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High Efficiency Blue and White Phosphorescent Organic Light Emitting Devices

Permanent Link: http://ufdc.ufl.edu/UFE0042393/00001

Material Information

Title: High Efficiency Blue and White Phosphorescent Organic Light Emitting Devices
Physical Description: 1 online resource (169 p.)
Language: english
Creator: Eom, Sang
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: array, blue, contact, device, diode, display, efficiency, emitting, external, extraction, fluorescent, hemispherical, high, internal, light, lighting, lithography, luminance, microlens, oled, organic, outcoupling, pdms, phosphorescent, polystyrene, power, quantum, soft, white
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic light-emitting devices (OLEDs) have important applications in full-color flat-panel displays and as solid-state lighting sources. Achieving high efficiency deep-blue phosphorescent OLEDs (PHOLEDs) is necessary for high performance full-color displays and white light sources with a high color rendering index (CRI); however it is more challenging compared to the longer wavelength light emissions such as green and red due to the higher energy excitations for the deep-blue emitter as well as the weak photopic response of deep-blue emission. This thesis details several effective strategies to enhancing efficiencies of deep-blue PHOLEDs based on iridium(III) bis(4?,6?-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which are further employed to demonstrate high efficiency white OLEDs by combining the deep-blue emitter with green and red emitters. First, we have employed 1,1-bis-(di-4-tolylaminophenyl) cyclohexane (TAPC) as the hole transporting material to enhance electron and triplet exciton confinement in Fir6-based PHOLEDs, which increased external quantum efficiency up to 18 %. Second, dual-emissive-layer (D-EML) structures consisting of an N,N -dicarbazolyl-3,5-benzene (mCP) layer doped with 4 wt % FIr6 and a p-bis (triphenylsilyly)benzene (UGH2) layer doped with 25 wt % FIr6 was employed to maximize exciton generation in the emissive layer. Combined with the p-i-n device structure, high power efficiencies of (25 plus or minus 2) lm/W at 100 cd/m2 and (20 plus or minus 2) lm/W at 1000 cd/m2 were achieved. Moreover, the peak external quantum efficiency of (20 plus or minus 1) % was achieved by employing tris3-(3-pyridyl)mesitylborane (3TPYMB) as the electron transporting material, which further improves the exciton confinement in the emissive layer. With Cs2CO3 doping in the 3TPYMB layer to greatly increase its electrical conductivity, a peak power efficiency up to (36 plus or minus 2) lm/W from the deep-blue PHOLED was achieved, which also maintains Commission Internationale de L?Eclairage (CIE) coordinates of (0.16, 0.28). High efficiency white PHOLEDs are also demonstrated by incorporating green and red phosphorescent emitters together with the deep-blue emitter FIr6. Similar to the FIr6-only devices, the D-EML structure with high triplet energy charge transport materials leads to a maximum external quantum efficiency of (19 plus or minus 1) %. Using the p-i-n device structure, a peak power efficiency of (40 plus or minus 2) lm/W and (36 plus or minus 2) lm/W at 100 cd/m2 were achieved, and the white PHOLED possesses a CRI of 79 and CIE coordinates of (0.37, 0.40). The limited light extraction from the planar-type OLEDs is also one of the remaining challenges to the OLED efficiency. Here we have developed a simple soft lithography technique to fabricate a transparent, close-packed hemispherical microlens arrays. The application of such microlens arrays to the glass surface of the large-area fluorescent OLEDs enhanced the light extraction efficiency up to (70 plus or minus 7)%. It is also shown that the light extraction efficiency of the OLEDs is affected by microlens contact angle, OLEDs size, and detailed layer structure of the OLEDs.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sang Eom.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042393:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042393/00001

Material Information

Title: High Efficiency Blue and White Phosphorescent Organic Light Emitting Devices
Physical Description: 1 online resource (169 p.)
Language: english
Creator: Eom, Sang
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: array, blue, contact, device, diode, display, efficiency, emitting, external, extraction, fluorescent, hemispherical, high, internal, light, lighting, lithography, luminance, microlens, oled, organic, outcoupling, pdms, phosphorescent, polystyrene, power, quantum, soft, white
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic light-emitting devices (OLEDs) have important applications in full-color flat-panel displays and as solid-state lighting sources. Achieving high efficiency deep-blue phosphorescent OLEDs (PHOLEDs) is necessary for high performance full-color displays and white light sources with a high color rendering index (CRI); however it is more challenging compared to the longer wavelength light emissions such as green and red due to the higher energy excitations for the deep-blue emitter as well as the weak photopic response of deep-blue emission. This thesis details several effective strategies to enhancing efficiencies of deep-blue PHOLEDs based on iridium(III) bis(4?,6?-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which are further employed to demonstrate high efficiency white OLEDs by combining the deep-blue emitter with green and red emitters. First, we have employed 1,1-bis-(di-4-tolylaminophenyl) cyclohexane (TAPC) as the hole transporting material to enhance electron and triplet exciton confinement in Fir6-based PHOLEDs, which increased external quantum efficiency up to 18 %. Second, dual-emissive-layer (D-EML) structures consisting of an N,N -dicarbazolyl-3,5-benzene (mCP) layer doped with 4 wt % FIr6 and a p-bis (triphenylsilyly)benzene (UGH2) layer doped with 25 wt % FIr6 was employed to maximize exciton generation in the emissive layer. Combined with the p-i-n device structure, high power efficiencies of (25 plus or minus 2) lm/W at 100 cd/m2 and (20 plus or minus 2) lm/W at 1000 cd/m2 were achieved. Moreover, the peak external quantum efficiency of (20 plus or minus 1) % was achieved by employing tris3-(3-pyridyl)mesitylborane (3TPYMB) as the electron transporting material, which further improves the exciton confinement in the emissive layer. With Cs2CO3 doping in the 3TPYMB layer to greatly increase its electrical conductivity, a peak power efficiency up to (36 plus or minus 2) lm/W from the deep-blue PHOLED was achieved, which also maintains Commission Internationale de L?Eclairage (CIE) coordinates of (0.16, 0.28). High efficiency white PHOLEDs are also demonstrated by incorporating green and red phosphorescent emitters together with the deep-blue emitter FIr6. Similar to the FIr6-only devices, the D-EML structure with high triplet energy charge transport materials leads to a maximum external quantum efficiency of (19 plus or minus 1) %. Using the p-i-n device structure, a peak power efficiency of (40 plus or minus 2) lm/W and (36 plus or minus 2) lm/W at 100 cd/m2 were achieved, and the white PHOLED possesses a CRI of 79 and CIE coordinates of (0.37, 0.40). The limited light extraction from the planar-type OLEDs is also one of the remaining challenges to the OLED efficiency. Here we have developed a simple soft lithography technique to fabricate a transparent, close-packed hemispherical microlens arrays. The application of such microlens arrays to the glass surface of the large-area fluorescent OLEDs enhanced the light extraction efficiency up to (70 plus or minus 7)%. It is also shown that the light extraction efficiency of the OLEDs is affected by microlens contact angle, OLEDs size, and detailed layer structure of the OLEDs.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sang Eom.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042393:00001


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1 HIGH EFFICIENCY BLUE AND WHITE PHOSPHORESCENT ORGANIC LIGHT EMITTING DEVICES By SANG HYUN EOM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Sang Hyun Eom

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3 To my f amily and all my f riends

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4 ACKNOWLEDGMENTS First of all, I would like to express my great respect to Dr. Xue for his brilliant advice and high expectati on which successfully led me to accomplish a PhD degree from University of Florida. I learn ed a lot from my research group and really enjoyed the whole time I spent in our lab. It is such an honorable moment to graduate from Dr. Xue s research group, and I am really excited to get the PhD from Materials Science and Engineering department in University of Florida. Also, it was my great honor to have Dr. So, Dr. Norton, Dr. Pearton, and Dr. Shanze as my committee members and I especially want to thank Dr. S o for his great advice in my first year of PhD. It was a very short meeting with him, but will be remembered forever in my life I have so many good friends in UF, mostly as a research colleague, but also sometimes as a friend, a competitor, and an adviso r. Ying Zheng and Teng Ku an Tseng helped me a lot to start my first experience in our lab, William T. Hammond and Jason D. Meyer always motivate d me with their creative thoughts and support for the equipment. Yixing Yang, Renjia Zhou, Weiran Cao, and Dr. W ei Zhao showed me their incessant endeavors for their research, which also gave me motivation for my own research. I also thank Edward Wreznieski as a research colleague as well as a friend who helped me a lot to understand the life in the US. It was great fun talking with Ed For the financial supports, I acknowledge two funding sources from the U.S. Department of Energy Solid State Lighting Programs and the Florida Energy Systems Consortium which led to the excellent collaboration s with Dr. So, Dr. Hollow ay, Dr. Douglas, and their students. Outside of my research group, I would like to show great gratitude to the people in Samsung who helped me start my PhD at UF with Samsung s support. Dr. Jong In Jeong and previous mentor Woon Hyun Choi provided me a gr eat chance to apply for

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5 the Samsung s support. Hwan Chul Kim in Samsung and Dr. Hae Gun Lee in Postech also gave me a motivation to pursue my PhD in the US. I also need to thank many Korean friends who helped me enjoy the life in Gainesville. Dr. Sang Soo Jee as my high school friend was the most thankful person who gave me a lot of help to settle down in Gainesville, and it was great fun with all the people I met in the University of Florida Korean Baseball League (UFKBL, now renamed as a GIBL). Seon Hoo Kim was the most intimate friend to me in Gainesville, and Dr. In Kook Jeon and Kyung Won Kim were also my beloved two friends in MSE department and in UFKBL as well. Besides, I want to share the great joy with all the guys such as Dr. Chan Woo Lee, Sung W on Choi, Byung Wook Lee, Dr. Kang Tek Lee, Dr. Jin Woo Kwak, Sang Jun Lee, and Dong Woo Song who studied together with me in UF. Finally, I would like to show my utmost gratitude to my lovely wife, Young Soo Park, as a mother of my lovely daughter, Kaile y Jihyo Eom, and with a consistent support for me Furthermore, my father and mother were always in my mind with my great respect and love and also I always missed my two sisters with their consistent comfort. My father and mother in laws also gave me a g reat support with their deeply considerate cheers. There might be many other friends who I missed to acknowledge here, but I would like to share my great joy with everyone who gave me their direct/indirect help.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBREVIATIONS AND SYMBOLS ................................ ................................ 16 ABSTRACT ................................ ................................ ................................ ................... 20 CHAPTER 1 INTRODUCTION TO ORGANIC LIGHT EMITTING DEVICES .............................. 22 1.1 Demand for High Efficiency Lighting Emitting Devices ................................ ..... 22 1.2 Advantages of Organic Light Emitting Devices ................................ ................. 24 1.3 Applications of Organic Light Emitting Devices ................................ ................ 27 1.3.1 Flat Panel Display ................................ ................................ .................... 27 1.3.2 Sol id State Lighting ................................ ................................ ................. 28 1.4 Demand for High Efficiency Blue and White PHOLEDs ................................ .... 30 1.5 Outline of Dissertation ................................ ................................ ....................... 32 2 PHOTO PHYSICAL PROPERTIES OF ORGANIC SEMICONDUCTORS ............. 34 2.1 Introduction ................................ ................................ ................................ ....... 34 2.2 Electro nic Structures ................................ ................................ ......................... 35 2.3 Charge Transport ................................ ................................ .............................. 37 2.3.1 Hopping Transport ................................ ................................ ................... 38 2.3.2 Band Transport ................................ ................................ ........................ 38 2.4 Transport vs. Optical Bandgaps ................................ ................................ ........ 38 2.5 Excitons ................................ ................................ ................................ ............ 39 2.5.1 Formation of Excitons ................................ ................................ .............. 39 2.5.2 Multiplicity of Excitons ................................ ................................ ............. 39 2.5.3 Metal Ligand Charge Transfer Exciton ................................ .................... 41 2.6 Intra Molecular Energy Transfer ................................ ................................ ....... 42 2.6.1 Absorption ................................ ................................ ............................... 42 2.6.2 Fluoresc ence ................................ ................................ ........................... 43 2.6.3 Intersystem C rossing ................................ ................................ ............... 44 2.6.4 Phosphorescence ................................ ................................ .................... 44 2.6. 5 Frank Condon Shift ................................ ................................ ................. 44 2.7 Inter Molecular Energy Transfer ................................ ................................ ....... 46 2.7.1 F rster Energy Transfer ................................ ................................ .......... 46 2.7.2 Dexter Energy Transfer ................................ ................................ ........... 46

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7 3 FUNDAMENTALS OF ORGANIC LIGHT EMITTING DEVICES ............................ 48 3.1 Device Struct ure of OLEDs ................................ ................................ ............... 48 3.2 Fabrication of OLEDs ................................ ................................ ........................ 49 3.3 Principle of OLEDs Operation ................................ ................................ ........... 50 3.4 Colorimetry ................................ ................................ ................................ ....... 52 3.4.1 Photometry vs. Radiometry ................................ ................................ ..... 52 3.4.2 Responsivity of Human Eyes ................................ ................................ ... 53 3.4.3 Plane vs. Solid Angles ................................ ................................ ............. 55 3.4.4 Lambertian Emission S ource ................................ ................................ ... 55 3.4.5 CIE C olor Space ................................ ................................ ...................... 56 3.4.6 Color R endering Index ................................ ................................ ............. 58 3.5 OLEDs Measurement ................................ ................................ ....................... 59 3.5.1 Lumina nce ................................ ................................ ............................... 60 3.5.2 Current Efficiency ................................ ................................ .................... 60 3.5.3 Luminous Power Efficiency ................................ ................................ ..... 61 3.5.4 External Quantum Efficiency ................................ ................................ ... 63 3.6 Theoretical Efficiency Limits of OLEDs ................................ ............................. 63 4 HIGH EFFICIENCY DEEP BLUE PHOSPHORESCENT O LEDS .......................... 67 4.1 Introduction ................................ ................................ ................................ ....... 67 4.2 Experiment ................................ ................................ ................................ ........ 71 4.3 High Triplet Ener gy Hole Transporting Material ................................ ................ 74 4.4 Single Emissive Layer p i n Structure ................................ ............................... 77 4.5 Dual Emissive Layer p i n Structure ................................ ................................ 81 4.6 Summary ................................ ................................ ................................ .......... 84 5 ELECTRON INJECTION AND TRANSPORT MATERIAL STUDY ON DEEP B LUE PHOSPHORESCENT OLEDS EFFICIENCY ................................ ............... 87 5.1 Introduction ................................ ................................ ................................ ....... 87 5.2 Experiment ................................ ................................ ................................ ........ 88 5.3 Electron Transporting Material for Deep Blue PHOLED s ................................ 90 5.4 Electron Injection Layer with Alkaline Metals ................................ .................... 94 5.4.1. Thin Interlayer for Efficient Electron Injection ................................ ......... 96 5.4.2. n Type Doped Layer ................................ ................................ ............... 98 5.5 Electron Injection Layer for Deep Blue PHOLEDs ................................ .......... 100 5.5.1. Effect of Alkaline Metal Doping into BPhen ................................ .......... 101 5.5.2. Effect of Alkaline Metal Doping into 3TPYMB ................................ ....... 104 5.6 A pplication o f Macrolens to Deep Blue PHOLEDs ................................ ......... 106 5.7 Summary ................................ ................................ ................................ ........ 107 6 HIGH EFFICIENCY WHITE PHOSPHORESCENT OLEDS ................................ 109 6.1 Introduction ................................ ................................ ................................ ..... 109 6.2 Experiment ................................ ................................ ................................ ...... 112

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8 6.3 Doping Concentration Optimization for Triple Emitte rs ................................ ... 114 6.4 High Efficiency Conventional D EML White PHOLEDs ................................ ... 117 6.5 High Efficiency p i n D EML White PHOLEDs ................................ ................ 121 6.6 Summary ................................ ................................ ................................ ........ 124 7 LIGHT EXTRACTION ENHANCEMENT IN OLEDS WITH HEMISPHERICAL MICROLENS ARRAYS ................................ ................................ ......................... 125 7.1 Introduction ................................ ................................ ................................ ..... 125 7.2 Experiment ................................ ................................ ................................ ...... 127 7.2.1 Microlens Fabrication ................................ ................................ ............ 127 7.2.2 OLEDs Fabrication ................................ ................................ ................ 129 7.2.3 Outcoupling Efficiency Measurements ................................ .................. 130 7.3 Characterization of Microlens Arra ys ................................ .............................. 132 7.3.1 Large Area and Close Packed Microlens Arrays ................................ ... 132 7.3.2 Control of Microlens Contact Angles ................................ ..................... 134 7.4 Characterization of OLEDs Efficiency with Microlens Arrays .......................... 136 7.4.1 Enhanced Light Extraction with Large Size OLEDs ............................... 136 7.4.2 Light Extraction Efficiency Dependence on Microlens Contact Angle ... 142 7.4.3 Light Extraction Efficiency Dependence on OLED Structure ................. 144 7.5 Summary ................................ ................................ ................................ ........ 144 8 CONCLUSIONS AND FUTURE WORKS ................................ ............................. 146 8.1 Conclusions ................................ ................................ ................................ .... 146 8.1.1 High Efficiency Deep Blue PHOLEDs ................................ ................... 146 8.1.2 High Efficiency White PHOLEDs ................................ ........................... 147 8.1.3 Light Extraction Enhancement in OLEDs via Microlens Arrays ............. 148 8.2 Future Works ................................ ................................ ................................ .. 149 8.2.1 What is the Limit of Outcou pling Efficiency in Bottom Emitting OLEDs? ................................ ................................ ................................ ....... 149 8.2.2 Microlens Simulation ................................ ................................ ............. 150 8.2.3 Scalability Issue of OLEDs ................................ ................................ .... 153 8.2.4 Stability Issue of OLEDs ................................ ................................ ........ 154 APPENDIX : LIST OF PUBLICATIONS AND PRESENTATION S ............................... 156 LIST OF REFERENCES ................................ ................................ ............................. 159 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 169

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9 LIST OF TABLES Table page 4 1 Co mparison of the turn on voltage ( V T ), external quantum efficiency ( EQE ), luminous efficiency ( L ), power efficiency ( P ), and CIE coordinates of four different devices ................................ ................................ ................................ 86 5 1 Conductivities of pure and alkaline metal doped BPhen and 3TPYMB films .... 102 5 2 Comparison of the turn on voltage ( V T ) drive voltage at 100 cd/m 2 ( V 100 ), external quantum efficiency ( EQE ), luminous efficiency ( L ), power efficiency ( P ), and CIE coordinates of various devices ................................ ................... 104 6 1 Comparison of the forward viewing external quantum ( EQE ) and power ( P ) efficiencies for white OLEDs (WOLEDs) without any outcoupling enhancement methods ................................ ................................ ..................... 110 6 2 List of device structures and layer thicknesses used for fabricatin g WOLEDs discussed in Chapter 6 ................................ ................................ ..................... 113 7 1 Comparison of the light extraction efficienc y enhancement factor ( f ) for small area (2 2 mm 2 ) and large area (12 12 mm 2 ) fluorescent and phosphorescent OLEDs ................................ ................................ ................... 145

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10 LIST OF FIGURES Figure page 1 1 Historical and predicted power efficiencies for various light sources .................. 24 1 2 Examples of flexible OLEDs applications ................................ ........................... 25 1 3 Different Commission Internationale de L Eclairage color coordinates of various phosphorescent cyclometalate platinum complexes. ............................. 26 1 4 Various OLEDs display applications ................................ ................................ ... 28 1 5 Various OLEDs lighting applications ................................ ................................ ... 29 2 1 Representative m olecular structures of organic semiconductors depend on the complexity of hydrocarbon conjugation length. ................................ ............. 34 2 2 Schematic illustration of electronic states of organic semiconductors ................ 35 2 3 Schematic illustration of splitting of energy levels by strong interaction between two molecules. ................................ ................................ ..................... 36 2 4 Molecular structures of the first five polyacenes, together with the wavelength of the main absorption peak ................................ ................................ ............... 36 2 5 Different energy level diagrams of a single molecule in gas phase, ionized electron and hole pairs in the solid crystal, and an disordered Gaussian density of states in an amorphous solid ................................ .............................. 37 2 6 Schematic illustration of two major types of excitons, classified by the binding energy ................................ ................................ ................................ ................ 40 2 7 Fluorescence from the singlet ex citon (left) vs. phosphorescence from the triplet exciton (right). ................................ ................................ ........................... 41 2 8 Schematic illustration of two organometallic compounds composed of heavy metal atoms in the core and organic molecules in the surrounding .................... 42 2 9 Schematic illustration of Jablonski energy diagram ................................ ............ 43 2 10 The configurational diagram of ground (S 0 ) and excited (S 1 ) states of a molecu le, respectively ................................ ................................ ........................ 45 2 11 Schematic illustration of nonradiative inter molecular energy transfer processes between molecules ................................ ................................ ............ 47 3 1 Typical structures of OLEDs. ................................ ................................ .............. 48

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11 3 2 Schematic illustration of organic thin film growth processes ............................... 50 3 3 Schematic energy band dia grams about OLEDs operation ................................ 51 3 4 Wide range of electromagnetic radiation either as a function of wavelength. ..... 52 3 5 Comparison b etween photometry and radiometry. ................................ ............. 53 3 6 Comparison of maximum luminous power efficiencies between photopic and scotopic responses ................................ ................................ ............................. 54 3 7 Schematic comparison betw een plane and solid angles ................................ .... 55 3 8 Schematic illustration of a Lambertian emission pattern for the 2 D area source. ................................ ................................ ................................ ................ 56 3 9 Three major tristimulus curves (x, y ,an d z) ................................ ........................ 57 3 10 CIE color space represented by (x,y) coordinates ................................ .............. 58 3 11 Color rendering difference under two different illumination l ight sources ............ 59 3 12 Schematic illustration of 2 D Lambertian source in area A and corresponding infinitesimal surface area ( dS ) to define the solid angle. ............. 61 3 13 Representative three different optical modes in bottom emitting OLEDs ........... 65 4 1 Most frequently used organic hole/electron transporting and host materials for green and red emitting PHOLEDs ................................ ............................... 68 4 2 Molecular structures of blue phosphorescent emitters ................................ ....... 69 4 3 Electroluminescent (EL) spect ra comparison between sky blue and deep blue PHOLEDs. ................................ ................................ ................................ .. 70 4 4 Quantum (circles) and power (squares) efficiencies versus current density for the deep blue baseline PHOLEDs ................................ ................................ ...... 71 4 5 Schematic molecular structures of all organic materials which are used in Chapter 4. ................................ ................................ ................................ ........... 72 4 6 Schematic illustration of all device structures used i n Chapter 4 and energy level diagram of D EML p i n structure ................................ ............................... 73 4 7 Electroluminescent (EL) spectra comparison in deep blue PHOLEDs with and without mCP layer ................................ ................................ ....................... 75 4 8 Schematic illustration of better electron and triplet exciton blocking by using different hole transporting layers. ................................ ................................ ....... 76

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12 4 9 External quantum ( EQE ) and power ( P ) efficiencies as a function of luminance ( L ) for conventional deep blue PHOLEDs using either NPB 33 or TAPC 102 as the HTL. ................................ ................................ .......................... 77 4 10 Luminance ( L ) current density ( J ) volta ge ( V ) and current efficiency ( L ) current density ( J ) characteristics between conventional and p i n structures for single emissive layer (S EML) deep blue PHOLEDs ................................ .... 79 4 11 Luminance ( L ) cu rrent density ( J ) voltage ( V ) characteristics and current ( L ) and power ( P ) efficiencies vs. current density ( J ) for S EML p i n deep blue PHOLEDs. ................................ ................................ ................................ .. 80 4 12 Electroluminescent (EL) spectra d ifference between conventional and p i n S EML deep blue PHOLEDs. ................................ ................................ ............. 81 4 1 3 Current density ( J ) voltage ( V ) and power efficiency ( P ) current density ( J ) characteristics varying the UGH2:FIr6 thickness for the S EML p i n deep blue PHOLEDs ................................ ................................ ................................ ... 82 4 14 Current density ( J ) voltage ( V ) and (b) power efficiency ( P ) current density ( J ) characteristics with either 20 or 15 nm thick UGH2 layers for S EML and D EML p i n deep blue PHOLEDs. ................................ ...................... 83 4 1 5 Summarized power ( P ) and external quantum ( EQE ) efficiencies as a function of luminance ( L ) for major parameters which contributed to the efficiency enhancement in deep blue PHOLEDs. ................................ ............... 85 5 1 Schematic energy level diagram of the dual emissive layer (D EML) p i n phosphorescent organic light emitting devices (PHOLEDs) and c hemical structures of three different electron transporting materials used ....................... 89 5 2 Schematic energy level diagram of the D EML conventional deep blue PHOLEDs ................................ ................................ ................................ ........... 91 5 3 Current density ( J ) voltage ( V ) and external quantum efficiency ( EQE ) luminance ( L ) characteristics for the D EML conventional deep blue PHOLEDs with different ETLs ................................ ................................ ............ 92 5 4 Power efficiency ( P ) vs. luminance ( L ) and EL spectra for the D EML conventional deep blue PHOLEDs using different ETLs. ................................ ... 94 5 5 Schematic illustration for p type (left) and n type (right) doping mechanisms in the organic charge transporting semiconductors. ................................ ........... 95 5 6 Conductivities ( ) as a function of F 4 TCNQ doping concentration into MeO TPD host molecule. ................................ ................................ ............................ 96

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13 5 7 Schematic energy level diagram for different electron injection interlayers and their current injection behaviors in OLEDs. ................................ ........................ 97 5 8 Schematic energy level diagram for the doped BPhen layer as an electron injection layer and their current injection behavior using different dopants ......... 99 5 9 Schematic device structure for D E ML p i n deep blue PHOLEDs and its current efficiency ( L ) characteristic as a function of current density ( J ). .......... 101 5 10 Current efficiency ( L ) vs. current density ( J ) and luminance ( L ) curr ent density ( J ) voltage ( V ) characteristics for the D EML p i n deep blue PHOLEDs with different ETL/EIL structures. ................................ .................... 103 5 1 1 Current density ( J ) voltage ( V ), and power ( P ) and external qu antum ( EQE ) efficiencies vs. luminance ( L ) for the D EML deep blue PHOLEDs with different 3TPYMB ETL/EIL structures ................................ ............................... 105 5 1 2 Schematic illustration of the D EML p i n deep blue PHOLED with a m acrolens on the surface of the glass substrate and their efficiency comparison ................................ ................................ ................................ ....... 107 6 1 Schematic molecular structures of organometallic Ir complexes as phosphorescent emitters ................................ ................................ .................. 113 6 2 Schematic energy level diagram of S EML PHOLEDs doped with blue or green or red emitters and EL spectra with either FIr6 and/or Ir(ppy) 3 dopants. 114 6 3 Current ( L ) and (b) power ( P ) efficiencies vs. current density ( J ) for S EML PHOLEDs using either FIr6 and/or Ir(ppy) 3 dopants. ................................ ........ 115 6 4 EL spectra and current density ( J ) volt age ( V ) characteristics of S EML PHOLEDs using either FIr6 or PQIr single dopant. ................................ .......... 116 6 5 Power efficiencies ( P ) vs. luminance ( L ), and EL spectra of triple doped WOLEDs. ................................ ................................ ................................ ......... 116 6 6 External quantum ( EQE ) and power ( P ) efficiencies of triple doped WOLEDs as a function of luminance ( L ) ................................ ................................ ........... 117 6 7 Possible energy flow in a dua l emissive layer (D EML) white phosphorescent organic light emitting device (PHOLED) ................................ ........................... 118 6 8 Current density voltage ( J V ) characteristics and p ower ( P ) and external quantum ( EQE ) efficiencie s as a function of luminance ( L ) of single and dual emissive layer (S EML and D EML, respectively) conventional white PHOLEDs. ................................ ................................ ................................ ........ 119

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14 6 9 Current efficiencies ( L ) as a function of current densit y ( J ) for conventional single emissive layer (S EML) WOLEDs ................................ .......................... 120 6 10 Current density voltage ( J V ) characteristics and power efficiency ( P ) comparison as a function of luminance ( L ) between conventional and p i n D EML white PHOLEDs. ................................ ................................ ...................... 121 6 11 EL spectra of a dual emissive layer p i n white PHOLED at different luminances of L = 10, 100, and 1000 cd/m 2 ................................ .................... 122 6 12 Comparisons of Commission Internationale de l Eclairage (CIE) coordinates of triple doped p i n D EML white PHOLEDs (this work) among different white light sources ................................ ................................ ............................ 123 7 1 Schematic illustration of the procedures for fabricating large area and close packed microlens arrays using a soft lithographic molding method .................. 127 7 2 Schematic illustrati on of creating a polystyrene (PS) monolayer using a convective and capillary assembly technique ................................ ................... 128 7 3 Schematic device structures of fluorescent and phosphorescent OLEDs (FOLED and PHOLED, re spectively). ................................ ............................... 130 7 4 Three measurement methods to measure the outcoupling efficiency ( out ) of the OLEDs between with and without microlens arrays ................................ .... 131 7 5 Optical images of a large area and highly close packed PS monolayer ........... 133 7 6 Measured scanning electron microscope (SEM) images of microlens fabrication processes ................................ ................................ ........................ 134 7 7 Cross sectional SEM images of concave PDMS molds fabricated using different sizes of PS spheres and schematic illustration of mechanically weak points in the PDMS mold when removing PS sphere s from the cured PDMS .. 135 7 8 Cross sectional SEM images of convex microlenses with different sizes of PS spheres ................................ ................................ ................................ ....... 137 7 9 Schemati c illustration of a small area OLED with microlens arrays and its efficiency enhancement results and light extraction mechanism ...................... 138 7 10 Schematic illustration for light extraction mechanism in a large area OLED with microlens array s, and EL intensity difference with and without microlens arrays for large area FOLEDs ................................ ................................ ......... 140 7 11 N ear field luminous intensity measurement set up and inte nsity profile measurement results across a large area F OLED ................................ ........... 141

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15 7 12 Summarized light extraction enhancement factor ( f ) for different parameters and angular emission patterns using different mi crolens contact angles .......... 143 8 1 Schematic HOMO/LUMO energy level diagram and external quantum efficiency ( EQE ) vs. current density ( J ) characteristics for green and blue gr een PHOLEDs ................................ ................................ ............................... 150 8 2 Schematic triplet energy level diagram describing possible Dexter energy transfer process from blue to green dopants in the blue green PHOLED. ........ 151 8 3 Summary of outcoupling efficiencies and conditions based on different simulation methods ................................ ................................ ........................... 152 8 4 Schematic modeling and analysis results of OLEDs between without and with mi crolens arrays on the glass surface ................................ .............................. 153

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16 LIST OF ABBREVIATION S AND SYMBOLS Ag silver Al aluminum AMOLED active matrix organic light emitting device BCP bathocuproine BPhen batho phenanthroline CB conduction band CBP 4,4' bis (carbazol 9 yl)biphenyl CCT correlated color temperature CDBP 4,4' bis(carba zol 9 yl) 2,2' dimethylbiphenyl CIE C ommission CRI color rendering index CRT cathode ray tube CVD chemical vapor deposition D EML dual emissive layer EIL e lectron injection layer EL Electroluminescent EML emissive layer ETL electron transporting layer FIrN4 iridium(III) bis (4,6 difluorophenylpyridinato ) 5 ( pyridine 2 yl) 1 H tetrazolate FIrpic iridium(III) bis(4,6 (difluorophenylpyridinato ) picolate F Irtaz iridium(III) bis (4,6 difluorophenylpyridinato ) 3 (trifluoromethyl) 5 ( pyridine 2 yl) 1,2,4 triazolate FIr6 iridium(III) bis(4 ,6 difluorophenylpyridinato)tetrakis(1 pyrazolyl)borate

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17 FOLED fluorescent organic light emitting device F 4 TCNQ tetraf luor o tetracyanoquinodimethane HIL h ole injection layer HOMO highest occupied molecular orbital HTL hole transporting layer IC internal conversion Ir(ppy) 3 fac tris (phenylpyridine) iridium ISC intersystem crossing ITO i ndium tin oxide LCD liquid crystal disp lay LUMO lowest uno ccupied molecular orbital MBE molecular beam epitaxy mCP dicarbazolyl 3,5 benzene MeO TPD N,N' diphenyl N,N' bis(3 methylphenyl ) [1,1' biphenyl] 4,4' diamine MLCT metal ligand charge transfer NPB N, N' bis(naphthalen 1 yl) N,N' bis (phenyl) benzidine NPD N, N' bis(naphthalen 1 yl) N,N' bis (phenyl) 2,2' dimethylbenzidine OLED organic light emitting device PDP plasma display panel PHOLED phosphorescent organic light emitting device PL photoluminescent PLD pulsed laser deposition PLED polymer based OLED P LQY photoluminescen ce quantum yield PQIr iridium(III) bis (2 phenylquinolyl N,C 2 ) acetylacetonate

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18 PQ2Ir iridium(III) bis (2 phenylquinoly N C )dipivaloylmethane S EML single emissive layer SMOLED small molecule based OLED SSL solid state lighting TAPC 1,1 bis (di 4 tolylaminophenyl)cyclohexane TCO transparent conducting oxide TCTA 4,4',4" tr is(carbazol 9 yl)triphenylamine TPBi 2,2',2" (1,3,5 benzinetriyl) t ris(1 phenyl 1 H benzimidazole) TPD N, N' bis(3 methylphe nyl) N,N' bis(phenyl) benzidine UGH2 p bis(triphenylsilyly)benzene VB valence band VDW van der Waals VTE vacuum thermal evaporation WOLED w hite organic light emitting device 3TPYMB tr is[3 (3 pyridyl)mesityl]borane conversion factor E f f ermi energy level f geometric factor G( ) photopic response G ( ) scotopic response I det photocurrent I D OLED device current J cu rrent density L luminance solid angle

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19 S 0 ground state S 1 excited singlet state S( ) spectrum T 1 excited triplet state mobility V voltage L current efficiency (or l uminance efficiency ) P l uminous power efficiency IQE i nternal quantum efficiency EQE e xternal quantum efficienc y out o utcoupling efficiency

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20 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy H IGH EFFICIENCY BLUE AND WHITE PHOSPHO RESCENT ORGANIC LIGHT EMITTING DEVICES By Sang Hyun Eom December 2010 Chair: Jiangeng Xue Major: Materials Science and Engineering Organic light emitting devices (OLEDs) have important applications in full color flat panel displays and as solid state li ghting sources. Achieving high efficiency deep blue phosphorescent OLEDs (PHOLEDs) is necessary for high performance full color display s and white light source s with a high color rendering index (CRI) ; however it is more challenging compared to the longer wavelength light emissions such as green and red due to the higher energy excitations for the deep blue emitter as well as the weak photopic response of deep blue emission. This thesis details several effective strategies to enhanc ing efficiencies of deep blue PHOLEDs based on iridium(III) bis(4 ,6 difluorophenylpyridinato)tetrakis(1 pyrazolyl)borate (FIr6) which are further employed to demonstrate high efficiency white OLEDs by combining the deep blue emitter with green and red emitters First, we have employed 1,1 bis (di 4 tolylaminophenyl) cyclohexane (TAPC) as the hole transporting material to enhance electron and triplet exciton confinement in Fir6 based PHOLEDs, which increased external quantum efficiency up to 18 %. Second, d ual emissive layer (D EML) structures consisting of an N,N dicarbazolyl 3,5 benzene (mCP) layer doped with 4 wt % FIr6 and a p bis ( triphenylsilyly ) benzene (UGH2) layer

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21 doped with 25 wt % FIr6 w as employed to maximize exciton generation in the emissive layer Combined with the p i n device structure, high power efficiencies of ( 25 2 ) lm/W at 100 cd/m 2 and ( 20 2 ) lm/W at 1000 cd/m 2 we re achieved Moreover the peak external quantum efficiency of ( 20 1 ) % was achieved by employing tris[3 (3 pyridyl)mesityl]borane (3TPYMB) a s the electron transporting material which further improves the exciton confinement in the emissive layer With Cs 2 CO 3 doping in the 3TPYMB layer to greatly increase its electrical conductivity, a peak power efficiency up to (36 2) lm/W from the deep bl ue PHOLED was achieved, which also maintain s H igh efficiency white PHOLEDs are also demonstrated by incorporating green and red phosphorescent emitters together with the deep blue emitter F I r6. Similar to the F I r6 only devices, the D EML structure with high triplet energy charge transport materials lead s to a maximum external quantum efficiency of ( 19 1 ) %. Using the p i n device structure, a peak power efficiency of ( 40 2 ) lm/ W and ( 36 2 ) lm/W at 100 cd/m 2 were achieved and the white PHOLED possesses a CRI of 79 and CIE coordinates of ( 0.37, 0.40 ) The limited light extraction from the planar type OLEDs is also one of the remaining challenges to the OLED efficiency. Here we have developed a simple soft lithography technique to fabricate a transparent, close packed hemispherical microlens arrays. The application of such microlens arrays to the glass surface of the large area fluorescent OLED s enhance d the light extraction effi ciency up to (70 7)% It is also show n that the light extraction efficiency of the OLED s is affected by microlens contact angle, OLED s size and detailed layer structure of the OLED s

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22 CHAPTER 1 INTRODUCTION TO ORGANIC LIGHT EMITTING DEVICES 1. 1 Demand f or High Efficien cy Lighting E mitting Device s W orldwide total energy consumption is steadily growing every year, especially with the recent increase of small mobile electronic s In order to satisfy both the limited energy re source problem and the environmen tally clean energy issue, there have been continuous efforts to harvest n atural energy sources such as water, wind, and s unlight Also, highly energy efficient rechargeable batteries such as Li ion batter ies and fuel /solar cell s are intensively under inves tigation to meet the modern sustainable energy initiatives 1 5 On the other hand low electrical power consum ing electronic devices are also expected to potentially reduce the demands of the energy grid. Therefore, research into high efficiency and low power consuming electronics is a high research priority. It was re port ed that a pproximately 22% of the total electricity consumed in the United States is transformed into lighting, 6 and i t is estimated that as much as 1,000 terawatt hours (TWh) of electricity will be consumed by lighting by the year of 2025. 6 If there were lighting s ource that could convert the electrical energy into the visible light with 50% co nversion efficiency it would save energy consumption in the US by approximately 620 billion kilowatt hours per year which can eliminate about 70 nuclear plants on the US soil. 7 The co mmon incandescent light bulb, for example, which was invented in the has a power conversion efficiency (PCE) as low as 5% (i.e. 95% of electrically supplied energy is lost as a heat). As a comparison the f luorescent tube an other common light source which excite s a c oated phosphor by discharging gas ha s a better PCE reaching as much as

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23 approximately 20% corresponding to high power efficiency ( P ) up to 100 lm/W at room temperature However, fluorescent tubes contain some amounts of mercury which is an enviro nmentally hazardous material and also typically have 40 ~ 6 0 lm/W at elevated temperature s 8 Solid state lighting (SSL) is a highly energy efficient light emitting device (LED) based on semiconducting materials Conventional LED s are comprised of inorganic semiconductor s mostly g roup III nitride which have direc t bandgaps, converting the electrical energy directly into the visible light with less indirect energy losses. Although almost 100% internal quantum efficiency could be achieved using inorganic LED s 9 there are still a few issues such as high material and fabrication co st low color rendering index (CRI) for white light source s and device scalability Nevertheless, the inorganic LED market has been incr easing at an enormous rate in the last a few years 10 Organic light emitting device s (OLED s ) convert the electrical power into light by using the organic semiconductors such as sm all molecule or polymer semiconducting materials Since the ir first introduc tion by Tang and Vanslyke in 1987 at Eastman Kodak, 11 extensive research ha s been conducted in academia and industry to achieve high efficiency and stable OLEDs. Compared to inorganic LEDs, OLED s can be cheaper, easily scalable, and even tunable in electrical/ optical propert ies suggesting an excellent next generation light source for either flat panel displays or SSL H o wever, low efficiency and device stability are the most important issues for OLEDs in order to replac e most existing light sources in the world. The classic power efficiencies and current progress of several white light sources including inorganic and organic LEDs are

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24 shown in Fig. 1 1, and the power efficiency of white OLEDs (WOL EDs), demonstrated in this dissertation with outcoupling enhancement methods is also indicated by the red star. 12 Figure 1 1 Historical and predicted power efficiencies for various l ight sources (source:http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_multiy ear_pla n.pdf) 12 The result presented in this dissertation is indicated by closed red star. 1 2 Advantages of Organic Light E mitting Devices There are many advantages to organic semiconducting materials. First of all, organic materials are more cost effective than inorganic semiconductors due to the nearly unlimited synthetic abundance of organic materials and the thinner film thickness, ty pically ~ 100 nm thick. Second, organic thin films c an be easily d eposited using

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25 various simple f abrication methods such as vacuum thermal evaporation (VTE) spin coating, inkjet printing, 13 14 and even roll to roll process 15 21 compared to the typical inorganic thin film growth methods such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD) 22 and pulsed laser deposition (PLD). Third, the extremely thin film thickness and flexibilit y of organic material s make OLEDs suitable for flexible device application s as shown in Fig. 1 2 23 24 Figure 1 2 Examples of flexible OLEDs applications. (Upper image) Flexible OLEDs by Universal display co rporation (Lower image) F lexible OLEDs on garments (source : h ttp://oled.pro/ )

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26 F urthermore, the optical and electrical properties of organic materials can be tuned via chemical structure modification of organic molecules For example, different vis ible emission spectra can be represented by tuning the chemical structure of phosphorescent cyclometalate platinum complexes as shown in Fig. 1 3 25 Figure 1 3 Different Commission color coordinates of various phosphorescent cyclometalate platinum complexes. 25 O ptical properties can be tuned via chemical structure modification of organic molecules (Reproduced with the permission from J Brooks et al., Inorg. Chem 41 3055 (200 2 ) )

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27 OLEDs al so have excellent display performances such as a fast response time, a wide viewing angle, a high contrast, and low power consumption, compared to the properties of their main competitor, liquid crystal display (LCD). 1 3 Application s of Organic Light E mit ting Devices There are two major application areas in OLEDs. T he first application area is the flat panel display device where OLEDs competes with the LCD and the other area is the next generation SSL source which could ultimately replac e fluorescent tube s Both of applications can be currently found in the commercial consumer market 1 3.1 F lat P anel D ispla y There have been many different types of display devices such as the cathode ray tube (CRT), the plasma display panel (PDP), and the LCD The LCD is t he most preva lent display device up to now, but the demand for the OLED display is tremendous ly growing due to the excellen t light emitting qualities compared with the LCD. The short radiative lifetime ( typically, from a few to thousand nanoseconds) of or ganic materials can provide much faster response time, compared to the slow response time of the LCD, which typically takes milliseconds (ms) to rotate the liquid crystal cell T herefore the OLED can be a better display device for watching sports and movi es The wide viewing angle (nearly 180) of OLEDs, which is one of the most important requirements for mobile display devices, is also another important advantage T he first 1, was released by Sony in 2007 (Fig. 1 4(a)) matrix OLED (AMOL E D) by Samsung mobile display (SMD Fig. 1 4(b) ). Although the relatively small size of OLED displays have been introduced to the market up to now, such large size prototype OLED TVs ( ) have been also continuously introduced in the consumer electronics show (CES)

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28 as shown in Fig. 1 4(c) Recently, Sony and Samsung demonstrated 3 dimensional (3 D ) di splay devices based on the LCD /PDP as shown in Fig. 1 4(d). However, it is expected that the OLEDs would be more compatible in realizing the 3 D images due to their fast response time in making different images Figure 1 4 Various OLEDs display applications: (a) 11 OLED display, (Sony, XEL 1 ) (b) 4 active matrix OLED display (Samsung M obile D isplay SMD ), (c) 31 OLED TV (SMD, prototype), (d) 3 D TV based on a LCD display (SMD) 1. 3 .2 Solid S tate L ighting Figure 1 5 shows various OLED lighting products introduc ed by Novaled, General Electric (GE), Philips, and Osram The OLED lighting market is not as large yet as

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29 display s but expected to grow significantly by 2015. 26 The requirement for the white light source for the lighting is somewhat different than that for the display ; h igh er brightness condition (luminance, L = 1,000 cd/m 2 for the lighting vs. L = 100 cd/m 2 for the display) good col or rendering index ( CRI ) of at least 70 matching of Commission similar to that of a blackbody radiator which is on the Planckian locus and a correlated color temperature (CCT) between 2500 K and 6000 K. Th e adjustment of these various light emitting properties for the efficient white light source can be easily tailored by selectively choosing different light emitting organic molecules and optimizing the device structure of the OLEDs, consequently resulting in a wide range of white light emissions such as warm or cool Figure 1 5 Various OLEDs lighting applications (Sources from Novaled, General Electric, Philips, and Osram).

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30 The development of phosphorescent organic emitters based on organometallic c ompounds brought almost 100% internal quantum efficiency ( IQE ), 27 28 leading to 20~ 25 % peak external quantum efficiency ( EQE ) and 30 ~ 60 lm/W peak power efficiency ( P ) in the planar structure W OLEDs. 29 31 However, white light emitting phosphorescent OLEDs (PHOLEDs) without any outcoupling efficiency enhancement methods still have a limitation for lighting application s which require much higher efficienc ies (60~ 10 0 lm/W) than those (10~20 lm/W) of displays. 8,32 1.4 Demand for High E fficiency B lue and W hite PH OLEDs The main OLED application areas such as flat panel display s and SSL require either individually blue (B) green (G) red (R) OLEDs for pixellation or WOLEDs which can be g enerated by multiple combinations of B G R emissions. Among many different color emitting OLEDs, high efficiency deeper blue emitting PHOLEDs are essential for realizing full color displays and high CRI white light sources. H o wever, blue phosphorescent emi tters typically have higher triplet energy levels as the blue emission goes deeper. As a result, it is much more difficult to effectively confine the excitons from the deeper blue emitters using the host and charge transporting materials which are typicall y used for green and red OLEDs. Due to the difficulty in developing large bandgap host and charge transporting materials, it is challenging to demonstrate high efficiency deep blue PHOLEDs. There have been several efforts to develop high efficiency deep b lue PHOLEDs based on a few different iridium complexes 33 34 Even though deep blue emissions could be obtained with CIE coordinates of (0.16, 0.26) using these deep blue emitters, still low external efficiencies of around 13% at peak were the main obstacle for further applications of these deep blue PHOLEDs to the full color displays or high CRI

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31 WOLEDs. 33 34 Therefore, it is necessary to enhance the efficiency of deep blue P HOLEDs by employing new materials for better triplet exciton confinement and developing new device structures which eventually enhance the internal quantum efficiency of such deep blue PHOLEDs. Since the first demonstration of white organic light emitting devices (WOLEDs) by Kido et al., 35 38 many different approaches to generating a white light emission from organics have been demonstrated. The initial devices exhibited efficiencies of less than 1 lm/W, 35 38 but significantly enhanced efficiencies have been reported over the last a few years. For example, Sun et al made a breakthrough in the WOLED efficiency with the employment of separate emissive layers for a blue fluor esc ent dopant and green and red phosphor escent dopants. 39 By exploiting singlet excitons only for a blue dopant and triplet excitons for green and red dopants, almost 100% internal quantum e fficiency and high CRI of 85 were obtained with the peak external quantum and power efficiencies of (18.7 0.5) % and (37.6 0.6) lm/W, respectively. Recently, Su et al. demonstrated the most efficient WOLEDs (among the published literatures without any outcoupling enhancement methods) by co doping FIrpic and iridium(III) bis (2 phenylquinoly N C )dipivaloylmethane (PQ2Ir) 3 0 D ual emissive layer (D EML) structure and wide bandgap hole and electron transporting layers (HTL and ETL, respectively) we re employed to maximize the exciton generation and confinement, leading to the peak external and power efficiencies of approximate ly 25% and 58 lm/W, respectively. However, their WOLEDs had a very low CRI of 68 due to the limited color gamut using the two dopant system including a sky blue emitter of FIrpic Although there are several reports from companies such as universal display corporation ( UDC ) and Novaled,

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32 demonstrating higher than 100 lm/W power efficiencies with outcoupling enhancement techniques most noteworthy WOLEDs in the published paper still struggle in achieving high power efficiencies along with a high CRI. Therefore it is highly recommended to formulate new device structures for high efficiency and high CRI WOLEDs. Moreover, conventional planar structure OLEDs suffer from very low light extraction efficiency as a large portion of the light emission inside an OLED e xists as waveguiding modes in the organic/transparent conductor layers and in the substrate. 27,40 Hence further improv ement of the light extraction efficiency in OLEDs will be required to ultimately achieve comme rcial viability 1.5 Outline of D issertation This dissertation endeavors to deliver the fundamental background of OLEDs as well as important design parameters for high efficiency OLEDs. First, Chapter 2 will explain the optical, electrical, and ph ysical properties of organic semiconductors Chapter 3 will help in understand ing more details about OLEDs fr o m the organic thin film growth to OLED measurements. The important terminologies and definitions for understanding the OLED operation and evaluati on will be described in Chapter 3 as well. In Chapter 4 and 5, high efficiency deep blue PHOLEDs will be demonstrated by employing high triplet energy hole/electron transporting materials, a dual emissive layer structure, and optimized doping concentration in the p i n structure. In Chapter 6, high efficiency W OLEDs will be demonstrated based on triple doped dual emissive layer p i n structure where design parameters for high efficiency as well as high CRI WOLEDs will be discussed. In order to further enha nce the OLED efficiency, a l ight extraction method using close packed and hemispherical microlens arrays will be applied to the OLEDs in Chapter 7. A rather simple microlens fabrication method will be established

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33 first, and important parameters will be fou nd for better light extraction in using microlens arrays. F inally, this dissertation will make conclusions and leave some future topics for further OLEDs research in Chapter 8.

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34 CHAPTER 2 PHOTO PHYSICAL PROPERTIES OF ORGANIC SEMICONDUCT ORS 2. 1 Intro duction Organic compounds are defined by the presence of carbon atoms, most often arranged as conjugated aromatic hydrocarbons Depend ing on the complexity of these conjugated hydrocarbon bonds, there are infinite combinations of organic molecules from a s imple hydrocarbon compound such as methane (CH 4 ) to the most complex human deoxyribonucleic acid (DNA) molecule as illustrated in Fig. 2 1. 41 Figure 2 1 Representative m olecular structures of organic semiconductors depend on the complexity of hydrocarbon conjugation length There are roughly four different types of bonding in solids; ionic bond, metallic bond, covalent bond, and van der Waals (VDW ) bond O rganic molecular solids are composed of discrete molecules held together by weak VDW forces t herefore, it is expected that the electronic structure and photo physical properties of organic molecules can be different compared with other solid mate rials 41 Hence, in Chapter 2

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35 the basic properties of organic semiconductors will be described to help understand the performances of OLEDs. 2.2 Electronic Structures An o rganic s emiconductor is classified as a highly conductive organic compound. Strong covalent bonding between conjugated carbon s forms sp 2 hybrid ized orbital s bond s ) and loosely connected p z orbital s which are perpendicular to the plane containing all the carbon atoms. The overlapping between the neighboring p z electron s of the carbon atoms forms the so called bond s as illustrated in Fig. 2 2 Figure 2 2 Schematic illustration of electronic states of organic semiconductors SP 2 hybridization generates strong bond s and loosely connected p z orbital s Typically hopping process is dominant for intermolecular electron transporting in conjugated organ ic compounds. The delocalized electrons are free to move within the molecule and enable charge transport in organic materials. Similar to the valence band (VB) and conduction band (CB) of inorganic semiconductor, organic semiconductor s possess the highes t occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) Interactions between the electrons and the electrons within molecules decide the gap between the HOMO and LUMO levels as shown in Fig 2 3 where the optical proper t ies of organic semiconductor s are decided by the transition. When a large number of electrons are involved, the energy levels may form a continuous band like a

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36 CB and VB of an inorganic semiconductor. A dditionally t he bandgap of the molecule is stro ngly affected by the degree of conjugation, i.e. larger molecule s ha ve smaller bandgap s as shown in Fig. 2 4 Figure 2 3 Schematic illustration of splitting of energy levels by strong interaction between two molecules. Figure 2 4 Molecular structure s of the first five polyacenes, together with the wavelength of the main absorption peak 42

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37 2.3 Charge Transport Charge transport in organic molecules should be considered as a pair of an electron and a hole which are formed from a neutral molecule. Compared to a single molecule in the gas phase, ionized electron and hole pairs in the solid crystal lower the bandgap due to the polarization energies as shown in Fig 2 5 42 In amorphous solids, locally different polarization energies create a Gaussian distributed de nsity of states for transporting sites. Figure 2 5 Different energy level diagrams of a single molecule in gas phase, ionized electron and hole pairs in the solid crystal and an disordered Gaussian density of states in an amorphous solid. 42 Organic sem iconductors have several orders of magnitude lower charge transport capabilities compared to the fast charge transport of their inorganic counterpart s A strong covalent bond is formed in inorganic semiconductors creating so called delocalized states and leading to the continuous transport band levels w hereas, a weak VDW interaction creat es discontinuous localized states in organic semiconductors

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38 contributing to less efficient charge transport. There are two major types of charge carrier transporting me chanism s in organic semiconductors. 2. 3 .1 Hopping T ransport Most organic thin films are in an amorphous solid state, and weak VDW interactions throughout the amorphous structure cannot provide a continuous band transporting path. Instead, an intermolec ular hopping mechanism dominates in the amorphous solid state. The typical charge carrier mobility ( ) of amorphous organic semiconductors is in the range of 10 3 ~ 10 10 cm 2 /V s. The mobility in amorphous organic semiconductors can be expressed as a function of electric field and temperature ; 43 W here F is a electric field, T is a temperature, is a activation energy for intermolecular hopping, k is a Boltzmann constant, and is a constant value 2. 3 .2 Band T ransport Some organic semiconductors can form a crystalline structure in the solid thin film forming a slightly delocalized band structure. Hence, relatively high charge carrier mobility of 1~ 10 2 cm 2 /V s can be realized in examples such as a pentacene crystal. 44 The mobility in crystalline organic semiconductor fol lows the temperature dependence. 45 2.4 T ransport vs. O ptical B andgap s Typically for inorganic semiconductors, there is a negligible d ifference (~ meV) between the optical bandgap ( E opt ) and transport bandgap ( E tr ) due to the small exciton

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39 binding energy in delocalized energy states. Whereas, for organic semiconductors, the E opt and E tr should be classified due to the strong exciton bind ing energy (~ eV) in the localized organic molecule. 41 The accurate relationship for E opt and E tr for organic semiconductors can be defined as ; E opt = ( E gap E p ) E ex = E tr E ex W here E gap is the energy level difference between the HOMO and LUMO in a single moelcule, E p is the energy loss due to the polarization, and E ex is the exciton binding energy. 2.5 Excitons 2. 5 .1 Formation of E xcitons The ex citon is define d as a bound electon hole pair by a c oulombic interaction. It can be treated like a chargeless quasi particle capable of diffusion. There are two major types of excitons, classified by the binding energy; the loosely bound Wanier exciton for inorganic semiconductors and the tighly bound Frenkel exciton for organic semiconductors as shown in Fig. 2 6 respectively. Compared to the delocalized Wanier exciton which has a large radius of ~ 100 with weak binding energy (~ 10 meV), the Frenkel ex citon is typically localized within one or two molecules (radius range of ~ 10 ) with strong binding energy (~ 1 eV). Another type of exciton is the charge transfer (CT) exciton which has a binding energy between the Wanier and Frenkel excitons, hence ele ctron and hole pairs can reside up to a few intermolecular distance s 2.5.2 Multiplicity of Excitons There are four possible spin states in the exciton (two spin states in each charge). The total wave function of a two electron system must be anti sy mmetric with the 46 Based on the possible spin

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40 Figure 2 6 Schematic illustration of two major types o f excitons, classified by the binding energy; (left) loosely bounded Wanier exciton for inorganic semiconductors and (right) tigh t ly bounded Frenkel exciton for organic semiconductors statistics of the excited electrons, the symmetric and anti symmetric w ave functions ( s a nd a respectively ) can be expressed as below; s = 1 2 s = 1 2 symmetric states ( spin =1, triplet ) s = 1 2 1 2 a = 1 2 1 2 anti symmetric state ( spin =0, singlet )

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41 w here n (n=1,2) is a spin function, and ( ) or ( ) represent the possible spin states of each electron. Figure 2 7 shows the radiative relaxation processes using single t and triplet excitons. Because of the difference in spatial symmetry, the single t state has a higher energy than that of the triplet state. The electron electron repulsion is smaller in the triplet exciton, leading to less potential energy. Also, very fast radiative lifetime of ~ 1 ns i s observed for fluorescence d ue to the symmetry conservation in the singlet exciton Whereas, very slow relaxation time of ~ 1 ms is observed for phosphorescence because the triplet exciton transition to the ground state is not preferable. Figur e 2 7 Fluorescence from the singlet exciton (left) vs. phosphoresce nce from the triplet exciton (right) 2.5.3 Metal Ligand Charge Transfer Exciton Organometallic compounds based on heavy metal s such as iridium, platinum, osmium, and ruthenium can expl oit singlet as well as triplet excitons due to very strong spin orbit coupling which is proportional to the atomic number (Z 4 ). 47 55 As shown in Fig. 2 8, strong spin orbit coupling based on heavier metal s such as iridium can effectively

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42 mix singlet and triplet states, generating emissive metal ligand charge transfer (MLCT) excitons. Therefore, it is theoretically possible to convert all the singlet and triplet excitons to phosphorescence leading to 100% photon con version efficiency using these organometallic compounds as emitters 27 Figure 2 8 Schematic illustration of two organometallic compounds composed of heavy metal atoms in the core and organic molecules in the sur rounding ; p latinum (left) and i ridium (right). 2. 6 Intra M olecular E nergy T ransfer Various photo physical processes in a typical molecule are illustrated in Fig 2 9 (called the Jablonski energy diagram). The absorption process is represented from t he ground state (S 0 ) to the excited singlet state (S 1,2 ), and nonradiative transitions such as internal conversion (IC), vibrational relaxation, quenching, and intersystem crossing (ISC) can happen before the radiative transitions such as a fluorescence (S 1 S 0 ) or a phosphorescence (T 1 S 0 ). 2.6. 1 Absorption The absorption process occur s with excitation energy larger than the bandgap energy (E b = hc / ), and there is a range of wavelength s that can lead to a transition

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43 between any two electronic state s (S 0 S 1,2 ). Thus, electronic absorption spectra generally occur with broad bands, rather than as single lines. Figure 2 9 Schematic illustration of Jablonski energy diagram (source : http://micro. magnet.fsu.edu/optics/timeline/people/jablonski.htm l ) The absorption process is represented from the ground state (S 0 ) to the excited singlet state (S 1,2 ), and nonradiative transitions such as internal conversion (IC), vibrational relaxation, quenching, and intersystem crossing (ISC) can happen before the radiative transitions such as a fluorescence (S 1 S 0 ) or a phosphorescence (T 1 S 0 ). 2. 6 2 Fluorescence Once exciton s are generated, they quickly relax to the lowest vibrational level of an exited singlet state via vibrational relaxation and internal c onversion processes. Then excitons can relax to the ground state by emitting a photon (S 1 S 0 ). Such light

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44 emission is called fl uorescence Fluorescence emission can only exploit the singlet excitons (25%) and has a very short radiative lifetime of approx imately 10 9 s to 10 7 s 2. 6 3 Intersystem C rossing Although a direct absorption from the ground state to the triplet state is not preferable a significant amount of energy can be transferred from the lowest excited singlet state to the excited triple t state (S 1 T 1 ). This process is referred to as intersystem crossing (ISC). The mechanism for ISC involve s vibrational coupling between the excited singlet state and a triplet state. 2. 6 .4 Phosphorescence Once an intersystem crossing has occurred the m olecule undergoes the IC process and falls to the lowest vi brational level of the triplet state. Since the di ffer ence in energy between the lowest vibrational level of the triplet state and the lowest vibrational level of the singlet state is large compare d to thermal energy, backward energy transfer from the triplet state to the singlet state is highly improbable. The transition from the lowest vibrational level of the triplet state to the ground state (T 1 S 0 ) is possible (typically forbidden process) wh en the spin orbit coupling breaks the selection rule. The molecules are therefore able to emit weakly and the emission continues long after the excitation. T he radiative lifetime of a triplet exciton (75%) is approximately 10 4 s ~ 1 s 2. 6 .5 Frank Condon S hift Most of the absorption and emission processes in organic molecules involve with the vibrational modes. The electronically excited molecule releases the energy very quickly toward stable energy state through either photon generation (fluorescence or phosphorescence) and/or phonon vibration (heat loss). The configurational diagram of ground and excited states of a molecule is shown in Fig. 2 10

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45 Figure 2 10 The configurational diagram of ground (S 0 ) and excited (S 1 ) states of a molecule respectiv ely. So called Frank Condon shift or Stokes shift happens in a molecule due to the fast and nonradiative vibrational relaxation s. The absorption is dominated by transition from the zero order vibrational mode of ground state to the higher order vibrati onal mode of the excited state (S a 0 S a*2 ). The excited electrons then experience a fast vibrational relaxation by releasing heat to the zero order mode (S a*2 S a*0 ). Light emission occurs from transition of the zero order vibrational mode of the excited state to various vibrational mode s of the ground state (S a*0 S c1 ). This so called Frank Condon shift (or Stokes shift) leads to the red shifted emission peak compared to the absorption peak. 41

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46 2.7 Inter M olecular E nergy T ransfer Inter molecular energy transfer is a nonradiative process between molecules, and it can be divided into two types depend ing on the range of transitions. The long range transit ion (~ 100 ) is called the F rster transfer, 56 whereas the short range transition (~ 10 ) is called the Dexter transfer. 57 T he mechanism of F rster and Dexter transitions is illustrated in Fig. 2 11 2. 7 1 F rster E nergy T ransfer The inter molecular interaction of the Frster transfer originate s from the resonant dipole dipole interaction and happens very fast (< 10 9 s) typically in singlet singlet transitions as shown in Fig 2 11 (a) 58 60 The overlap (the amount is de fined as J ) between the emission spectrum of donor and absorption spectrum of the acceptor is necessary The Frster transfer rate constant k D A is given by: Where K is an orientation factor, n is the refractive index of the medium D is the radiative lifetime of the donor, r is the distance (cm) between donor and a cceptor 2. 7 .2 Dexter E nergy T ransfer Dexter energy transfer involves electron exchange between molecules, and requires very short range wave function overlap between d onor and acceptor as shown in the Fig 2 11 (b). 61 It is the dominant mechanism in triplet triplet transitions. The transfer rate constant, k ET is given by:

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47 W here r is the distance between do nor and acceptor, L and P are constants not easily related to experimentally determinable constants, and J is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed. Figure 2 11 Schematic illustration of nonradiative i nter molecular energy transfer processes between molecules ; (a) Frster energy transfer with t he long range transition (~ 100 ) and (b) Dexter energy transfer with the short range transition (~ 10 )

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48 CHAPTER 3 FUNDAMENTALS OF ORGA NIC LIGHT EMITTING DEV ICES 3.1 D evice S tructure of OLED s OLED architecture can be divided into two groups depend ing on the light emitting direction, the bottom emitting OLED (BOLED) and the top emitting OLED (TOLED) G eneral device structure s of both OLEDs are shown in Fig 3 1 The BOLED is the conventional OLED structure where the light i s directed out from the emissive layer (EML) to the air through the glass substrate (bottom direction), as opposed to through the transparent top electrode for the TOLED. Figure 3 1 Typic al structures of OLEDs; (a) the bottom emitting OLED (BOLED) and (b) top emitting OLED (TOLED), where HTL = hole transporting layer, ETL = electron transporting layer, and EML = emissive layer. Indium tin oxide (ITO) is generally used as an anode with hig h transmitivity (~90%) patterned on the glass substrate. A h ole transporting layer (HTL) facilitates hole injection from the anode to the emissive layer (EML) and sometimes functions as an electron blocking layer (EBL) as well. Similarly,

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49 the electron transporting layer (ETL) helps better electron injection from the cathode to the EML, typically with a thin electron injection layer (EIL) between the ETL and the cathode. Injected holes and electrons from the anode and cat hode, respectively, recombine and create excitons in the EML, and emit the light via the radiative relaxation process. Highly reflective metals such as silver (Ag) and aluminum (Al) can be typically used as the cathode material due to their high reflectivi t ies and low work functions (4.1 ~ 4.2 eV). In this dissertation, the BOLED structure will be discussed primarily 3.2 F abrication of OLEDs D epend ing on the size of the organic molecules which are used for the thin film growth OLED s can be divided into tw o types; small molecule OLEDs (SMOLEDs) and polymer OLEDs (PLEDs) Small molecule is defined by a mole cular weight approximately less than 1000 g/mol whereas polymer has long and complicated structures with the molecular weight approximately more than 100 0 g/mol There have been many different methods to grow organic thin films such as vacuum thermal evaporation (VTE) spin coating, organic vapor jet printing, spraying method, inkjet printing and even a roll to roll process ing Among those methods, the V T E is commonly used to fabricate SMOLED s, whilst the spin coating is most widely used for research into PLED s. Figure 3 2 (a) illustrates the VTE system in which a sufficient amount of electrical current through the source boat heats up and sublimes organic materials in a vacuum environment. The ballistic molecular thin film deposition is possible with tens of centimeter scale mean free path under typical chamber pressure of 10 6 ~ 10 7 Torr. A quartz crystal monitor (QCM, coated with a thin gold layer) provi de s accurate film thickness control up to the 0.1 /s. Figure 3 2 (b) schematically shows the spin coating

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50 process where polymer materials are completely dissolved in a specific solvent and dropped onto the substrate in order to evenly disperse the polymer solution. A thermal annealing process is typically required to completely evaporate the solvent residue after spin coating. The thickness of a polymer thin film can be accurately controlled by adjusting the spin speed and the polymer concentration. The VTE has an advantage for fabricating OLEDs with multilayered structure and doped layer s whereas the spin coating method is more cost efficient than the VTE Figure 3 2 Schematic illustration of organic thin film growth processes ; (a) a vacuum thermal eva poration (VTE) and (b) a spin coating method s 3.3 P rinciple of OLED s O peration Fig ure 3 3(a) ~ (d) illustrate the energy band diagram s for the operation processes of the OLED The OLED is simplified as a single organic layer structure with the anode and the cathode at both electrical contacts. Fermi level s ( E F ) are not aligned b efore the electrical contact by anode and cathode as shown in Fig. 3 3 (a). After the electrical contact, there is an equilibrium state with aligned E F However, charge injection i nto the organic layer is still not preferable due to the built in potential barrier shown in Fig. 3 3 (b).

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51 Figure 3 3 Schematic energy band diagrams about OLEDs operation. F or simplicity, single organic layer is assumed between two electrodes; (a) befo re the electrical contact, (b) a fter the electrical contact, (c) w ith the applied voltage bias through both electrodes, charge carriers are ready to be injected at V=V bi and (d) charge carriers are finally injected in to the organic layer at V>V bi forming excitons and emitting the light.

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52 When the voltage bias is applied to the electrodes, charge carriers are ready to be injected into the organic layer at V=V bi as shown in Fig. 3 3 (c), and finally injected to the organic layer at V>V bi as shown in Fig. 3 3 (d). Holes move upward through the HOMO level and electrons transport downward through the LUMO level of the organic layer respectively. Once both charge c arriers meet each other in the organic layer, they become energetically bounded each other (Coulombi c interaction) form excitons, and generate light emission through the radiative relaxation in the organic layer. 3.4 C olorimetry Light is defined as radiation of electromagnetic wave s which can be observed by the human eyes Th is wide range of electromag netic radiation is identified by frequencies or wavelengths as shown in Fig. 3 4 T he visible light is located in the wavelength ranges between 380 nm and 780nm whereas u ltraviolet (UV) and infrared (IR) radiation s both are invisible to the human eyes are next to the violet and red regions respectively Figure 3 4 Wide range of electromagnetic radiation either as a function of wavelength. 3.4.1 Photometry vs. R adiometry P hotometry is defined only for the visible wavelengths ( = 380 ~ 780 nm), whereas a radiometry is applied to the entire spectrum of electromagnetic radiation. 62

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53 The terminologies and definitions for photometry and radiometry are summarized in Fig. 3 5 Throughout this dissertation, photometry will be mainly discussed Figure 3 5 Comparison between photometry and radiometry. A photometry is defined only for the visible wavelength s whereas a radiometry is applied for the entire wavelengths including visible and invisible wavelengths which is a more general concept t o the electromagnetic radiation. 3.4.2 Responsivity of H uman E ye s The illuminance of daytime is about 100,000 lux (lx), as opposed to about 0.0003 lx at night. In order to detect such a wide range of illuminance, the size of the pupil and the responsivity of the retina of the human eye change s according to the brightness of the environment. 63 In a relatively bright environment, the human eye detect s the light based on the photopic response, whilst scotopic response is dominant in the re latively dark environment. The p hotopic response can be considered for brightness approximately over 3 cd/m 2 whereas the scotopic response is dominant at less than 0.0003 cd/m 2 In the middle of these two extremes, there is another so called mesopic respo nse. 63

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54 The photopic response G( ) has the peak luminous power efficiency of 0 = 683 lm/W at = 555 nm, whereas the scotopic response G ( ), has the peak value of 0 = 1700 lm/W at = 507 nm as shown in Fig. 3 6 So, the photopic and scotopic responses can be expressed as: W here g( ) and g ( ) are the normalized photopic and scotopic responses, respectively which has a unity intensity at the peak wavelength position. Throughout this dissertation, the photopic response will be generally applied for calculating the luminance of the OLED. Figure 3 6 Comparison of maximum luminous power efficiencies between photopic and scotopic responses. (source : http://www.visual 3 d.com/Education/LightingLessons )

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55 3.4.3 Plane vs. S olid A ngle s A plane angle ( ) is defined as a ratio of the length of the arc to the radius of the circle in 2 D, whereas a s d as a ratio of the spherical surface area to the square of the radius of the sphere in 3 D as shown in Fig 3 7 The SI units for the plane angle and solid angle are radian (rad) and steradian (sr), respectively. Figure 3 7 Schematic comparison between plane and solid angles; (a) a plane angle in 2 D and (b) a solid angle in 3 D. 3.4.4 Lambertian Emission S ource An isotropic light emitting with equal luminance into any solid angle is defined as a Lambertian source as shown in Fig 3 8 There is a cosine law for luminous intensity (cd or lm/sr) from the Lambertian source when observed from the surface normal. However, the luminance (cd/m 2 or lm/sr m 2 ) at each angle is exactly the same because the observed light emitting area is also reduced by the same amount.

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56 Figure 3 8 Schematic illustration of a Lambertian emission pattern for the 2 D area source 3.4.5 CIE C olor S pace The Co color space wa s created in 1931 and can be defined with (x,y,z ) coordinates In order to obtain (x,y,z) color coordinates, tristimulus values of (X,Y,Z) should be calculated by integrating the tristimulus curve s (see Fig. 3 9 ) over a color spectrum S( ).

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57 Figure 3 9 Three major tristimulus curves (x, y ,and z). T ristimulus values of (X,Y,Z) should be calculated by integrating the tristimulus curves in order to get CIE coordinates. (sour ce : http://en.wikipedia.org/wiki/CIE_1931_color_space ) Then, CIE color coordinates can be obtained: Typically the color space with (x,y) coordinates is used to define specific colors because there is a relationship of x+y+z=1. The example of CIE color space represented by (x,y) coordinates is shown in Fig 3 10

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58 Figure 3 10 CIE color space represented by (x,y) coordinates Planckian locus is also illust r ated with the correlated color temperature (T C ) 3.4.6 Color R endering I ndex The definition of the color rendering index (CRI) is how well the object can represent color s when illuminated by a certain light source, compared to that illuminated by a reference l ight source. Therefore, the CRI is a relative value ranging from 0 to 100, and larger CRI value means similar color rendering under a certain light source compared to that under a reference light source. B lackbody radiation source s which are on the Plancki an locus can show a CRI = 100 so an incandescent light bulb is typically used as a reference light source. There are up to 14 different colors to get the individual

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59 CRI 1~14 values, but CRI 1~8 from 8 colors are commonly averaged out to get the so called CR I. Figure 3 11(a) and (b) shows the color rendering difference under two illumination light sources. The color under a light source with a low CRI does not represent the real color as shown in Fig. 3 11 (a), whereas the color looks more natural under a light source with a high CRI as shown in Fig. 3 11 (b). A CRI at least higher than 70 is typically necessary for the purpose of illumination. Figure 3 11 C olor rendering difference under two different illumination light sources ; (a) Illuminated by a light source with a CRI of 62 and (b) Illuminated by a light source with a CRI of 90 3.5 O LED s M easurement The measurement of OLED characteristics is important to provide consistent comparison criteri a among different devices. For this purpose, t he ac curate measurement method for the OLED has been suggested previously 64 The below definitions and equations are similarly derived from the basic definitions and measurement concepts in the previous report 64 Although the measurement set up

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60 might be somewhat different among other research groups, understanding the following subsections can provide a consistent OLED measure ment Based on the dedicated measurement set up, l uminance ( L ) current density ( J ) voltage ( V ) measurements were carried out in ambient conditions using an Agilent 4155C semiconductor parameter analyzer and a calibrated Newport silicon detector (818 UV ) The luminance was calibrated using a Konica Minolta LS 100 luminance meter (with No. 110 close up lens) assuming a Lambertian emission pattern. Electroluminescent (EL) spectra were taken using an Ocean Optics Jaz spectrometer. L J V and spectrum measur ement of OLEDs will derive all the detailed properties of OLEDs as follows below. 3.5.1 Luminance L uminance is defined as the luminous flux per unit area and per unit solid angle (or luminous intensity per unit area). The unit is commonly expressed as cd/ m 2 ( = lm/sr m 2 or nit) and it contains the photopic response (compare with t he radiance W/sr m 2 which does not include the photopic response) In order to get the actual luminance in the OLED measurement, the measured photocurrent ( I det ) of the OLED ligh t emission is converted into the actual luminance ( L ) by multiplying a conversion factor ( ) which is obtained from the luminance meter measurement 3.5.2 Current E fficiency Current efficiency is also called luminance efficiency ( L ) because it means a ratio of luminance to the injected current to the OLED as shown in the below equation It can

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61 be simply obtained by measuring L J V characteristics of the OLED ( L is converted from the photocurrent measurement). The unit of L ca n be expressed as cd/A. 3.5.3 Luminous P ower E fficiency Luminous p ower efficiency ( P ) of the OLED can be obtained assuming the 2 D area emission source of the OLED as a Lambertian source as shown in Fig 3 12 The derivation of the P is shown in the equation below ; where A is the total luminous flux (lm) from the light source with a certain area (A) P is the electrical power (W) injected into the OLED, and the solid angle is derived for the hemispherical Lambertian area source. Figure 3 12 Schematic illustration of 2 D Lambertian source in area A and corresponding infinitesimal surface area ( dS ) to define the solid angle.

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62 Also, P of the OLED can be defined as a multiplication of the radiant ef ficacy (or wall plug efficiency) and the l uminous efficacy as defined in the below Therefore, high optical power with less electrical power and /or more overlap between the spectrum of the OLED and the photopic response will lead to the high power efficien cy. The unit of P is expressed as lm/W. W here, G( ) is the photopic response of the human eye, S( ) is the optical power of the OLED, I D is the device current injected into the OLED, V is the applied voltage bias to the OLED, 0 is the constant o f 683 lm/W, I 0 is the absolute optical power intensity at the peak, g( ) is the normalized photopic response of the human eye, s( ) is the normalized optical power spectrum of the OLED, I det is the photocurrent of the OLED measured by a photodetector, f is the geometric factor of the OLED measurement system, and R( ) is the responsivity of the photodetector.

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63 3.5.4 External Q uantum E fficienc y The external quantum efficiency ( EQE ) physically means a ratio of the number of photons coming out to the air from the OLED to the number of charge carriers injected into the OLED as defined in the below equation. The photopic response, G( ) is not considered in the EQE so the measure ment of EQE should be done with the careful choice of photodetector depend ant on the light emitting wavelength (for example, Si detector for visible wavelength emission, whereas InGaAs detector for infrared ( IR ) wavelength emission) The unit of EQE can be expressed as a percentage (%) by multiplying 100 times to the ratio of EQE W here q is an electric charge (1.602 10 19 coulombs), h is Planck s constant (6.626 10 34 J s) and c is a speed of the light (299,792,458 m/s). 3.6 Theoretical E fficiency Limits of O LED s There are several important parameters which generally affect the external quantum efficiency ( EQE ) of the OLED. As shown in the equations below the EQE can be increased by maximizing the internal quantum efficie ncy ( IQE ) and the outcoupling efficiency ( out ) separately The IQE is defined as the ratio of total number of photons generated in the OLED to the total number of electrons injected into the OLED. For achieving high IQE three major characteristics sh ould be considered

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64 First, the charge recombination efficiency ( R ) should be maximized by balancing the charge s inject ed from the anode and cathode, respectively. Balanced charge injection can maximiz e the exciton generation in the middle zone of the E ML and also minimiz e the possible nonradiative exciton polaron (excited charge carrier) quenching s 65 68 Second, both the singlet and triplet excitons should be exploited to maximize the exciton generation. The flu orescent emission can convert only singlet (25%) exciton s into photon s whereas the phosphorescent emission can convert singlet (25%) as well as triplet (75%) excitons into photon s leading to theoretical ly 100% photon conversion efficiency. 39,69 Third, t he photoluminescen t quantum yield (PLQY) of a light emitting molecule is another factor affecting IQE T he PLQY of organic dyes can be as high as over ~ 90% in a solution matrix, 70 but the solid state thin film PLQY reduces significantly due to strong intermolecular interaction s 71 So, higher PLQY can ultimately provide higher IQE but also the higher electroluminescent quantum yiel d of the emissive layer in the OLED should be optimized by alternatively choosing appropriate host guest systems and /or adjusting the guest emissive dopant concentration.

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65 Although 100% internal quantum efficiency can be obtained by tailor ing the OLED design, the external quantum efficiency of BOLED is typically limited to ~ 20% due to total internal reflection losses (TIRs) at multiple interfaces. 27,40 The OLED has a multi layered structure consisted of a metal reflector, organic layers, ITO, and a glass substrate. Hence, the isotropic dipole s generated in the emissive organic layer must go through the multiple interfaces in order to finally com e out to the air as shown in Fig 3 13 Figur e 3 13 R epresentative three different optical modes in bottom emitting OLEDs (BOLEDs). Light emitting dipoles are assumed to be generated in the middle of the organic layers and travel through multiple interfaces until they are finally coming out to the air; (1) is the air modes (~ 20%), (2) is the glass substrate modes (~ 30%), and (3) is the localized plasmonic and organic/ITO waveguiding modes (~ 50%) There are three different optical modes depend ing on the optical confinement mechanisms. The first mo de is the organic/ITO waveguiding modes where almost 50 ~ 60% of the generated photons are confined near the metal cathode and organic/ITO

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66 layers due to the strong localized electric field on the metal cathode and TIR s mainly at the interface between ITO a nd glas s substrate. 72 74 The second glass substrate mode is around 20 ~ 30%, where photons are trapped and lost in the thick glass substrate due to the TIR s at the glass/air interface. Therefore, it is believed tha t only around ~ 20% (1/2n 2 ) of the third air mode can contribute to the out in a planar structure BOLED system.

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67 CHAPTER 4 HIGH EFFICIENCY DEEP BLUE PHOSPHORESCENT OLEDS 4.1 I ntroduction Since the introduction of phosphorescent organic light emitting devices (PHOLEDs), 27 28,75 76 t here have been significant progresses in maximizing efficiencies of green and red PHOLEDs. 49,76 86 Green and red emitting phosphorescent organic dyes such as fac tris (phenylpyridine) iridium [ Ir(ppy) 3 ] and ir idium(III) bis (2 phenylquinolyl N,C 2 ) acetylacetonate ( PQIr ), for example, have low triplet energy ( T 1 ) levels of 2.4 eV and 2.1 eV, respectively 81,87 Hence, the efficient exciton confinement is possible for gre en and red emitters by using several different host, HTL, and ETL materials which also have low T 1 levels ( T 1 = 2.3 eV ~ 2.6 eV see Fig. 4 1. f or detailed molecular structures of these materials ). 85,88 93 Whereas, blue emitting phosphorescent emitters ( see Fig. 4 2 for the molecular structure s) such as iridium(III) bis[(4,6 difluorophenyl) pyridinato N C ]picolinate (F Irpic) 79,94 99 iridium(III) bis(4,6 difluo rophenylpyridinato ) 3 (trifluoromethyl) 5 ( pyridine 2 yl) 1,2,4 triazolate (FIrtaz), 34 iridium(III) bis(4,6 difluorophenylpyridinato) 5 (pyridine 2 yl) 1H tetrazolate (FIrN4), 34 and difluorophenylpyridinato)tetrakis(1 pyrazolyl)borate (FIr6) 33,100 have higher T 1 levels of 2.62 eV ~ 2.72 eV, 34,101 which are higher than t hose of host and charge transporting materials for green and red emitters Therefore, the exciton confinement for such blue emitters is very inefficient using general host and charge transporting materials for green and red emitters A lso the excitat ion of such large bandgap blue emitters typically requires high turn on voltages, leading to the low er power efficienc ies

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68 Figure 4 1 Most frequently used organic hole/electron transporting and host materials for green and red emitting PHOLEDs; NPB = N, N' b is(naphthalen 1 yl) N,N' bis(phenyl) benzidine NPD = N, N' b is(naphthalen 1 yl) N,N' bis(phenyl) 2,2' dimethylbenzidine TPD = N, N' b is(3 methylphenyl) N,N' bis(phenyl) benzidine TCTA = 4,4',4" t ris(carbazol 9 yl)triphenylamine CBP = 4,4' b is(carbazol 9 yl)biphenyl CDBP = 4,4' b is(carbazol 9 yl) 2,2' dimethylbiphenyl BCP = bathocuproine, BPhen = bathophenanthroline, TPBi = 2,2',2" (1,3,5 b enzinetriyl) tris(1 phenyl 1 H benzimidazole) There have been significant efficiency enhancements us ing a sky blue emission organic dye, FIrpic, which already achieved higher than EQE of 20% and P of 59 lm/W at 100cd/m 2 with low turn on voltage ( V T ) of 2.7V. 94,96,99 However, PHOLEDs based on a sky blue FIrpic d ye are not enough to generate genuine blue color which is necessary for the full color displays and white lights with high CRI. 95 As shown in the e lectroluminescent (EL) spectra in Fig. 4 3 FIrpic bas ed PHOLEDs have two major

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69 Figure 4 2 Molecular structures of blue phosphorescent emitters. FIrpic is a sky blue emitter, whereas FIrtaz, FIrN4, and FIr6 are deep blue emitters. vibronic peaks at 475 nm and 500 nm, leading to CIE coordinates of app roximately (0.17, 0.34), whereas FIr6 based devices show deep blue emissions with major vibronic peaks at 458 nm and 489 nm and CIE coordinates of (0.16, 0.26). 33,102 However, the PHOL E Ds based on the deep blue FIr 6 ( T 1 = 2.72 eV) 101 requires host and neighboring charge transporting layers with even much higher than T 1 of FIr6 in order to effectively confine triplet excitons on the electrophosphorescent dye molecules So, the most materials which are developed for FIrpic ( T 1 = 2.62 eV) 94 based PHOLEDs are not compatible for high efficiency deep blue PHOLEDs based on FIr6. Also note that the deeper blue emission of FIr6 based devices leads to an a pproximately 20% lower photopic sensitivity by the human eyes compared to that of FIrpic devices. Holmes et al previously demonstrated the maximum EQE of 12% and peak P of 14 lm/W at low luminances as shown in Fig. 4 4. 33 A wide band gap material

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70 Figure 4 3 Electroluminescent (EL ) spectra comparison between sky blue and deep blue PHOLEDs The FIrpic dye is used for the sky blue emission, whereas the FIr6 dye is used for the deep blue emissio n. p bis(triphenylsilyly)benzene (UGH2, T 1 = 3.5 eV) 101 was used as the host material for FIr6, and a dicarbazolyl 3,5 benzene (mCP, T 1 = 2.9 eV) 94 layer was inserted between the HTL and the EML to serve as the electron/exciton blocking layer (E/XBL). Although host and blocking materials with higher T 1 than that of FIr6 are used, the efficiencies were still not as high as the PHOLEDs based on the FIrpic. Therefore, it is nec essary to enhance the efficiencies of such deep blue PHOLEDs by employing new materials and devel oping new device structures Here, we demonstrate high external quantum and power efficienc ies of deep blue PHOLEDs by employing new HTL with high triplet energy optimized doping concentration of FIr6 dye in the p i n structure, and dual emissive layer ( D EML) structure with optimized EML thicknesses. As a result, the EQE was improved from (1 2 1) % to (1 7 1) % for such deep blue PHOLED s and t he peak P of (2 5 2) lm/W was

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71 achieved at 100 cd/m 2 with a low turn on voltage of 3. 2 V The EL spectrum and t he CIE coordinat es change d negligible compared to the previous results. 33 Figure 4 4 Quantum (circles) and power (squares) efficienc ies versus current density for the deep blue baseline PHOLEDs; device structure is ITO/NP B (40 nm)/mCP ( 15 nm)/UGH1 (closed symbols) or UGH2 (open symbols): 10% FIr6 (25 nm)/BCP (40 n m)/LiF (0.5 nm)/Al (50nm) OLEDs. Inset: EL at 10 mA/cm 2 originating solely from FIr6 in UGH1 (broken line) and UGH2 (solid line). (Reproduced with the permission from R. J. Holmes et al. Appl. Phys. Lett 83 3818 (2003). 33 4.2 E xperiment Glass substrates precoated with an indium tin oxide (ITO) anode (sheet resistance ) were degreased in detergent and de ionized water, and cleaned with ultrasonic baths of acetone and isopropanol consecutively for 15 minutes each. The

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7 2 substrates were then exposed to an ultraviolet ozone ambient for 15 minutes immediately before loading into a high vacuum thermal evaporation system (background pressure ~ 3 10 7 torr). All the organic and metal layers were deposited successively without breaking the vacuum. The molecular structures of organic materials used in C hapter 4 are show n in Fig. 4 5 Figure 4 5 Schematic molecular structures of all organic materials which are used in C hapter 4 ; hole injection layer (HIL), hole transporting layer (HTL), hosts and exciton /electron blocking layer (XBL), and electron transporting layer (ETL).

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73 Most organic materials are purchased from the Lumtec and directly used without further purification process. A ll device structures employed in C hapter 4 are illustrated in Fig. 4 6 (a), and the energy level diagram of the D EML p i n blue PHOLEDs is schematically shown in Fig. 4 6 (b) (energy levels are taken from the literature) 33,101,103 104 Figure 4 6 Schematic illustration of all device structures used in C hapter 4 and energy level diagram of D EML p i n structure ; single emissive layer (S EML) and dual emissive layer (D EML). Energy levels are taken from the literature. 33,101,103 104

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74 For conventional devices, a 40 nm thick bis[N (1 naphthyl) N phenyl a mino]bip henyl (NP B ) or 1,1 bis (di 4 tolylaminophenyl)cyclohexane (TAPC) along with a 40 nm thick BCP are used as the HTL and ETL, respectively W hereas for p i n devices, a 20 nm thick N, N' diphenyl N,N' bis(3 methylphenyl) [1,1' biphenyl] 4,4' diamine (MeO TP D) layer doped with 2 mol% tetrafluoro tetracyanoquinodimethane (F 4 TCNQ) and a 20 nm thick Li doped 4,7 diphenyl 1,10 phenanthroline (BPhen) layer (with a molar ratio of 1:1) were used as the p type hole injection 104 and n type electron injection 104 105 layer along with 20 nm thick undoped TAPC and BPhen layer s as the HTL and ETL, respectively. In the single EML (S EML) devices, on ly the UGH2 layer was doped with FIr6 with a concentration of 10 ~ 25% by weight whereas, i n the D EML devices, the mCP layer was also doped with FIr6 though at a much lower concentration than in UGH2. Finally, a 1 nm thick layer of LiF followed by a 50 n m thick layer of Al were deposited through in situ shadow masks as the cathode, forming active device areas of 4 mm 2 Luminance ( L ) current density ( J ) voltage ( V ) measurements were carried out in ambient using an Agilent 4155C semiconductor parameter analyzer and a calibrated Newport silicon detector. The luminance was calibrated using a Konica Minolta LS 100 luminance meter assuming Lambertian emission pattern. Electroluminescent (EL) spectra were taken using an Ocean Optics HR4000 high resolution s pectrometer. The luminous, power, and external quantum efficiencies ( L P and EQE respectively) were derived based on the recommended methods. 64 4.3 H igh T riplet E nergy H ole T ransporting M aterial In the previous report by Holmes et al. 33 a 15 nm thick mCP layer was inserted between the NPB (HTL) and UGH2 :FIr6 (EML) layers for the purpose of effective

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75 exciton and electron blocking. The LUMO energy (= 2.4 eV) and T 1 (= 2.9 eV) levels of the mCP are higher than those (LUMO = 3.1 eV and T 1 = 2.72 eV, respectively) of the FIr6 emitter in the UGH2 host by 1.1 eV and 0.18 eV. These energy barri ers should prevent electron injection and exciton diffusion from the EML to the HTL. However, the small amount of fluorescent NPB emission at the wavelength of 43 5 nm was observed as shown in Fig. 4 7 (a), 33,102 and much stronger NPB fluorescent emission was Figure 4 7 Electroluminescent ( EL ) spectra comparison in deep blue PHOLEDs with and without mCP layer; (a) with mCP layer (b) without mCP layer between the HTL and the UGH2:10%FIr6 Device structure is ITO/H TL(40 nm)/mCP(15 nm)/ UGH2 :10%FIr6(25 nm)/BCP(40 nm)/LiF(0.5 nm)/Al : HTL=NP B (solid line) or TAPC (dashed line) (Inset of (a) ) schematic energy level diagram of the devices. (Reproduced with the permission from Y. Zheng et al. ,Appl. Phys. Lett. 92 223301 (2008). 102

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76 demonstrated without the mCP blocking layer as shown in Fig. 4 7 (b), which implies significant amount of electron injection or triplet exciton diffusion into the NPB layer as illustrated in Fig. 4 8 (a) 102 When the TAPC is employed instead of the NPB with 0.3 eV higher LUMO (2.0 eV vs. 2.3 eV) and 0.57 eV higher T 1 (2.87 eV vs. 2.3 eV), clearly no TAPC fluorescent emission is appeared at wavelength of 360 nm as shown in Fig. 4 7 (b) implying complete electron and triplet exciton blocking by the TAPC eve n without the mCP layer as illustrated in Fig. 4 8 (b). Figure 4 8 Schematic illustration of better electron and triplet exciton blocking by using different hole transporting layers: (a) NPB and (b) TAPC. Even though it is very difficult to conclude wh ich process between electron blocking and exciton blocking is dominant by the TAPC, higher external quantum efficiency was obtained using the TAPC layer even with thinner EML and mCP thicknesses.

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77 Figure 4 9(a) shows the EQE improvement from (1 2 1) % to (1 5 1) % by using the TAPC as the HTL and P enhancement is also shown in Fig. 4 9(b). The P at 100 cd/m 2 which is the important luminance level for display s, was significantly improved from (8 1 ) lm/W to (13 1 ) l m/W for such deep blue PHOLED s without significant changes of EL spectrum and CIE coordinat es Figure 4 9 External quantum ( EQE ) and power ( P ) efficiencies as a function of luminance ( L ) for conventional deep blue PHOLEDs using either NP B 33 or TAPC 102 as the HTL. 4.4 S ingle E missive L ayer p i n S tructure The power efficiency of the OLEDs is decided by the external quantum effic iency as well as the device operating voltage. Most amorphous organic semiconductors have discontinuous transporting paths due to the weak VDW interaction so that there is a significant voltage drop even across very thin organic semiconductors. In order to enhance the conductivity ( = nq ) of the semiconductor, either charge carrier density ( n ) or mobility ( ) should be increased. While is the intrinsic character istic of the organic semiconductors as a function of electric filed and temperature n can be

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78 increased by the extrinsic doping with either electron donors for the n type semiconductor or electron acceptor for the p type semiconductor. In many p i n PHOLEDs, an organic dopant, F 4 TCNQ, is exceptionally used as a strong electron acceptor with very high electron affinity (EA) level of 5.24 eV, 106 107 well matching with the ionization potential of 5.1 eV for hole transporting materials such as 4,4',4'' tris (3 methylphenylphenylamino)triphenylamine (m MTDATA) or MeO TPD 104 105 W hereas highly reactive alkaline metals such as lithium (Li) or cesium (Cs) is commonly used as the electron donor. 108 111 Previously, there have be en many efforts to reduce the device operating voltage of PHOLEDs based on p i n structure s 104 105,112 Although very low turn on voltage could be really achieved with the optimized dop ing concentration in p type o r n type transporting layers most PHOLEDs also experienced significant quenching problems due to the huge amount of charge carriers in the p i n structure, leading to the decreased external quantum efficiency. Huge increased number of charge carriers in t he very thin organic layers induces more vigorous interactions between charge carriers and singlet/triplet excitons, leading to the annihilation loss and/or thermal agitation loss in organic molecules. 41,113 114 Here, similarly to the previous studies, the p i n structure is applied to the deep blue PHOLEDs in order to further increase the power efficiency. The 2mol% of F 4 TCNQ is doped into the MeO TPD as a p type injection layer, whereas L i metal ion is doped into the BPhen (with 1:1 molar ratio) as an n type injection layer. The S EML p i n deep blue PHOLED structure is described in Fig 4 6 (a), where the FIr6 doping concentration is varied from 10wt% to 25wt% in the UGH2 host.

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79 Figure 4 10 (a) compares the L J V characteristics between the conventional (i.e., without p and n doped charge injection layers) and the p i n S EML devices with 10wt% FIr6 doping concentration in UGH2 It can be seen that with the same FIr6 concentration (10 wt%) in the EML, the p i n device has higher current density than the conventional device at the same voltage. H o wever, a s shown in Fig. 4 10 (b) L of the p i n device showed similar or slightly lower efficiencies than that of conventional device, especially at low current injection ranges. T his low current efficiency might be attributed to the quenching effects and/or charge im balance between p /n doped layers, leading to the low internal quantum efficiency. Figure 4 10 Luminance ( L ) current density ( J ) voltage ( V ) and current efficiency ( L ) current density ( J ) characteristics between conventional and p i n structures for single emiss ive layer (S EML) deep blue PHOLEDs. The 10wt% FIr6 is doped in UGH2 host for both devices. A s the FIr6 doping concentration increases further from 10wt% to 25wt% both J and L at a given voltage are significantly increased as shown in Fig 4 11 (a) and the turn on voltage ( V T defined as the voltage corresponding to L = 0.1 cd/m 2 ) is reduced from V T = 4.4 V (10 wt%) to 3.7 V (25 wt%). Moreover, L of the p i n devices also increases with the increased FIr6 doping concentration, from a maximum of (24 2)

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80 cd/A for the 10 wt% device to a maximum of (30 2) cd/A for the 25 wt% device as shown in Fig. 4 11 (b) Figure 4 11 Luminance ( L ) curren t density ( J ) voltage ( V ) characteristics and current ( L ) and power ( P ) efficiencies vs. current density ( J ) for S EML p i n deep blue PHOLEDs The FIr6 doping concentration is varied from 10wt% to 25wt% in UGH2 host. This dependence is consistent with the previous report that charge transport in the UGH2:FIr6 EML is through FIr6 molecules. 33 A higher FIr6 concentration leads to better charge transport in the EML, which is necessary to reduce any charge built up at the interfaces between the charge transport layers and the EML as the incorporation of the p and n doped charge injection layers results in improved charge injection from the contacts into the charge transport layers. The combination of lower drive voltage and higher L in p i n devices with higher FIr6 concentrations lead s to a maximum P = (20 2) lm/W in the 25 wt% device. Although this only represents an approximately 10% increase from the maximum P of the conventional device (18 lm/W), the improvement is much more significant at higher luminance levels ( L 100 cd/m 2 ) which are more relevant to practical display or lighting applications. For example, P at 100 cd/m 2 is improved from 13 lm/W for the conventional device to 19 lm/W for the best p i n device,

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81 up by approximately 50%. The CIE coordinates of the p i n devi ces are maintained at (0.17, 0.29), which are slightly shifted from (0.16, 0.27) for the conventional device as shown in Fig. 4 1 2 Figure 4 1 2 Electroluminescent ( EL ) spectra difference between conventional and p i n S EML deep blue PHOLEDs 4 .5 D ual E missive L ayer p i n S tructure For deep blue PHOLEDs, it is believed that mCP and UGH2 have lower charge carrier mobilities compared to those of HTL and ETL, resulting in the most significant voltage drops in mCP and UGH2 layers In order to furth er enhance the power efficiency of deep blue PHOLEDs, t he thickness of mCP as an electron/exciton blocking layer could be reduced from 15 nm to 10 nm by employing the TAPC (instead of NPB) as the HTL, which c ould effectively b lock the electron insertion an d/or e xciton diffusion along with the thin mCP layer due to the high triplet energy (2.87 eV) and small electron affinity (2.0 eV) levels. 102

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82 Similarly, the thickness of the UGH2:FIr6 layer was reduced from the 20 nm to 15 nm in the S EML p i n deep blue PHOLEDs. As shown in Fig. 4 1 3 (a) the current injection was increa sed at the given voltage due to the reduced series resistance with the thinner UGH2:FIr6 layer however, there was a significant EQE drop so that even much lower power efficiency was obtained as shown in Fig. 4 1 3 (b). Figure 4 1 3 Current density ( J ) voltage ( V ) and power efficiency ( P ) current density ( J ) characteristics varying the UGH2 :FIr6 thickness for the S EML p i n deep blue PHOLEDs. The FIr6 doping concentration in UGH2 was 25wt% in both cases. This low EQE is attributed to the incomplete recombination of charge carriers in the thinner UGH2:FIr6 EML layer of the p i n structure The incomplete charge recombinatio n in the thin UGH2 EML may suggest that a significant portion of electrons enter the mCP layer, which may recombine with holes injected from the TAPC HTL and form excitons. In that case, doping FIr6 in the mCP layer in addition to in the UGH2 layer to for m the D EML structure would maximize the generation of excitons on the FIr6 dye molecules.

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83 The D EML structure has been previously used to demonstrate green PHOLEDs with external quantum efficiency of 19% and a drive voltage of only 2.65 V at 100 cd/m 2 104 We fabricated D EML p i n deep blue PHOLEDs with 7 nm thick mCP layer doped with 4 wt% FIr6. As shown in Fig. 4 1 4 (a), the D EML devices have slightly higher current den sities than the S EML devices with the same UGH2 thicknesses, mostly due to the slightly thinner mCP layer used (7 nm vs. 10 nm). The FIr6 doping concentration in mCP does not affect the J V characteristics appreciably and there was no further efficiency i mprovement with more than 4 wt% FIr6 in mCP. With a 20 nm thick UGH2:FIr6 (25 wt%) layer, the D EML and S EML devices have very similar P as shown in Fig. 4 1 4 (b). Figure 4 14 Current density ( J ) voltage ( V ) and (b) power efficiency ( P ) curren t density ( J ) characteristics with either 20 or 15 nm thick UGH2 layers for S EML and D EML p i n deep blue PHOLEDs However, very differently from the S EML devices, the D EML device with a 15 nm thick UGH2:FIr6 layer has a significantly higher P than th e device with a 20nm thick UGH2:FIr6 layer. This clearly demonstrates the effectiveness of the D EML structure in harvesting excitons generated in the two emissive layers. The better D EML device here

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84 possesses a low V T = 3.2 V and a maximum L = 35 cd/A, corresponding to an external quantum efficiency of 17%. The maximum power efficiency of this device is P = (25 2) lm/W, achieved at L 100 cd/m 2 which is only slightly reduced to P = (20 2) lm/W at L = 1,000 cd/m 2 It needs to be mentioned that t he effectiveness of the D EML structure is enabled by the use of TAPC as the HTL, which as we discussed previously can more effectively block electrons and confine excitons than using NP B as the HTL. 102 4.6 S ummary Figure 4 1 5 (a) and (b) show a comparison of the power and external quantum efficienc ies, respectively, as a function of luminance for all significant devices described in Chapter 4. The comparison of the turn on voltage, efficiencies, and CIE coordinates of these devices is also summarized in Table 4 1 The p i n devices not only possess higher peak P but als o the luminance corresponding to the peak P shifts toward higher luminances, which are very important for use in displays ( L ~ 100 cd/m 2 ) and lighting s ( L ~ 1000 cd/m 2 ). At these luminances, P of the best D EML p i n is more than three times higher than the conventional device with an NP B HTL, 33 or approximately twice or more than that of the conventional device with a TAPC HTL. The EL spectra do not display appreciable changes among these four devices, and the CIE coordinates of these devices are nearly identical (see Table 4 1 ). In co nclusion, w e have demonstrated significantly enhanced power efficiencies especially at high luminances for deep blue PHOLEDs by employing a D EML p i n device structure. The improved performance is attributed to the use of p and n charge injection layers to reduce drive voltage, the increased FIr6 concentration in UGH2 to

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85 maximize charge recombination and reduce drive voltage, and a FIr6 doped mCP layer to maximize exciton generation by serving as the second emissive layer. Power efficiencies of (25 2) lm/W at 100 cd/m 2 and (20 2) lm/W at 1,000 cd/m 2 are demonstrated with a turn on voltage of only 3.2 V and CIE coordinates maintained at (0.16, 0.28). Figure 4 1 5 Summarized p ower ( P ) and external quantum ( EQE ) efficienc ies as a function of luminan ce ( L ) for major parameters which contributed to the efficiency enhancement in deep blue PHOLEDs.

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86 Table 4 1. Comparison of the turn on voltage ( V T ), external quantum efficiency ( EQE ), luminous efficiency ( L ), power efficiency ( P ), and CIE coordinates of four different devices See text for detailed device structures. Device V T at L =0.1 cd/m 2 EQE at peak P at ( peak, 100cd/m 2 1000cd/m 2 ) CIE (x, y) at 1mA/cm 2 Conventional NP B 4.5 V 12 1 % ( 14, 8, 6 1 ) lm/W (0.16, 0.28) Conventional TAPC 4.1 V 15 1 % ( 18, 13, 8 1 ) lm/W (0.16, 0.27) S EML p i n 3.7 V 15 1 % ( 20, 19, 13 1 ) lm/W (0.17, 0.29) D EML p i n 3.2 V 17 1 % ( 25, 25, 20 2 ) lm/W (0.16, 0.28)

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87 CHAPT ER 5 ELECTRON INJECTION A ND TRANSPORT MATERIA L STUDY ON DEEP B LUE PHOSPHORE SCENT OLEDS EFFICIENCY 5 .1 Introduction In C hapter 4 several strategies to enhance the external quantum and power efficiencies for deep blue PHOLEDs have been investigated. The significant amount of EQE enhancement was originated from the employment of high triplet energy level TAPC HTL ( T 1 = 2.87 eV) and the D EML structure using UGH2 and m CP two wide bandgap host s. Most of electron s and triplet excitons could be blocked by the TAPC HTL, and the exc iton formation could be maximized in the D EML. Also, the p i n structure was optimized with the D EML structure to reduce the device operating voltage s while maintaining the high external quantum efficiency As a result, the peak EQE = ( 1 7 1 ) % and P = ( 25 2) lm/W w ere demonstrated for such deep blue PHOLEDs based on the FIr6 dye. In C hapter 5 the effect of the electron transport and injection layers (ETL and EIL, respectively) on the performances of deep blue PHOLEDs is investigated The ETL san dwiched between the EML and the cathode, has many different purposes such as an electron transporting, hole blocking, exciton blocking, and efficient electron injection if directly contacted to the cathode. Whereas the EIL can be intentionally i nserted be tween the ETL and cathode in order to m ake a better electrical contact and/or increase the number of electron charge carriers First, three major electron transporting materials BCP, BPhen, and tris[3 (3 pyridyl)mesityl]borane (3TPYMB) 114 117 are compared for the performance of deep blue PHOLEDs. Second, the doping of alkaline metal s such as Li or Cs into the above ETLs is studied to minimize the device drive voltage s and quenching problems, which are

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88 very import ant for achieving a high power efficiency p i n PHOLEDs We show that EQE = (20 1 ) % and maximum P = (36 2) lm/W can be achieved for such deep blue PHOLEDs with 3TPYMB as the electron transport material and 3TPYMB:Cs as the electron injection layer. The se high external quantum and power efficiencies are attributed to the very high triplet energy level of 3TPYMB, T 1 = 2.95 eV 117 and the significantly increased conductivit y of the 3TPY MB:Cs layer. 5. 2 Experiment ) were degreased in detergent and de ionized water, and cleaned with ultrasonic baths of acetone and isopropanol consecutively for 15 minutes each. The substrates were then exposed to an ultraviolet ozone ambient for 15 minutes immediately before being loaded into a high vacuum thermal evaporation system (background pressure ~ 3 10 7 torr), where all the organic and metal layers were deposited successively without breaking the vacuum. T he schematic energy level diagram of the D EML p i n deep blue PHOLEDs and chemical structures of three electron transporting materials are shown in Fig. 5 1 (energy levels are taken from the literature) 33,101,103 104 The D EML structure composed of 4 wt% FIr6 doped mCP (7 nm thick) and 25 wt% FIr6 doped UGH2 (15 nm thick) layers was used to maximize the exciton generation within the broadened charge recombination zone 118 TAPC serves as the hole transporting/electron blocking layer (HTL), whereas an undo ped layer of BCP, BPhen, or 3TPYMB was used as the electron transporting/hole blocking layer (ETL). The thicknesses of both HTL and ETL were x nm with x varying from 5 to 20. A (40 x ) nm thick MeO TPD layer doped with 2 mol%

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89 Figure 5 1. Schematic energ y level diagram of the dual emissive layer (D EML) p i n phosphorescent organic light emitting devices (PHOLEDs) and chemical structures of three different electron transporting materials used (BCP, BPhen, and 3TPYMB). F 4 TCNQ was used as the p type hole i njection layer ( HIL ), 104 whereas Li or Cs was doped into the ETL material to serve as the n type electron injection layer (EIL), whose thi ckness was also (40 x p i n type, devices were also fabricated, which did not contain the p and n doped charge injection layers. The thicknesses of HTL and ETL in the conventional devices were both 40 nm. For the alkaline doping of the electron transport materials, lithium metal was used as the Li source, whereas cesium carbonate (Cs 2 CO 3 ), which decomposes during thermal evaporation to generate cesium atoms (2Cs 2 CO 3 2 + 2CO 2 ) 119 was used as the Cs source. The cathode consisting of a 0.5 nm thick layer of LiF followed by a 50 nm thick Al was deposited through an in situ shadow mask, forming

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90 active device area of 4 mm 2 3TPYMB, mCP, UGH2, and FIr6 were purchased from Luminescence Technology Corp., and used as obtained without further purification. Luminance ( L ) current density ( J ) voltage ( V ) measurements were carried out in ambient using an Agilent 4155C semiconductor parameter analyzer and a calibrated Newport silicon detector. The luminance was calibrated using a Konica Minolta LS 100 luminance meter assu ming Lambertian emission pattern. Electroluminescent (EL) spectra were taken using an Ocean Optics HR4000 high resolution spectrometer. The luminous, power, and external quantum efficiencies ( L P and EQE respectively) were derived based on the recommended methods 64 The conductivities of nominally undoped or alkaline doped electron transport materials were obtained from the ohmic regions of the J V characteristics of 100 nm thick films sandwiched between two Al electrodes. 5 .3 Electron T ransporting M aterial for D eep B lue PHOLEDs BCP and BPhen are the most commonly used electron transporting materials in PHOLEDs due to their relatively high triplet energies ( T 1 = 2.5 eV) and good electron transporting properties ( electron mobility, e = 10 6 cm 2 /V s and 10 4 cm 2 /V s for BCP and BPhen, respectively ). 114,120 However, lower T 1 = 2.5 eV for BCP and BPhen than T 1 = 2.72 eV for FIr6 is not sufficient to effectively block the triplet exciton di ffusi on from the EML to the ETL, resulting in a low internal quantum efficiency. Hence, it is suggested that the employment of novel electron transporting material which has higher triplet energy than that of FIr6 and at least similar electron mobility to those of BCP and BPhen provide a higher internal quantum efficiency for deep blue PHOLEDs. R e cently, Tanaka et al. introduced a new electron transporting material, 3TPYMB as shown in Fig. 5 1 117 Due to the vacant p orbital of the b oron atom and electron

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91 withdrawing pyridine units, which mak e a high electron affinity pro perty, 3TPYMB acts like a strong electron acceptor Also, wide bandgap can be designed by introducing the twisted molecular structure with mesityl group, leading to the high triplet energy level. 117 Previously, t he application of 3TPYMB as the ETL for FIrpic based blue PHOLEDs already showed EQE = 21% and P = 39 lm/ W at L = 100 cd/m 2 along with other high triplet energy host and HTL materials implying nearly 100% internal quantum efficiency. 96 Here, 3TPYMB is similarly employed as the ETL in order to maximize the triplet exciton confinement for deep blue PHOLEDs, and compared with the PHOLEDs using the BCP and BPhen as the ETL. Figure 5 2 illustrates the energy level diagram of the D EML conventional deep blue PHOLDs used here, and HOMO and LUMO levels are varied depend o n the used ETL materials. Figure 5 2 Schematic energy level diagram of the D EML conventional deep blue PHOLEDs. Three different electron transporting materials (ETLs) are used (BCP, BPhen, and 3TPYMB). Energy levels are taken from the literatures. 33,101 J V characteristics using three different ETLs showed a consistency depend on their electron mobilities assuming very similar electron injections into the ETL from the

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92 LiF/Al cathode. The e of BPhen is around 10 4 cm 2 /V s, two orders of magnitude higher than that ( e = 10 6 cm 2 /V s) of BCP, and the e of 3TPYMB is ~ 10 5 cm 2 /V s which is in between those of BPhen and BCP. As a result, t he current injection behavior using the same device st ructure except the ETL domina ted by the electron mobilit ies as shown in Fig 5 3(a). Figure 5 3 Current density ( J ) voltage ( V ) and external quantum efficiency ( EQE ) luminance ( L ) characteristics for the D EML conventional deep blue PHOLEDs with different ETLs Figure 5 3(b) compar es EQE of three conventional D EML deep blue PHOLEDs with different ETLs. A maximum EQE of 14% wa s achieved using BCP as the ETL, whereas higher EQE of 16% could be obtained with the BPhen instead of BCP. Furthermore when the 3TPYMB wa s used as the ETL EQE wa s substantially higher than in the other two devices, and reache d the maximum of 20%. As the light extraction efficiency in these planar type OLEDs is generally believed to be 20% (although there have been rece nt evidences that it could be slightly higher), 27,121 this suggests that the internal quantum efficiency ( IQE ) could be very close to 100% with 3TPYMB as the ETL

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93 Considering the hole mobility of TAPC ( h = 10 2 c m 2 /V s), 122 there might be significant charge imbalances, generating the charge recombination zones in the EML close to the ETLs. Therefore, it is believed that the significant EQE en hancement is attributed to the different properties of ETLs. Among the most important properties for an ETL in a PHOLED are the electron mobility ( e ), the energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respe ctively), and the triplet energy. The electron mobility affects the charge balance in the device, hence the location of the charge recombination zone. The HOMO and LUMO energies affect the hole blocking and electron injection properties, respectively, at t he ETL/EML interface, whereas the triplet energy of the ETL significantly impacts the exciton confinement within the EML. As BCP and BPhen have similar HOMO/LUMO (6.5/3.0 eV) 116 and T 1 (2.5 eV) 114 115 the higher EQE with BPhen c ompared to that with BCP should be attributed to the more balanced charge injection due to the higher e of BPhen than that of BCP (~ 10 4 cm 2 /V s vs. ~ 10 6 cm 2 /V s) 116,123 (the hole mobility of the TAPC HTL is ~ 10 2 cm 2 /V s) 122 However, only charge balance cannot explain much higher EQE with 3TPYMB because the electron mobility of 3TPYMB ( e ~ 10 5 cm 2 /V s) 117 is approximately one order of magnitude lower than that of BPhen. Hence, much higher external quantum efficienc y with 3TPYMB ETL should be attributed to the much higher T 1 = 2.95 eV of 3TPYMB 117 enabling a better confinement of triplet exciton in the EML, although the 0.3 e V lower HOMO of 3TPYMB (6.8 eV) than that of BCP/BPhen (6.5 eV) also results in more effective hole blocking at the EML/ETL interface. 96 As shown in Fig. 5 4(a), t he power efficiencies using BPhen or 3TPYMB as the ETL significantly en hanced over 20 lm/W at 100 cd/m 2 mainly due to the much

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94 lower drive voltage with BPhen or better triplet exciton confinement wi th the 3TPYMB, maximizing up to 30 lm/W at 1 cd/m 2 with 3TPYMB. A lthough there we re some minor differences in the relative intensities of these vibronic peaks possibly due to the different optical properties of the three ETLs and the slightly different pos ition of the recombination zone in these three devices t he EL spectra of these three devices ( measured at 1 mA/cm 2 ) we re very similar exhibiting multiple vibronic peaks and the highest deep blue color purity with CIE coordinates of (0.15, 0.25) wa s obtai ned using 3TPYMB ETL as shown in Fig. 5 4(b) Figure 5 4 P ower efficiency ( P ) vs. luminance ( L ) and EL spectra for the D EML conventional deep blue PHOLEDs using different ETLs. 5. 4 Electron I njection L ayer with A lkaline M etal s Similar to the inorganic dopants as an electron donor ( n type) or acceptor ( p type), either organic do pant or alkaline metal can be dispersed into the organic charge transporting material s As illustrated in Fig. 5 5, electrons can be donate d to the LUMO level of the host molecule from the HOMO level of the n type dopant, or electron s can be extracted from the HOMO level of the host (leaving behind the holes in the host) to

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95 th e LUMO level of the p type dopant. Hence, the energy level alignment between the host and the dopant is well matched for efficient doping process. Figure 5 5 Schematic illustration for p type (left) and n type (right) doping mechanisms in the organic charge transporting semiconductors The F 4 TCNQ is a strong electron acceptor which is well known as a p type organic dopant. As shown in Fig. 5 6, t he measured conductivity ( ) of pu re MeO TPD wa s around 10 10 S/cm, whereas the wa s increased significantly up to ~ 2.6 10 6 S/cm with 2mol% F 4 TCNQ in MeO TPD Further F 4 TCNQ doping in MeO TPD did not proportionally increase the reaching around 3.9 10 5 S/cm with 9mol% F 4 TCNQ. As opposed to the stable organic p type dopant, it is difficult to find a stable organic n type dopant. For the efficient n type dop ing the HOMO level of the dopant should be energetically close to the LUMO level of the host material, making the dopant

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96 e asily oxidize in the air environment. Instead of the unstable organic dopant, alkali metals such as Li and Cs have been frequently used to achieve efficient n type doping. Figure 5 6 Conductivities ( ) as a function of F 4 TCNQ doping concentration into MeO TPD host molecule. 5 4 .1 Thin I nter layer for E fficient E lectron I njection The most commonly used alkali metal compound as an electron injection inter layer is the LiF. 124 125 H igh temperature condition during the Al metal deposition on top of ETL/LiF layer will cause the chemical reaction between Al and LiF, dissociating the Li + ion which then diffus e s into the ETL as an electron donor (ETL / LiF + Al ETL / 3 Li + + AlF 3 ) 125 127 Similarly, two Cs ion compounds (Cs 2 CO 3 or CsCl) we re inserted between

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97 BPhen and Al as an electron injection interlayer as shown in Fig. 5 7 (a), and compared with LiF for the current injection b ehavior. The different work functions of Li and Cs metals are also shown in the figure for the reference. As shown in Fig 5 7(b), the current injection behavior using the Cs ion compound interlayer showed slightly better than that using a LiF interlayer. Figure 5 7 Schematic energy level diagram for different electron injection interlayers and their current injection behavior s in OLEDs. The work functions of Li and Cs metals are described for a reference Electron injections using Cs ion compounds show ed slightly better than using LiF interlayer.

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98 In the case of CsCl compound, similar to the LiF, the chemical reaction of ( BPhen + CsCl + Al BPhen + 3 Cs + + AlCl 3 ) can provide active Cs + ion to diffuse into the BPhen layer donat ing electrons to the BP hen acceptor. Whereas, using a Cs 2 CO 3 it i s suggested that the chemical reaction of ( BPhen + Cs 2 CO 3 B P hen + Cs + + CsO + CO 2 ) provides a strong CsO junction path at the interface between Al and BPhen for an efficient electron injection. 111,128 129 Although further investigation is needed to find out the better electron injection mechanism using Cs ion compound rather than LiF via such as x ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectro scopy (UPS) it is shown here that the Cs ionic compound can be also used as an efficient n type electron injection donor leading to the better power efficiency than using a common LiF interlayer as long as there is no decrease in internal quantum efficie ncy 5 4 .2 n T ype D oped L ayer Instead of using a thin alkaline metal interlayer, direct doping into the organic electron transporting material can significantly enhance the electron injection. As shown in Fig. 5 8(a), pure alkaline metal (Li) or alka line metal compound (LiF or Cs 2 CO 3 ) was coevaporated into the BPhen layer as an electron injection layer. 130 The doping concentration was kept at 5 wt% for all dopants, and there was no additional interlayer deposition between BPhen:dopant layer and Al cathode. The corresponding current injection behaviors using different n type dopants were compared with those of using an interlayer (LiF or Cs 2 CO 3 ) as shown in Fig. 5 8(b). First of all, the current injection using the EIL based on LiF dopi ng in BPhen showed lower current injection rather than a LiF interlayer, whereas the use of pure Li doping into BPhen showed nearly twice higher current injection than using a LiF interlayer. This is probably attributed to the how well Li

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99 Figure 5 8 Schematic energy level diagram for the doped BPhen layer as an electron injection layer and their current injection behavior using different dopants. All dopants are coevaporated into the BPhen with 5wt% amount. ion is dissociated from the LiF, and donate the electron to the BPhen. For the LiF doping into the BPhen, it is less probable that Li ion is able to be separated from the strongly bonded LiF compound leading to a low current injection behavior, whereas LiF can be highly reactive to the hot Al meta l deposition so that LiF monolayer between BPhen and Al cathode can provide a better electron injection behavior. Instead, i n the case of pure Li doping, highly reactive Li ion can be randomly dispersed in the BPhen

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100 so that it can well donate electrons to the BPhen molecule s increasing the electron carrier density. Cesium carbonate (Cs 2 CO 3 ) compound ha s been largely used either as an efficient electron injection interlayer or dopant, 108 109,111 and its effectivene ss as an interlayer h as been characterized by XPS and UPS by Huang et al, 108 suggesting better electron injection can be achieved by generating a strong bonding of Cs O Al with the low work fun ction of Cs metal. As shown in F i g. 5 8(b), better electron injection was observed using a Cs 2 CO 3 interlayer than using a LiF interlayer, probably showing a consistency with the assumption of the lower work function of Cs metal (2.1 eV vs. 2.9 eV for Cs vs Li, respectively) as well as a strong bonding through the oxygen between Cs and Al. Furthermore, Cs 2 CO 3 doping into the BPhen showed a significantly enhanced current injection behavior than any other dopants or interlayer s Although more accurate XPS and UPS studies are required to investigate the exact mechanism, a lot lower electron injection barrier, tunneling injection, and increased charge carrier density could attribute to the much higher current injection using Cs 2 CO 3 as a n type dopant 5. 5 Electron I njection L ayer for D eep B lue PHOLEDs It has been previously shown that the operating voltages of the deep blue D EML PHOLEDs can be significantly reduced by employing the p i n device structure that incorporates highly conductive p and n doped charge injection layers to reduce the total resistance of the organic layers and enhance the charge injection from the electrodes. 93,118 In that work BPhen was dope d with Li as the n type EIL and a thin, n ominally undoped BPhen layer was sandwiched between the doped BPhen layer and the EMLs as the ETL. Here the effect s of different ETLs and alkaline metal (Li or Cs)

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101 dop ed EIL s are investigated in order to improve the performance of the p i n deep blue PHOL EDs 5 5 1 E ffect of A lkaline M etal D oping into B Phen In order to further reduce the operating voltage f or devices with BPhen:Li as the EIL the layer thickness of the undoped BPhen ETL as well as the TAPC HTL is reduced from 20 nm to 10 nm as shown in Fig. 5 9 (a). T he current injection at a given voltage ( V > 4V) is improved by approximately an order of magnitude with the reduced series resistance of thicker doped EIL However, as shown in Fig. 5 9 (b), L wa s drastically reduced from 35 cd/A to 11 c d/A. W hen the Cs 2 CO 3 wa s used as the dopant in BPhen with the same structure except the dopant the PHOLED showed slightly Figure 5 9 Schematic device structure for D EML p i n deep blue PHOLEDs and its current efficiency ( L ) characteristic as a funct ion of current density ( J ). Pure 10 nm thick TAPC and BPhen layers are used for ETL buffer layer to prevent dopant diffusion to EML. Either Li or Cs 2 CO 3 was doped in BPhen layer with 5wt%, respectively. Significantly lower efficiency is observed using a Li dopant, due to the Li ion diffusion through the 10 nm thick pure BPhen layer.

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102 higher current injection than using a Li as shown in Table 5 1 This could be attributed to the difference in the electrical conductivity of the doped EILs. As summarized in Tab le 5 1 the conductivity of BPhen:Cs (with a molar ratio of 1:0.2) is measured to be = (6 2) 10 6 S/cm, approximately doubling that of BPhen:Li (molar ratio = 1:1), although both are approximately four orders of magnitude higher than that of undoped BPhen. Table 5 1. Conductivities of pure and alkaline metal doped BPhen and 3TPYMB films ETL host Dopant Molar ratio (ETL:Dopant) Conductivity (S/cm) BPhen N one ( 4 1 ) 10 10 Li 1 : 1 (3 1 ) 10 6 Cs 2 CO 3 1 : 0.2 (6 2) 10 6 3TPYMB N one (2 1 ) 10 13 Li 1 : 4 (2 1 ) 10 6 Cs 2 CO 3 1 : 0.3 (4 2) 10 6 More importantly, however, as the buffer layer thicknesses (both pure TAPC and BPhen layers) are reduced from 20 nm to 10 nm and further 5 nm, the PHOLEDs with the Cs 2 CO 3 dopant sh owed a significant efficiency drop with a 5 nm thick buffer layer thickness, which is different with using Li dopant as shown in Fig. 5 10(a). The e fficiency drop for very thin ETLs can be attributed to the diffusion of the alkaline

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103 dopants from the EIL to the EBL/EML interface or even into the EML where they act as luminescence quenching centers for excitons in the EML. The results here also suggest that Cs has a lower diffusivity than Li, consistent with the apparent difference in their atomic sizes 109 Figure 5 10 Current efficiency ( L ) vs. current density ( J ) and luminance ( L ) current density ( J ) voltage ( V ) characteristics for the D EML p i n deep blue PHOLEDs with different ETL /EIL structures; where b uffer layer ( x nm) as an ETL and Cs 2 CO 3 doped BPhen layer (40 x nm) as an EI L were used Using a Cs 2 CO 3 dopant, w hile the PHOLEDs with 10 nm thick buffer layer has almost identical L as the PHOLED with 20 nm thick buffer layer, the thinner undoped HTL/ETL leads to a reduction of device operating voltage as shown in Fig, 5 10(b) therefore an increase in P by approximately 15%, from a maximum of 25 lm/W to 29 lm/W (also see the comparison of device parameters summarized in Table 5 2 ). The EL spectra of Li and Cs doped p i n PHOLEDs are slightly different, and the CIE coordinates are shifted from (0.16, 0.28) using a Li dopant to (0.18, 0.30) using a Cs 2 CO 3 dopant possibly due to the different optical properties of BPhen:Li and BPhen: Cs 2 CO 3 and/or shift of recombination zone toward HTL with a higher conductivity of BPhen: Cs 2 CO 3. 131

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104 Table 5 2 Comparison of the turn on voltage ( V T defined as the drive voltage at 0.1 cd/m 2 ), drive voltage at 100 cd/m 2 ( V 100 ), external quantum efficiency ( EQE ), luminous efficiency ( L ), power efficiency ( P ), and CIE coordinates of various devices. See text for detailed device structures. ETL EIL V T V 100 (V) EQE (%) (at peak) L (cd/A) (at peak) P (lm/W) (at peak, 100cd/m 2 ) CIE (x, y) (at 1mA/cm 2 ) BPh en (40 nm) N one 3.4, 4.5 16 1 30 2 25, 21 2 (0.19, 0.27) BPhen (20 nm) BPhen:Li (20 nm) 3.2, 4.3 17 1 35 2 25, 25 2 (0.16, 0.28) BPhen (10 nm) BPhen:Cs 2 CO 3 (30 nm) 3.1, 3.7 17 1 35 2 29, 28 2 (0.18, 0.30) 3TPYMB (40nm) N one 3.7, 4.7 20 1 36 2 30, 23 2 (0.15, 0.25) 3TPYMB (20 nm) 3TPYMB:Li (20 nm) 3.3, 4.4 20 1 41 2 32, 28 2 (0.16, 0.27) 3TPYMB (10 nm) 3TPYMB:Cs 2 CO 3 (30 nm) 3.0, 3.8 20 1 41 2 36, 32 2 (0.16, 0.28) 5 .5.2 E ffect of A lkaline M etal D oping into 3TPYMB To understand the effect of alkaline metal doped 3TPYMB EILs on device efficiencies, two p i n devices were fabricated with Li or Cs 2 CO 3 doped 3TPYMB EIL as well as a p i i device with a 40 nm thick undoped 3TPYMB ETL but no EIL. Similar to BPhen based dev ices, a 20 nm thick undoped 3TPYMB ETL is used for Li doped EIL, which is reduced to 10 nm with Cs 2 CO 3 doping. T he electrical conductivities of undoped and alkaline metal doped 3TPYMB films were also investigated As listed in Table 5 1 while the conducti vity of pure 3TPYMB, = 2 10 13 S/cm, is much lower than that of pure BPhen ( = 4 10 10 S/cm), the conductivities of Li or Cs 2 CO 3 doped 3TPYMB are very similar to those of doped BPhen films, on the order of 10 6 S/cm. Although a

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105 complete study of the doping concentration dependence has not been described here it appears that Cs 2 CO 3 doped 3TPYMB films can also achieve approximately twice higher conductivities than the Li doped films. It is apparent from Fig. 5 11 (a) that the device with Cs 2 CO 3 has th e best charge injection due to the high conductivity of 3TPYMB: Cs 2 CO 3 and the very thin undoped ETL layer. The external quantum efficiencies of these three devices, however, are nearly identical over a wide range of luminances (up to L = 10 4 c d /m 2 ) with ma ximum values at EQE = 20%, as shown in Fig. 5 11 (b). Figure 5 1 1. Current density ( J ) voltage ( V ), and power ( P ) and external quantum ( EQE ) efficiencies vs. luminance ( L ) for the D EML deep blue PHOLEDs with different 3TPYMB ETL/EIL structures. Pu re 3TPYMB is used for the ETL, and alkaline metals (Li or Cs 2 CO 3 ) is used as a dopant for the EIL. 20 nm thick ETL is deposited with 20 nm thick Li doped EIL, whereas 10 nm thick ETL is deposited with 30 nm thick Cs 2 CO 3 doped EIL. This suggests that the apparent difference in charge balance for these three devices does not affect exciton generation/recombination appreciably, which is in fact consistent with the superior exciton confinement/charge blocking provided by the TAPC and 3TPYMB charge transport l ayers. The power efficiencies of these three 3TPYMB based devices, however, do show appreciable differences due to the differences in device

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106 operating voltages. The d evice with Cs 2 CO 3 as a dopant especially has a very low turn on voltage of 3.0 V, and need s a bias of 3.8 and 4.3 V to achieve a luminance of 100 cd/m 2 and 1,000 cd/m 2 respectively. Based on the high external quantum efficiency and low device operating voltage, the power efficiencies of D EML p i n deep blue PHOLEDs using 3TPYMB ETL and Cs 2 CO 3 doped EIL were achieved up to 36 lm/W and 32 lm/W at peak and 100 cd/m 2 as shown in Fig. 5 11 (b). The CIE coordinates did not show appreciable changes keeping at (0.16, 0.28) for all three devices. 5. 6 A pplication of M acrolens to D eep B lue PHOLEDs Note that typically for a planar structure OLED, only about 20% of the overall emission can escape the device in the forward viewing directions, while the rest is trapped in the glass substrate and in the ITO and organic layers as waveguiding modes 27,121 While there have been active research in enhancing the light extraction efficiencies in OLEDs 132 133 here a near ly hemispherical lens (BK 7 plano convex lens, diameter 25.4 mm and height 9.5 mm) was simply attached to the glass substrate of the D EML p i n deep blue PHOLED using a refractive index matching gel (n=1.517) as shown in Fig. 5 12(a) Attaching the macrolens to the glass substrate of the PHOLED, where the size of the macrolens should be larger than the light emitting area of the PHOLED, should be able to extract all the substrate mode s to the air mode s leading to the much enhanced external quantum efficiency. As shown in Fig. 5 12 (b), attaching the macrolens to the P HOLED le d to the maximum EQE = (34 2) %, or approximately 70% enhanced outcoupling efficiency enhancement, compared to the device without the macrolens, which is in general agreement with previous reports 134 The power efficiency of the D EML p i n deep blue PHOLED with the macro lens reache d the peak P = (61 4) lm/W

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107 at L 10 cd/m 2 slightly reducing to (57 3) lm/W at L = 100 cd/m 2 and (44 2) lm/W at L = 1,000 cd/m 2 With more complicated optical designs to achieving 2.3 times enhancement in light extraction 132 power efficiencies up to 80 lm/W could be achieved in the FIr6 based deep blue PHOLEDs. Figure 5 1 2 Schematic illustration of the D EML p i n deep blue PHOLED with a macrolens on the surface of the gla ss substrate and their efficiency comparison ; (a) Dimensions are not scaled. There should be no total internal reflection with the attachment of macrolens as long as the refractive indices are exactly same between the glass and the macrolens. Hence, most o f glass mode (~ 20%) should be further extracted into the air mode, eventually enhancing the theoretical outcoupling efficiency limit up to ~ 40%. (b) P ower ( P ) and external quantum ( EQE ) efficiencies as a function of luminance ( L ) for the D EML deep blu e PHOLEDs with and without the macrolens on the glass substrate. N e arly 70% enhanced outcoupling efficiency is confirmed with the attachment of the macrolens on the PHOLED using an integrating sphere system. 5 7 Summary It has been shown that the efficien cies of FIr6 based deep blue PHOLEDs significantly depend on the properties of the electron transport and injection materials. A

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108 maximum EQE = 20% was achieved using an ETL of 3TPYMB, substantially higher than those of similar devices with BCP or BPhen as the ETL, respectively. This is attributed to the nearly perfect charge and exciton confinement provided by 3TPYMB as the ETL and the cou nterpart TAPC as the HTL It was also show n that alkaline metal doping (Li and Cs 2 CO 3 ) in the 3TPYMB ETL can effectively increase its conductivity by several orders of magnitude, similar to the effect in the more widely studied BPhen ETL. Incorporation of such doped layers in p i n devices led to the significantly improved charge injection and lower device driv ing voltage s As a result, peak power efficiency of 36 lm/W and a low turn on voltage of only 3.0 V were achieved for such deep blue PHOLEDs with CIE coordinates of (0.16, 0.28). The se efficiencies were further improved to the maxima of EQE = 34% and P = 61 lm/W by attaching a near hemispherical macro lens to the light emitting surface of the glass substrate, and P has the potential to be further imp roved to up to 80 lm/W by using more complicated optical designs to enhance the light extraction efficiency

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109 CHAPTER 6 HIGH EFFICIENCY WHITE PHOSPHORESCENT OLEDS 6 .1 Introduction High efficiency w hite organic light emi tting devices (WOLEDs) have been on e of the most practical issue s in the OLED research fields due to their great potential as flat panel display s or solid state lighting (SSL). 32,133,135 White light emission typically requires a combination of multi ple colors of blue (B) green (G) and red (R ) T h is complexity of mixed color system makes it challenging to achieve both high efficiency and balanced white color purity Since the first introduction of WOLED s with fluorescent organic dyes by Kido et al. which showed very low power efficiency ( P ) of less than 1 lm/W 35 38 t here have been many active researches focusing on improving the WOLED efficiency to beyond that of fluorescent tubes (60 90 lm/W) and achiev ing a high CRI. 29 30,32,39,87,133,136 145 Table 6 1 summarizes recent significant progresses of WOLEDs without any additional outcoupling enhancement methods published in the journal. Even though it was possible to fabricate very simple structure WOLEDs w ith a single active oligothiophene compound which shows a broad blue green light emission and red shifted narrow peak due to the dimer emission, 138 the efficiency of these devices turned out too much low to be used in the practical applications. T he most impressive efficiency progress surpassing that of incandescent light bulbs (~ 20 lm/W) was achieved by emplo ying three phosphorescent emitters a deep blue dye, FIr6 101 in addition to the red dye, PQIr and the green dye, Ir(ppy) 3 resulting in P of 26 lm/W and CRI of 80. 87,133 Although high CRI could be achieved by employing R, G, B three organic emitters, the power efficiency was too low to surpass that of fluorescent lights. This is mainly attributed to

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110 Table 6 1 Comparison of the forward efficiencies for white OLEDs (WOLEDs) without any outcoupling enhancement methods The CIE (x,y) coordinates and CRI are also shown for comparison. WOLED architecture EQE (%) (at peak) P (lm/W) (at peak) CIE (x, y) CRI S ingle oligothiophene blue green emitter with red shifted dimer emission 138 0.35 % (0. 31 ,0.4 2 ) Phosphorescent triple doped single emissive layer 87 12 % 26 lm/W (0.43,0.45) 80 Fluorescent blue and phosphorescent green/red emissive layers with triple stacks 139 33 3 % 14 1 lm/W (0.38,0.44) 82 Fluorescent blue and phosphorescent green/red emissive layers with an interlayer 145 17.4 lm/W (0.47,0.42) 85 Management of singlet and triplet excitons 39 11.0 0.3 % 22.1 0.3 lm/W (0.38,0.40) 85 Three separate phosphorescent emission layers 29 16.6 0.8 % 32 1 lm/W (0.37,0.41) 81 Phosphorescent two dopants in dual emissive layers 30 25 % 58 lm/W (0.34,0.40) 68 Multiple exc iton generation in three separate emissive layers 140 15.5 0.8 % 37 2 lm/W (0.37,0.41) 81 Three separate phosphorescent emission layers with p i n structure 32 ~ 16.4 % (at 1,000 nits) ~ 37.5 lm/W (at 1,000 nits) (0.44,0.46) 80 the high triplet energy of FIr6 ( T 1 = 2.72 eV), 101 which makes it more difficult to efficiently generate and confine excitons on the deep blue emitting FIr6 molecule s, leading to a relatively low external quantum efficiency ( EQE ) of 12 % at peak. 33,87,133 Recently, Su et al. reported high efficiency white PHOLEDs with a peak P of 58 lm/W using two phosphorescent dopants, a greenish blue emitter, F Irpic 94 and a red emitter, iridium(III) bis (2 phenylquinoly N,C )dipivaloylmethane (PQ2Ir). They

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111 employ ed such higher triplet energy hole and electron transporting materials, 2,2 bis( m di p tolylaminophenyl) 1,1 biphenyl (3DTAPBP T 1 = 2.68 eV ) and 1,3 bis(3,5 dipyrid 3 yl phenyl)benzene (BmPyPB T 1 = 2.69 eV ) than that of FIrpic ( T 1 = 2.62 eV ) 30,146 Therefore, it is believed that high external quantum efficiency ( EQE ) of 25 % can be achieved by efficient triplet exciton blocking by HTL and ETL. However, those devices possessed a low CRI of 68 despite the high efficiencies due to the limited spectral coverage of the two emitters. 30 Hence, in order to enhance the CRI property, much deeper blue emitting organic dye such as FIr6 is necessary. Also, those 3DTAPBP HTL and BmPyPB ETL are not sufficient to effectively confine triplet excitons of FIr6 due to the higher triplet energy level of FIr6 ( T 1 = 2.72 eV ) 101 In C hapter 5 i t has been shown that the p eak EQE of FIr6 based deep blue PHOLEDs c ould be improved up to 20% by using TAPC as the HTL and 3TPYMB as the ETL. 93,102,118 The efficiency improvement was attributed to the enhanced charge and exciton confinement in the EML provided by the charge transport layers. Furthermore, D EML structure and p i n device structure were employed to maximize exciton generation and minimize the device operating voltage s respectively, leading to P = 36 lm/W at peak for such deep b lue PHOLEDs. 93,118 Here, high efficien cy white PHOLEDs are demonstrated by incorporating green and red phosphorescent dopants along with FIr6 into the D EML p i n PHOLEDs. Doping concentrations of blue, green, and red emitters were optimized into the UGH2 host material for the main white emission with balanced color purity, and additional triple doping concentration wa s adjusted in the mCP host material. Based on the high efficiency D EML deep blue PHOLED s tructure various doping combinations (either

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112 binary or triple doping in the S EML) into two emissive layers were investigated and finally optimized for not only high efficiency but also stable color purity at various luminances. The peak EQE = (19 1 ) % and P = ( 40 2) lm/W were achieved without any outcoupling enhancement technique w ith balanced white light emission from the triple dop ed D EML structure W hite light emission ha d a CRI of 7 9 and CIE coordinates of (0.3 7 0. 40 ) without showing appreciable chang es at different luminances 6 .2 Experiment The OLEDs were fabricated on g lass substrates precoated with an ITO anode ) using vacuum thermal evaporation following procedures published previously. 93,118 The general structure for p i n devices is ITO/HIL/HTL/EML/E TL/EIL/cathode, in which the p type HIL and the n type EIL con sisted of MeO TPD doped with 2 mol% F 4 TCNQ and 3TPYMB doped with Cs (with a molar ratio of 1:0.3), respectively, whereas TAPC and 3TPYMB served as the HTL and ETL, respectively. Conventional de vices without the HIL and EIL were also fabricated as a comparison. A wide band gap host UGH2 33,87 is doped with the three phosphorescent dopants (FIr6, Ir(ppy) 3 and PQIr ) for S EML device In addition, another wid e gap material mCP 93,118,133 is also doped with three dopants with relatively small amounts to form the D EML structure whereas the mCP layer was undoped in the S EML devices. The molecular structures of three B,G and R organometallic Ir complexes are shown in F i g. 6 1. The cathode consisting of a 1.0 nm thick LiF followed by a 50 nm thick Al was deposited through an in situ shadow mask, forming active device area of 4 mm 2 The structures and layer thicknesses of the specific devices to be discussed below are summarized in Table 6 2

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113 Figure 6 1 Schematic molecular structure s of organometallic Ir complexes as phosphorescent emitters. The FIr6, Ir(ppy) 3 and PQIr are for deep blue, green, and red emissions, respectively. Table 6 2 List of device structures and layer thicknesses used for fabricating WOLEDs discussed in C hapter 6. The general device structure is ITO/HIL/HTL/EML/ETL/EIL/cathode, where HIL = hole injection layer, HTL = hole transport layer, E ML = emissive layer, ETL = electron transport layer, and EIL = electron injection layer Device HIL HTL EML ETL EIL C onventional S EML 40 nm S EML a 40 nm C onventional D EML 40 nm D EML b 40 nm p i n D EML 30 nm 1 0 nm D EML b 1 0 nm 30 nm a S EML = m CP(10 nm undoped )/UGH2(20 nm doped ) b D EML = mCP (7 nm doped )/UGH2(10 nm doped ) L J V and EL spectra l measurements were carried out in ambient condition without encapsulation. The current power, and external quantum efficiencies ( L P and EQE respectively) were derived based on the previously published methods. 64 Note that all efficiency values referred to here are for emissions in the forward viewing directions without any specific ou tcoupling enhancement technique. 2

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114 6.3 Doping Concentration Optimization for Triple Emitters In order to satisfy the white emission purity such as CIE coordinates and CRI, the doping concentration of triple (B, G, R) emitters should be optimized in the emissive layer. Based on the previous conventional S EML deep blue PHOLED structure with 15wt% FIr6 doping, only 0.5wt% Ir(ppy) 3 green dopant is co doped in the UGH2 layer as shown in Fig. 6 2(a). The addition of 0.5wt% Ir(ppy) 3 did not change current inj ection behavior appreciabl y because most charge carriers preferably travel through the 15wt% FIr6 dopants due to its charge transporting property. 33 Figure 6 2 Schematic energy level diagram of S EML PHOLEDs doped with blue or green or red e mitters and EL spectra with either FIr6 and/or Ir(ppy) 3 dopants. However, t he EL spectrum of B G PHOLEDs showed a well balanced blue and green emission pattern as shown in Fig. 6 2 (b) f urthermore, the current and power efficiencies were significantly enhanced compared to the d evice either with the single FIr6 or Ir(ppy) 3 dopan t as shown in Fig. 6 3 (a) and (b). These high current and power efficiencies are attributed to the much spectral overlap with the photopic response of human eyes as well as higher external quantum efficiency from 12% for the FIr6 PHOLED to the 15% for the FIr6:Ir(ppy) 3 PHOLED (data not shown here).

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115 Figure 6 3 Current ( L ) and (b) power ( P ) efficiencies vs. current density ( J ) for S EML PHOLEDs using either FIr6 and/or Ir(ppy) 3 dopants. Similarly, PQIr doped PHOLED is compared with the device doped with FIr6, and EL spectra and current injection behavior are shown in F ig. 6 4 (a) and (b), respectively. Different with Ir(ppy) 3 doped PHOLEDs, the PHOLED with PQIr showed significantly reduced current injection even with less than 1wt% PQIr doping concentration. Also, less significant overlap with photopic response resulted in low power efficiencies for PQIr doped PHOLEDs. Therefore, it was strongly recommended to optimize the doping concentration of FIr6, Ir(ppy) 3 and PQIr in order to balance the white emission spectrum and maximize the power efficiency. F i gure 6 5(a) comp ares the power efficiencies of WOLEDs with slight different doping concentration of Ir(ppy) 3 and PQIr. Slight increase of Ir(ppy) 3 doping increases photopic response overlap, and slight reduction in PQIr doping reduce the device driving voltage, leading to the higher power efficiency. Also, well balanced EL spectra was obtained as shown in Fig. 6 5(b), with CIE coordinates of (0.37, 0.42) and CRI of 77.

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116 Figure 6 4 EL spectra and current density ( J ) voltage ( V ) characteristics of S EML PHOLEDs using eit her FIr6 or PQIr single dopant. Figure 6 5 Power efficienc ies ( P ) vs. luminance ( L ), and EL spectra of triple doped WOLEDs. Slight change of Ir(ppy) 3 and PQIr doping concentration changes the shape of EL spectrum and current injection property, leadi ng to the different power efficiencies. Also, the FIr6 doping concentration was increased from 15wt% to 25wt%. As a result, the current injection was slightly increased (data not shown here), furthermore, the external quantum efficiency was much significa ntly enhanced from maximum 16% to 18% as shown in Fig. 6 6 (a), leading to the peak power efficiency of 30 lm/W at 1

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117 cd/m 2 as shown in Fig. 6 6 (b). This higher current injection and better EQE are attributed to the charge transporting property of FIr6, maximize d exciton generation in FIr6 and efficient cascading Dexter energy transfer to the Ir(ppy) 3 and PQIr. Due to the strong charge trapping by the green and red dopants leading to ineffi cient exciton formation on the blue dopant doping concentration of green and red dopants should be carefully adjusted. The EL spectr a at various luiminances did not show appreciable changes, maintaining the CIE coordinat es o f (0.37, 0.42) and CRI value of 77. Figure 6 6 External quantum ( EQE ) and power ( P ) efficiencies of triple doped WOLEDs as a function of luminance ( L ). The doping concentration of deep blue dopant, FIr6, is changed from 15wt% to 25wt%, keeping Ir(ppy) 3 and PQIr doping concentration at the same ratio. 6. 4 High Efficie ncy C onventional D EML W hite PH OLEDs In section 6.3, the doping concentra tions for B, G, and R emitters we re optimized for achieving well balanced white emission. Triple dopants (B,G,R) system for white light emission can provide broad range of color adjus tability as well as a good CRI, whereas it is more difficult to control the optical/electrical properties of OLEDs due to its systematic and physical complexities. As shown in Fig. 6 6(a) and (b) the triple doped

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118 S EML WOLED achieved EQE = 18 % and P = 30 lm/W at peaks, respectively, with a high CRI of 77. However, th e s e peak efficienc ies dramatically went down to around 1 3 % and 12 lm/W at 1,000 cd/m 2 respectively which are not sufficient for lighting applications. T herefore, furt her enhancing the efficiencies especially at high luminance s is required. Similarly to the strategies for deep blue PHOLEDs discussed in Chapter 4 and 5, D EML structure wa s employed for maximizing the exciton generation with the high triplet energy TAPC H TL and 3TPYMB ETL. Figure 6 7 illustrates possible energy transfer paths among the dopants in the two wide gap hosts, as well as exciton confinement by TAPC and 3TPYMB (energy levels are taken from the literature ) 10 1 102,117 Figure 6 7 Possible energy flow in a dual emissive layer (D EML) white phosphoresce n t organic light emitting device (PHOLED) Energy levels are taken from the literature.

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119 The triplet energies of TAPC ( T 1 = 2.87 eV) 102 and 3TPYMB ( T 1 = 2.95 eV) 117 are higher than that of FIr6 ( T 1 = 2.72 eV) 101 leading to effective confinement of triplet excitons within the two emissive layers. 118 The optimized D EML structure consists of a 10 nm thick UGH2 layer and a 7 nm thick mCP layer where the FIr6, Ir(ppy) 3 and PQIr doping concentrations are 25 wt%, 2.5 wt%, 2.3 wt%, respectively, in UGH2, and 5.0 wt%, 1.3 wt%, 1.0 wt%, respectively, in mCP. Figure 6 8 (a) compares J V characteristics of conventional S EML and D EML white PHOLEDs, whereas P and EQE of these two devices are compared in Fig. 6 8 (b). Figure 6 8 Current density voltage ( J V ) characteristics and p ower ( P ) and external quantum ( EQE ) efficiencies as a function of luminance ( L ) of single and dual emissive layer (S EML and D EML, respectively) conventional white PHOLEDs. The peak external quantum efficiencies of these two devices are almost the same, EQE max = (19 1) % probably due to the excellent charge/exciton blockings by TAPC HTL and 3TPYMB ETL However, the maxim um power efficiency is P, max = (31 2) lm/W for S EML WOLEDs and (38 2) lm/W for D EML WOLEDs and furthermore, P at 100 cd/m 2 and 1,000 cd/m 2 for D EML WOLEDs showed much higher than those of S EML WOELDs (28 lm/W and 19 lm/W for D EML vs. 19 lm/W a nd 12 lm/W for S EML).

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120 These higher power efficiencies for D EML structure is attributed to the greatly reduced less conducting UGH2 thickness from 20 nm for S EML to 10 nm for D EML resulting in huge decrease in turn on voltage, V T from 4.0 V to 3.2 V Although reducing the UGH2 layer thickness in the S EML structure to below 20 nm le d to the significant decrease in EQE (or L with negligible spectrum changes) due to the incomplete charge recombination in the thinner EML as shown in Fig. 6 9(a) 118 adjacent another emissive layer with mCP host in the D EML structure c ould additionally enhance the light emission by capturing leaked electron charge carriers a nd diffusive triplet excitons from the UGH2 layer. Also, d ifferent from the UGH2 layer in which charge transport is through the dopant molecules, 33 the mCP molecules participate in charge transport, leading to a less steep efficiency roll off and reduced opera ting voltages at high luminances as shown in Fig. 6 9(b) Figure 6 9 Current efficiencies ( L ) as a function of current density ( J ) for conventional single emissive layer (S EML) WOLEDs; (a) varying the thickness of UGH2 host layer (b) different host materials (UGH2 vs. mCP)

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121 Note that while the CIE coordinates of the D EML white PHOLED at 100 c d/m 2 are (0.3 7 0. 40 ) which are similar to those of fluorescent tubes a much higher CRI of 79 than that of fluorescent tubes is obtained. 147 6. 5 High Efficiency p i n D EML W hite PH OLEDs Although the conventional D EML WOLEDs show a high EQE and relatively low turn on voltage of V T = 3.2 V, the peak P is achieved at a low luminance of L < 10 cd /m 2 In order to improve P at higher luminances, the p i n device structure is employed to further reduce the device operating voltage. 93,118 As shown in Fig. 6 10(a) Figure 6 10 Current density voltage ( J V ) characteristics and power efficiency ( P ) comparison as a function of luminance ( L ) between conventional and p i n D EML white PHOLEDs. the current de nsity of the p i n device is one to two orders of magnitude higher than that of the conventional d evice (at V > 3 V). The p i n device has a lower V T = 2.9 V and the operating voltages at 100 cd/m 2 and 1,000 cd/m 2 are 3.5 V and 4.0 V, respectively. Even th ough the peak EQE of p i n device is slightly reduced to (18 1 ) %, a higher maximum P of (40 2 ) lm/W is achieved as shown in Fig. 6 10(b). The power efficiencies at L = 100 and 1 000 cd/m 2 are P = (36 2 ) and (26 2 ) lm/W, respectively, approximately 30% highe r than those for conventional device at the same

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122 luminances. Moreover, these high power efficiencies of p i n D EML WOLEDs are more than twice higher than those of the triple doped conventional S EML WOLEDs reported 87 in which P = 11 lm/W at L = 1,000 cd/m 2 The pe rformance of dual triple doped p i n structure WOLEDs also shows improvement over that of the device by Sun and Forrest, 5 in which the same three dopants were separately doped into three adjacent emissive layers and showed P = 38 lm/W at peak and 20 lm/W at L = 1,000 cd/m 2 Figure 6 11 shows the EL spectra of p i n D EML WOLEDs at different luminances. With the increase of luminance, the relative intensity of the blue emission slightly increases while that of the red emission slightly decreases, leading to a slight shift of the CIE coordinates from (0.39, 0.41) at L = 10 cd/m 2 to (0.35, 0.40) at L = 1,000 cd/m 2 It should be noted that there are negligible white light emission spectra changes in triple doped D EML structure, compared with other WOLED stru ctures which have separate B/G/R dopants in individually adjacent emissive layers. 29,140 Figure 6 11 EL spectra of a dual emissive layer p i n white PHOLED at different luminance s of L = 10, 100, and 1000 cd/m 2

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123 Figure 6 12 shows the c omparison of CIE coordinates of triple doped p i n D EML white PHOLEDs (this work) among different white light sources such as daylight, incandescent bulbs, and fluorescent tubes The CRI value of 7 9 1 can be obtained due to the deeper blue and red dopants (FIr6 and PQIr, respectively) in the triple dopants device which is consistent with the previous result using the same three dopants, 5, 87 but is significantly higher than that of the device by Su et al. 3 0 based on the two dopants system Figure 6 12 Comparisons of c oordinates of triple doped p i n D EML white PHOLEDs (this work) among different white light sources. Excellent CRI value is obtained with s imilar CIE coordinates with the fluorescent lamp.

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124 6 6 Summary H igh efficiency white PHOLEDs have been demonstrated by codoping three B,G,R phosphorescent dopants into two adjacent emissive layers. F i rst, the doping concentrations of three individual dopa nts were optimized in order to maximize the exciton photon conversion as well as represent the balanced white color purity. Second, the D EML structure was employed with the high triplet energy charge transporting materials resulting in a nearly 100% inte rnal quantum efficiency Also, d ue to the strong charge trapping onto the red and efficient energy transfer from the blue to the green and red dopants in WOLEDs thinner UGH2 host layer thickness could be employed compared to that for the deep blue PHOLED leading to the high er power efficiency. Finally, dual triple doped p i n structure WOLEDs showed h igh external quantum efficiencies of ( 1 8 1) % and a peak power efficiency of ( 40 2) lm/W Also, a high CRI value of 7 9 1 was obtained, much exceeding th at of fluorescent tubes, and more importantly, the CIE coordinates were maintained at (0.37 0.02, 0.40 0.0 1 ) without showing appreciable changes in EL spectra at different luminances.

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125 CHAPTER 7 LIGHT EXTRACTION ENHANCEMENT IN OLEDS WITH HEMISPHER ICAL MICROLENS ARRAYS 7 .1 Introduction Nearly 100% internal quantum efficiencies have been demonstrated for deep blue and white PHOLEDs in Chapter 5 and 6, respectively assuming approximately 20% outcoupling efficiency in the bare OLED structure. T here have been lots of efforts to maximize the internal quantum efficiency of the OLED 148 All th ese approaches such as exciton/charge confinement by large bandgap material, 93,102 double emissive layer structure for maximizing exciton recombination, 93,118 and good char ge balance had led 65 However, externa l quantum efficiency ( EQE ) of the planar structure PHOLED is typically limited to the ~ 20% due to the total internal reflection losses (TIRs) at multiple interfaces. 149 Recently, universal display company (UDC) and some OLED research gr oups have reported that outcoupling efficiency over 30% is really achievable for the bare PHOLED through the application of high PLQY organic emitters an extremely well balanced charge recombination, and a superior exciton confinement. A l though it might b e ultimately possible to further increase the internal quantum efficiency close to real 100% and corresponding external quantum efficiency up to 30%, these efforts seem to be much more difficult than focusing on the other external outcoupling efficiency en hancement methods. In order to further increase the outcoupling efficiency ( out ), many approaches such as a resonant microcavity, embedded low index grids, and a photonic crystal have been applied to the OLEDs. 132,150 151 Through the application of these methods to the OLEDs, t he signific ant amounts of photons which could be trapped and lost in substrate,

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126 ITO/organic, and localized plasmonic modes, can be partially extracted to the air mode leading to the enhanced light extraction efficiency exceeding ~20 %. However, these methods typica lly require internal modification of the OLED structure, making a fabrication process complicated and expensive. Whereas, simple texturing technique on the surface of the glass substrate or attaching a macrolens or a microlens array to the glass substrate 152 157 can easily enhance the photon extraction from substrate modes. For example, Sun et al. recently demonstrated highly ordered hemispherical microlens arrays using an imprint lithography method, showing high l ight extraction enhancement of (1.68 0.09) 132 However, complicated photolithography and wet etching processes in making such a microlens mold is still considered to be an issue for the commercialization of microlens. Here, a rather simple microlens fabrication method is introduce d to create really h emisp herical microlens arrays. A large area and close packed polystyrene ( PS ) monolayer wa s created using a convective and capillary assembly 158 and a soft lithographic molding technique wa s employed to generate a concave polydimethylsiloxane (PDMS) template for a reproducible polymer microlens. We found that high microlens contact angle of ( c = 85 5) and array fill factor of ( FF = 85 3 )% c ould micro spheres The application of such microlens arrays to the glass surface of the large area OLED enhances the light extraction efficiency up to (70 7)% We also sh ow that the light extraction efficiency of the OLED is affected by microlens contact angle, OLED size and detailed layer structure of the OLED.

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127 7 .2 Experiment 7. 2 .1 Microlens F abrication The entire microlens fabrication procedures based on a soft litho graphy molding technique are illustrated in Fig. 7 1. Figure 7 1 Schematic illustration of the procedure s for fabricating large area and close packed microlens arrays using a soft lithograph ic molding method ; (a) Create a polystyrene (PS) monolayer on Si substrate (coated with thin SiO 2 layer) ; ( b ) pour polydimethylsiloxane (PDMS) precursor onto the PS monolayer; ( c ) remove the substrate after thermal curing of PDMS; ( d ) remove the embedded PS spheres from the cured PDMS with scotch tape; ( e ) apply 2~3 drops of optical adhesive on the concave PDMS mold and place the glass side of the OLED on top of optical adhesive; ( f ) release the PDMS mold after UV curing of the optical adhesive leaving behind microlens arrays See text for details.

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128 PS microspheres solids content) were purchased from Thermo Fisher Scientific. All PS microspheres ( PS dispersed using a convective and capillary assembly on a pre cleaned h ydrophilic UV ozone treated glass or SiO 2 substrate 158 F i gure 7 2 schematically illustrates how the PS monolayer can be continuously created using a convective and capillary assembly. 159 Figure 7 2 Schematic illustration of creating a polystyrene (PS) monolayer using a convective and capillary assembly technique. The V C is the growth rate of PS microspheres and J P J S and J E are the particle flux, th e solvent flux, and the solvent evaporation flux, respectively (Reproduced with the permission from B G. Prevo and O. D. Velev, Langmuir 20 2099 (2004) ) 159 Based on the transient mass transport model previously proposed by Dimitrov et al., 160 the PS monolayer growth rate, V C can be controlled to create a continuous film by below equation with the convention flow, V W on top of the PS layer. W here, and h are the porosity and the height of the PS sphere, is the volume fraction of the PS spheres in water suspension. The K is dependent on the rate of evaporation, typically kept as a constant as long as the temperature and the humidity are maintained. 159 In order to a pply the convection flow to the PS sphere suspension, t he substrate was initially slightly tilted (less than 10 ) creat ing a downward convection

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129 flow and the aqueous solution with PS microspheres was continuously dropped, eventually leading to the PS mono layer (without convection flow, multi layered PS arrays are expected). When water evaporates from the PS monolayer side, strong capillary forces between PS microspheres can make tightly closed packed PS arrays. In order to make a reproducible soft template for a microlens array, the PDMS (Sylgard 184 purchased from Dow Corning) was mixed with 10:1 weight ratio (base polymer : curing agent) and poured onto the PS monolayer. The substrate was kept in vacuum for 3 hours to completely remove the gas from the PD MS precursor, and thermally cured for 2 hours at 60 C in a vacuum oven. A highly transparent Norland optical adhesive (NOA 61, refractive index ~ 1.56 in cured polymer) is used for the polymer microlens material, and directly dropped onto the concave surf ace of PDMS template a few times. The glass substrate of an OLED was then subsequently placed on top of NOA 61 and left for 2 minutes without any pressure to well spread out the NOA 61 before the UV curing process ( = 365 nm, 200 mW/cm 2 for 3 minutes). Se parating the concave PDMS mold from the cured NOA 61 leaves behind the convex microlens arrays on the glass substrate of the OLED. The soft PDMS mold could be reused for many times without any breakage 7. 2 2 OLEDs F abrication The light extraction efficie ncy enhancement of OLEDs was examined by attaching the large area (covering a circle approximately 2 cm in diameter) microlens arrays to the glass surface of OLEDs which have been fabricated using a vacuum thermal evaporation method following procedures pu blished elsewhere 31,93,118 Also, t wo different sizes of OLEDs, 2 2 mm 2 (small area) and 12 12 mm 2 (large area) defined by the overlapping area of two electrodes in the cross bar geometry, were fabricated.

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130 Th e multi layer device structures of fluorescent and phosphorescent OLEDs (FOLEDs and PHOLEDs, respectively) are schematically illustrated in Fig. 7 3 (a) and (b), respectively. Figure 7 3 Schematic device structures of fluorescent and phosphorescent OLE D s (FOLED and PHOLED respectively ). Refractive indices are measured by ellipsometry, at their respective peak emission wavelengths ( 525 nm for the FOLED and 510 nm for the PHOLED ) The FOLED structure was consisted of glass/ITO/N, N' bis(naphthalen 1 yl) N,N' bis(phenyl) benzidine (NPB, 60 nm)/tris(8 hydroxy quinolinato)aluminium (Alq 3 60 nm)/ Cs 2 CO 3 ( 1 nm)/Al (100 nm), and t he PHOLED structure was glass/ITO/NPB (40 nm)/8.0wt% fac tris (phenylpyridine) iridium [ Ir(ppy) 3 ] doped 4,4' N,N' dicarbazole bipheny l (CBP) (20 nm)/bathocuproine (BCP, 20 nm)/Alq 3 (40 nm)/ Cs 2 CO 3 (1 nm)/Al (100 nm). 7. 2 3 Outcoupling E fficiency M easurements Three different methods were applied to measure the outcoupling efficiency ( out ) enhancement between with and without microlens a rrays. First, Si photodetector (818

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131 UV, diameter = 1cm) was used to measure the far field luminous intensity at the normal direction assuming a Lambertian source as shown in Fig. 7 4(a) and calibrated using a luminance meter (Minolta LS 100 with No. 110 c lose up lens). Figure 7 4 Three measurement methods to measure the outcoupling efficiency ( out ) of the OLEDs between with and without microlens arrays ; (a) far field luminous intensity measurement using a normal set up (b) luminous flux measurement using an integrating sphere (c) near field luminous intensity measurement using a optical fiber.

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132 J V characteristics of the OLEDs were measured using an Agilent 4155C semiconductor parameter analyzer Second, OLEDs were aligned and attached to the entrance port of a Newport integrating sphere (819D 3 Port) and an Ocean Optics Jaz spectrometer was co nnected to the outlet port to measure the total luminous flux from the OLEDs as shown in Fig. 7 4(b) Only the light emitting device area was opened toward the inside of the integrating sphere, while all the other non emitting areas were completely blocked Third, an optical fiber sensor ( ) was used to measure the near field luminous intensity at the normal direction as shown in Fig. 7 4(c) and the angular distribution of the luminous intensity was measured using a small area Si spot photodetector (818 UV) at 10 cm away from the device (corresponding to a half angle of ~ 0.5 ). 7 .3 Characterization of Microlens Arrays 7 .3.1 L a rge A rea and C lose P acked M icrolens A rrays A large ricated by a convective and capillary assembly is shown in Fig. 7 5 (a). Up to a few square inches of PS monolayer could be created. Although grain boundaries are observed between individually close packed PS domains, hexagonally close packed PS microsphere arrays are well formed as shown in Fig. 7 5 (b). It is mentioned that PS monolayer could challenging to generate a large th e increased impact of the capillary force. Figure 7 6 (a) is a cross sectional scanning electron microscopic (SEM) image showing the completely embedded PS microspheres in the cured PDMS after removing the substrate. The subsequent application of scotch ta pe could well remove the

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133 Figure 7 5 Optical images of a large area and highly close packed PS monolayer; ( a ) Convective and capillary assembly is employed to form a large area polystyrene (PS) microsphere monolayer on a 3 Si wafer (coated with a thin SiO 2 layer). (b) O ptical microscop e image of a PS microsphere monolayer 10 0 m size PS microspheres are used in both (a) and (b). embedded PS microspheres and PDMS residues as shown in Fig. 7 6 (b), leading to a concave PDMS mold successfully as shown in Fig. 7 6 (c). A large area microlens array using a PDMS mold is shown in Fig. 7 6 (d) and highly close packed microlens arrays

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134 are also shown in the inset of Fig. 7 6 (d). The measured FF of this microlens array (from 100 i s (85 3)%, whi ch is very close to the ideal FF of two dimensional hexagonally close packed spheres (91%). Figure 7 6 Measured scanning electron microscope (SEM) images of microlens fabrication processes ; ( a ) Cross sectional image of embedded PS spheres in cured PDM S. (b) The PDMS mold after (intentional) incomplete removal of PS spheres. (c) Concave PDMS template for microlens arrays after completely removing PS spheres. (d) L arge area microlens array s subsequently made with the concave PDMS mold. Inset: hexagonally close packed lens arrays are observed. Scale bar is 10 0 m in (a) and (d), and 2 0 m in (b) and (c). 7.3. 2 Control of M icrolens C ontact A ngles The same microlens fabrication method could be applied to different sizes of PS microspheres; however, the conta ct angle c of the PDMS mold is dependent on PS as

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135 shown in the cross sectional SEM images of Fig. 7 7 (a) ~ (c). Figure 7 7 Cross sectional SEM images of concave PDMS molds fabricated using different sizes of PS spheres and schematic illustration of mechanically weak points in the PDMS mold when removing PS spheres from the cured PDMS; (a) PS = 5 m, (b) PS = 20 m, and (c) PS = 100 m. (d) The PS separation is expected along the a a plane for 100 m size PS spheres, but along a lower plane (such as the b b plane ) for smaller size PS spheres due to the (unwanted) removal of PDMS in the mechanically weak points As the size of PS microspheres was increased, the shape of PDMS mold was closer to the perfect hemisphere, showing almost hemispherical s hape for PS We believe that this is related to the mechanical robustness of the PDMS mold in the narrow regions between individual features. When the PDMS precursor was poured onto the PS microspheres in order to make a concave template, it coul d completely cover the PS microspheres, penetrating all the way down to the SiO 2 surface through the interstitials among the PS microspheres (see Fig. 7 6 (a) ), which after curing form columns connecting PDMS below and above the PS microspheres. When the sc otch

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136 tape was applied to remove the PS microspheres from the thermally cured PDMS, it is ideal that PDMS breaks along the a 7 7 (d), leaving concave hemisphere features in the mold. We believe that PDMS columns using PS mechanically robust and broken at the narrowest neck area along the a However, narrower columns using smaller PS are more easily torn off at locations away from the narrowest neck area (for example, along the b than the PS microsphere radius, and in turn lower contact angles for the PDMS mold. The contact angles of microlens using different PDMS molds clearly showed the difference in c as shown in Fig. 7 8 (a) ~ (c). Here, c averaged over more than 10 lenses was obtained by drawing a tangential line at the edge of a lens and measuring its angle with the substrate surface We obta ined c = (45 5 ) with PS is similar to previous reports 161 162 As the PS c was increased to (65 5 ) and (85 5 ), respectively, as shown in Fig. 7 8 (d). The semi circle drawn in the cross sectional microlens image for PS the hemispherical shape of lens. The FF of a microlens array was estimated by counting the number of lenses per unit area and measuring the base diameter of lens ( D lens ). The FF for the microlens array from PS 3)% and (85 3)% for PS 7 8 (d). We believe that the larger D lens / PS ratio for larger PS in Fig. 7 8 (d) corroborates higher contact angle and FF for larger PS 7. 4 Characterization of OLEDs Efficiency with Microlens Arrays 7. 4 1 Enhanced L ight E xtraction with L arge S ize OLEDs The light extraction enhancement by microlens array s ( fabricated from 20 m or

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137 Figure 7 8 Cross sectional SEM images of convex microlenses with different sizes of PS spheres ; (a) PS = 5 m, (b) PS = 20 m, and (c) PS = 100 m. In (c), the dashed semi circle line is drawn for comparison (d) The PS sphere size ( PS ) depende nce for the contact angle ( c ) of microlens, lens array fill factor ( FF ), and the ratio of the lens base diameter ( D lens ) to the PS microsphere size 1 00 m PS spheres) is investigated based on tw o different sizes of FOLEDs For small area ( 2 2 mm 2 ) FOLE Ds with microlens arrays which entirely covered the FOLED as shown in Fig. 7 9(a), the external quantum efficiencies ( EQE ) with and without microlens arrays based on 100 m PS spheres were obtained using the far field luminous intensity measurement method As shown in Fig. 7 9(b), a light extraction enhancement factor (defined as the light output with the microlens array to that without the microlens array) of f = ( 1.35 0.04) was achieved in the entire current injection range. This enhancement factor, wh ile confirming the applicability of using microlens arrays to enhance the light extraction in OLEDs, is somewhat lower than the previously reported enhancement factors using microlens arrays fabricated using other methods, which is in the range of 1.5 1.7 132,157

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138 Figure 7 9 Schematic illustration of a small area OLED with microlens arrays and its efficiency enhancement results and light extraction mechanism; (a) OLED is entirely covered with large area microlens arrays on the surface of the glass substrate. (b) Ex ternal quantum efficiencies ( EQE ) of small area FOLEDs with and without microlens arrays. Light extraction enhancement factor ( f ) of ( 1.35 0.04) is calculated as the ratio of the light intensity with the microlens array to that without the microlens array (c) S chematic illustratio n for light extraction mechanism in a small area OLED with microlens array s attached to the glass su bstrate.

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139 It is believed that s uch a n OLED size dependence on f is attributed to the Al cathode size When photons are reached at the air/microlens interface some photons may be reflected back towards the cathode instead of escaping the front surface as shown in Fig 7 9(c). Hence, there is a significant backward air mode, leading to low light extraction efficiency into the forward viewing direction On the co ntrary, a larger metal cathode can re direct those back ward traveling photons again to the forward directions enhancing the chance of forward light extraction as illustrated in Fig. 7 10(a) Fig ure 7 10(b) compares the absolute luminous intensities for a large area ( 12 12 mm 2 ) FOLED with or without a microlens array ( PS = 100 m ) The FOLED is operated at J = 5 mA/cm 2 in both cases, and measured using an integrating sphere system By integrating the area under the peak in each spectr um a light extraction enhancement factor of f = (1.70 0.07) was obtained with the microlens a rray, which was significantly higher than the result for the small area FOLED The enhancement factor for large area devices was also confirmed by the near field luminous intensity measurement As shown in Fig. 7 11(a) only a half of a large area FOLED w as covered with a microlens array ( PS = m ) With the device operated at J = 3 mA/cm 2 t he near field optical intensity was recorded across the entire light emitting area ( between the bare side and the side covered with the microlens array ) using an optical fiber which is perpendicular to th e device As illustrated in Fig. 7 11(b) we see that the electroluminescent ( EL ) intensity in the microlens covered region is approximately 1.69 times higher than that in the bare side of the OLED. This is in

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140 good agreement with the integrating sphere me asurement results for large area FOLEDs. Figure 7 10 Schematic illustration for light extraction mechanism in a large area OLED with microlens array s, and EL intensity difference with and without microlens arrays for large area FOLEDs ; microlenses a re fabricated from PS = 100 m and EL intensities are measured with the device operated at 5 mA/cm 2 in both cases using an i ntegrating sphere system Inset: A photo of the large area FOLED covered with a microlens array on the glass surface ( PS = 100 m)

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141 Figure 7 11 N ear field luminous intensity measurement set up and intensity profile measurement results across a large area F OLED ; The right h alf of the FOLED was covered with a microlens array while the other half was bare. The FOLED was operated at 3 mA/cm 2 and negligible degradation was confirmed before/after the measurement.

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142 7. 4 2 Light E xtraction E fficiency D ependence on M icrolens C ontact A ngle The light extraction efficiency enhancement in FOLEDs using microlens arrays fabricated from 2 0 m PS micro spheres is also investigated. As summarized in Fig. 7 1 2(a) enhancement factors of f = (1.25 0.04) and (1. 4 5 0.0 5 ) were obtained for small area and large area devices, respectively. In addition to the device area dependence as observed with larger microlenses we also see that the absolute enhancement was somewhat lower in this case, as compared to using microlenses from 100 m PS micro spheres We attribute this to the lower contact angles of microlens es from 2 0 m PS micro spheres rather tha n similar FF as shown in Fig. 7 8(d) L ight extraction efficiency can be enhanced significantly at larger angles than ~ 41.8 (critical angle for total internal reflection (TIR) at the air( n = 1.0)/glass( n = 1.5) interface with the planar glass) with the a ttachment of microlens arrays on the surface of glass substrate, and previous results showed that higher contact angle microlens can extract photons more effectively than by the lower contact angle microlens 132,157 which is consistent with the results here. Figure 7 1 2(b) shows t he angular emission patterns for the OLED with the microlens attachment ( PS = m ) High c of microlens is obtained using standard procedure (see experiment section) with low viscosity optical adhesive (NOA61, 300 cps), whereas low c is intentionally fabricated by dropping high viscosity optical adhesive (NOA 68, 5000 cps) onto the glass surface of the OLED and instant curing after putting down the PDMS mold. Although bare OLED showed slightly wider angular emission pattern than a Lambertian source due to the measurement set up, slightly forward directional and a negligible pattern change were observed with high and low

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143 Figure 7 12 Summarized light extraction enhancement factor ( f ) for different parameters and angular emission patterns using different microlens contact angles; (a) parameters = device size (small vs. large areas), device structures (FOLEDs vs. PHOLEDs), and microlens arrays fabricated from two different sizes of pol ystyrene microspheres ( PS = 20 m and 100 m) (b) Normalized optical power intensity as a function of the viewing angle for a bare F OLED ( red open squares ) and FOLEDs with low and high contact angle microlens arrays (blue open and solid circles, respecti vely), as well as that for a Lambertian source ( black solid line)

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144 microlenses, respectively, compared to that of bare OLED. We believe tall height of high c microlens arrays block s the light emission at larger angles and redirect s it forward which is opposite what Sun et al. demonstrated using 7 m microlens arrays with the enhanced luminous intensities at larger angles (especially at > 4 0 ) 157 7. 4 3 Light E xtraction E fficiency D ependence on OLED S tructure The close packed hemispherical microlens arra ys ( PS = 100 m) were also attached to the large area PHOLEDs, and the light extraction efficiency was examined using the near field luminous intensity measurement method. Interestingly, somewhat lower f = (1.56 0.06) was observed for PHOLEDs compared w ith that for FOLEDs ( f = 1.69 0.04) as shown in Fig. 7 12(a) T his difference is attributed to the more significant optical modes confinement for the PHOLED with higher refractive index of the emissive layer ( n CBP:8%Ir(ppy)3 = 1.83 at peak = 510 nm) tha n those of neighboring layers ( n NPB = 1.81 and n BCP = 1.74), measured by ellipsometry (see Fig. 7 3 (b)). Therefore, it is expected that less amount of photons could reach the glass substrate for the PHOLED, eventually leading to lower light extraction enha ncement by microlens arrays. 7 5 Summary It is shown that the light extraction efficiency of fluorescent OLEDs can be improved up to 70% using large area, close packed hemispherical microlens arrays fabricated by a soft lithography method. The major param eters affecting to the light extraction efficiency of the OLED are summarized in Table 7 1. Different contact angles of the microlens are achieved with different sizes of PS microspheres, and contact angle of (85 5 ) and fill factor of (85 3)% are obt PS microspheres It is also found that the large size OLED is necessary to maximize the light extraction by microlens arrays due to the photon re direction into forward by the

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145 large area aluminum cathode. For multi layered phosphorescent OLEDs, slightly low light extraction efficiency is observed with microlens arrays due to the significant optical modes confinement in the emissive layer. The microlens fabrication technique described here has a great advantage for application to the large area lighting systems, instead of pixelated small area display devices. Table 7 1 Comparison of the light extraction efficiency enhancement factor ( f ) for small area (2 2 mm 2 ) and large area ( 12 12 mm 2 ) fluorescent and phosphorescent OLEDs (FLOEDs a nd PHOLEDs, respectively) using microlens arrays fabricated from different sizes of polystyrene microspheres ( PS ). The a ctual microlens base diameter ( lens ), contact angle ( C ), and array fill factor ( FF ) are also listed. PS m ) lens m ) C () FF (% ) f FOLEDs PHOLEDs s mall area l arge area l arge area 5 4.6 0.1 45 5 6 7 3 20 18.4 0.4 65 5 77 3 1.25 0.04 1.45 0.05 1.44 0.05 100 9 8 2 85 5 85 3 1.35 0.04 1.70 0.07 1.56 0.06

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146 CHAPTER 8 CONCLUSIONS AND FUT URE WORK S 8 .1 Conclusions 8 1 .1 High E fficiency D eep B lue PHOLEDs First, a significant amount of research work ha s been dedicated to develop ing high efficiency deep blue PHOLEDs. D eeper blue emitting OLEDs are necessary for enriching the color gamut in dis play s and a high CRI in lighting. However, it is more difficult to effectively generate and confine high energy blue excitons, limiting their OLED efficiencies to lower than those of green or red emitting OLEDs. In order to enhance the efficiency of deep blue PHOLEDs, several strategies are tackled based on the physical understanding of organic semiconductors and OLEDs In Chapter 4 t he employment of the high triplet energy hole transporting material TAPC, significantly improved the blue exciton confine ment leading to the external quantum efficiency of 15%. Also, a dual emissive layer structure with optimized FIr6 doping concentration in two neighboring large bandgap host materials led to the EQE of 17% suggesting the importance of a broad charge reco mbination zone for maximizing exciton generation V ery low device operating voltage was achieved by applying the p i n structure, and high EQE of 17% could be maintained due to the excellent electron blocking by TAPC and maximized exciton generation in D EML structure, leading to a peak power efficiency of P = 25 lm/W. In Chapter 5 3TPYMB was compared with the common ETL, BCP or BPhen, to further maximiz e the triplet exciton confinement in deep blue PHOLEDs. Compared to the low triplet exciton energy lev el of BCP or BPhen (2.5 eV), the employment of higher triplet exciton energy 3TPYMB (2.95 eV) as the ETL led to a m u ch enhanced EQE up to

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147 20%, implying a nearly 100% internal quantum efficiency for such deep blue PHOLEDs. Additionally the electron transp orting property of 3TPYMB is improved by doping alkaline metals. T he electron mobility of pure 3TPYMB is one order of magnitude lower than that of pure BPhen, therefore, better electron injection layer is required to achieve high power efficiency. In order to accomplish this several alkaline metal compounds were investigated either as a n interlayer between ETL and cathode or as a dopant in the ETL The cesium carbonate compound (Cs 2 CO 3 ) doped 3TPYMB layer as the EIL showed significantly enhanced conductivi ty, similar to the level of Cs 2 CO 3 doped BPhen, leading to the very high power efficiency of 36 lm/W at peak The attachment of a macrolens on the glass substrate of the highest efficiency deep blue PHOLEDs enabled extraction of substrate waveguiding mode s which are typically trapped in the glass substrate in the planar structure OLEDs, leading to peak P = 61 lm/W with the 70% enhanced light extraction efficiency. In summary, throughout the Chapter 4 and 5, internal quantum efficiency was significantly improved up to nearly 100% by broadening the charge recombination region and maximizing the triplet ex citon confinement. This could be achieved by employing the D EML structure and high triplet ener g y TAPC HTL and 3TPYMB ETL. Also, very low device turn on voltage, V T = 3.0V, was obtained by optimizing the p i n structure, leading to the highest power effic iency for such deep blue PHOLEDs. 8 1 2 High E fficiency W hite PHOLEDs There have been many attempts to achieve WOLEDs having both a high efficiency and high color purity. Previous ly, very high efficiency WOLEDs showing peak P = 58 lm/W without any light extraction methods have been published, 30 but the CRI of such WOLED s was only 68 which is not appropriate for a good white light source. Also,

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148 several different device structures were suggested to satisfy both the high efficiency as well as the high colo r purity. H o wever, it was still problematic to achieve high efficiencies and/or consistent spectra at different luminances. Previously, the S EML doped with t riple emitters was introduced to minimize the spectra changes, but it showed low efficiency due to the inefficient exciton generation/confinement. So, in Chapter 6, several design parameters for WOLEDs were investigated and finally p i n triple (B G R) doped D EML WOLEDs were suggested in order to overcome most described problems above leading to peak P = 40 lm/W without any outcoupling enhancement methods as well as maintaining the CIE coordinates = (0.3 7 0.02 0.4 0 0.01 ) at different luminances with negligible changes to CRI of ( 79 1) 8 1 3 Light E xtraction E nhancement in OLEDs via M icrolen s A rrays In planar structure OLEDs, the light outcoupling efficiency is generally believed to be ~ 20% due to the existence of waveguiding modes in the organic active layers and the substrate Microlens arrays directly deposited on the light emitting surfa ce of the glass substrate have been shown to reduc e the total internal reflection at the glass/ air interface, leading to enhanced light outcoupling. In Chapter 7, a relatively simple and cost effective soft lithography technique has been employed to fabric ate large area, close packed hemispherical polymer microlens arrays. A close packed monolayer of polystyrene (PS) microspheres was formed using a convective and capillary assembly method, which was employed to generate a p olydimethylsiloxane (PDMS) concave template for the fabrication of transparent microlens arrays based on a Norland optical adhesive Both t he microlens contact angle and array fill factor showed depend ence on the size of PS microspheres, and nearly close packed, hemispherical microlens ar rays with

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149 microlens contact angle of (85 5) and array fill factor of (85 3)% were obtained with when such fabricated microlens arrays were attached to the light emitt ing glass surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large area (12 mm 12 mm) fluorescent OLED with a near close packed hemispherical microlens array, a maximum enhancement of (70 7)% in the light extraction efficiency was achieved. 8 2 Future Works 8 2 1 What is the Limit of O utcoupling E fficiency in Bottom Emitting OLEDs ? Based on classical electromagnetic calculations 27,163 170 the max imum achievable outcoupling efficienc ies ha ve been reported to be around 20 ~ 2 2 % for bottom emitting O LEDs limiting the oretically achievable external quantum efficiencies to below 22 % 27,163,165 168,170 Recently, h owever, phosphorescent OLEDs (PHOLEDs) which are showing EQE of up to 29% have been reported, 83,97 98,171 172 suggesting that the maximum achievable outcoupling efficiency for OLEDs is likely higher than 2 2 %. In my research work (not seriously presented here), h igh triplet energy materials such as TAPC, mCP, and 3TPYMB we re employed to green PHOLEDs in order to completely confine triplet exciton s onto the green dopant, Ir(ppy) 3 Assuming 100% PLQY of Ir(ppy) 3 in an mCP host the achieved peak EQE = 22% was believed to be close to the classical outcoupling efficiency limit H o wever, when the deep blue dopant, FIr6, wa s codoped with Ir(ppy) 3 in the mCP host (25wt% vs. 0.5 wt% concentrations) as shown in Fig. 8 1(a), much higher peak EQE = ( 2 7 1) % and less significant roll off were achieved as shown in Fig. 8 1(b).

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150 Figure 8 1 Schematic HOMO/LUMO energy level diagram and external quantum efficiency ( EQE ) vs. current density ( J ) characteristics for green and blue green PHOLEDs Pure mCP lay er is inserted as a additional exciton blocking layer It is believed that most charge carriers go through the FIr6 dopant sites first, directly generating triplet excitons in the FIr6, and then efficiently transfer ring triplet excitons to the Ir(ppy) 3 gr een dopants (Dexter energy transfer) maximizing the exciton generation. Schematic triplet energy level diagrams and possible energy transfer flow equations are shown in Fig. 8 2 (a) and (b), respectively Although further investigation should be conducted in order to completely demonstrate the theoretical/ experimental maximum limit of outcoupling efficiency in BOLEDs, this high EQE = ( 2 7 1) % suggest s that there is still a chance of further improving the OLED efficiency, possibly exceeding EQE = 30% in a n optimized BOLED structure as summarized by Kim et al. in Fig. 8 3 40 8 2 2 Microlens Simulation The theoretical light extraction enhancement using microlens arrays attached to th e glass sur f ace of OLEDs have been calculated mostly based on classical ray optics. While classical ray optics can provide some essential information it is not sufficient to

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151 Figure 8 2 Schematic triplet energy level diagram describing possible Dexter energy transfer process from blue to green dopants in the blue green PHOLED. High triplet energy materials are used for better triplet exciton confinement onto dopants. B and G subscripts mean blue and green, respectively. The density of triplet excito n in the emitting layer ( N ), triplet exciton generation rate ( G ), the density of charge carriers in the emitting layer ( N ch ), radiative triplet exciton rate ( k r ), triplet triplet annihilation rate ( k TT ), and triplet exciton polaron quenching rate ( k TP ) are expressed in the equation. accurately account for the emission in OLEDs. 173 174 For example, the total thickness of the organic layers, typically on the order of 100 nm, is smaller than the emiss ion wavelength, which somewhat invalidates ray optics. Therefore, more accurate analysis should be executed based on the considering the optical interference effect s in the multi layer OLED structure and isotropic ally oscillating dipol e generation in the amorphous organic emissive layer. 175 Figure 8 4 shows 3 D modeling and light extraction enhancement analysis of OLEDs with and without microlens arrays on the surface of a glass substrate, resulting in 41% light extraction enhancement via

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152 Figure 8 3 Summary of outcoupling efficienc ies and conditions based on different simulation methods. All results are found in literature s Reproduced with t he permission from S. Y. Kim and J. J. Kim, Org Electron 11 (6), 1010 (2010). 40

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153 hemispherical microlens arrays with 91% array fill factor. A trial version of Lumerical FDTD si mulation software was used for this modeling and calculation based on the finite difference time domain (FDTD) method. It is necessary to further test and investigate the accuracy of this FDTD program, it might suggest better analysis about the effects of lens contact angle, fill factor, and even other parameters in OLEDs, which are not clearly known so far through the use of wave optic calculations. Figure 8 4 Schematic modeling and analysis results of OLEDs between without and with microlens arrays o n the glass surface. Lumerical FDTD simulation software was used for this calculation. 8 2 3 Scalability Issue of OLEDs OLEDs have been recently commercialized mostly for the small area (around 4 ) display devices, but there is a very strong demand for the development of large area (larger than 15 ) OLEDs. Samsung Mobile Display (SMD) is about to launching mass production for 7 display OLEDs in 2011 based on 5.5 generation line and planning to

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154 invest on 8 th generation line by 2015 which will be used to build OLED TV (expected at least larger than 15 ). Fabrication of large area OLEDs is much more difficult than making small area OLEDs due to the luminance uniformity and bad pixels over the entire large area as well as the huge amount of investment (at least several billions of dollars) for equi pment scalability. Also, lots of process conditions would be very difficult to optimize for high production yield. Therefore, better OLEDs fabrication methods and their process conditions should be developed for l arge area OLEDs. For example, printing method 14 or roll to roll (R2R) 15 21 process have been introduced for revolutionizing the typical manufacturing process, vacuum thermal evaporation (VT E), of organic electronics. Although the device efficiencies based on printing or R2R method are not as high as that based on the VTE, further investigation on these fabrication conditions is expected to provide better fabrication environment for large are a OLEDs. 8 2 4 Stability Issue of OLEDs High efficiency PHOLEDs exhibiting very long lifetime over 100,000 hours (operated at the display device condition) have been already reported, 176 177 however, it i s still strongly recommended to further investigate on high efficiency and long lifetime PHOLEDs. There are many possible factors affecting to the OLEDs stability such as electrochemical properties, thermal properties, charge balance, extrinsic impurities in organic materials encapsulation condition, operating conditions, interface morphologies, and etc. 178 For example, the electrochemical property of the emissive layer layer (EML) is more important than hole/electron transporting layers (HTL/ETL) because holes and electrons exist together in the EML, which can lead to the chemical reaction s causing rapid device degradation. Mostly small amount of phosphorescent emitters are doped in

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155 the host materials in PHOLEDs so that it is believed that the electrochemical properties of host materials mainly decide the device stability. Also, multilayer structures are typical in PHOLEDs in order to effectively block charge carriers and excitons. Although high efficiencies can be obtained based on the multilayer structure, the excessive accumulation of charge carriers and excitons at the interfaces can cau se electrochemical reactions as well which is detrimental to the device stability. Therefore, more investigation should be focused on the device stability along with the high efficiencies for PHOLEDs which can eventually replace low efficiency FOLEDs with high efficiency PHOLEDs.

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156 APPENDIX LIST OF PUBLICATION S AND PRESENTATION S Publication s 1. S. H. Eom E. Wrzesniewski and J. Xue, Efficiency Blue and White Phosphorescent Organic Light Emitting Devices for Solid Mat Sci Eng R (in preparation ) 2. S. H. Eom E. Wrzesniewski, and light emitting devices via hemispherical microlens arrays fabricated by soft accepted Journal of Photonics for Energy 2010) 3. S. H. Eom E. Wrzesniew ski, and packed hemispherical microlens arrays for light extraction enhancement in organic light (submitted to Organic Electronics 2010 ) 4. E. Wrzesniewski, S. H. Eom oxide/me tal/oxide trilayer electrode for use in top emitting organic light emitting 2010) 5. Y Yang, P Cohn, A. L. Dyer, S. H. Eom J. R. Reynolds, R. K. Castellano, and J. Violet Electroluminescenc of Mater. 22 3580 (2010) 6. J. Xue, S. H. Eom Y. Zheng, E. Wrzesniewski, N. Chopra, J. Lee, and F. So, 7415 36 (2009) 7. J. Lee, N. Chopra, S. H. Eom Y. Zheng, J. Xue, and F. So, energy confinement by charge transporting layers on blue phosphorescent 7051 70511T (2008). 8. Y. Yang, R. T. Farley, T. T. Steckler, S. H. Eom J. R. Reynolds K. S. Schanze infrared organic light emitting devices based on low 106 044509 (2009). 9. S. H. Eom Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So and J. Xue, nt organic light emitting devices with dual triple doped 94 15 3303 (2009). 10. N. Chopra, J. Lee, Y. Zheng, S. H. Eom J. Xue Balance on High Efficiency Blue Phosphorescent Organic Light Em itting CS Appl. Mater. Inter. 1 1169 (2009). 11. S. H. Eom Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So and J. Xue, blue phosphorescent organic light emitting devic 10 686 (2009).

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157 12. Y. Yang, R. T. Farley, T. T. Steckler, S. H. Eom J. R. Reynolds, K. S. Schanze emitting devices based on donor acceptor 93 163305 (2008). 13. J. Lee, N. Chopra, S. H. Eom Y. Zheng, J. Xue, and F. So energies and transporting properties of carrier transporting materials on blue Appl. Phys. Lett. 93 123306 (2008). 14. N. Chopra, J. L ee, Y. Zheng, S. H. Eom J. Xue and F. So, High efficiency blue phosphorescent organic light emitting device Appl. Phys. Lett. 93 14 3307 (2008). 15. S. H. Eom Y. Zheng, N. Chopra, J. Lee, F. So very high efficiency deep blue phosphorescent organic light Appl. Phys. Lett. 93 13 3309 (2008). 16. Y. Zheng, S. H. Eom blue phosphorescent organic light emitting device with im proved electron and exciton confin 92 223301 (2008). Patent 1. J.D. Meyer, S. H. Eom Presentation s 1. S H. Eom E. Wrzesniewski and J. Xue, ing the light outcoupling efficiency in OLEDs using large area close packed hemispherical microlens 2010 Optics + Photonics SPIE conference, Aug. (2010) ( Invited Talk ) 2. S. H. Eom Y. Zheng, E. Wrzesniewski J. Lee N. Chopra, F. So, and J. Xue, Emitting (2009) 3. S H Eom Y Zheng, and J Xue emit ting Materials Research Society, S ymposium H: Physics and Technology of Organic Semiconductor Devices Dec. (2008). 4. S H Eom Y Zheng, N Chopra, J Lee, F So, and J Xue B lue Phosphorescent Organic Light Materials Research Society, S ymposium E : Materials and Processes for Flat Panel Displays Jun. (2008).

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158 5. S H Eom Y Zheng, J Lee, N Chopra, F So, and J Xue High Efficiency Deep Blue Phosphorescent Organic Light Emitting Florida Chapter of the AVS Science and Technology Society (2008).

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169 BIOGRAPHICAL SKETCH Sang Hyun Eom was born in Seoul, South Korea He grew up with the desir e to be either a baseball player or an engineer until he entered into middle school. He majored in m echanical e ngineering for his B.S. degree at Hanyang University from 1993 to 1999 (including a Korean Army duty ), and continued his M.S. degree at Pohang Un iversity of Science and Technology (POSETCH) for 2 year s after 1999 with a specialty in structural design for robust cold rolling system s based on a finite element method (FEM). Upon graduating i n early 2001, he joined the Samsung SDI as a mechanical desi gn engineer and figured out many structural problems in display devices. He also trained for a Six Sigma specialist in Samsung SDI and acquired a black belt qualification in 2003. In 2003, he was transferred to the new display device development team and p articipated in launching a new display device and managing a small research group. In early 2006, he was promoted to the principal manager in Samsung SDI and also dispatched to the department of Materials Science and Engineering in University of Florida to get more in depth experience in OLEDs. He spent about 4 years in the organic electronics group where he mostly contributed to the development of high efficiency blue and white PHOLEDs including light extraction enhancement methods. Upon completion of his Ph.D. program, he is planning to go back to the central research center in Samsung SDI, South Korea, where he is going to continue his research focusing on the dye sensitized solar cell (DSSC) and/or Li ion battery (LIB). He has a wife named Young Soo Park and a 1 year old daughter named Kailey Jihyo Eom.