Seminar Report

on

OLED

Submitted in partial fulfillment for the requirement of the award of Bachelor of Technology Degree in Electronics and Communication

Engineering

Submitted by:

Abhishek-2307222

Under the supervision of

Er. Gaurav Walia

Lecturer ECE Department, ACE

Submitted to:

Department of Electronics and Communication Engineering

Ambala College of Engineering and Applied Research,

Devsthali. (Affiliated to Kurushetra University, Kurushetra)

CERTIFICATE

Certified that this seminar report entitled “OLED” is the bonafide work of “ABHISHEK (2307222) of B.Tech 8th Semester, Electronics and Communication Engineering, Ambala College of Engineering and Applied Research, Devsthali, Ambala ”, who presented the seminar under my supervision.

Seminar Coordinator

(Er.Gaurav Walia) (Er. Ashok Kumar)Lecturer Associate Professor and HeadECE Department ECE DepartmentACE ACE

ACKNOWLEDGEMENT

I wish to record out esteem and profound sense of gratitude to Er. Gaurav Walia, Lecturer Electronics & Communication Engineering Department, of Ambala College of Engineering & Applied Research for giving me a lot of help in writing this report.I am also thankful to Associate Prof. Ashok Kumar, Head of the department (Electronics & Communication Engineering) Ambala College of Engineering & Applied Research, and faculty members for their co-operation and advice, which have brought many improvements in the quality of this report.I am very thankful to Dr. J.K. Sharma, Director of Ambala College of Engineering & Applied Research, Ambala who gave me an opportunity to expose myself.

Abhishek (2307222)

ABSTRACT

The seminar is about polymers that can emit light when a voltage is applied to it. The structure comprises of a thin film of semiconducting polymer sandwiched between two electrodes (cathode and anode).When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light .The band gap, i.e. The energy difference between valence band and conduction band determines the wavelength (color) of the emitted light.

They are usually made by ink jet printing process. In this method red green and blue polymer solutions are jetted into well defined areas on the substrate. This is because, OLEDs are soluble in common organic solvents like toluene and xylene .The film thickness uniformity is obtained by multi-passing (slow) is by heads with drive per nozzle technology .The pixels are controlled by using active or passive matrix.

The advantages include low cost, small size, no viewing angle restrictions, low power requirement, biodegradability etc. They are poised to replace LCDs used in laptops and CRTs used in desktop computers today.

Their future applications include flexible displays which can be folded, wearable displays with interactive features, camouflage etc. Unlike other flat panel displays OLED has a wide viewing angle (upto 160 degrees), even in bright light. Their low power consumption (only 2 to 10 volts) provides for maximum efficiency and helps minimize heat and electric interference in electronic devices. Because of this combination of this features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space. Their application in numerous devices is not only a future possibility but a current reality.

LIST OF FIGURES

Figure No. Title Page No.

2.1 OLED schematic 6

2.2 OLED working principle 7

3.1 Alq3 9

3.2 poly (p-phenylene vinylene) 10

3.3 Ir (mppy)3 11

3.4 OLED structure 12

3.5 Schematic of the ink jet printing 13

3.6 Active and passive matrices 15

3.7 Conjugation of π 18

3.8 Series of orbital diagrams 19

4.1 Active matrix OLED Structure 22

4.2 Passive matrix OLED structure 23

4.3 Transparent OLED structure 24

4.4 Top-emitting OLED structure 25

4.5 Foldable OLED 26

4.6 White OLED 27

6.1 A Sony PSP having foldable OLED display 32

6.2 Toshiba Laptop having Transparent OLED 33

CHAPTER - 1

INTRODUCTION

INTRODUCTION

1.1 History

Eastman Kodak Company & Universal laboratories, USA ha started the research towards the OLED technology in the mid of 90’s but the cup of victory gone to the Kodak researchers have made a number of major breakthroughs which led to patents on OLED material, device structure, dopping techniques to drastically improve efficiency and colour control, thin film deposition method, patterning methods as well as design & fabrication methods for both active & passive matrix OLED panels.

The OLED technology initially grew from research on organic electronic devices used in solar cells & electrophotography. At this time Kodak is the world’s only company who has patent on this OLED technology. The intrinsic quality of this technology is superb because of its high brightness & efficiency, low drive voltage fast response. Low cost manufacturing methods are already in use for passive matrix OLED display. The advance of the complementary low temperature polySi technology has enabled the fabrication of high resolution, full colour, active matrix OLED display. An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and PDAs, watches, advertising, information and indication. OLEDs are also used in light sources for space illumination and in large-area light-emitting elements. Due to their early stage of development, they typically emit less light per unit area than inorganic solid-state based LED point-light sources.

An OLED display functions without a backlight. Thus, it can display deep black levels and can be thinner and lighter than liquid crystal displays. In low ambient light conditions such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCD using either cold cathode fluorescent lamps or the more recently developed LED backlight.

1.1 Evolution With the imaging appliance revolution underway, the need for more advanced handheld devices that will combine the attributes of a computer, PDA, and cell phone is increasing and the flat-panel mobile display industry is searching for a display technology that will

revolutionize the industry. The need for new lightweight, low-power, wide viewing angled, handheld portable communication devices have pushed the display industry to revisit the current flat-panel digital display technology used for mobile applications. Struggling to meet the needs of demanding applications such as e-books, smart networked household appliances, identity management cards, and display-centric handheld mobile imaging devices, the flat panel industry is now looking at new displays known as Organic Light Emitting Diodes (OLED).

Over the time there are many changes came into the field of output/display devices. In this field first came the small led displays which can show only the numeric contains. Then came the heavy jumbo CRTs (Cathode Ray Tubes) which are used till now. But the main problem with CRT is they are very heavy & we couldn’t carry them from one place to another the result of this CRT is very nice & clear but they are very heavy & bulky & also required quiet large area then anything else.

Then came the very compact LCDs (Liquefied Crystal Displays). They are very lighter in weight as well as easy to carry from one place to the other. But the main problem with the LCDs is we can get the perfect result in the some particular direction. If we see from any other direction it will not display the perfect display. To overcome this problems of CRTs & LCDs the scientist of Universal Laboratories, Florida, United States & Eastman Kodak Company both started their research work in that direction & the overcome of their efforts is the new generation of display technologies named OLED (Organic Light Emitting Diode) Technology.

In the flat panel display zone unlike traditional Liquid-Crystal Displays OLEDs are self luminous & do not required any kind of backlighting. This eliminates the need for bulky & environmentally undesirable mercury lamps and yields a more thinner ,more compact display. Unlike other flat panel displays OLED has a wide viewing angle (up to 160 degrees), even in bright light. Their low power consumption (only 2 to 10 volts) provides for maximum efficiency and helps minimize heat and electric interference in electronic devices. Because of this combination of this features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space. Their application in numerous devices is not only a future possibility but a current reality.

An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and PDAs, watches, advertising, information and indication. OLEDs are also used in light sources for space illumination and in large-area light-emitting elements.

Due to their early stage of development, they typically emit less light per unit area than inorganic solid-state based LED point-light sources.

An OLED display functions without a backlight. Thus, it can display deep black levels and can be thinner and lighter than liquid crystal displays. In low ambient light conditions such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCD using either cold cathode fluorescent lamps or the more recently developed LED backlight.

There are two main families of OLEDs: those based upon small molecules and those employing polymers. Adding mobile ions to an OLED creates a Light-emitting Electrochemical Cell or LEC, which has a slightly different mode of operation. OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, and can make higher resolution and larger size displays possible.

CHAPTER 2

WORKING PRINCIPLE

WORKING PRINCIPLE

A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of special organic molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors. The first, most basic OLEDs consisted of a single organic layer, for example the first light-emitting polymer device synthesized by Burroughs et al involved a single layer of poly(p-phenylene vinylene). Multilayer OLEDs can have more than two layers to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted.

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode.

1. The battery or power supply of the device containing the OLED applies a voltage across the OLED.

2. An electrical current flows from the cathode to the anode through the organic layers (an electrical current is a flow of electrons).

3. The cathode gives electrons to the emissive layer of organic molecules. 4. The anode removes electrons from the conductive layer of organic molecules. (This is the

equivalent to giving electron holes to the conductive layer.) 5. At the boundary between the emissive and the conductive layers, electrons find electron

holes.

Figure 2.1 OLED schematic

6. When an electron finds an electron hole, the electron fills

Figure 2.2 OLED working principle

the hole (it falls into an energy level of the atom that's missing an electron). When this happens, the electron gives up energy in the form of a photon of light (see How Light Works).

7. The OLED emits light. 8. The color of the light depends on the type of organic molecule in the emissive layer.

Manufacturers place several types of organic films on the same OLED to make color displays.

9. The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.

CHAPTER – 3

CONSTRUCTION

CHAPTER – 3

CONSTRUCTION

CONSTRUCTION

3.1 Material technologies

3.1.1 Small molecules

Figure 3.1 Alq3, commonly used in small molecule OLEDs.

Efficient OLEDs using small molecules were first developed by Dr. Ching W. Tang et al. at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use.

Molecules commonly used in OLEDs include organometallic chelates (for example Alq3, used in the organic light-emitting device reported by Tang et al.), fluorescent and phosphorescent dyes and conjugated dendrimers. A number of materials are used for their charge transport properties, for example triphenylamine and derivatives are commonly used as materials for hole transport layers. Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used. Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.

The production of small molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive and of limited use for large-area devices than other processing techniques. However, contrary to polymer-based devices, the vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures. This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs.

Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated. The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.

3.1.2 Polymer light-emitting diodes

Figure 3.2 poly(p-phenylene vinylene), used in the first PLED.

Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage. They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced.

Vacuum deposition is not a suitable method for forming thin films of polymers. However, polymers can be processed in solution, and spin coating is a common method of depositing thin polymer films. This method is more suited to forming large-area films than thermal evaporation. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing. However, as the application of subsequent layers tends to dissolve those already present, formation of multilayer structures is difficult with these methods. The metal cathode may still need to be deposited by thermal evaporation in vacuum.

Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer backbone may determine the colour of emitted light or the stability and solubility of the polymer for performance and ease of processing.

While unsubstituted poly(p-phenylene vinylene) (PPV) is typically insoluble, a number of PPVs and related poly(naphthalene vinylene)s (PNVs) that are soluble in organic solvents or water have been prepared via ring opening metathesis polymerization.

3.1.3 Phosphorescent materials

Figure 3.3 Ir(mppy)3, a phosphorescent dopant which emits green light.

Phosphorescent organic light emitting diodes use the principle of electrophosphorescence to convert electrical energy in an OLED into light in a highly efficient manner, with the internal quantum efficiencies of such devices approaching 100%.

Typically, a polymer such as poly(n-vinylcarbazole) is used as a host material to which an organometallic complex is added as a dopant. Iridium complexes such as Ir(mppy)3 are currently the focus of research, although complexes based on other heavy metals such as platinum have also been used.

The heavy metal atom at the centre of these complexes exhibits strong spin-orbit coupling, facilitating intersystem crossing between singlet and triplet states. By using these phosphorescent materials, both singlet and triplet excitons will be able to decay radiatively, hence improving the internal quantum efficiency of the device compared to a standard PLED where only the singlet states will contribute to emission of light.

Applications of OLEDs in solid state lighting require the achievement of high brightness with good CIE coordinates (for white emission). The use of macromolecular species like polyhedral oligomeric silsesquioxanes (POSS) in conjunction with the use of phosphorescent species such as Ir for printed OLEDs have exhibited brightnesses as high as 10,000 cd/m2.

OLED is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film of semiconducting polymer sandwiched between two electrodes (anode and cathode) as shown in fig.1. When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light that escapes through glass substrate. The bandgap, i.e. energy difference between valence band and conduction band of the semiconducting polymer determines the wavelength (colour) of the emitted light.

Figure 3.4 OLED structure

Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxides typically used for the anode and aluminum or calcium for the cathode. Fig.2.1(a) shows the structure of a simple single layer device with electrodes and an active layer.

In order to manufacture the polymer, a spin-coating machine is used that has a plate spinning at the speed of a few thousand rotations per minute. The robot pours the plastic over the rotating plate, which, in turn, evenly spreads the polymer on the plate. This results in an extremely fine layer of the polymer having a thickness of 100 nanometers. Once the polymer is evenly spread, it is baked in an oven to evaporate any remnant liquid. The same technology is used to coat the CDs.

Single-layer devices typically work only under a forward DC bias. Fig.2.1(b) shows a symmetrically configured alternating current light-emitting (SCALE) device that works under AC as well as forward and reverse DC bias.

3.2 INK JET PRINTING PROCESS

Although inkjet printing is well established in printing graphic images, only now are applications emerging in printing electronics materials. Approximately a dozen companies have demonstrated the use of inkjet printing for PLED displays and this technique is now at the forefront of developments in digital electronic materials deposition. However, turning inkjet printing into a manufacturing process for PLED displays has required significant developments of the inkjet print head, the inks and the substrates .Creating a full color, inkjet printed display requires the precise metering of volumes in the order of pico liters. Red, green and blue polymer solutions are jetted into well defined areas with an angle of flight deviation of less than 5º. To ensure the displays have uniform emission, the film thickness has to be very uniform.

Fig. 3.5 Schematic of the ink jet printing

For some materials and display applications the film thickness uniformity may have to be better than ±2 per cent. A conventional inkjet head may have volume variations of up to ±20 per cent from the hundred or so nozzles that comprise the head and, in the worst case, a nozzle may be blocked. For graphic art this variation can be averaged out by multi-passing with the quality to the print dependent on the number of passes. Although multi-passing could be used for PLEDs the process would be unacceptably slow. Recently, Spectra, the world’s largest supplier of industrial inkjet heads, has started to manufacture heads where the drive conditions for each nozzle can be adjusted individually – so called drive-per-nozzle (DPN). Litrex in the USA, a subsidiary of CDT, has developed software to allow DPN to be used in its printers. Volume variations across the head of ±2 per cent can be achieved using DPN. In addition to very good volume control, the head has been designed to give drops of ink with a very small angle-of-flight variation. A 200 dots per inch (dpi) display has colour pixels only 40 microns wide; the latest print heads have a deviation of less than ±5 microns when placed 0.5 mm from the substrate. In addition to the precision of the print head, the formulation of the ink is key to making effective and attractive display devices. The formulation of a dry polymer material into an ink suitable for PLED displays requires that the inkjets reliably at high frequency and that on reaching the surface of the substrate, forms a wet film in the correct location and dries to a uniformly flat film. The film then has to perform as a useful electro-optical material. Recent progress in ink formulation and printer technology has allowed 400 mm panels to be colour printed in under a minute. However, turning inkjet printing into a manufacturing process for PLED displays has required significant developments of the inkjet print head, the inks and the substrates Creating a full color, inkjet printed display requires the precise metering of volumes in the order of pico liters. Red, green and blue polymer solutions are jetted into well defined areas with an angle of flight deviation of less than 5º are used. In order to manufacture the polymer, a spin-coating machine is used that has a plate spinning at the speed of a few thousand rotations per minute.

3.3 ACTIVE AND PASSIVE MATRIX

Many displays consist of a matrix of pixels, formed at the intersection of rows and columns deposited on a substrate. Each pixel is a light emitting diode such as a PLED, capable of emitting light by being turned on or off, or any state in between. Coloured displays are formed by positioning matrices of red, green and blue pixels very close together. To control the pixels, and so form the image required, either 'passive' or 'active' matrix driver methods are used.

Pixel displays can either by active or passive matrix. Fig. 2.1.2 shows the differences between the two matrix types, active displays have transistors so that when a particular pixel is turned on it remains on until it is turned off.

The matrix pixels are accessed sequentially. As a result passive displays are prone to flickering since each pixel only emits light for such a small length of time. Active displays are preferred, however it is technically challenging to incorporate so many transistors into such small a compact area.

Fig 3.6 Active and passive matrices

In passive matrix systems, each row and each column of the display has its own driver, and to create an image, the matrix is rapidly scanned to enable every pixel to be switched on or off as required. As the current required to brighten a pixel increases (for higher brightness displays), and as the display gets larger, this process becomes more difficult since higher currents have to flow down the control lines. Also, the controlling current has to be present whenever the pixel is required to light up. As a result, passive matrix displays tend to be used mainly where cheap, simple displays are required.

Active matrix displays solve the problem of efficiently addressing each pixel by incorporating a transistor (TFT) in series with each pixel which provides control over the current and hence the brightness of individual pixels. Lower currents can now flow down the control wires since these have only to program the TFT driver, and the wires can be finer as a result. Also, the transistor is able to hold the current setting, keeping the pixel at the required brightness, until it receives another control signal. Future demands on displays will in part require larger area displays so the active matrix market segment will grow faster.

PLED devices are especially suitable for incorporating into active matrix displays, as they are processable in solution and can be manufactured using ink jet printing over larger areas.

3.4 BASIC PRINCIPLE AND TECHNOLOGY

Polymer properties are dominated by the covalent nature of carbon bonds making up the organic molecule’s backbone. The immobility of electrons that form the covalent bonds explain why plastics were classified almost exclusively insulators until the 1970’s.

A single carbon-carbon bond is composed of two electrons being shared in overlapping wave functions. For each carbon, the four electrons in the valence bond form tetrahedral

oriented hybridized sp3 orbitals from the s & p orbitals described quantum mechanically as geometrical wave functions. The properties of the spherical s orbital and bimodal p

orbitals combine into four equal , unsymmetrical , tetrahedral oriented hybridized sp3

orbitals. The bond formed by the overlap of these hybridized orbitals from two carbon atoms is referred to as a ‘sigma’ bond.

A conjugated ‘pi’ bond refers to a carbon chain or ring whose bonds alternate between single and double (or triple) bonds. The bonding system tend to form stronger bonds than might be first indicated by a structure with single bonds. The single bond formed between two double bonds inherits the characteristics of the double bonds since the single bond is

formed by two sp2 hybrid orbitals. The p orbitals of the single bonded carbons form an effective ‘pi’ bond ultimately leading to the significant consequence of ‘pi’ electron de-localization.

Unlike the ‘sigma’ bond electrons, which are trapped between the carbons, the ‘pi’ bond electrons have relative mobility. All that is required to provide an effective conducting band is the oxidation or reduction of carbons in the backbone. Then the electrons have mobility, as do the holes generated by the absence of electrons through oxidation with a dopant like iodine.

3.5 LIGHT EMISSION

The production of photons from the energy gap of a material is very similar for organic and ceramic semiconductors. Hence a brief description of the process of electroluminescence is in order.

Electroluminescence is the process in which electromagnetic(EM) radiation is emitted from a material by passing an electrical current through it. The frequency of the EM radiation is directly related to the energy of separation between electrons in the conduction band and electrons in the valence band. These bands form the periodic arrangement of atoms in the crystal structure of the semiconductor. In a ceramic semiconductor like GaAs or ZnS, the energy is released when an electron from the conduction band falls into a hole in the valence band. The electronic device that accomplishes this electron-hole interaction is that of a diode, which consists of an n-type material (electron rich) interfaced with p-type material (hole rich). When the diode is forward biased (electrons across interface from n to p by an applied voltage) the electrons cross a neutralized zone at the interface to fill holes and thus emit energy.

The situation is very similar for organic semiconductors with two notable exceptions. The first exception stems from the nature of the conduction band in an organic system while the second exception is the recognition of how conduction occurs between two organic molecules.

With non-organic semiconductors there is a band gap associated with Brillouin zones that discrete electron energies based on the periodic order of the crystalline lattice. The free electron’s mobility from lattice site to lattice site is clearly sensitive to the long-term order of the material. This is not so for the organic semiconductor. The energy gap of the polymer is more a function of the individual backbone, and the mobility of electrons and holes are limited to the linear or branched directions of the molecule they statistically inhabit. The efficiency of electron/hole transport between polymer molecules is also unique to

polymers. Electron and hole mobility occurs as a ‘hopping’ mechanism which is significant to the practical development of organic emitting devices.

PPV has a fully conjugated backbone (figure 2.2.1), as a consequence the HOMO (exp link remember 6th form!) of the macromolecule stretches across the entire chain, this kind of situation is ideal for the transport of charge; in simple terms, electrons can simply "hop" from one π orbital to the next since they are all linked.

Figure 3.7 A demonstration of the full conjugation of π

PPV is a semiconductor. Semiconductors are so called because they have conductivity that is midway between that of a conductor and an insulator. While conductors such as copper conduct electricity with little to no energy (in this case potential difference or voltage) required to "kick-start" a current, insulators such as glass require huge amounts of energy to conduct a current. Semi-conductors require modest amounts of energy in order to carry a current, and are used in technologies such as transistors, microchips and LEDs.

Band theory is used to explain the semi-conductance of PPV, see figure 5. In a diatomic molecule, a molecular orbital (MO) diagram can be drawn showing a single HOMO and LUMO, corresponding to a low energy π orbital and a high energy π* orbital. This is simple enough, however, every time an atom is added to the molecule a further MO is added to the MO diagram. Thus for a PPV chain which consists of ~1300 atoms involved in conjugation, the LUMOs and HOMOs will be so numerous as to be effectively continuous, this results in two bands, a valence band (HOMOs, π orbitals) and a conduction band (LUMOs, π* orbitals). They are separated by a band gap which is typically 0-10eV (check) and depends on the type of material. PPV has a band gap of 2.2eV (exp eV). The valence band is filled with

all the π electrons in the chain, and thus is entirely filled, while the conduction band, being made up of empty π* orbitals (the LUMOs) is entirely empty).

In order for PPV to carry a charge, the charge carriers (e.g. electrons) must be given enough energy to "jump" this barrier - to proceed from the valence band to the conduction band where they are free to ride the PPV chain’s empty LUMOs.

Figure 3.8 a series of orbital diagrams.

In this model, holes and electrons are referred to as charge carriers, both are free to traverse the PPV chains and as a result will come into contact. It is logical for an electron to fill a hole when the opportunity is presented and they are said to capture one another. The capture of oppositely charged carriers is referred to as recombination. When captured, an electron and a hole form neutral-bound excited states (termed excitons) that quickly decay and produce a photon up to 25% of the time, 75% of the time, decay produces only heat, this is due to the the possible multiplicities of the exciton. The frequency of the photon is tied to the band-gap of the polymer; PPV has a band-gap of 2.2eV, which corresponds to yellow-green light.

Not all conducting polymers fluoresce, polyacetylene, one of the first conducting-polymers to be discovered was found to fluoresce at extremely low levels of intensity. Excitons are still captured and still decay, however they mostly decay to release heat. This is what you may have expected since electrical resistance in most conductors causes the conductor to become hot.

Capture is essential for a current to be sustained. Without capture the charge densities of holes and electrons would build up, quickly preventing any injection of charge carriers. In effect no current would flow.

A diatomic molecule has a bonding and an anti-bonding orbital, two atomic orbitals gives two molecular orbitals. The electrons arrange themselves following, Auf Bau and the Pauli Principle. A single atom has one atomic obital. A triatomic molecule has three molecular orbitals, as before one bonding, one anti-bonding, and in addition one non-bonding orbital. Four atomic orbitals give four molecular orbitals. Many atoms results in so many closely spaced orbitals that they are effectively continuous and non-quantum. The orbital sets are called bands. In this case the bands are separated by a band gap, and thus the substance is either an insulator or a semi-conductor. It is already apparent that conduction in polymers is not similar to that of metals and inorganic conductors , however there is more to this story! First we need to imagine a conventional diode system, i.e. PPV sandwiched between an electron injector (or cathode), and an anode. The electron injector needs to inject electrons of sufficient energy to exceed the band gap, the anode operates by removing electrons from the polymer and consequently leaving regions of positive charge called holes. The anode is consequently referred to as the hole injector.

CHAPTER-4

TYPES OF OLED

TYPES OF OLED

4.1 Active-matrix OLED - AMOLED

AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image. AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large screen TVs and electronic signs or billboards.

Active-matrix OLED displays provide the same beautiful video-rate performance as their passive-matrix OLED counterparts, but they consume significantly less power. This advantage makes active-matrix OLEDs especially well suited for portable electronics where battery power consumption is critical and for displays that are larger than 2” to 3” in diagonal

Figure 4.1 Active matrix OLED Structure

An active-matrix OLED (AMOLED) display consists of OLED pixels that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that illuminate light upon electrical activation. In contrast to a PMOLED display, where electricity is distributed row by row, the active-matrix TFT backplane acts as an array of switches that control the amount of current flowing through each OLED pixel.

4.2 Passive-matrix OLED - PMOLED

PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.

Figure 4.2 Passive matrix OLED structure

PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitry. PMOLEDs are most efficient for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as those you find in cell phones, PDAs and MP3 players. Even with the external circuitry, passive-matrix OLEDs consume less battery power than the LCDs that are currently used in these devices.

4.3 Transparent OLED

Transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate. When a transparent OLED display is turned on, it allows light to pass in both directions. A transparent OLED display can be either active- or passive-matrix. This technology can be used for heads-up displays.

Figure 4.3 Transparent OLED structure

Top emission: Using the same transparent structure, TOLED technology can also be used for top-emitting structures for active-matrix displays and with opaque substrates. Especially desirable for high-resolution, active-matrix OLED applications, a top-emittingstructure can improve the effective active area and the power consumption of the display by directing the emitted light away from the thin film transistor (TFT) backplane rather than through it (see schematic below). Top-emitting OLEDs can also be built on opaque surfaces such as metallic foil and silicon wafers. To illustrate this point, the video (to the right) shows an icon-format TOLED demonstrator that Universal Display Corporation built on metallic foil with Palo Alto Research Center (PARC), a subsidiary of Xerox Corporation, and Vitex Systems, Inc. Potential TOLED applications include smart cards or displays on furniture, automotive parts and other opaque surfaces, to suggest a few.

4.4 Top-emitting OLED

Top-emitting OLEDs have a substrate that is either opaque or reflective. They are best suited to active-matrix design. Manufacturers may use top-emitting OLED displays in smart cards. TOLED transparent and top-emitting OLED technology uses a proprietary transparent contact structure to create displays that can be transparent, that is, top- and bottom-emitting or, selectively, top-emitting only.

Figure 4.4 Top-emitting OLED structure

TOLEDs can significantly enhance display performance and open up many new product applications. Transparency: TOLEDs can be 70% to 85% transparent when switched off, nearly as clear as the glass or plastic substrate on which they are built. To better picture this, please refer to the video (to the right) where a simple transparent OLED pixel is shown turning on and off. This feature paves the way for TOLEDs to be built into vision-area applications, such as architectural windows for home entertainment and teleconferencing purposes, and automotive windshields for navigation and warning systems. TOLEDs may also enable the development of novel helmet-mounted or "heads-up" systems for virtual reality, industrial and medical applications.

4.5 Foldable OLED

Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable OLED displays can be sewn into fabrics for "smart" clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.

Figure 4.5 Foldable OLED

FOLED flexible OLEDs are organic light emitting devices that are built on flexible substrates such as plastic or metallic foil. FOLED displays can offer significant performance advantages over LCD displays that are typically built on rigid glass

substrates and contain a bulky backlight. Today, the primary substrate candidates are thin plastics, such as PET and PEN polyester films. While these materials offer many attractive features, they also currently impose limitations with respect to thermal processing and barrier performance. Companies are developing coatings for these substrates as well as new plastic substrates to compensate for these constraints. Universal Display Corporation is actively working with a number of these companies.

4.6 White OLED

A white organic LED (OLED) incorporating a blue phosphorescent dye and a down-conversion phosphor has achieved a luminous efficacy of 25 lm/W. This high-efficacy device was enabled by lowering the device operating voltage, increasing the outcoupling efficiency, and incorporating highly efficient phosphorescent emitters.

Figure 4.6 White OLED

Solid-state white lighting using PHOLED, TOLED and FOLED technologies represents a true breakthrough for next-generation lighting. Among the exciting advances in white OLED lighting technology are the PHOLED technology and materials present the potential to combine the power efficiencies of fluorescent tubes with the pleasing color quality associated with incandescent bulbs in a thoroughly new flat form factor. In collaboration with Toyota Industries Corporation, at the 2004 Society for Information Display Symposium and Exhibition, we reported record-breaking white PHOLED performance exceeding 18 lm/W at an operating voltage of < 6.5 V, brightness of 1000 cd/m2 and CIE color coordinates of (0.38, 0.38).

CHAPTER- 5

ADVANTAGES & DRAWBACKS

ADVANTAGES & DRAWBACKS

OLEDs offer many advantages over both LCDs and LEDs:

• Require only 3.3 volts and have lifetime of more than 30,000 hours. • Low power consumption. • Self luminous. • No viewing angle dependence. • Display fast moving images with optimum clarity. • Cost much less to manufacture and to run than CRTs because the active material is

plastic. • Can be scaled to any dimension. • Fast switching speeds that are typical of LEDs. • No environmental draw backs. • No power in take when switched off. • All colours of the visible spectrum are possible by appropriate choose of polymers. • Simple to use technology than conventional solid state LEDs and lasers. • Very slim flat panel. The plastic, organic layers of an OLED are thinner, lighter and more

flexible than the crystalline layers in an LED or LCD. • Because the light-emitting layers of an OLED are lighter, the substrate of an OLED can

be flexible instead of rigid. OLED substrates can be plastic rather than the glass used for LEDs and LCDs.

• OLEDs are brighter than LEDs. Because the organic layers of an OLED are much thinner than the corresponding inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered. Also, LEDs and LCDs require glass for support, and glass absorbs some light. OLEDs do not require glass.

• OLEDs do not require backlighting like LCDs. LCDs work by selectively blocking areas of the backlight to make the images that you see, while OLEDs generate light themselves. Because OLEDs do not require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is especially important for battery-operated devices such as cell phones.

• OLEDs are easier to produce and can be made to larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets. It is much more difficult to grow and lay down so many liquid crystals.

• OLEDs have large fields of view, about 170 degrees. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles. OLEDs produce their own light, so they have a much wider viewing range.

OLED seems to be the perfect technology for all types of displays, but it also has some problems:

Lifetime - While red and green OLED films have longer lifetimes (46,000 to 230,000 hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours. The major drawback is the limited lifetime of organic materials. This problem still needs to be solved to push OLED technology to be more successful in the future. Blue OLEDs have only a lifetime of around 5,000 hours, when used in flat panel displays, which is much lower than the typical lifetimes of LCDs or plasma displays. But there are various experimentations to increase the lifetime, some are reporting that they already reached a lifetime up to 10,000 hours and above.

Water - Water can easily damage OLEDs. Organic materials can easily be damaged by water intrusion into the displays. Therefore an improved sealing process is necessary for OLED displays.

Vulnerable to shorts due to contamination of substrate surface by dust. Voltage drops. Mechanically fragile. Potential not yet realized. The development of the technology is restrained by patents held by Kodak and other

companies. For commercial development of OLED technology it is often necessary to acquire a license.

CHAPTER 6

APPLICATIONS & FUTURE SCOPE

APPLICATIONS & FUTURE SCOPE

Applications of OLEDs

OLEDs have been proposed for a wide range of display applications including magnified micro displays, wearable, head-mounted computers, digital cameras, personal digital assistants, smart pagers, virtual reality games, and mobile phones as well as medical, automotive, and other industrial applications. This OLEDs with its full color displays will replace today’s liquid crystal displays (LCDs) used in laptop computers and may even one day replace ordinary CRT-screens. OLED technology is already used in some devices. On this page we will name some products that are powered by OLED displays. Most of them are cellular phones or portable music players, but also other products use this new technology. Cellular/mobile phones There are many mobile phones that use OLED displays. Samsung has several models like the SGH-E700, E715 or E730. All these models use an external OLED screen with different resolutions (64 x 96, 96 x 96 pixels) and different color depths (either 256 colours or 65k colours). The Samsung SGH-X120 uses a main OLED screen with 128 x 128 pixels. The S88 phone from BenQ-Siemens uses a two inch active-matrix OLED display with about 262k colors and 176 x 220 pixels. LG Electronic offers several mobile phones with an OLED technology. LG LP4100 has an external display powered with the new technology.

Figure 6.1 A Sony PSP having foldable OLED display

LG's model VX8300 has an organic light-emitting diode display with 262,000 colors and a resolution of 176 x 220 pixels.

Other mobile phone manufacturers like Motorola, Nokia, Panasonic or Sony Ericsson are also using organic light emitting diodes for their external displays. MP3 players MobiBLU ships an mp3 player that features an OLED display, the DAH-1500i model. The popular Creative Zen Micro has also an organic LED display with 262k colors. The Sony NW-A3000 and NW-A1000 both have an OLED display. The Zen Sleek music player from Creative has a new 1.7 inch organic LED display. The Giga beat audio player from Toshiba features also an OLED screen. The Kodak Easy Share LS633 is the world's first digital camera with an organic LED display. The Sanyo Xacti HD1 is a high definition camera that features an OLED display. Other digital cameras with an OLED screen are from Hasselblad (H2D-39 and 503CWD for example).

Figure 6.2 Toshiba Laptop having Transparent OLED

Future scope of OLEDs

In OLEDs as crystalline order is not required, organic materials, both molecular and polymeric, can be deposited far more cheaply than the inorganic semiconductors of conventional LED’s. Patterning is also easier, and may even be accomplished by techniques borrowed from the printing industry. Displays can be prepared on flexible, transparent substrates such as plastic. These characteristics form the basis for a display technology that can eventually replace even paper, providing the same resolution and reading comfort in a long-lived, fully reusable (and eventually recyclable) digital medium.

CONCLUSION

OLED is emerging as the new technology for thin panel displays. It can be used for mp3 players, cell phones, digital cameras or hand-held gaming devices. The field of applications for OLED displays has a wide scale. According to a report of Maxim Group revenues will rise from 600 million dollars in 2005 to more than five billion dollars by 2009. Other reports have shown that the total number of sold OLED units grew up to over fifty percent in the past year. It is expected that this number will rise up to 80 or 90 percent in the following year. One of the future visions is to roll out OLEDs or to stick them up like post-it notes. Another vision is the transparent windows which would function like a regular window by day. At night it could be switched on and become a light source. This could be possible because OLED allows transparent displays and light sources. It will take considerably longer, of course, for OLED to keep its promise of cheap manufacturing costs. The challenge is to compete against the industrial powers that overwhelmingly support LCD and therefore achieve massive price advantages. Currently, the wallpaper screen is nothing more than a vision, a clever one though. For future, further improvement of Lifetime will be necessary while improving power efficiency. If a device of longer Lifetime is realized, the foot of the application spreads out greatly. We hope that the development discussed in this paper opens up a course to practical use of OLED as lighting sources for illumination use, backlights and others.

REFERENCE

1. www.Whatis.com2. www.Infopedia.com3. www.Wikipedia.com4. www.Answers.com5. www.Webopedia.com6. www.About.com 7. www.Engiguide.com