investigation of dna nucleobases for bio-organic light emitting diodes
TRANSCRIPT
INVESTIGATION OF DNA NUCLEOBASES FOR
BIO-ORGANIC LIGHT EMITTING DIODES
A DISSERTATION THESIS SUBMITTED TO THE
GRADUATE FACULTY OF THE UNIVERSITY OF CINCINNATI
IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY (PH.D.)
OF
ELECTRICAL ENGINEERING
ELIOT FRENCH GOMEZ
UNIVERSITY OF CINCINNATI
DEPARTMENT OF ELECTRICAL AND COMPUTING SYSTEMS
COMMITTEE CHAIR: ANDREW J. STECKL, PH.D.
SUBMITTED ON
JANUARY 13TH, 2015
i
ABSTRACT
Natural electronics is the field that incorporates biological molecules in organic electronic
devices to create inexpensive, renewable, performance-enhancing, and environmentally safe
alternatives for the electronics industry. Natural DNA, for example, has been incorporated as an
electron blocking layer (EBL) to improve device efficiency and luminance in organic light emitting
diodes (OLED). OLEDs require a diverse set of materials with optical and electrical properties
that meet the rigorous design requirements of the device. DNA, being one of the few materials in
OLEDs, lays the groundwork for other natural material to be explored.
The nucleic acid bases from the DNA and the RNA (adenine, guanine, cytosine, thymine,
uracil) are excellent options for the next steps in natural OLED electronics. The bases form thin
films directly by thermal evaporation, unlike DNA that requires a surfactant and solution
processing. The bases were shown to have a wide range of opto/electronic properties such as
refractive index, dielectric constant, resistivity, and electron/hole transport making them a good
candidate for OLEDs. The thin film properties and performance of the bases were explored by
depositing the individual bases as the EBL and hole blocking layer (HBL) in place of conventional
OLED material. It was shown that adenine and guanine performed well as EBLs, exceeding the
efficiency of the baseline device (52 vs 39 cd/A), which contained non-biological material. It was
also demonstrated that OLEDs with very high efficiency can be obtained using a thin layer of
thymine as an EBL, resulting in a peak efficiency of 76 cd/A and a higher maximum luminance
(132,000 cd/m2) than the baseline OLED (100,000 cd/m2). In the hole blocking layer, uracil
performed well by transporting electrons and blocking hole transport to provide the highest
emission efficiency of the bases.
The final set of experiments demonstrated that adenine increased the hole injection of gold
electrodes due to the natural affinity the base has with gold, corresponding to a 4-7× increase in
luminance. Thin film gold is an attractive electrode alternative for OLEDs on plant-based cellulose
substrates since it does not require high temperature annealing and has high conductivity. Gold
cannot be directly evaporated on the rough cellulose substrate, therefore, a template stripping
ii
procedure was employed using epoxy to lift off the gold electrode from Si wafers, in combination
with adenine as a hole injector to yield high quality and efficient OLEDs. Nucleic acid bases are a
diverse set materials that result in performance-enhancing, inexpensive, and natural-based OLEDs.
iii
iv
ACKNOWLEDGEMENTS
Good work is only accomplished with the support of great people, and there are many I am
indebted to over the course of my graduate studies. I would like to thank my academic advisor,
Dr. Andrew Steckl who has been incredibly supportive guiding me up and down every mountain
of this journey with persistence, diligence, and the best intentions to see me succeed. Dr. James
Grote whose financial support and knowledgeable direction made this project possible. I would
like to thank my dissertation committee who have, not only provided feedback and discussion on
my work, but have been inspirational professors throughout my academic career at UC. At the
Nanoelectronics Laboratory, I am grateful to my colleagues past and present, especially former
student, Dr. Hans Spaeth, who raised me up from a young graduate student to where I am today.
Other colleagues, Vishak Venkatraman, Dr. Han You, Adam Zocco, and Sumit Purandare who
have provided good discussion, aided in experiments, and were good friends during my time in
the lab. I would also like to thank Dr. Necati Keval in Chemistry whose friendship and help over
the years has been most appreciated.
I would like to express my deepest gratitude and love for my wife, Melanie, who has sacrificed
so much over these past years to get where I am now. She has truly been the solid foundation
behind this work and has been my greatest support. I would like to thank my parents, Elias and
Sarah, who have supported me unwaveringly both financially and in spirit on my academic
endeavors: my father who inspired in me a firm work ethic and love for engineering, my mother
who helped so much caring for the twins while we worked and has been very supportive, along
with Ciss and Michael Beatty. My brother, Andre Gomez, whose artistic eye has complemented
my engineering brain. Finally, I would like to acknowledge the deep friendships we have formed
over the years in Cincinnati, especially the Church of Missio Dei. They have walked, encouraged,
prayed, and celebrated with us through every trial and experience we have been through, and they
have helped me understand how to do all work done in light of the gospel of Jesus Christ. Soli Deo
Gloria.
v
“Call to Me and I will answer you
and tell you great and unsearchable
things you do not know”
Jeremiah 33:3
vi
MEMBERS OF THE DISSERTATION COMMITTEE
Dr. Andrew J. Steckl
(Advisor & Committee Chair)
School of Electronics & Computing Systems
Nanoelectronics Laboratory
University of Cincinnati
Cincinnati, Ohio
Dr. James G. Grote
(Co-Advisor)
Air Force Research Laboratory
Materials and Manufacturing Directorate
Wright-Patterson Air Force Base
Dayton, Ohio
Dr. Fred R. Beyette, Jr
School of Electronics & Computing Systems
Point-of-Care - Center for Emerging Neurotechnologies
University of Cincinnati
Cincinnati, Ohio
Dr. Peter B. Kosel
School of Electronics & Computing Systems
GaAs Devices and ICS Lab
University of Cincinnati
Cincinnati, Ohio
Dr. Ian Papautsky
School of Electronics & Computing Systems
Bio Micro Systems Lab
University of Cincinnati
Cincinnati, Ohio
vii
TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................................................ ix
LIST OF TABLES ............................................................................................................................... xiv
COMMON ABBREVIATION & SYMBOLS ............................................................................................ xv
Chapter 1. Introduction to Natural Electronics .......................................................................... 1
1.1. Natural Electronics ................................................................................................................ 2
1.2. OLEDs to Bio-OLEDs .......................................................................................................... 6
1.3. Nucleobases and motivation .................................................................................................. 8
1.4. Summary and Thesis Outline .............................................................................................. 11
Chapter 2. OLEDs and Experimental Methods ........................................................................ 12
2.1. Overview of OLEDs ............................................................................................................ 12
2.2. General OLED Fabrication and Characterization Procedures .............................................. 17
2.2.1. OLED Fabrication ...................................................................................................................................17
2.2.2. OLED Characterization ........................................................................................................................19
2.3. Summary ............................................................................................................................. 21
Chapter 3. Nucleobases and Thin Film Properties .................................................................... 23
3.1. Nucleobase Origin and Synthesis ........................................................................................ 23
3.2. Thin Film Properties ............................................................................................................ 25
3.2.1. Thermogravimetric analysis ..............................................................................................................25
3.2.2. AFM and SEM ............................................................................................................................................28
3.3. Optical and Electrical Properties ......................................................................................... 30
3.3.1. Optical Spectroscopy .............................................................................................................................30
3.3.2. Ellipsometery ............................................................................................................................................31
3.3.3. Dielectric constant..................................................................................................................................32
3.3.4. HOMO/LUMO levels ...............................................................................................................................33
3.4. Summary of Nucleobase Properties ..................................................................................... 35
Chapter 4. Nucleobase Bio-OLEDs ............................................................................................ 36
4.1. Nucleobases as an EBL/HTL .............................................................................................. 37
4.1.1. EBL Results ................................................................................................................................................37
4.1.2. Discussion of EBL results .....................................................................................................................40
4.2. Nucleobases as an HBL/ETL .............................................................................................. 41
viii
4.2.1. HBL Results ................................................................................................................................................42
4.2.2. HBL Discussion .........................................................................................................................................43
4.2.3. HBL Optimization ...................................................................................................................................45
4.3. Optimization of EBL ........................................................................................................... 46
4.3.1. Thin EBL OLED Experiments..............................................................................................................46
4.3.2. AFM Results of Thin Films ...................................................................................................................50
4.3.3. Discussion of Thymine EBL Optimization .....................................................................................52
4.4. Conclusions ......................................................................................................................... 53
Chapter 5. Cellulose and Au for Natural-Based OLED substrates .......................................... 56
5.1. Cellulose substrates ............................................................................................................. 56
5.1.1. Cellulose for OLEDs ................................................................................................................................59
5.1.2. Cellulose challenges ...............................................................................................................................60
5.2. Gold electrodes in OLEDs ................................................................................................... 61
5.3. Template Stripping of Gold Electrodes ............................................................................... 63
5.3.1. Au Template Stripping Procedure ...................................................................................................63
5.3.2. Cellulose/Au Properties and quality ...............................................................................................65
5.4. Summary ............................................................................................................................. 68
Chapter 6. Flexible Nucleobase Bio-OLED ............................................................................... 70
6.1. Adenine as a Hole Injection layer ........................................................................................ 70
6.1.1. OLED fabrication with Adenine as HIL ..........................................................................................70
6.1.2. Adenine as HIL Results ..........................................................................................................................72
6.1.3. Discussion of Cellulose vs Glass Substrate ....................................................................................75
6.1.4. Discussion of Adenine as HIL..............................................................................................................77
6.2. Cost Analysis of Nucleobase OLEDs .................................................................................. 79
6.3. Summary ............................................................................................................................. 81
Chapter 7. Summary and Future Work..................................................................................... 83
7.1. Conclusion of Dissertation .................................................................................................. 83
7.2. Future Work ........................................................................................................................ 84
7.3. Closing remarks................................................................................................................... 85
Appendix A – Error Analysis of Experiments ................................................................................ 87
Appendix B – Cost Analysis of Natural Materials ......................................................................... 92
REFERENCES .................................................................................................................................... 94
ix
LIST OF FIGURES
Figure 1-1 Examples of biomaterials from natural sources integrated as thin film components
in bioelectronic devices................................................................................................................... 3
Figure 1-2 Advantages of natural electronics in materials, device, and toxicity and the corollary
of applications. ................................................................................................................................ 5
Figure 1-3 Several OLEDs applications on (a) flexible cellulose substrates,39 (b) lighting
panels,83 and (c) flexible displays.84 ............................................................................................... 6
Figure 1-4 (a) DNA complexed with a surfactant that dissolves in alcohols and spin coats into
thin films. (b) Photographs of phosphorescent OLEDs with DNA-CTMA resulting in high
efficiency red, green a,d blue emission.87 ....................................................................................... 7
Figure 1-5 The DNA double helix comprises of two long nucleotide chains fused together by
hydrogen bonds. Nucleotides contain a pentose sugar, phosphate group, and a nucleobase. ........ 9
Figure 1-6 (a) Nucleobase powder as-received; (b) thin film cytosine deposited on Si to 200
nm by thermal evaporation. .......................................................................................................... 10
Figure 2-1 (a) Device structure and operation of a simple electroluminescent device. (b)
Molecular orbital levels of the same structure to define work function, electron affinity, and
ionization potential relative to the vacuum level. ......................................................................... 13
Figure 2-2 (a) A two layer heterostructure OLED design with an EBL. (b) Large energy barriers
prevent electrons from leaving the emitting layer and increase recombination efficiency. ......... 14
Figure 2-3 Light emission in fluorescent molecules (CBP) limited to singlet spin states.
Phosphorescent molecules receive energy by spin coupling and emit light with singlet and triplet
spin states. ..................................................................................................................................... 15
Figure 2-4 (a) The reference OLED device stack without nucleobases employed for this study;
(b) Energy level diagram with layer thickness. ............................................................................ 16
Figure 2-5 (a) A photograph of the CBP:Ir(ppy)3 light emission. (b) Wavelength spectrum of
the phosphorescent Ir(ppy)3 molecule. (Inset) CIE graph showing green emission. .................... 16
x
Figure 2-6 (a) Commercially patterned ITO on glass substrates; (b) photograph of PEDOT:PSS
during spin coating; (c) result of the PEDOT after bake; (d) SVT high vacuum deposition system;
(e) result of the organic thin films (inset) organic shadow mask; (f) Al electrodes deposited on top
of the organics to complete the stack (inset) electrode shadow mask. ......................................... 18
Figure 2-7 Typical graphs of the baseline OLED performance (a) current density versus
voltage; (b) luminance versus voltage; (c) current efficiency versus luminance; (d) luminous
efficiency versus current density. ................................................................................................. 19
Figure 2-8 Total internal reflection results in a decrease in external quantum efficiency (light
out-coupled) from the original internal quantum efficiency (photons per recombination). ......... 21
Figure 3-1 Chemical structures of the nucleobases consisting of nitrogen (blue), oxygen (red),
and hydrogen (gray). G and A are purines with fused pyrimidine and imidazole rings. C, T, and
U are pyrimidines with a single heterocyclic ring. ....................................................................... 24
Figure 3-2 Thermogravimetric analysis from 30 to 600 °C in Ar showing temperature stability
of: (a) nucleic acid bases; (b) additional nucleic acids and their complex with CTAC; (c) reference
OLED materials. ........................................................................................................................... 26
Figure 3-3 (a) AFM analysis of nucleobases (b) sectional line scan samples from AFM results.
....................................................................................................................................................... 29
Figure 3-4 SEM images of thymine thin film on Si at (a) 1000× and (b) 20,000× magnification.
....................................................................................................................................................... 30
Figure 3-5 Electromagnetic absorption spectra for UV and visible light from 200-700 nm. . 31
Figure 3-6 Refractive index of nucleobases versus wavelength determined by ellipsometry 32
Figure 3-7 Dielectric constant device stack and measurement. .............................................. 33
Figure 3-8 Molecular orbital energy levels of the nucleic acids compared to the reference
OLED.104 Energy levels from Faber et al.130 and are shown as a black solid line (—) are compared
to the results from Lee et al.46 shown in the dotted grey lines (···). The DNA-CTMA energy level
obtained via UPS from Lin et al.131 give an electron affinity of 1.6 eV compared to the typical
reported value 0.9 eV.21 ................................................................................................................ 34
xi
Figure 4-1 OLED configurations to study nucleobase charge transport: (a) baseline device
without the bases to establish a reference; (b) nucleobase in the EBL/HTL configuration; (c)
nucleobase in the HBL/ETL configuration. .................................................................................. 36
Figure 4-2 The performance of the nucleobases as an EBL/HTL: (a) current density versus
voltage, (b) luminance versus voltage, (c) luminous efficacy versus current density, (d) current
efficiency versus luminance. ......................................................................................................... 38
Figure 4-3 Illumination of the OLED with adenine as the EBL. ............................................ 39
Figure 4-4 Possible charge transport mechanism in the EBL OLED for (a) guanine, (b) adenine,
and (c) uracil. ................................................................................................................................ 40
Figure 4-5 The performance of the nucleobases as an HBL/ETL: (a) current density versus
voltage, (b) luminance versus voltage, (c) luminous efficacy versus current density, and (d) current
efficiency versus luminance. ......................................................................................................... 42
Figure 4-6 Mechanisms of charge transport in the HBL OLED for (a) guanine, (b) adenine,
and (c) uracil. ................................................................................................................................ 44
Figure 4-7 HBL optimization of the U and the reference (BCP) showing the effects on
performance on (a) current density versus voltage and (b) current efficiency vs luminance. ...... 45
Figure 4-8 (a) Device stack of the EBL configuration. (b) The energy levels comparing the
four different EBL NPB, A, T, and DNA-CTMA and the adjacent layers to the EBL in the OLED.
....................................................................................................................................................... 47
Figure 4-9 The effect of varying the thickness of the nucleic acids in the EBL configuration
compared to the reference. ............................................................................................................ 48
Figure 4-10 Results of the EBL at 10 nm for baseline, DNA-CTMA, A, and T: (a) current
density versus voltage; (b) current efficiency versus luminance; (c) luminance versus current
density; (d) current efficiency versus current density. .................................................................. 49
Figure 4-11 (Left) AFM scans of each EBL deposited to 10 nm and CBP deposited on silicon
to 30 nm. Also shown are AFM scans on CBP deposited on top of each EBL film. Scan length is
1 µm; (Right) sectional views of each AFM result plotted on vertical/horizontal axes with each
xii
CBP paired to its respective layer to elucidate how each EBL affects the growth of the emitting
layer............................................................................................................................................... 51
Figure 4-12 (a) Simplified mechanism of hole and electron transport for T(10 nm) showing
charge transport concentrated at the valleys of the BCP and T. (b) The smaller roughness of the A
layer has more uniform charge injection. ..................................................................................... 52
Figure 5-1 The cell wall of plants form a mesh of fibrils. Fibrils are composed of microfibrils
made from bundles of cellulose chains. Cellulose chains can be processed to reform into different
materials. ....................................................................................................................................... 57
Figure 5-2 (a) Reconstituted cellulose film with excellent optical transparency, (b) compared
with glass and conventional copy paper. ...................................................................................... 58
Figure 5-3 Template stripping method of evaporated Au on Si transferred to cellulose via UV
curable epoxy. ............................................................................................................................... 64
Figure 5-4 The adhesion properties of Au on (a) glass and (b) cellulose substrate. ............... 66
Figure 5-5 Quality analysis of cellulose surface before and after template stripping through a
microscope. (a) Photograph of plain cellulose substrate; (b) Au (20 nm) directly evaporated onto
the cellulose; (c) SEM image of the plain cellulose substrate showing rough texture; (d) photograph
of the template stripped Au on cellulose; (e) photograph of quality of template stripped showing
high quality electrode; (f) SEM image of template stripped Au on cellulose. ............................. 67
Figure 5-6 (a) OLED fabricated on Au directly deposited on the rough cellulose; (b) OLED
fabricated on a template stripped Au on cellulose. ....................................................................... 67
Figure 5-7 Transmission spectra of the substrates: glass, UV epoxy, ITO, cellulose, and Au.
(Inset) Photo of template stripped substrate on lettering to show transparency. .......................... 68
Figure 6-1 (a) Shadow masks for the experiments with Au and cellulose. (b) Photo of the 30x30
mm cellulose attached to a glass substrate. ................................................................................... 71
Figure 6-2 (a) Device stack of the OLED on cellulose; (b) energy levels of the OLED with
adenine. ......................................................................................................................................... 71
xiii
Figure 6-3 Results of phosphorescent OLED on a glass substrate with adenine (squares) and
without adenine (circles) on Au (a) luminance and current density versus voltage; (b) current
efficiency versus luminance. ......................................................................................................... 73
Figure 6-4 Results of phosphorescent OLED for template stripped Au on a cellulose substrate
with adenine (diamond) and without adenine (triangles) (a) luminance and current density versus
voltage; (b) current efficiency versus luminance. ......................................................................... 74
Figure 6-5 Luminance versus current density for all four device types: glass and cellulose
substrates with and without adenine. ............................................................................................ 75
Figure 6-6 Contour microscopy of Au on cellulose................................................................ 76
Figure 6-7 (a) A clean Au substrate surface with the arrow indicating the presence and
orientation of the dipole field. (b) Adenine adsorbs on the gold and interacts with the Au orbitals
that redistribute the dipole orientation, creating a favorable electron transfer between the electrode
and the adenine. ............................................................................................................................ 77
Figure 6-8 Photograph of bio-OLED on cellulose substrate using adenine as a HIL. ............ 78
Figure 6-9 (a) Estimated cost of organics for the reference OLED versus the bio-OLED; (b)
Cost of the glass substrate with ITO and PEDOT compared to the cellulose substrate with Au and
A. ................................................................................................................................................... 81
xiv
LIST OF TABLES
Table 1-1 Natural materials in bioelectronics.........................................................4
Table 3-1 Summary of nucleobase optoelectronic properties. ..............................35
Table 4-1 Summary of the nucleobase performance in OLEDs in the (a) EBL
configuration at 17 nm; (b) HBL configuration at 12 nm; (c) EBL
optimized at 10 nm. .............................................................................55
Table 5-1 Review of OLEDs using cellulose as a substrate. ................................59
Table 5-2 Summary of Au and ITO
properties......................................................62
Table 6-1 Cost of organic and cathode material for OLED. .................................79
Table 6-2 Cost of substrate and anode for OLED. ................................................79
xv
COMMON ABBREVIATIONS & SYMBOLS
OLED Organic Light Emitting Diodes
EBL Electron blocking layer
HBL Hole blocking layer
HIL Hole injection layer
EIL Electron injection layer
EL Emission layer
G Guanine
A Adenine
C Cytosine
T Thymine
U Uracil
DNA Deoxyribonucleic acid
RNA Ribonucleic acid
CTMA Cetyltrimethylammonium
CTAC Cetyltrimethylammonium chloride
ATP Adenosine 5′-triphosphate
OPV Organic Photovoltaic
OTFT Organic thin film transistor
HOMO Highest occupied molecular orbita
LUMO Lowest unoccupied molecular orbita
PDMS Poly(dimethylsiloxane)
ITO Indium tin oxide
PEDOT:PSS Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
NPB N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine
CBP 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl
Ir(ppy)3 Tris[2-phenylpyridinato-C2,N]iridium(III)
BCP Bathocuproine
Alq3 Tris-(8-hydroxyquinoline)aluminum
MBD Molecular beam deposition
xvi
XPS or UPS X-ray or Ultraviolet photoemission spectrometer
UV Ultraviolet
SEM Scanning electron microscopy
AFM Atomic force microscopy
TGA Thermogravametric analysis
eV Electron volts
V Voltage
I Current
J Current density
L Luminance
ε Dielectric constant
ε0 Permeability of free space
φ Work function
λ Wavelength
n Refractive index
ηint Internal quantum efficiency
ηext External quantum efficiency
ηI Current efficiency
ηlum Luminous efficacy
cd Candelas
lum Luminous flux
1
Chapter 1. Introduction to Natural Electronics
The electronics industry has a bountiful and largely unexplored harvest of materials available
for a new age of electronic devices. Innumerous materials found in nature derived from trees,
produce, plants, and animals have material properties and molecular structures equipped for
electronic devices beyond Si.1,2 Though a unification between the wet biological environment with
the dry solid-state world seems uncanny, the electronics industry would benefit from renewable
and environmentally responsible resources. Biology may even surpass the performance of
traditional electronic devices in optical applications, electronic transport, signal transduction, or
even data storage. Considering the physiological complexity and efficacy of the human body,
biology is fine-tuned with remarkable functionality unsurpassed by human engineering that could
have unprecedented value for electronic devices. The deoxyribose nucleic acid (DNA) molecule,
for example, is a beautiful structure thousands of times smaller than a human hair that is calculated
to have the data storage capacity of hundreds of exabytes per gram of material.3 Bioelectronics is
the field that seeks to capitalize on the unique properties and functionality of the natural realm and
connect it with modern electronics.4-6
While DNA hard drives are far from market potential, DNA7,8 and other types of common
biological molecules (biomaterials) have already been integrated into many organic electronic
devices to replace synthetic carbon-based (organic) molecules to enhance thin film devices such
as the organic light emitting diodes (OLED), organic thin film transistor (OTFT), and organic
photovoltaic (OPV). Replacing synthesized organic molecules with naturally derived materials
could inspire electronic devices that are inexpensive and environmentally sustainable.9 Biological
molecules have been used to even enhance device performance and have demonstrated OTFTs
created from all-natural materials.10 The new breed of all-natural electronics comes with unique
advantages and applications unavailable to other types of technology, such as high volume use-
and-toss devices as well as biocompatible implantable sensors.
The OLED is a light emitting device with organic materials for displays or lighting. The
number of natural materials available for the OLED is limited. Displays are becoming increasingly
Chapter 1 – Introduction to Natural Electronics
2
prevalent in society, and expanding the list of natural materials available for OLEDs would result
in a positive environmental impact with potentially new applications. This work introduces natural
occurring nucleic acid bases as a new biomaterial for OLEDs. The nucleic acid bases, called
nucleobases or simply bases, are smaller components of the large DNA molecule that readily adapt
to standard OLED fabrication. They are renewable resources that come from a variety of food
sources. It will be shown how nucleic acid bases have intrinsic optical and electrical properties
that can replace conventional materials, while still meeting or exceeding luminance and efficiency
criteria in OLEDs. It will also be demonstrated how the bases have great affinity and charge
injection capability on gold electrodes. Gold is an excellent electrode choice to fabricate OLEDs
on cellulose, a flexible and natural substrate, due to its high conductivity and low temperature
processing.
This chapter will continue with a brief discussion on natural electronics and its current status.
Afterwards, the work done in DNA-based OLED will be summarized, which leads to the
motivation of using nucleobases.
1.1. Natural Electronics
The transition from a biological environment to a fully functional electronic device begins with
understanding how biological materials (biomaterials) perform as the basic building blocks of
organic electronics: semiconductors, dielectrics, substrates, and conductors. The list of
biomaterials that can replace such components is constantly growing. Figure 1-1 shows a few
biomaterials that have been integrated in electronics. Pigments11-13 from plant dye and β-
carotene12,46 found in carrots have been used as a semiconductor in OTFTs. Caffeine from coffee,14
adenine found in bee pollen,14 and albumen in egg whites15 have been used as dielectrics. Even the
device substrate has been reimagined using natural biodegradable materials such as food starch16
or plant cellulose.14 Silk is an excellent biocompatible material for implantable sensors17 and has
recently been used to generate electricity to power small devices.18 Many of these could be used
for ingestible devices or metabolizable electronics.19 Table 1-1 has a summary of natural
electronics that has been compiled from other reviews.2,4,20
A fundamental understanding of the electronic properties of biomaterials leads to enhanced
operation and expanded application in many different types of devices. DNA, for example, has
been a widely studied molecule in bioelectronics for the past decade and has been integrated in
Chapter 1 – Introduction to Natural Electronics
3
many different electronic devices. Early work has shown the biopolymer can enhance efficiency
in OLEDs21, and research has expanded its potential to many other types of devices: organic thin
film transistors (OTFT),22,23 organic photovoltaic (OPV),24 memory transistors,25 waveguides,26
optical fibers,27 and lasers.28,29 Its structural properties have also found benefits in nanotechnology
such as nanostructures30-32 and photolithography.33 DNA, once strictly a biological molecule, has
become an important tool in natural electronics because its natural properties and structure have
been leveraged in electronic devices. Similarly, other biomaterials such as silk, cellulose, and
melanin are no longer perceived as an unusual electronic components, but rather as a staple for
future applications.
Once a material is optimized, the building blocks become the foundation to construct a device
derived entirely from natural sources. An all-natural laser has been demonstrated using flavin
mononucleotide and vitamin B2 fabricated on a poly-L-lactic acid substrate34 and a similar device
on flexible silk substrates.35 Another notable work replaced every component of OTFT, including
Figure 1-1 Examples of biomaterials from natural sources integrated as thin film components in
bioelectronic devices.
Chapter 1 – Introduction to Natural Electronics
4
Table 1-1 Natural materials in bioelectronics.
Biomaterial Natural Origin Device Function Ref
starch potato, corn, wheat substrate 14
cellulose plants substrate 16,20,36-40
leather animal skin substrate 41
silk silkworms substrate / energy harvest 17,18,35,42,43
poly(L-lactide-co-glycolide) lactic acid substrate 20,34
polyhydroxyalkanoates microorganisms substrate 16
gelatin proteins substrate 14
shellac natural resin from beetles substrate 12
glucose sugar from plants substrate, dielectric, fuel cell 14,44,45
aurin (roslic acid) plant roots smoothing layer 14
adenine fruit dielectric 14
guanine fish skin, bat excretion dielectric 14,46
caffeine coffee dielectric 14
albumin protein from eggs dielectric 15
aleo vera plant dielectric 47
indanthrene yellow G natural anthraquinone semiconductor 14
indanthrene brillant orange natural anthraquinone semiconductor 14
perylene diimide natural anthraquinone semiconductor 14,48
β-carotene carrots, sweet potatoes semiconductor 14,49
indigo natural dye from plants semiconductor 11,12
melanins pigments in mammals semiconductor/conductor 20,50
deoxyguanosine DNA derivative semiconductor 22
natural rubber tree sap memory transistors 51
flavin mononucleotide organ tissue laser gain 34
amyloid fibrils bovine insulin OLEDs 52
DNA salmon sperm/organ tissue OLEDs 21,53-66
“ “ dielectric/OFETs 23,67-71
“ “ capacitor 72,73
“ “ solar cells 24,74-76
“ “ memory transistors 25,77-82
“ “ waveguide/optical fiber 26,27
“ “ photovoltaic
“ “ lasers 28
the substrate, with kitchen ingredients, e.g. sugars, caffeine, starch, and dyes.14 Whether to enhance
device performance or create all-natural electronics, the benefits and applications of natural
electronics expand a diverse range of disciplines.
Figure 1-2 illustrates the advantages of natural electronics in three major areas: material, device
performance, and toxicity. All of the biomaterials in Table 1-1 can be harvested safely from
renewable and often inexpensive sources. Most of the materials are plant based, which can be
grown or harvested to minimize or even ameliorate ecological impact. Natural materials often
improve device performance due to their intrinsic electrical properties or have biological affinities
that can be leveraged for unique applications in bioelectronics. Biomaterials could offer low
toxicity and biocompatibility42 that conventional electronics cannot offer. Most of the materials
Chapter 1 – Introduction to Natural Electronics
5
listed are already recognized by the U.S. Food and Drug Administration as safe substances.34
Materials that are degradable and non-hazardous during decomposition9 would ease the burden of
the environmental impact of electronic waste.
Figure 1-2 Advantages of natural electronics in materials, device, and toxicity and the corollary of
applications.
The three areas mentioned in material, performance, and low toxicity could lead to applications
shown in Figure 1-2. Material that is inexpensive and renewable is a prime opportunity for large-
volume “use-and-toss” devices. One-time-use displays or sensors fabricated on low cost substrates
(paper or bio-plastics) could be used for advertising, food labels, business cards, brochures, or
newspapers. Another significant area of impact would be the medical field. Bioelectronics has
made great strides interfacing and implanting electronics with the body6 by using biomaterials that
Chapter 1 – Introduction to Natural Electronics
6
are non-toxic to the body and bridge cellular activity to signals and systems.5 Devices that interface
with the body could aid in health monitoring, tissue repair, and drug delivery.4 Finally, low toxicity
and renewable sources are ingredients for a sustainable future by using electronic material that
decompose quickly without environmental damage. Sustainability is becoming increasingly
relevant as consumer electronics continues to proliferate and assimilate into home appliances.
1.2. OLEDs to Bio-OLEDs
OLEDs are staged to become the future of displays and lighting due to their ability to be grown
on low-cost, lightweight, and flexible substrates85 in a variety of applications. Figure 1-3 shows
OLED fabricated as small pixels for displays or large-area light panels for solid-state lighting.83,84
They offer better color control and consume less power than traditional luminaires.86 OLEDs are
composed of several thin-films (typically 1 to 50 nm) deposited by thermal evaporation or solution
deposition methods. As the diversity of biomaterials for natural electronics grows, there are
opportunities to replace many of the conventional organic materials in OLEDs with the materials
Figure 1-3 Several OLEDs applications on (a) flexible cellulose substrates,39 (b) lighting panels,83 and (c)
flexible displays.84
Chapter 1 – Introduction to Natural Electronics
7
of natural origin. The OLED is a complex structure with demanding optical and electrical
properties that will prove to be an arduous journey towards an all-natural OLED.
DNA has been the primary natural molecule in OLEDs. The path of integrating DNA from a
wet biological environment to dry thin films for OLEDs is not straightforward. DNA is harvested
from the milt and roe sacs from salmon. It passes through several purification steps to finally dry
as a powder.88 The powder is not immediately ready for fabrication, since it is only soluble in
water, which is difficult to form thin films. The biopolymer must be complexed with a cationic
surfactant, cetyltrimethylammonium chloride (CTAC). The surfactant binds to the negatively
charged DNA backbone to form DNA-cethylmethylammonium (DNA-CTMA), which is further
purified by dialysis.89 The new complex is soluble in polar organic alcohols (e.g. butanol, ethanol,
Figure 1-4 (a) DNA complexed with a surfactant that dissolves in alcohols and spin coats into thin films.
(b) Photographs of phosphorescent OLEDs with DNA-CTMA resulting in high efficiency red, green a,d
blue emission.87
Chapter 1 – Introduction to Natural Electronics
8
and methanol) and is ready for solution deposition. Spin coating DNA creates a well-controlled
film with thicknesses variable from nanometers to micrometers. Figure 1-4(a) shows the cationic
surfactant bound to the backbone of the DNA and a photograph of DNA-CTMA spin coating from
a solution to form a thin film layer in OLEDs.
The history of DNA in OLEDs began when it was believed to conduct charge like a nanowire.90
To further elucidate its conductive properties, DNA-CTMA was first inserted into OLEDs in
several device configurations by Hirata.58 Hagen from the Nanoelectronics Laboratory showed
that OLED device performance could be improved by using DNA-CTMA as an electron-blocking
layer (EBL), a layer that confines charge to the light emitting layer (EL), to improve efficiency.21
Spaeth continued the work and used more efficient phosphorescent emitting layers with the DNA-
CTMA as the EBL layer that resulted in bright and highly efficient red, green, and blue OLEDs,87
as seen in Figure 1-4(b).
The results from the Nanoelectronics Laboratory spurned others to explore different types of
bio-OLED devices. DNA-CTMA was used as an EBL to increase the efficiency in several similar
structures such as quantum-dot LED,65 polymer LED,61,64,66 and fluorescent OLED.53,57 One
application used DNA/PAn/Ru(bpy)3 to create a color-tunable fluorescent OLED.62 DNA has also
been coupled with luminophores59 (light-emitting molecules), which has led to white luminescence
from DNA nanofibers,91 and phosphorescent hosts in OLEDs.54
1.3. Nucleobases and motivation
Nucleobases are nitrogenous heterocyclic rings found in the DNA and ribonucleic acid (RNA)
shown in Figure 1-5. DNA is found in the cell nucleus of every living organism and has billions
of base pairs that hold the genetic instructions for biological life. The RNA transcribes the DNA
nucleotide chain and translates the sequence into a protein, which are the building blocks in
biology.92 The DNA chain is composed of two strands of successive nucleotide units organized in
a double helix bonded together by the bases. A nucleotide unit consists of pentose attached to a
phosphate group and one nucleobase. The DNA bases are known as guanine (G), adenine (A),
cytosine (C), and thymine (T). The RNA contains the bases G, A, C, and uracil (U). The nucleic
acids have specific affinity for each other: G has three hydrogen bonding sites to pair with C, and
A has two hydrogen bonding sites to pair with either T or U.
Chapter 1 – Introduction to Natural Electronics
9
Nucleic acid bases have several advantages over DNA in organic electronics. Firstly, they are
small molecules that thermally evaporate directly from powder form into a thin film, shown in
Figure 1-6. Evaporation is often advantageous because it integrates with microfabrication
procedures and thickness is controllable with nanometer precision. Thermal deposition systems
produce pristine film quality because evaporation is done under vacuum with minor contamination.
Secondly, evaporation eliminates wet deposition methods and extensive DNA processing that
is required to create thin DNA films. The bases, unlike DNA, require no further modification or
surfactants to evaporate from their powder form. DNA must be sonicated to the desired molecular
weight, combined with CTAC, and dissolved in alcohols before it can be processed in thin films.
Spin coating with alcohols creates problems when spin coating on top of organic materials and
creates additional complications during fabrication. The surfactant further increases the
Figure 1-5 The DNA double helix comprises of two long nucleotide chains fused together by hydrogen
bonds. Nucleotides contain a pentose sugar, phosphate group, and a nucleobase.
Chapter 1 – Introduction to Natural Electronics
10
complexity, cost, and requires extensive filtration procedures.93 Nucleobases are ready to
implement into OLED devices directly by evaporation.
Figure 1-6 (a) Nucleobase powder as-received; (b) thin film cytosine deposited on Si to 200 nm by thermal
evaporation.
Thirdly, nucleobase offer better purity and more controllable properties compared to DNA.
Natural DNA has an arbitrary sequence of bases with varying molecular weights and the added
complexity of CTAC. The diverse molecules found in DNA-CTMA have negative implications
for OLED research, especially for reproducibility and charge transfer. For example, it has been
studied that charge carriers in a guanine rich strand traverse differently than a strand with random
composition of the bases.94 It is also known that the nucleobases have different optical properties.95
The sequence of bases cannot be controlled when harvested from natural sources, hence the
properties and results may vary from run to run. Therefore, the purity of the nucleobases allows
more precise knowledge and control over the optoelectronic properties of the thin-films.
Nucleobases will expand the availability of natural materials for bio-based OLEDs. DNA and
protein microfibrils52 (used in the emitting layer) have been the only reports of biomaterials
available for OLEDs. Work has been done on the OTFTs to replace nearly every component with
a natural material, but OLEDs only have few materials at its disposal. It will be shown in this
thesis how the diverse properties of the nucleobases will expand the availability of biomaterial for
different layers in the OLED.
Chapter 1 – Introduction to Natural Electronics
11
1.4. Summary and Thesis Outline
In conclusion, natural electronics is the field that searches for biological materials that have
innate optical or electrical properties to use in electronic devices. The growing list of biomaterials
are expanding as suitable building blocks for organic electronics, such as dielectrics, substrates,
semiconductors, and conductors. The biomaterials are typically renewable, non-toxic,
performance enhancing, biocompatible, and inexpensive for a variety of applications. The device
components come together to create all-natural devices that have many useful applications in use-
and-toss displays, implantable devices, and environmentally sustainable industries.
DNA has been an important biomaterial for bioelectronics and has been implemented in
OLEDs, OTFTs, and OPVs. However, DNA is a difficult polymer to work with because it requires
extensive processing and purification. Nucleobases, on the other hand, readily adapt to traditional
fabrication procedures (low temperature evaporation). They also have simpler molecular
structures that lend to reproducibility and specific properties.
The outline of the thesis will be briefly described. Chapter 2 will continue with a brief
explanation of OLEDs, followed by fabrication and characterization procedures for this work in
order to provide a foundation for further discussion. A chapter will then be devoted to
understanding nucleobases and their thin film properties as they pertain to OLEDs (Chapter 3).
The properties will lay the groundwork for the experiments of nucleobases as an electron blocking
layer and a hole blocking layer in OLEDs (Chapter 4). Afterwards, attention will shift to finding
natural based materials for the substrate and the electrode of the OLED, specifically cellulose and
Au, and a new method will be presented for fabricating high-quality electrodes for OLEDs on
cellulose (Chapter 5). In Chapter 6, an OLED will be fabricated on cellulose and adenine will be
used as a novel hole injector for gold electrodes to improve the device performance ~5x. Chapter
7 will conclude with a summary and recommendations for future work.
12
Chapter 2. OLEDs and Experimental Methods
Light emitters have been reimagined and reconstructed over the century with new materials
and designs to perfect the luminaire. Incandescent, halogen, fluorescent, plasma, and light-emitting
semiconductors currently dominate the industry, but OLEDs are quickly maturing and staged to
become the future of lighting technology.86 A foundational knowledge for OLEDs will be given
to provide contextual language for further discussion of the nucleobase OLEDs. The initial
overview of OLEDs will highlight major advances in OLED technology. The focus will then shift
to general fabrication procedures along with characterization techniques for the reference OLED
structure implemented for this work.
2.1. Overview of OLEDs
OLEDs are beginning to emerge as the next generation for flexible and curved displays. The
thin organic layers (hydrogen-oxygen-nitrogen based) form amorphous films bonded together by
weak van der Waals forces, contrary to inorganic light emitting diodes (LEDs) that are hard and
brittle with covalent molecular bonds. Therefore, OLED displays can be fabricated on inexpensive
and lightweight substrates, such as plastic or other polymers. They can flex or curve while
operating and adapt to a variety of microfabrication techniques, including thermal evaporation,96
but it may also eventually lead to mass fabrication by inkjet printing97 or roll-to-roll.98,99
Fabrication such as roll-to-roll will support flexible application designs and could eliminate costly
microfabrication assembly lines to thrust OLEDs to the forefront of lighting technology.85
The OLED layers are designed to convert electrical energy to photon energy.
Electroluminescence is the process of converting electron energy to light. Lumophores emit
photons when electrons relax from a high-energy state and recombine with a hole (a vacant
electron site) in a low energy state. The energy difference between the states is equivalent to the
energy of the photon that is emitted, resulting in different color light. Simple electroluminescence
can be observed by depositing a layer (~100 nm) of electroluminescent material between two
electrodes (anode and cathode) with a voltage potential as shown in Figure 2-1(a). Helfrich and
Chapter 2 – OLEDs and Experimental Methods
13
Schneider first observed electroluminescence in organics for the molecule anthracene.100 Electrons
are injected into the organic material from the cathode and holes are injected from the anode. In
organic material, charge travels by charge hopping between molecular sites until they eventually
recombine at an active site. The structure of a single electroluminescent layer is simple but it
requires very large voltages (>100 V).
Figure 2-1(b) is the energy diagram containing the molecular orbitals of the organics and work
functions of the metals. Molecular orbital theory provides a better understanding of charge
transport in organic electronics.101 Every organic material has intrinsic energy levels that determine
the energy required for a charge carrier to transfer to another material102 based on the molecular
orbital of the molecule. The orbital is the electron energy in electron volts (eV) required for an
electron to escape from its state in the molecule into the vacuum level, which is the reference at 0
eV considered a large distance away from the molecule. The two molecular orbitals that are of
most concern in organics electronics are the lowest unoccupied molecular orbital (LUMO) and the
highest occupied molecular orbital (HOMO). The LUMO is the electron affinity energy with
respect to the vacuum level that determines electron charge transfer energy. Similarly, the HOMO
is the ionization potential for hole transfer. These are analogous to the conduction and valence
band gap in inorganic semiconductors except there is no continuous energy band. The metal
Figure 2-1 (a) Device structure and operation of a simple electroluminescent device. (b) Molecular orbital
levels of the same structure to define work function, electron affinity, and ionization potential relative to
the vacuum level.
Chapter 2 – OLEDs and Experimental Methods
14
electrodes do not have orbitals but instead has a work function (φ). The anode is typically a
transparent electrode with a high work function such as indium tin oxide (ITO) (φ = 4.7 eV) to
inject holes into the HOMO of the organic layers. The cathode is a metal with a low work function,
such as aluminum (Al) (φ = 4.1 eV) to inject electrons into the LUMO of the organic layers.
Although other material properties for OLEDs are important, such as resistivity and refractive
index, the energy levels help give a fundamental understanding of OLED design. Energy levels
must have a miniscule eV barrier between adjacent layers to decrease the electric field potential
required to overcome the barriers, leading to better OLED designs. A significant breakthrough in
the OLED structure was the implementation of the two layer heterostructure by Tang and
VanSlyke103. A similar model of their design is shown in Figure 2-2(a) and its corresponding
energy levels in Figure 2-2(b). The structure employed a hole injection layer (HIL) with a nearly
matched HOMO level to the ITO work function to facilitate better hole injection into the organics.
Similarly, the Al work function matches the LUMO of the adjacent organic to efficiently inject
electrons into the device. Additionally, the low LUMO adjacent to the emitting layer served as an
electron blocking layer since the energy barrier was too high for electrons to penetrate to the
adjacent layer. The EBL contains the electrons near the emitting layer to increase the probability
of recombination corresponding to an improvement in efficiency.
Figure 2-2 (a) A two layer heterostructure OLED design with an EBL. (b) Large energy barriers prevent
electrons from leaving the emitting layer and increase recombination efficiency.
Chapter 2 – OLEDs and Experimental Methods
15
Another major milestone for OLEDs was the development of the electro-phosphorescent
system by Baldo et al.104,105 Up until this point, fluorescence was the only mechanism for
electroluminescence in OLEDs. A fluorescent molecule does not always produce a photon when
an electron recombines with a hole. In Figure 2-3, recombination in CBP results in a fluorescent
photon emission only if the electron-hole pair has a singlet state (excited states with a total angular
of momentum equal to zero), which has a 25% probability of occurring. The triplet states (total
angular of momentum equal to 1) have a 75% probability of occurring and results in no photon
emission (non-radiative), which is lost efficiency. The phosphorescent emitters utilize a host-guest
system that exploit both singlet and triplet state for photon emission. The host (CBP) transfers
spin states to the phosphorescent guest (Ir(ppy)3) by inter-system crossing. The electron relaxation
occurs in the phosphorescent molecule and results in light emission regardless of the spin state,
triplet or singlet, and thus the theoretical quantum efficiency is 100%. The electro-phosphorescent
system propelled OLEDs to compete with traditional luminaire technology.
Figure 2-3 Light emission in fluorescent molecules (CBP) limited to singlet spin states. Phosphorescent
molecules receive energy by spin coupling and emit light with singlet and triplet spin states.
Chapter 2 – OLEDs and Experimental Methods
16
Figure 2-4 (a) The reference OLED device stack without nucleobases employed for this study; (b) Energy
level diagram with layer thickness.
Figure 2-5 (a) A photograph of the CBP:Ir(ppy)3 light emission. (b) Wavelength spectrum of the
phosphorescent Ir(ppy)3 molecule. (Inset) CIE graph showing green emission.
The OLED design shown in Figure 2-4 is the baseline device for this works without
nucleobases. The emitting layer is the phosphorescent system CBP doped with 10wt% of Ir(ppy)3.
The OLED is grown on ITO patterned on a glass substrate. The conductive polymer, PEDOT:PSS,
is spin-coated on top of the ITO to enhance hole injection from the electrode to the organics. NPB
is the electron blocking layer and hole transport layer. BCP is a hole-blocking layer to prevent
holes from leaving the emitting layer. Alq3 facilitates electron injection from the cathode. The
Chapter 2 – OLEDs and Experimental Methods
17
LiF and Al serve as the cathode. LiF has a dual purpose of enhancing electron injection and is
necessary to prevent aluminum diffusion into organics106.
Figure 2-5 shows the phosphorescent reference device, emitting green light with a peak
wavelength of ~512nm, from the Ir(ppy)3 that has an orbital energy difference of 2.6 eV. Other
phosphorescent emitters are available with different energy levels that produce a variety of colors.
The inset in Figure 2-5 has the CIE color space that is useful for relating the electromagnetic
spectrum with how colors are perceived by the eye.
2.2. General OLED Fabrication and Characterization Procedures
2.2.1. OLED Fabrication
OLEDs are fabricated on 2 inch round glass substrates that are received with commercial-grade
pre-patterned ITO (90 nm; 20 Ω/□). Each wafer contains four 4 mm2 OLED devices. The area
where the ITO, organics, and aluminum overlap is the active area. Each device run can hold a
maximum of three round wafers, therefore a baseline device is usually run alongside two other
devices.
The wafer preparation begins by scrubbing the substrates with a dust-free wipe and rinsing
with organic solvents (acetone, methanol, isopropyl alcohol) and deionized water. The wafers are
blown dry and placed in an oven at 100°C for 15 min to remove any remaining moisture. The
wafers are then exposed to an oxygen plasma preening system (Plasma-Preen, Terra Universal
Inc.) for 10 min as the final step in the cleaning process. Figure 2-6(a) has a picture of the
glass/ITO substrate after cleaning. This cleaning method is considered the standard cleaning
process for all wafers.
The PEDOT:PSS (Heraeus Materials Technology) is the first layer deposited by spin coating
(Laurell Technologies WS-400B-6NPP/Lite spincoater) on the ITO (Figure 2-6(b)). The solution
is filtered with a 0.45 μm PVDF syringe filter and 1 mL of PEDOT:PSS is dropped on the wafer.
The spin coating parameters is 500 rpm for 8 s followed by 2000 rpm for 20 s to produce ~40 nm
thin film. The wafers are placed in an oven at 125°C for 15 mins, resulting in a uniform film
~40nm thick that covers the entire wafer (Figure 2-6(c)). The ITO leads are cleaned with a
methanol wipe to prevent the PEDOT:PSS from shorting the anode and cathode. Afterwards, the
Chapter 2 – OLEDs and Experimental Methods
18
wafers are transferred to the vacuum deposition system (Figure 2-6(d)) to deposit the remaining
organic layers by thermal evaporation.
The organic materials and cathode are thermally deposited in a molecular beam deposition
(MBD) system in ultra-high vacuum at 10-9 Torr. The MBD system (SVT Inc.) contains eight
thermal effusion cells containing the organic and electrode materials. A mechanical arm transfers
the wafers to the main chamber where they rest on a rotating stage to ensure uniform deposition.
Each effusion cell is individually heated to the sublimation or evaporation temperature of the
material. The shutter opens to expose the wafer and a quartz crystal monitor (Inficon XTC/2)
measures deposition rate in the chamber for precise thickness control. Each layer is grown at a
deposition rate of ~0.05-0.2 nm/s. The organic layers are deposited through a shadow mask onto
the active device area (Figure 2-6(e)). The wafers are briefly removed from vacuum to apply a
shadow mask for the cathode. The wafers are placed back into the system and the LiF and the Al
are deposited through the new mask (Figure 2-6(f)). The final OLED structure is as follows: ITO
Figure 2-6 (a) Commercially patterned ITO on glass substrates; (b) photograph of PEDOT:PSS during
spin coating; (c) result of the PEDOT after bake; (d) SVT high vacuum deposition system; (e) result of the
organic thin films (inset) organic shadow mask; (f) Al electrodes deposited on top of the organics to
complete the stack (inset) electrode shadow mask.
Chapter 2 – OLEDs and Experimental Methods
19
[90nm] / PEDOT:PSS [40nm] / NPB [17nm] / CBP:Ir(ppy)3 (10wt%) [30nm] / BCP [12nm] / Alq3
[25nm] / LiF [<1nm] / Al [40nm].
2.2.2. OLED Characterization
After OLED fabrication, the wafers are removed from vacuum to be characterized. All
characterizations are done in a homemade glove box in a controlled nitrogen environment to
prevent moisture from degrading the device. A DC power supply (HP 6634B) is connected to the
OLED and an automated LabView program controls the voltage and records the current. The
voltage increases from 0 V to 20 V at 0.25 V intervals. The Konica Minolta CS-200 color and
luminance meter records luminance and color data (also controlled by LabView). The CS-200 is
set to 0.1° measurement angle and 0.5 s record time to quickly measure the OLED and to minimize
heating the device at higher currents. The device is on for 1 s at each voltage interval while it is
Figure 2-7 Typical graphs of the baseline OLED performance (a) current density versus voltage; (b)
luminance versus voltage; (c) current efficiency versus luminance; (d) luminous efficiency versus current
density.
Chapter 2 – OLEDs and Experimental Methods
20
measured, then OLED is turned off for 3 s. Luminance is measured directly from the meter in
luminous intensity, candela per unit area (cd/m2).
Figure 2-7 shows the results of the standard baseline OLED (the control without biomaterials).
Voltage (V) and current density (J) plots in Figure 2-7(a) compare how the current changes versus
voltage. Figure 2-7(b) shows voltage and luminance (cd/m2) values for everyday luminaires.107
Current efficiency, ηI (cd/A), (Figure 2-7(c)) is useful for understanding electron/hole
recombination efficiency and is often plotted versus luminance or current density. Current
efficiency is calculated directly from luminance, L (cd/m2), and current density, J (A/m2), viz.
𝜂𝐼 =𝐿
𝐽 (1)
Luminous efficacy, ηlum, is the amount of luminous flux emitting per watt (lumens/W) shown in
Figure 2-7(d). It is particularly useful for comparing different lighting applications. The efficacy
of a tungsten lightbulb, for example, is ~15 lum/W. Luminous flux (lum) differs from luminance
(cd) in that it accounts for the light intensity in a particular direction. It can be converted by
assuming that OLEDs emits uniformly in all directions.105 Therefore, candela (cd) can be
approximated to lumens (lum) by multiplying by steradians in the forward direction (π), and
luminous efficacy (lum/W) is calculated, viz.
𝜂𝑙𝑢𝑚 =𝑐𝑑 ∙ 𝜋
𝐼𝑉 (2)
Ideally, every electron injected into the device should recombine with one hole to produce one
photon that exits out of the device to the viewer. However, various incidents do not produce a
photon that lower the efficiency. Recombination could occur in non-emitting layers; the electron
relaxation path could prohibit photon generation, such as the triplet state in fluorescent emitters;
the electron loses its energy to an impurity in the device. In these cases, the energy is typically
converted to a phonon resulting in vibrational energy and loss efficiency. Internal quantum
efficiency (IQE) measures the efficiency of the electron to photon conversion. Simply, IQE is the
ratio of photons generated by the number of electrons injected into the device. IQE depends on the
multiplication of the charge carrier balance factor (γ), explicitly the ratio of electrons/holes at the
recombination region; the probability of exciton formation (ηs); lastly, the photoluminescence
quantum efficiency of a material (𝜙), which is 100% for phosphorescent materials106.
Chapter 2 – OLEDs and Experimental Methods
21
𝜂𝑖𝑛𝑡 = 𝛾 × 𝜂𝑠 × 𝜙𝑓 (3)
Furthermore, a generated photon could fail to reach the viewer. Since glass (n=1.46) has a
higher refractive index than air (n=1.0), light attempting to exit the glass above the critical angle
of 43.2° normal to the surface will experience total internal reflection and remain in the device or
exit out the sides. Total internal reflection typical results in ~80% of the loss efficiency108 shown
in Figure 2-8. The external quantum efficiency (EQE) is the ratio of the total photons produced
by the total photons that actually reach the viewer. EQE is calculated by the ratio of the internal
quantum efficiency by the measured luminance in the direction of the viewer. Alternatively, EQE
can be calculated with luminance, wavelength emission, and current,109 which was the method
used in this work.
2.3. Summary
An overview has been presented on the fundamentals of OLEDs including the basic structure,
mechanism, and characterization. OLEDs rely on organic electroluminescent material to convert
electron energy to photon energy. More efficient OLEDs use additional layers such as
hole/electron injection layers (HIL and EIL) to enhance charge injection from the electrode and
electron or hole blocking layers (EBL and HBL) that contain charge to the emitting layer. A major
milestone in OLED efficiency was the discovery of phosphorescent emitters that enhanced the
efficiency of OLEDs to 100% internal quantum efficiency.
Figure 2-8 Total internal reflection results in a decrease in external quantum efficiency (light out-
coupled) from the original internal quantum efficiency (photons per recombination).
Chapter 2 – OLEDs and Experimental Methods
22
Fabrication techniques for the baseline device used in this work involved a wet spin coating
process and dry thermal evaporation. Characterization methods and a discussion on device
performance and efficiency were presented, as well as a discussion on quantum efficiency. The
baseline device will be used to compare to the performance of the nucleobases OLEDs in the
proceeding chapters.
23
Chapter 3. Nucleobases and Thin Film Properties
This chapter will discuss the properties of the nucleobases in their thin film state. The first
section will be on the discovery of the nucleobases and how they can be extracted from natural
materials or artificially synthesized. The second section will look at the thin film quality using
microscopy and investigate temperature stability. Finally, additional tests will explore
opto/electrical characteristics such as optical spectroscopy, dielectric constants, refractive indices,
and molecular orbital levels. The properties will be the foundation for the subsequent experiments
when the bases are inserted into the OLEDs.
3.1. Nucleobase Origin and Synthesis
Most of the initial work of the nucleic acid bases was done at the turn of the 20th century. They
were first observed by the Swiss physician Johannes Miescher110 and later isolated by the German
biochemist Albrecht Kossel.111 In the early 1870’s, Miescher examined bandages from soldiers in
the Crimean War. He observed large molecules in the nuclei of white blood cells from the bandage
and named it ‘nuclein’ because it appeared to come from the cell nuclei. Shortly after, Kossel
isolated the first nucleic acids from natural sources, which he later received the Nobel Prize in
Medicine.112 The etymology of the bases hint where the first material were extracted from113:
guanine from bird feces (guano), adenine from the pancreas gland (aden) of an ox, cytosine from
cellular tissue (cyto), thymine from a thymus gland, and uracil (urea from uric acid) extracted from
the hydrolysis of herring sperm.112 The knowledge of their biological significance containing the
code for life came nearly 50 years later.114,115
The nucleobases are categorized into two chemical systems: purines and pyrimidines. Adenine
and guanine are part of the purine system. Thymine, cytosine, and uracil have the pyrimidine
system. Figure 3-1 illustrate the chemical structure of the bases. The purines have a two-ring
structure comprising of a pyrimidine ring fused with an imidazole ring. The purine family is the
most widely nitrogenous heterocyclic molecule found in nature.116 Some of the more well-known
natural purines are adenine, guanine, hypoxanthine, xanthine, theobromine, caffeine, uric acid, and
Chapter 3 – Nucleobases and Thin Film Properties
24
isoguanine. The pyrimidines contain only a single heterocyclic ring similar to the pyridine
molecule. Other well-known natural pyrimidines include thiamine (vitamin B1) and alloxan112.
Nucleic acid bases can be created chemically by enzymatic reactions or metabolically
created.112,116 There are numerous methods for artificially synthesizing nucleobases. For example,
adenine can be created from ammonia and hydrogen cyanide through chemical processes.117 Most
of the bases can be synthesized by a process known as Fischer–Tropsch synthesis, which requires
heating a gas mixture of CO, H2, and CH3 to 600 ºC with a nickel/iron catalyst.118
But as demonstrated in the early 20th century, nucleobases can also be isolated from the natural
sources, which is important for natural electronics. They can be extracted from many different
natural resources, such as wheat germ, meat, plants, and fish by chemical isolation techniques or
enzymatic processes.10,114,119,120 For example, organ tissue can be dissolved by chemical processes,
filtered, and dried into a powder.121 Adenine can come from plants and non-animal sources.122
Bee pollen is considered a food rich in adenine.123 Extraction from yeast is another common
method to create adenine by converting it from phosphoribosyl pyrophosphate (PRPP) through
Figure 3-1 Chemical structures of the nucleobases consisting of nitrogen (blue), oxygen (red), and
hydrogen (gray). G and A are purines with fused pyrimidine and imidazole rings. C, T, and U are
pyrimidines with a single heterocyclic ring.
Chapter 3 – Nucleobases and Thin Film Properties
25
enzymatic processes.124,125 The nucleobases used in this study were purchased from Sigma-
Aldrich, and although the source of the bases could not be verified, it is quite possible that they
were synthetically created. However, similar to DNA, nucleic acid bases have the potential to be
harvested from the byproduct of another industry or unwanted biological wastes leading to their
low cost and renewability. As the bases continue to be studied in thin film electronics, it may be
more practical to switch from a synthetic origin to a natural origin weighing the cost and
environmental impact.
3.2. Thin Film Properties
The thin film properties of the nucleobases were investigated to explore their potential in
opto/electronic devices including thermal stability, film quality, dielectric constants, refractive
index, and energy levels. The bases (Sigma-Aldrich) were received as white to slightly yellowish
powder. All the nucleobases had a purity of ≥99% except for guanine, which was available at 98%
purity, and used without further purification. They are not soluble in aqueous solutions except in
the presence of a weak acid. Thermogravimetric analysis was first done on the nucleobase powders
to determine temperature stability. Afterwards, the nucleobases were loaded one at a time into an
effusion cell in the MBD system and the bases were thermally evaporated onto a clean substrate
for thin film analysis.
3.2.1. Thermogravimetric analysis
Thermogravimetric analysis (TGA) was performed (Netzsch STA 409 PC Luxx) to measure
mass loss as a function of increasing temperature. The bases were loaded ~10 mg from powder
form into a metal crucible and placed into the TGA system. The test was performed in an argon
environment and the temperature was increased from 30 °C to 600 °C at a rate of 10 °C/min. For
the sake of comparison, temperature stability is defined here as the point that the mass decreases
5% from its original mass.
The results of the nucleobases were compared to other nucleic acids: DNA (200kDa),
adenosine, adenosine triphosphate (ATP), and their complexes DNA-CTMA and ATP-CTMA.
Adenosine is a nucleoside consisting of an adenine molecule and pentose sugar without the
phosphate group. The ATP has the adenine, sugar, and three phosphate groups attached. Since
Chapter 3 – Nucleobases and Thin Film Properties
26
Figure 3-2 Thermogravimetric analysis from 30 to 600 °C in Ar showing temperature stability of: (a)
nucleic acid bases; (b) additional nucleic acids and their complex with CTAC; (c) reference OLED
materials.
Chapter 3 – Nucleobases and Thin Film Properties
27
both DNA and ATP have a negative charge they can be complexed with CTAC to form DNA-
CTMA and ATP-CTMA done according to the literature procedure126.
Figure 3-2(a) has the results of the TGA for the nucleobases. T and U have nearly equal thermal
stability points of 260 and 270 °C, respectively. The rapid mass loss is due to thermal evaporation,
and the molecules are completely evaporated by 340 °C. The purine A is nearly identical to the
two pyrimidines with a slightly higher stability temperatures at 290 °C completely evaporating by
360 °C. The C has the third highest thermal stability point of 325 °C and rapidly loses mass until
340°C, most likely due to thermal evaporation. Beyond 340 °C the mass loss for the C sample
occurs much more gradually. Upon removing the C from the system, it was clear that the sample
had carbonized causing the rate of evaporation to decrease. Finally, G displayed the highest
stability temperature of 465 °C. The G sample has high thermal stability due to its high crystal
lattice energy attributed to the presence of oxo and amino groups that facilitate intermolecular
hydrogen bonding.127 After 540 °C, the G slowed its rate of evaporation and began to carbonize
similar to C.
The nucleosides and nucleic acids are given in Figure 3-2(b). Water retention was very
apparent in DNA, DNA-CTMA, ATP, and ATP-CTMA even after oven drying, exhibiting a mass
loss of more than 5-10% before ~150 °C. Since DNA showed the greatest premature mass loss,
DNA powder was dried for 1 week in high vacuum (10-7 Torr). The TGA experiment was repeated
with the same results. The high water retention may have significant impact on DNA-based
OLEDs, especially since the material is spin coated and not typically oven dried before using as
an EBL. These four materials all became thermally unstable (defined here as a significant change
in mass loss rate above water removal temperature) between 200-220 °C. Adenosine did not show
evidence of water retention and became thermally unstable at 280 °C. All the samples were
carbonized upon removal from the system. Interestingly, the nucleobases have a higher thermal
stability than the nucleic acids, which is beneficial device fabrication. DNA-CTMA has been
shown to result in irreversible structure change of the thin film above 160 °C, attributed most likely
to the thermal denature of the DNA helix.128
TGA was done on conventional organic material used for the baseline OLED structure shown
in Figure 3-2(c). BCP had the lowest thermal stability at 300 °C and was completely evaporated
by 380 °C. Three of the molecules, CBP, NPB, and Alq3 had nearly identical thermal stability at
Chapter 3 – Nucleobases and Thin Film Properties
28
380 °C and completely evaporated by 460 – 480 °C. The inorganic salt, LiF, was stable up through
600 °C.
3.2.2. AFM and SEM
A silicon wafer was cleaned using the standard cleaning process described in section 2.2.1.
Each nucleobase was individually evaporated on a silicon wafer at a rate of 0.1 nm/s to a thickness
of ~100 nm. Atomic force microscopy (AFM) (Veeco Dimension) measures height distribution
of samples by recording the vertical deflection of a Si cantilever along the surface. AFM was set
to tapping mode for all experiments. The images are given in Figure 3-3(a) for all five bases. A
cross-section view was created by sampling a 5 µm line scan from each AFM image and the results
plot vertical deflection versus horizontal distance in Figure 3-3(b). The horizontal line scan was
plotted on the same height scale (40 nm) so that the difference could be easily compared. Thymine
was the exception being nearly 10x larger in height, requiring a larger scale (400 nm).
The G thin film had the highest quality of the five bases evident by the AFM image and line
scan. The small granules had a surface roughness of 0.4 nm. A had grains several times larger
than G with a roughness of 3.4 nm. The line scan showed A grains had very consistent horizontal
width of 100-150 nm and an average height of 10-15 nm. C had a slightly larger roughness (5.0
nm) with string-like features. The line scan for C was similarly in height to A (~10-15 nm), but
unlike A the horizontal width of the long strings were nearly 1-2 μm in length. U was almost twice
the roughness (7.0 nm) as C with smaller grain sizes. Its height distribution was 20-25 nm with a
crystallite width of 150-200 nm. T stood out above the rest with the most unusual growth. T
formed tall pillar structures with peaks 200-250 nm in height and nearly 1 μm in diameter, and the
film was extraordinarily rough at 81.7 nm. The AFM result for T are similar with a study from
literature10 showing large columnar structures for T . U was plotted on the same line graph as T
to show the difference. It is important to note that the film quality was only observed at 100 nm
to understand how the film layer grows with thickness. The film quality for thicknesses between
10-20 nm is addressed for some of the bases in section 4.3.2.
Due to the high surface roughness of T, further analysis on its thin film was done with a
Scanning Electron Microscopy (Evex Mini-SEM, SX-3000). SEM observes the reflection of high-
energy electrons on the surface to construct a 2D morphology of the surface. T was deposited to
thickness of 100 nm on a silicon wafer. The settings for the scan were 30 kV. The results are shown
Chapter 3 – Nucleobases and Thin Film Properties
29
Figure 3-3 (a) AFM analysis of nucleobases (b) sectional line scan samples from AFM results.
Chapter 3 – Nucleobases and Thin Film Properties
30
Figure 3-4 SEM images of thymine thin film on Si at (a) 1000× and (b) 20,000× magnification.
in Figure 3-4. At 1000× the surface was uniform and had no distinguishing features. At 20,000×,
the roughness and pillar features that were observed in the AFM were distinguishable. Vibration
from the system is apparent at 20,000×.
3.3. Optical and Electrical Properties
3.3.1. Optical Spectroscopy
Optical spectroscopy was done on thin film nucleobases to observe optical absorption between
the ultra-violet (UV) through the visible spectrum (Perkin Elmer Lambda 900). Optical
spectroscopy verified that the nucleobases were transparent in visible spectrum so as not to hinder
OLED light output. The bases also have unique absorption peaks in the UV region. These peaks
correlate well with previous optical spectroscopy reports of the bases in aqueous solution129 and
give confidence that the evaporated bases are not destroyed or altered during thermal evaporation.
Each nucleobase was thermally evaporated onto a clean quartz wafer to a thickness of ~100
nm. DNA-CTMA was dissolved in butanol at 1 wt% and spin coated (6000 rpm for 20 seconds)
onto a quartz wafer and dried at room temperature, resulting in ~20 nm film. The measurements
were taken between the wavelengths 200 – 700 nm for the UV and visible light and the results
were normalized. It was observed that all of the nucleic acid thin films were transparent in the
visible range (400 – 700 nm). Figure 3-5 contains the results of the optical spectroscopy analysis.
Chapter 3 – Nucleobases and Thin Film Properties
31
3.3.2. Ellipsometery
Ellipsometry instruments detect minuscule changes to circularly polarized light as it travels
through a medium to measure properties such as roughness, thickness, and refractive index.
Ellipsometry (WVASE32, J.A. Woollam Co) was used in this work to determine refractive index
and film thickness. Individual bases were deposited on a Si wafers to 17 nm, the thickness used
for OLED fabrication (see section 2.2.1) and the crystal monitor was calibrated according to the
Figure 3-5 Electromagnetic absorption spectra for UV and visible light from 200-700 nm.
Chapter 3 – Nucleobases and Thin Film Properties
32
results of this analysis. Measurements were taken from 400-900 nm at 65, 70, and 75 degrees. The
oxide of the Si wafer was measured to be 2.3 nm, which was used in the curve fit model. A Cauchy
curve fit was used. The data of the polarization changes is not given, but the model gives the
refractive index of the material as a function of wavelength of light, which is shown in Figure 3-6
for the nucleobases between 400 – 700 nm. At 580 nm, G had the highest refractive index at 1.96.
A, C, and U had similar refractive indices, 1.73, 1.76, and 1.67, respectively. T had the lowest at
1.50, comparable to glass.
Figure 3-6 Refractive index of nucleobases versus wavelength determined by ellipsometry
3.3.3. Dielectric constant
The dielectric constants were measured by fabricating a thin film parallel plate capacitor. The
commercially available and pre-patterned ITO (section 2.2) was used as the bottom electrode.
Each base was deposited to a thickness of ~100 nm monitored by a crystal monitor. The same
organic and electrode masks used for the OLEDs were used for the capacitor. The nucleobases
were deposited on top of the ITO, and the Al was patterned as the top electrode so that the active
area was 4 mm2 as shown in Figure 3-7. The capacitance was measured with an HP4275A LCR
meter at 1 MHz. The relative permittivity (εr) was calculated according to the capacitance of a
parallel plate capacitor, viz.
Chapter 3 – Nucleobases and Thin Film Properties
33
Figure 3-7 Dielectric constant device stack and measurement.
𝜀𝑟 =𝐶𝑑
𝜀0𝐴 (4)
C is the measured capacitance (F); A is the device area (m2); εo is the permittivity of free space
(8.854×10-12 F/m), and d is the thickness of the nucleobase layer (m).
The dielectric constant (relative permittivity, εr) for the purines G and A were 4.0 and 3.4,
respectively. The pyrimidines C, T, and U were calculated to be 4.3, 2.0, and 1.6, respectively.
The results are slightly lower (0.3 – 0.4) than those reported in a similar study10 that reported the
values at a lower frequency (1 kHz). Another study46 reported the dielectric constant of G to be
5.03.
3.3.4. HOMO/LUMO levels
The molecular orbital energy levels are an important parameter to understand charge transport
between molecules. Energy level measurements require an X-ray or ultraviolet photoemission
spectrometer (XPS or UPS) that exert short electromagnetic wavelengths to perturb the electron
and detect electron kinetic energy in the orbitals, measured in electron volts (eV). Due to limited
access to such expensive and complex systems, a literature survey was done from groups who have
explored orbital levels by XPS, UPS, or by computational simulations.
Chapter 3 – Nucleobases and Thin Film Properties
34
Figure 3-8 Molecular orbital energy levels of the nucleic acids compared to the reference OLED.104 Energy
levels from Faber et al.130 and are shown as a black solid line (—) are compared to the results from Lee et
al.46 shown in the dotted grey lines (···). The DNA-CTMA energy level obtained via UPS from Lin et al.131
give an electron affinity of 1.6 eV compared to the typical reported value 0.9 eV.21
Lee et al. worked closely with the bases and has done the most comprehensive measurement
of the nucleobase orbital levels.46,132 The thin film nucleobases were evaporated on ITO and
aluminum electrodes and measured with UPS. Their initial work only highlighted A and T
HOMO/LUMO levels but a more recent work included G and C. The U was not included in any
of the studies. The study revealed that the energy level gaps are wide (3.8 – 4.1 eV). The ionization
potential (HOMO) increases accordingly, G < A < C < T, such that G has the lowest (6.3 eV) and
T has the highest (7.0 eV).
Another group, Faber et al., did computational simulations of all the bases.130 The results had
similar energy gaps (3.6 – 3.9 eV) but placed the HOMO/LUMO levels 0.5 – 0.7 eV lower than
the results from Lee. However, the relative energy levels of the nucleobases still follow the trend
G < A < C < T < U. G had the lowest ionization potential (HOMO) of 5.7 eV and an electron
affinity (LUMO) of 1.8 eV. The U had the highest ionization potential of 6.7 eV and the highest
electron affinity of 3.0 eV.
There are many other studies that have investigated the energy levels of the bases.133-137 The
variation in results is most likely due to different measurement techniques (UPS vs simulation)
and under different conditions (gas phase, single molecule, and thin film). A better understanding
of the energy levels pertaining to OLEDs may aid in further understanding of the nucleobases.
Nevertheless, all of the studies are in accord with the general trend (G < A < C < T < U). It implies
Chapter 3 – Nucleobases and Thin Film Properties
35
that G is a strong hole acceptor while prohibiting electron transport that could serve as an
EBL/HTL in OLEDs. Conversely, U is a strong electron acceptor while prohibiting hole transport
that could serve as an HBL/ETL in OLEDs. The energy levels of the other bases increase stepwise
between G and U creating ample flexibility in device design. The energy levels quoted from Faber
and Lee are compared to the baseline OLED in Figure 3-8 showing the similarities to the EBL/HTL
or HBL/ETL layers of the baseline.
3.4. Summary of Nucleobase Properties
This chapter has explored some of the fundamental properties of thin film nucleobases
pertaining to optoelectronic devices. A summary of the results is presented in Table 3-1. The bases
have diverse attributes that will show potential in OLED design. The G and T (and U) have the
largest disparity of properties. The G has the highest thermal stability (465 °C) while T and U have
the lowest thermal stability (~260 °C). The surface roughness of thin film deposited bases
fluctuates from extremely smooth (G – 0.5 nm roughness) to unusually coarse (T – 81.7 nm
roughness). The optical transmission is transparent in visible range for all the bases, and the index
of refraction ranges from 1.96 for G and 1.5 for T at 580 nm. Dielectric constants are diverse from
4.0 to 4.3 (for G and C, respectively) to 1.6 (for U). Finally, the energy levels of the bases expand
the gamut from EBL/HTL (for G) to a HBL/ETL (for U) following the trend G < A < C < T < U.
The substantial difference in thin film attributes will be considered in the next chapters during the
nucleobase-OLED characterizations. It will be shown that the properties offer flexibility in
creating efficient and optimized OLEDs.
Table 3-1 Summary of nucleobase optoelectronic properties.
G A C T U
Refractive index (580nm) 1.96 1.73 1.76 1.50 1.67
Relative dielectric constant (1MHz) 4.0 3.4 4.3 2.0 1.6
HOMO (eV)130 5.7 6.0 6.2 6.5 6.7
LUMO (eV)130 1.8 2.2 2.6 2.8 3.0
Molecular orbital gap (eV) 3.9 3.8 3.6 3.7 3.7
Thermal stability (°C) (95% remaining mass)
465 290 325 260 270
36
Chapter 4. Nucleobase Bio-OLEDs
The diverse properties and wide range in energy levels offer rich opportunities for nucleobases
in all-natural organic electronics by allowing more flexibility in device design. Several nucleobase
OLEDs were fabricated to observe charge transport behavior and demonstrate their potential in
organic electronics. The reference device and the nucleobase configurations explored in this
chapter are shown in Figure 4-1. The first type of device was a reference device to establish the
control. The second device replaced the NPB (the EBL/HTL layer in the reference) to demonstrate
that G and A are EBL/HTL, while C, T, and U fail to transport holes to the emitting layer. The
third device substituted the BCP (the HBL/ETL in the reference) with nucleobases to show the
reverse trend: U, T, and C are HBL/ETL while G and A fail to transport electrons into the emitting
layer.
This chapter also investigates the optimal thickness of the nucleobase in the EBL and HBL
configuration. U was varied in thickness as the HBL-device to show that 12 nm produced the
greatest performance increase and several suggestions are given to improve results. For the EBL-
device, thin layers of T (<12 nm) resulted in a two-fold increase in efficiency over the reference,
even though T has HBL tendencies. AFM analysis revealed that the T created large surface
morphology changes in the emitting layer, which was the proposed reason for the large efficiency
gain.
Figure 4-1 OLED configurations to study nucleobase charge transport: (a) baseline device without the bases
to establish a reference; (b) nucleobase in the EBL/HTL configuration; (c) nucleobase in the HBL/ETL
configuration.
Chapter 4 – Nucleobase Bio-OLEDs
37
4.1. Nucleobases as an EBL/HTL
Nucleobases were deposited as the EBL/HTL (hereafter called EBL for simplicity though hole
transport has a significant role) in the standard phosphorescent OLED. The device structure for the
EBL-OLED shown in Figure 4-1(b) was ITO [90nm] / PEDOT:PSS [40nm] / nucleobase [17nm]
/ CBP:Ir(ppy)3 (10wt%) [30nm] / BCP [12nm] / Alq3 [25nm] / LiF [<1nm] / Al [40nm]. A standard
reference device (Figure 4-1(a)) was fabricated in each experiment to ensure consistency among
the runs. The reference contained the conventional EBL and HBL materials NPB and BCP. The
fabrication process followed the standard procedure described in section 2.2, except that the wafers
required an extra step to deposit the nucleobases separately from the baseline, which exposed the
wafers to air temporarily during the wafer exchange. The results of each experiment were repeated
at least 3 times and the luminance and current at each particular voltage were averaged (see
Appendix A for a discussion on calculating experimental error). The efficiencies were calculated
based on the averaged values.
4.1.1. EBL Results
The results of the baseline and each nucleobase OLED in the EBL configuration are compiled
in Figure 4-2(a-d) showing current density, luminance, luminous efficacy, and current efficiency,
respectively. The current density in Figure 4-2(a) shows that the current decreases sequentially in
the order G, A, C, T, U as predicted by the nucleobase energy levels. G had the largest current
compared to the other bases and confirmed its utility as a hole transporting layer. The current for
G was only slightly below the reference, which was expected since it has a slightly higher HOMO
level than the NPB, ~5.7 to 5.4, respectively. Conversely, U impedes hole transport, because it has
the largest HOMO level (~6.7 eV), resulting in the lowest current density. The U will have more
much potential as an HBL/ETL (discussed in section 4.2) due to its large orbital levels.
The current density for G was slightly lower than the baseline, also resulting in a later turn-on
voltage as seen from the luminance graph in Figure 4-2(b). The turned-on voltage was defined as
the point of detectable optical emission. The baseline turned on at 3.75 V, while G and A turned
on at 4.75 and 5.0 V, respectively. The higher driving voltage was likely due to the higher HOMO
energy levels of G and A compared to NPB (~Δ0.3-1.2 eV), diminishing hole injection from the
electrode ITO/PEDOT. At low bias voltage, the G-EBL produced slightly higher luminance than
the A-based device. However, at higher voltages the luminance does not increase as quickly, and
Chapter 4 – Nucleobase Bio-OLEDs
38
the A-EBL surpassed G at ~11 V. Although G was an excellent hole transport, it produced a
maximum luminance of only 17,191 compared to 82,289 cd/m2 for A.
Figure 4-2 The performance of the nucleobases as an EBL/HTL: (a) current density versus voltage, (b)
luminance versus voltage, (c) luminous efficacy versus current density, (d) current efficiency versus
luminance.
Chapter 4 – Nucleobase Bio-OLEDs
39
Figure 4-3 Illumination of the OLED with adenine as the EBL.
Figure 4-2(c)(d) give the efficiencies in terms of luminous efficacy and current efficiency
versus luminance. The A-based device outperformed all the bases including the baseline device in
current efficiency. A photo of an operational device with the A as an EBL is shown in Figure 4-3.
The peak current efficiency for A was 51.8 cd/A compared to 38 cd/A for the baseline. Moreover,
the A had well-controlled efficiency roll-off and retained its high efficiency even at high luminance
(40 cd/A at ~40,000 cd/m2). The luminous efficacy of A was slightly below the baseline, 21.1 vs
22.2 lum/W, due to its high driving voltage at lower current density, but surpassed the baseline
after 1 mA/cm2. The G had the second best performance in efficiency. G began with efficiency
above the baseline (44.7 cd/A) but had a large roll-off that quickly dropped below the baseline. Its
lower driving voltage boosted its power efficacy to nearly match the baseline at 22.0 lum/W.
The performance diminished rapidly for the pyrimidines as an EBL following the energy level
increase: C, T, and U. The C was more similar to the purines. It turned on at 5.0 V and reached
efficiencies of 36.1 cd/A and 14.5 lum/W. However, the maximum luminance of C was only 5,646
cd/m2 as recombination shifted away from the emitting layer. The T and U have a large HOMO
levels that halt hole current evident by the significantly higher turn-on voltages (7.75 V) and large
Chapter 4 – Nucleobase Bio-OLEDs
40
decrease in current density (Figure 4-2(a)). T exhibited only moderate luminance performance
with a maximum of ~2,000 cd/m2 that resulted in a modest maximum efficiencies of ~23 cd/A and
~7 lum/W. The U EBL device was not functional and barely generated any detectable luminance
(<10 cd/m2). It is evident by the current density that the higher HOMO level of the pyrimidines
quench the hole transport to the emitting layer and the low efficiencies show that the high LUMO
level failed to efficiently block the electrons.
4.1.2. Discussion of EBL results
The G-EBL was a more efficient hole transporter, but G diminished the OLED performance
shown by the lower luminance output and lower current efficiency compared to A especially at
higher voltages. A possible explanation is that G shifts the recombination away from the emitting
layer. It has been shown that G is an excellent “hole getter” in OFET dielectrics and has a great
Figure 4-4 Possible charge transport mechanism in the EBL OLED for (a) guanine, (b) adenine, and (c)
uracil.
Chapter 4 – Nucleobase Bio-OLEDs
41
affinity for holes.46 By placing the G slightly away from the semiconductor in the dielectric layer,
the “hole getting” properties of G enhance the performance in OFETs. It has been stated
elsewhere138 that G is a “sink” for holes, owing to its lower oxidation potential. In the OLED, the
high current is evidence that the holes are received by the G layer, however, it is possible that the
G fails to transport holes to the emitting layer, as shown in Figure 4-4(a), due to its affinity to
oxidize that results in non-radiative recombination most likely in the G layer.
The exceptional performance of A as an EBL is attributed to its HOMO energy level and wide
energy gap. The HOMO level of A is nearly commensurate with the CBP (~6.0 eV) to permit
effective transport of holes into the emitting layer. The wide energy gap places the LUMO level -
Δ0.7 compared to the CBP for effectively electron blocking to produce more radiative
recombination, demonstrated in Figure 4-4(b). Additionally, the A has a small rolloff in efficiency,
i.e. maintains high efficiency at high luminance. Decreased rolloff is typically a result of reducing
quenching effects that occur when the emitting layer is over saturated with charge carriers and
excitons at higher electric fields.126,127 It has been shown that materials with a wide energy gap can
lead to an increase in efficiency by limiting charge accumulation at the emitting layer interface to
prevent quenching.139,140
Figure 4-4(c) also shows the explanation of the decrease in performance for the pyrimidines.
As the HOMO level increases beyond the energy level of CBP, hole injection is quelled. The
pyrimidines will show much more potential as an HBL due to the large energy levels.
4.2. Nucleobases as an HBL/ETL
Nucleobases were deposited as the HBL/ETL (hereafter called HBL) in place of the baseline
with BCP as illustrated in device stack in Figure 4-1(c). A baseline was grown with each run and,
similar to the EBL fabrication, an extra step was necessary to deposit the nucleobases separately
from the baseline device. The nucleobases were deposited at the same thickness as the known
optimized BCP thickness (~12 nm). The results of each experiment were repeated at least 3 times
and the results of the luminance and current averaged at each voltage. The efficiencies were
calculated based on the averaged values. The energy levels suggested that the trend for the HBL
case should be opposite to the EBL case. The G and A, which performed well as an EBL, should
perform poorly as an HBL with low luminance and efficiency. Conversely, C, T, and U should
serve much better performance as HBLs than as EBLs.
Chapter 4 – Nucleobase Bio-OLEDs
42
4.2.1. HBL Results
The results of the nucleobases as the HBL are presented in Figure 4-5. The predicted trend in
performance (U>T>C>A>G) was especially evident in the efficiency (Figure 4-5(c)(d)). While
there was some unexpected variance in the trend for current density (Figure 4-5(a)) and luminance
(Figure 4-5(b)), the performance of the purines was low. The result for G as an HBL was a low
current density, nearly zero luminance, and negligible efficiencies (<1 cd/A and lum/W). The G
Figure 4-5 The performance of the nucleobases as an HBL/ETL: (a) current density versus voltage, (b)
luminance versus voltage, (c) luminous efficacy versus current density, and (d) current efficiency versus
luminance.
Chapter 4 – Nucleobase Bio-OLEDs
43
has a small electron affinity that prohibited electron injection into the emitting layer evident by the
low current. Similarly, the low ionization potential fails to efficiently block holes. The results for
A as an HBL were somewhat perplexing. The current density for A was the highest of all of the
bases. The turn-on voltage was 5.5 V and maximum luminance was 215 cd/m2, similar to some
of the pyrimidines. Although the luminance was relatively high, it is clear that A is not an HBL
since its efficiencies do not exceed 1 cd/A and lum/W.
The pyrimidines sequentially increased in performance in the expected trend C, T, and U. The
C had the lowest performance of the pyrimidines. It turned on at 5.5 V and the peak efficiencies
were 5.2 cd/A and 2.1 lum/W with a maximum luminance of 217 cd/m2. The T was the second
best HBL of the bases. Its turn-on voltage was equivalent to C (5.5V) but the efficiencies were
greater with a maximum of 15 cd/A and 5 lum/W, suggesting that the higher energy levels
effectively retained the holes in the emitting layer. The maximum luminance of T was slightly
higher at 362 cd/m2. The U had highest the performance of the bases and appeared to be the most
promising HBL. It turned on noticeably lower than the other bases at 4.25 V. The high current
density and higher maximum luminance (~4,000 cd/m2) indicated that more efficient electron
transport was occurring into the EL. The higher current efficiency of 16 cd/A and luminous
efficacy of 7.4 lum/W was due to effective hole blocking.
4.2.2. HBL Discussion
Figure 4-6 shows possible mechanisms for select nucleobases in the HBL configuration. The
G is undoubtedly prohibiting electron transport into the emitting layer, indicative of the low
current, and failing to block holes as shown in Figure 4-6(a). The A-HBL in Figure 4-6(b) illustrate
that its low efficiency was most likely due to the low ionization potential that fails to contain holes
to the emitting layer. The HOMO level from Faber calculates A to be 6.0 eV, which is the
equivalent to CBP, and is unable to block holes. Lee measures the HOMO level for A at 6.6 eV,
which is the same energy level as the BCP (the HBL in the baseline structure). If A had similar
HOMO levels as the BCP it would be expected that the A have a much higher current efficiency,
which was not observed.
But the high current density for A remained puzzling. The luminance reached relatively high
values and the current is the highest among the bases as an HBL. Based on the energy levels, it
seems most plausible that the electron/hole pairs are recombining in the Alq3 layer, illustrated in
Chapter 4 – Nucleobase Bio-OLEDs
44
Figure 4-6(b). The matching energy levels of the emitting layer and A provide effective hole
transport and the low electron affinity of A confines most of the electrons to the Alq3. The
effective recombination in the Alq3, a fluorescent molecule emitting a green color, may result in
an increase in luminance and current. Nevertheless, it is clear from the low efficiencies that the A
HBL resulted in poor hole blocking ability and is most suitable as an EBL.
U had the greatest potential as an HBL due to its high energy levels. The U appears to be
matched in electron affinity with the Alq3 and emitting layer. Furthermore, the relatively high
current efficiency confirmed that the high HOMO confined holes to the emitting layer. Additional
discussion will be presented in the subsequent section.
Figure 4-6 Mechanisms of charge transport in the HBL OLED for (a) guanine, (b) adenine, and (c) uracil.
Chapter 4 – Nucleobase Bio-OLEDs
45
4.2.3. HBL Optimization
While U demonstrated the greatest potential as an HBL, the results do not reach the
performance of the reference device. In order to investigate the potential cause of the diminished
operation, U was inserted in the HBL OLED configuration and varied in thickness between 8-17
nm and the BCP was varied in the reference OLED from 0-17 nm. The results are given in Figure
4-7. Figure 4-7(a) shows the current density versus voltage and Figure 4-7(b) shows the current
efficiency versus luminance for the two devices with varying thicknesses. U(8nm) had a large
current suggesting the layer was too thin to affect the charge carriers (holes or electrons). U(8nm)
had a relatively high maximum luminance of 19,627 cd/m2 with poor current efficiency 7.7 cd/A,
which resembled the results of the BCP at 0 nm. A mere 4 nm increase in thickness, U(12nm)
decreased the current density nearly 1-2 orders of magnitude over the voltage range of operation.
The large decrease in current seemed to indicate that the “bulk” properties of the U was being
observed, that is U began to show its effect on charge carriers. U(12) had a higher current
efficiency of 16.3 cd/A, however, the luminance decreased to 4,043 cd/m2. U(17nm) caused a
slightly lower current density and resulted in a corresponding decrease in peak luminance (1,625
cd/m2) and efficiency (14.5 cd/A).
Additionally, a reference OLED with BCP as the HBL was varied between 0-17 nm.
BCP(12nm) was the standard reference device that had a luminance of 95,179 cd/m2 and current
Figure 4-7 HBL optimization of the U and the reference (BCP) showing the effects on performance on (a)
current density versus voltage and (b) current efficiency vs luminance.
Chapter 4 – Nucleobase Bio-OLEDs
46
efficiency of 38.5 cd/A. Upon increasing the layer thickness, the BCP(17nm) resulted in negligible
change in performance. Removing the HBL, BCP(0nm), caused the current density to rise nearly
half of a decade across the entire voltage range, and the maximum efficiency fell to 13 cd/A, which
was expected since the device was without hole blocking ability. However, the larger current still
maintained the high luminance output resulting in only a minor decrease in maximum luminance
from the reference to 83,500 cd/m2.
The change in thickness had similar results for the U and the BCP. U(8nm) and BCP(0nm)
both result in high current, relatively high maximum luminance, and low efficiency due to little or
no hole blocking ability. At U(12nm), the hole blocking ability is effective evident by the rise in
efficiency. However, the current density for U remained far below the reference, which affected
the overall performance. The electron affinity for U is 3.0 eV and there should be no electron
barrier to the CBP. It is most probable that the U has high resistivity that caused the electrons to
transfer inefficiently to the emitting layer.
More exploration using U as a HBL is required, especially to verify resistivity and energy
levels of the thin film. It is possible that using a different emitter or EIL could better match the
energy levels with U. Unfortunately, U was not as high performing as the HBL configuration,
which presently is a major limitation for an all-natural OLED. Nevertheless, the efficiency of U as
an HBL showed that it is a hole blocking layer, which was the first successful attempt for a natural
HBL material. The study could open doors to further investigation.
4.3. Optimization of EBL
The EBL-OLED (section 4.1, Figure 4-1(a)) was further investigated. The thickness of the
EBL was varied for select nucleobases, A and T, in order to optimize the full potential of the
nucleobases in the EBL configuration. The results were compared with DNA in the EBL
configuration and showed that the nucleobases exceeded the performance of the DNA and the
reference device in efficiency.
4.3.1. Thin EBL OLED Experiments
The thickness of the EBL was varied from ~5 to 30 nm in the EBL-OLED configuration
described in section 4.1. In order to simplify the experiments, only two nucleobases were initially
chosen: A (to represent the purines) and T (to represent the pyrimidines). U was later investigated
Chapter 4 – Nucleobase Bio-OLEDs
47
for EBL thicknesses between 5 and 17 nm since it is most similar to T. DNA-CTMA and NPB
were also done in order to compare the effects of their thickness. Freeze-dried DNA (200kDa)
was combined with CTAC following procedures described in literature126 and the resulting DNA-
CTMA was dissolved in butanol at 0.25 wt% (for spin coating an 8 nm layer) and 0.5 wt% (for
16nm) and the solutions were mixed overnight. After the PEDOT:PSS baking step, DNA-CTMA
solution was spin coated as the EBL on top of the PEDOT:PSS layer at 6000 rpm for 20 s and
allowed to air dry for 10 min. The device was then transferred to the evaporation system for the
deposition of the remaining layers excluding the NPB layer.
Figure 4-8 (a) Device stack of the EBL configuration. (b) The energy levels comparing the four different
EBL NPB, A, T, and DNA-CTMA and the adjacent layers to the EBL in the OLED.
The device stack and a comparison of the energy levels for the four different EBL adjacent to
the HIL and EL are given in Figure 4-8(a) and (b), respectively. Figure 4-9 plots the results of the
maximum current efficiency versus the corresponding thickness of the device. T had a surprising
result producing the greatest performance at 10 nm reaching 76 cd/A. Increasing the T beyond 10
nm diminished performance due to its large HOMO level, as was observed in section 4.1.1.
Decreasing the T layer less than 10 nm also diminished efficiency. The A device had little
fluctuation in efficiency with thickness variation, peaking to 54.7 cd/A at ~17 nm and remaining
above 40 cd/A for all other thicknesses. DNA-CTMA was only efficient at ~8nm (0.25 wt. %)
and significantly decreased performance with larger layers. U, although similar to T in energy
levels, had a peak efficiency of 48 cd/A and showed a stark drop in efficiency after 8 nm. The
NPB was less affected by thickness variation but had its peak performance at ~17nm.
Chapter 4 – Nucleobase Bio-OLEDs
48
Figure 4-9 The effect of varying the thickness of the nucleic acids in the EBL configuration compared to
the reference.
The high performance of the T was further investigated at 10 nm. Representative I-V
characteristics are given in Figure 4-10 for the case of the A, T, DNA-CTMA, and NPB. The turn-
on voltage for the reference device was 3.75 V, the DNA device turned on at 4.0 V, the A device
at 4.5 V, and the T device had a much larger turn-on voltage of 5.5 V. The increase in driving
voltage is related to the increasing HOMO level since both the A and the T have higher ionization
potentials than NPB and DNA, requiring a higher driving voltage. However, after the T device
turned on, the current density had a much steeper slope and surpassed the A device. The DNA
was higher than the bases in current density, due to the low HOMO level, but the current was not
translated to emission.
Figure 4-10(b) contains the current efficiency plots for all the devices with respect to
luminance. The peak efficiency for all the devices occurred between 200-400 cd/m2. The T had
the highest peak efficiency of 76 cd/A at ~200 cd/m2 that remained high throughout the entire
range: 51 cd/A at 10,000 cd/m2, 36 cd/A at 100,000 cd/m2, and 26 cd/A at its maximum luminance
of 132,000 cd/m2 (obtained at 14 V). The A-based OLED had a maximum efficiency of 48 cd/A
at 300 cd/m2 with a slight roll-off, decreasing to only 42 cd/A at 10,000 cd/m2. The NPB OLED
had an efficiency of 37 cd/A at 150 cd/m2, which was less than half of the efficiency for the T
device at the same luminance.
Chapter 4 – Nucleobase Bio-OLEDs
49
Luminance as a function of current density is given in Figure 4-10(c). T had the largest
luminance value at any given current density followed by A, while NPB had a smaller luminance
output. The T device had a maximum luminance of 132,000 cd/m2 followed by the NPB with a
maximum of 113,000 cd/m2. The A device reached 93,000 cd/m2. Figure 4-10(d) is a similar plot
to Figure 4-10(b) with respect to current density.
The DNA-CTMA as the EBL layer showed only modest improvements over the reference, but
the bases were higher in performance. DNA-CTMA had a peak current efficiency of 43 cd/A at
230 cd/m2, which was higher than the baseline but not the nucleobase OLEDs. Its maximum
Figure 4-10 Results of the EBL at 10 nm for baseline, DNA-CTMA, A, and T: (a) current density versus
voltage; (b) current efficiency versus luminance; (c) luminance versus current density; (d) current
efficiency versus current density.
Chapter 4 – Nucleobase Bio-OLEDs
50
luminance was the lowest of all the devices at 62,000 cd/m2. While the current efficiency for DNA-
CTMA was respectably high, the A and T-based OLEDs were the preferred materials for bio-
OLEDs, not only due to their higher performance but in addition to their advantage of simpler
fabrication processes.
The higher recombination efficiency for T was unexpected. Therefore, the subsequent section
used AFM studies to investigate possible mechanisms for the higher efficiencies.
4.3.2. AFM Results of Thin Films
AFM were studies were done to examine the surface morphology of the EBL films and the
changes in the emitting layer (CBP) deposited on top of each EBL. The EBL materials T, A, and
NPB were deposited on Si and AFM was first done on the EBL layers to observe the differences
in morphology. Next, a CBP layer was deposited on top of each EBL, and AFM was done on CBP
to observe the film formation of the emitting layer with the different EBL layer underneath. A
layer of CBP was also deposited on Si to know its standard thin film quality. The parameters were
grown under the same conditions as the OLED fabrication: 10 nm for each EBL and 30 nm for the
CBP layers.
The column on the left of Figure 4-11 has AFM images of the EBL (T, A, NPB) on silicon,
the reference CBP film on silicon, and the CBP scans with the EBL underneath (T/CBP; A/CBP;
NPB/CBP). The horizontal scan resolution for each run was 1 µm and the vertical scale was set to
20 nm. T had a roughness of 1.76 nm and revealed a range of random crystallites in both surface
periodicity and height distribution. The A scan had a similar roughness, 1.83 nm, but had a more
uniform distribution of crystallites in periodicity and height distribution. The NPB layer had a
relatively large roughness (3.40 nm) with similar grain size as A.
The CBP on Si, A, and NPB had similar results, but CBP on T had vastly different morphology.
CBP on A had a similar roughness (1.89 nm) to that of CBP on Si. CBP on NBP had the same
roughness as NBP on Si but had the grain size of CBP on Si. The CBP on both A and NPB had
relatively similar grain size, but the CBP on T differed significantly. The CBP deposited on T had
much higher roughness (3.25 nm) than T on Si (1.75 nm) and its morphology exhibited relatively
deep (20+ nm) and wide (50-100 nm) craters uniformly dispersed throughout the layer. Horizontal
line scans were sampled from each AFM images to create the illustration in the column on the
right in Figure 4-11. Each CBP scan was stacked on top of its corresponding EBL to observe a
Chapter 4 – Nucleobase Bio-OLEDs
51
Figure 4-11 (Left) AFM scans of each EBL deposited to 10 nm and CBP deposited on silicon to 30 nm.
Also shown are AFM scans on CBP deposited on top of each EBL film. Scan length is 1 µm; (Right)
sectional views of each AFM result plotted on vertical/horizontal axes with each CBP paired to its
respective layer to elucidate how each EBL affects the growth of the emitting layer.
Chapter 4 – Nucleobase Bio-OLEDs
52
sectional view. Again, it is apparent that the CBP on T is considerably different from the other
samples. It is interesting to compare the results of the CBP with the large columnar-like structures
of T, as was shown previously (section 3.2.2) and in AFM scans. Though the thin T film does not
appear to have tall aspect ratios in this AFM study, the CBP seems to create these pillar-like
structures.
4.3.3. Discussion of Thymine EBL Optimization
The most plausible explanation for the high efficiency in the T-based EBL device is the large
craters of the CBP layer created by the T as shown by the AFM. The deep craters of the CBP
allow for enhanced electron injection from the BCP. Additionally, the irregular and non-periodic
morphology of the T layer, along with the suspected higher resistivity of the material, causes hole
traps that create a better electron/hole ratio. Figure 4-12 simplifies the explanation by estimating
the peaks as high and low steps assuming that the large valleys in the CBP align with the valley of
the T layer. The electrons from the BCP layer approach the CBP layer funneling towards the
valleys of the CBP. The aggregation of electrons at these valleys encourage electron injection and
recombination in the CBP, hence increasing recombination efficiency. Concurrently, the hole
injection from the PEDOT into the T layer preferentially travel via the T valleys, due to the higher
Figure 4-12 (a) Simplified mechanism of hole and electron transport for T(10 nm) showing charge
transport concentrated at the valleys of the BCP and T. (b) The smaller roughness of the A layer has more
uniform charge injection.
Chapter 4 – Nucleobase Bio-OLEDs
53
resistance and HOMO level of the T. The preferential injection at the valleys accomplishes two
things: firstly, it places hole injection nearest the point of the electron aggregation, that is the
valleys of the BCP; secondly, the peaks of T acts as a hole trap thereby controlling the injection
into the CBP and providing a better electron/hole ratio. Hole traps have been noted in other work
to enhance efficiency due to a surplus of holes.141
The thin T layer (8-12 nm) is optimized for the effect of hole traps and enhanced hole/electron
injection at the valleys. As the thickness of T increases (see Figure 4-9), the T layer becomes too
thick to inject holes into the CBP due to the high HOMO level of the T and the current efficiency
drops dramatically. Conversely, the A layer shows a much more constant variation in current
efficiency as an EBL. This is supported by the relatively smooth morphology of the A that
facilitates a more uniform injection of the electrons and hole into the CBP layer. Variation in A
thickness merely influences layer resistivity, but the enhanced hole/electron injection and hole
trapping seen in the thin T layer are not observed for A or any of the other nucleobases.
It is misleading to believe that a higher HOMO level of T is the entire reason for regulated
hole current to provide a better hole balance for a better electron/hole balance. Higher HOMO
levels do not necessarily imply high efficiency, as was shown in Figure 4-9, a thin U layer does
appreciably raise the efficiency, but the efficiency of U is ~20% less than T. Interestingly, the
thickness variation in Figure 4-9 shows the nucleobases and the baseline reaching no greater than
~55 cd/A.
In summary, there may be several factors contributing to the high efficiency of T including
high HOMO levels and higher resistivity, but the craters appear to be unique to the efficiency
increase for T. Much more investigation is necessary, especially to decrease the driving voltage,
but the increase in OLED efficiency is a promising result for natural electronics because it reveals
that the nucleobases are capable of achieving high efficiency and luminance.
4.4. Conclusions
The nucleobases were deposited first as an EBL to replace the NPB and then as an HBL to
replace the BCP in a conventional phosphorescent OLED structure. For the EBL case, the current
decreased accordingly, G<A<C<T<U, which is caused by the increase in energy levels. It is A
that has the highest luminance and efficiency due to its matched HOMO level to CBP. The
pyrimidines do not function as an EBL because of their high HOMO level.
Chapter 4 – Nucleobase Bio-OLEDs
54
Conversely, the pyrimidines serve well as an HBL. U had the highest performance, while T
and C had the second and third highest efficiencies due to their hole blocking/electron transporting
ability, as predicted by the energy levels. While A had unpredictably high current and luminance
as an HBL, most of the recombination in the A-HBL was suspected to be in the Alq3, due to the
electron blocking and hole transport ability. Nevertheless, A and G yielded small efficiencies due
to its inability to trap holes. U was further investigated as an HBL by optimizing the thickness. It
was found that U appears to be a resistive material or unmatched energy levels to the emitting
layer that quenches charge transport. Further investigation into U as an HBL may improve
performance.
Finally, a thin layer of T as an EBL produced exceptionally high current efficiencies. Upon
investigation with AFM, it was found that T (unlike A or NPB) produced large craters in the CBP
layer that caused an increase in efficiency and luminance. The efficiency is due to a combination
of improved hole/electron injection at the valleys of the CBP and the controlled hole current from
hole traps in peaks of the T layer. A summary of the results of all the experiments in this chapter
is presented in Table 4-1.
The work in this chapter provides a good foundation for future organic electronic devices that
incorporate nucleobases. While there still remain many other layers in the OLED structure to
experiment with natural materials, the results in this chapter show that the five bases offer a diverse
range of current control for hole/electron blocking/transport and have the potential for highly
efficient OLEDs. The next chapter will explore natural options for the device substrate, electrode,
and hole injection layer to further advance the all-natural OLED.
Chapter 4 – Nucleobase Bio-OLEDs
55
Table 4-1 Summary of the nucleobase performance in OLEDs in the (a) EBL configuration at 17 nm; (b)
HBL configuration at 12 nm; (c) EBL optimized at 10 nm.
(a) EBL (17nm) Turn-on (V) Max Lum (cd/m2)
Max Current Eff (cd/A)
Max Lum Eff (lum/W)
Quantum Efficiency (%)
Ext Int
Ref 3.75 95,179 38.5 22.3 10.7 59.4
G 4.75 17,191 44.3 21.9 12.3 68.3
A 5.0 82,289 51.8 21.2 14.3 79.4
C 5.0 5,646 36.1 14.5 10.0 55.6
T 7.75 3,844 22.6 6.9 6.3 35.0
U 7.0 21 3.3 1.2 0.9 5.0
DNA-CTMA 3.75 60,061 43.3 25.6 12.0 66.7
(b) HBL (12nm) Turn-on
(V) Max Lum (cd/m2)
Max Current Eff (cd/A)
Max Lum Eff (lum/W)
Quantum Efficiency (%)
Ext Int
G 6.0 16 1.3 0.6 0.4 2.2
A 5.5 215 1.0 0.4 0.3 1.6
C 5.5 217 5.2 2.1 1.5 8.3
T 5.5 362 15.1 5.0 4.2 23.3
U 4.25 4,045 16.3 7.4 4.6 25.6
(c) EBL (10nm) Turn-on
(V) Max Lum (cd/m2)
Max Current Eff (cd/A)
Ref 3.75 113,000 37
DNA-CTMA 4.0 62,000 43
A 4.5 93,000 48
T 5.5 132,000 76
56
Chapter 5. Cellulose and Au for Natural-Based OLED substrates
One of the primary advantages of OLED is the ability to fabricate it on flexible substrates. In
addition to exciting new applications, flexible substrates will help to reduce the cost of OLEDs by
taking advantage of roll-to-roll or printed processing on lightweight and low cost substrates.85
Plastic is typically the material of choice for flexible OLEDs because of its low cost and durability.
Unfortunately, many plastics (such as PET) derive from petroleum, which is a non-renewable
resource and has many environmental concerns including overwhelming waste and toxicity.16
Plastic takes thousands of years to fully degrade142 and has accounted for 10% of landfill waste.143
Many new bio-based polymers and biodegradable plastics16 are responding to the environment
problems and offer alternatives for flexible device substrates. Cellulose, the age-old organic
polymer that forms paper, is the material considered here for the flexible substrate for natural
electronics.
Cellulose is a natural material derived from plants that has many properties that are attractive
as a substrate for natural electronics. The origin and properties of cellulose substrates are explored
in this chapter. A review on the current research of OLEDs devices on cellulose will be presented.
Gold, a natural element, was chosen as the electrode to replace ITO due to its flexibility, non-
annealing process, and high conductivity. A lift-off technique to transfer gold electrodes to
cellulose using epoxy was employed to overcome some fabrication challenges of paper such as
rough surfaces and its low glass transition temperature. The cellulose/gold substrate was used in
the final bio-OLED device in the next chapter.
5.1. Cellulose substrates
Paper is not the typical candidate for flexible electronics due to its initial challenges. Paper is
less durable, more hydroscopic, and conventional paper is rougher than most plastic.144 However,
its advantages could inspire new applications in electronics unmatched by its competitors. Paper
is made from plant cellulose, which is the most abundant organic polymer material on Earth,145
therefore, cellulose is a nearly inexhaustible source of raw biomaterial and a tenth of the cost of
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
57
most plastic.144 Paper is already equipped with one of the most advanced deposition lines of
modern times, roll-to-roll printing, which can print micrometer features on wide reams at
astounding rates.37,146 It is a renewable and biodegradable resource147 unlike most plastics. As
flexible electronics continue towards commercialization, advancing paper as a substrate could
ensure its place in future electronic devices. Use-and-toss displays on paper could be used in
advertising, newspapers, product packaging, and brochures as a low cost substrates.
Figure 5-1 The cell wall of plants form a mesh of fibrils. Fibrils are composed of microfibrils made from
bundles of cellulose chains. Cellulose chains can be processed to reform into different materials.
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
58
Cellulose is a polysaccharide substance that gives structure to plant cell walls. The origin of
cellulose is shown in Figure 5-1. Every cell wall in plants is composed of elongated fibrils (~100
nm in diameter) that are made of bundles of microfibril (10-30 nm in diameter).148 The microfibrils
are further made from twines of β-glucose polymers that are bound together by a complex
hydrogen bonding network149 of the hydroxyl groups. Cellulose has been used to make paper for
centuries and today is found in cardboard, clothing, and building materials. It is primarily extracted
by separating wood or cotton by mechanical or chemical processes producing a pulp of suspended
cellulose fibers, but it also can be produced by the secretion of some bacteria.148 After it is
extracted, the cellulose pulp undergoes a process of breaking and reforming the hydrogen bonds.
Cellulose can take different forms depending on additives, temperature, β-glucose chain length,
pressure, and fillers during the processing. The various recipes generate a chasm of material
properties that determine the quality and type of the paper: transparent or opaque, soft or rigid,
water resistant or hydroscopic, smooth or rough surface.37
Figure 5-2 (a) Reconstituted cellulose film with excellent optical transparency, (b) compared with glass
and conventional copy paper.
Cellulose is naturally transparent,150 but conventional copy paper is opaque. Microfibril are
aligned, but the fibril planes are stacked randomly with air gaps in between that scatter light.
Hence, copy paper is not practical for bottom-emitting OLEDs since its opacity would significantly
hinder light output. Cellulose films, however, can take on transparent forms by reconstituting the
cellulose fibers. Cellulose reconstitution is the process in which cellulose is dissolved in a
dimethylacetamide solution and a salt is added during the film formation to prevent O-H bonds
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
59
from reforming between the stacked planes. As a result, the cellulose chains form dense and highly
aligned layers rendering the film with transparency comparable to glass, as seen in Figure 5-2. The
process and study of reconstituted cellulose is described in literature.151
5.1.1. Cellulose for OLEDs
The work that has been done for cellulose in electronics is quite extensive.37,38,144,152 It has
primarily been used as a substrate, but there has been considerable amount of research in the
electronics field that seeks to use cellulose as a dielectric material, energy storage, or embedded
with conductive nanoparticles.37 Cellulose substrates have been sparse in OLEDs. A summary is
shown in Table 5-1.
Table 5-1 Review of OLEDs using cellulose as a substrate.
Ref Smoothing Layer
OLED type Electrode/HIL Luminance (cd/m2)
Efficiency (cd/A)
Legnani SiO2 NPB fluorescence ITO 1,200 N/A
Min none NPB fluorescence Ni 620 0.97
Ummartoyotin Resin CBP fluorescence Cu/MoO3 200 0.085
Najafabdi NPB Top-emitting Ir(ppy)3 Au/MoO3 75,000 53.7
Purandare Parylene Ir(ppy)3 phosphorescence
ITO/PEDOT 10,000 50
The earliest notable work for OLEDs on cellulose was done by Legnani et al.153 A fluorescent
NPB OLED was constructed on top of bacterial cellulose membranes. The cellulose had a
roughness of 120 nm and SiO2 was sputtered before the ITO to reduce the roughness to 45 nm.
The ITO was sputtered very thick (185 nm) to achieve good conductance. The ITO directly
sputtered on cellulose was 48 Ω/□ and the ITO on the cellulose/SiO2 smoothing layer was 27 Ω/□.
However, due to the thick ITO and the slightly brownish tint of the bacterial cellulose, the
transmission in the visible region was only 40%. The result of the NPB OELD on cellulose with
the SiO2 as the smoothing layer was 1,200 cd/m2, which was a ~50% decrease from the same
device on glass.
Another NPB device fabricated on wood-cellulose used thin metal Ni as the transparent anode
instead of ITO.154 Ni was thermally evaporated onto cellulose through a shadow mask. The
cellulose was found to be very smooth, better than PET, and did not require a smoothing layer.
The performance of the OLED was very low for an NPB device. The maximum luminance was
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
60
620 cd/m2 at 15 V and a maximum efficiency of 0.97 cd/A. However, the device could be
improved by optimizing the device and using a hole injection layer.
In 2012, Ummartoyotin et al. used bacterial cellulose with a Cu anode in a fluorescent CBP
device,155 which emits blue without a phosphorescent guest. The Cu anode was very thick (200
nm). A polyurethane based resin was used as the smoothing layer to reduce the roughness from
2.7 μm to 33 nm. The performance of the OLED was low with a maximum luminance and
efficiency of 200 cd/m2 and 0.085 cd/A, respectively.
The aforementioned devices constructed on cellulose were focused primarily on substrate
composition and characterization rather than OLED performance. Najafabdi et al. has recently
implemented their highly efficient top-emitting OLED structure156 on cellulose nanocrystal
substrates.156 The structure is the same phosphorescent emitter, CBP:Ir(ppy)3, but is a top-emitting
OLED with a semitransparent gold electrode with several layers differing from this work. The
device obtained a maximum luminance of 75,000 cd/m2 and a current efficiency of 53.7 cd/A. The
device still required a very thick (400 nm) α-NPD on top of the cellulose nanocrystal as a
smoothing layer.
Purandare from the Nanoelectronics Laboratory has done additional work on optimizing the
luminance and efficiency for OLEDs on cellulose paper for bottom-emitting devices.39 Purandare
utilized the phosphorescent emitter CBP:Ir(ppy)3 with the conventional OLED stack, the reference
device described in Chapter 2, to attain emission efficiencies of ~50 cd/A and 20 lm/W with a
maximum luminance of 10,000 cd/m2. The cellulose substrate was found to be very smooth (~4.4
nm), but a protective layer of parylene-C was deposited on top of the cellulose by chemical vapor
deposition to protect it from wet spin-coating processes. PEDOT:PSS was used to enhance the
hole injection efficiency. While the efficiency and the luminance continue to improve from many
of the previous efforts, the ITO could not be annealed due to the thermal sensitivity of paper
resulting in very high sheet resistance. Furthermore, the wet process is unsuitable for hydroscopic
cellulose. Different methods for electrodes, surface smoothing, and hole injection are necessary to
continue the progress of cellulose-based OLEDs.
5.1.2. Cellulose challenges
Based on the review of OLEDs on cellulose substrates, there are several challenges that
confront OLEDs:
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
61
(1) Cellulose typically has a rough surface that requires special smoothing layers for good
device operation. Lamination and special coating can overcome rough surfaces, but require
complex sputtering or chemical deposition procedures. For this work, smoothing was done by a
simple UV curable epoxy step without the use of complex systems.
(2) ITO is not the preferred electrode for cellulose substrates since annealing is required to
obtain high conducting electrodes. Furthermore, ITO is a rigid electrode that is not suitable for
flexibility.157 For this work, ITO was replaced with gold as a semi-transparent and highly
conductive electrode.
(3) PEDOT:PSS is a wet deposition process that is difficult to work with on paper since water
can absorb into the substrate and destroy the device. PEDOT also has a baking step that can deform
or degrade due to the temperature sensitivity of cellulose. PEDOT:PSS was eliminated in the final
device and was replaced with adenine as the hole injection layer (see Chapter 6).
5.2. Gold electrodes in OLEDs
ITO is the de facto electrode for bottom-emitting OLEDs because of its optical transparency
and excellent charge injection into common organic semiconductors. But thin metal electrodes
(Au, Pd, Pt, Ag) have been proposed as an alternative electrode in OLEDs because they are highly
conductive and flexible.158 Gold was chosen as the bottom-emitting to replace the ITO electrode
on cellulose. Although gold has several challenges to overcome, there have been very successful
OLED devices demonstrated with gold electrodes159 and it remains a viable option for state-of-
the-art OLEDs. Gold has several key advantages over ITO160 that are essential for OLEDs on
cellulose substrates. Gold requires no annealing, it is highly flexible, adaptable to flexible
processes, and has high conductivity. A comparison of the two electrodes is summarized in Table
5-2.
The conductivity of gold is orders of magnitude higher than ITO and does not require
annealing. High conductivity is crucial for uniform emission from large area OLEDs. A gold
electrode requires only 20 nm of gold to obtain a sheet resistance of 5 Ω/□, which aids in decreasing
its relatively high cost. ITO electrode purchased commercially at 90 nm thick is only ~20 Ω/□
after annealing. Gold does not require annealing, which is the primary advantage for cellulose
substrates. The annealing temperature of ITO is ~300°C, which is beyond the temperature cellulose
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
62
can withstand. The absence of annealing ITO proved to be problematic for previous OLED devices
on cellulose.39
Table 5-2 Summary of Au and ITO properties
Gold ITO
Electrode Thickness 20 nm 90 nm
Mass density 19.28 g/cm3 6.8 g/cm3
Transparency (λ=515nm) 58% 91%
Sheet Resistance 5.5 Ω/□ 20 Ω/□
Refractive Index161 0.467 + 2.415i, 1.8-2.0
Work Function ~5.1 ~4.7
Annealing None required ~300 C
Electrical Conductivity 9×104 S/cm 5×103 S/cm
Typical Deposition Methods
Thermal
evaporation, e-beam,
roll-to-roll,
sputtering
Sputtering, e-
beam, CVD
Toxicity161 None Low
Poisson ratio161 0.44 0.35
Young’s modulus162,163 35 GPa 140-200 GPa
It has already been stated that gold is a flexible material,159 unlike ITO, which is a brittle
material.157 Flexibility not only is important for plastic and cellulose substrates but also extends
into fabrication. Gold is currently being deposited on large plastic substrates by roll-to-roll
processing with commercial assembly lines.159 In this work, the flexible nature of gold is used for
lift-off procedures. In addition, gold can be patterned using traditional fabrication techniques such
as thermal evaporation, e-beam, and sputtering.
Finally, its optoelectronic properties have some advantages over OLEDs. Gold has a higher
work function (5.1 to 5.3 eV) than ITO (4.7 eV) for better hole injection. ITO also has been known
to vary depending on surface treatment and conditions. Gold does not alter its properties and is
even temperature independent. Gold does not react with organic layers, a frequent problem with
ITO that has reduced OLED lifetime.160
Despite all these advantages, gold has not surpassed ITO as the de facto electrode for OLEDs.
Although gold is semitransparent in the visible region (section 5.3.2), thin gold is a highly
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
63
reflective metal with a low index of refraction. The reflective surface forms a weak optical cavity
when used as a bottom-emitting electrode. The optical cavity can increase the output in the forward
direction, but the emission intensity strongly depends on viewing angle, which is undesirable for
displays that require a high viewing angle. The challenge of overcoming the optical cavity is not
addressed here since it is beyond the immediate interest of natural electronics. However, it should
be noted that this challenge can be overcome with high refractive index materials that outcouple
light and thickness optimization of the optical cavity to produce some of the highest performing
OLEDs to-date.164
The second and most difficult challenge of gold electrodes is the poor metal-organic contact
caused by the dipole of the gold electrode. Organic materials such as NPB have been specifically
engineered for good hole injection with ITO.159 It is well known that gold forms a strong interfacial
dipole101 and creates an energy barrier with common hole injection layers such as NPB. The
interfacial dipole dramatically decreases the hole injection efficiency. C60 or MoO3 are commonly
deposited on top of gold to overcome poor hole injection and improve device performance.158,160,165
In this work, it is shown how adenine deposited on Au improves OLED performance behaving
similarly to C60 with similar mechanisms discussed in subsequent section 6.1.3.
5.3. Template Stripping of Gold Electrodes
Template stripping gold electrodes was adapted from similar methods.166,167 Epoxy was used
to lift-off gold from a silicon template. The gold conforms to the roughness of the silicon and not
to the cellulose resulting in creating some of the smoothest gold surface available,167 which is not
possible by directly evaporating gold on cellulose. Similar work has been done on PDMS,168 but
the technique has not been implemented for cellulose. On cellulose, UV curable epoxy acts as a
smoothing layer and also adheres very well to the gold. The substrate cellulose/epoxy/Au
fabrication procedure is discussed and analyzed in this chapter. The next chapters integrates the
substrate in the final device.
5.3.1. Au Template Stripping Procedure
The fabrication procedure is presented in Figure 5-3. Transparent cellulose was purchased
(BKK, LLC) and used without further processing or cleaning. Each substrate cost ~$0.03/sheet
and was cut in half to a final dimension of 33x35 mm with a thickness of ~25 μm. The Si wafer
was cleaned by oxygen plasma (Plasma-Preen, Terra Universal Inc., 500W) for 10 minutes and
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
64
Figure 5-3 Template stripping method of evaporated Au on Si transferred to cellulose via UV curable
epoxy.
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
65
placed in a thermal evaporation system (Edwards Coating E306A) with the anode shadow mask.
Different electrode and organic masks were used for cellulose devices (see section 6.1). Au was
purchased (Kurt J Lesker, 99.999%) and deposited onto the Si at a pressure of ~5x10-6 Torr. The
thickness was deposited to 20 nm, which offered the best transparency for the highest conductivity
determined by previous work.160
After evaporation, the Si with the patterned gold was removed from vacuum and a small
amount (~200-500 μL) of UV curable epoxy (Loctite 352) was dispensed directly on top of the
gold. Although the epoxy is not a natural material, it is considered to be safe after
polymerization.169 The cellulose substrate was placed on top of the epoxy and a quartz wafer was
used to compress and spread the epoxy uniformly. The quartz wafer and the silicon template
sandwiched the substrate held together by clips during UV exposure. Prior to exposure, the device
was placed in vacuum (~10-3 Torr) to mitigate air bubbles from the epoxy for ~10 min. The
substrate was removed from vacuum and placed under a UV lamp for 2 minutes to cure the epoxy.
The thickness of the epoxy was measured to be ~100 μm.
The quartz wafer was removed and a razor blade was used to separate a corner of the
cellulose/epoxy from the silicon wafer and care was taken not to damage the gold. The remaining
substrate was easily peeled from the Si template onto the cellulose revealing a smooth electrode.
There was no gold left on the Si template and any residue from the epoxy could be cleaned with
organic solvents and the wafer could be reused. The sheet resistance of the cellulose/epoxy/Au
was measured to be 5.5 Ω/□, which was the same as Au evaporated on glass.
5.3.2. Cellulose/Au Properties and quality
The substrate with template stripped gold showed immediate benefits compared to direct
evaporation. The gold had excellent adhesion to the cellulose/epoxy, which was a vast
improvement to the poor adhesion gold has on glass and plastics. Adhesion of gold on the
cellulose/epoxy/Au was compared to gold evaporated to 20 nm on glass. A tissue was used to
gently wipe the glass/gold substrate and damage resulted in an unusable device demonstrated in
Figure 5-4(a). The same procedure was done on the cellulose/epoxy/Au substrate and no visible
damage was evident (Figure 5-4(b)). Sheet resistance before and after wiping was similar (~5.5
Ω/□). The epoxy offers a simple way to improve adhesion without using thin metal films such as
Cr or Ni that further decrease transmission.
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
66
Figure 5-4 The adhesion properties of Au on (a) glass and (b) cellulose substrate.
The cellulose surface in Figure 5-5(a) was compared to template stripped gold on cellulose in
Figure 5-5(d) by several microscopy experiments. The plain cellulose surface is on the left column
in Figure 5-5 and the template stripped gold is on the right column. Figure 5-5(b) is a photograph
of 20 nm of gold evaporated onto the plain cellulose substrate through a microscope showing non-
uniform electrodes. When viewed at the same settings, the template stripped gold was featureless,
as shown in Figure 5-5(e). SEM of the bare cellulose substrate at 700x without evaporated gold
revealed (Figure 5-5(c)) large trenches in the cellulose film over 25 μm wide and scratches that
may have been created during the film production. SEM of the template stripped gold resulted in
very smooth electrodes (Figure 5-5(f)) at the same magnification.
A reference OLED was fabricated (see section 6.1.1 for details) on the gold thermally
evaporated on cellulose and also on the smooth template stripped gold. The OLED with thermally
evaporated cellulose showed large defects in the OLED similar to the trenches that were observed
in the cellulose substrate. A photograph is shown in Figure 5-6. The OLED fabricated on the
template stripped gold was featureless.
The transmission spectrum (Perkin-Elmer Lambda 900) was observed for each layer in the
cellulose/epoxy/Au substrate. A spectrum was also taken for glass and ITO used in the reference
device. The cellulose, glass, and epoxy were measured with air as the background. The epoxy was
measured by sandwiching the epoxy on a Si wafer with a quartz wafer and exposed to cure the
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
67
Figure 5-5 Quality analysis of cellulose surface before and after template stripping through a microscope.
(a) Photograph of plain cellulose substrate; (b) Au (20 nm) directly evaporated onto the cellulose; (c) SEM
image of the plain cellulose substrate showing rough texture; (d) photograph of the template stripped Au on
cellulose; (e) photograph of quality of template stripped showing high quality electrode; (f) SEM image of
template stripped Au on cellulose.
Figure 5-6 (a) OLED fabricated on Au directly deposited on the rough cellulose; (b) OLED fabricated on a
template stripped Au on cellulose.
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
68
film. The film easily peeled off of the Si wafer and transmission spectrum was done on the epoxy
film. The gold (20 nm) and ITO (commercial grade, 90 nm) were measured on glass with glass
subtracted as the background. The spectrum were measured from the near-UV through the visible
range to near IR, 300-800 nm, the results are shown in Figure 5-7. The Au electrode showed a
peak transmission of 57.5% at 515 nm, which is the peak emission of Ir(ppy)3. It is known164 that
Au reflects most of the light and little is absorbed, which generates an optical cavity in the OLED.
ITO was much higher in transmission, 91% at 515 nm. The cellulose film had transmission as high
as 88-90% throughout the entire visible range, which was only slightly lower than the glass at 90-
93%. The cellulose only decreased slightly to 80% in the near-UV range. The glass transmission
decreased significantly to 20% by 300 nm. The epoxy film had >95% transmission throughout the
entire visible range declining sharply in the near-UV. The results of the transmission spectra show
that cellulose/epoxy is optically comparable to glass.
Figure 5-7 Transmission spectra of the substrates: glass, UV epoxy, ITO, cellulose, and Au. (Inset) Photo
of template stripped substrate on lettering to show transparency.
5.4. Summary
In this chapter, an overview of cellulose and gold was given with an analysis of their potential
for OLEDs. Advantages of both cellulose and gold were presented along with their inherent
challenges pertaining to OLEDs. A template stripping fabrication process was presented as an
Chapter 5 – Cellulose and Au for Natural-Based OLED substrates
69
easy method to lift-off gold onto cellulose using UV curable epoxy. The result overcomes several
challenges of the cellulose including temperature sensitivity and surface roughness. The procedure
takes advantage of the highly conductive metal electrodes and has excellent adhesion to the
cellulose. Cellulose and gold paired with the epoxy lift-off offer a dynamic combination for a
natural-based substrate for bioelectronics. The final challenge that will be addressed is
overcoming the metal-organic interface of the gold electrode, which will be done using the adenine
as the natural hole injection layer.
70
Chapter 6. Flexible Nucleobase Bio-OLED
After the quality of the cellulose substrate with gold electrodes was analyzed, attention shifted
to the gold electrodes to address the problem of hole injection at metal-organic interface. It has
been extensively studied that the nucleobases have great affinities for gold,170-173 especially
adenine,174-177 but this affinity has not been investigated for applications in OLEDs or any type of
solid-state device. The final bio-OLED fabricated on cellulose substrates incorporated adenine as
a HIL on gold to overcome dipole barriers and enhance current efficiency and luminance. Finally,
a cost analysis of nucleobase OLEDs and the cellulose substrate will show how natural materials
can reduce the cost of organic electronics.
6.1. Adenine as a Hole Injection layer
6.1.1. OLED fabrication with Adenine as HIL
The template-stripped gold on cellulose substrate was fabricated according to the procedures
in section 5.3.1. After peeling the electrodes from the Si substrate, no further cleaning or
processing was done on the gold electrodes. The cellulose substrate, however, was too thin and
flexible for fabrication and the undulations caused non-uniform deposition due to shadowing
effects from the substrates. Therefore, the cellulose substrate was adhered to a glass substrate with
UV epoxy. The epoxy was cured with a UV lamp for 5 minutes. The entire substrate was then
ready to be loaded into the MBD system for the deposition of the OLED.
New shadow masks and wafers were used for these sets of experiments. The wafers (glass or
cellulose) were 30×30 mm. Four wafers could be grown in one run and each wafer contained four
devices. The wafer masks and the device are shown in Figure 6-1. To characterize adenine as a
hole injection layer, several devices were created on cellulose and glass with and without adenine.
The adenine based OLED on cellulose (called Cellulose-A) consisted of the following: cellulose-
epoxy / Au [20nm] / adenine [10nm] / NPB [17nm] / CBP:Ir(ppy)3 [30nm, 10wt%] / BCP [12nm]
/ Alq3 [25nm] / LiF [<1nm] / Al [40nm]. A reference device (Cellulose-ref) was fabricated with
the same parameters but excluded the adenine layer and did not contain PEDOT:PSS, viz.
Chapter 6 – Flexible Nucleobase Bio-OLED
71
Figure 6-1 (a) Shadow masks for the experiments with Au and cellulose. (b) Photo of the 30x30 mm
cellulose attached to a glass substrate.
Figure 6-2 (a) Device stack of the OLED on cellulose; (b) energy levels of the OLED with adenine.
Chapter 6 – Flexible Nucleobase Bio-OLED
72
cellulose-epoxy / Au [20nm] / NPB [17nm] / CBP:Ir(ppy)3 [30nm, 10wt%] / BCP [12nm] / Alq3
[25nm] / LiF [<1nm] / Al [40nm]. The deposition of the organic layers and cathode followed
procedures in section 2.2.1. The final device stack for adenine is shown in Figure 6-2(a) with the
energy level diagram in Figure 6-2(b).
The same two device structures (with and without adenine) were also fabricated on glass
substrates. Plain glass substrates without ITO were cleaned with O2 plasma for 10 minutes and
then Au was evaporated on glass to a thickness of 20 nm as described in section 5.3.1. The
glass/Au was loaded into the MBD system without further cleaning. Both an adenine device on
glass substrates (Glass-A) and a reference device without adenine (Glass-ref) were fabricated.
6.1.2. Adenine as HIL Results
Results for the glass devices with and without adenine are shown in Figure 6-3. The luminance
and current density with respect to voltage are shown in Figure 6-3(a). The results show an
improvement of OLED performance when a thin layer of adenine is deposited on Au (Glass-A)
compared to the device without the nucleobase (Glass-ref). Glass-A turned on at 4.25V, while
Glass-ref turned on slightly later at ~4.50-4.75 V. After turn-on, Glass-A increased sharply from
100 cd/m2 at 5.5 V until it reached a maximum value of 45,223 cd/m2 at 13 V. The Glass-A
luminance output was nearly 10x greater than the Glass-ref over the entire range. The Glass-ref
reached 100 cd/m2 at 7.25, which was 2 V higher than Glass-A for the same luminance, and had a
maximum luminance of only 12,513 cd/m2.
The current density (Figure 6-3(a)) of Glass-A was slightly higher than Glass-ref, suggesting
that adenine led to improved hole injection from Au. The hole injection is strongly supported by
paralleled increase in current efficiency for Glass-A. Figure 6-3(b) shows the current efficiency
of Glass-A reached a peak efficiency of 31.7 cd/A and Glass-ref achieved only 4.5 cd/A. The
device with adenine remained 3-7x higher in efficiency over the entire range, which was
considerable evidence of enhanced hole injection from gold.
Chapter 6 – Flexible Nucleobase Bio-OLED
73
Figure 6-3 Results of phosphorescent OLED on a glass substrate with adenine (squares) and
without adenine (circles) on Au (a) luminance and current density versus voltage; (b) current
efficiency versus luminance.
Chapter 6 – Flexible Nucleobase Bio-OLED
74
Figure 6-4 Results of phosphorescent OLED for template stripped Au on a cellulose substrate with
adenine (diamond) and without adenine (triangles) (a) luminance and current density versus voltage; (b)
current efficiency versus luminance.
The corresponding results of OLED on cellulose had similar enhancements from adenine,
presented in Figure 6-4. Cellulose-A and Cellulose-ref turned on roughly the same voltage at 4.75
and 4.5 V, respectively, similar to that on glass. The luminance of Cellulose-A increased rapidly
and was nearly 10-fold higher in emission output over the voltage range compared to Cellulose-
ref. Cellulose-A reached 100 cd/m2 at ~7.0 V and reached a maximum luminance of 8,378 cd/m2
at 16 V. Cellulose-ref had a luminance of 100 cd/m2 at 10 V and a maximum luminance of 2,042
cd/m2 at 18 V.
Chapter 6 – Flexible Nucleobase Bio-OLED
75
The improvement in current density in Figure 6-4(a) for Cellulose-A over Cellulose-ref was
noticeable. The current density for Cellulose-A was ~2 times higher than Cellulose-ref, indicating
a notable increase in hole transport. This is also confirmed by the increase in recombination
efficacy in Figure 6-4(b). The Cellulose-A reached a peak efficiency of 13.9 cd/A, while
Cellulose-ref was less than 3 cd/A throughout most of the range. The adenine improved the current
efficiency on cellulose nearly 5-7 times over the entire range.
6.1.3. Discussion of Cellulose vs Glass Substrate
The cellulose devices had decreased performance compared to the same devices on glass, such
as higher driving voltage and lower efficiency, as seen by comparing Figure 6-3 and Figure 6-4.
However, a plot of luminance versus current density with all four types of devices in Figure 6-5
reveals the luminance output for the adenine on gold was nearly identical for both the glass and
cellulose substrates. Furthermore, the adenine on both glass and cellulose surpassed the devices
without adenine in luminance emission ~5-7x over nearly entire range of operation of the device,
confirming that the performance enhancement of the adenine is independent of the device
substrate.
Figure 6-5 Luminance versus current density for all four device types: glass and cellulose substrates with
and without adenine.
Chapter 6 – Flexible Nucleobase Bio-OLED
76
Figure 6-6 Contour microscopy of Au on cellulose.
The increase in driving voltage for the OLEDs on cellulose is likely due to delamination of the
electrode and/or non-planar gold electrodes created by undulations of the cellulose film. Joule
heating at high current cause the electrode to delaminate. Figure 6-5 shows that Glass-A and
Cellulose-A are similar in output luminance except at high current densities. Cellulose-A began to
deviate from the Glass-A results after 10 mA/cm2 and became more pronounced after 100 mA/cm2.
Glass-ref and Cellulose-ref were nearly identical in output luminance until Cellulose-ref deviated
after 100 mA/cm2. Both cellulose devices burn out at ~200 mA/cm2. The higher current causes
higher operating temperatures that could cause delamination from the cellulose substrate or burn
the cellulose substrate, which prohibits high luminance emission. The lower luminance output on
cellulose substrates is not unusual, as was shown in section 5.1.1 from reports of other OLEDs on
cellulose.
The flexible substrate may also cause problems during device fabrication. It was shown by
SEM images and sheet resistance measurements that Au electrode for transferred cellulose is
featureless with high quality (see section 5.3.2). However, contour microscopy of the Au on
cellulose shows that the morphology of the gold electrode on cellulose has undulations on a larger
Chapter 6 – Flexible Nucleobase Bio-OLED
77
scale. Contour microscopy (Bruker Contour GT-K1) uses optical interferometry to construct a 3D
image of the substrate surface. Contour microscopy revealed that the surface of the gold electrode
on cellulose over a 600 x 400 μm area has a vertical displacement of ~1 µm. The non-planar
electrode could cause thickness variations in the organic layers, which may require higher driving
voltage of the OLED. Thicker and more rigid cellulose substrates could potential prevent this.
6.1.4. Discussion of Adenine as HIL
The increase performance from adenine is explained by interactions that the nucleobase has
with gold electrodes. As was previously discussed, gold forms an interfacial dipole with NPB that
causes an increase in the hole barrier. The dipole can be overcome by depositing a thin layer of
C60 on top of the gold.160 It has been proposed that the primary reason for the improved
performance of C60 is the strong chemical reaction it has with the gold.158,165 The chemical affinity
causes a short physical separation between the metal-organic interface, which consequently
reverses the dipole and enhances hole injection. Similar to C60, it has been shown that the
nucleobases have a strong physical adsorption affinity to Au. Adenine has the strongest physical
adsorption to Au175 and the others have relative adsorption strengths in the order A>G>C>>T. The
mechanism is illustrated in Figure 6-7. The electron cloud extends to surface that create a dipole
moment at the surface. The dipole causes large barrier for holes to be injected into organics. When
A is deposited onto Au, the strong affinity causes the A orbitals to overlap with the Au to reverse
Figure 6-7 (a) A clean Au substrate surface with the arrow indicating the presence and orientation of the
dipole field. (b) Adenine (n+) adsorbs on the gold and interacts with the Au orbitals (e-) that redistribute
the dipole orientation, creating a favorable electron transfer between the electrode and the adenine.
Chapter 6 – Flexible Nucleobase Bio-OLED
78
the dipole moment, thus the metal-organic energy barrier reduces and hole injection is improved
as in similar mechanisms as C60, consequently improving performance of the bio-OLED.
Furthermore, in addition to absorption affinities, the A and C60 have very similar HOMO
levels, ~6.0 and 6.2 eV, respectively. It has been suggested158 that the large HOMO level C60 is
likely transferring “hot” holes to deeper states in the NPB at the interface. It is likely that a similar
mechanism is occurring at the adenine/NPB interface. Despite the large energy difference between
Au and the hole injection layers, A and C60 (~Δ0.7-0.9 eV), the deep HOMO levels aid hole
injection into the organic stack.
A photograph of the adenine device on cellulose is shown in Figure 6-8. The OLED is
operational while bending and the quality of the OLED is uniform. The natural cellulose, highly
conductive gold, and improved hole injection of adenine is a promising substrate/electrode for
natural electronics with a practical fabrication process for bio-based substrates.
Figure 6-8 Photograph of bio-OLED on cellulose substrate using adenine as a HIL.
Chapter 6 – Flexible Nucleobase Bio-OLED
79
6.2. Cost Analysis of Nucleobase OLEDs
Cost is one of the major advantages in natural electronics as seen in Table 6-1. All of the
nucleobases are a tenth to a hundredth of the cost of synthesized organics for OLEDs. Similarly,
the cost benefits are apparent when replacing the glass/ITO/PEDOT with the cellulose-
epoxy/Au/adenine device as the substrate/HIL. A detailed analysis of the material cost for each
layer is provided in Appendix B. The cellulose substrate is a hundredth of the cost compared to
the glass per unit area, shown in Table 6-2. Admittedly, natural electronics is not the entire answer
to decreasing the cost of the OLED. Material cost shares a portion of OLED expenditures along
with display drivers, fabrication equipment/processing costs, and encapsulation. But decreasing
material cost will continue to aid in decreasing the overall cost of OLED displays.
Table 6-1 Cost of organic and cathode material for OLED.
Material Price/g OLED thickness
(nm) OLED Type
Uracil $0.38 12 HBL
Thymine $1.20 12 HBL
Adenine $1.62 17 EBL
Guanine $0.70 17 EBL
Cytosine $6.78 17 EBL
Al $2.80 40 Electrode
LiF $4.00 1 EIL
Alq3 $62.00 35 EIL
CBP $170.00 30 EL
Ir(ppy)3 $620.00 3 EL
BCP $260.00 12 HBL
NPB $40.00 17 HBL
Table 6-2 Cost of substrate and anode for OLED.
Material Price
PEDOT:PSS $1.83/mL
ITO $12.27/g
Glass $2.50/wafer
Epoxy $0.08/wafer
Cellulose $0.02/wafer
Au $74.07/g
Chapter 6 – Flexible Nucleobase Bio-OLED
80
As an example, the cost benefit was calculated for replacing the conventional EBL and HBL
with nucleobases (sections 4.1 and 4.2). The cost to deposit each organic layer depends on the
material thickness, for example, Ir(ppy)3 is $620/g but only 3 nm of material is deposited each run
and hence the material is not consumed as rapidly as the other layers. In order to provide a fair
estimate of the material cost per run, an equation was constructed that accounted for layer thickness
to provide a fair comparison, viz.
𝐶𝑙𝑎𝑦𝑒𝑟 𝑐𝑜𝑠𝑡 = 𝑀 ($
𝑔) × 𝐷 (
𝑔
𝑛𝑚) × 𝑡 (𝑛𝑚) (5)
The M is the material cost per gram from the supplier; D is an estimate of the amount of
material consumed for every nanometer of material deposited during one device run; t is the layer
thickness. The constant D is difficult to calculate because it depends on the system and deposition
procedures. In order get a numerical (dollar) value, it was estimated that 1 mg of material is
consumed for each nanometer that is thermally evaporated for the OLED, which was loosely based
on how many OLED runs were possible with a gram of NPB material. To simplify the
calculations, D = 0.001 g/nm was extended as the constant for all other thermally evaporated
materials, a reasonable approximation since deposition procedure was similar for each material.
Of course, this value may vary with material, deposition techniques, different systems, or even
different system operators; the equation provides a close comparison for material cost per device
run. The final cost of material per device calculated here is not intended to reflect the actual cost
of the OLED for every system, but is intended to be a relative comparison between the thermally
evaporated materials for the OLED fabricated in this work.
The substrate costs are shown in Table 6-2. Quotes were obtained for the cellulose and glass
substrates at 30x30 mm. The materials Au, ITO, and A used equation 4 with D = 0.001 g/nm. The
PEDOT and the epoxy were measured to consume ~1 mL and 0.4 mL, respectively, for each device
run which would vary depending on wafer size.
Figure 6-9(a) compiles the estimated cost of the organic and cathode material for reference and
the bio-OLED used in Chapter 4, that is the cost of replacing BCP and NPB in the reference OLED
with U and A, respectively, excluding the cost of the substrate. The cost of the reference device
without the substrate is $13.04. Replacing the EBL and HBL with nucleobases reduces the entire
cost of the OLED to $9.30, or a 29% reduction of cost. Figure 6-9(b) shows the cost benefits of
Chapter 6 – Flexible Nucleobase Bio-OLED
81
replacing the glass substrate with the cellulose substrate. The cost of a typical glass substrate with
ITO and PEDOT is $5.43. The cellulose, epoxy, and A offset the relatively high cost of Au and
reduces the overall substrate to $1.59, an 70% cost reduction. Although the analysis of the cost is
estimate, it was intended to show the potential reduction in savings for natural materials.
Figure 6-9 (a) Estimated cost of organics for the reference OLED versus the bio-OLED; (b) Cost of the
glass substrate with ITO and PEDOT compared to the cellulose substrate with Au and A.
6.3. Summary
Adenine was used as a novel HIL on gold electrodes in OLED. A device was fabricated on the
cellulose/epoxy/gold substrate that was discussed in Chapter 5 to demonstrate the potential of the
flexible and natural substrate. Depositing adenine on gold electrodes resulted in an improvement
in OLED performance. The efficiency of the device was increased 4-7 times compared to the
device that did not have adenine. The increase in hole transport is due to the chemical and physical
affinity that adenine has to gold. The affinity causes a reverse in dipole that minimizes the energy
barrier between gold and adenine, similar to C60. The adenine also has a HOMO level similar to
C60, which serves to inject holes into the NPB.
Finally, the cost of a bio-OLED based on the work presented in this work was analyzed.
Replacing the conventional EBL and HBL with nucleobases (Chapter 4) resulted in 29% decrease
in material cost. Using the cellulose substrate reduced the material cost by 70% over the glass/ITO
Chapter 6 – Flexible Nucleobase Bio-OLED
82
substrate. The confluence of low cost, renewable, and biodegradable material and substrates is a
promising first step for future natural electronics.
83
Chapter 7. Summary and Future Work
7.1. Conclusion of Dissertation
In conclusion, the work presented here was an initial investigation into nucleobases as
biomaterials for OLED. Nucleobases are a versatile set of materials with intrinsic properties that
serve to substitute conventional organics while enhancing the performance of OLEDs.
Nucleobases have distinct advantages over DNA in fabrication in performance. It was shown that
nucleobases thermally evaporate into thin films directly into the OLED structure, unlike DNA that
requires surfactants and wet processing. Nucleobases are much simpler molecules that have
specific properties, unlike the unknown nucleobase composition of natural DNA.
Thin film nucleobases showed a diverse set of properties for opto/electronic devices. All of
the bases are optically transparent in the visible spectrum. They have thermally stable
temperatures between 260 - 465 °C. Their relative permeability ranged from 1.6 to 4.0. The
refractive indices vary from 1.50 to 1.96. Analysis of the thin films revealed that all of the
nucleobases deposit with diverse morphologies. G has a very smooth film (<1 nm) while T has
large crystallites that grow as columnar structures, which was used to enhance OLED emission.
Lastly, it was confirmed from literature that the nucleobases have similar energy gaps with increase
ionization potential, G<A<C<T<U.
The energy levels were confirmed by depositing nucleobases in a phosphorescent OLEDs as
an EBL (replacing the NPB) and also as an HBL (replacing the BCP). The performance of the
nucleobases as an EBL diminished the current injection into the emitting layer according to their
energy levels such that G was the best hole transporter and U was the most inefficient hole
transporter. In the HBL case, the performance of the OLED reversed such that G was the least
efficient electron transporter and U was the best. Although G was the most efficient hole
transporter, the best EBL-OLED was A, which reached 51.8 cd/A and 82,000 cd/m2, surpassing
the baseline device in efficiency (38 cd/A). The efficiency increase was attributed to A having a
matched HOMO level with the emitting layer. While U was a decent HBL, further investigation is
required to improve luminance.
Chapter 7 – Summary and Future Work
84
Optimization of the EBL showed that T, typically an HBL, improved the current efficiency 2x
over the baseline from 36 cd/A to 76 cd/A when it was decreased from 17 nm to 10 nm in the EBL
configuration. The maximum luminance output was increased from 113,000 cd/m2 to 132,000
cd/m2 with T as an EBL. The high performance was attributed to the large peaks and valleys in
the emitting layer caused by T at 10 nm. The morphology caused enhanced charge injection into
the emitting layer at the valleys and subsequently creating a balanced electron hole pairs using T
as a hole trap layer.
Finally, transparent cellulose was implemented as plant-derived substrate into the final device.
To overcome the high annealing temperatures of ITO, thin film Au was used instead as the bottom-
emitting electrode. UV epoxy lift off the Au electrodes from Si onto the cellulose substrate,
creating high quality electrodes on a rough cellulose substrate. The Au had good adhesion to the
cellulose and defect-free OLEDs were realized. To overcome poor interfaces to the Au electrode,
a thin layer of A was deposited on top of the Au electrode and the luminance and efficiency were
improved as much as 7x over the same device without A. The A has exceptional chemical and
physical affinity to the Au, which can reverse the dipole of the Au and enhance hole injection into
the OLED.
7.2. Future Work
The work presented here covers a broad range of initial exploration of the nucleobases in
organic electronics. Just like the history of DNA in natural electronics, its investigation began as
one layer in OLEDs and then expanded to many different devices and applications. The
foundation of nucleobase in thin film electronics unfolded in this work present many opportunities
to explore the nucleobases in other structures. Recommendations for future work will be given
for the all-natural OLED and secondly for self-assembly of nucleobases in electronics.
There are several potential next steps for the all-natural OLED. The natural substrate on
cellulose has been demonstrated here, but future work for the cellulose includes using a more
rigid or thicker substrate to ensure uniform deposition, which may enhance performance. The UV
curable epoxy, although a safe polymer, could be replaced by a natural epoxy. The Au electrode
requires light extraction to unlock its full potential. Other work164 uses a material with high
refractive index such as TiO5 (n=2.2) deposited before the Au. G has a high refractive index
(n=2.0), which may be an interesting candidate for light extraction. The HIL would be A, which
Chapter 7 – Summary and Future Work
85
couples well to the gold though further experiments optimizing its thickness could prove
beneficial. The EBL could be G or a thicker layer of A. The emitting layer serves as a large
challenge. DNA as a phosphorescent host in the emitting layer has been demonstrated.54 There
are fluorescent and phosphorescent emitters in nature but a natural electrophosphorescent remains
to be explored. U as a HBL requires more investigation into it energy levels or resistivity. U
could improve by adjusting adjacent energy levels, but a new natural HBL may be necessary to
improve efficiency. Other pyrimidines are potential candidates for an HBL. Molecules with
additional heterocyclic rings may increase conductivity.178 A practical place to start would be the
expanded bases, a derivative of the nucleobases, which have additional heterocyclic rings.179
Other pyrimidines may serve to replace the Alq3. Of course, this work focuses on nucleic acids,
but there are other natural molecules, such as indigo and β-carotene, that are semiconductors and
could enhance charge transport.11,14
The second area of future work for nucleobases in organic electronics is the self-assembly
properties of the nucleobases. There has been a considerable amount of work done on the self-
assembly of nucleobases on electrodes. Though much of this work has been for the purpose of
detecting nucleobases in biotechnology, little has been explored for leveraging the incredible self-
assembly of the bases for enhanced charge injection into devices. The self-assembly and
absorption of nucleobases on a variety of metals have been explored, such as Cu,180 Ag,181
graphene,182 carbon nanotubes,183 and Au(111).172,184-189 Self-assembly could be beneficial for
enhancing charge mobility.
7.3. Closing remarks
The path to the all-natural OLED will require much more investigation in natural materials,
and even still, the current method is merely a hybrid bioelectronic OLED, that is natural molecules
as dry solid-sate films. The hybrid OLED, however, is important in understanding how the natural
materials function and how traditional solid-state electronics interface with the biological world.
The current design could eventually pave the way towards devices with all-natural materials. The
immediate benefits would be sustainable, low cost, and potentially higher performing electronic
devices. However, the future of bioelectronics may not resemble the longstanding design of the
thin film solid-state. It is fascinating to consider the biological realm, which does not rely on thin-
films, electrodes, and electron blocking, but rather on chemically driven processes such as
Chapter 7 – Summary and Future Work
86
bioluminescence found in many organisms. The dreams of natural electronics are ambitious but
the reward would be incalculable. Nature has proven that light can be grown instead of fabricated,
that material can be harvested and not synthesized, and decompose after its life cycle to begin
anew. The remarkable truth is that since the genesis of life biological systems have been reading
and writing DNA, assembling and repairing with proteins, relaying and sensing data with electrical
networks, and the night has always been illuminated with bioluminescence without ever once
stepping inside a laboratory cleanroom.
87
Appendix A – Error Analysis of Experiments
The major cause of variation in device performance is non-uniformities in layer thickness.
Since the nucleobases are sensitive to layer thickness (see section 4.3.1), runs were repeated
several times and data were averaged together. The standard deviation was calculated for
luminance and current density. From the standard deviation, the standard error could be calculated
based on the number of runs.
𝑆𝐸 =𝑠
√𝑛 (1-A)
The value s is the standard deviation and n is the number of the unique devices observed. Error
analysis was done for the three major experiments: the EBL/HBL (sections 4.1 and 4.2), the EBL
optimization (section 4.3), and the adenine hole injection (section 6.1). The standard error for the
EBL at 17 nm is shown in Figure A-1 for the current density and luminance versus voltage. The
number (n) of unique samples recorded for the EBL experiment were Ref = 5, G = 12, A = 8, C
= 10, T = 5, and U = 6. The error analysis for the HBL at 12 nm is shown in Figure A-2. The
number (n) of unique samples recorded for the HBL experiment were U = 6, T = 5, C = 11, A =
10, and G = 7. The calculated error for the optimization of the EBL (10 nm) is shown in Figure A-
3. The sample size (n) is ref = 3, A = 12, T = 5, DNA-CTMA = 10. Finally, the standard error
was calculated for the adenine as a hole injection layer. Figure A-4(a) shows the results of the
adenine compared to no adenine for luminance and current density on glass and Figure A-4(b)
shows the same on cellulose. The sample size for the glass substrates were Glass-A = 9 and Glass-
ref = 6. The sample size for the cellulose substrates were Cellulose-A = 9 and Cellulose-ref = 2.
Thymine was the most difficult base to control the thickness since crystal monitor readings
were not always consistent between device runs, most likely due to its rough film quality.
However, as thymine was varied between 8 – 35 nm, (see Figure 4-9 in section 4.3.1) it was clear
that there is a transition period for thymine as the EBL between enhanced OLED operation (high
efficiency) and hole blocking (low efficiency). Though the error is larger for the thymine
compared to the other bases, the trend as the thickness increased and decrease was very clear.
Appendix A – Error Analysis of Experiments
88
The errors for all the device experiments are within the range to make a confident comparison
of device performance of the nucleobases in OLEDs. Smaller wafer substrates could minimize
gradient effects of the MBD system and could further decrease error.
Figure A-1 Standard error of the reference device (NPB = 17 nm) and the nucleobases in the EBL
configuration (17 nm) for (a) current density and (b) luminance versus voltage.
Appendix A – Error Analysis of Experiments
89
Figure A-2 Standard error of the nucleobases in the HBL configuration (12 nm) for (a) current density and
(b) luminance versus voltage.
Appendix A – Error Analysis of Experiments
90
Figure A-3 Standard error of the optimized EBL OLED with T, A, DNA, and NPB at 10 nm for (a) current
density versus voltage and (b) luminance versus current density.
Appendix A – Error Analysis of Experiments
91
Figure A-4 Standard error of the experiments with adenine as a HIL showing luminance and current density
versus voltage for the (a) OLEDs on a glass substrate with and without adenine; (b) OLEDs on a cellulose
substrate with and without adenine.
92
Appendix B – Cost Analysis of Natural Materials
A detailed analysis of the cost of each component in the OLED is presented. The final cost is
intended to be a point of comparing the relative cost of natural material to conventional materials
for OLEDs. It is not meant to be the cost of an OLED in an industrial setting. The cost of the
components were determined as follows:
Glass
The glass wafers were quoted at $2.50 from PG&O for 30x30mm in a quantity of 100.
ITO
ITO is from Kurt J. Lesker for a 2.00 in diameter x 0.125 in, or a volume of 6.435 cm3. ITO
has a density of 7.12 g/cm3 resulting in a total mass of 45.8 g. ITO requires a Cu backing plate for
sputtering and was included in the total of cost $562. Therefore, the estimated cost to sputter a
gram of ITO (including the Cu backing) is $12.27 / g.
Cellulose
Cellulose was quoted from BKK LLC, available for $39.99 for 33x70 mm sheets (qty. 1200),
which was divided in half to result in $0.016 for one 33x35 mm.
Epoxy
The UV curable epoxy (Loctite 352) is available on McMaster for $19.85 for a 50 mL bottle.
It was measured that one device required 200µL to coat a 33x35 mm cellulose substrate), resulting
in a cost for one device at $0.08. The cost would increase or decrease depending on the wafer size.
Gold
Au was available at Kurt J Lesker for 1.00 in diameter x 0.125 in thick, or a volume of 1.608
cm3, which cost $2,300. The density of gold is 19.31 g/cm3 resulting in a total mass of 31.05g.
No Cu backing is required unlike ITO, which aids in decreasing the cost. Therefore, the cost to
sputter (or evaporate) a gram of gold is $74.07/g.
Appendix B – Cost Analysis of Natural Materials
93
Organics
The OLED organics are quoted from Lumtec resulting in a direct cost per gram. The
nucleobases, Al, and LiF were obtained from Sigma Aldrich. The PEDOT:PSS is obtained from
Heraus at $365 / 200mL. It was measured that each device required ~1mL of PEDOT:PSS to
provide enough for spin coating a 2 in wafer, which would increase or decrease depending on the
wafer size required.
94
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