-DPQ) light-emitting hybrid/organic complexes were explored in solid-state,

Figure 10.1 Thermal ellipsoidal model of mer-Alq3 and fac-Alq3 [24].


Figure 10.2 Chemical structure of typical blue host and doping materials. (A) 1,3,5-tris(N-Phenylbenzimidizol-2-yl) benzene (TPBI) [25]; (B) 9, 10-di (2-napthil) Anthracene (β-DNA) [26]; (C) Bathophenanthroline(BPhen) [25]; (D) tetrakis(t-butyl)perylene(TBPe) [25]; and (E) n-Anthril carbazolyl derivative [27].


polymer matrices and also in different basic and acidic solvents. The opti-cal absorption and emission spectra of the synthesized complexes were carried out on a Perkin Elmer LAMBDA 3517 and photo-luminescence excitation spectra were carried out on a Hamamatsu F-4500 RF5301 spectrofluorometer.

10.2.1 Synthesis of RGB PhosphorOrganic light-emitting diodes have shown huge superiorities as light sources due to advantages such as high electroluminensce efficiency, fast response, low driving voltage, simplicity of fabrication, environmental friendliness, and no harm to eye. Thus there has been rapid development of organic/hybrid red, green, blue (RGB) phosphor that can be synthe-sized with ease at low cost and that are environmently friendly. When taken in suitable proportion, these colors manifest into white light emission. An overview of the synthesis and characterization of novel RGB phosphor is well illustrated from prior state of art in the preceding sections.

Synthesis Scheme of Eu ComplexesThejo Kalyani et al. [32] synthesized volatile Eu(TTA)3Phen,Eu(x)Y(1−x) (TTA)3Phen and Eu(x)Tb(1−x)(TTA)3Phen(x = 0.4, 0.5) hybrid organic complexes by solution technique using the following steps: Step 1: For the synthesis of Eu (TTA)3Phen hybrid organic complex,

6.63 m mol (1.4731 g) of TTA and 2.21 m mol (0.4381 g) of Phen was dissolved in 20 ml of ethanol.

Step 2: pH value of the dissolved solution was found to be in between 5 and 6. The KOH solution was added drop by drop until its pH value reached 7.

Step 3: 2.21 m mol (0.5709 g) of EuCl3 was dissolved in 10 mL of double- distilled water.

Step 4: The solution obtained in step 1 and the one obtained in step 3 (i.e., EuCl3 solution) were mixed.

Step 5: The mixed solution was stirred continuously for 1 hour at a temperature of 60°C on a heating mantle with a magnetic stirrer.

Step 6: With the help of filter paper, precipitate (pale yellow) was col-lected. This precipitate was then washed with ethanol and distilled water to purify the precipitate.

Step 7: This precipitate was dried for 2 hrs at 80°C in an oven.Similarly Eu(x)Y(1−x)(TTA)3Phen and Eu(x)Tb(1−x)(TTA)3Phen com-

plexes (where x = 0.4, 0.5) were synthesized by taking total combination


of Eu(x)Y(1−x) and Eu(x)Tb(1−x)as 2.21 mol by dissolving in 10 mL of double-distilled water in another flask, taken individually. The composition of the chemical constituents for the synthesis of Eu(x)Y(1−x)(TTA)3Phen and Eu(x)Tb(1−x)(TTA)3 Phen complexes are shown in Table 10.1.

From Fig. 10.3(A), it is clear that 1:10 phenanthroline and TTA are bidendate. Eu3+ is associated with three molecules of TTA and one mol-ecule of Phen. Eu3+ has 8 coordinates (6 with TTA, which are shown to the right of Eu3+ and the other two with Phen, which are shown to the left of Eu3+). Similarly, Eu(x)Ln(1−x)(Y/Tb)is associated with three molecules of TTA and one molecule of Phen as shown in Fig. 10.3 (B). Hence, for the formation of Eu(TTA)3Phen, Eu(x)Y(1−x)(TTA)3Phen, and Eu(x)Tb(1−x)(TTA)3Phen complexes, the stichiometry of chemical com-pounds must be in the 1:3:1 ratio.

10.2.1.1 Synthesis Scheme of Organo Metallic Alq3 ComplexAlq3 is a molecule widely utilized in OLEDs as an electron-transporting and/or a highly efficient emitting material of green light since 1980s [15]. Since then, interest in Alq3 and other metal/chelate systems to produce electroluminescence-indifferent spectra regions for display applications has increased considerably. The fundamental principle of the synthesis of Alq3 is to combine HQ anion with Al3+ in its aqueous solution. Alq3 is a coor-dination complex, where aluminum is bonded in a bidentate manner to the conjugate base of three 8-hydroxyquinolineligands [34]. Variations in the substituents on the quinoline rings affect its luminescence properties [35]. Minaevet et al. [36] synthesized green light-emitting tris(8-hydroxy-quinoline) aluminum (Alq3) metal complex by simple precipitation at room temperature using the following steps:

Table 10.1 Composition of the chemical constituent for the synthesis of Eu(x)Y(1−x)(TTA)3 and Eu(x)Tb(1−x)(TTA)3 Phen complexesStichiometric ratio TTA in (g) Phen in (g) EuCl3 in (g) YCl3 in (g)

Eu(x)Y(1−x)(TTA)3Phen

x = 0.4 1.4731 0.4381 0.2283 0.2589x = 0.5 1.4731 0.4381 0.2854 0.2157

Eu(x)Tb(1−x)(TTA)3Phen

x = 0.4 1.4731 0.4381 0.22831 0.3517x = 0.5 1.4731 0.4381 0.28544 0.2931


Step 1: Take 25 mL of double-distilled water and 25 mL of acetic acid in a beaker.

Step 2: Dissolve 5 g of 8-hydroxyquinoline in a mixture of double- distilled water and acetic acid; stir vigorously until the orange transpar-ent solution is obtained (solution I).

Step 3: Dissolve 4.3069 g of Al(NO3)3.9H2O in double-distilled water. Step 4: Stir it well until clear solution is obtained say (solution II). Step 5: Mix solutions I and II for 10 min and add the NH4OH solution

to this mixture drop by drop with continuous stirring.

Figure 10.3 Chemical structure of (A) Eu(TTA)3Phen and (B) Eu(x)Ln(1−x)(TTA)3Phen, where x = 0.4, 0.5, Ln = Y/Tb [33].


Step 6: Filter the yellow–green precipitate and wash the precipitate with double-distilled water 8–10 times.

Step 7: Place the precipitate in a oven at 40–50°C for 45 minutes to evaporate the moisture in the complex.The chemical reaction during the synthesis of 8-hydroxyquinoline

with aluminum (III) is [37]:

C9H7NO+Al3++ OH−→Al (C9H6NO)3+H2O.

The chemical structure of tris(8-hydroxyquinoline) aluminum mole-cule is shown in Fig. 10.4.

10.2.1.2 Synthesis Scheme of P-Acetyl biphenyl Cl-DPQ Organic ComplexSuyash et al. [38] synthesized blue-light-emitting 2-([1, 1′-biphenyl]-4-yl)-6-chloro-4-phenylquinoline (P-acetyl biphenyl Cl-DPQ) organic complex using the following steps: Step 1: The mixture of 2 amino 5 chloro benzo phenone

(C13H10ClNO), (2 g), 4 acetyl biphenyl (C14H12O) (2 g), diphenyl phosphate (2 g), and M-cresol (3 mL) was added to a round 3 neck flask and the glass stirrer was fixed at the middle neck of the round flask to stir the compound.

Step 2: The oil bath was maintained at constant temperature at 90°C for 1 hour and then at 140°C for 4 hours. After completing the heating and stirring process, the flask was taken out of the oil bath for cooling.

Step 3: In the purification process, 60 mL dichloromethane (methylne chloride) and then 60 mL NaOH solution with a 10% concentration were added and the mixture was kept for 8 hours.

Step 4: Two layers formed in the flask were separated by the funnel and then washed with 50 mL distilled water (three times). It was again washed with 20 mL of hexane (three times).

Figure 10.4 Chemical structure of Tris(8-hydroxyquinoline) aluminum molecule [37].


Step 5: The resulting precipitate was kept in an oven at 45°C for 10 hours to remove water from the synthesized complex.

Step 6: The powder precipitate was collected on butter paper and dried at room temperature to remove the moisture.

Step 7: Finally, milky white color powder with compound weight 2.56 g was obtained. The scheme for the synthesis of 2-([1, 1’-biphenyl]-4-yl)-6-chloro-4-phenylquinoline (P-acetyl biphenyl Cl-DPQ) is shown in Fig. 10.5.

10.2.2 R/G/B Phosphor in PMMA/PS MatrixThe increased use of polymer thin film in integrated optical technology makes it interesting to study the incorporation of organic/hybrid organic complexes in polymers [39]. These complexes when doped in polymers are mechanically flexible and could be easily spin-coated and thermally converted into uniform films. When incorporated into several matrices including zeolites, glass films, and semiconductors [40], micro- or meso-porous materials [41,42], and polymers [43–46] these complexes prevent luminescent concentration quenching, enhance thermal and mechani-cal stabilities, and improve processing ability. In these blends, conven-tional polymers with excellent optical and mechanical properties like poly (methyl methacrylate) (PMMA), polystyrene (PS), polyethylene (PE), polyvinylchloride (PVC), or polycarbonate (PC) can be used as host matrices. Thejo Kalyani et al. used polymethylmetacrylate (PMMA) and polystyrene (PS) as model polymers as they have good film-forming prop-erties with high glass-transition temperatures of 105°C and 100°C, respec-tively. Polymethylmetacrylate is a common polymer, stable in air with high formability, and has no absorption and fluorescence in the visible region.

Figure 10.5 2D-Synthesis scheme of 2-([1, 1′-biphenyl]-4-yl)-6-chloro-4-phenylquino-line [38].


Figure 10.6 Molecular structure of (A) PMMA and (B) Polystyrene [49].

Table 10.2 Physical and chemical parameters of PMMA and PS [50]Chemical formula (C5O2H8)n (C8H8)n

Synonyms Polymethylmethacrylate PMMA poly (methyl methacrylate)

Polystyrene PS

Molecular mass varies variesDensity 1.19 g/cm³ 1.05 g/cm³Melting point 180°C 237.5°CBoiling point 200.0°C -Glass transition 114°C 100°CRefractive index 1.492 (λ = 589.3 nm) 1.519(λ = 589.3 nm)Transmission 80–90% -

The molecular structure of PMMA is shown in Fig. 10.6A. Although it is a linear polymer, polymethylmetacrylate forms quasi-cross-linked struc-ture through strong dipole–dipole interaction, preventing it from crystal-lization [47,48]. Polystyrene is a lightweight cellular plastic foam material composed of carbon and hydrogen atoms. The molecular structure of PS is shown in Fig. 10.6B. When undergoing glass transition, PS becomes flexi-ble rather than rigid. As both polymers have good film-forming properties with high glass-transition temperature, the synthesized Eu complexes were molecularly doped in PMMA and PS. The physical and chemical param-eters of PMMA and PS were shown in Table 10.2.

10.2.2.1 Preparation of Blended FilmsCommercially available polymers PMMA/PS have been used to make blended films of synthesized complexes. Polymer (PMMA/PS) matrix is prepared by dissolving 0.5 g of polymer (PMMA/PS) in 15 mL of chlo-roform with vigorous stirring for 15 minutes at room temperature. Later, 0.05 g of the synthesized complexes are taken individually and dissolved in the same solvent in separate beakers. Then the complex solution is


mixed with PMMA/PS matrix at room temperature under vigorous stir-ring for 15 minutes to obtain the homogeneous mixture. The resulting homogeneous mixture is then poured on to a glass. The solvent is allowed to evaporate in air for 2–3 hours at room temperature and then pilled up from the substrate. The obtained films are placed in a vacuum dry oven at room temperature overnight to remove any residual solvent. The obtained blended films are homogeneous and show excellent optical transparency. No visible phase separation is detected.

10.2.3 Photo-Physical Properties of Red Light-Emitting Eu ComplexesThe photo-physical properties of red light-emitting europium hybrid organic complexes are explored in solid state, polymer matrices and also in different basic and acidic solvents in order to compare the spectral data in solid state, polymer matrix PMMA and in various basic and acid solvents.

10.2.3.1 Photo-Luminescence Spectra of Eu(TTA)3PhenThe excitation and emission spectrum of Eu(TTA)3Phen in solid state was excited in the wavelength range of 250–500 nm as shown in Fig. 10.7. When the emission spectrum was recorded by excit-ing Eu(TTA)3Phen at 370 nm. An intense red emission at 611 nm has

Figure 10.7 Excitation and emission spectra of Eu(TTA)3Phen [52].


been registered. This can be attributed to the 5D0→7F2 transition, while the shoulders at 551, 557, and 643 nm can be attributed to the 5D0→7Fj (j = 0, 1, 3) transitions [51].

The fluorescence of Eu(TTA)3Phen was generated by the transition of excitation energy from ligand TTA to the Eu3+ ion in the excited state. The synthesized Eu complex was based on a synergistic effect in which the ligand can absorb UV light and transfer energy from Eu(III), leading to enhancement of intensity.

10.2.3.2 Photo-Luminescence Spectra of Eu(x) Y (1−x)(TTA)3PhenThe excitation and emission spectra of Eu(x)Y(1−x)(TTA)3Phen (x = 0.4 and 0.5) is shown in Fig. 10.8 along with the emission spectra of Eu(TTA)3Phen for comparison. Both complexes were excited at 370 nm, monitoring emission at 611 nm.

The emission peaks, registered at 551, 557, 611, and 643 nm, are due to 5D0→7Fj ( j = 0, 1, 2, 3) transitions, respectively. When the photo-luminescence emission intensity of Eu(x)Y(1−x)(TTA)3Phen is compared with Eu(TTA)3Phen, enhancement of intensity is observed, which may be due to nonradiative energy transfer from enhancing ion Y3+ to Eu3+ through TTA. Maximum photo-luminescence intensity is observed for

Figure 10.8 Photo-luminescence spectra of Eu(x)Y(1−x)(TTA)3Phen, where x = 0.4, 0.5 [52].


Eu0.4Y0.6(TTA)3Phen. It is well known that Y3+ doesn't possess the elec-tronic structure of 4 f shell. The lowest excited energy level of Y3+ is more than 32 × 10−3 cm−1. These energy levels are much higher than the lowest excited triplet level T (about 20.3 × 10−3 cm−1) of the β-diketone TTA [53,54]. The excited state 5D0 of Eu3+ has long lifetime, up to millisec-onds, which leads to efficient energy transfer from excited Y3+ to Eu3+, resulting in enhancement of photo-luminescence.

10.2.3.3 Photo-Luminescence Spectra of Eu(x) Tb(1-x)(TTA)3PhenFig. 10.9 shows the photo-luminescence spectra of EuTb(1−x)(TTA)3 Phen(x = 0.4 and 0.5) in solid state. Both complexes show a sharp emis-sion band peaking at 611 nm when excited at a wavelength of 370 nm. This sharp emission is due to the electric-dipole 5D0→7F2 transition. The maximum photo-luminescence emission intensity was observed for Eu0.4Tb0.6(TTA)3Phen. When this series is compared with Eu(TTA)3Phen, enhancement in photo-luminescence intensity is observed in Eu(x)Tb(1−x)(TTA)3Phen. The lowest excited energy level of Tb3+ is near to lowest excited triplet level of TTA, hence the energy transfer from TTA to Eu3+ through Tb3+ ion results in enhancement of PL emission intensity.

Figure 10.9 Photo-luminescence spectra of Eu(x)Tb(1−x)(TTA)3Phen, where x = 0.4, 0.5 [55].


The model relaxation and energy transfer processes of Eu(TTA)3Phen and Eu(x)Tb(1−x)(TTA)3Phen are shown in Fig. 10.10. The dependence of luminescent intensity of Eu3+ on the content of Tb3+ is relevant to the triplet–triplet energy transfer TTA in Eu(x)Tb(1−x)(TTA)3Phen to that of Eu(TTA)3Phen. An important factor that accounts for the fluores-cence enhancement in Eu3+ luminescence is the energy transfer between Eu3+and Tb3+ [52]. As shown in Fig. 10.5 the resonance energy level of Eu3+ (5D0, 17,500 cm−1) is lower than that of Tb3+ (5D4, 20,400 cm−1). In the mixed complex Eu3+ diverts a large portion of the energy from the 5D4 level of the Tb3+ and thus promotes luminescence enhancement of the Eu3+.

Tb3+ acts as an energy bridge to enhance the energy transfer between the ligands and Eu3+ ion. The intramolecular energy transfer from Tb3+ to Eu3+ was studied by a time-resolved spectrum, which gave direct evidence of the intramolecular energy transfer between the two lanthanide ions [52].

10.2.3.4 Absorption Spectra of the Eu Complexes in PMMA MatrixThe absorption spectra of Eu complexes doped in PMMA matrix for 10 wt% are characterized by two absorption peaks centered at 334 nm and 280 nm as shown in Fig. 10.11.

The absorption peak at 334 nm is attributed to the n–π* transition of β-diketonate ligand TTA, while the other peak at 280 nm can be attrib-uted to the π–π* transition of Eu3+ β-diketonate (TTA) moieties, show-ing that all compounds have the same tris-chelated core. As the absorption

Figure 10.10 Model relaxation and energy transfer process of Eu(TTA)3Phen and Eu(x)Tb(1-x)(TTA)3Phen [52].


wavelength is characteristic of the aromatic group of β-diketonate (TTA), λmax in all complexes appear at the same position with a change in optical density.

10.2.3.5 Energy Gap of Eu Complexes in PMMA MatrixUsing the procedure described by Morita et al. [57,58] for energy gap determination, the relationship between energy and (energy×absorbance)2 was plotted for complexes in PMMA as shown in Fig. 10.12. The cut-ting edge of the plot on the x-axis is taken as the energy gap. The val-ues obtained for the energy gap (Eg) were 3.41, 3.35, and 3.43 eV for the blended films of Eu(TTA)3Phen, Eu0.5Y0.5(TTA)3Phen, and Eu0.5Tb0.5(TTA)3Phen in PMMA, respectively.

10.2.3.6 Photo-Luminescence Spectra of Eu (TTA)3 Phen in PMMATo study the effect of rigidity on the efficiency of energy transfer from the ligand to Y3+and Eu3+ ion/Tb3+ and Eu3+ ion, Eu0.5Y0.5(TTA)3Phen, and Eu0.5Tb0.5(TTA)3Phen complexes were incorporated in PMMA matrix. The excitation and emission spectra of Eu(TTA)3Phen complex in PMMA solid matrix are shown in Fig. 10.13. Eu(TTA)3Phen is an electro-active and photo-active complex made of small molecules that can interact with the polymer (PMMA) in the excited state. Hence, careful consideration is essential to interpret the photo-luminescence spectra. The 5D0→7F2 emission bands of Eu(TTA)3Phen in solid state/PMMA show

Figure 10.11 Absorption spectra of Eu(TTA)3Phen, Eu0.5Y0.5(TTA)3 Phen, and Eu0.5 Tb0.5(TTA)3Phen blended films in PMMA [56].


sharp red emission λemi centered at 611 nm, suggesting potential applica-tion of the complexes as red-emitting materials in OLEDs. The complex doped in PMMA shows a sharp peak at 611 nm and comparatively the same bandwidth as in solid state.

The intensity of the electric-dipole transition 5D0→7F2 transition is much stronger than the magnetic dipole-allowed 5D0→7F1 transition, indicating that the Eu3+ ions in these systems occupy very low symmetric

Figure 10.12 Determination of energy gap of Eu(TTA)3Phen, Eu0.5Y0.5(TTA)3Phen and Eu0.5Tb0.5 (TTA)3 Phen in PMMA [56].

Figure 10.13 Photo-luminescence spectra of Eu(TTA)3Phen in PMMA matrix [56].


sites. This hypersensitive 5D0→7F2 transition is very sensitive to the inter-mediate environments around the Eu3+ ions. The profiles of the 5D0→7F2 emission bands of Eu(TTA)3Phen complex in solid state as well as in PMMA are similar due to the fact that the β-diketonate ligand (TTA) used is the same in both cases. Enhancement in intensity was found when Eu(TTA)3Phen was molecularly doped in PMMA matrix as compared with the emission intensity in solid state, due to the fact that rare earth ions can easily form aggregates. It has been reported that this cofluores-cence effect is caused by intermolecular energy transfer between the two complex types [59,60]. Considering the Foster and Dexter theory [61], energy can be transferred to molecules in short distance through intermo-lecular energy transfer and the efficiency of this transfer depends on the proximity of donor and acceptor. PMMA enwraps the Re3+ complexes and keeps the donors and acceptors much closer [62] as it has oxygen atom, which can freely donate electrons with ease. Fluorescence enhance-ment factor F, a factor of luminescence enhancement expressing the effi-ciency of energy transfer [63], is the ratio of the luminescent intensity of the film that contains the Eu complex to that which does not.

10.2.3.7 Photo-Luminescence Spectra of Eu0.5Y0.5(TTA)3Phen in PMMAThe emission spectra of Eu0.5Y0.5(TTA)3Phen complex doped in PMMA are shown in Fig. 10.14. This system also exhibits strong luminescence,

Figure 10.14 Photo-luminescence spectra of Eu0.5Y0.5(TTA)3Phen in solid state and the complex in PMMA [56].


which is similar to the spectral characteristics of Eu3+, indicating the exis-tence of triplet–triplet energy transfer between Eu3+ and Y3+ through TTA, which enhances the fluorescence intensity of the complexes. The emission bands have very narrow width emission with full-width at half-maximum <10 nm. The lowest excited energy level of Y3+ is the lower excited triplet level T (about 20.3 × 10−3 cm−1) of the β-diketonate (TTA) [64,65].

Electric-dipole 5D0→7F2 transition of Eu3+ is responsible for the sharp spectral line centered at 611 nm. The excited state 5D0 of Eu3+ has a long lifetime, up to milliseconds, due to this long lifetime, the efficient energy transfer from excited Y3+ to Eu3+ is possible. This results in enhancement of photo-luminescence. Hence, when doped in PMMA the emission bands of this complex have comparatively high intensity.

10.2.3.8 Photo-Luminescence Spectra of Eu0.5Tb0.5 (TTA)3Phen in PMMAFig. 10.15 shows the excitation and emission spectra of Eu0.5Tb0.5(TTA)3Phen in PMMA. The 5D0→7F2 emission bands of Eu0.5Tb0.5(TTA)3Phen in solid state and PMMA matrix have the same profile, and have sharp peaks λemi at 611 nm with weak shoulders on either side of the main peak. Efficient energy transfer from the ligand (TTA) to Eu3+ ion through Tb3+ ion results in enhancement of luminescence from Eu3+ ion that becomes pronounced in PMMA solid matrix. Generally, the closer the ionic radius of the enhancing ion to that of the luminescent

Figure 10.15 Photo-luminescence spectra of Eu0.5Tb0.5(TTA)3Phen in solid state and the complex in PMMA [56].


ion, the greater the luminescence enhancement in the complex. As the radius of Tb3+ is closer to that of the Eu3+ ion, the fluorescence enhance-ment phenomena were observed and hence this complex when doped in PMMA has comparatively high intensity. As the radius of Tb3+ (0.923A0) is closer to that of the ionic radius of Eu3+ (0.947 A0), the interaction between both the ions becomes easier and leads to fluorescence enhance-ment phenomena. The other reason could be that terbium is a fluorescent lanthanide, it can accept energy from the ligand in the close proximity, provided the excited state of the donor is above the emitting level of the ions and even some times terbium donate the energy it has absorbed anal-ogous to the fluorescent complexes of Eu(III) through TTA.

Fluorescence in Eu0.5Tb0.5(TTA)3Phen/PMMA is enhanced, indicat-ing the existence of triplet-triplet intermolecular energy transfer between Eu3+ and Tb3+ through TTA, leading to enhancement of the fluorescence intensity of Eu0.5Tb0.5(TTA)3Phen/PMMA system. These blended films show a red shifted emission band from 611 to 611.4 nm, which may be due to dominant intramolecular energy transfer within the complex. The optical parameters of the synthesized complexes in solid state and PMMA are shown in Table 10.3.

10.2.3.9 Thermal Annealing Effect on Photo-LuminescenceThin films of molecularly doped Eu(TTA)3Phen, Eu0.5Y0.5(TTA)3Phen, and Eu0.5Tb0.5(TTA)3Phen complexes in PMMA were

Table 10.3 Optical parameters of synthesized Eu complexes in solid state and PMMACompound λmax

(nm)λext (nm)

λemi (nm)

Stokes shift (cm−1)

F Energy gap (eV)

Eu(TTA)3Phen complex – 370 611 – – –Eu complex in PMMA 280 381 611 3.610 2.65 3.41

334Eu0.5Y0.5(TTA)3

Phen complex– 370 611 – – –

Complex in PMMA

280 387 611 3.610 1.53 3.35334

Eu0.5 Tb0.5(TTA)3 Phen Complex

– 380 611 – – –

Complex in PMMA 280 380 611.4 3.605 2.16 3.43334


subjected to annealing at 50°C and 80°C for 2 hours. The degradation in photo-luminescence intensity was observed with emission centered at 611 nm for Eu(TTA)3Phen and 612 nm in Eu0.5Tb0.5(TTA)3Phen and Eu0.5Y0.5(TTA)3Phen complex as shown in Fig. 10.16A–C. The relative intensity of the emission bands arising from Eu3+ ion decreases with increase in temperature. The dependence of the relative intensity of 5D0→7F2 transition band on temperature can be clearly observed. The rea-son could be that the energy absorbed by the ligand couldn’t be trans-ferred to Eu3+ effectively due to dispersion of emitting molecules in the matrix with thermal treatment. The energy obtained by Eu3+ from TTA may be transferred to Phen further through a metal to ligand by an intra-molecular energy transfer process [66].

10.2.3.10 Energy Gap of Solvated Eu ComplexesSolvated electrons have been the subject of great interest due to their large absorption cross-sections, making them amenable to study by ultra-fast spectroscopy [67–71]. These hybrid Eu organic materials are soluble in most basic and acidic solvents; hence desired thickness can be deposited on the substrate by spraying these solvated organic complexes by solu-tion techniques. With time, the solvent evaporates and a film of organic layer can be obtained easily. However, the choice of solvent is signifi-cant as the emission intensity in that particular solvent depends on the absorption spectra of the solvated complex. Considering these facts, the absorption spectra of Eu(TTA)3Phen, Eu0.5Y0.5(TTA)3Phen, and Eu0.5Tb0.5(TTA)3Phen complexes were studied in basic organic solvents (chloroform, toluene, tetra hydro furan) and acidic solvents (acetic acid and formic acid) and their optical band gaps calculated.

All the synthesized Eu complexes are low-molecular-weight hybrid organic complexes with charge-transport properties. These complexes have good solubility in most organic solvents (basic media) as well as in acidic media. These complexes are dissolved in different solvents according to molar ratio. For example, to obtain 10−3 molar solution of the complex, 0.0055 gm of Eu(TTA)3 Phen is dissolved in 5 mL of the solvent. To get a solution of 10−4 molar ratio, 1 mL of 10−3 solution is dissolved in 10 mL of the solvent. Similarly 10−5, 10−6 molar solutions can be obtained. The solution is taken in a cuvette to record absorption spectra on an Analytic Jena split-beam spectro photo-meter SPECORD-50. Using the procedure described by Morita et al. [57] for energy gap determination, the values obtained for energy gap (Eg) were 3.42, 3.41, 3.39, 3.38, and 3.35 eV for


540 560 580 600 620 640 660 6800

200

400

600

800

1000(A)

(B)

(C)

Inte

nsi

ty (

a.u

)

Wavelength (nm)

Eu(TTA)3 Phen Complex in PMMAFilm at 50 CFilm at 80 C

550 600 650 7000

200

400

600

800

1000

Inte

nsi

ty (

a.u

)

Wavelength (nm)

Eu0.5 Y0.5 (TTA)3 PhenComplex in PMMAFilm at 50 CFilm at 80 C

540 560 580 600 620 640 6600

200

400

600

800

1000

Inte

nsi

ty (

a.u

)

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Eu0.5 Tb0.5 (TTA)3 PhenComplex in PMMAFilm at 50 CFilm at 80 C

Figure 10.16 Photo-luminescence graphs of (A) Eu(TTA)3Phen, (B) Eu0.5Y0.5(TTA)3, and (C) Eu0.5Tb0.5(TTA)3Phen in PMMA at 50°C and 80°C.


Eu(TTA)3Phen in chloroform, toluene, THF, acetic acid, and formic acid, respectively, as calculated from Fig. 10.17A. In energy gap determination of solvated Eu0.5Y0.5(TTA)3Phen complex in chloroform, toluene, THF, acetic acid, and formic acid, the values obtained for energy gap (Eg) were 3.43, 3.42, 3.41, 3.36, and 3.29 eV, respectively, as shown in Fig. 10.17B. In energy gap determination of solvated Eu0.5Tb0.5(TTA)3Phen complex in

Figure 10.17 Energy gap determination of (A) Eu(TTA)3Phen, (B) Eu0.5Y0.5(TTA)3Phen, and (C) Eu0.5Tb0.5(TTA)3Phen in different solvents [72].

3.0 3.2 3.4 3.60

1

2

3

4

5

6

7

[En

erg

y x

abso

rban

ce]2

Energy (eV)

ChloroformTouleneTHFAcetic acidFormic acid

3.0 3.2 3.4 3.60

1

2

3

4

[En

erg

y x

abso

rban

ce]2

Energy (eV)

ChloroformTolueneTHFAcetic acidFormic acid

(A)

(B)


chloroform, toluene, THF, acetic acid, and formic acid, the values obtained for energy gap (Eg) were 3.42, 3.41, 3.39, 3.34, and 3.28 eV, respectively, as shown in Fig. 10.17C. The energy gaps of all synthesized Eu complexes in different solvents are summarized in Table 10.4.

10.2.4 Photo-Physical Properties of Green Light-Emitting Alq3 ComplexThe photo-physical properties of green light-emitting metallo-organic tris(8-hydroxyquinoline) aluminum (Alq3) complex are explored in the solid state and polymer matrices of PMMA at different wt% (1and 10 wt%). These films are then investigated for absorption and photo-lumi-nescence spectra to calculate energy gap and Stokes shift.

Table 10.4 Energy gap of synthesized Eu complexes in various solvents [72]Complex Solvent Eu(TTA)3Phen Eu0.5Y0.5

(TTA)3PhenEu0.5Tb0.5 (TTA)3Phen

Energy gap(eV) Chloroform 3.42 3.43 3.42Toluene 3.41 3.42 3.41THF 3.39 3.41 3.39Acetic acid 3.38 3.36 3.34Formic acid 3.35 3.29 3.28

3.0 3.2 3.4 3.60

1

2

3

4

[En

erg

y x

abso

rban

ce]2

Energy (eV)

ChloroformTolueneTHFAcetic acidFormic acid

(C)

Figure 10.17 (Continued).


10.2.4.1 Photo-Luminescence Spectra of Alq3

Fig. 10.18 shows the excitation and emission spectra of Alq3 crystalline powder in the range of 250–600 nm, monitored at 385 nm. When the powder is excited at 385 nm, it emits green light at wavelength 506 nm. The photo-luminescence spectrum shows that any excitation photon energy in the spectral range extending from the 1La to the 1Bb electronic transition band contributes to the broad green photo-luminescence. This behavior is due to fast nonradiative relaxation from the 1Bb to 1La state with subsequent radiative recombination to the ground state, which means fast energy transfer processes are at work among the various orbital states with final relaxation at the LUMO states.

10.2.4.2 Absorption Spectra and Determination of Stokes ShiftThe absorption spectra of Alq3 display-prominent absorption bands in the range of 270–350 nm, corresponding to the 1La and 1Bb electronic transi-tion bands of solvated Alq3 molecules in chloroform at 10−3 molar con-centration. The peak at 360 nm (larger wavelengths) corresponds to the optical transition between the HOMO and LUMO states. The energy-band at 294 nm (shorter wavelengths) is due to optical transitions from the ground state to higher-energy molecular orbitals, and its asymmetry clearly indicates a complex structure for these states. Stokes shift is the difference between positions of the band maxima of the absorption and emission spectra of the same electronic transition. When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited

Figure 10.18 (a) Excitation and (b) emission spectra of the powder of Alq3.


state. One way for the system to relax is to emit a photon, thus losing its energy. When the emitted photon has less energy than the absorbed pho-ton, this energy difference is considered as Stokes shift. Alq3 has broadband structured photo-luminescence emission spectra with a Stokes shift rang-ing from 105–150 nm, which is characteristic of intermolecular excimer emission. The Stokes shift between absorption and emission spectrum of Alq3 is shown in Fig. 10.19.

10.2.4.3 Determination of Optical Energy-Band Gap in ChloroformThe optical energy-band gap for the designed complex was calcu-lated using the graphical method given by Morita et al. [57]. The plot of energy × absorbance2 against the energy in eV is used to find the optical energy gap. The straight line portion of the graphs when extrapolated to zero gives the value of optical energy gap of Alq3 complex in chloroform as shown in Fig. 10.20.

10.2.4.4 Photo-Luminescence Spectra of Alq3+PMMAExcitation and emission spectra of Alq3 in solid state and in PMMA matrix at different wt% are shown in Fig. 10.21 for comparison. In the blended thin film of Alq3: PMMA with different concentration ratios, nei-ther annealing nor sublimation at high temperature was carried out, hence the structure of the Alq3 molecule remains the same in the blended thin

Figure 10.19 Stokes shift between absorption and emission spectrum of Alq3.


film as it was in the Alq3powder. However, a slight blue shift of 4 nm in a blended thin film of 1 wt% concentration and a red shift of 12 nm in a blended thin film of 10 wt% concentration was observed. The observed blue shift of emission may be attributed to cross-relaxation [73] whereas the red shift of emission may be attributed to the change from a 2D exci-tation state (surface component) to a 3D excitation state (bulk compo-nent) [74].

Figure 10.21 Excitation emission spectra of Alq3 +PMMA in different wt% [75].

Figure 10.20 Determination of optical energy-band gap of Alq3.


10.2.5 Photo-Physical Properties of BlueLight-Emitting P-Acetyl Biphenyl Cl-DPQTo ensure the suitability of P-acetyl biphenyl Cl-DPQ for fabricating blue OLEDs, assorted characterization techniques such as optical absorption spectra and photo-luminescence spectra in solid state were carried out. To further probe its compatibility in polymer matrix and various acidic and basic solvents, the synthesized complex was molecularly doped in polysty-rene (PS) polymer matrix and its photo-luminescence studied at different wt%. The energy-band gap of P-acetyl biphenyl Cl-DPQ was determined in various acidic and basic solvents at 10−3 M concentration.

10.2.5.1 UV-Visible Absorption SpectraIn order to scan the reallocation of π→π* and the n→π* optical tran-sitions in P-acetyl biphenyl Cl-DPQ, different luminescent solutions of 10−3 molar concentrations were prepared in basic (chloroform, dichlo-romethane) and acidic (acetic acid, formic acid) media. The absorption spectra of solvated DPQ polymeric compound in chloroform exhibits two absorption peaks at 250 and 342 nm, while dichloromethane exhibits peaks at 235 and 305 nm, respectively, which is attributed to the π→π* transition of the conjugated polymer main chains and the n→π* optical transitions attributed to the conjugated side chains in DPQ moieties. The absorption spectra of acetic acid and formic demonstrates strong absorp-tion peaks at 284 and 373, 271, and 373 nm, respectively, with a noticeable red shift with hypochromic effect (decrease in optical density) as shown in Fig. 10.22. The red shift of the absorption spectrum of DPQ in acids as compared to bases may be due to protonation of the imine nitrogen of the quinoline ring to form the quinolinium ion.

10.2.5.2 Determination of Band Gap in Various SolventsUsing the procedure described by Thejo Kalyani et al. [34] the energy gap of P-acetyl biphenyl Cl-DPQ was determined by extrapolating the straight portion of the plots to zero as shown in Fig. 10.23. The energy gaps of the polymeric compound were found to be 3.03, 3.02, 3.40, and 3.37 eV in formic acid, acetic acid, dichloromethane, and chloroform, respectively.

10.2.5.3 Photo-Luminescence SpectraIn order to probe the excitation and emission wavelengths of the syn-thesized P-acetyl biphenyl Cl-DPQ polymeric compound in solid state,


Figure 10.23 Determination of optical energy-band gap of P-acetyl biphenyl Cl-DPQ in different solvents [38].

Figure 10.22 UV-visible spectra of P-acetyl biphenyl Cl-DPQ in different acidic and basic media [38].

photo-luminescence excitation (PLE) spectra was carried out in the range of 220–500 nm. When excited at 377 nm, the emission spectra displays a prominent sharp blue intense emission peak centered at 397 nm with a weak shoulder as shown in Fig. 10.24.


10.2.5.4 Photo-Luminescence Spectra of P-Acetyl Biphenyl Cl-DPQ in PS MatrixFig. 10.25 shows the excitation and emission spectra of P-acetyl biphenyl Cl-DPQ+PS at 10 wt%, plotted in wavelength scale ranging from 200 to 500 nm. This spectrum displays emission peak at 390 nm, which falls near the UV region. In solid state, P-acetyl biphenyl Cl-DPQ displays emission at 397 nm, while in PS matrix, its emission is near the UV region of the electromagnetic spectrum. Thus P-acetyl biphenyl Cl-DPQ can be used as raw emissive material for near UV-blue LEDs.

Figure 10.24 Photo-luminescence spectra of P-acetyl biphenyl Cl-DPQ [38].

Figure 10.25 Excitation and emission spectra P-acetyl biphenyl Cl-DPQ+PS at 10 wt% [38].


As shown the intensity of P-acetyl biphenyl Cl-DPQ decreases with concentration. No accountable shift in λemi was noticed, but change in optical density, which depends on the concentration of Eu3+ ion, was noticed. Thus the interaction between the complex and the host polymer is likely less at higher concentration.

10.3 CONCLUSIONS

Volatile Eu complexes, namely Eu(TTA)3Phen, Eu(x)Y(1−x)(TTA)3Phen, and Eu(x)Tb(1−x)(TTA)3Phen were synthesized by maintaining stoichio-metric ratio. These complexes showed pinkish red emission under UV radiation. When these complexes were excited by UV light in the range of 250–500 nm, a broad excitation peak was observed at about 370 nm with a weak shoulder at 254 nm. Among all the complexes studied, Eu0.4Y0.6(TTA)3Phen showed excellent intense emission at 611 nm with narrow full-width at half-maximum in comparison with the other syn-thesized complexes. This may be due to nonradiative energy transfer from enhancing ion Y3+ to Eu3+ in the doped complexes. The emission intensity increased in the order of Eu(TTA)3Phen<Eu0.5Tb0.5(TTA)3Phen<Eu0.4 Tb0.6(TTA)3Phen<Eu0.5Y0.5(TTA)3Phen<Eu0.4Y0.6(TTA)3Phen, proving their potential application in OLEDs.

The complexes doped in PMMA showed strong absorption peaks at 334 and 336 nm, respectively, which was attributed to the n–π* transi-tion of β-diketonate ligand TTA, with another peak at 280 nm in both complexes attributed to the π–π* transition of the TTA component. The close absorptivities of all the complexes indicate the same concentration of the β-diketonate (TTA) in the polymer, in accordance with all com-pounds having the same tris-chelated core. Among the three complexes doped in PMMA, Eu0.5Tb0.5(TTA)3Phen complex showed a hyper-chromic shift than the other two complexes leading to enhancement in luminescent intensity. The fluorescence intensity in PMMA matrices increased in the order of Eu(TTA)3Phen<Eu0.5Y0.5(TTA)3Phen<Eu0.5

Tb0.5(TTA)3Phen. The 5D0→7F2 emission bands of all the doped systems with similar profiles have a sharp peak at 611–611.4 nm in PMMA. The relative intensities of the sharp peak as well as the shoulders decreased with increase in temperature. These results show that the microenviron-ments around Eu3+ ions in the doped systems at temperatures of 50°C and 80°C may be somewhat different from that of the doped systems at room temperature and pure complex. Thermal treatment may result


in dispersion of emitting molecules in the matrix. For Eu(TTA)3Phen, Eu0.5Tb0.5(TTA)3Phen, and Eu0.5Y0.5(TTA)3 Phen, the emission peak was observed at 611 nm at room temperature as well as at higher tempera-tures. But for Eu0.5Y0.5(TTA)3Phen the emission center shifted slightly to 612 nm at higher temperature (80°C). From the results it can be observed that the emission behavior of the doped systems changed with tempera-ture, clearly reflecting the effect of temperature on the variation of inten-sity. On the basis of absorption spectra of solvated Eu complexes, the optical energy-band gap was determined, which was found to be almost the same in both acidic and basic media. Alq3 was synthesized by a sim-ple precipitation method at room temperature. When excited at 385 nm, it emitted green light at a wavelength of 506 nm. The photo-luminescence revealed that any excitation photon energy in the spectral range extending from the 1La to the 1Bb electronic transition band contributed to the broad green photo-luminescence. This 8-hydroxyquinoline derivative Al metal complex showed broadband structured photo-luminescence emission spectra with a Stokes shift of 150 nm, which is characteristic of intermo-lecular excimer emission. The films of Alq3+PMMA at 10, 5, and 1 wt% excited at 385 nm showed green light emission at 510, 512, and 511 nm, respectively. The films of Alq3+PS at 10, 5, and 1 wt% emitted intense green light at 511, 510, and 508 nm, respectively, when excited at 385 nm. All the films of Alq3 with PMMA and PS showed a small red shift when compared with Alq3 powder. Full-width at half-maximum of pure Alq3 is 75 nm and Alq3 with PMMA at different concentrations of 10, 5, and 1 wt% are 81, 80, 76 nm and for Alq3 with PS at different concentration of 10, 5, and 1 wt% were 82, 76, and 74 nm.

Novel P-acetyl biphenyl Cl-DPQ, i.e., 2-([1,1′-biphenyl]-4-yl)-6-chloro-4-phenylquinoline, was synthesized according to Friedlander condensation. The shift in π→π* and the n→π* optical transitions in P-acetyl biphenyl Cl-DPQ, different luminescent solutions of 10−3 molar concentrations were studied in basic (chloroform, dichloromethane) and acidic (acetic acid, formic acid) media. The red shift of the absorp-tion spectrum in acids as compared to chloroform and dichloromethane of DPQ was observed, which may be due to protonation of the imine nitrogen of the quinoline ring to form the quinolinium ion. The energy gaps of the synthesized polymeric compound were found to be 3.03, 3.02, 3.40, and 3.37 eV in formic acid, acetic acid, dichloromethane, and chloro-form, respectively. When excited at 377 nm, the emission spectra displayed prominent sharp blue intense emission peak centered at 397 nm. The


blended films of polymeric compound prepared by molecular doping of 2-([1, 1′-biphenyl]-4-yl)-6-chloro-4-phenylquinoline (P-acetyl biphenyl Cl-DPQ) in PS by different wt% (10, 5, and 1 wt%) showed good com-patibility. The photo-luminescence spectra of P-acetyl biphenyl Cl-DPQ revealed a larger shift in protic solvents as compared to aprotic and nonpo-lar solvents. Thus all these complexes in solid state can be employed as raw red emissive materials for the fabrication of OLEDs by vacuum deposi-tion. On the other hand, the same complexes are also suitable for fabrica-tion of OLEDs by solution techniques, when dispersed in polymer matrix and the solvated in various basic and acidic solvents. The combination of tricolor phosphor in suitable proportion manifests white light, which can be used for energy-efficient and ecofriendly SSL.

REFERENCES [1] N. ThejoKalyani, S.J. Dhoble, Importance of Eco-friendly OLED LightingDefect and

Diffusion Forum, 357, Trans Tech Publications, Switzerland, 2014, pp. 1–27. [2] N. Thejo Kalyani, S.J. Dhoble, R.B. Pode, J. Biol. Chem. Lumin. 28 (2) (2013) 183–187. [3] Dipti Chitnis, N. Thejo Kalyani, S.J. Dhoble, Bio Nano Front. Mater. Sci. 2 (2012)

346–348. [4] N. Sabatini, M. Guardigli, J.-M. Lehn, J. Coord. Chem. Rev. 123 (1993) 201. [5] G.F. de Sa, O.L. Malta, C. de, M. Donega, A.M. Simas, R.L. Longo, P.A. Santa-cruz, et

al., J. Coord. Chem. Rev. 196 (2000) 165. [6] M. Nogami, T. Hayakawa, Phys. Rev. 56 (1997) R14235. [7] R.W. Corkery, J.P.D. Martin, J. Lumin. 82 (1999) 1. [8] H. Hanzawa, D. Ueda, G. Adachi, K. Machida, Y. Kanematsu, J. Lumin. 94-95 (2001)

503. [9] S.A. Bhagat, S.V. Borghate, N. Thejo Kalyani, S.J. Dhoble, Luminescence 30 (2015)

251–256. [10] S.A. Bhagat, S.V. Borghate, N.S. Koche, N. Thejo Kalyani, S.J. Dhoble, Optik 125

(2014) 3813–3817. [11] S.A. Bhagat, S.V. Borghate, N. Thejo Kalyani, S.J. Dhoble, Lumin. J. Biol. Chem. Lumin.

29 (2014) 433–439. [12] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi, J. Am.

Chem. Soc 122 (2000) 21. [13] Q. Le, L. Yan, Y. Gao, M. Mason, D. Giesen, C. Tang, J. Appl. Phys. 87 (2000) 375. [14] T. Kugler, M. Logdlund, W. Salaneck, Acc. Chem. Res 32 (1999) 225. [15] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [16] B.C. Baker, D.T. Sawyer, Anal. Chem 40 (1968) 1945. [17] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Mascioccihi, A. Sironi, J. Am.

Chem. Soc. 122 (2000) 5147. [18] A. Curioni, W. Andreoni, IBM J. Res. Dev. 45 (2001) 101. [19] R. Larsson, O. Eskilsson, Acta. Chem. Scand. 22 (1968) 1067. [20] K. Sano, Y. Kawata, T.I. Urano, Y. Mori, J. Mater. Chem. 2 (1992) 767. [21] M. Brinkmann, G. Gadret, M. Muccini, C. Masciocchi, A. Sironi, J. Am. Chem. Soc. 122

(2000) 5147. [22] M. Colle, J. Gmeiner, W. Milius, H. Hillebrecht, W. Brutting, Adv. Funct. Mater (2003) 13.


































[23] F. Talylor, T. Wilkins, Chem. Soc., Dalton Trans (1973) 87. [24] P.W. Yawalkar, S.J. Dhoble, N. Thejo Kalyani, R.G. Atram, N.S. Kokode, Lumin. J. Biol.

Chem. Lumin. 28 (2013) 63–68. [25] X.H. Zhang, M.W. Liu, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, et al., Chem. Phys.

Lett. 369 (2003) 478. [26] Ying Kan, Liduo Wang, Lian Duan, Yuanchuan Hu, Guoshi Wu, Yong Qiua, Appl. Phys.

Lett. 84 (2004) 1513. [27] I.D. Parker, Q. Pei, UhZ4X Corporation, 537.5 Overpass Road, Santa Barbara,

California 93111. [28] N.Thejo Kalyani, S.J. Dhoble, Renewable Sustain. Energy Rev. 16 (2012) 2696–2723. [29] S.Y. Mullemwar, G.D. Zade, N.Thejo Kalyani, S.J. Dhoble, Optik 127 (2016)

10546–10553. [30] S.A. Jenekhe, L. Lu, M.M. Alam, Macromolecules 34 (2000) 7315. [31] C.J. Tonzola, M.M. Alam, B.A. Bean, S.A. Jenekhe, Macromolecules. 37 (2000) 3554. [32] N. Thejo Kalyani, R.G. Atram, S.J. Dhoble, Lumin. J. Biol. Chem. Lumin. 29 (2014)

674–678. [33] Wang Dongmei, Cao Wenbo, Fan Jian, Sci. ChinaChem. 57 (6) (2014) 791–796. [34] M. Amati, F. Lelj, J. Phys. Chem. A 107 (2003) 2560. [35] M.D. Halls, H.B. Schlegel, Chem. Mater. 13 (2001) 2632. [36] Boris Minaev, Xin Li, Zhijun Ning, He Tian, Hans Ågren, Organometallic Mater.

Electrolumin. Photovoltaic Devices 3 (2011) 61–100. [37] Hairong Li, Zhang Fujia, Yanyong Wang, Daishun Zheng, Mater. Sci. Eng. B 100

(2003) 40–46. [38] S.Y. Mullemwar, G.D. Zade, N.Thejo Kalyani, S.J. Dhoble (in press), Lumin. J. Biol.

Chem. Lumin. [39] L.H. Slooff, A. Polman, S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. VanVeggel, et al.,

Opt. Mater. 14 (2000) 101. [40] N. Sabatini, M. Guardigli, J.-M. Lehn, J. Coord. Chem. Rev. 123 (1993) 201. [41] G.F. de Sa, O.L. Malta, C. de, M. Donega, A.M. Simas, R.L. Longo, P.A. Santa-cruz, et

al., J. Coord. Chem. Rev. 196 (2000) 165. [42] M. Nogami, T. Hayakawa, Phys. Rev. 56 (1997) R14235. [43] R.W. Corkery, J.P.D. Martin, J. Lumin. 82 (1999) 1. [44] K.A. Gschneiduer, Jr., J.-C.G. Bünzli, , V.K. Pencharsky, Hand book on physics and

chemistry of Rare earth, 35, Elsevier (2005),H. Maas, A. Currao, G. Calzaferri, Angew. Chem. Int. Ed 41 (2002) 2495.

[45] Q. Xu, L. Li, X. Liu, R. Xu, Chem. Mater. 14 (2002) 549. [46] P.N. Minoofar, R. Hernandez, S. Chia, B. Dunn, J.I. Zink, A.-C. Franville, J. Am. Chem.

Soc. 124 (2002) 14388. [47] R.T. Morrison, R.N. Boyd, Organic Chemistry, 3rd Edition, by Allyn and Bacon, New

York, 1973. [48] G. Ma, Y. Yang, G. Chen, Mater. Lett. 34 (1998) 377. [49] N.Thejo Kalyani, S.J. Dhoble, R.B. Pode, Int. J. Knowl. Eng. 3 (1) (2012) 47–49. [50] N.Thejo Kalyani, S.J. Dhoble, Renewable Sustain. Energy Rev. 16 (2012) 2696–2723. [51] M. Metlay, J. Electrochem Soc. 111 (1964) 1253. [52] N.Thejo Kalyani, S.J. Dhoble, R.B. Pode, Lumin. J. Biol. Chem. Lumin. 28 (2) (2013)

183–187. [53] G.A. Crosby, R.E. Whan, R.M. Alire, J. Chem. Phys 34 (1961) 743. [54] L. Chen, J. Bao, C. Gao, S.X. Huang, J. Comb. Chem. 6 (2004) 699. [55] N. Thejo Kalyani, S.J. Dhoble, Gajendra Singh, Optik Int. J. Light Electron Optics 124

(2013) 3614–3617. [56] N. Thejo Kalyani, S.J. Dhoble, R.B. Pode, Indian J. Phys 86 (7) (2012) 613–618.


















































[57] S. Morita, T. Akashi, A. Fujji, M. Yoshida, Y. Ohmori, K. Yoshimato, et al., Synth. Met. 69 (1995) 433.

[58] N. Thejo Kalyani, S.J. Dhoble, R.B. Pode, J. Korean Phys. Soc. 57 (4) (2010) 746–751. [59] N. Thejo Kalyani, S.J. Dhoble, R.B. PodeAdvanced, Mater. Lett. 2 (1) (2011) 65–70. [60] Y.Y. Xu, I. Hemmila, T.N.E. Lovgren, Analyst 117 (1992) 1061. [61] T. Froster, Ann. Phys. 2 (1948) 55. [62] J.J. Ding, H.F. Jiu, J. Bao, J.C. Lu, W.R. Gui, Q.J. Zhang, J. Comb 14 (1976) 91. [63] G. Zhong, K. Yang, Langmiur 14 (1998) 5502. [64] V.S. Pathak, N.Thejo Kalyani, S.J. Dhoble, Int. J. Knowl. Eng. 3 (1) (2012) 124–126. [65] V. Bekiari, G. Pistolis, P. Lianos, Mater. Chem 11 (1999) 3189. [66] V. Bekiari, P. Lianos, J. Sol-Gel Sci. Technol 26 (2003) 887. [67] L.-H. Wang, W. Wang, W.-G. Zhang, E.-T. Kang, W. Huang, Chem. Mater. 12 (2002)

2212. [68] M.S. Pshenichnikov, A. Baltuska, D.A. Wiersma, Chem. Phys. Lett. 389 (2004) 171. [69] C. Silva, P.K. Walhout, P.J. Reid, P.F. Barbara, J. Phys. Chem. 102 (1998) 5701. [70] X.L. Shi, F.H. Long, K.B. Eisenthal, J. Phys. Chem. 99 (1995) 6917. [71] M. Assel, R. Laenen, A. Laubereau, J. Chem. Phys. 111 (1999) 6869. [72] N. Thejo Kalyani, S.J. Dhoble, R.B. Pode, J. Korean Phys. Soc. 57 (4) (2010) 746–751. [73] H.G. Liu, X.S. Feng, K. Jang, S. Kim, T.J. Won, S. Cui, et al., J. Lumin. 127 (2) (2007)

307–315. [74] Y.F. Xu, H.J. Zhang, H.Y. Li, S.N. Bao, P. He, Appl. Surf. Sci. 252 (2006) 2328–2333. [75] P.W. Yawalkar, S.J. Dhoble, N. Thejo Kalyani, R.G. Atram, N.S. Kokode, Lumin. J. Biol.

Chem. Lumin. 28 (2013) 63–68.


























http://dx.doi.org/10.1016/B978-0-08-101213-0.00019-9

CHAPTER 11

Future Prospects of Organic Light-Emitting Diodes

11.1 INTRODUCTION

Even in this technology-driven world, there are still millions of people who live without electricity and lighting in their daily lives. The history of lighting can be viewed as the expansion of increasingly efficient tech-nologies for generating visible light in the desired spectral region. The technologies available today have saturating efficiencies in the 1–25% range. These artificial lights increase night sky luminance, creating the most visible effect of light pollution and artificial sky glow. According to the world atlas of artificial sky luminance developed by Falchi et al. [1] more than 80% of the world and more than 99% of the United States and European populations live under light-polluted skies. The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans. Moreover, 23% of the world’s land surfaces between 75°N and 60°S, 88% of Europe, and almost half of the United States experience light-polluted nights. Unless careful consideration is given to color and lighting levels, this tran-sition could unfortunately lead to a 2–3-fold increase in sky glow on clear nights. In contrast, the newly developed solid-state lighting (SSL) by organic light-emitting diodes (OLEDs) has the potential to generate white light with reduced pollution and energy usage by nearly one half and contribute significantly to the worldwide economy. Hence, OLEDs have become one of the major players in the field of lighting and flat-panel display technology. Once the challenges associated with OLEDs are addressed, this technology has the potential to meet our lighting needs and design ultra-thin displays.

11.2 CURRENT STATUS OF OLEDs

At present, the two most notable features in the field of OLED include the (1) rapid progress and (2) high degree of integration of different


disciples: OLED research encompasses expertise in electrical engineering, synthesis chemistry, optics, electrochemistry, and material science for an up-to-date monograph that covers the current state of progress in OLED research. There is also an emphasis on the technical aspects necessary for the development of viable OLED products from the fundamentals to the practical considerations of device manufacturing. The above has trans-formed organic emissive materials into handheld displays currently avail-able commercially.

Currently OLED technology is facing off against something called quantum dot LCD technology [2]. While there are similarities between the two, the production methods and potential costs for each type of dis-play are actually very different. OLEDs have traditionally been more challenging to make, which has contributed to their limited production. However, successful OLED flat-panel displays have been released by LG Electronics, who presented its new G6 signature television at a press con-ference at the CES 2016 conference. The G6 is designed to essentially be a picture on glass that measures about 2.57 mm thin. OLED technol-ogy does not require backlighting for its display, allowing televisions and other electronics with these screens to be built with very narrow pro-files. Speakers and other circuitry are built into the G6’s stand to slim it down even further. Screens built with OLEDs can also be flexible and are highly energy efficient. LG has also developed a prototype OLED televi-sion screen that can be rolled up like a newspaper. However, there is some debate about whether OLED-based screens ultimately produce superior images compared with its rivals. The contrast ratio and color accuracy is higher in OLED displays. The result and final test must be the content that looks real to the human eye. Companies involved in SSL are releas-ing their first products utilizing OLEDs. Both of these applications ben-efit from the unique properties of the technology to produce a thin, flat source of diffuse light and take advantage of the most appealing feature of OLEDs, their flexibility.

11.3 FUTURE PROSPECTS OF OLEDs

OLEDs have been at the forefront of lighting and flat-panel display tech-nology from a while now. Some of the products being introduced include wall-mount lighting sources, handheld smart displays, large paper-thin TV panels, and many more.

Future Prospects of Organic Light-Emitting Diodes 289

11.3.1 Small DisplaysSmart devices make life easier and more convenient. Some of the recently introduced small display devices include: Pebble smart watch: They offer a way to read color e-paper display, which

works with Android and iPhone smartphones. Netflix: A video-steaming service, offering subscriptions. Users can use

the service on most any device. Atlanta Beta 350: This act as air purifier and effectively helps combat

the polluted air indoors. Redminote 3: A display device with a metal body that comes packed

with 4050 mAh of power and a snap dragon 650 processor.

11.3.2 Large DisplaysThe trend toward higher-resolution displays is evident in every segment of the display industry. Smartphones and tablets have taken the lead in this trend, and higher resolutions are coming to televisions and computer monitors. More is the pixel density (measured in terms of pixels per inch), more is the display resolution and thus detailed text and elements can be observed on the screen. OLED technology paved the way for new innova-tion in the TV industry and provides a solid picture, facilitates boundless contrast, and offers an ideal backdrop for generating vivacious colors with no light noise. In addition, this advanced technology can turn every indi-vidual pixel on or off, avoiding the necessity of backlighting. As noted by researchers, the advanced OLED TV market has gigantic growth potential. In fact, OLED large display panels can even serve as novel light sources by mounting them directly onto a wall or ceiling. Interactive white boards, which work on display vision technology, have become popular in class-rooms today.

11.3.3 Flexible DisplaysOLED technology is expected to reshape the entire display industry over the next few years due to the fact that these displays can be paper thin, transparent, and flexible. Double-sided extremely thin and flexible OLED displays are also expected in the future [3]. Currently, mass production is already underway. This will open the way further for electronic news-papers, construction plans, architecture plans, etc. However, the first ver-sions will not be fully flexible because the control electronics are not very


flexible yet. But within 2–3 years, mass production of OLED displays that can be rolled up (rollable displays) will hit the market. Manufacturers have already launched curved screens that offer an enhanced viewing experi-ence and an immersive panorama effect. The curve also removes the prob-lem of screen-edge visual distortion and detail loss [4,5].

11.4 OLEDs RESEARCH TRENDS IN PAST, PRESENT, AND FUTURE

In the past, OLEDs were primarily used for testing but by 2005, this tech-nology began to be integrated into numerous applications and electronic devices such as mobile phones and tablets. Today more than 80 companies, 70 universities, and many other nonindustrial laboratories are currently engaged in fabricating highly reliable, efficient, and long-life OLEDs with simpler and cheaper manufacturing technology, better encapsulation tech-niques, and optimum device processes. White OLEDs are also being stud-ied worldwide as sources of general illumination. Today, OLEDs are still significantly more expensive than conventional lights and are thus primar-ily suitable for premium sectors such as hotels and restaurants, but this is expected to change in the future.

Researchers are also still addressing some of the inherent problems of OLEDs including low durability and shorter lifespan, which will limit their use in many applications. However, if these challenges are addressed, this technology has the potential to be used for heads-up displays, car displays, billboard-type displays, flat-panel TVs, and even for flexible dis-plays. With the high price of the coating material used to make OLEDs for these applications, researchers are trying to develop and study poten-tial new materials for reliable mass production of these devices. Through the widespread use of new substrates and encapsulation processes, sig-nificant cost reductions can be expected. The efficiency, service life, and color quality of OLEDs have already reached the level of halogen lamps. Presently, OLEDs are used in small displays in cell phones, car stereos, digital cameras, etc. The rapidly growing market for OLED displays and lighting is driving research of both advanced materials and improved man-ufacturing processes. Companies working in the field of OLED devices and displays and hot spots of OLED research and development and manu-facturing are depicted in Figs. 11.1 and 11.2, respectively.

In 2015 some companies moved from OLED TVs back to the old LCDs, but with the use of a technology called “quantum dots” to produce


very bright and colorful images that come close to the range OLEDs offer [3], since they are relatively easy to make and are less expensive to pro-duce. In a TV that uses quantum dots a blue LED is used as a backlight, replacing the white LED used in most LCD TVs. To fabricate a white LED a blue LED is normally used with different phosphors to change the

Figure 11.1 Companies working in the field of OLED devices and displays.

Figure 11.2 Hot spots of OLED research and development and manufacturing around the world.


blue light to white light, which is normally not as pure as and as a result the TVs produced this way are not as bright. In a quantum dot TV the white LED is replaced with a blue one and then special liquids are used that glow in a specific color when purged with light. The glowing colors depend on the sizes of the dots; larger dots are used for red and smaller ones for blue. Because these colors are reasonably precise, the output needs less filtering, which helps with the output levels and accuracy. The tech-nology, most often called quantum light-emission displays (QLEDs), uses this layer of quantum dots to illuminate the LCD panel, i.e., the quan-tum dots produce both the light and the picture itself. While QLEDs are similar to OLEDs, they are simpler and cheaper to make and have a lon-ger lifespan. Like all TV pictures, red, green, and blue LEDs are arranged together in “pixels.” The amount of each of the three colors affects the color of the pixel and thus the final image. While the future holds many possibilities for this technology, today the largest OLED display is from LG’s at 77 inches [3].

While there are OLED TVs available on the market today, the prices are still high and thus the technology will need to figure out how to come down in order for consumers to get fully onboard [2].

11.5 OLEDs: FUTURE PERSPECTIVES

While today just about all cell phones in use are touch screen, this tech-nology was actually only developed about 5 years ago when the Apple launched capacitive touch technology and multitouch. The next milestone in OLED display devices was the incorporation of tactile—or haptic—technology. Tactile screens can imitate different surface structures on a panel. In a tactile touch screen, a separate layer provides feedback to your finger when touched. It can imitate physical buttons and textures dynami-cally on the screen surface such as sandpaper or dirt. One of the most rec-ognized augmented reality projects is Google’s Project Glass where a small device projects visual information onto the user’s eye. It is not an actual screen but rather a mini projector that sits in front of the eye. In the future, light levels will be increased nearly two times.

Homes with OLED films applied to windows are also on the horizon. One of the most practical demonstrations of OLED’s potential outside the display market is in display films used for virtual curtains, which could be activated by the touch of a button to block sunlight. The screen could also be used to show the temperature, humidity level, and extended forecast.


It is anticipated that transparent screens could even be used in refrigerator doors and in shops [3].

Today, OLED TVs have achieved a maximum brightness of about 400 nits (sun reflections are of up to 800–1200 nits). While some research-ers predict that a brightness level of 1000 nits is possible today, it would be disproportionately expensive for consumer products. However, as the technology is improved, including using organic material inside pixels, even higher levels could be obtained. A 4K OLED screen at an afford-able price is expected in the near future. A 4K ultraHD TV has a picture with 3840 × 2160 pixels, more than 8 million pixels in total, which is four times the number in a normal high-definition (1920 × 1080) screen [6]. There would be so much detail and depth that the picture would almost appear 3D. While the benefits of a 4K over a 1080p seem obvious, a 4K TV only improves the picture quality if we are (1) watching native 4K content and (2) sitting close enough to the TV to notice the difference. A 4K TV does not improve the picture quality of lower-quality content such as that of 1080p Blu-rays that are currently available, but due to its flat design it offers better viewing angles than its curved counterparts as well as an impressive picture when viewing HDR content (see Fig. 11.3) [4].

After 4K resolution, the next step is 8K, which again has four times as many pixels as 4K and thus 16 as many as full HD. As TV sizes move toward 100 inches or more, suppliers will start implementing 8K. The step after that is a full “wallpaper” display with no bezels that can fill an entire wall of a house. The fascinating journey of OLEDs from past to future is shown in Fig. 11.4.

Figure 11.3 Comparison of the pixels of normal DVD (720 × 480) pixels with the futur-istic 4K (3840 × 2160) pixels [6].


The unique features of OLED lighting are also being explored by designers, who are exploring various OLED applications such as windows, curtains, automotive light, decorative lighting, and wallpapers [8]. This technology has the potential to transform the way we light our world [7].

11.6 OLEDs IN THE OVERALL LIGHTING SECTOR

Lighting technology is going through a period of technological change with semiconductor-based technologies such as organic and inorganic light-emitting diodes with new designs, emitting light at very few volts, creating the lighting solutions for a wide range of customer require-ments. The field of OLEDs is wide open and looks very promising for ecofriendly SSL in the next 3–5 years. The vision of SSL has largely been driven by the desire to reduce energy consumption by almost 50% and pollution-free light generation. IDTechEx Research estimated the market share of OLEDs per lighting market segment, calculated the total lighting area per sector, estimated the lumen output per segment, and forecasted the equivalent number of units sold per sector. The market segments include residential, office, industrial, outdoor, hospitality, shop, and auto-motive. Each sector attaches a different degree of importance to upfront cost, energy efficiency, lifetime, light intensity, color warmth, and design

Figure 11.4 OLED roadmap [7].


features. This explains why the technology mix in each sector is differ-ent. Combining all their analysis the monetary value of the market was forecasted at a module level per market segment. The relative monetary contributions of each lighting market segment to the total OLED market between 2013 and 2023 are shown in Fig. 11.5.

While OLED lighting has the potential to efficiently emit warm light across large surfaces and to bring new and novel products to the light-ing sector, it still has to be compared with LEDs in order to determine its strengths and weaknesses. A comparison of LEDs and OLEDs is pre-sented in Fig. 11.6 [10]. Due to the fact that inorganic LED lighting was first introduced onto the market, its technology, cost structure, and sup-ply base have dramatically improved, creating a large performance and cost gap between LEDs and the younger OLEDs. Since the introduction of OLEDs the performance gap has not drastically narrowed despite progress in the performance of OLEDs. The challenge facing OLEDs is therefore to identify paths for differentiation, but the prediction is that OLEDs will reach 2.2 billion USD by 2026. Since OLEDs are becoming more impor-tant for use in flexible displays differentiation for OLEDs should be aimed at creating flexible displays and related technology [11,12].

Figure 11.5 Relative monetary contribution of each lighting market segment to the total OLED market between 2013 and 2023 [9].


11.7 INDUSTRIAL CHALLENGES

There are many industrial challenges preventing the use of OLEDs in displays and in SSL including the following.

11.7.1 LifespanTypical plasma TVs have a lifespan of 20,000–30,000 hours, which equates to about 20 years of use if the TV is powered on for 4 hours a day. The lifespan of an LCD TV is typically 50,000–60,000 hours, or about 40 years of running 4 hours daily. The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs his-torically had a lifetime of around 14,000 hours to half original bright-ness (5 years at 8 hours a day) when used for flat-panel displays, which is lower than the typical lifetime of LCD, LED, or plasma technology, which are currently rated for about 60,000 hours to half brightness, depend-ing on manufacturer and model. However, some manufacturers aim to increase the lifespan of OLED displays, pushing their expected life past that of LCD displays by improving light outcoupling, thus achieving the same brightness at a lower drive current, but the limited lifespan of the organic materials is still a challenge to be overcome. Red and green OLEDs have longer lifetimes as compared with blue organics, when used

Figure 11.6 OLED versus LED [10].


for SSL and flat-panel display applications. Research on possible ways for macro-encapsulation or nano-encapsulation as protection against photon degradation is currently underway in Belgium and South Africa. These techniques might help to extend the lifespan of these displays. The latest generation of OLED TVs have a lifespan of 100,000 hours [13].

11.7.2 FabricationThe complexity of fabricating organic materials is still challenging due to the costs and steps involved in the OLED manufacturing process including the (1) cost of substrate manufacturing, (2) organic film deposition, and (3) encapsulation. Vacuum-based techniques currently used in the deposition of small-molecule-based OLEDs are expensive and thus vacuum evaporation is seen as a transitional deposition technique that will eventually be replaced by more cost-efficient, solution-based deposition techniques such as print-ing. The potential for lower fabrication costs that result from the solubility of polymer-based OLED materials in common solvents has been the motiva-tion for their use. The goal is to produce a 4K OLED screen at an afford-able price for everyone. Many new OLED materials are currently being developed to increase the efficiency of OLEDs, and researchers believe these new OLED materials will expand the horizons of this technology [14–16]. With the aim of producing emitters of blue light molecules with excellent fluorescence characteristics, materials such as anthracene, pyrene, and fluo-rene as single core or side moieties [17–23] have been tested. Anthracene and pyrene can form excimers through packing due to their flat molecular structures, but this packing reduces EL efficiency and degrades color purity [24,25]. Bulky side groups to these molecules were added by researchers in an attempt to disrupt packing of anthracene and pyrene [26–28]. With the same goal, Lee et al. [29] substituted various bulky side groups into anthra-cene [30–32]. To overcome these problems with a different approach, they recently reported the synthesis of new dual-core chromophore derivatives containing anthracene and pyrene that include various side groups [33,34]. There is a dihedral angle of approximately 90° between the two corechro-mophores in a dual core comprised of anthracene and pyrene, which disrupts packing. The efficiency of an EL device made with such a dual-core com-pound with a substituted side group has been shown to be higher than that of a device made from a single-core moiety and maybe a good future can-didate to improve emission efficiency. The dual-core compounds with side groups also have other excellent characteristics such as high glass transition


temperature (Tg) and decomposition temperature (Td) values compared to the dual core without side groups [34]. Fig. 11.7 illustrates a typical structure of an OLED lighting panel [35], which includes a substrate on which all lay-ers are coated, a cover glass or metal that protects the multiple organic layers from the environment, and an external light extraction film, which may both improve the panel’s efficacy and create a more esthetically pleasing appear-ance. The organic layers are sandwiched between a transparent anode, such as indium tin oxide (ITO), and a metal cathode, which is typically aluminum or silver. The metal cathode material may be either highly reflective or trans-parent, depending on whether the panel is intended to emit light through the substrate only or through both the substrate and a transparent cover. While transparent OLEDs are not yet commercially available, they represent a unique opportunity in applications where a combination of window and light source is desired.

11.7.3 Degradation IssuesDegradation of the organics in OLEDs is another major challenge with this technology [36,37]. Degradation can be attributed to various mech-anisms such as electrochemical reactions at the electrode/organic inter-face, crystallization of organic solids, and migration of ionic species. Water can instantly damage the organic materials of these types of displays, thus improved sealing processes are important for practical manufacturing. This issue can be controlled to certain extent by the process of bond-ing a metal sheet onto the substrate glass using UV-cured epoxy called encapsulation. Developing sufficiently durable and flexible OLEDs will

Figure 11.7 Illustration of OLED layers [35].


require better materials and further development of manufacturing tools and processes [38]. An exciting opportunity for OLED lighting panels and displays is the potential to use flexible substrates to provide prod-ucts that can be curved, rolled, or even folded [35]. The most commonly selected substrate material for nonrigid OLED applications is some form of plastic, such as polyethylene terephthalate. However, the rate at which oxygen and moisture permeates plastics can challenge OLEDs. Various thin-film encapsulation techniques have been developed and imple-mented, both as methods to eliminate the need and cost of a cover glass and as a way to enable flexible devices on plastic substrates [39]. Thin, flexible glass substrates are another material that may open opportunities for OLEDs. As these technologies improve, and the price to implement them drops, nonrigid OLED lighting products may offer differentiation and a competitive advantage over LED and other lighting technologies. Flexible plastic substrates need improved barrier layers to protect OLEDs from moisture and oxygen. Thin-film encapsulation is also needed to create thin and flexible metal- and glass-based OLEDs. These advances ultimately may lead to very flexible OLED panels for both display and lighting products, ensuring that any surface area—flat or curved—will be able to host a light source. Recent demonstrations by display and lighting companies already have hinted at the potential of flexible OLED tech-nology. Substantial development efforts are being invested in this area and, if successful, flexible OLED panels may become commercially available as early as the last half of this decade.

Several encapsulation methods have been explored that maintain a thin profile and low weight. As illustrated in Fig. 11.8, a sheet of plastic with a multilayer barrier or ultrathin glass can be laminated on top of the upper electrode [40,41]. Special care must be taken to prevent the ingress of oxygen and moisture through the edges. Adhesive materials with barrier or absorbing properties are available from several companies. For down-ward-emitting structures, a thin metal can be used as a cover, providing some mechanical stability as well as an effective surface barrier. This solu-tion has already been implemented by some display manufacturers. The edge effects can be minimized by suitable deposition of a thin-film bar-rier. High-temperature processes must be avoided to prevent damage to the underlying layers. Patterning is needed to avoid coating the electrical contacts at the edge, which can be accomplished during deposition, e.g., by inkjet printing or slot-die coating [40].


11.7.4 Power-Conversion EfficiencyIn order to improve the power-conversion efficiency, reducing driving voltage is crucial. The problem of driving voltage can be overcome by the following: Using low work function metals as cathode Reducing the energy barrier between ITO and hole transport layer

(HTL) Inserting an effective buffer layer between the organic layer and anode Doping strong electron acceptor and donor materials as dopants into

HTL and electron transport layerA concept is currently developed for an outdoor OLED luminaire

using solar energy for lighting pedestrian areas [42]. This novel lumi-naire concept will increase OLED energy savings by reducing the energy used by using solar energy exclusively for power, demonstrating how smart controls and communication can be used with OLED lighting and increasing public awareness of OLED lighting by showcasing its proper-ties in public places. Importantly, this effort will also show the poten-tial for outdoor lighting cost reduction primarily due to eliminating the large costs associated with burying suitable electrical wires and reduc-ing operating costs by integrating advanced operational controls that,

Figure 11.8 Alternative approaches to OLED encapsulation [40,41].


today, are simply not available in commercial luminaires designed for this market.

In future applications, beam-shaping may be required to focus the light where it is most needed or to avoid glare [40]. It seems unlikely that this will be accomplished within the panel, so exterior optical elements may be needed in the luminaire. Though some light-shaping optics may be cost effective in high brightness OLED luminaires, in many applications the bare panel will remain sufficient, providing an advantage in reducing the cost-scaling factor in going from light source to luminaire.

11.7.5 Improved Contrast RatioThough OLED-based displays have matured into commercial prod-ucts, there are still some performance gaps compared with low-cost high-resolution and high-contrast displays with long lifespan. Singh et al. addressed various techniques for increasing the ambient contrast ratio of OLED displays, which greatly depends on the ambient illuminance [43]. In general, most of the techniques used for enhancing contrast also reduce brightness. Therefore in order to achieve the required brightness, it is nec-essary to drive display with higher currents. As a consequence, there is a loss in lifetime. In light of this, black matrix, anti-reflection (AR) coatings, or a combination of both are attractive choices.

11.7.6 ScalingAnother challenge for commercial applications is scaling hybrid white OLEDs to large areas. The efficacy, lifetime, and color uniformity for large-area devices are quite different from those of devices with small areas (few mm2). The key issue is to ensure that the square resistance of both electrodes in the large devices is low enough to avoid the increase of operation voltage and the heterogeneity of current across the active area. Encouragingly, successful techniques, such as introducing a metal grid on top of ITO and employing silver nanowire material as transpar-ent electrode, have been utilized to improve the performance of large-area OLEDs [44,45]. However, improvements are still needed [40].

The first stage involves the formation of the underlying layers onto which the organics are deposited. This usually involves the substrate, with barrier layers if necessary, extraction enhancement layers, and the anode structure. Careful inspection of the processed substrate is necessary so that any defects or contaminants can be identified and repaired before the thin


organic layers are added. The second stage is devoted to deposition of the organic layers. The third stage is the metal deposition to create the second electrode (usually the cathode). Finally, the panel is encapsulated and tested. The standard approach to the fabrication of OLED lighting panels pro-cesses separate substrate sheets. Glass substrates of thickness 0.3 mm or more are self-supporting, whereas ultrathin glass, thin metalfoils, and plastic sub-strates may need to be attached temporarily to a rigid frame during manu-facture to avoid distortion. Around 20 deposition chambers are arranged in line, with the substrate passing at a uniform rate through the whole sequence. This requires coordination between the deposition rates for each layer, some of which are much thicker than others, as shown in Fig. 11.9.

One interesting feature of this arrangement is that the start and fin-ish of the deposition segments are close together. This is primarily so that the substrate carriers can be returned quickly, but it is possible that the partly processed substrates could be directed back through the cycle for the addition of further layers. As shown in Fig. 11.10, cluster configura-tion has been explored, which is more common in the integrated circuit industry. The flexibility offered by this approach will possibly reduce both capital cost and floor space.

Roll-to-roll (R2R) processing is attractive for flexible substrates, because tension can be applied to keep the substrate flat and in the correct position. Simple operations can be carried out at high speed, and many

Figure 11.9 OLED panel production line in Aachen, Germany [40,43].


have indicated that the approach will lead to significant savings. For exam-ple, at the DOE OLED community meeting in 2015, it was estimated that the R2R processing of ultrathin glass could lead to cost reductions of around 30% by eliminating the need to laminate the fragile material to a rigid carrier and then release without damaging the OLED structure. However, all the processing steps must be synchronized to match the rate of web motion. The rolling and unrolling process also introduces the risk of contamination or other damage to the panel surfaces. Thus difference in the yield of good products must be taken into account in comparing manufacturing costs for the two approaches. The development of R2R methods for fabricating OLEDs has been led by Asian and European lab-oratories, with participation by US suppliers of equipment and material. A prototype line is used at the Fraunhofer Institute FEP in Dresden, which hosts many projects supported by the European Commission, the German Federal Government, and the State of Saxony. The approach is almost entirely based on vacuum processing, and it has used both flexible glass and plastic substrates.

The Holst Centre in Eindhoven has focused on coating techniques that can be applied at full atmospheric pressure and so may lead to less expensive equipment bills. To guard against contamination, Holst employs several clean room levels and has the capability to carry out some opera-tions in a nitrogen atmosphere [47].

The Industrial Technology Research Institute (ITRI) of Taiwan has been developing prototype OLED panels for several years and launched the OLED Lighting Commercialization Alliance in July 2014 [48]. The Alliance—which involves suppliers of materials, such as Corning and

Figure 11.10 Cluster configuration for OLED panel manufacturing [40,46].


Merck, and potential manufacturers, such as WiseChip—intends to offer panels with color correlated temperature (CCT) of 1900K to minimize the blue-light impact in health facilities and bedrooms [49]. The ITRI has announced the construction of a R2R production line with low-volume production of panels on flexible glass to begin in 2017 [50].

11.7.7 CompetitionToday display manufacturers are racing to resolve the challenges of OLED technology in order to expand its applications, since OLEDs are in a sense still difficult to produce for large displays. Smaller devices such as phones have been using them for a while now, with great success, but issues arrive when creating products such as large-screen TVs. However, steps can be taken to address these challenges including: Substrates for OLEDs play a critical role in ensuring that they meet

cost efficiency and lifetime requirements. Thus novel and cost-effective flexible substrates that offer dramatically new possibilities are needed.

Separate hole and electron layers provide efficient charge injection and recombination by permitting optimization of electrons, hole injection, and transport simultaneously.

Developing OLED materials and devices that overcome the intrinsi-cally high resistivity of organic materials while achieving balanced charge injection from electrodes into organics.

Employing hetero structure device architecture could confine charges to facilitate efficient recombination and emissive excitation formation. These structures should satisfy two criteria: firstly, high charge mobility of the bulk material and secondly, frontier orbital alignment at the het-ero structure surface.

High phosphorescence and efficiency and microsecond lifetime can be achieved by incorporating a heavy metal atom into the dopant, whose spin–orbit coupling efficiency promotes intersystem crossing between singlet and triplet states.

The options for glowing clothing, creative signage, vehicle bodies, light-emitting roll-up window shades, retail backlighting, wallpaper, and more will explode when R2R manufacturing is perfected for OLEDs. The potential for architectural lighting is enormous. A few suggestions were offered by a panel of lighting designers at the OLED Summit in 2015 [35]. Combine LEDs and OLEDs in lighting systems, so that the LEDs do

directional lighting work and the OLEDs do the diffuse work. This


is done by optimizing the performance of the OLED and LED light sources in a single luminaire. The contemporary luminaire combines the OLEDs for their soft and calm diffused lighting (in direct-view downlight) with the LEDs’ high-lumen performance and precise optical control for the up-light distribution and as the primary source of ambient illumination [51].

Use OLED panels like ceramic tiles, used as durable wall covering, but with digital control to create changing patterns, subtle lumi-nance changes, or decorative elements that also contribute to ambi-ent light levels. A shallow, electrified frame would allow tiles to be popped in and out of a matrix, making an electrical connection as it snaps in, so it would be easy to install and rearrange the tiles as desired.

Use OLED panels as large, low-luminance surfaces. An upper band on walls at the ceiling line could be a functional decorative architectural element.

Use OLED panels as luminous ceiling panels, suspending 2 × 2 ft., 4 × 4 ft., 4 × 8 ft., or larger panels at different planes below the ceil-ing. At low luminances, these OLED planes would deliver pleasant light without visual discomfort or significant reflected glare in dis-play screens.

Use OLEDs as shelf lighting, which could be manufactured as a single unit, either for uplighting translucent merchandise (think champagne glasses) or downlighting expensive leather goods. The OLED panel could also be used to backlight objects on a shelf, or be a plane of light mounted in niches or lightboxes in retail, resi-dential, or museum applications. The wire harness could be inte-grated into the shelving stanchions, offering flexibility for the retailer or display director.

Use OLEDs as marker lights for aircraft cabins, paths of egress in buildings, decorative markings for corporate logos, and similar purposes.

If R2R manufacturing becomes a reality, use OLEDs as lighted lam-inates. Imagine dining-room table tops or bar counters that glow. The question here will be: Can the OLEDs be field-cuttable for custom applications [35]?

Use OLEDs as linear recessed or surface-mounted luminaires with little-to-no recess depth. This is a possibility as long as drivers can be min-iaturized or easily concealed, and if optical films are available to


modify the normal cosine light distribution for wall-washing, nar-row beams, bat-wing distribution, and other lighting effects.

Create shape-changing panels or luminaires using flexible OLEDs con-trolled by servomotors. These would offer a creative medium, espe-cially when curved or folded shapes are possible.

Develop color-changing OLED panels tuneable to deliver the desired CCT. Architectural lighting OLED panels with user-selectable CCT would not only allow color selection according to the appli-cation and time of day, it would also reduce the number of SKUs manufacturers would need to stock.

Apply OLEDs as a lighted wrap for architectural elements: columns, walls, beams, and facades.

Locate OLEDs as luminaires mounted in front of windows on clear sub-strates that simulate daytime light direction even when the sun has gone down.

Consider OLEDs as off-grid lighting solutions for developing countries.

11.8 CONCLUSIONS

Small things can make a difference provided all of us do our part. To pro-tect our environment and make the most of our energy resources, we need to replace incandescent and fluorescent lamps with energy-efficient LED and OLED lamps. Technology like this could not only ensure that people all over the world have access to SSL, but we can also reduce the carbon foot-print by offering increased energy savings and reducing pollution. OLEDs and SSL technologies has the potential for use as light sources, wall deco-rations, OLED display drivers, luminous cloth digital cameras, flat-panel displays, flexible displays, computer displays, mobile phones, televisions, etc. We also predict that 4K OLED screens will soon be affordable enough for everyone. A bright future of flat-panel displays is expected. What’s next?

REFERENCES [1] F. Falchi, P. Cinzano, D. Duriscoe, C.C.M. Kyba, C.D. Elvidge, K. Baugh, et al., Sci. Adv.

2 (6) (2016) 1600377. [2] Xconomy New York, The next phase of the TV tech battle: quantum dot vs. OLED

display. <http://www.xconomy.com/new-york/2016/01/15/the-next-phase-of-the-tv-tech-battle-quantum-dot-vs-oled-display/>, 2016 (accessed 11.07.16).

[3] Exclusive: LG talks future of OLED & first details on 2016 OLED TVs. <http://www.flatpanelshd.com/news.php?subaction=showfull&id=1446462482>, 2015 (accessed 11.07.16).



http://www.xconomy.com/new-york/2016/01/15/the-next-phase-of-the-tv-tech-battle-quantum-dot-vs-oled-display/

http://www.xconomy.com/new-york/2016/01/15/the-next-phase-of-the-tv-tech-battle-quantum-dot-vs-oled-display/

http://www.flatpanelshd.com/news.php?subaction=showfull&id=1446462482

http://www.flatpanelshd.com/news.php?subaction=showfull&id=1446462482


[4] OLED TV: everything you need to know. <http://www.whathifi.com/advice/oled-tv-everything-you-need-to-know>, 2015 (accessed 11.07.16).

[5] The future of TVs may not be OLED after all, but something quite different. <http://www.techradar.com/news/television/the-future-of-tvs-may-not-be-oled-after-all-but-something-quite-different-1296520>, 2015 (accessed 11.07.16).

[6] 4k vs 1080p and upscaling: Is UHD worth the upgrade? <http://www.rtings.com/tv/reviews/by-resolution/4k-ultra-hd-uhd-vs-1080p-full-hd-tvs-and-upscaling-com-pared>, 2014 (accessed 11.07.16).

[7] N. Thejo Kalyani, S.J. Dhoble, Defect and Diffusion Forum, vol. 357, Trans Tech Publications, Switzerland, 2014, pp. 1–27.

[8] D. Chitnis, N.T. Kalyani, H.C. Swart, S.J. Dhoble, Renew. Sust. Energy Rev. 64 (2016) 727–748.

[9] OLED lighting opportunities 2013–2023: forecasts, technologies, players. <http://www.idtechex.com/research/reports/oled-lighting-opportunities-2013-2023-fore-casts-technologies-players-000339.asp?viewopt=showall>, 2014 (accessed 12.07.16).

[10] OLED lighting opportunities 2016–2026: forecasts, technologies, players materials, manufacturing costs, modules and LED competitive assessment. <http://www.idte-chex.com/research/reports/oled-lighting-opportunities-2016-2026-forecasts-tech-nologies-players-000472.asp>, 2016 (accessed 27.07.16).

[11] E.K. Park, S. Kim, J. Heo, H.J. Kim, Appl. Surf. Sci. 370 (2016) 126–130. [12] G. Mone, Commun. ACM 56 (6) (2013) 16–17. [13] LG: OLED TV lifespan is now 100,000 hours. <http://www.flatpanelshd.com/

news.php?subaction=showfull&id=1465304750#wJm5EHppfBf94auh.99>, 2016 (accessed 27.09.16); <http://techtalkradio.com.au/OLED-tv.phpJune>, 2016 (accessed 18.10. 16).

[14] A.S. Kalyakina, V.V. Utochnikova, E.Y. Sokolova, A.A. Vashchenko, L.S. Lepnev, R.V. Deun, et al., Organ. Electron. 28 (2016) 319–329.

[15] K.-J. Choi, J.-Y. Lee, D.-K. Shin, J. Park, J. Phys. Chem. Solids 95 (2016) 119–128. [16] J. Liu, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 149 (2015) 48–53. [17] H. Park, J. Lee, I. Kang, H.Y. Chu, J.I. Lee, S.K. Kwon, et al., J. Mater. Chem. 22 (2012)

2695–2700. [18] Z.Q. Wang, C. Xu, W.Z. Wang, L.M. Duan, Z. Li, B.T. Zhao, et al., New J. Chem. 36

(2012) 662–667. [19] M.S. Cong, H.S. Lee, Y.M. Jeon, J. Mater. Chem. 20 (2010) 10735–10746. [20] C.H. Wu, C.H. Chien, F.M. Hsu, P.I. Shih, C.F. Shu, J. Mater. Chem. 19 (2009)

1464–1470. [21] S.L. Lai, Q.X. Tong, M.Y. Chan, T.W. Ng, M.F. Lo, S.T. Lee, et al., J. Mater. Chem. 21

(2011) 1206–1211. [22] T.M. Figueira-Duarte, P.G.D. Rosso, R. Trattnig, S. Sax, E.J.W. List, K. Mullen, Adv.

Mater. 22 (2010) 990–993. [23] A. Thangthong, N. Prachumrak, R. Tarsang, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk,

et al., J. Mater. Chem. 22 (2012) 6869–6877. [24] B. Balaganesan, W.J. Shen, C.H. Chem, Tetrahedron Lett. 44 (2003) 5747–5750. [25] D. Mansell, N. Rattray, L.L. Etchells, C.H. Schwalbe, A.J. Blake, E.V. Bichenkova, et al.,

Chem. Commun., 41 (2008) pp. 5161–5163. [26] J. Cornil, A.J. Heeger, J.L. Bredas, Chem. Phys. Lett. 272 (1997) 463–470. [27] G. Mu, S. Zhuang, W. Zhang, Y. Wang, B. Wang, L. Wang, et al., Organic Electron. 21

(2015) 9–18. [28] Z. Wang, C. Liu, C. Zheng, W. Wang, C. Xu, M. Zhu, et al., Organic Electron. 23

(2015) 179–185. [29] S. Lee, B. Kim, H. Jung, H. Shin, H. Lee, J. Lee, et al., Dyes Pigments 136 (2017)

255–261.

http://www.whathifi.com/advice/oled-tv-everything-you-need-to-know

http://www.whathifi.com/advice/oled-tv-everything-you-need-to-know

http://www.techradar.com/news/television/the-future-of-tvs-may-not-be-oled-after-all-but-something-quite-different-1296520



http://www.rtings.com/tv/reviews/by-resolution/4k-ultra-hd-uhd-vs-1080p-full-hd-tvs-and-upscaling-compared







http://www.idtechex.com/research/reports/oled-lighting-opportunities-2013-2023-forecasts-technologies-players-000339.asp?viewopt=showall



http://www.idtechex.com/research/reports/oled-lighting-opportunities-2016-2026-forecasts-technologies-players-000472.asp





http://www.flatpanelshd.com/

http://techtalkradio.com.au/OLED-tv.phpJune





























[30] S.K. Kim, Y.I. Park, I.N. Kang, J.H. Lee, J. Park, J. Mater. Chem. 17 (2007) 4670–4678. [31] S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J. Park, J. Mater. Chem. 18 (2008) 3376–3384. [32] S.K. Kim, B. Yang, Y. Park, Y. Ma, J. Lee, H. Kim, et al., Organic Electron. 10 (2009)

822–833. [33] B. Kim, Y. Park, J. Lee, D. Yokoyama, J.H. Lee, J. Kido, et al., J. Mater. Chem. C 1 (2013)

432–440. [34] H. Lee, B. Kim, S. Kim, J. Kim, J. Lee, H. Shin, et al., J. Mater. Chem. C 2 (2014)

4737–4747. [35] OLED lighting products: capabilities, challenges, potential report prepared in support

of the U.S. DOE Solid-State Lighting Program, NJ Miller, FA Leon, May 2016. [36] K.F. Jeltsch, G. Lupa, R.T. Weitz, Organic Electron. 26 (2015) 365–370. [37] J. Zhang, W. Li, G. Cheng, X. Chen, H. Wu, M.-H. Herman Shen, J. Luminesc. 154

(2014) 491–495. [38] Future of OLEDs. <http://www.novaled.com/oleds/future_of_oleds/2016>,

(accessed 27.09.16). [39] OLED-Info (n.d.). OLED encapsulation: technological introduction and market status.

Available from: <http://www.oled-info.com/oled-encapsulation> (accessed 05.16). [40] J. Brodrick (Ed.), DOE Solid State Lighting Program, “R&D Plan,” 2016. [41] M. Riva, SAES Getters, OLED Summit, Berkeley, CA, October 2015. [42] <http://energy.gov/eere/buildings/downloads/outdoor-oled-luminaire-using-solar-

energy-lighting-pedestrian-areas> (accessed 10.16). [43] R. Singh, K.N. Narayanan Unni, A.S. Deepak, Opt. Mater. 34 (2012) 716–723. [44] J. Chen, F. Zhao, D. Ma, Mater. Today 17 (4) (2014) 175–183. [45] U. Hoffman, China International OLED Summit, Beijing, 2015. [46] J. Kreis, Aixtron, OLEDs World Summit, San Diego, CA, 2016. [47] P. Groen, Roll-to-roll processing for solution processed OLED devices, in DOE SSL

Workshop, San Francisco, CA, 28 January 2015. [Online]. Available from: <http://www.energy.gov/sites/prod/files/2015/02/f19/groen_r2roled_sanfrancisco2015.pdf> (accessed 05.16).

[48] Industrial Technology Research Institute, OLCA Aims to Lead the Next-Generation Solid-State Lighting Industrialization, 2014. [Online]. Available from: <https://www.itri.org.tw/eng/DM/PublicationsPeriods/620414544734011541/content/activity3.html> (accessed 05.16).

[49] OLED-info, Wisechip Blue-Light Free OLED Program Update, 2015. [Online]. Available from: <http://www.oled-info.com/wisechip-blue-light-free-oled-program-update> (accessed 05.16).

[50] Electronics Weekly, Taiwan’s ITRI Building OLED Lighting Line, 2016. [Online]. Available from: <http://www.electronicsweekly.com/news/products/led/taiwans-itri-building-oled-lighting-line-2016-04/> (accessed 05.16).

[51] <http://www.electronics-eetimes.com/news/luminaire-blends-leds-and-oleds> (accessed 10.16).












http://www.novaled.com/oleds/future_of_oleds/2016

http://www.oled-info.com/oled-encapsulation

http://energy.gov/eere/buildings/downloads/outdoor-oled-luminaire-using-solar-energy-lighting-pedestrian-areas

http://energy.gov/eere/buildings/downloads/outdoor-oled-luminaire-using-solar-energy-lighting-pedestrian-areas



http://www.energy.gov/sites/prod/files/2015/02/f19/groen_r2roled_sanfrancisco2015.pdf



https://www.itri.org.tw/eng/DM/PublicationsPeriods/620414544734011541/content/activity3.html



http://www.oled-info.com/wisechip-blue-light-free-oled-program-update

http://www.oled-info.com/wisechip-blue-light-free-oled-program-update

http://www.electronicsweekly.com/news/products/led/taiwans-itri-building-oled-lighting-line-2016-04/

http://www.electronicsweekly.com/news/products/led/taiwans-itri-building-oled-lighting-line-2016-04/

http://www.electronics-eetimes.com/news/luminaire-blends-leds-and-oleds

309

INDEX

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

AAccent lighting, 90Activator, 25Active matrix OLED (AMOLED), 33–34,

221Aluminum metal-chelate compounds, 254Ambient lighting, 89Antenna chromophore, 9–10Antenna effect, 9–10, 10f

in lanthanides and organic chelate, 9–10Artificial lighting, 90–91

origin and impact, 90–91flaming torch, 90gas lighting, 90incandescent lightbulb, 90–91natural oil lamps, 90primitive lamps, 90

Atlanta Beta 350, 289

BBack-energy transfer, 18Backlight, 210–211Band-transport mechanism, 54–59, 56fBathochromic shift, 28B-diketonates, 66–68Bicolor LED, 121Bis[2, 3-diphenylquinoxalinato-N,C2¢]

iridium(III) 5-methylpyrazinate (dpq)2 Ir(mprz), 72–73

Blue-light-emitting materials for OLEDs, 76–79, 256f

anthracene derivatives, 76–79approaches, 76–78

bilayer or trilayer films, 76–78organic dyes and polymers, 76–78

3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1, 2, 4-triazole (TAZ), 188

9, 10-bis(2-naphthyl)-2-t-butylanthracene (TBADN), 189–190

bistriphenylenyl, 76–78

2-(4-bromophenyl)-4-phenylquinoline (Br-DPQ), 79

chemical structures of, 79f9, 10-di(2-naphthyl)anthracene (ADN),

188–1899, 10-di-(2-naphthyl) anthracene (ADN),

76–78distyrylarylene derivatives (DSA), 76–78fluorene-based, 188–189hydroxyphenyl–pyridine beryllium

complex, 76–78by introducing spiro group, 190–192LiB(qm)4, 78literature review, 188–192lithium tetra-(8-hydroxy-quinolinato)

boron complex, 78luminance efficiency, 188–1892-(4-methoxy-phenyl)-4-phenyl-

quinoline (OMe–DPQ), 792-(4-methyl-phenyl)-4-phenylquinoline

(M-DPQ), 79nondoped, 190–192oligo(p-phenylenevinylene)s, 188–189P-Acetyl biphenyl Cl-DPQ, 255–257,

283–284determination of band gap in various

solvents, 279, 280fexcitation and emission spectra, 281fphoto-luminescence spectra, 279–280,

281fphoto-physical properties of, 279–282in PS matrix, photo-luminescence

spectra of, 281–282synthesis scheme of, 260–261, 261fUV-visible absorption spectra, 279,

280fpoly(p-phenylene) (PPP), 188spirobifluorene-cored conjugated

compounds, 76–78tris(acetylacetonato)-monophenanthroline

Tm complex, 78

Index310

unidentateorganolithium complex, 76–78

using p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), 190–192

using perylene as a dopant, 188–189using SBAF-type anthracene derivatives,

190–192Zn(hpb)2, 78–79

Boundary orbitals. See Highest occupied molecular orbital (HOMO); Lowest unoccupied molecular orbital (LUMO)

CCarbon nanotubes (CNTs), 29–30Cathode materials for OLEDs, 155Cathode ray tubes (CRTs), 68, 205–206,

212–213, 213fCerium (Ce), 6–7Charge transfer transitions, 6Chromophore, 25Climate-smart lighting (CSL), 115Coil-coil filament, 97–98Color centers, 24Color gamut, 209Color rendering index (CRI), 95–96, 96f,

245Colors, perception by human eye, 88–89,

88fCommission International de l’Eclairage

(CIE) coordinatesOLED, 243–244, 244f, 244twhite light, 94–95

Compact fluorescent lamps (CFLs), 20–24, 87, 103

Concentration quenching, 25, 26fConjugated dendrimers, 152–153P-Conjugated rigid poly(quinoline)s,

65–66, 255Contrast ratio, 209Correlated color temperature (CCT),

245–246, 247f

DDavy, Humphry, 90–91D–d transitions, 6

Dendrimer LEDs, 153properties of, 154t

Dexter energy transfer, 11–12, 12fDiphenyl (10-phenyl-10

H-spiro[acridine-9, 9¢-fluoren]-2¢-yl)phosphine oxide (POSTF), 80–81

Discharge lamps, 99–103compact fluorescent lamps, 103high-pressure, 99–100linear fluorescent lamps, 102–103mercury vapor lamps, 99metal halide lamps, 99–101sodium lamps, 99, 102

Discrete LED, 120Display colors, 210Display devices, 207–208

active, 208, 208tpassive, 208, 208t

Display port, 210Displays, 32–34, 206–207

basic parameters ofcolor capability, 207projection technology, 207screen size, 206sharpness, 207viewing angle, 207

categorieslarge, 211medium, 211microdisplays, 211small, 211superlarge, 211

E-paper, 223–224future outlook, 223–224history of technology, 211–219LED, 32–33, 33fOLED, 33–34, 34ftechnology landscape of LCDs, LEDs,

PDPs, FEDs, and OLEDs, 222tterminology

aspect ratio, 208backlight, 210–211color gamut, 209contrast ratio, 209display colors, 210display port, 210display resolution, 209

Blue-light-emitting materials for OLEDs (Continued)

Index 311

encapsulation, 211lifetime, 210luminance, 210pixel, 209refresh rate, 210response time, 210screen burn-in, 210

ultrathin, 223Dopant, 25Dysprosium (Dy3+), 6–7

EEdison, Thomas, 87, 90–91Einstein’s law, 2Electrically powered lamps, 97–99

comparison of different parameters of, 110t

flowchart, 94fluminescent lamps, 99–103

Electroluminescence (EL)devices, 68in organic materials, 59–60phenomenon from organic materials,

171–174spectrum, 242, 243f

Electron-injecting materials for OLEDs, 155

Electron-stimulated luminescence lamps, 99Electron-transport layer (ETL), 171–174Electron-transport materials, 154–155

structure, physical, chemical, thermal, and optical properties of, 156t

Emissive materials for OLEDs, 149–154small molecules, 149–151, 151f

Encapsulation, 211E-paper, 223–2241-Ethyl-2-(2-pyridyl) benzimidazole

(EPBM), 174–175Eu(BA)(TTA)2 phen, 72–73Europium complex, 72–73

b-diketonate complexes, 72Eu(TTA)3 (DPPz), 175–176Eu (TTA)2 (N-HPA) Phen, 176–177,

177fIII b-diketonate complexes, 66–68, 74f,

174–175Europium (Eu3+), 6–7Excimer, 25

Exciplex, 25Excited triplet state, 17Exciton, 26External quantum efficiency (EQE), 61–62

FF-Block lanthanide and actinide series, 6Field emission displays (FEDs), 29–30,

205–206, 214–215Flat-panel displays (FPDs), 205–207Flexible OLED displays, 289–290Fluorescence, 16–17Fluorescent lamps, 93Fluorescent materials, 60–61Flux density, 3–4Förster (dipole–dipole) mechanism, 11–124K ultra HD TV, 293Fridrich, Elmer, 98–99Frontier orbitals. See Highest occupied

molecular orbital (HOMO); Lowest unoccupied molecular orbit (LUMO)

GG6 signature television, 288Green-light-emitting materials for OLEDs,

73–76, 77fAl–Cu alloy, 1851, 2-bis(1-methyl-2, 3, 4,

5,-etraphenylsilacyclopentadienyl) ethane (2PSP), 73–75

bis(2-methyl 8-hydroxiquinoline) aluminum hydroxide (Almq 2 OH), 185

9, 9-diarylfluorene-terminated 2, 1, 3-benzothiadiazole (DFBTA), 185–186

2, 5-di-(3-biphenyl)-1, 1-dimethyl-3, 4-diphenyl silacyclo pentadiene (PPSPP), 73–75

9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9 H-carbazoleas ligands, 75–76

heteroleptic iridium (III) complexes using carbene and pyridine-triazole, 186–187

literature review, 182–1889-((6-Phenylpyridin-3-yl) methyl)-9

H-carbazole, 75–76

Index312

phosphorescent dendrimers, 183–185phosphorescent Pt(II) complexes

(Pt1–Pt3), 187–188poly(2,bquinoline vinylene) (PQV),

182–1839-silafluorene-9-spiro-1¢-(2¢, 3¢,

4¢, 5¢-tetraphenyl)-1¢-H-silacyclopentadiene (ASP), 73–75

Tb(MTP)3 Phen, 73–75Tb-tris-(acetylacetonato), Tb(acac)3,

73–75, 182–183Tb-tris(tetradecylphethalate)

phenanthroline complex Tb(MTP)3 Phen, 182–183, 183f

tris(8-hydroxyquinoline) aluminum (Alq3), 75–76, 78–79, 189–190, 254, 260f

absorption spectra of, 276–277determination of optical energy-band

gap in chloroform, 277, 278fdetermination of Stokes shift, 276–

277, 277fexcitation and emission spectra of,

276ffac-Alq3 and mer-Alq3, 254photo-luminescence spectra of,

276photo-physical properties of,

275–279in PMMA, photo-luminescence

spectra of, 277–278, 278f, 282–283synthesis scheme of organo metallic,

258–260Znq2 .2H2O crystal, 75–76

HHalogen lamps, 98–99Heeger, Alan J., 41Heterojunction device, 54–55High-energy visible (HEV) light, 97Highest occupied molecular orbital

(HOMO), 39–40, 50–54, 50fHigh-intensity discharge (HID) lamps, 93High-pressure discharge lamps, 99–100

High-pressure mercury vapor lamps, 99, 101–102

High-pressure sodium lamps, 99Hole, 26Hole-blocking layer (HBL), 171–174Hole-injecting materials, 149

physical, chemical, thermal, and optical properties of, 150t

Hole-transport materials (HTL), 147–149, 171–174

structure, physical, chemical, thermal, and optical properties of, 148t

Holonyak Jr., Nick, 117–118Hopping mechanism of charge transport,

56–57, 57fHybrid white organic light-emitting diodes

(HWOLEDs), 196–198Hydroxobis b-diketonates, 66–68Hyperchromic shift, 28Hypochromic shift, 28Hypsochromic shift, 27

IIlluminance, 92Incandescence, 3–5

demonstration of, 4finverse square law, 5f

principle of, 97sources, 3–5

Incandescent bulb, 5Incandescent filament displays (IFDs),

205–206, 214Incandescent lamps, 97–98Infrared (IR) radiation, 1Inner-transition metals, 6Inorganic luminescent materials, 62Intensity of illumination, 92Internal conversion, 26–27Intersystem crossing, 26Intraligand charge-transfer (ILCT) states, 11ITO glass substrate, 229–231, 229f

JJablonski diagram, 10–13, 11f

fluorescence and phosphorescence, 18fJ-V-L curve characteristics of OLEDs,

242, 243f

Green-light-emitting materials for OLEDs (Continued )

Index 313

KKiller impurities, 25

LLamp phosphors, 29Lanthanides

excited state of, 7–9intraconfigurational f–f transitions, 7–9luminescence in, 7–9magnetic-dipole and electric-dipole

transitions of, 7–9physical, chemical, thermal, and optical

properties of, 15tLanthanum (La), 6–7Large display devices, 289Lasers, 29LED chip technologies, 117–118LED lamps, 104–107, 104f. See also Light-

emitting diodes (LEDs)advantage of, 104advantages of, 106approaches to obtaining white light from,

105–106, 106fcomponents, 105construction of filaments, 107, 108fdevelopment of brighter, 105–106filament bulb, 106–107, 107finfrared, 105–106surface-mount devices (SMD), 105

Lifetime, 92, 210Ligand-enhanced lanthanide luminescence

mechanism, 10–13Ligand-to-metal charge transfer (LMCT),

6, 11–13Light, 1–2

importance of, 88–89perception of, 1–2velocity of, 2

Light-conversion efficiency, 5Light emissions, 1

mechanism of, 3–24Light-emitting dendrimers, 152–153Light-emitting diodes (LEDs), 20–24,

29–30, 31f, 32–33, 33f, 65–66, 115, 120–123. See also LED lamps

advantages, 126–128alphanumeric displays, 122–123

applications, 129basic configurations, 125bicolor, 121clusters and lights, 122, 122fcolor shift, 128configurations and manufacturing,

125correct voltage and current, 129discrete, 120displays, 220encapsulation of, 125fenergy efficiency, 126–128generation of light from, 124flight-emission parameters of

various semiconductors used for, 127t

light pollution, 129materials, 126operating environment, 128physical function, 123–125polarity, 128seven-segment display, 123shapes and sizes, 121fsingle-color, 120–121tricolor, 122, 122fvs OLED, 137

Light-emitting polymers (LEPs), 116Lighting, 89

classification, 89–90accent lighting, 90ambient lighting, 89artificial lighting, 90–91task lighting, 89–90

climate change and globalization, effects on lighting systems, 116–117

day, 89active, 89passive, 89

terminology, 91–92Light pollution, 92Light sources, 92–94

artificial, 93–94milestones in evolution of, 93fnatural, 92–93spectral distribution of different, 97

Linear fluorescent lamps, 102–103

Index314

Liquid crystal displays (LCDs), 205–206, 216–219, 216f

active-matrix technology, 219advantages and disadvantages, 217,

218tcategories, 217–218color, 218graphical, 218monochrome character/segment, 218passive-matrix technology, 218–219

Lowest unoccupied molecular orbit (LUMO), 39–40, 50–54, 50f

Low-pressure discharge lamps, 99Low-pressure mercury vapor lamp,

101–102Lumen, 91Luminaire, 91–92

efficiency, 91Luminance, 92, 210Luminescence, 3, 5–16

in actinides, 13applications, 21t–23tbased on source of excitation, 20based on time lag, 16–20centers, 24in electron–hole centers, 13–16extended defects, 16in heavy metals, 13innovation in technology of, 28–34in lanthanides, 7–9originating in triplet states, 17quantum yield, 158–159in rare earth metal complexes, 6–13sources, 20–24terminology associated with, 24–28in transition metal ions, 6

Luminescent organic compounds, 59–62Luminosity, 92Luminous density, 92Luminous efficacy, 91

of a halogen lamp, 98–99of incandescent bulb, 97–98

Luminous energy, 91Luminous exposure, 92Luminous flux, 91Luminous intensity, 91Lux, 92

MMcDiarmid, Alan G., 41Mercury vapor lamps, 99, 101–102Metal halide lamps, 99–101Metal 8-hydroxyquinoline (Mqn) chelates,

254Metal to ligand charge transfer excited state

(MLCT), 61–62Microdisplays, 211Minimum excitation energy, 2Moby, Fredrick, 98–99Molecular organic light-emitting diodes

(MOLEDs), 183–185Monomer, 42–44

NN,N-dimethyl formamide (DMF), 72–73Nakamura, Dr. Shuji, 117–118Nanophosphors, 29–30Netflix, 289

OOrganic light-emitting diodes (OLEDs),

20–24, 29, 31, 33–34, 34f, 59–60, 65–66, 87, 107–109, 115, 141–142, 227

advantages, 135–136, 161–162anatomy of, 130–132, 131fanode of, 146–147applications, 164–167available from LG display, 248–249, 250f

TM-30–15 color metrics and graphics, 246–248, 248f

blue, 107–109blue-light-emitting materials for, 76–79challenges, 136characterization techniques of, 240–251

CIE coordinates, 243–244, 244f, 244tcolor characteristics, 246–249color rendering index (CRI),

245correlated color temperature (CCT),

245–246, 247feffects of the hole-modulating layer

(HML), 245–246, 246felectroluminescence (EL) spectrum,

242, 243f

Index 315

J-V-L curve characteristics, 242, 243flifetime measurements, 249–251V-I characteristics, 241–242, 242f

CSL with, 130–136current status, 287–288device architectures, 160–161, 171–174,

172fbottom-light-emitting OLEDs, 161,

162ftop-light-emitting LEDs, 162ftop-light-emitting OLEDs, 160–161

device structure of, 241fdisplays, 220–223

active-matrix (AMOLED), 221advantages of, 220–221passive-matrix (PMOLED), 221

efficiency of, 155–1608K OLED screen, 293emission color, 107–109energy-saving and ecofriendly models

for lighting, 166ffabrication, 240t

design of the electroluminescence (EL), 227–228

Doosan DND fabrication system, 228–230

general specifications, 228–229on prepatterned ITO substrate,

230–231procedure, 229–230, 231fsolution techniques, 237–240technologies, 231–240, 232fvacuum deposition techniques,

232–2374K OLED screen, 293framework of primary single-layer and

double-layer, 143–145, 143ffuture prospects, 288–290, 292–294Germany-based blue-TADF, 250–251, 250fgreen, 107–109green-light-emitting materials for, 73–76HOMO, LUMO, and energy gap of

some materials used for, 157tindustrial challenges, 296–306

competition, 304–305complexity of fabricating organic

materials, 300

contrast ratio, 304degradation issues, 300lifespan, 300power-conversion efficiency, 300scaling, 304–306

influence in lighting sector, 294–295monetary contributions, 295f

large displays, 167LED vs, 137, 296flight-emitting mechanism, 132, 134f,

145–146light-extraction efficiency in, 158luminescence quantum yield, 158–159materials for, 146–155, 180f

Al-doped zinc oxide, 147emissive layer (EML) materials,

149–154fluorine-doped tin oxide, 147hole-injecting materials, 149hole-transport materials (HTL),

147–149substrates, 146transparent conductive oxides, 147

multulayer, 131–132notable features of, 287–288organic layers, 130organic over inorganic

advantages of, 136comparison of different parameters,

136t, 137foverall thickness of active, 143–145P-i-N, 171–174power efficiency, 158purpose and basic requirements of

different layers in, 133tquantum efficiency, 159recombination efficiency, 158red, 107–109red-light-emitting materials for, 66–73requirements of different layers in, 173tresearch hurdles and challenges, 162–164,

290–292, 291fdark spots on, 163fdegradation, 162–163lifetime, 164

roadmap, 294fsmall-area full-color, 167f

Index316

small displays, 166–167solution-processed devices, 232stacked, 161, 162fstructure of, 142–145techniques to improve the efficiency of,

160appropriate organic materials, 160codoping, 160device structure, 160endothermic energy transfer, 160harvesting triplet states, 160layer thickness, 160

transparent, 161, 162ftypical characteristics of RGB, 166tas versatile light sources, 164–166white-light-emitting materials for,

80–81working principle of, 144f

Organic materialscharge transport in, 54–59

band transport, 54–59, 56fhopping transport, 56–57tunneling transport, 57–58

mechanical and chemical properties, 39–40

optoelectronic devices based on, 39–40organic compounds, 40–45

based on functional group, 44based on presence of heteroatoms, 41based on size, 41–44characterization techniques and

targeted outcomes, 46tluminescent, 59–62neutral, 44physical properties of, 44polymers, 42–44small molecules, 42, 43fsolubility, 44

organic semiconductors, 45–52applications, 51tcharge transport in, 49–52comparison of crystalline/inorganic

and molecular/organic solids, 49tHOMO and LUMO in, 52–54mobility of, 56

molecular orbitals and energy level in, 53f

p-type and n-type semiconductors, 52ftraditional vs polymeric, 48t

PPassive matrix OLED (PMOLED), 33, 221Pebble smart watch, 289P-electron delocalization, 151–152Phenyl quinoline, 255Phosphorescence, 17–20Phosphorescent materials, 61–62Phosphorescent OLEDs (PhOLEDs), 179–

182, 185–186Phosphorescent (persistent) phosphor, 18Phosphors, 24

quantum efficiency, 27Physical vacuum deposition, 235–237, 236f,

237fadvantages, 236–237disadvantages, 237

Physical vacuum deposition (PVD), 231–232

Pixel, 209Planck’s black-body emission theory, 3Planck’s constant, 2Plasma display panels (PDPs), 205–206,

219–220Polarons, 56Polyannaline, 153fPoly (9, 9¢-dioctlyfluorene), 153fPolymer electroluminescence, 151–152Polymeric semiconductors, 48tPolymerization, 42–44Polymer light-emitting diodes (PLEDs),

151–152, 227, 253properties of, 154t

Polymers, 42–44semiconducting behavior of, 42–44

Poly(p-phenylene), 153fPoly(p-phenylenevinylene), 153fPoly(propyleneamine) dendrimer, 153fPoly(pyrrole), 153fPope, Martin, 142Power efficiency, 158PQM (poly(3, 7-N-octyl phenothiozinyl

cyanoisophthalylidene)), 176–177

Organic light-emitting diodes (OLEDs) (Continued)

Index 317

PQP (poly(3, 7-N-octyl phenothiozinyl cyanotere-phthalylidene)), 176–177

2-(2-Pyridyl)benzimidazole (HPBM), 174–175

QQuantum-caged atoms (QCAs), 29–30Quantum dot LCD technology, 288Quantum efficiency, 27, 159Quantum yield of a radiation-induced

process, 27Quenching, 25

RRare earth b-diketonates, 66–68, 67f

europium Eu(III) b-diketonates, 66–68, 174–175

samarium Sm(III) b-diketonates, 66–68Rare earth metals

descriptive classification of, 8tluminescence in, 6–13

Recombination efficiency, 158Red, green, and blue (RGB), 1–2, 29–30,

60–61OLED devices, 32fphosphorescent P-i-N homojunction

devices, 189–190Red-light-emitting lanthanide b-diketonate

complex, 72Red-light-emitting materials for OLEDs,

66–73cationic iridium (Ir) complexes,

179–181C545T dopant, 178–1794-(dicyanomethylene)-2-t-butyl-6-

(8-methoxy-1, 1, 7, 7-tetra methyljulolidyl-9-

enyl)4 H-pyran (DCJMTB), 69–704-(dicyanomethylene)-2-t-butyl-

6-(8-methoxy-1, 1, 7, 7-tetra methyljulolidyl-9-enyl) 4 H-pyran (DCJMTB), 174–175

Eu complexes doped in PMMA matrixabsorption spectra of, 266–267, 267fenergy gap of, 267, 268foptical parameters of synthesized, 271t

EuTb(1–x) (TTA)3 Phen, 265–266, 265f

model relaxation and energy transfer process of, 266, 266f

Eu(TPBDTFA)3 Phen, 70–71Eu(TTA)3 Phen, 69–70, 174, 181–182,

255–257, 258t, 282emission, 72–73excitation and emission spectra of,

263–264, 263ffluorescence of, 264model relaxation and energy transfer

process of, 266, 266fphoto-luminescence spectra of, 264–

265, 264fin PMMA, photo-luminescence

spectra of, 267–271, 268f, 270fin PMMA, thermal annealing effect

on, 271–272red-light-emitting with, 72synthesis scheme of Eu complexes,

257–258, 259fheteroleptic iridium (III) complexes,

179–181literature review, 174–182methyl methacrylate (MMA), 70phosphorescent silicon-cored

spirobifluorene derivative (SBP-TS-PSB), 71–72

poly(N-vinylcarbazole) (PVK), 70PQM (poly(3, 7-N-octyl

phenothiozinylcyanoisoph-thalylidene)), 70–71

PQP (poly(3, 7-N-octyl phenothiozinylcyanotereph-thalylidene)), 70–71

rare earth b-diketonates, 66–68, 67fred phosphorescent iridium (III)

complexes, 177–178red phosphorescent Ir (III) complexes,

181–182solvated Eu complexes, energy gap of,

272–275, 273f, 274f, 275tTb(DBM)3 Phen, 70–71

Redminote 3, 289Red phosphorescent heteroleptic-tris-

cyclometalated-iridium complex, 71–72

Reinhoudt’s empirical rule, 18

Index318

RGB light-emitting phosphors, 255experimental details, 255–282photo-physical properties

of blue light-emitting P-acetyl biphenyl Cl-DPQ, 279–282

of green light-emitting Alq3 complex, 275–279

of red light-emitting europium hybrid organic complexes, 263–275

in PMMA/PS matrix, 261–263, 262fmolecular structure of PMMA, 262–

263, 262fphysical and chemical parameters of,

262tpreparation of blended films, 262–263

synthesis of, 257–261scheme of Eu complexes, 257–261

SSafe limits of blue-light, 88Screen burn-in, 210Screening effect, 9Semiconductor–air interference, 124–125Sensitization process, 9–10Sensitizer, 25Shirakawa, Hideki, 41Single-color LED, 120–121Singlet state, 25Small display devices, 289Small molecular organic light-emitting

devices (SMOLEDs), 233Small molecules, 42, 43f, 149–151, 151f

fluorescent, 60–61for OLEDs, 60f

properties of, 154tSodium lamps, 102Solar photovoltaic (PV), 115Solid-state lighting (SSL), 29, 31–32, 65–66,

87, 103–109, 115, 117f, 253, 287advantages, 87brief history, 116future perspectives, 109illumination, 116in LED lamps, 104–107with LEDs, 117–130

blue LED covered with yellow-emitting phosphor, 118–119

combination of RBG and yellow for white light, 119

near UV or blue LED and RGB phosphors, 119–120

right proportion of RGBY generating white light, 120f

selection of right fraction of tricolors (RBG), 118

strategies for, 118–120in OLED lamps, 107–109requisite of, 116–117

Solution techniques, 237–240ink-jet printing, 239, 239flaser-induced thermal image, 240screenprinting, 240spin-coating, 238, 238f

Stilbenoid dendrimer, 153fStokes, George G., 27Stokes shift, 27, 27fSuper luminescent diode (SLD), 30Surface-conduction electron-emitter

displays (SEDs), 205–206, 215–216Swan, Joseph Wilson, 90–91

TTask lighting, 89–90Tb-tris-(acetylacetonato), Tb(acac)3,

73–75Terbium (Tb3+), 6–7Thermally activated delayed fluorescence

(TADF) OLEDs, 187–188Thermal quenching, 25Transparent conductive oxides (TCO),

147Transport bands in organic crystals, 62Trap, 26Tricolor LED, 122, 122fTriplet-singlet transitions, 17Triplet states, 25Tris(8-hydroxyquinoline)-aluminum (Alq3),

65–66Tunneling mechanism of charge transport,

57–58, 58f

UUltrathin displays, 223Ultraviolet (UV) light, 1

Index 319

VVacuum deposition techniques, 232–237

physical vacuum deposition, 235–237, 236f, 237f

advantages, 236–237disadvantages, 237

vacuum thermal evaporation, 233–235, 234f, 235f

advantages, 234disadvantages, 234–235

Vacuum fluorescent displays (VFDs), 205–206, 213–214

Vacuum thermal evaporation, 233–235, 234f, 235f

advantages, 234disadvantages, 234–235

VIBGYOR, 1–2V-I characteristics of OLEDs, 241–242,

242fVisible light, 1Visible (VIS) spectrum, 1–2, 2f

wavelength range and band width of different colors of, 2t

WWhite light, quality of

chromaticity diagram, 95fCIE coordinates, 94–95color rendering index (CRI), 95–96, 96fcomparison of CIEs, CRIs, and CCTs

for, 247tcorrelated color temperature (CCT),

96–97lighting-source categorization and

applications based on, 96twhite-light illumination, 96

White-light-emitting materials for OLEDsfrom blue-light-emitting zinc

complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)2 ], 80–81

chemical approaches, 80–81

by dispersing blue, red, and green fluorescent dye in polyvinylcarbazole (PVK), 192–193

with Eu (aca)3 Phen binuclear complexes, 192–193

graphene/PEDOT:PSS conductive film, 196–198, 199f

iridium complex Ir(dfbppy)(fbppz)2 and yellow-emitting osmium complex Os(bptz)2 (dppee), 195–196

iridium (III) complexes, 196–198literature review, 192–198phosphorescent dopants, 193–194by sequential energy transfer between

different layers, 192–193using a blend of luminescent

semiconducting polymers and organometallic complexes, 194–195

using multiple emitters, 193fusing phosphorescent dopants, 193–194from Zn(hpb)2, 80–81

White-light generation, 134–135combination of RBG and yellow for

white light, 119down-conversion phosphor system, 135exciplex emission structures, 135host–guest systems, 134microcavity structures, 135multilayer structures, 135right proportion of RGBY, 120fsingle-molecule structures, 134–135

White organic light-emitting diodes (WOLEDs), 141–142, 192–193

White polymer light-emitting devices (WPLEDs), 80–81, 141–142

Wiedemann, Eilhard, 5–6Wiley, Emmett, 98–99

YYellow Zn(hpb)mq-emitting materials,

78–79Ytterbium (Yb), 6–7

principles and applications of organic light emitting diodes (oleds)

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