Investigation of Dna Nucleobases for Bio-Organic Light Emitting Diodes

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

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

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.

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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.

“Call to Me and I will answer you and tell you great and unsearchable things you do not know” Jeremiah 33:3

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

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

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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

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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 1-5 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

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 3-2 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

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

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 5-2 (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

xii

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

xiii

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

xiv

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

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

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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 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

Figure 1-1 Examples of biomaterials from natural sources integrated as thin film components in bioelectronic devices. 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

Chapter 1 – Introduction to Natural Electronics

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 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 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

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 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,

Chapter 1 – Introduction to Natural Electronics 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 62 application used DNA/PAn/Ru(bpy)3 to create a color-tunable fluorescent OLED. 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

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.

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

Chapter 1 – Introduction to Natural Electronics 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

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.

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

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

Chapter 2 – OLEDs and Experimental Methods

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. 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.

Chapter 2 – OLEDs and Experimental Methods

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

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

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

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

Chapter 2 – OLEDs and Experimental Methods

[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

Chapter 2 – OLEDs and Experimental Methods 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.

퐿 휂 = (1) 퐼 퐽

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.

푐푑 ∙ 휋 휂 = (2) 푙푢푚 퐼푉

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 (휙), which is 100% for phosphorescent materials106.

Chapter 2 – OLEDs and Experimental Methods

휂푖푛푡 = 훾 × 휂푠 × 휙푓 (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.

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).

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.

Chapter 2 – OLEDs and Experimental Methods

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.

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

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. 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 118 heating a gas mixture of CO, H2, and CH3 to 600 ºC with a nickel/iron catalyst.

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

Chapter 3 – Nucleobases and Thin Film Properties 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

Chapter 3 – Nucleobases and Thin Film Properties 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

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

Figure 3-3 (a) AFM analysis of nucleobases (b) sectional line scan samples from AFM results.

Chapter 3 – Nucleobases and Thin Film Properties

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

Figure 3-5 Electromagnetic absorption spectra for UV and visible light from 200-700 nm.

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

Chapter 3 – Nucleobases and Thin Film Properties 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

Figure 3-7 Dielectric constant device stack and measurement.

퐶푑 휀푟 = (4) 휀0퐴

2 C is the measured capacitance (F); A is the device area (m ); 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

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 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

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) 465 290 325 260 270 (95% remaining mass)

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.

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.

Chapter 4 – Nucleobase Bio-OLEDs

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 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.

Chapter 4 – Nucleobase Bio-OLEDs

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 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 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

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

Chapter 4 – Nucleobase Bio-OLEDs 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

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

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).

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.

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

Chapter 4 – Nucleobase Bio-OLEDs 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 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

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

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

Chapter 4 – Nucleobase Bio-OLEDs 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

Chapter 4 – Nucleobase Bio-OLEDs 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

Chapter 4 – Nucleobase Bio-OLEDs 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

Chapter 4 – Nucleobase Bio-OLEDs

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

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. Quantum Max Lum Max Current Max Lum Eff (a) EBL (17nm) Turn-on (V) Efficiency (%) (cd/m2) Eff (cd/A) (lum/W) 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

Quantum Turn-on Max Lum Max Current Max Lum Eff (b) HBL (12nm) Efficiency (%) (V) (cd/m2) Eff (cd/A) (lum/W) 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

Turn-on Max Lum Max Current (c) EBL (10nm) (V) (cd/m2) 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

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

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. 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.

Chapter 5 – Cellulose and Au for Natural-Based OLED substrates

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 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 OLED type Electrode/HIL Luminance Efficiency Layer (cd/m2) (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 ITO/PEDOT 10,000 50 phosphorescence 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 2 the SiO2 as the smoothing layer was 1,200 cd/m , 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

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 156 substrates. 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

(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 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/□ 161 Refractive Index 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 Thermal evaporation, e-beam, Sputtering, e- Typical Deposition Methods roll-to-roll, beam, CVD sputtering 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 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

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 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

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

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 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 164 peak transmission of 57.5% at 515 nm, which is the peak emission of Ir(ppy)3. It is known 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 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.

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

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 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

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

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

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 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 160 C60 on top of the gold. It has been proposed that the primary reason for the improved 158,165 performance of C60 is the strong chemical reaction it has with the gold. 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

Figure 6-8 Photograph of bio-OLED on cellulose substrate using adenine as a HIL. 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 158 levels, ~6.0 and 6.2 eV, respectively. It has been suggested 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.

Chapter 6 – Flexible Nucleobase Bio-OLED

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. OLED thickness Material Price/g OLED Type (nm) 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

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.

$ 푔 퐶 = 푀 ( ) × 퐷 ( ) × 푡 (푛푚) (5) 푙푎푦푒푟 푐표푠푡 푔 푛푚

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 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 substrate. The confluence of low cost, renewable, and biodegradable material and substrates is a promising first step for future natural electronics.

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

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

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 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 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.

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. 푠 푆퐸 = (1-A) √푛

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

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

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

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

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.

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

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.

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