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Light Extraction of Organic Light-Emitting Diodes Using Corrugated Structure

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Title:
Light Extraction of Organic Light-Emitting Diodes Using Corrugated Structure
Creator:
Youn, Wooram
Place of Publication:
[Gainesville, Fla.]
Florida
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University of Florida
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english
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1 online resource (125 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Materials Science and Engineering
Committee Chair:
SO,FRANKY FAT KEI
Committee Co-Chair:
XUE,JIANGENG
Committee Members:
HOLLOWAY,PAUL H
NORTON,DAVID P
RINZLER,ANDREW GABRIEL
Graduation Date:
5/2/2015

Subjects

Subjects / Keywords:
Antinodes ( jstor )
Electrons ( jstor )
Light refraction ( jstor )
Periodicity ( jstor )
Photonics ( jstor )
Quantum efficiency ( jstor )
Sapphire ( jstor )
Wave diffraction ( jstor )
Waveguides ( jstor )
Wavelengths ( jstor )
Materials Science and Engineering -- Dissertations, Academic -- UF
extraction -- light
Genre:
bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Materials Science and Engineering thesis, Ph.D.

Notes

Abstract:
Light extraction is critical to the efficiency of organic light-emitting diodes (OLEDs). Because of the internal reflection, only 20~30% of generated light in OLEDs is emitted across the top interface and detected by observers. The balance is controlled by the refractive indices of each side of the interface and the angle at which light is incident upon the interface. In particularly, the generated light in OLEDs is dominated by three different modes: air mode, substrate mode, waveguide mode because of the presence of two interfaces of air (n=1)-substrate (n=1.52) and substrate (n=1.52)/indium tin oxide (ITO) (n=1.8). To enhance the light extraction efficiency in OLEDs, intensive amount of studies and researches has been followed. The air/glass interface in OLEDs may be modified using microlens arrays, sand-blasting, or light scattering films, which can extract the substrate mode by a factor of 2. For waveguide mode extraction, the light extraction technique called corrugated structures can be utilized. Therefore, increase of light extraction based on the corrugated structures in OLEDs is focus of this research. Corrugated structures with quasi-periodic patterns were created on fluorescent green OLEDs. The effects of a peak periodicity of 0.5 and 1 um was both measured experimentally and predicted by model. The smaller periodicity of 0.5 um was found to give 35% higher in extraction efficiency. To quantify the effects of internal reflection, materials with different indices were chosen for the substrate, for examples, glass (n=1.52) and sapphire (n=1.76). And this results in external quantum efficiency (EQE) of 63% in OLEDs using both corrugated structure and macro lens. Finally, we demonstrated an EQE of 67% by adding lithium fluoride (LiF) layer with refractive index of 1.39 between glass and ITO layers by increasing of the refractive index contrast for diffraction mechanism. Proper choice of materials with refractive index in corrugated OLEDs can increase the light extraction efficiency by a factor of more than 2. ( en )
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.
Thesis:
Thesis (Ph.D.)--University of Florida, 2015.
Local:
Adviser: SO,FRANKY FAT KEI.
Local:
Co-adviser: XUE,JIANGENG.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2016-05-31
Statement of Responsibility:
by Wooram Youn.

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UFRGP
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Applicable rights reserved.
Embargo Date:
5/31/2016
Classification:
LD1780 2015 ( lcc )

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LIGHT EXTRACTION OF ORGANIC LIGHT EMITTING DIODES USING CORRUGATED STRUCTURE S By WOORAM YOUN 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 2015 1

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2015 Wooram Youn 2

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To my family 3

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ACKNOWLEDGMENTS First of all, I would like to thank my advisor Dr. Franky So for his leadership and his belie f in me. Dr . So has given me the opportunity to work in organic electronics and light extraction. I will be always truly grateful for our valuable discussions. I would also like to acknowledge the help from my supervisory committee members, Dr. Xue, Dr. No r ton, Dr. Holloway , and Dr. Rinz ler for taking the time to offer guidance and be on this committee. I would like to thank our formal post Dr. Wonhoe Koo for guiding me to push my research forward. I would like to thank my friend, Dr . Jinhyung Lee for sharing and developing our research. Also, I would like to mention Dr. Jaewong Lee, Dr. Sungwook Mhin , and Dr. Seuonghwan Yeo for their sincere care about me. I also want to acknowledge my friend Shingyeong Yeo for the support. I would like to express my most sincere gratitude to my family in Korea for their continuous support and love. I am forever indebted to my parents Moo sup Yoon and Kyoyae Lee for allowing me to pursue any academic interest that I wanted to follow and for always being encouraging. My bro ther Boram Yoon has always been there to support and cheer. I would also like to thank my par ents in law, Sangjin Pyeon and Jeomsook Cho for their countless belief in me. Above all I would like to express thanks and love to my wife, Yujeong, for her love, encouragement, support and praying for me. Thanks to her dedication, I could complete my PhD work without any problems. All family’s love and belief in me has shaped me into who I am today. I couldn’t imagine myself without their strong support behind me. 4

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Finally, I would like to thank God for his endless love, help, and faithfulness to me. 5

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TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 9 LIST OF FIGURES ........................................................................................................ 10 ABSTRACT ................................................................................................................... 13 CHAPTER 1 IN TRODUCTION .................................................................................................... 15 1.1 General Development of OLEDs ...................................................................... 15 1.2 Development for Internal Quantum Efficiency in OLEDs .................................. 16 1.3 The Role of Total Internal Reflection in Bottom Emission OLEDs .................... 16 2 PARAMETERS TO DETERMINE THE EFFICIENCY OF OLEDS .......................... 20 2 .1 OLED Operation ............................................................................................... 20 2 . 2 Quantum Efficiency ........................................................................................... 20 2.2.1 External Quantum Efficiency ( ext) ........................................................... 20 2.2.2 Internal Quantum Efficiency ( int) ............................................................. 21 2.2.2.1 Charge balance () ......................................................................... 21 2.2.2.2 Radiative transition ( ex) ................................................................. 22 2.2.2.3 Photoluminescence quantum yield (p) .......................................... 22 2.2.3 Outcoupling efficiency ( ph) ..................................................................... 23 3 REVIEW OF LIGHT EXTRACTION TECHNIQUES IN OLEDS .............................. 26 3.1 Light Extraction Techniques for Substrate Mode .............................................. 26 3.1.1 Microlens Array ....................................................................................... 26 3.1.2 External Scattering Layer ........................................................................ 27 3.1.3 SandBlasting .......................................................................................... 28 3.2 Light Extraction Techniques for Waveguide Mode ............................................ 28 3.2.1 Internal Scattering Layer ......................................................................... 29 3.2.2 Low Index Grid ........................................................................................ 30 3.2.3 HighRefractive Index Substrate ............................................................. 31 3.2.4 Photonic Crystal ...................................................................................... 32 3.3. Limitation of Light Extraction Techniques ........................................................ 33 4 MOTIVATION AND OUTLINE ................................................................................ 40 5 RE VIEW OF CORRUGATED STRUCTURES FOR WAVEGUIDE MODE EXTRACTION ......................................................................................................... 42 6

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5.1 Corrugated Structure ........................................................................................ 42 5.1.1 Pattern Effect ........................................................................................... 42 5.1.2 Periodicity ................................................................................................ 45 5.1.3 Corrugation Depth ................................................................................... 46 5.1.4 Refractive Index Contrast ........................................................................ 48 5.2 Fabrication Methods ......................................................................................... 51 6 LIGHT EXTRACTION OF GREEN FLOURESCENT OLEDS BY DEFECTIVE HEXAGONAL CLOSE PACKED ARRAY WITH DIFFERENT PERIODICITY ........ 65 6.1 Review and motive............................................................................................ 65 6.2 Experimental Procedure ................................................................................... 66 6.2.1 Defective HCP Grating Fabrication ......................................................... 66 6.2.2 Device Fabrication and Measurement ..................................................... 67 6.3 Result And Discussion ...................................................................................... 68 6.3.1 Fabrication and Characterization of Defective HCP Structure ................. 68 .................................................... 72 6.4 Summary .......................................................................................................... 76 6.5 Acknowledgements ........................................................................................... 76 7 LIGHT EXTRACTION OF CORRUGATED OLEDS FABRICATED ON HIGHREFRACTIVE INDEX SAPPHIRE SUBSTRATE ................................................... 84 7.1. Review and motive........................................................................................... 84 7.2 Experimental procedure .................................................................................... 85 7.2.1 Fabrication of Corrugated Sapphire Structure ......................................... 85 7.2.2 Device Fabrication and Measur ement ..................................................... 87 7.3 Result and Discussion ...................................................................................... 88 7.3.1 Optical Simulation of OLEDs Fabricated on Sapphire Substrate ............. 88 7.3.2 Device Characterization .......................................................................... 88 7.4 Summary .......................................................................................................... 91 7.5 Acknowledgements ........................................................................................... 92 8 LIGHT EXTRACTION OF CORRUGATED OLEDS WITH REFRACTIVE INDEX CONTROL .............................................................................................................. 97 8.1. Review and motive........................................................................................... 97 8.2 Experimental Procedure ................................................................................... 99 8.3 Result and Discussion .................................................................................... 101 8.3.1 OLED Configuration . ............................................................................. 101 8.3.2 Corrugated First Antinode OLEDs ......................................................... 102 8.3.3 Dispersion Curve and Electric Field Intensity Distribution of Waveguide Modes for First Antinode Corrugat ed OLEDs ........................... 104 8.3.4 Corrugated Second Antinode OLEDs .................................................... 105 8.3.5 Full Extraction with Extra Substrate Mode Extractor for First Antino de OLEDs ......................................................................................................... 106 8 .4 Summary ........................................................................................................ 107 7

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8.5 Acknowledgements ......................................................................................... 108 9 CONCLUSIONS ..................................................................................................... 97 LIST OF REFERENCES ............................................................................................. 116 BIOGRAPHICAL SKETCH .......................................................................................... 125 8

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LIST OF TABLES Table page 8 1 The maximum current efficiency of corrugated OLED with first antinode condition ........................................................................................................... 112 8 2 The maximum current efficiency and EQE from the LC and HC grating devices with an additional macrolens ............................................................... 115 9

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LIST OF FIGURES Figure Page 1 1 Basic OLED configurations ................................................................................. 18 1 2 Total internal reflection and three allowed optical modes in OLEDs ................... 19 2 1 Schematic energy band diagram with OLED consisted of well known NPB/Alq3 bi layer structure .................................................................................. 24 2 2 Schematic process of excitation, radiation, and nonradiation process and definition of PL quantum yield. ............................................................................ 25 3 1 Microlens array for light extraction. ..................................................................... 34 3 2 External scattering layer for the substrate mode. ............................................... 34 3 3 Sandblasting techniqu e . .................................................................................... 35 3 4 Refractive index and transmittance control as function of 25NPs concentration of the external scattering film . ...................................................... 36 3 5 L ow index grid. ................................................................................................... 37 3 6 Efficiency as a function of ETL thickness for OLED fabricated on high refractive index substrate (n>1.7) ....................................................................... 38 3 7 Photonic crystal (PC) structure. .......................................................................... 39 5 1 Schematic of a corrugated OLED. ...................................................................... 55 5 2 Schematic of the OLED incorporating with corrug ated PEDOT:PSS and optical and physical properties. .......................................................................... 56 5 3 OLED incorporating with perfectly periodic corrugated ITO. ............................... 57 5 4 Corru gated OLED incorporating with quasi periodic structure ............................ 58 5 5 Calculated dispersion and relation between grating period and emission wavelength for the thin film mode and the SP mode. ......................................... 59 5 6 Corrugation depth on electrical effect. ................................................................ 60 5 7 Depth effect on device performance ................................................................... 61 5 8 Characteristics of PC structure with different refractive index contras t . .............. 62 10

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5 9 Calculated and experimental results for relation between periodicity and outcoupled emission wavelength of the waveguide mode .................................. 62 5 10 Schematic of OLEDs employing PC with different refractive index contrast ....... 63 5 11 Schemat ic of corrugated structure fabrication process for silver nanoparticles and AFM image .................................................................................................. 64 6 1 SEM images of silica array embedded in polystyrene layer before and after heat treatment .................................................................................................... 78 6 2 AFM images, line profiles and power spectral density on corrugated structure with different periodicity. ..................................................................................... 78 6 3 Calculated electric fi eld distribution in OLEDs .................................................... 79 6 4 Electrical and optical characteristics of 1periodic structure. ........................ 80 6 5 Electrical and optical characteristics of both corrugated structure with different periodicity. ............................................................................................ 81 6 6 Electrical and optical characteristics of 0.5periodic structure ...................... 82 6 7 Angular dependence of emitting light with and without hemisphere lens f or corrugated OLEDs. ............................................................................................. 83 7 1 Schematic diagram of the fabrication process for the quasi periodic hemisphere pattern sapphire substrate. ............................................................. 93 7 2 SEM and AFM characterization on the corrugated sapphire substrate with nominal periodicity of 260 nm. ............................................................................ 94 7 3 Optical and electrical characteristics of OLED fabricated on corrugated and planar sapphire substrate. .................................................................................. 95 7 4 EL at normal direction for corrugated and planar OLEDs fabricated on sapphire substrate .............................................................................................. 96 8 1 AFM image of the top view and the depth of the corrugated resin structure fabricated on the glass substrate ...................................................................... 109 8 2 Simulated power distrbution as a function of ETL thickness ............................ 110 8 3 Electrical and optical charateristics of corrugated OLED with first antinode ..... 111 8 4 Electric field intensities of one SP( TM 0) mode and one ITO/organic (TE0) mode mode as a function of the position inside of the devices ......................... 113 8 5 Electrical and optical characteristics of second antinode corrugated OLEDs ... 114 11

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8 6 Angular distribution of corrugated OLEDs with first antinode with and without macro lens ........................................................................................................ 114 12

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Abstract of Dissertation Presented to the Graduate School of the University of Flo rida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy LIGHT EXTRACTION OF ORGANIC LIGHT EMITTING DIODES USING CORRUGATED STRUCTURE S By Wooram Youn May 2015 Chair: Franky So Major: Materials Science and Engineering Light extraction is critical to the efficiency of organic light emitting diodes (OLEDs). Because of the internal reflection, only 20~30% of generated light in OLEDs is emitted across the top interface and detected by observers. The balance is co ntrolled by the refractive indices of each side of the interface and the angle at which light is incident upon the interface. In particularly, the generated light in OLEDs is dominated by three different modes: air mode, substrate mode, waveguide mode because of the presence of two interfaces of air (n=1) substrate (n=1.52) and substrate (n=1.52)/indium tin oxide (ITO) (n=1.8). To enhance the light extraction efficiency in OLEDs, intensive amount of studies and researches has been followed. The air/glas s interface in OLEDs may be modified using microlens arrays, sandblasting, or light scattering films, which can extract the substrate mode by a factor of 2. For waveguide mode extraction, the light extraction technique called corrugated structures can be utilized. Therefore, increase of light extraction based on the corrugated structures in OLEDs is focus of this research. Corrugated structures with quasi periodic patterns were created on fluorescent green OLEDs. The effects of a peak periodicity of 0.5 an d 1 um was both measured 13

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experimentally and predicted by model. The smaller periodicity of 0.5 um was found to give 35% higher in extraction efficiency. To quantify the effects of internal reflection, materials with different indices were chosen for the substrate, for examples, glass (n=1.52) and sapphire (n=1.76). And this results in external quantum efficiency (EQE) of 63% in OLEDs using both corrugated structure and macro lens. Finally, we demonstrated an EQE of 67% by adding lithium fluoride (LiF) layer with refractive index of 1.39 between glass and ITO layers by increasing of the refractive index contrast for diffraction mechanism. Proper choice of materials with refractive index in corrugated OLEDs can increase the light extraction efficiency by a fac tor of more than 2. . 14

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CHAPTER 1 IN TRODUCTION 1.1 General Development of OLEDs OLEDs have been rapidly developed since Tang and VanSlyke introduced green OLED with high brightness.[ 1 ] T he structure of OLED is organic bilayer of 8 hydroxyquinoline aluminum (Alq3) as electron transfer layer (ETL) /emitting layer (EML) and an aromatic diamine as hole transport layer (HTL) sandwiched by transparent ITO anode and reflecting alloyed metal cathode comprised of m agnesium (Mg) and silver (Ag) that injects holes and electro ns respectively . Such noble OLED can operate at high brightness, which can meet the requirement for display and lighting applications. Unfortunately, poor injection property from the electrodes led to the high operating voltage for the brightness of 1000 c d/m2, which is standard reference for display applications , resulting in undesirable high power consumption and very short lifetime of the OLED . I n 1997, Hung et al . put forward much improved injection property for the OLEDs using a bilayer cathode that consist of very thin 1~2 nm thick lithium f luoride (LiF) layer adjacent to a ETL and aluminum, resulting in low operating voltage at high brightness .[ 2 ] This improved injection property is attributed to reduced energetic barrier between ETL and cathode, almost like forming Ohmic contact, that can inject the charge carrier much better. And similar effort at the other side to reduce the barrier between anode and HTL was demonstrated that simple ultraviolet o zone (UVO) treatment on pre coated ITO on the glass substrate can facilitate the hole injection better .[ 3 , 4 ] T he UVO treatment plays an important role to increase work function of ITO by removing carbon contaminants on the ITO surface and simultaneously leaving a tindeficient a nd oxygenrich surface . Another well known treatment regarding on hole injection property 15

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is to insert poly(3,4ethylenedioxythiophene):poly(4styrenesulphonate) (PEDOT:PSS) layer developed by BAYER AG Corp. between anode and HTL.[ 5 7 ] T he high work function and hole mobility of PEDOT:PSS is credited for improved injection behavior. 1.2 Development f or Internal Quantum Efficiency in OLEDs Although such efforts pushed OLED further steps close to commercialization, the poor efficiency of device performance was remained as challenge. T he parameter to directly influence the efficiency was the fraction of radiative excitons formed through the recombination of electrons an d holes. According to quantum electronic spin statistics, excitons can be categorized into two states of singlet and triplet with generation ratio of 1:3. The contribution to generate photon is only from singlet exciton relaxation as known as fluorescence, while another radiative transition from triplet excitons to the ground state, as known as phosphorescence , is forbidden due to disallowed spin configuration by Pauli exclude principle. Meanwhile, the energy of triplet is attenuated by nonradiative transi tion, which eventually generates unwanted heat that can degrade organic materials .[ 8 ] Thus, such limited efficiency of OLED cannot exceed more than 25% in internal q uantum efficiency (IQE) which is the ratio of the number of photon generated to the number of electron injected. Fortunately, Baldo et al. suggested that using phosphorescent organic emitters enable to achieve 100% of IQE from both singlet and triplet .[ 9 ] Such forbidden transition from triplet can be allowed by spin orbit coupling due to presence of heavy metal components like platinum (Pt) and iridium (Ir) in emitting small molecule compounds .[ 1015] 1.3 The Role of Total Internal Reflection in Bottom Emission OLEDs In the view of efficiency of how to utilize the generated photon, only 20~30% of total generated photon can be exploited in conventional bottom emitting OLED st ructure 16

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as shown in Figure 11B .[ 13, 1618 ] Since the OLED structures consist of transparent electrode, organic layers , bottom metallic electrode, and glass substrate, the TIR due to refractive index mism atch between air (n=1), glass (n=1. 5 2 ), ITO/organic layers (n=1.7~2) allows only 20~30% of the generated photon inside of OLED to be outcoupled into air, as show n in F igure 1 2. The rest of significant amount of 70~80% generated photons are confined and trapped in substrate (substrate mode) and ITO/organic layer (waveguide mode), accordingly. Obviously the outcoupling efficiency is highly influenced by the configuration of the OLED structure. T opemitting OLEDs, as shown in F igure 1 1A , can be a solution since generated light will only experience the TIR at the interface between air and transparent electrode. However , physical damage on soft organic layers by sputt ering top transparent ITO is quiet severe, which makes such configuration unfavorable .[ 19 ] A lthough u sing thin and transparent metal layer to replace the damageinducing ITO was suggested, both comparable transparency and proper energy alignment of such electrode with organic layer for injection is difficult to be satisfied .[ 20 ] T hus, bottom emitting OLED configuration has been utilized as conventional OLED structure since transparent conducting ITO can easily offer excellent transmittance in vi sible wavelength and also inject charge carrier to adjacent organic layer with ideal Ohmic contact. 17

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Figure 11. Basic OLED configurations; A) Topemitting, B ) Bottom emitting A B 18

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Figure 12 . Total internal reflection and three allowed optical modes in OLED s 19

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CHAPTER 2 PARAMETERS TO DETERMINE THE EFFICIENCY OF OLEDS 2 .1 OLED O peration The operating principle of light emission from OLED s is recombination of injected holes and electrons from each their electrode. Once electrons and holes meet and are bound by columbic force, excitons are formed. T hese excitions with excited st ate s lose their energy by emitting the light. Figure 21 shows well known bilayer structure of OLED with flat band condition described by electronic energy band diagram. O nce forward voltage is applied at onset bias in the OLED , hole carriers are injected into HTL like (N,N’bis(naphthalene1 yl) bis (phenyl)benzidine) (NPB) from a anode like ITO, while electron carriers are also injected into ETL like tris (8 hydroxyquinoline) aluminum (Alq3) from a cathode like Al . Eventually, these both carriers form ex citons at an emitting molecule, Alq3, which leads to emission of photons in OLEDs. To understand how effectively such photon generation happens, we shall understand the measure of efficiency of OLED, which is external quantum efficiency.[ 21] 2 . 2 Quantum Efficiency 2.2.1 External Quantum Efficiency ( ext) External q uantum efficiency (EQE) is defined as the number of photons emitted through the front face of the device per the number of injected electrons. It can be also written as ,int ph ext ext is the external quantum efficiency, int is the internal quantum efficiency , and ph is the out coupling efficiency . int indicates how photons are generated by injected 20

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electrons. And ph is a fraction of light that eventually escapes the struc ture and contribute to useful light source from total generated photons inside OLEDs. 2.2. 2 Internal Quantum E fficiency ( int) Internal quantum efficiency (IQE) is defined as the number of photon generated per the number of electron injected and expressed as Equation 22, which is .intpex the charge balance factor, ex the radiative transition, and p the photoluminescence quantum yield, determine the internal quantum efficiency . 2.2. 2 .1 Charge b alan ce ( The charge balance factor is a ratio between injected hole and electron carriers. Excitons are formed by the recombination of hole and electron carriers. To form excitons with 100% efficiency, it is required same number of injected electron and hole carriers. In order to achieve the ideal in OLEDs , injection characteristics of OLEDs are very important. By forming O hmic contact at electrode/organic interfaces, the injection of carriers can ’ t be limited. For anode side, ITO is the commonly used anode for injecting the hole carriers. By using UVO t reatment, Chlorinated ITO, very thin MoOx , HAT CN, or PEDOT:PSS, the ideal injection at the organic/anode interface can be achieved.[ 6 , 2228] For cathode, using alloyed metal like Mg:Ag was introduced earlier for the ideal electron injection.[ 1 ] However, the strong oxidation of Mg degrades the OLED very quickly. To replace suc h sensitive metallic cathode, the bilayer electrode consisting of Al and a nanometer thin LiF was introduced with ideal injection efficiency and enhanced stability of OLEDs.[ 2 ] 21

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2.2. 2 .2 Radiative t ransition ( ex) T he radiative transition ratio is defined by the fraction of the excitons which lead to radiative path. O nce injected electronhole pairs recombine and become excitions, the formed ex citons can be categorized into singlet and triplet excitons due to t heir quantum electronic spin status. And t he generation ratios of single t and triplet are 1:3, as calculated by quantum spin statistics.[ 29] For fluorescent emitters , only singlet excitons can decay as emission transition, while the radiative transition from triplet state to the ground state is forbidden due to the incoherent spin status , result ing in that the ex for fluorescence is only 25% of total number of generated excitons . However, for phosphorescent emitters, both singlet and triplet excitons are able to follow radiative decay transition, which leads to 100% of ex. Organic molecules consisting of heavy metal element like Ir, Pt, or Ru can use spinorbit coupling, making the disallowed radiative transition from triplet excitons possible.[ 9 , 10, 13, 30] Furthermore, according to Adachi ’ s discovery, the utilization of 100% radiative transition from fluorescent emitte r can be realized through thermally activated delayed fluorescent mechanism . [ 31] 2.2. 2 .3 P hotoluminescence quantum y ield ( p) Photoluminescence (PL) quantum yield is a measure of photophysical property of illuminating materials . Generally speaking, it is the ratio of the number of emitted photons per the number of absorbed photons. As shown in Figure 22 , the formed excitons decay eventually occurs by radiative or nonradiative transition. As reported, the novel phosphorescent emitter, for example, Ir(ppy)3 was found with the PL quantum yield close to 100% , implying that all generated excitons are losing their energy by emission process.[ 32, 33] In general, the PL quantum yield is highly sensitive to concentration of exciton.[ 34, 35] Thus, doping of such ‘ guest ’ materials at which the 22

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excitons form into ‘ host ’ materials with efficient energy transfer property between the guest and host can eliminate such problem.[ 35 , 36] 2.2.3 Out coupling efficiency ( ph) The out coupling efficiency is defined as the number of photons emitted through the front surface of substrate per the number of photons generated within the devices. To a firs t degree approximation, the out coupling eff iciency ph is given by E quation 2 3 , 2 / 12nph ph

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Figure 21. Schematic energy band diagram with OLED consisted of well known NPB/Alq3 bilayer structure 24

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Figure 22 . Schematic process of excitation, radiation, and nonradiation process and definition of PL quantum yield. 25

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CHAPTER 3 REVIEW OF LIGHT EXTRACTION TECHNIQUES IN OLEDS As addressed previous ly in Chapter 2, the most limiting factor for OLED efficiency is the outcoupling efficiency. Since the TIR is inevitabl y present at two major interfaces: air/glass and glass/ITO, the maximum outcoupling efficiency is only 20~30% in typical bottom emitting OLED configuration. In order to recover those 70~80% of optical power loss in OLEDs, various and innovative approaches have been introduced . First, th e light extraction techniques aimed to the outcoupling of the substrate mode, the light confined inside of glass substrate, will be discussed . Second, the introduction and working principle of light extraction techniques for the waveguide mode, the light confined and guided inside of highindex (n=1.7~1.9) ITO/organic layers, will be followed. 3.1 Light Extraction Techniques f or Substrate Mode For substrate mode, the photon generated in the highindex organic layers escapes with the incident angle less than the critical angle induced by the ITO/glass substrate but is eventually confined inside of the substrate since the incident angle of the escaped photon is still larger than another critical angle introduced by air/glass interface. Thus, 20~30% of generat ed photon is trapped and guided inside of the substrate. To utilize the lost fraction of light trapped as the substrate mode, the modification of the interface at air/glass can extract such trapped light into air by eliminating the TIR at the air/glass int erface. 3.1.1 Microlens Array By forming array of well ordered hemispherical like lens having micrometer size on the back side of glass substrate as shown in Figure 31A and B , the light that is 26

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originally trapped in the glass substrate can escape into air . Since entering all the light to the lens/air interface will have the normal incident angle, such trapped photons do not suffer the TIR at the interface any more, making substrate mode extractable. Moller et al . has introduced the array of ordered microlens on the back side of glass substrate with an efficiency enhancement factor of 1.5 for OLEDs.[ 37 ] According to Eom et al., it can be improved further by optimizing the contact angle between the lens and the surface.[ 38, 39] I t was c onfirmed that using the array of 10 m diameter microlens with the contact angle of 85 degree enhanced the OLED efficiency 70% more compared to that without the microlens array. 3.1.2 External Scattering Layer As another approach for the substrate mode ex traction, the texturing on the air glass interface can extract the substrate mode by light scattering as illustrated in F igure 3 2 A . Cheng et al . demonstrated the meshed scattering layer can diffuse the trapped substrate mode, resulting in the enhancement of 46% in outcoupling efficiency .[ 40] F igure 3 2 B shows the light scattering layer made of poly(dimethyl siloxane) ( PDMS ). Self organized porous anodic aluminum ox ide is utilized for scattering pattern via electrochemical process .[ 41] And Figure 33C shows the photo image of the external layer applied on the glass substrate wi th the strong scattering effect using relatively sim ple fabrication process.[ 42] T he outcoupled light with incident angle greater than the critical angle induced at the ai r /glass interface mainly contributed to such enhancement factor proving that the light that would trap in the substrate mode was successfully coupled out by the scattering layer. However, it should be also pointed out that the scattering phenomenon occur s also negatively on the light that would esca pe the 27

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interface (air mode), making full extraction of both air mode and substrate modes difficult. 3.1.3 SandBlasting T he working principle of sand blasting technique is the light scattering as same as the external scattering layer . Instead of applying such functional layer through additional fabrication process , sand blasting technique creates the scattering center s by directly roughening the glass substrate.[ 4345]. And Figure 33 A shows the optical microscope image of the surface of glass treated by the sandblasting . OLEDs using the treated glass substrate shows blurred emission, indicating the strong light scattering of bot h air mode and substrate mode as show n in Figure 33 C . Chen et al. suggested that a dditional roughening on the edge of t he glass substrate can even increase ~10% more of light extraction in forward direction, as shown in Figure 33B .[ 43] Thus, the sandblasting on both edge and surface of the glass substrate was found total 20% enhanced light extraction efficiency. Although the enhancement is not large enough to extract substrate mod e out effectively, relative simple and low cost process can be employed for such as OLED lighting applications 3.2 Light Extraction Techniques for Waveguide Mode F or the waveguide mode, a significant amount of the 40~60% photons are confined and guided in side of the highindex ITO and organic layers. Since the total thickness of the high index ITO and organic layers is 300 ~ 400 nm, only selective optical modes can be excited: these include thin film and surface plasmon (SP) modes. The thin film guided mode, or ITO/organic mode, is trapped and guided by the ITO/glass interface due to the mismatch of refractive indices between them. When the photons or electrons in the highindex dielectric media are incident upon the metal layer 28

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consisted of a density of fr ee electrons on its surface, surface plasmon polaritons (SPPs) in the forms of electromagnetic waves propagating along the dielectric/metal interface can be excited. B ecause the OLEDs consist of organic and metal layers, the condition for excitation of SP mode by the incidence of generated photons can be satisfied. T he in plane components of wave vectors of the SP mode are different than those of a photon at the same frequency in vacuum. Thus, the energy can’ t be coupled into the radiation process, but will eventually be dissipated to the metal layer, resulting in an increase in the operational temperature of the device. Unlike the substrate mode light extraction techniques, which are applied on the backside of the substrate where the OLED operation is not affected, the light outcoupling methods involved to engineer the ITO/glass interface can cause electrical perturbation of OLEDs . This make s the waveguide mode extraction more difficult. F ortunately, light extraction techniques satisfying such complicated r equirement have been introduced. Here, the light extraction methods for the waveguide mode will be introduced and discussed in detail. 3.2.1 Internal Scattering Layer T he internal scattering layer is normally placed between the substrate and transparent ox ide electrode, ITO, in order to recover the light trapped in the ITO/organic layer . In contrary to methods for extracting substrate mode , this method regarding to the waveguide mode extraction is require d to take into account on the aspect of OLED fabricat ion and operation in addition to the light extraction ability , because the location of such functional layers affect also the electrical properties of the ITO and organic active layer directly. Chang and coworkers demonstrated the internal scatter layer comprised of nanocomposite materials, which is based on relatively simple solution 29

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process and resulted in twofold enhancement in optical out coupling efficiency .[ 46] Also, the introduced external lay er is as flat as ITO layer, indicating that the electrical property of OLEDs is hardly altered. It was also reported that the smooth scattering layers were made by spincoa ting of the mixture of 25/250nm sizeTiO2 nanopar ticles (NPs) (n=2.6) and relatively low refractive index photoresist (n=1.5) .[ 47] Since the 25 nm NPs are too small to interact with the emission wavelength of OLE D s (400~700 nm in visible range of emission wavelength) , it mainly works for controlling the refractive index and flatt ening the film surface as shown in Figure 34. On the other side, the 250nm NPs are very strong light scattering centers in that mixture. Thus, by balancing the ratio, they were able to fabricate such scattering layers with refractive index similar to the ITO layer, so that more light can easily enter to the scatter layer and the entered light will experience strong scattering effect, resul ting in two times higher in light output than one without the scattering layer. However, this technique is only targeting to the thin film guided mode by controlling the ITO/glass interface. Thus, t he SP mode induced by the organic/metal interface remains still as significant optical power loss channel. 3.2.2 Low Index Grid Forrest group first introduced the way to extracting waveguide mode by embedding low index grids (LIG) between the ITO and organic layers, as illustrated in F igure 3 5A .[ 48 ] T he light that would be trapped and guided as the waveguide mode will enter the low index region and be redirec t ed toward the substrate normal . In addition, the LIG does not affect the light that originally escapes to the substrate. The role of refractive index of the LIG plays an important parameter on the redirected light. T he lower the index, the higher incident angle light can be refracted and propagated into the substrate normal, eventually into air mode. According to their follow ing work, the 30

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efficiency was fur ther enhanced up to 2.3 times using the OLED with ultra low index grid (n=1.10~1.15) than one without the grid (see Figure 35B ) .[ 49] F igure 3 5 C presents SEM images of the ultra low index grid made of porous SiO2 by glancing angle vapor deposition.[ 50, 51] Such low refractive index of the LIG is contributed by the very small size of the pore size in the range of ~10nm . Furthermore, it does not distort the original emission spectrum since the width of the grid is not in the range of the wavelength of emitting light so that the interference with respect to the low index materials can ’ t occur . However, the microsize patterning process for the LIG is not that simple and also not applicable for largearea application , which makes this approach not practical for general lighting applications 3.2.3 High RefractiveIndex Substrate Th e relatively low refractive index of the conventional glass substrate (n=1.52) consequently become the bou ndary that confine the wav e guide mode in the high refractive index layers of ITO/organic. Fortunately, the effort to replace it with highrefractive i ndex substrate (n>1.7) has been followed.[ 5254] Leo group successfully demonstrated the OLED fabricated on such highrefractive index sapphire substrate , resultin g in disappearance of the thin film guided mode.[ 54] The fraction of energy lost to the thinfilm guide mode is now transferred to the substrate mode. By using the substrate mode extractor, they demonstrated the peak 45% EQE from red emitting phos phorescent iridium ( III) bis [ _2 methyldibenzo( f, h ) quinoxaline]( acetylacetonate) [ Ir ( MDQ )2( acac )] based OLEDs, which is consistent with simulated result , as shown in F igure 3 6 . However, the strong loss by the SP mode , corresponding 20~30% of generated pow er , due to interaction between free electrons on the metal surface and electromagnetic field generated by OLED s, remain as absolute efficiency loss channel . 31

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To minimize the loss by the SP mode, b y placing the emitting zone away from the metal layer, the intensity of SP mode exponentially drops while the EQE of the OLEDs increased further up to 55% with the substrate mode . However, from the aspect of fabrication cost for the high index substrate , it is much more expensive than conventional glass substrate . 3.2.4 Photonic Crystal P hotonic crystal (PC) structures c onsist of emission wavelengthscale periodic modulation between highand low index refractive materials . In that way, light propagating in lateral direction like the waveguide mode in the PC struct ure can’ t be allowed, instead will be diffracted out of the structure due to the formation of photonic band gap, a range of frequencies in which the propagation of light is forbidden. And Figure 3 7 A and B show SEM image and schematic configuration of the PC structure . In 2003, Lee et al. first introduced the PC structure within Alq3based green fluorescent OLEDs with efficiency enhancement factor of 1.5.[ 55] T he PC used in this work consisted of SiNx (n=1.9) and glass (n=1.50) and were fabricated between glass and ITO. Another approach utilizing higher depth of PC layer of 430 nm demonstrated that the PC assisted polymer red LED increases in efficiency 2.3 times higher than one without PC. [ 56]However, such high enhancement was because significantly low efficiency of control device, less than 0.1 cd/A. And the Figure 37D and (e) show far field radiation intensity measured on top of emitting glass substrate. Especially, the PC OLED showed non uniform emitting surface due to the limited directions at which PC effect occurs . Furthermore , due to well define periodicity of the structure, the outcoupling enhancement is limited at certain emission wavelength and angle, as shown in F igure 3 7C . These drawbacks of PC s for OLED light extraction application must be solved for 3 2

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practical light extraction techniques that require uniform emitting pixel and independence on emission wavelength and angle. 3.3. Limitation of Light Extraction Techniques As described, the light extraction techniques have showed enhancement outcoupling between 50% and 230%. For the substrate mode extraction techniques, most of them can be applied as practical use in OLED light extraction bec ause their characteristic extraction such as uniform enhancement all over the area of emitting surface of OLED, emission wavelength, and angle, is attracting enough to be considered in practical light extraction techniques for OLEDs . Additionally , their si mple fabrication process is another advantage. For the waveguide mode, however, it still challenges to become as potential light extraction method since their applications are still not optimized due to the complicated process, dependence on specific emis sion wavelength and angle, and limited extraction efficiency. Most importantly, it is impossible for largearea application like TV display and lighting applications. 33

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Figure 31. Microlens array for light extraction; A ) schematic of light extracti on scheme by microlens array, B ) SEM image of top view of microlens array on the back side of substrate, (inset: tilted cross section SEM image of microlens array attached on the substrate). Figure 31B is from the reference 37. Fi gure 32. External scattering layer for the substrate mode; A ) schematic of ray tracing inside substrate with a nd without the scattering layer, B ) SEM image of the film on top , (inset: cross se ction SEM image of the film), C ) photo of scattering film on the glass substrate. Figure 32 B and C is from the reference 41 and 42, respectively. A B A B C 34

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Figure 3 3. Sandblasting technique; A ) optical microscope image o f rough surface of the glass, B) edge sandbasting C) surface sandblasting D ) untreated. Figure 3 3 is from the reference 43. A B D C 35

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Figure 3 4 . Refractive index and transmittance control as function of 25NPs concentration of the external scattering film. Figure 34 is from the reference 47. 36

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Figure 3 5 . Low index grid; A ) schematic of light extraction by low index grid, B ) refractive index of ultralow index porous SiO2 fabricated by glanced deposition technique, C) SEM images of tilted and cross section view of the porous low index SiO2. F igure 35A is from the reference 48 and Figure 35B and C are from the reference 49. A B C 37

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Figure 3 6 . Efficiency as a function of ETL thickness for OLED fabricated on highrefractive index substrate (n>1.7) Figure 36 is from the reference 54. 38

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Figure 3 7 . Photonic crystal (PC) structure; A ) S EM image of photonic crystal, B) schematic of PC OLED, C) EL of PC OLED, (D ) Farfield radiation image of light emission from one without PC, E ) Far field radiation image of light emission from one with PC. Figure 37 is from the reference 55. A B C D E 39

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CHAPTER 4 MOTIVATION AND OUTLINE OLEDs have been notified as one of revolution in technologies: flexibility, intuitively simple process, excellent color tunability, and fast response. But, OLEDs faced critical challe nge of limited efficiency of 20~30% EQE, that is, outcoupling efficiency. To understand the limiting factor to the low outcoupling efficiency , it is very critical that presence of the optical interfaces induced by the difference in refractive indices withi n the OLED device play a key role for the efficiency. For a typical bottom emission OLED structure, it consists of organic/ITO lay ers with refractive index of 1.7~2 and glass substrate with refractive index of 1.52. Since two major optical interfaces at gl ass/ITO and air/glass exist, the TIR induced by such interfaces allows only 20~30% of total generated photons to escape and contribute as useful light sources, which is the definition of the outcoupling efficiency .[ 16, 57] Otherwise, the rest of light is confined and tra pped as substrate and waveguide modes . As discussed in Chapter 31, the recovery of the substrate mode is no longer a problem for the low efficiency since the extraction techniques have been developed with e outcoupling efficiency, simple fabrication process, and largearea applicability. F or the waveguide mode, accounting for significant amount of 40~60% total power, there was not so a clear solution, because the all mentioned methods for the waveguide mode are not effective, and strong ly dependen t on selective emission wavelength and angle, limited application size, and complicated process like using holographic, electronbeam, and nanoimprint lithography techniques. However, the light extraction scheme utilizing corrugated OLEDs can overcome such limitations.[ 5868] S imilar as the PC structure, the corrugation of all ITO, organic, and metal layers in OLEDs can use the diffraction for the 40

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extraction of the thinfilm guided mode. Moreover , the corrugated nature of the metallic electrode can provide extra in plane wave vectors onto the SP mode, allowing the radiation of the SP mode possible. In addition to the extraction of both thinfilm mode and SP mode, the outcouple d EL is not dependent on the emission wavelength and angle by using quasi periodic corrugated structures. Also, various and simple fabrication methods exploited for corrugated structures are another advantage. And the detail working principle, important parameters and fabrication methods regarding the corrugated OLEDs will be r eviewed and discuss ed in C hapter 5. This dissertation focuses on efficiency of OLEDs incorporating corrugated structure with quasi periodic pattern by changing key parameters ; 1 ) b y fabricating the corrugated structures with different periodicity, the dependence of efficienc y of corrugated O LEDs are discussed in Chapter 6, 2) t he effect of refractive index of corrugated substrate for OLED light extraction is discussed in Chapter 7, and 3) the refractive index contrast effect between glass and ITO on corrugated device efficiency is discussed in Chapter 8. 41

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CHAPTER 5 REVIEW OF CORRUGATED STRUCTURES FOR WAVEGUIDE MODE EXTRACTION 5.1 Corrugated Structure Buckling OLEDs, or corrugated OLEDs, consist of all OLED stacks including transparent dielectric anode, organic layers , an d a reflecting metal electrode with an uneven surface, as illustrated in Figure 51A . Due to the corrugated interface between the ITO and glass, the diffusing reflection at that interface will take place, hindering the TIR which is a significant limiting f actor when determining the outcoupling efficiency . In addition, the corrugated metal reflector contributes to light scattering of the waveguide mode, increasing the light extraction efficiency . In this chapter, key parameters to determine the corrugated O LED performance and working principle will be discussed. 5.1 . 1 Pattern Effect As expected, depending on characteristic patterns of the corrugated structures, the light extraction can be controlled. Therefore, the corrugated OLEDs can be categorized by the pattern characteristics: random oriented, periodic and quasi periodic structures, as illustrated in Figure 5 1B, C and D . The randomized pattern for corrugated OLEDs can be useful for scattering the waveguide mode into the air. Several reports have shown t hat OLEDs utilizing random light scattering corrugated structures improve efficiency without deterioration of the emission spectrum or the angle profile. And owing to the naturally graded refractive index between the ITO and glass by the corrugation, improved transmittance is also attributed to the enhancement of the OCE as shown in Figure 5 2 C . 42

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Recently, the green phosphorescent OLED with a holeinjecting PEDOT:PSS layer having a random corrugated pattern was demonstrated with an enhancement factor of 1.7 .[ 63] Using an additional microlens array to extract the substrate mode, the total EQE was increased up to 41.1%. And due to the randomly oriented light scattering of the waveguide modes, the enhanced EL spectrum and angular intensity profile did not deviate from the control planar OLED , as shown in Figure 5 2 D and E . Secondly, the outcoupling mechanism , using perfect periodicity for the corrugated OLED , is based on the Bragg diffraction grating. Since the periodic refractive index modulation in in plane directions are present near two interfaces of ITO/glass and organic/metal, the condition for Bragg diffraction can be satisfied for coupling the waveguide mode into the air, as described by the diffraction grating equation: = = and ( 5 1) = 2 / , ( 5 2 ) w here denotes the absolute value of inplane component of wave vectors in free spaces, is the polar angle with respect to the surface normal, is the inplane component of the waveguide mode in corresponding medium, is a n integer, and is the grating wave vectors, which is directly related to the periodicity of the structures: shorter periodicity, larger grating wave vector. By using appropriate periodicity, the diffraction of both thinfilm and SP modes can be coupled out. Fujita et al. demonstrated that the OLED with perfect 300 nm periodic corrugated structure shows corresponding optical enhancement in emission spectrum ; the outcoupled thin film mode and SP m ode at ~530 nm, ~510 nm, and ~480 nm in emission wavelengths, 43

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respectively , as shown in Figure 5 3 D .[ 69 ] The maximum enhancement was obtained at a wavelength of ~ 530 nm with an enhancement factor of 2.3. However, corrugated structures with periodic and random patterns have critical drawbacks. Since the random structure utilizes randomly oriented light scattering, effective light outcoupling of the waveguide mode, especially the SP mode, is still limited. In terms of the periodic structure, the diffracted light is significantly dependent on emission wavelength and angle, because single or dual periodicity can only work on specific angles and emission wavelengths. C onsidering that typical OLED emissions display a broad spectrum, the enhanced emission spectrum by the periodic corrugated OLED is different from the original emission spectrum , as can be seen in Figure 5 3 D . Potentially, light extraction techniques must be applied for broadband white color ; distortion in the emission and angle profile can’ t fulfill such requirement s. To solve problems induced by these two types of corrugated OLEDs, the corrugated structures having quasi periodic patterns show effective l ight outcoupling of both the thin film mode and SP mode based on the diffraction mechanism, while maintaining the original emission spectrum and Lambertianlike angle profile. This is because, as shown in Figure 5 4 A , the broadly distributed periodicity ca n generate with broad ranges, which causes the waveguide mode to be coupled out into “broad” angle with “broad” emission wavelength. As reported by Koo et al. (2010), the buckling pattern with distributed periodicity, or quasi periodic, was successfully demons trated, proving the corrugated OLEDs with an optical enhancement factor of 2.2 and broadly enhanced EL spectrum as shown in Figure 5 4 B and C .[ 58] Additionally, Figu re 5 4A shows that such distinctive fast furrier transform (FFT) images suggest that the diffraction in all azimuthal 44

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angles can provide further optical enhancement, compared to perfect periodic structures which have only limited azimuthal direction for di ffraction. Many of similar works using such semi periodic corrugated structures for OLEDs were followed with enhanced efficiency and independence on angle and emission wavelength of the outcoupled light. 5.1.2 P eriodicity As briefly discussed in the prev ious section , periodicity is a key parameter used to determine the optical characteristic of the corrugated OLEDs incorporating Bragg diffraction grating. As indicated from the equation, grating wave vector is inversely proportional to the periodicity. For the outcoupling of the waveguide mode into normal direction by the first order diffraction, the Bragg diffraction equation is modified to : = . ( 5 3) Therefore, the grating wave vector must be equal to the inplane component of the waveguide mode . To characterize the waveguide modes in the structure of OLED, the inplane co mponents of the waveguide can be calculated using a well known transfer matrix method, taking into account the information about refractive indices and the thickness of each layer of OLED .[ 7072] T he calculated dispersion relationship between the inplane components of wave vector and the frequency are shown in Figure 5 5A . Since the values of the waveguide modes are different over all ranges of frequency, using a single periodicity can extract the waveguide mode on specific emission wavelengths into normal direction. And a typical EL spectrum of OLED employing singleperiodicity structure is shown in Figure 5 3D . As mentioned previously, Koo et al. also studied the relationship between periodicity and emission wavelength being coupled out into a normal direction for the waveguide modes using the calculated 45

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dispersion data, as shown in Figure 5 5 B . For the first order diffraction condition of t he waveguide mode into the normal direction , periodicity must be between 250 ~ 500 nm for outcoupling the waveguide mode with visible emission wavelengths (400~700 nm) . For quasi periodic corrugated OLEDs, the same principle of diffraction law is applied. As discussed previously, unlike the single value of the grating wave vector, the characteristic distribution of the periodicity can provide the grating wave vectors with a broad range, which produces the waveguide mode extraction over broad emission wavel engths (Figure 5 4 B ). According to the comparison study from the quasi periodic corrugated OLEDs between 1um nominal periodicity, the Alq3periodicity showed better optical enhancement with a factor close to 2 compared t o 1periodic corrugated OLED with an enhancement factor of 1.5. This is because the m periodicity can provide direct diffraction of the m periodicity is not enough to diffr act the waveguide mode into air . 5.1.3 C orrugation Depth To determine the optical enhancement factor, the corrugation depth is equally as important as periodicity. This is because the proportion of the propagating electromagnetic waves being perturbed i s dependent on the grating height .[ 7376] W hen the height of the corrugated structure increases, the efficiency of Bragg diffraction increases in a parabolic like p rofile. Then, the diffraction efficiency saturates up to 80~90%. However, for the electric driven devices like OLEDs, the effect caused by the depth control is not only limited on the optical characteristics, but on the electrical properties of the OLEDs as well. This is because the OLEDs are fabricated on the grove 46

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surface of the corrugated structure, so that the induced electrical field is not uniform over the surface of the OLEDs .[ 69 , 77, 78] In particular , the difference in the electrical field is much stronger inside the organic layer since the electrical condu ctivit ies of the organic materials are relatively small. Also, Fujita et al. reported a simulation work based on static electric field s on 2D models for electrical characteristics of the corrugated OLED with different ratios between periodicity and depth.[ 69] Figure 56 A shows the result. The induced electric field at the locally thinner organic layer in a corrugated OLED is higher than that of a conventional planar OLED. Moreover, when the ratio of the periodicity to the depth decreases (or depth increases), the intensity of electric field produced in that thinner organic region of the corrugated OLEDs increases. Thus, the stronger electric field at the locally th inner organic layer due to the depth of the corrugated structure in OLEDs will facilitate the injection and transport of charge carriers better, compared to that of planar OLEDs. And current density voltage characteristics of OLEDs from Figure 5 6 B shows clearly that the current density of corrugated OLEDs at a fixed voltage is much higher than that of the planar OLED. Therefore , such electrical effects owing to the corrugation depth allow the corrugated OLEDs to operate with smaller amounts of power. T his partially explains why the power efficiency enhancement of the corrugated OLED is generally higher than current efficiency enhancement (see Figure 54 C ). However, if the depth of the corrugation increases too much, the stronger electric field at the thinner organic layer can’ t hold any more, resulting in an electrical shortage that will damage the charge balance between both electron and hole charge carriers and eventually decrease device efficiency. As 47

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reported by Kim ’ s group, when the depth increase d more th an 50 nm of corrugated OLED , the optical enhancement dramatically decreased, as shown in Figure 57 D .[ 79 ] The crosssection dual beam focusedion beam (DB FIB) image s of corrugated OLEDs with different depths of 15, 50, and 100 nm are shown in 5 7 A . In particular , the gap between organic and Al layers was found from the OLED with 50 and 100 nm depth, which might provide the possible leakage current paths that mi ght cause the damage of OLED operation. To confirm the presence of the leakage current, the optical enhancement factor was measured with respect to injected current density and power density (See Figure 57 B and C ). For the 80and 100nm depth devices, op tical enhancement with respect to the injected current were 19% and 27%, respectively, compared to the planar reference , which indicates that the degree of loss by interrupted charge balance is more severe than that of the outcoupling effect of the corrugated structure. In contrast, the optical enhancement with respect to the injected power density for devices with corrugation depth of 15nm and 50nm were 10% and 50%, addressing that the optimum corrugation depth for light extraction in corrugated OLEDs mu st consider both electrical and optical characteristics. 5.1.4 Refractive Index Contrast The diffraction outcoupling mechanism in waveguid ing environments like OLED s is triggered by the periodic modulation of the structures. In other words, the periodic m odulation of the refractive index is a key to implementing the diffraction phenomenon. The direct expression for the refractive index contrast effect as a function of the diffraction strength is predicted by Fresnel reflect ion , as described below: ( /), ( 5 4) 48

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where is the diffraction efficiency, n is the difference in refractive indices consisting of the diffraction grating, and nav is th e average refractive index between the two components of the diffraction grating.[ 8084] According to the simulation work using the plane wave expansion method , when n increases more than 2, complete diffraction will occur on the electromagnetic waves propagating in the structure by forming a photonic band gap , as shown in Figure 58 A .[ 84 ] From the calculated photonic band diagram shown in Figure 58 B , the refractive index difference of low index (n=1) and highindex (n=3.4) materials are enough to generate the photonic band gap where light propagation in the structure is forbidden in every direction, which will eventually lead to the coupling of the waveguide mode out of the structure via the Bragg diffraction. For a structure with periodic modulation of refractive index between 1.7 and 2.0, however , the contrast is not enough to open the gap , as shown in Figure 5 8 C . Therefore, it is predicted that for corrugated OLEDs, the region near the interface of glass (n~1.5) and ITO (n~1.8) would provide wea k diffraction on the thinfilm guided mode. However, the regions between the organic and metal layers in the corrugated OLED is expected to have a stronger diffraction upon the waveguide mode, especially SP mode, due to the much higher contrast of refractive index between organic (n~1.7) and metal layers (n~0.9 for Al) . To confirm the diffraction efficiency at two major corrugated interfaces due to the refractive index contrast , Ishihara et al. conducted an experimental comparison study using green fluorescent corrugated OLEDs with perfect periodicity of 300 nm.[ 85 ] A s shown in Figure 59 A , the calculated grating periodicity as a function of the outcoupled emission wavelength for the first order diffraction into the normal direction shows that 49

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the designed 300 nm single periodicity can outcouple the SP mode (transverse magnetic mode with zero order (TM0)) and thinfilm modes (transverse electric mode with zero order (TE0) and T M1)) at ~550, ~530, and ~480 nm, respectively . T his was confirmed by EL measurement comparison between the corrugated and planar devices, as shown in Figure 5 9 B . Compared to the enhancement responsible for the extracted thin film modes at ~ 480 and ~530 nm in emission wavelength, much higher enhancement responsible for outcoupling of the SP mode at ~550 nm in emiss ion wavelength was observed. This can be explained by the electric field intensity distribution of the waveguide modes with respect to the position in the OLED structure as shown in Figure 59 C . The electric field intensity of the SP mode is highly concent rated at the Alq3/Al interface due to the distinctive nature of the surface wave of the SP mode. Thus, the Bragg diffraction for the SP mode comes from the corrugated metal surface which has periodic modulation of the refractive index between the metal and organic materials. For the thinfilm guided modes (TE0 and TM1), the corresponding electric field intensities are concentrated around the glass substrate and IZO transparent electrode due to the TIR at the glass/IZO interface. So, the weak diffraction ind uced by such small periodic variations of 0.3~0.4 in the refractive index between glass (n=1.52) and IZO (n=1.8~1.9) in the corrugated OLED resulted in low enhancement at ~480 and 530 nm in emission wavelength from the corrugated device. To enhance the dif fraction efficiency upon the thinfilm guided mode, Kim et al. introduced a photonic crystal structure with a low index material called spinonglass (SOG) into OLEDs.[ 86] A systematic comparison study was performed between the SOGassisted (n=1.28) and SiO2assi sted (n=1.52) PC OLEDs. Figure 510 A and B 50

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show the schematic structures of PC OLEDs utilizing the PC structures with components of SiNx/SiO2 and SOG/SiNx. As shown in 5 10C , the focused ion beam SEM images of a cross section view showed the OLED fabricated on top of planar PC structure which consisted of SiNx and SOG. Figure 54 D shows the current efficiency vs. the current density of the OLEDs using the PC structures. Compared to the conventional planar OLEDs with current efficiency of 10.7 and 11.7 cd/A measured at current density of 20 mA/cm2, the OLEDs with SiNx/SiO2 and SOG/SiO2 PC structures show current efficiency of 17.4 and 21.6 cd/A, respectively , indicating that the light extraction efficiency using PCs was improved by 63% and 85% compared to those of conventional OLEDs. An additional enhancement of around 20% in current efficiency of the OLED with PC consist ing of SOG/SiO2 was achieved due to the improved diffraction efficiency by increasing the refractive index contrast in the PC structure. In a ddition to the increase of diffraction efficiency, Jeong et al. demonstrated that using higher contrast at the interface between a glass substrate and a low index amorphous fluoropolymer, poly [ perfluoro( 4 vinyloxyl butene) ] ( PPFVB) with refractive index o f 1.34 for a corrugated structure can improve the directionality of outcoupled waveguide modes. [ 87] 5.2 Fabrication Methods T he corrugated OLEDs showed various and attractive advantages for extraction of the waveguide modes, which accounted for 40~60% of the total generated power in the OLEDs . Recently, the focus related to such outcoupling techniques has been moved to the fabrication techniques for the corrugated structures. Typically, electronbeam, holographic, and nanoimprint lithography techniques are used to fabricate the corrugated structures.[ 69, 85, 8793] For electronbeam lithography, direct control of the 51

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electronbeam with high resolution can generate nanoand micropattern structures. And the holographic lithography employs the interference effect of two or more coherent optical beams. By controlling the wavelength and angle of the incident beams, the interference pattern with different periodicity and depth will be produced on target substrates. In contr ast to th e se two techniques, the nanoimprint lithography generates such corrugated structures by using a pre fabricated mold containing corrugated patterns. The key element to the fabrication process of nanoimprint technique is to press the mold over the t arget substrate with a temperature over its glass transition temperature. As a result, the identical pattern is transferred onto target substrates. However, all the lithography techniques are limited to a relatively small area, which is critical for such p rocesses as largeareademanding display and lighting applications. In addition, it requires complicated fabrication processes , which will inevitably increase the fabrication cost. To overcome th e se challenges, several innovative approaches have been intr oduced. As demonstrated by Koo et al . in 2012, the buckling structure with nanometer scale periodicity was successfully fabricated by employing the mechanism based on the difference in thermal coefficient constants between two adjacent aluminum and PDMS layers , as shown in Figure 5 4 A . [ 58 , 60, 9497] Using a simple the rmal evaporator, a 10nm thick aluminum layer was deposited on the thermally expanded PDMS, which was preheated to 100 C by an external radiation source. After cooling the sample down to ambient temperature, the compress stress was induced at the interfac e between the aluminium and PDMS due to the difference in thermal coefficients of the two layers, generating a buckling structure simultaneously. Figure 552

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4 A shows the AFM images of the fabricated buckling structures with distributed periodicity around 450 nm. In general, the depth D and periodicity of the buckling structure is proportional to each other, according to: .[ 98100] ( 5 5) To independently control the depth and periodicity, the compress stress play s a key role. Through another deposition of 10nm thick aluminum applied directly onto the b uckled PDMS and the cooling process, additional compress stress can be induced, which impacts only on an increase of the depth without change of periodicity. As an optimized depth of the buckling structure for light extraction, the three time deposition pr ocess of the 10 nm thick aluminum layer generated the buckling structure with 50~70 nm depth while the main periodicity was not different from the buckling structures generated by oneand two time deposition process, as shown in Figure 5 4A . Compared to the corrugated structures with well defined periodicity fabricated by typical lithography techniques, such corrugated structures formed by this technique show periodicity with broad distribution. Such characteristic distribution of the periodicity can effe ctively outcouple the waveguide modes in all emission wavelengths and all azimuthal angles by broadly distributed grating wave vectors, as discussed previously. In addition , the corrugated structures were fabricated simultaneously all over the substrate, ( 2.5 x 2.5 cm2), which indicates that the scalability of the corrugated structures can be significantly improved. Recently, it was reported that the OLED fabricated directly on the buckled aluminum/PDMS structures on a glass substrate can eliminate the extr a pattern transfer process, which makes this approach more practical. [ 60] 53

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Recently, another fabrication method was reported in 2014 by Zhou et al.[ 63] As illustrated in Figure 511A , corrugated structure was prepared by aggregated silver NPs on Si wafer through rapid annealing process. By heating a 10nm thick silver film spin coated on the Si wafer with temperature to 400 C for 1 min, the silver film is aggregated and becomes such silver NPs with different size. Using reactive ion etchi ng method, the silver NPs are used as etching mask, resulting corrugated structure directly on Si wafer. Using soft lithography, the pattern is transferred onto perfluoropolyether (PFPE) mold. A nd AFM image of corrugated structure prepared on PFP E mold is shown in Figure 511B . To fabricate the corrugated structure with different size and depth, the size of silver NPs and etching time are controlled.[ 64, 68] 54

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Figure 5 1. Schematic of a corrugated OLED; A ) Cross section view, and top view of B) well orient periodic, C) quasi periodic, D ) random patterns. A B C D 55

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F igure 5 2. Schematic of the OLED in corporating with corrugated PEDOT:PSS and optical and physical properties; A ) Schematic diagram of the OLED using corrugated PEDOT:PSS hole injecting layer with random like pattern and microlens array on the back side of the substrate, B ) AFM images of the corrugated PEDOT:PSS layers with different roughness, C ) Transmittance of the incident light from ITO side to the PE DOT:PSS deposited substrates, D ) EL at normal direction of c orrugated and planar devices, E ) Angular intensity profiles of the corrugated a nd planar OLEDs (dashed line is a reference for Lambertian profile). Figure 52 is from the reference 63. A B C D E A A A A 56

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Figure 53. OLED incorporati ng with perfectly periodic corrugated ITO; A ) schematic configuration of corrugated OLED using periodic ITO pat tern, B ) SEM images of corrugated ITO layer with periodic pattern, C ) c ross section SEM image of the OLED deposi ted on the corrugated ITO layer, D ) EL spectra of the corrugated and planar OLED measured at normal direction. T hree distinctive enhancement at ~470 nm, ~510 nm, and 530 nm is a result of Bragg diffraction coupling of the thinfilm and SP modes . Figure 5 3 is from the reference 69. A B D C 57

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Figure 5 4 . Corrugated OLED incorporating with quasi periodic structure; (a) AFM images of quasi periodic struct ure with three different depths and the characteristic periodicity distribution confirms the same periodicity of those three structures. In the inset, circular like pattern indicate the diffraction can occur all azimuthal directions , (b) EL spect ra of a planar (black) and two different buckling (red for smaller depth and blue for higher depth) OLEDs show the optical enhancement is coming from all emission wavelengths and for corrugated OLEDs. The angular profiles of both planar and corrugated devi ces show similar Lambertian pattern, indicating uniform light intensity over all polar angles, (C) c urrent efficiency and power efficiency of devices show maximum 120% and 190% enhancement compare to the planar OLED. Figure 54 is from the reference 23. A B C 58

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Figure 55. Calculated dispersion and relation between grating period and emission wavelength for the thin film mode and the SP mode; A ) d ispersion curve of waveguide modes in conventional bottom emitting OLED fabricated on the glass substrate; thinf ilm guided mode (black line) with zeroorder transverse electric (TE) polarization with zero order and SP mode (red line) with zeroorder transverse magnetic polarization are excited inside of highrefractive in dex ITO/organic lay er (n=1.7~2), B ) relation between grating periodicity for first and secondorder diffraction for normal direction and the outcoupled emission wavelength for thinfilm and SP mode. The dotted horizontal line is the nominal periodicity of 400 nm used for the corrugated devices. Figu re 5 4 is from the reference 23. A B 59

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Figure 56. Corrugati on depth on electrical effect; A ) s imulated static electric field intensity distributions , B ) device electrical characteristics of planar and corrugated OLEDs with different periodicity from 3 00 nm to 1 m . Figure 56 is from the reference 69. A B 60

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Figure 57. Depth effect on de vice performance; A ) crosssection DB FIB images of fabricated corrugated OLED s with depth of 15, 50, and 100 nm, B ) efficiency as a function of current density f or different depth , C ) efficiency as a function of power density for different depth, D ) enha ncement ratio of current and power efficiency to the corrugation height . Figure 57 is from the reference 79. A B C D A A A 61

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Figure 5 8 . Characteristics of PC structur e with different refractive index contrast ; A ) simulated photonic band gap size as a function of refractive index contrast , (t he required ‘onset’ contrast for opening the photonic band gap begins from 2 ), B ) photonic band diagram with PC consisting of n1= 1.7 and n2=2 , C ) photonic band diagram with PC consisting of n1=1 and n2= 3.4, (a ll simulated data are calculated by a plane wave expansion method). Figure 58A is from the reference 84 and Figure 58B and C are from 69. Figure 59 . Calcu lated and experimental results for relation between periodicity and outcoupled emission wavelength of the waveguide mode; A ) calculated grating periodicity for normal direction first order diffraction with respect to the outcoupled emission wavelength for the thinfilm modes (TE0 and TM1) and SP mode (TM0), (solid square, triangle, and circular symbols are exper imental points), B) EL measurement of both planar and corrugated OLEDs with perfect periodicity of 300 nm, C) e lectric field intensities of TM0, TE0 , and TE1 with respect to position inside of OLEDs . Figure 59 is from the reference 85. A B C A B C 62

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Figure 510. Schematic of OLEDs employing PC with different refractive index contrast ; A ) PC with SiNx and SiO2, B ) PC with SOG and SiNx. C ) f ocused ion beam crosssection SEM image of OLED fabricated on SOG/SiNx PC layer, D ) Current efficiency as function of current density for conventional and PC OLEDs, (u sing higher contrast in refractive index of PC structure shows highest efficiency among them ) . Figure 510 is from the reference 86. A B C D 63

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Figure 511. Schematic of corrugated structure fabrication process for silver nanoparticles and AFM image; A ) s chematic fabrication process of corrugated structure fabricated on Si wafer by aggregated silver nanoparticles through rapid annealing process in nitrogen environment , B ) AFM image of corrugated structure transferred onto PFPE mold. Figure 511 is from the reference 63. A B 64

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CHAPTER 6 L IGHT EXTRACTION OF GREEN FLOURESCENT OLEDS BY DEFECTIVE HEXAGO NAL CLOSE PACKED ARRAY WITH DIFFERENT PERIODICITY 6.1 Review and motive Light extraction in organic light emitting diodes (OLEDs) has been an active area of research because of signifi cant power loss due to light wave guiding in the indium tin oxide/organi c layers (ITO/organic mode) and the glass substrate (substrate mode). While the substrate mode can easily be outcoupled by attaching a commercial microlens array or a light extraction fi l m on the back of the substrate, extraction of the ITO/organic mode is much more challenging to achieve without deteriorating the device performance because it requires a modifi cation of the internal structure inside the device such as low index grids outside t he transparent electrodes,[ 48 ] internal scattering structure of the organic layer,[ 101] preferred orientation of the transition d ipole moments of the emitter fi lm,[ 102] and Bragg diffraction gratings.[ 69, 85 , 8791] Bragg diffraction grating requiring incorporation of corrugated structure into OLEDs have been widely studied as one of most promising approaches, but it still has many issues to be solved for practical applications. For most photonic and grating structures used for light extraction, the outcoupled light is strongly direc tional depending on the specifi c emission wavelength, polar angle, and azimuthal angle because of their perfectly periodic microstructure.[ 69 , 85, 8791] In addition, fabrication of these corrugated structures requires electron beam lithography which is not compatible with low cost manufacturing. Recently, Koo et al. reported that a spontaneously formed buckling structure with a non directional emission profi le might provide a new approach to randomize with the directionality in photonic crystal structures.[ 58, 94, 96, 97 ] In this chapter , we demonstrate a light extraction scheme with an emission profi le close to a Lamber tian emitter by introducing defects into a 65

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hexagonal close packed (HCP) silica sphere a rrays and thereby randomizing the directionality and broadening the periodicity. The defective silica sphere array having locally HCP structure but lack of long ordering was fabricated by simple low cost rapid convective deposition. Although Hyun et al. reported two dimensional (2D) titanium oxide (TiO2) photonic crystal by the simple convective deposition for light extraction in polymer light emitting diodes, they did no t address the effects of the defects involved in their periodic structure on light extraction, their 2D TiO2 photonic crystal with f l at surface was unable to outcouple the surface plasmon mode propagating at the interface between the organic and cathode layers.[ 103] Here, we demonstrate the effects of the defective HCP array on light extraction experimentally and theoretically, and fi nally report 70% and 90% enhance ments in current and power effi ciencies, respectively, without introducing particular spectral change over emission angles. 6.2 Experimental Procedure 6.2.1 Defective HCP Grating Fabrication Silica sphere array templates were fabricated by depositing a sus pension consisting of either 1.0 or 0.5diameter silica spheres and 100 nm diameter polystyrene spheres on glass substrate by rapid convective deposition. In these experiments, we employed 14% silica and 4% polystyrene binary suspension prepared by di spersing the 1.0and 0.5nm diameter polystyrene nanospheres into distilled water. Then, the suspension was immersed in the ultrasonic bath for 1 hour and thoroughly shaken by the vortex for 1 mininute. Prior to dep osition, the glass substrate was cleaned by using piranha solution (volume ratio of 5:1 for sulfuric acid/hydrogen peroxide) and distilled water. The back and bottom edges of glass deposition blade were treated for ensuring hydrophobic surfaces by adding a 66

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thin coating with parafilm in order to control the wetting region of the meniscus droplet. During the deposition, a 7.0injected between the substrate and the blade forming a wedge with the subst rate at the linear motor. After deposition, the sample was heated at 140 C to melt the polystyrene spheres, which would fill up the gap between the silica microspheres to form the planar polystyrene layer. By using this process, we fabricated silica/polystyrene arrays templates with silica s phere diameters of 1.0 and 0.5 m employing polystyrene here array substrates were used as a template on which PDMS was poured and cured at 60C for 2 hours to form the PDMS replica. To fabricate the resin coated substrate for OLED fabrication, a UV curable resin layer (Norland Optical Adhesive 81) was spincoa ted on the 0.1mm thick glass substrates and subsequently cured by UV irradiation for 5 minutes, a drop of resin was placed on the resincoated substrates. Then the silica sphere array patterns of the PDMS replicas were transferred to UV curable resin layers by imprinting technique. Finally, the corrugated resin layer and a fl at resin layer coated glass substrates were used for grating and reference devices, respectively. 6.2.2 Device Fabrication and Measurement For device fabrication, the following layers were deposited on the corrugated and flat resin layers: a120nm thick ITO, a 50 nm bis(naphthalene1 yl) bis (phenyl)benzidine)), a 60nm thick Alq3 (tris (8 hydroxyquinoline) aluminum), a 1.0 nm thick lithium fluoride (LiF), and a 100nm thick aluminum (Al). The devices with the emitting area of 2 mm 2 mm were encapsulated with glass and UV curable sealant in a glove box under N2 atmosphere. In order to extract the substrate mode, a 67

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hemisphere lens with a diameter of 3 mm was attache d on the back of the glass substrate. EL spectra were recorded with a source meter (Keithley 2400) and a spectrometer (Ocean Optics HR4000). The angular emission patterns were measured by integrating the EL spectra of devices according to emission angles. A perfect hemisphere lens with a diameter of 5 mm and a refractive index of 1.52 was attached on glass substrate of devices by an index matching gel. 6.3 Result and Discussion 6.3.1 Fabrication and Characterization o f Defective HCP Structure The HCP silica sphere array was prepared by the rapid convective deposition process which is applicable for low cost and large scale production without using lithography.[ 104107] 14% silica and 4% polystyrene binary suspension prepared by dispersing the 1.0and 0.5 diameter silica microspheres and 100nm diameter polystyrene nanospheres into distilled water were deposited with the deposition blade on glass substrates. Figure 6 1 A shows the 0.5 diameter silica sphere with the 100 nm diameter polystyrene after deposition. After heat treatment of the sample at 140 C for 4 minutes, the polystyrene spheres were melted to form a thin fi lm embedded with a monolayer of silica spheres, thereby forming corrugated structures with periodicities corresponding to the size of the silica spheres, as seen in Figure 6 1 B and C . The depth of the corrugated structure was tuned by controlling the heat treatment time to reduce the polystyrene layer thickness, similar to the process described in Reference 107 . While the diffraction effi ciency of grating generally increases with the depth of grating, we selected the depth of 185 nm and 75 nm for the 1.0and 0.5 because too high depth of grating causes a device failure due to high leakage current 68

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paths. To form a corrugated structure for OLED fabrication, a PDMS replica was fi rst made using the silica sphere arra y substrate as a template. Subsequently, the PDMS replica was stamped onto a UV curable resin coated glass substrate. Finally, the corrugated resin layer having locally HCP structure but lack of long ordering and a fl at resin layer on the 0.1 mmthick glas s substrates were used for grating and reference devices, respectively . Figure 6 2 A and B show the AFM images of the 1.0and 0.5m diameter silica sphere array templates respectively along with the FFT patterns as inserts. Although the local HCP areas retain the hexagonal FFT patterns, defects in the array break the long range hexagonal symmetry and generate a ring pattern over the entire area. The FFT patterns indicate that the defective HCP array pattern forms the grating vectors in all azimuthal angles, allowing diffractions of waveguided light in all azimuthal angles.[ 58, 94, 96, 97] The line profiles in the AFM images show that the local HCP areas in the corrugated templates have the periodicities of 1.0 and 0.5 m corresponding to the diameters of the silica spheres. However, the power spectral density from the whole area in Figure 6 2 C indicate s m templates actually hav e peak periodicities templates were 180190 and 70 80 nm for the depths of the corrugated resin layers imprinted from the template were reduced sl ightly to 170180 and 60 70 nm respectively, while their periodicities of 0.46 0.04 and 0.9 maintained. The calculated surface area ratios, expressing the increment of the corrugated surface area relative to the flat surface area, were 12% and 8% m corrugated resin layers. 69

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6 .3.2 m p eriodicity To fabricate t he OLEDs, the following layers were deposited on the corrugated and fl at resin layers coated glass substrates: a 120 nm thick ITO, a 50nm thi ck NPB (N,N’bis(naphthalene1 yl) bis (phenyl )benzidine)), a 60nm thick Alq3 (tris (8 hydroxyquinoline) alumi num), a 1.0 nm thick lithium fl uoride (LiF), and a 100nm thick aluminum (Al). Figure 6 3 A shows the device structure and Figure 6 3 B shows the distributions of the electric field intensities for one transverseelectric (TE0 ) and two transversemagnetic (TM 0 and TM1) waveguide modes, which were calculated by the tran s fer matrix method.[ 58] Generally, the diffraction effi ciency of a grating is proportional to the electric fi eld intensity in the grating region. [ 88] Because the corrugated structure is maintained over all layers from the resin to the Al layer, effec tive outcoupling of the thin fi lm guided modes is expected. The t ypical current densities (mA/cm2) and luminances (cd/m2) for the devices plotted as a function of applied voltage in Figure 6 4 A . The higher luminance at a constant voltage compared with the reference device. The leakage current of the grating device below turnon voltage showed little difference with that of the reference device on a log log scale, indicating smooth surface of the corrugated structure in the grating device (Figure 6 5A ) . Because the surface area of the grating dev ic e is increased slightly by 12%, the much higher enhancemen t of th e current density in the grating device cannot be explained by considering only the surface area enhancement . In general, it is reported that a corrugated OLED have a higher current density because of the enhanced electric fi eld due to nonuniformity of th e organic layer thicknesses in a 70

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corrugated structure.[ 58 , 77, 87] Despite of the increased current density, the higher enhancement of the luminance in the grating device represents the extraction of the waveguide modes. Th e current (cd/A) and power effi ciencies (lm/W) at a current density of 40 mA/cm2 are 2.6 cd/A and 1.64 lm/W for the reference device and 3.5 cd/A and 2.5 lm/W for the grating device (Figure 6 4 B ). The grating device shows 35% and 50% enhancements in the current and power effi ciencies compared to the reference devices. The higher enhancement in the power efficiency than the current effi ciency is due to the lower operating voltage in the grating devices, as shown in Figure 6 4 A . The EL spectra of the grating and reference devices at normal direction show that there is no spectral change due to the 1.0of Figure 6 4 C ). Dividing the EL intensity of the grating device by that of the reference device, the enhancement ratio for the emission wavelengths is obtained and the results are shown in Figure 6 4 C . The enhancement ratio is fairly uni form across the EL spectrum with slightly higher intensities at around 500 and 650700 nm. In order to elucidate the dependence of the enhancement ratio on the emission wavelength, we calculated the dispersion curves of the TE0, TM0 and TM1 modes and the results are shown in Figure 6 4 D . The dispersion curve with the light lines for air and glass substrate can be divided into three regions: air , substrate , and ITO/organic modes. According to the Bragg grating E quation 5 1 , the large in plane wave vectors o f the TE0, TM 0 and TM 1 modes can be reduced by the grating vector which is inversely proportional to the periodicity of grating.[ 58, 94 , 96, 97] The length of the small arrows in Figure 64 D represents the magnitude of the grating vectors by the periodicity of 0.9 and 0.8 71

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due to the large periodicity is only able to transfer the ITO/organic mode to the substrate mode by the first order diffraction, a fraction of the enhanced substrate mode can be outcoupled to the air mode by multiple scattering with the corrugated Al layer on a thin glass substrate (0.1 mm).[ 97] grating is mainly caused by the extraction of the substrate mode by scattering, which gives rise to the broad enhancement of the EL intensity over all emission waveleng ths in Figure 64 C . The slightly higher intensities at around 500 and 650 700 nm in Figure 6 4 C are re lated to the transfer of the TM 1 mode to the substrate mode up to below 520 nm along with the TE 0 and the secondorder diffraction of TE0 by the grating v ectors from k0.8 m to k0.9 m above 630 nm, respectively. Since the TM0 mode as the SP mode has the characteristics of the strong absorption, the large periodicity of mode effectively .[ 96] 6 .3.2 OLED Devices with 0.5 m P eriodicity m grating, the TE 0 , TM 0 and TM 1 modes can be more effectively outcoupled because of the larger grating vector to counter their large inp lane wave vectors. Figure 6 6 A shows the current densities and luminance grating grating device exhibits a higher current density and higher luminance than those of the reference device. However, the enhancement of the current density grating device because of the lower surface area ratio of the 12%). The current and power effi ciencies a t a current density of 40 mA/cm2 in Figure 6 6 B are 2.57 cd/A and 1.58 lm/W for the reference devices, and 4.38 cd/A and 2.95 lm/W for the 72

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grating devices , respectively. Enhancements of 70% and 90% were achieved for the cu rrent and power effi ciencies due to the with the spectra of the grating and reference devices at normal direction again show a broad enhancement over all emission wavelengths (inset of Figure 6 6 C ). The enhancement ratio in Figure 6 6 C represents two strong intensities at around 470 and 700 nm. It should be noted that the enhancements at both ends of the spectrum are slightly larger compared to the 1.0 grating device. According to the dispersion curve in Figure 6 6 D , the having the periodicity range of m can extract the TE 0 and TM 0 modes into the normal direction through the secondorder diffraction by the grating vectors f rom k0.46 m to k0.5 m at around 470 nm and the fi rst order diffraction by the grating vectors from k0.42 m to k0.46 m at around 700 nm, while those modes at all emission wavelengths can be outcoupled to various angles through the first and secondorder diffractions by the wide range of the grating vectors. The substrate mode also can be outcoupled through the multiple scattering between the corrugated Al cathode layer and the thin glass substrate, contributing to the overall enhancement for all emission wavelengths, as seen in Figure 6 6 C . 6.3.4 Angular d ependence of e mitting l ight with and w ithout h emisphere l ens and 60 are shown in Figure 6 7 A enhanced over all emission wavelengths irrespective of the emission angles because 73

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the IT the broad periodicity and the random orientation. In order to more effectively extract the substrate mode, a hemisphere lens was used. By integrating the EL spectrum, the angul ar dependences of emitting light for the reference, 1.0, and m grating devices with (fi lled circle) and without (open circle) the hemisphere lens are shown in Figure 67 B , C , and D , respectively. The emitting light intensities of each device for all e mission angles were normalized by the intensity of each device without the hemisphere lens at normal direction. Both the reference the hemisphere exhibit nearly Lambertian emission pattern. Since the grating vector by the grating is too small to directly extract the ITO/organic mode by the fi rst order diffraction, the indirect extraction of the enhanced substrate mode via the multiple scattering gives rise to the non characteristic emission profi le in Figure 6 7 C . H grating device shows the broader distribution, compared with the Lambertian emission pat tern, particularly at around 40 which corresponds the fi rst order diffraction angle of the TE0 and TM0 modes around the main emission peak of 530 nm by the grating periodicity of conventional grating or photonic crystal structure with a short and long range order hexagonal symmetry give distinct butterfl y wing emission patterns,[ 79, 87 ] the emission profi les of our OLEDs fabricated on defective HCP array patterns do not show a particular polar and azimuthal angle dependence because of the broadening of the periodicity and the random orientation from the defective array pattern. in Figure 67 C and D represent different characteristic emission profi les from t he reference device in Figure 74

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6 7 B . These indicate that the grating on the thin glass redistributes the substrate m ode by transferring the ITO/organic mode to the substrate mode and extracting the substrate mode, depending on its periodicity. In order to understand the mode distribution, we defi ne the enhancement ratio by dividing the EL intensity of the device with th e hemisphere lens by that of the device without the hemisphere lens. Figure 67 E shows the enhancement ratios for the reference grating devices. Please, note that the smaller enhancement of the grating devices than the reference device in Figure 67 E is caused by the higher effi ciency of the grating devices without the hemisphere lens than the reference device without the hemisphere lens. While the enhancement ratio of the reference device with the hemisphere lens generally decreases with increasing emission angles, that of the with the hemisphere lens increases r ather than decreases, possibly because the fi rst order diffraction by the ITO/organic mode to the substrate mode. The device with the hemisphere lens increases up to 20 30 and th en decreases as the emission angle increases further, reflecting the extraction of the ITO/organic mode by the grating. The reference device with the hemisphere lens shows the enhancement of 77% in the integrated inte nsity over all emission angles, compared to that without the grating devices with the hemisphere lens show the enhancements of 67% and 48%, compared to each device without the hemisphere lens, indicating that a fraction of the ITO/organic modes in the grating devices are indeed extracted to the air mode. Finally, the power effi grating devices with lens were enhanced by a factor 75

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o f 2.5 ( = 1 reference 1.6 power effi ciency 1.67 lens ) and 2.8 ( = 1 reference 1.9 power effi ciency 1.48 lens ), respectively, compared with the reference device without the hemisphere lens . 6.4 Summary In summary, a defective silica sphere array pattern having locally HCP structure , but lack of long ordering was fabricated by rapid convective deposition. The defective silica array pattern was incorporated into the devices as grating to extract the ITO/organic modes and additionally the substrate mode via a thin glass su bstrate. Despite of the insuffi cien t grating vector, and 50% enhancements in the current and power effi ciencies by transferring the ITO/organic modes to the substrate mode and then scattering the substrate mode. The the stronger grating vector were able to effectively outcouple the ITO/organic modes by the fi rst and secondorder diffractions with the scattered substrate mode, thereby improving the current and power effi ciency by 70% and 90%, respectively, without spectral changes and direc tionality. With the low cost and largearea processing, the defective HCP silica array pattern can supply a practical solution for light extraction in the fi eld of OLED applications. 6.5 Acknowledgements The authors would like to acknowledge the support of Department of Energy Solid State Lighting Program (Contract number: DE FG0207ER46464). The authors would like to acknowledge the support of the Department of Energy Office of Energy Effi ciency and Renewable Energy (Contract Number: DE EE0001522). The authors would also like to acknowledge Dr. Renbo Song and Mr. Le Zhao of 76

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Department of Electrical and Computer Engineering at Lehigh University for their assistance in preparing part of the microlens arrays template, and Prof. James F. Gilchrist and Dr. Pisist Kumnorkaew of Department of Chemical Engineering at Lehigh University for technical assistance in colloidal suspension preparation. 77

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Figure 6 1 . SEM images of silica array embedded in polystyrene layer before and after heat treatment; A ) SEM image o n the 0.5diameter silica sphere mixed with the 100 nm polystyrene sphere before heat treatment , B) and A ) cross sectional view of SEM images on the 0.5 and 1.0diameter silica array template after heat treatment at 140 C for 4 minutes, respectively . Figure 61 is from the reference 58. Figure 6 2 . AFM images, line profiles and power spectral density on corrugated structur e with different periodicity; A) 1.0and B ) 0.5 diameter silica sphere arrays template (d2) . Inset: The FFT pattern of each image, (the solid line in the image corresponds to the l ine profile below the image), C ) power spectral density from FFTs as a function of wavelength Figure 62 is from the reference 58. A B C A B C A A A 78

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Figure 6 3 . Calculated electric field distribution in OLEDs; A ) schematic of co rru gated device structure, B ) distributions of electric fi eld intensity across the layers for TE 0 ( ), TM0 ( —), and TM 1 ( ) waveguide modes in a f l at str ucture, calculated by transfer matrix method. Figure 63 is from the reference 58. A B B A B 79

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Figure 6 4 . Electrical and optical characteristics of 1periodic structure ; A ) current density (mA/cm2) and luminance (cd/m2) , B ) current effi ciency (cd/A) and power effi ciency (lm/W) for corrugated (fi lled symbols) and reference (open symbols) devices , C ) e nhancement ratio of EL intensity, plotted by dividi ng the spectrum of the grating device by that of the reference devices. Inset: EL spectra of the grating ( —) and reference ( ) devices , D ) d ispersion curves of the TE 0 ( ), TM0 ( —), and TM 1 ( ) modes by transfer matrix method. It was assumed that the refr active index of a resin layer ( n = 1.56) is equal to that of a glass substrate ( n = 1.52). The thin solid lines represent the light lines for air and glass substrate. The small arrows indicate the grating vectors k0.9 m and k0.8 m from the periodicity distribution of 0.9 grating, and m = 1, 2 means the fi rst and second order diffraction. A large arrow describes the extraction of the substrate mode, irrespective of the length of the arrow. Figur e 64 is from the reference 58. A B D C A 80

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Figure 6 5. Electrical and optical characteristics of both corrugated structure with different periodicity; A ) c urrent density (mA/cm2, left axis) and luminance (cd/m2, right axis) as a function of a pplied voltage in log scale for the devices with (filled circle) and without (open circle) the 1.0 m grating, B ) the devices with (filled circle) and without (open circle) the 0.5 m grating . Figure 65 is from the reference 58. A B 81

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Figure 6 6. Electrical and optical characteristics of 0.5periodic structure ; A ) current density (mA/cm2) and luminance (cd/m2) , B ) current effi ciency (cd/A) and power effi ciency (lm/W) for corrugated (fi lled symbols) and reference (open symbols) devices , C ) e nhancement rati o of EL intensity, plotted by dividing the spectrum of the grating device by that of the reference devices. Inset: EL spectra of the grating ( —) and reference ( ) devices , D ) d ispersion curves of the TE 0 ( ), TM0 ( —), and TM 1 ( ) modes . Figure 6 6 is from the reference 58. A B C D A 82

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Figure 6 7. Angular dependence of emitting light with and without hemisphere lens for corrugated OLEDs; A ) EL spectra of the reference (open symbols) and the angles of 0 (square), 20 (circle), 40 (triangle), and 60 (diamond). Angular dep endence of emitting light for B) the referenc m grating devices with (fi lled circle) and without (open circle) the hemisphere lens. The solid line represents a Lambertian emission pattern. The emitting light intensities of each device with and without lens for all emission angles were normalized by the intensity of each device without lens at normal direction. E) enhancement ratio of the EL intensity by the hemisphere lens for by dividing the EL intens ity of the device with the hemisphere lens by that of the device without the hemisphere lens in B D . Figure 67 is from the reference 58. A B C D E A A A A A 83

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CHAPTER 7 L IGHT EXTRACTION OF CORRUGATED OLEDS FABRICATED ON HIGHREFRACTIVE INDEX SAPPHIRE SUBSTRATE 7.1 Re view and motive OLEDs have become a promising candidate for lighting applications due to their flexibility, low power consumption and excellent color quality. However, the low extraction efficiency in OLEDs still remains as the biggest challenge for lighting applications . Although the internal quantum efficiency has reached close to 100% using phosphorescent emitters , the external quantum efficienc y is typically about 25% .[ 11, 17 , 18 ] The low extraction efficiency in OLEDs is due to the difference in refractive indices between air (n=1), the glass substrate (n~ 1.5) and the organic/ITO (1.7~2) layer s. In conventional OLEDs, only 20~30% of the generated light can escape into air and another 20~30% is trapped in the substrate ,[ 108] and t he rest of the emitted light is trapped and guided in the organic/ITO layer s. Depending on where the optical modes are located in the ITO/organic layer s, they can be categorized into waveguide mode s that mostly concentrate closed to the ITO layer and SP mode that propagates as surface waves along the metal organic interface, accounting for close to 5 0% of the total light output in an optimized OLED.[ 109] To improve the light extraction efficiency, various techniques have been introduced. The substrate mode can be extracted effectively using microlens array s and light scattering layer s.[ 38, 40, 43] W aveguide mode s can be extracted using photonic crystal s, low index grids, and highindex substrates.[ 48, 5255, 85, 110] Especially, u sing high index substrates with a refractive index similar to that of the organic/ITO layer s, the waveguide mode s can be significantly suppressed. Leo et al. reported white OLEDs fabricated on high refractive index substrates , resulting in a ~50% enhancement in 84

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EQE .[ 54 ] To minimize the efficiency loss due to the SP mode, one can either get rid of the metal electrode or use corrugated structures. To minimize the presence of the SP mode, Kim el al. have fabricated transparent OLEDs without the metal electrode and achieved an EQE of 65%. To extract the SP mode, corrugated OLEDs have been shown to be very ef fective .[ 58, 59, 96, 102 , 111, 112] Specifically, corrugated OLEDs fabricated on buckling structures with a semi random periodicity have shown to give significant e nhancements in efficiency across the entire visible spectrum.[ 58] While the techniques mentioned above are effective to extract either the thinfilm guided modes or the SP modes, there has not been a demonstration of effective out coupling of both thinfilm and SP modes simultaneously. Here, we report a scheme to extract the waveguide, SP and substrate modes simultaneously in an OLED. By fabricating OLEDs on corrugated highrefractive index sapphire substrates (n~1.8) , extraction of both the waveguide and SP modes was fully exploited. Using an additional macro lens on the back side of the substrate to extract the substrate mode, we were able to realize a green phosphor escent OLED with an EQE of 63% and a current efficiency of 225 cd/A . Unlike the conventional photonic crystal s which are only effective for light extraction at specific wavelengths, the quasi periodic corrugated structure can enhance the extraction in all wavelengths with an emission profile similar to that of a Lambertian emitter. 7.2 Experimental P rocedure 7.2.1 Fabrication of C orrugated S a pphire S tructure The fabrication process for the corrugated sapphire substrates consists of three steps: (i) coating a monolayer of silica sphere arrays by Langmuir Blodgett (LB) technique on sapphire substrates, (ii) transferring the pattern of silica sphere arrays 85

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onto the sapphire substrate by inductively coupled plasma reactive ion etching (ICP RIE), and (iii) remov ing the silica spheres with buffered oxide etch. The schematic diagrams of these processing steps are illustrated in Fig ure 7 1 . To fabricate the sacrificial masks for patterning the sapphire substrate, monodispersed silica spheres with a diameter of 275 nm were self assembled to form a quasi periodic array on the substrates using a LB trough, and the details have been documented in previous work .[ 59] To fabricate the corrugated structure on the sapphire substrat es , 10 sccm of BCl3, 25 sccm of Cl2 and 10 sccm of Ar were used in a plasma etcher at a radio frequency power of 450 W and a coil power of 125 W to etch both the silica spheres and the sapphire substrates at the same time. To maximize the depth of the corr ugated structure, the substrate was exposed to the plasma until the silica spheres were completely removed. Using this process, hemispheres are formed on the sapphire substrate with a periodicity close to the diameters of the silica particles. The corrugat ed structure was characterized using SEM and AFM , and the results are shown in Fig ure 7 2 A and B . The AFM and SEM images show the quasi periodic pattern fabricated. The diffused ring pattern in the FFT patterns shown in the inset of Fig ure 7 2 B confirms t hat the grating wave vectors are over all azimuthal directions , allowing outcoupling of the SP mode over all azimuthal angles . Fig ure 7 2 C represents the power spectrum of the corrugated structure, which shows a nominal periodicity of 260 nm with a full wi dth at half maximum of 50 nm, indicating a broad range of grating wave vectors. The SEM image in the inset of Fig ure 7 2 A shows the cross sectional view corrugated structure of the sapphire substrate with a depth of about 80~90 nm which can effectively dif fract the 86

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SP modes. F or direct comparison, green phosphorescent OLEDs were fabricated on both corrugated and planar highrefractive index substrates . 7.2.2 Device F abrication and M easurement For device fabrication, a sputtering system and a thermal evapor ator were used for depositing the ITO electrodes and organic layers on both corrugated and planar sapphire substrates. The OLED has the following structure: a 120nm thick ITO, a 45nm Cyclohexylidenebis( N , N bis(4 methylphenyl)benzenamine) ), a 15nm Bis( N carbazolyl) biphenyl) doped with Ir(ppy)3 ( fac tris(2 phenylpyridyl)Ir(III) ) and, 15 nm thick TCTA (Tris(4 carbazoyl 9 ylphenyl)amine) doped with Ir(ppy)3, a 60nm thick 3TPYMB (Tris(2,4,6 triMethyl 3 (pyridin 3 yl)phenyl)borane, a 1nm thick lithium fluoride (LiF), and a 100nm thick aluminum (Al). The emitting area of devices is 2 2 mm2 for both devices. And they were encapsulated with cover glass and UV curable sealant in a nitrogenambient glove box. A sapphire hemisphere lens with a diameter of 4 mm and index matching gel with refractive index of 1.76 were attached on the backside of the sapphire substrate for extracting the substrate mode. For a conventional glass substrate, a 4mmdiameter hemisphere macro lens and the index matching gel with both refractive index of ~1.5 and were used as well. For device characterization, a 10inch dimeter integrating sphere (Labsphere) with photodiode was used for collecting all photons emitted from the devices driven by a sou rce meter (Keithley 2400). EL spectra with and without a linear polarizer were recorded with same source meter and a spectrometer (Ocean Optics HR4000). The emission profile of the OLEDs was done by mounting the devices on a rotating stage. 87

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7.3 Result and D iscussion 7 . 3 . 1 Op tical S imulation of OLEDs Fabricated on Sapphire S ubstrate To analyze the optical modes in OLEDs fabricated on sapphire substrates , the dispersion curve was calculated using transfer matrix method and the results are shown in Figure 7 3 A . Here, the refractive indices of ITO and Al were taken from our previous work and published literatures .[ 72 , 110, 113, 114] For simplicity, the effect of the LiF layer was ignored due to the extremely thin layer used (1 nm) compared with other layers and the th icknesses of the substrate and the Al layer were considered semi infinite.[ 77] The calculated dispersion curve shows that only the SP mode can be excited inside o f OLEDs on highrefractive index substrates. It should be noted that for the devices fabricated on sapphire substrates, the thinfilm modes are not present since they are coupled into the sapphire substrate, and hence more light is expected to be trapped i nside the substrate. With the presence of an inplane component of the grating wave vector in the quasi periodic corrugated OLEDs, the SP mode can be effectively extracted. Specifically, f or emitt ers such as Ir ( ppy )3 with an emission wavelength maximum at 515 nm, the optimum periodicity of the grating for normal direction diffraction on the SP modes was determined to be in the range of 260270 nm by transfer matrix method.[16, 17] Here, we chose silica spheres with a periodicity of 2 75 nm to fabricate the corrugated structure on the sapphire substrates, and the resulting corrugated structure has a nominal periodicity of 260 nm . 7.3 . 2 Device C haracterization F igure 7 3B shows the LuminanceVoltageCurrent density (LV J) characteristics of OLEDs fabricated on corrugated and planar sapphire substrates with 88

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a macro lens. Thus, total EQE can be obtained by extracting all air mode, substrate mode, and waveguide mode. It is generally believed that the corrugated surface might lead to shorting in the devices. How ever, our data show s that even with a corrugation depth of about 80~90 nm , it has no effect on the device leakage current and hence shorting is not a problem in our corrugated devices. On the other hand, the corrugation does increase carrier injection resulting in a higher current density in the corrugated OLEDs. Such an enhancement in current density has been attributed to the thickness variation and the enhanced electric field in the organic layers due to the corrugated structures .[ 58, 59 ] In addition to the increase of current density , Figure 7 3B shows the enhanced luminance in the corrugated device due to extractio n of the SP mode . Figure 3C shows both the current efficiency and EQE for devices . The c orrugated OLED show s a high current efficiency value of 225 cd/A and a high peak EQE value of 63%, while the planar device shows 178 cd/A and 52% in peak current effici ency and EQE, respectively. Compared to the OLED fabricated on planar sapphire substrates, the enhancement is ~25% in both current efficiency and EQE , compared to planar OLED. Since both devices were fabricated on sapphire substrates, the thin film guided mode is not present and the enhancement in the corrugated device must be coming from extraction of the SP mode. To confirm the enhanced substrate mode, planar OLEDs were fabricated on both conventional glass and sapphire substrates. When a macro lens was used for the devices, as shown in the inset of Figure7 3 D , 120% enhancement in light output was observed from the sapphire substrate while only 70% enhancement from the glass substrate was observed, confirming that the enhanced substrate mode in the sapphi re substrate is attributed to the transfer of the waveguide 89

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mode into the substrate mode in devices fabricated on sapphire substrates . A s shown in Figure 7 3D , t he corrugated OLED shows a Lambertianlike emission profile , indicating that the outcoupled SP mode is not angular dependent due to the randomly distributed grating wave vectors with a broad magnitude. To further understand the origin of the light outcoupling due to the corrugated device, EL at normal direction were measured on both corrugated and planar devices with the macro lens applied, and the results are shown in Figure 7 4A . It should be noted that the EL spectra for both devices are almost identical, further confirming the broad distribution of the grating wave vectors in the corrugated structure. To verify that the enhanced outcoupling is due to Bragg diffraction of the SP modes into the normal direction, the enhancement factor of the corrugated OLED , which is the ratio of the EL intensity of the corrugated OLED to that of the planar OLED , w as measured, and the results are shown in Figure 7 4B . Th is figure shows more pronounced enhancements at ~500 nm and ~650 nm. To understand the origin of the light outcoupling enhancements, we calculated the inplane propagation vectors of the SP modes by transfer matrix method and plotted the grating periodicity as a function of the emission wavelength of the outcoupled light as shown in Figure 7 4 C . Considering a nominal periodicity of 260 nm in the corrugated structure, the first order (m=1) diffraction for the SP mode is responsible for the enhancement at wavelengths of ~500 nm. Similarly, the second order diffraction for the SP mode is responsible for the enhancement at wavelengths of ~650 nm as indicated in the blue region in Figure7 4C . Thus, the strong enhancement in outcoupling at these two wavelengths from the corrugated OLED is a direct evidence of SP mode extraction via the first and secondorder diffractions. Additionally, we also 90

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performed polarized EL measurements to determine the enhancement factor of the transverse magnetic( TM ) and transverse electric( TE ) polarized modes of the EL emission, and the results are shown in in F igures 7 4D and E . Generally, the excitation of the SP mode comes from the incident TM polarized light on the metal organic interface. As expected, the enhancement factor regarding TM polarization of the corrugated OLED from F igure 7 4 D show a similar wavelength dependence compared to the data measured without the polarizer as shown in F igure 7 4 B , satisfying the first and s econd order diffraction conditions. However , similar enhancements were also observed from the TE polarized EL as shown in F igure 7 4 E . Since the ITO/organic mode ( TE mode ) is not expected to be present in devices fabricated on sapphire substrates, the enhancements of the TE polarized light is due to polarization conversion resulting from diffraction in a quasi periodic structure. This socalled conical diffraction [ 115, 116] from quasi periodic structure has been previously reported in corrugated OLEDs.[ 96 ] Due to the randomly oriented grating wave vector s in all azimuthal angles, diffraction can convert the polarization from T M polarized light to T E polarized light , further confirming the extraction of the SP mode. 7.4 Summary In summary, the outcoupling of thin film g uided mode and SP modes were demonstrated with OLEDs fabricated on high refractive index corrugated substrates. With a macro lens to extract the substrate and thin film guide modes, the resulting OLEDs have a very high external quantum efficiency of 63 % an d a current efficiency of 225 cd/A. With the nature of corrugated structure with broad periodicity for OLEDs , the light extraction enhancement is independent of wavelength and the emission profile is 91

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similar to a Lambertian emitter, demonstrating that this device is promising for OLED lighting applications. 7.5 Acknowledgements The authors would like to acknowledge the support of Department of Energy Solid State Lighting Program (Contract number: DE FG0207ER46464). 92

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Figure 7 1 . Schematic diagram of the f abrication process for the quasi periodic hemisphere pattern sapphire substrate. 93

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Figure 7 2 . SEM and AFM characterization on the corrugated sapphire substrate with nominal periodicity of 260 nm; A ) t op view of SEM image on corrugated sapphire substrate, ( Inset: crosssection view of the nanostructured sapphire substrate and i t shows an optimized corrugation depth of 80~90 nm ), B ) AFM image of the corrugated sapphire substrate fabricated with 275nm diameter 2) , (i nset: F ast Fo urier T ransform (FFT) pattern of the corresponding image), C ) p ower spectra from FFT as a function of periodicity for the patter n with n ominal periodicity of 260 nm with a full width at half maximum (FHMW) of 50 nm , indicat ing a broad di stribution of the periodicity . A B C 94

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Figure 7 3 . Optical and electrical characteristics of OLED fabricated on corrugated and planar sapphire substrates; A ) d ispersion curves calculated by transfer matrix method. (t he solid lines represent the light lines for air and sapphire substrate modes ), B ) L V J curves for both plain (bl ack) and corrugated (red) OLEDs , C ) current efficiency (cd/A) and EQE (%) for the 260 nm periodicity grating (red) and reference (black) devices D ) Angular measurements in normal direction for both OLEDs with planar (black) and corrugated structures (red) , (f or comparison, the ideal Lambertian pattern (green) is plotted as well ), Inset: substrate mode extraction of devices fabricated on both planar glass (dot) and sapphire (d ash) substrates by a macro lens and m ore light output from the sapphire substrate by the substrate mode extractor indicates that the power of the waveguide mode is transferred to the substrate mode. A B C D 95

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Figure 7 4 . EL at normal direction for corrug ated and planar OLEDs fabricated on sapphire substrate ; A) EL spectra for both OLEDs with planar (black) and corrugated structures (red) measured in normal direction, B ) EL intensity enhancement ratio between planar and corrugated OLED s, plotted by dividing the EL spectrum of the grating device by that of the planar device, indicating that s tronger enhancements at ~500 nm and ~650 nm are due to first order (m=1) and secondorder (m=2) diffraction , respectively , C ) g rating periodicity for normal direction di ffraction on the SP mode as a function of emiss ion wavelength, (f irst (black) and secondorder (red) diffraction condition are satisfied by distributed periodicity of 260 nm grating structure (blue rectangular box) ), D) and E ) e nhancement ratio of EL inte nsity of TM and TE polarization. A B C D E 96

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CHAPTER 8 L IGHT EXTRACTION OF CORRUGATED OLEDS WITH REFRA C TIVE INDEX CONTROL 8 .1. Review and motive Organic light emitting diodes (OLEDs) have been the focus of significant development over the past 30 years. Due to their attractive characteristics such as color quality, fast response, flexibility, and low cost processing, OLEDs are recognized as promising candidates for the next generation light sources. Although intensive research has resulted in the OLEDs with 1 00% internal quantum efficiency, the 20~30% outcoupling efficiency still remains as the biggest challenge.[ 13, 17, 18 ] Substantial amount of efforts have been devoted to increase the light outcoupling efficiency. In particular , by sandblasting or attaching a microlens array or a light extraction film to the re verse side of the substrate has led to efficient and effective extraction of the light rapped in the substrate.[ 38, 40 , 43] To recover the light confined and guided in a highindex indium tin oxide and organic layers in OLEDs, also known as waveguide mode, corrugated structures, photonic crystals, low index grids, anisotropic dipole orientation, and doubleside transparent electrodes have been introduced.[ 48, 5255, 5860, 85, 110, 117, 118] In particular, the corrugated structures incorporating with Bragg diffraction have been exploited as an effective approach to recover the power loss to the waveguide mode. Like photonic crystal structures, the corrugated Bragg grating structure can extract the thin film mode guided by the glass/ITO interface by forming the photonic band gap composed of microscale periodic change of refractive index. In addition, the periodically corrugated metal can outcouple the SP mode associated with organic/metal interface by modifying the inplane wave vector component of SP mode via Bragg grating diffraction. Recently, corrugated structures with broad distributions of 97

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periodicity can even provide the enhanced light extraction efficiency with independence on b oth emission spectrum and angle, thereby enabling this technique practical for white OLED applications.[ 58] The availability of simple fabrication processes and larg e area compatibility of corrugated structures are additional advantages.[ 59, 60 ] However, it is believed that most of the enhancement from the corrugated structure can be attributed to the SP mode extraction.[ 69, 96] First, the location of the grating structure is very important for outcoupling of the waveguide mode. Given that grating efficiency depends on the strength of the electric field intensity of the waveguide mode in the grating region. However, the maximum electric field intensity of the SP mode is located within the metallic grating region, while the electric field intensity of the thinfilm waveguide mode (ITO/organic mode) is maximum at the center of the ITO layer and is much weak er in the ITO/substrate grating region , making effective diffraction outcoupling of the ITO/organic mode difficult . Secondly, the refractive index contrast between the substrate (typically glass with refractive index of 1.52) and ITO layer (n~1.8) in the corrugated structures is not large en ough to induce strong diffraction efficiency on the ITO /organic mode.[ 69, 87] In this study , we evaluate the light extraction efficiency of each ITO/organic mode and SP mode using quasi periodic corrugated grating OLEDs with first and second antinode conditions. Furthermore, we suggest a scheme to improve the corrugated structure for OLED light extraction by using a low refractive index layer, LiF, inserted between the conventional glass substrate and ITO layer. The enhanced contrast in refractive index between the LiF and ITO provides higher diffraction efficiency with better outcoupl ing of the ITO/organic mode, resulting in 61% enhancement in efficiency , 98

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compared to planar OLED s. With an additional substrate mode extractor, we were able to present a corrugated green phosphorescent OLED with extremely high efficiency ( 210 cd/A and 67 % EQE ) and without deterioration of original emissive spectrum . 8.2 Experimental P rocedure As described in C hapter 7, a corrugated structure with a nominal periodicity of 500 nm was fabricated on sapphi re substrate . By depositing 0.5diameter silica spheres on the sapphire substrate, hexagonal close pack ed (HCP) silica monolayer array was formed with self assembly characteristics. Using the array as a sacrificing mask for the inductively coupled plasma reactive ion etching process (ICP RIE) , the corrugated structure with periodicity corresponding to the diameter of silica particle was finally obtained and detailed experimental conditions can be found in Experimental section. Since the diffraction efficiency is highl y sensitive with corrugation depth, we controlled the etching time to give an optimum depth of 70~80nm , which can expect effective outcoupling of the waveguide mode and avoid the electrical short age in OLEDs due to the corrugated surface during device oper ation. To fabricate the corrugated structure on the conventional glass substrate, a polydimethylsiloxane (PDMS) stamp was first prepared using the corrugated sapphire substrate as a template. After a drop of UV curable resin was placed on the planar glass substrate, the stamp was applied on top of the substrate. After UV treatment and removal of the PDMS stamp, the corrugated resin (n=1.52) structure fabricated on 0.1mmthick glass was used for the corrugated grating OLEDs. For comparison, planar resin co ated glass substrates were used for planar reference OLED. Figure 81 show s the atomic force microscopy (AFM) images of the corrugated structure fabricated on the resincoated glass substrate and corresponding periodicity 99

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and depth profile. Although the corrugated structure shows HCP array in short range order, the break of long range HCP structure inevitably introduces broadening of the periodicity and random orientation. The line profile in the AFM image show ed that the periodicity in the local HCP areas was 0.5 m, which is the same as the diameter of silica particles used for the fabrication process. However, lack of long range order of HCP symmetry suggests that periodicity was rather distributed around the main periodicity of 0.5 m. The depth of the r esin corrugated structure on the glass substrate was 70~80 nm, which was the same as that of the corrugated sapphire template, indicating exact transfer of the structure. 8.2.2 Device Fabrication and M easurement For device fabrication, a 90nm layer of LiF was deposited on the corrugated and planar structures on the substrate using a thermal evaporator. ITO electrode and organic layers for OLED device were fabricated by sputter ing and the same thermal evaporator. The emitting area of devices were 2 2 mm2 for both corrugated and planar devices. T hey were encapsulated with cover glass and UV curable sealant in a nitrogenambient glove box before measurement s. For device characterization, a black box mounted with a calibrated photodiode was used for collecti ng forward direction light emitted from the devices driven by a source meter (Keithley 2400). And Minolta LS200 as luminance meter was used for calibrati on. EL spectra were recorded using a spectrometer (Ocean Optics HR4000) with same source meter at 0.1 m A/cm2 for all devices . The emission profile of the OLEDs was measured by mounting the devices on a rotating stage with the spectrometer located at fixed position. For substrate mode extraction, a 4mm diameter 100

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fused silica hemisphere macro lens and the index matching gel with both refractive index of ~1.5 (Norland 81) were used. Simulation: the software Setfos 3.4 was used for calculating the efficiency as a function of ETL thickness for Irppy3 emission wavelength based on classical dipole theory and Fresnel equation by numerical computation . 8.3 Result and D iscussion 8 .3 . 1 OLED Configuration. To improve the diffraction efficiency on the thinfilm guided mode, a 90nm thick layer of lithium fluoride (LiF) was deposited on the corrugated substrate prior to th e 90 nm thick anode ITO deposition process. Because of the low refractive index of the LiF, 1.39, the higher contrast between the LiF and ITO (n~1.8) from the corrugated grating OLED was expected to further extract the thin film guided mode, compared to th e index contrast between the glass (n=1.52) and ITO layer in conventional corrugated OLEDs. For OLED configuration , we designed the OLEDs with first and second antinode conditions for peak emission wavelength at 515 nm based on the simulation result using the commercially available software , Setfos, taking into account classic dipole theory and Fresnel reflection. As shown Figure 8 2 A , the calculated optimum out coupling efficiency (air mode) can be achieved by locating the emitting molecules at the antin odes ( first or second ) of the electromagnetic field from the metallic reflector by simply changing the thickness of the electron transporting layer (ETL) in the bottom emitting architectures of OLEDs. When the emitter is located at the first antinode, ( ~ 60 nm of ETL in OLED) , it was calculated that the fraction of power lost to thinfilm mode is similar to the fraction lost to the SP mode. However, in the second antinode condition, ( ~240 nm of ETL in OLED ) , from Figure 8 2A, the power fraction of the SP m ode 101

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decreases exponentially as the thickness of ETL increases , whereas that of the thinfilm guided mode increases and becomes dominant. In other words, the OLEDs constructed on first and second antinode conditions have similar outcoupling efficiency (air mode) but totally different waveguide mode condition s. F or first antinode, both thinfilm mode and SP mode are produced, but second antinode condition, it has only thinfilm guide mode. By incorporating the corrugated grating structure with these two different OLED configurations, direct evaluation of the diffraction efficiency on the thinfilm mode and SP mode in OLEDs can be possible. For OLED fabrication, the following layer were deposited for both OLEDs with two different optimized conditions : a 45 nm thick TAPC Cyclohexylidenebis( N , N bis(4 methylphenyl)benzenamine)) as HTL , and a 15nm Bis( N carbazolyl) biphenyl) doped with 12 wt.% Ir(ppy)3, and a 15nm thick TCTA (Tris(4 carbazoyl 9 ylphenyl)amine) doped with 12 wt.% Ir(ppy)3 as double EML . For the ETL configurations, we used a 55nm thick 3TPYMB (Tris(2,4,6 triMethyl 3 (pyridin 3 yl)phenyl)borane for first antinode devices , as shown in Figure 82 B . And for second antinode OLED , as shown in Figure 82C , we deposited a 20nm thick 3TPYMB and a 220 nm thick Cs doped 3TPYMB as ETL to avoid potential issues related to the distortion of charge balance between hole and electron charge carriers and high power consumption of device operation. For cathode electrodes, 1nm LiF and 100nm aluminum (Al) were deposited for both first and second antinode OLEDs. 8.3.2 Corrugated First A ntinode OLEDs The current densities (mA/cm2) and luminances (cd/m2) for devices constructed for the first antinode condition are plotted as a function of a pplied voltage in Figure 8 3 B . For corrugated grating devices with the low index LiF interlayer (LiF grating) and without LiF interlayer (grating), the current densities are higher than that of the 102

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reference planar OLED at a fixed voltage. The enhanced current is attributed to the stronger electric field induced by the locally thin organic layer in corrugated structure.[ 69] T he leakag e current s of corrugated devices before the turnon voltage are close to those of two planar devices, indicating that 70~80 nm depth of corrugated structure does not degrade the electrical propert ies of OLEDs. For optical characteristics, both grating dev ices show higher luminances because of the outcoupling of the trapped light in the highindex organic/ITO layers via Bragg diffraction. T he electroluminesce nce (EL) measurement in the normal direction is shown in Figure 8 3 C . According to Bragg diffraction , the grating wave vectors modify the inplane wave vector component of the waveguide mode so that it can be coupled out into an air mode. T he grating wave vector is inversely proportional to the periodicity of the grating structure.[ 58] Thus, the distributed grating wave vectors induced by the periodicity with broad distribution are eventually applied on the waveguide mode with broad range of emission wavelengths. As a result, the spectrum shift from the original emission profile is not severe, as shown in F igure 8 3C . T he current efficiency (cd/ A ) plotted as a function of luminance is in Figure 8 3 A , show ing that both corrugated grating devices have higher efficienc i es in all range of luminances compared to the planar devices. Table 8 1 shows the peak current efficiency for LiF grating, grating, LiF planar, and planar reference devices. The planar OLED with the low index layer does not show any difference in efficienc y compared to that of the planar reference device without additional LiF layer. Furthermore, the electrical and optical characteristics of the planar OLED structures from Figure 8 3B are almost identical , indicating that adding the LiF layer does not affec t the device performance in the planar OLED structures. However, the corrugated OLED with the LiF 103

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interlayer show s higher efficiency compared to the device without the LiF layer (peak efficiency of 144 and 127 cd/A for LiF grating and grating devices ) . Com pared to 89 cd/A for the reference devices, there is 61% enhancement in the efficiency for the LiF grating device, while the conventional grating devices show only 43% enhancement. The extra 18% enhancement from the LiF grating device indicates that the enhanced refractive index contrast improves diffraction efficiency upon the waveguide mode. However, the origin of the extra enhancement is still not clear , because the outcoupling of ITO/organic mode by the broadly distributed grating wave vectors is diffic ult to differentiate from the outcoupling of the corresponding SP mode. 8 .3.3 ElectricField I nten sity Distribution of W aveguide M odes for First Antinode C orrugated OLED s To determine possible optical environment variations due to the introduced low index layer, the electric field intensities for transverse electric (TE) and transverse magnetic (TM) waveguide modes were calculated for planar OLED devices with and without the LiF interlayer by the transfer matrix method, as shown in Figure 8 4 A and B . For th e simulation, thickness and refractive indices of all layers consisting of the OLEDs were used for the calculation and taken from our previous work in Chapter 7. For both devices, SP (TM0) mode associated with organic/cathode interface shows significantly high electrical field intensity at that interface, while the maximum intensity of the ITO/organic (TE0) mode is broadly distributed about ITO layer with relatively small magnitude. Since the diffraction efficiency is proportional to the location and the in tensity of the electric field, strong SP mode outcoupling via grating diffraction is expected. From the optical perspective, the presence of the LiF interlayer between the glass substrate and ITO layer for both devices does not affect the SP mode. 104

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Because of the higher contrast in refractive index between LiF (n=1.39) and ITO (n~1.8) compared to glass (n=1.52) and ITO (n~1.8) it was expected that the ITO /organic guided mode (TE0 mode) would be more confined to the ITO/organic layer . However, it was not the effect significant due to the small variation in refractive index . Thus, the outcoupling ef ficienc ies from both planar devices with and without LiF interlayer are similar , as shown in Figure 8 3 A . 8.3.4 Corrugated Second A ntinode OLEDs As described earlier , in the second antinode condition, where the emitting source is located far from the metallic interface, the power fraction dissipated to SP mode excitation becomes almost negligible. Therefore, the ITO/organic mode confined by total internal reflection at the ITO/glass interface can be excited only in the high index ITO/organic layers in such condition. By outcoupling only the ITO/organic mode with the corrugated grating structure, direct evaluation of the diffraction efficiency for the ITO/organic mode as a function of the index contrast can be achieved using second antinode condition and F igure 8 5A shows the performance of devices with diff erent refractive index contrast . Similar to the corrugated OLED with first antinode condition, the leakage currents for corrugated devices are well controlled, showing no such difference in current density before turnon voltage. T he LiF grating OLED shows the maximum efficiency of 126 cd/A while the conventional grating OLED without the LiF interlayer shows an efficie ncy of 113 cd/A (Figure 8 5B ) . It must be emphasized again that the general device efficiency of second antinode corrugated system is lower than that of first antinode condition , because the diffraction efficiency related to SP mode outcoupling in the firs t antinode is stronger due to the strong reflection of the corrugated metallic structure. We also eliminated the possibility that the reduced corrugated depth 105

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of the second antinode grating OLED may cause relatively lower efficiency by confirming that the corresponding AFM image does not show any significant difference with the corrugated OLED with first antinode condition. Compared to the current efficiency of the reference planar second antinode OLEDs ( 9 0 cd/A ) , the enhancement ratio for the LiF grating and grating OLEDs are 40% and 25% , respectively. The extra 15% enhancement for the second antinode LiF grating device is fairly consistent with that of the LiF grating device with first antinode condition, indicating that the improved diffraction efficienc y of the thin film guided mode is a result of the higher index contrast between LiF and ITO . 8.3.5 Full Extraction with Extra Substrate Mode E xtractor for First A ntinode OLEDs To evaluate the total light outcoupling efficiency, we also used a hemispherical lens on the back side of the substrate for extracting the substrate mode. To make coherent contact between the lens and substrate, an optical matching gel with refractive index of 1.5 was applied between them. Figure 86 A shows the angular intensity for t he LiF grating and grating devices with and without the lens. It clearly shows that with the lens about 50% additional enhancement is achieved from both corrugated OLEDs. Compared to the Lambertian distribution, which is a measure of uniform light output a s a function of polar angle, both corrugated OLEDs show stronger enhancement with macro lens attachment at around 20~30 This corresponds to the first order diffraction angle for the waveguide mode around the emission peak at 515 nm by the 0.5 peak periodicity of our corrugated structure. OLEDs fabricated on conventional photonic crystal s or grating s having well ordered symmetry show emission profiles with a distinctive butterfly pattern.[ 69, 85] However, our OLEDs fabricated on corrugated 106

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structure s with random orientation and broad distribution of periodicity do not show any particular emission angle dependence. From Figure 8 6 B , the EL spectra of both corrugated devices show as similar to that of the planar reference devic es, indicating that the enhancement is not limited to certain emission wavelengths. To calculate the outcoupling efficiency for the corrugated devices with a macro lens substrate mode extractor, the Lambertian correction factor was also considered, becaus e the angular pattern of both grating devices is slightly deviated from Lambertian pattern.[ 119 ]. Finally , we calculated the total outcoupling efficiency , as sh own in Table 8 2 , t he peak current efficiency of 210 cd/A and EQE of 67 % for the first antinode LiF grating device with the substrate mode extractor is the highest efficiency reported so far among the conventional bottom emitting OLED s fabricated on the conventional glass substrate. 8 .4 Summary In conclusion, we evaluated the outcoupling efficiency enhancement of each thinfilm mode and SP mode using OLED configurations with first and second antinode conditions. Furthermore, we demonstrate d that controllin g the index contrast for the corrugated grating structure in OLEDs can increase the diffraction efficiency of the ITO/organic waveguide mode . Addition of the low index LiF layer between the glass substrate and the ITO layer result ed in total 61% enhancemen t in light outcoupling of the device. By using an additional substrate mode extractor, we were finally able to produce a corrugated green phosphorescent OLED fabricated on a conventional glass substrate with extremely high efficienc ies of 210 cd/A and 67% EQE. 107

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8.5 Acknowledgements The authors would like to acknowledge the support of Department of Energy Solid State Lighting Program (Contract number: DE FG0207ER46464) 108

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Figure 8 1. AFM image of the top view and the depth of the corrugated resin structure fabricated on the glass substrate; t he nominal periodicity of 500 nm was observed from the corrugated structure and its depth was 70~80 nm 109

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Figure 8 2. Simulated power distrbution as a function of ETL thickness; A ) simulated efficiency charact eristics with single frequency of 515 nm as a function of ETL thickness by clasiccal dipole thoery method, (a round 60 and 240 nm of ETL (first and second antinode condtion, B ) first and, C ) second antinode OLED configurations for fabricated devices. A B C 110

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Figure 8 3. Electrical and optical charateristics of corrugated OLED with first antinode; A ) LuminanceVoltageCurrent density , B ) current efficiency curves , and C ) EL spectrum at normal direction for HC , LC grating, LiF planar and planar refer ence devices with first antinode condtion. A B C 111

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Table 8 1. The maximum current efficiency of corrugated OLED with first antinode condition Devices Planar reference a Planar with LiF G rating LiF grating Peak current efficiency [cd/A] 89 89 127 144 En hancement [%] 43 61 a The enhancement percent is based on the planar reference device. 112

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Figure 8 4 . Electric field intensities of one SP( TM 0) mode and one ITO /organic (TE0) mode mode as a function of the position inside of the devices; A) with and B ) without the 90nm thick LiF interlayer calculated by the transfer matrix method. A B 113

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Figure 8 5 . Electrical and optical characteristics of second antinode corrugated OLEDs ; A ) Luminanc e VoltageCurrent density and B ) current effici ency curves for HC, LC grating, LiF planar and planar reference devices Figure 8 6 . Angular distribution of corrugated OLEDs with first antinode with and without macro lens ; A ) a ngular intensity measurement for first antinode grating devices with and without a substrate mode extractor and B ) EL spectrum of devices measured at normal direction with macrolens A B A B 114

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Table 8 2. The maximum current efficiency and EQE from the LC and HC grating devices with an additional macrolens Devices Substrate mode enhanceme n t factor a Peak current efficiency [cd/A] Peak EQE [%] G rating 1.48 187 53 LiF grating 1.4 6 210 6 7 a Substrate mode enhancement factor can be obtained by the ratio between the EL intensity of OLED with and without a macrolens applied. 115

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CHAPTER 9 CONCLUSIONS Light extraction has been a key factor to determining the efficiency of OLEDs because the TIR generated by refractive index mismatching between air (n=1) , substrate (n=1.52 for glass) , and ITO/organic layers (n=1 .7~2) can only allow 20~30% of light to escape the device and contribute as useful light source. Especially, for the light trapped and guided in highindex ITO/organic layers, light extraction techniques using corrugated structures have shown effective out coupling characteristics of the light lost to the waveguide mode. In this study, we were able to understand the light extraction mechanism of quasi periodic corrugated grating structure in OLEDs and also developed new fabrication technique with simple and low cost process for the pattern structure. We fabricated the corrugated HCP array with silica particles embedded in the PS layer on glass substrate using LB based methods. By introducing the defects in HCP array, the break of long range order of HCP symm etry provide the broadening of periodicity and diffraction direction in all azimuthal angle for the diffraction of the waveguide mode in OLEDs. Varying the size of silica particles and the depth of PS layer in the silica/PS corrugated structure, we were able to control the peak periodicity and depth independently to optimize the light extraction enhancement in OLEDs. Because of distributed periodicity in the structure, the enhancement by the outcoupled waveguide mode in visible emission wavelength was fairl y uniform and did not show such particular angular dependence. Also, b y using distributed periodicity around 0.5 um in corrugated structure in OLEDs , the outcoupling of waveguide mode was realized with 70% enhancement in current efficiency, compared to 35% enhancement in OLEDs using 1 116

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um periodicity . By generating large enough grating wave vector from 0.5 um periodic structure, the direct out coupling of waveguide mode was demonstrated, while 1 um periodicity was not sufficiency enough to provide the direct outcoupling of the waveguide mode. Second, we studied corrugated OLEDs with two different substrates: normal glass and sapphire substrates. Using sapphire substrate with refractive index of 1.76, the optical interface between substrate and ITO can be el iminated. Thus, the thinfilm guide mode is no longer present in OLEDs. Furthermore , the SP mode was outcoupled by corrugated metal surface due to the diffraction mechanism. We were able to demonstrate the green phosphorescent OLED with efficiency of 63% E QE using both corrugated structure and substratemode extracting macro lens. Compared to 50% EQE from planar OLED fabricated on sapphire substrate with substrate mode extractor, 25% of enhancement is directly contributed from SP mode outcoupling . Fin ally, we studied refractive index contrast effect between substrate and ITO in corrugated OLED efficiency. Since the diffraction outcoupling from thinfilm guide mode is relatively limited due to the low contrast in refractive index between glass and ITO, the larger contrast using LiF with refractive index 1.39 between glass and ITO can provide better extraction of the thinfilm guide mode in corrugated OLEDs. Attaching the macro lens for extracting the substrate mode, we were successful to fabricate corrug ated grating OLED on conventional glass substrate with efficiency of 67% EQE, which is highest efficiency reported so far. 117

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BIOGRAPHICAL SKETCH Wooram Youn was born in March of 1983, in Incheon, Korea. He graduated from Inha University , Incheon, Kor ea with a b achelor’s degree in materials science and e ngineering in February o f 2009. He was admitted to the m aterials scie nce and e ngineeri ng at University of Florida , Gainesville and he joined Dr. So ’s research group in the fall of 2009. His PhD research was in the area of light extraction using quasi periodic corrugated structure in OLEDs. He has planned to join an optoelectronic industry to demonstrate hi s research professional skills and knowledge acquired from his PhD life. He received his PhD from University of Florida on May of 201 5 . 125