1 HIGH EFFICIENCY BLUE PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODES By NEETU CHOPRA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORID A IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009
2 2009 Neetu Chopra
3 To my Family and Sushant
4 ACKNOWLEDGMENTS A dissertation is almost never a solitary effort and neither is this one. As Ludwig Wittgenstein wisely said knowledge in t he end is based on acknowledgement Hence, writing this dissertation would be meani ngless without thanking everyone who has contributed to it in one way or the other. First and foremost, my thanks are due to my advisor Dr. Franky So, without whose guidance and encouragement none of this work would have been possible. He has been a great advisor and has always been patient through the long paths of struggle finally leading towards significant results. This work is a fruit born out of many st imulating discussions with Dr. So and my group members Jaewon Lee, Kaushik Roy Choudhury, Doyoung Kim, Dongwoo song, Cephas Small, Alok Gupta, Galileo Sarasqueta, Jegadesan S ubbiah, Mike Hartel, Mikail Shaikh, Song Chen, Pieter De Somer, Verena Giese, Dani el S. Duncan, Jiyon Song, Fredrick Steffy, Jesse Manders, Nikhil Bhandari and their contribution to these pages cant be acknowledged enough. I am especi ally thankful to Dr. Jiange ng Xue, Dr. Paul Holloway and their group members Sang Hyun Eom, Ying Zheng, Sergey Maslov and Debasis Bera who were an indispensable part of our DO E project team. I am also indebted to Dr. Rajiv Singh, Dr. Henry Hess and Dr. Kirk Schanze for agreeing to serve on my thesis committee. I would also like to express my sincer e thanks to the OLED team managers Dan Gaspar and Mark Gross at Pacific Northw est National Laboratory for giving me the wonderful experience of working at PNNL. I am especially thankful to my mentor Asanga B. Padmaperuma for explaining most of the chemistry involved in devices. None of the ambipolar host or mixed host wo rk would have possible without the help of James S. Swensen, Eugene Polikarpov, Lelia Cosimbescu, Charles C. Bonham, Liang
5 Wang, Phillip Koech, James Rainbolt, Amber Von Ruden. I was indeed very fortunate to collaborate with all of them. I would also like to thank my friends in Richland, WA for brining so much joy to my stay in PNNL. Jim, Sylvia, Nami ta, Margarita, Marcel, Tina, Marianna, Mikolaj, Tomonori, Eric, Jeremy, Byuongsu thank you all for all the wonderful times I spent with you guys. Gainesville and its enthusiastic spirit also had a profound impact on my thesis. I still remember vividly arriving here in an alie n land wondering if I will ever fit in and I found my place right from the start. For giving me this comfort, a la rge number of friends played a very big role and I would like to thank everyone Purushottam, Vibahva, Richa, Aniruddh, Arul, Praneetha, Ka ramjit, Ashutosh, Shirshant, Shweta, Preeti, Amee, Mamta and many others who have been a part of my life in Gainesville. I would especially like to thank my roommate and one of my best friends Toral for her support and company in all the fun as well as tough ti mes. I probably would have never made it to Gainesville, if it was not for t he unwavering support and unquestioned trust of Sushant in me. He has been my friend for over eight years now and he has always advised me in all my decisions, always stood by me in trouble or happiness, been patient though periods of my insanity and no words would be ever enough to express my gratitude to him. Every page of this thesis reflects the belief that he had in me. If it was not for his constant support and encour agement, I would not be obtaining this degree. Lastly, I would like to thank my family, mu mmy, papa and didi for their belief in me through all these years, for always encourag ing me to pursue my dreams, for enduring
6 this distance from me for my sake, for making me who I am today. I owe everything in my life to them. To them and to Sushant I dedicate this thesis.
7 TABLE OF CONTENTS page ACKNOWLEDG MENTS .................................................................................................. 4 LIST OF TABLES .......................................................................................................... 10 LIST OF FIGURES ........................................................................................................ 11 LIST OF ABBR EVIATIONS ........................................................................................... 15 ABSTRACT ................................................................................................................... 17 CHA PTER 1 INTRODUCTION: BRIEF REVIEW OF ORGANIC ELECTRON IC MATERIALS .... 19 1.1 Organic Semiconductors ................................................................................... 19 1.1.1 Interest in Or ganic Semicon ductors ........................................................ 21 1.1.2 Organic Semiconductors: T wo Gener al Classes ..................................... 22 1.1.3 Why Organic Semiconductors: Advantages and Di sadvantages ............. 23 1.2 Organic Semiconductor De vices ....................................................................... 25 1.2.1 Organic Light Em itting Diodes (OLEDs) .................................................. 25 1.2.2 Organic Photovol taic Device s (OPVs) ..................................................... 27 1.2.3 Other Devices Based on Organic Semi conducto rs ................................. 28 1.3 Physics of Organic Semiconductors: Fun dam entals and Pr ocesses ................ 29 1.3.1 Optical Properties .................................................................................... 29 126.96.36.199 Electronic proc esses in mo lecules ................................................. 30 1.3.2 Electrical Properties ................................................................................. 31 188.8.131.52 Charge ca rrier tr anspor t ................................................................. 31 184.108.40.206 Ex citons ......................................................................................... 33 220.127.116.11 Energy and charge transfer in molecules ....................................... 34 1.4 Summa ry .......................................................................................................... 35 2 ORGANIC LIGHT EMI TTING DI ODES ................................................................... 46 2.1 Need fo r OLEDs ............................................................................................... 46 2.1.1 Display Applications ................................................................................ 46 2.1.2 Lighting Applications ................................................................................ 47 2.2 OLED Lighting: Standar d Terms and Definitions .............................................. 47 2.2.1 Intr oduction .............................................................................................. 47 2.2.2 Basic Concept s ....................................................................................... 48 18.104.22.168 Luminance fl ux ............................................................................... 48 22.214.171.124 Lamberti an sour ce ......................................................................... 49 126.96.36.199 Human eye re s ponse ..................................................................... 49 188.8.131.52 Color correlate d temper ature ......................................................... 50 184.108.40.206 Color rendering i ndex ..................................................................... 50
8 220.127.116.11 CIE colo r coordi nates ..................................................................... 51 2.3 Device Parameters and Device Efficiency Measurements ................................ 52 2.3.1 Current Effici ency .................................................................................... 53 2.3.2 Power Effici ency ...................................................................................... 53 2.3.3 External Q uantum Effi cien cy ................................................................... 54 2.4 OLEDs: From History to Cu rrent State of the Art OLEDs .................................. 54 2.4.1 Phosphoresc ent mate rials ....................................................................... 55 2.4.2 Weak Link in High Effi ciency White OL EDs: Blue ................................... 56 2.4.3 Blue Phosphor escent OLEDs .................................................................. 58 2.5 Review of Factors Limi ting Blue OLED Performance ........................................ 59 3 EFFECT OF TRIPLET ENERGY CO NFINEMENT ON PERFORMANCE OF BLUE PHOPHORE SCENT OL EDS ........................................................................ 65 3.1 The Problem: Triplet Energy Conf inement ........................................................ 65 3.2 Triplet Exciton Confinement with Charge Transport Layers .............................. 68 3.2.1 Hole Tr ansport Layer ............................................................................... 68 3.2.2 Electron Tr ansport Layer ......................................................................... 70 3.3 Host-Dopant Effect............................................................................................ 71 3.4 Summa ry .......................................................................................................... 73 4 EFFECT OF GUEST-H OST INERACTIONS .......................................................... 82 4.1 FIrpic Doping Conc entration in Different Host Systems .................................... 82 4.2 Investigating GuestHost Inter actions ............................................................... 83 4.2.1 mCP Host ................................................................................................ 83 4.2.2 UGH2 Host .............................................................................................. 83 4.2.3 Effect on De vice Effi ciency ...................................................................... 84 4.2.4 Photoluminescence Quantum Yield Measurem ents (PLQY) ................... 85 4.3 Summa ry .......................................................................................................... 86 5 IMPORTANCE OF CHARGE BALA NCE IN BLUE PHOSPHORESCENT OLEDS .................................................................................................................... 91 5.1 Investigating Charge Ba lance ........................................................................... 91 5.2 Probing the Location of Recombinat ion Zone ................................................... 92 5.3 Importance of Tuning Char ge Balance for Blue PHO LEDs ............................... 93 5.4 Tuning Charge Balance wit h Doped Trans port Layers ..................................... 94 5.5 Tuning Charge Balance with High Tr ip let Energy High Mobility Electron Transport Ma terial ............................................................................................... 96 5.6 Triplet Energy or Mob ility? ................................................................................ 97 5.7 Improving Charge Transport and Ba lance in the Emitting Lay er ..................... 100 5.8 Summa ry ........................................................................................................ 101 6 ACHIEVING CHARGE BALANCE USING AMBIPOLAR HOST MATERIALS ...... 113 6.1 Charge Bal ance in EML .................................................................................. 113 6.2 Designing Concept for Ambipolar Host ........................................................... 114
9 6.3 Investigating Charge balance in PO Based Ambipolar Host Ma terial ............. 116 6.4 Evidence for Broad Recombinat ion Zone with Am bipolar Host ....................... 119 6.5 Summa ry ........................................................................................................ 123 7 USING MIXED HOST ARCHITECTURE TO ACHIEVE CHARGE BALANCE ...... 131 7.1 Introdu ction ..................................................................................................... 131 7.2 Blue PHOLED Devices with Mix ed Host Ar chitecture ..................................... 132 7.2.1 Unipolar Host De vices ........................................................................... 132 7.2.2 Devices with Mixed Host EML ............................................................... 134 7.3 Summa ry ........................................................................................................ 136 8 CONCLUSIONS AND FU TURE WORK ............................................................... 144 8.1 Summa ry ........................................................................................................ 144 8.1.1 Triplet Exci ton Confi nement .................................................................. 144 8.1.2 Charge Balance in Blue PH OLEDs ....................................................... 145 8.1.3 High Efficien cy Blue PH OLEDs ............................................................. 146 8.2 Light Ex traction ............................................................................................... 146 8.3 Down C onversion............................................................................................ 148 8.4 Future Work .................................................................................................... 149 LIST OF RE FERENCES ............................................................................................. 152 BIOGRAPHICAL SKETCH .......................................................................................... 163
10 LIST OF TABLES Table page 3-1 Table listing the triplet energies of various host materials used in this study and their corresponding efficienc y comparis on at the same current density of 1.5 mA/cm2. ........................................................................................................ 81 5-1 Energy levels, triplet energy and mob ility par ameters for different electron transport materials used in this study ............................................................... 112 7-1 Device parameters for devices fabr icated with the same structure using PO15, TAPC, or TAPC-PO15 mixed host as the host in the EML .................... 143
11 LIST OF FIGURES Figure page 1-1 Formation of and bonds with sp2 hybridization. ............................................ 37 1-2 Schematic representation of sp2 hybridization leading to formation of delocalized bond which further paves the way for hopping transport in organic semiconductors as illustrated. ................................................................ 37 1-3 Atomic orbitals (AOs) (s, p) combi ne to for m molecular orbitals (MOs) ( ). ... 38 1-4 Molecular structures of some of commonly used small molecules for organic electronic devices ............................................................................................... 38 1-5 Structures of some of the polymers widely used in solut ion processed organic optoelectr onic devic es. .......................................................................... 39 1-6 CIE coordinates of phosphorescent cyclomated platinum complexes with slight alteration in mo lecular stru cture. ............................................................... 39 1-7 Schematic showing a typical OLED device stack with various layers. ................ 40 1-8 Sonys 11 OL ED televi sion.. .............................................................................. 40 1-9 A picture of Samsung bendable OLED display. .................................................. 41 1-10 Schematic showing two prevalent device architectures used for so lar cells. ...... 41 1-11 Plots showing comparison of device performance for typical planar and bulk heterojunction photovoltaic cell.. ......................................................................... 42 1-12 Schematic showing the steps involved in fl uorescence. ..................................... 43 1-13 Example of absorption and emission s pectra for an organic material (PFO) showing flor escence. .......................................................................................... 43 1-14 Schematic showing steps involved in phosp horescence. ................................... 44 1-15 Jablonski energy diagram summarizing the processes involved in fluorescence and phos phorescenc e. .................................................................. 44 1-16 Schematic depicting the energy diagram for hopping transport. ......................... 45 1-17 Schematic showing various types of ex citons in materials. ................................ 45 2-1 Photopic and scotopic response curves fo r human eye. .................................... 61 2-2 Mesopic response cu rve for hum an eye. ............................................................ 61
12 2-3 CIE chromaticity di agram. .................................................................................. 62 2-4 A picture of LIV se tup used in this study for measurements on OLED devices. .............................................................................................................. 62 2-5 Experimental geometry used for effici ency measurements and calculations in this dissertation ................................................................................................... 63 2-6 Different approaches used for producing wh ite light. .......................................... 63 2-7 Structure of common blue phos phorescent dopants and their peak emission wavelengt h. ........................................................................................................ 64 3-1 Studying triplet ex citon conf inement .................................................................. 74 3-2 Illustrating the effect of host trip let energy .......................................................... 74 3-3 Illustrating the effect of triple t energy of HTL on dev ice performance ................. 75 3-4 Illustrating the effect of triplet energy of ETL. ..................................................... 76 3-5 Device structure of blue PHOLEDs for studying effect of triplet energy confinement fro m HTL ........................................................................................ 77 3-6 Current efficiency and power efficien cy of blue PHOLEDs by changing HTLs. .. 77 3-7 Device structure used for studying e ffect of different electron transport materials ( ETMs). ............................................................................................... 78 3-8 Comparing different ET Ls for blue PHOLEDs. ................................................... 78 3-9 Device structure used for comparing di fferent host materials and to study the guest-host inte ractions ....................................................................................... 79 3-10 Device structure used for comparing different host materials and to study the guest-host inte ractions ....................................................................................... 79 3-11 Current efficiency of devices fabr icated with different host mate rials. ................ 80 4-1 Photoluminescence efficiency for thin films of phos phorescent dopants. ........... 88 4-2 Device structure used for investigating guest-host intera ctions. ......................... 88 4-3 J-V characteristics for devices with mCP as host and different doping concentration of FI rpic mole cules ....................................................................... 89 4-4 J-V characteristics for devices wi th UGH2 as host and different doping concentration of FI rpic mole cules ....................................................................... 89
13 4-5 Concentration dependence of device efficiency for devices with mCP and UGH2 as host (lines are used only as a guide to the ey e). ................................. 90 5-1 Device structures for single ca rrier devices used in this study.......................... 103 5-2 Current density-voltage (J-V) characteri stics for single carrier devices shown in Figure 5-1 using TAPC as HTL and BCP as ETL. ........................................ 104 5-3 Devices fabricated for probi ng the recombi nation zone.. .................................. 104 5-4 Device probing the location of rec ombination zone in the dev ice. .................... 105 5-5 Illustrating the location of recombinat ion zone and triplet exciton co nfinement for blue PH OLEDs. ........................................................................................... 105 5-6 Device structure for UGH2 hos t c ontrol device and p-i-n dev ice. ..................... 106 5-7 Efficiency comparison for three sets of devices: control device, p-i-n device and n-doped dev ice. ......................................................................................... 106 5-8 Comparison of device efficiency for tw o p-i-n devices with different FIrpic doping concentrati ons: 10% and 15%. ............................................................. 107 5-9 JV characteristics of electron only devices with BCP, 3TPYMB and BPhen as the ET L. ............................................................................................................ 107 5-10 Efficiencies for OLED devices wi th BCP, 3TPYMB and BPhen as the ETL ..... 108 5-11 Device structure of blue PHOLED us ed in this study with UGH2 as the host with BCP or 3TPYMB used as ETL. ................................................................. 108 5-12 UGH2 host based OLED devices with BCP and 3TPYMB as ETL.. ................. 109 5-13 Energy diagram for device C showing the energy levels for respective layers. 109 5-14 Comparing device performance fo r devices A, B, C, an d D ............................. 110 5-15 Efficiency comparison for UGH2 de vices with 10% and 20% FIrpic doping concentra tion. ................................................................................................... 110 5-16 Illustrating the effect of higher dop ant concentration on transport in EML. ....... 111 6-1 Design criteria used for designing hos t materials to optimize charge injection and charge carrier blocking in het erostructure bl ue PHOLEDs. ....................... 124 6-2 Design strategy used for designing ambipolar host materi als by combination of hole transport and electron tran spor t moiety using an aryl linkage. .............. 124 6-3 Chemical structures of mole cules evaluated fo r host des ign. ........................... 125
14 6-4 HOMO and LUMO energy levels for the four evaluated host molecules as computed at B3LYP/ 6-31G* le vel. .................................................................... 125 6-5 Illustrating ambipolar trans port in HMA hos t material ....................................... 126 6-6 EQE as a function of varying th icknesses of an undoped HM-A1 interlayer between the HTL and the EM L.. ....................................................................... 127 6-7 J-V characteristics for the hole on ly and electron only devices using the ambipolar host HM-A1. ..................................................................................... 127 6-8 Efficiency of optimized device with HM-A1 as host. .......................................... 128 6-9 Chemical structure of another host synthesized on t he same design strategy and another electron transport/hole blocking material used in this study. ........ 128 6-10 Hole-only and electron-only single carrier devices for HM-A5, PO12 and HMA1 host mate rials. ............................................................................................. 129 6-11 Recombination zone study in PO based host materials. .................................. 130 7-1 Device structure for heteros tructu re and mixe d host devices. .......................... 138 7-2 Device structure used for fabric ati ng unipolar host devices where EML consisted of either TAPC or PO15 as host doped with FIrpic. .......................... 138 7-3 Efficiency characteristics for devic es fabricated with TAPC as host doped with varying concentration of FIr pic emitter. ..................................................... 139 7-4 Dependence of operation voltage on dopi ng concentration of FIrpic in devices with T APC as host. .............................................................................. 139 7-5 Schematic illustrating the device arch itecture of devices with a mixe d host EML. ................................................................................................................. 140 7-6 Dependence of device efficiency on TAPC concentration in emitting layer. ..... 140 7-7 Plot showing the tr end of operation voltage of mixed hos t devices with varying concentration of TAPC in the emi tting layer. ........................................ 141 7-8 Plot showing the comparison of current densityluminancevoltage characteristics for devi ces with TAPC, PO15 as host and the optimized mixed host device. ............................................................................................ 141 7-9 Plot showing the repr esentative current and power efficiency characteristics for the opti mized mi xed host device. ................................................................ 142 7-10 Plot showing the comparison of devic e efficiency as a function of brightness for the TAP C host, PO15 host and optimized mixed host devices. ................... 142
15 LIST OF ABBREVIATIONS OLED Organic Light Emitting Diode PHOLED Phosphorescent Or ganic Light Emitting Diode HOMO Highest Occupied Molecular Orbital LUMO Lowest Unoccupied Molecular Orbital HIL Hole Injection Layer HTL Hole Transport Layer EML Emitting Layer ETL Electron Transport Layer HBL Hole Blocking Layer FIrpic iridium (III) bis[(4 ,6-difluorophenyl)-pyridinatoN C2]picolinate mCP 1,3-Bis(carbazol-9-yl)benzene CBP 4,4'-Bis(carbazol-9-yl)biphenyl CDBP 4,4'-Bis(9-carbazolyl)-2,2'-dimethyl-biphenyl UGH2 1,4-Bis(tri phenylsilyl)benzene Alq3 Tris(8-hydroxy-quinolinato)aluminium MEH-PPV poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] CuPc Copper Phthalocyanine NPB N,N'-Bis(naphthalen-1-yl )-N,N'-bis(phenyl)-benzidine TAPC Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane HM-A1 4-(diphenylphosphoryl)-N,N-diphenylaniline HM-A5 9-(6-(diphenylphosphoryl)pyridin-3-yl)-9 H -carbazole mer-Ir(Pmb)3 mer-Iridium(III) Tris(1-phenyl3-methylbenzimidazolin-2-ylideneC,C2') ETm Electron transport moiety HTm Hole transport moiety
16 FIr6 Bis(2,4-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate iridium III Ir(ppy)3 Tris(2-phenylpyridine)iridium(III)
17 Abstract of Dissertation Pr esented to the Graduate School of the University of Florida in Partial Fulf illment of the Requirements for t he Degree of Doctor of Philosophy HIGH EFFICIENCY BLUE PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODES By Neetu Chopra December 2009 Chair: Franky So Major: Materials Science and Engineering Organic light emitting diodes are touted as a promising candidate for solid state lighting. Keeping in mind the ener gy situation world is facing today, it is imperative to have a low cost, large area, low energy consumption lighting al ternative and thats where organic light emitti ng diodes become very important. Also, OLEDs are very attractive from the standpoint of full co lor display application because of desirable properties such as wide viewing angle an d easy fabrication. For both of these applications, white light is desirable which can be obtained from the combination of basic color components i.e. RGB. Phosphorescent organic mate rials are inherently four times more efficient compared to fluoresc ent materials. Using these phosphorescent materials, very high efficiencies have been achieved for red and green OLEDs. However, blue emitting phosphorescent devices were still lagging behind in terms of device efficiency until very recently. The focus of this work has been to remove this weak link in development of high efficiency white OLEDs by studying the materials and device properties. For understanding factors impacting t he device performance, effect s of material properties such as triplet energy and mobility on the device performance of blue phosphorescent
18 organic light emitting diodes (PHOLEDs) were in vestigated. The effect of triplet energy of different charge transport materials and the host materials were studied systematically. Also, the device performance was correlated with the mobility and transport properties for the materials used. Single carrier devices were fabricated to study and compare the transport of hole and el ectrons. It was found that these devices are largely hole dominant. Hence, it was ex pected that the recombination zone is located on the interface of the emitting la yer and the electron transport layer which was verified experimentally by probing the recombination zone. The charge balance becomes even more significant in case of PH OLEDs as most of t he conventionally used electron transport materials have lower triple t energy than that of blue phosphorescent dopants. Based on these findings, two majo r challenges were identified in these devices namely, 1) low triplet energy of t he electron transport materials and 2) charge imbalance in the devices. Two approaches were used to get around these problems: 1) improving the electron trans port in the device by use of doped transport layers and using high triplet energy high mobility electron transport material to confine the triplet excitons and tune the char ge balance in the device and 2) using a mixed host architecture or ambipolar host materials to achieve charge balance in emitting layer. Based on these studies, vey high efficiency devices were fabricated an efficiency of 50 lm/W. In the course of fabr icating these high efficiency devices we discuss the device physics and the correlation of materials properties such as the energy level alignment of different layers on the device performanc e and characteristics of these blue phosphorescent devices.
19 CHAPTER 1 INTRODUCTION: BRIEF REVIEW OF ORGANIC ELECTRONIC MATERIALS The field of organic semiconductors is very interesting largely because of the their numerous advantages over conventional inorganic semiconductor materials such as potential low cost, large area fabrication, ease of proces sing and fabrication and their tailor-ability for specific application s. Howeve r, there are still some issues inhibiting large scale commercialization of organic devices. The aim of this chapter is to make the reader aware of these advant ages as well as disadvantages of organic semiconductors along with highlighting their numerous applicati ons. This brief review will also highlight current state of the art and associated challenges for large scale production and commercialization of these devices. 1.1 Organic Semiconductors Since the invention of transistor i n 1947, inorganic semiconductors like silicon or germanium have dominated the scene in electroni c industry. It is the key component in all modern electronic devices which were in conceivable few years ago. Now, we are looking at a new electronics revolution whic h has become possible due to the advent of a new class of materials known as organic semiconductors. Hence, this section is focused on understanding this class of materials. A chemical compound is said to be organic if it contains carbon. This definition is very broad and covers a large number of materials that are know n to us. In fa ct, to date, about two million organic compounds have been made which constitutes nearly 90% of all known materials. These organic molecules range from simple hydrocarbons like methane, ethane, etc to comple x polymer chains, proteins and biological molecules that
20 can contain thousands of carbon atoms in co mbination with variou s other elements like hydrogen, nitrogen, oxygen, sulfur, etc. Solids are generally classified based on the type of bonding that exists between the constituent units (i.e. atoms or molecule s); these include covalent, ionic, metallic, or molecular. Organic solids are molecular[2, 3] solids composed of discrete molecules held together by van der Waals forces wher eas the individual molecules consist of atoms bonded together by covalent bonds. O wing to these weak van der Waals interactions these organic molecular solids retain very similar electrical and optical properties of the constituent molecules when compared to the case of covalent solids, such as inorganic semiconductors Also due to these weak van der Waals intermolecular forces, molecular solids are generally soft materials with low melting points and poor electrical quality. However, of the nearly in finite number of possible organic molecules, those with chains or rings of alternating single and double bonds (conjugated molecules) are particularly inte resting, as they show semiconducting properties. In such molecules, the four outer shell electrons in the carbon atom (i.e. the two s electrons and two p electr ons) combine to form three sp2 orbitals, leading to the formation of and -bonds as shown in Figure 1-1. The delocalization of electrons due to formation of these -bonds leads to the semiconducting properties of conjugated organic materials. Figure 1-2 shows how the delocalization of these electrons in the plane perpendicular to the molecule and the bonds leads to a continuous path for electron transport in these mole cules The overlapping of these -orbitals creates degeneracy which leads to the formation of filled bands known as highest occupied molecular orbital (HOMO or bonding molecu lar orbital) and unfilled bands known as
21 lowest unoccupied molecular orbital (LUMO or antibonding molecular orbital). The formation of these bands are illustrated in Figure 1-3 where the molecular orbital denotes the HOMO and denotes the LUMO. In the ca se of a large number of electrons, these and levels broaden into conti nuous bands, with the HOMO-LUMO energy gap becoming analogous to the valance band-conduction band gap of inorganic semiconductor materials. 1.1.1 Interest in Organic Semiconductors The interest in organic semiconductors is not completely new, the first studies date back to early 20th century[4-6]. Discovery of electroluminescence in single crystals of anthracene lead to research efforts being focu sed on molecular crystals in the later half of 20th century[2, 7, 8]. Although these early on studies helped lay the groundwork for organic semiconductors, the devices fabricated at that time were largely inefficient and unstable. Further developments in synt hesis and doping led to development of conjugated polymers which wa s honored with the Nobel Prize in Chemistry in year 2000. Conducting polymers al ong with organic photoconductors initiated the first applications of organic materials as conducti ve coatings. The interest in undoped organic semiconductors was revived by f abrication of first organic heterojunction photovoltaic cell Also the first reports of organic thin film transistors based on polymers[12, 13] and o ligomers were published around the same time. However, the demonstration of high efficiency organic light emitting diodes by both vacuum sublimation of organic molecules[15, 16] and by solution processing of polymers rekindled extensive interest in th e field of organic semiconductors.
22 1.1.2 Organic Semiconduc tors: Tw o General Classes As briefly discussed above, organic semico nductors are generally classified into two major classes: 1) low molecular weight molecules better known as small molecules and 2) polymers. Both these classes show the sp2 hybridization of C-orbitals with delocalization due to formation of bonds. Small molecules typically consist of several to a few hundred atoms. Small molecule materials were the focus of most of the initial studies as outlined above as they are easy to purify and grow into crystals or thin films using a variety of vapor phase techniques. Figure 1-4 shows the structure of some of the common small molecule materials used in OLEDs, organic solar cells and transistors. Alq3 (tris(8-hydroxyquinolinato)aluminum) is a common material used in OLEDs, pentacene is used in fabrication of organic thin film transistors (OTFTs) and it can be grown into single crystals or polycrystalline thin films, CuPC (copper phthalocyanine) and C60 (Fullerene) are small molecule materials us ed widely in organic photovoltaic devices and photo-detectors. One of the major differences between the two classes of materials lies in the way these materials are processed into thin film s for device fabrication. Small molecules are usually deposited from the v apor phase by sublimation or evaporation. The packing of molecules in these thin films is determined by the shape of the molecule and the degree of stearic hindrance structure these molecules stack differently in films. For example, Alq3 has an irregular non symmetric shape and hence forms amorphous thin films whereas pentacene which has a regular or symme tric structure packs efficiently to give regular lattices forming crystalline films. Hence, some of these have been studied extensively as model systems.
23 Polymers are characterized by a molecular structure of long chains consisting of a basic repeat structural unit called monomer connected by covalent bonds. These long polymer chains with a large degree of polym erization can not be vapor deposited as they decompose before reaching theoretic al boiling temperatures. Hence, these polymeric materials are processed from solu tion by using various techniques such as spin coating, drop casting, doctor blading or printing. The growth of crystal domains or the film morphology is usually optimized using a number of parameters such as choice of solvents, baking and post processing conditi ons. The controlled growth of thin films of polymers having desired morphology is still a s ubject of ongoing research and is critical for various device applications. Figure 1-5 shows some of the commonly used polymers for fabr ication of organic optoelectronic devices. Poly [2-methoxy-5 -(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is a common orange emitting polym er, whereas poly (9,9-dioctylfluorene) (PFO) and poly(N-vinyl carbazole) PVK em it in green and blue respectively. Regioregular poly(3-hexylthio phene) (P3HT)phenyl-c61-butyric acid methyl ester (PCBM) is a commonly used donor acceptor combination used for fabricating high efficiency polymer solar cells. 1.1.3 Why Organic Semiconductors : Advantages and Disadvantages Organic semiconductors have numerous advantages over their inorganic counterparts. The most attrac tive advantages for these mate rials are potential of low cost and large area manufacturing processe s. Both vacuum sublim ation of small molecule and solution processing of polymer thin films require significantly lower processing temperature compar ed with thin film fabricat ion processes of inorganic semiconductors such as MOCVD (metal-or ganic chemical vapor deposition), MBE
24 (molecular beam epitaxy), sputtering or plasma enhanced chemical vapor deposition (PECVD). Especially some of the polymer deposition techni ques such as gravure or inkjet printing are compatible with large-area, roll-to-roll proc essing potentially leading to very high-throughput, large scale and low cost manufacturing. Also all these organic thin film deposition techniques are compatible with various low-cost and flexible substrates such as plastic leading to various novel applications wh ich are not possible with conventional inorganic materials. Another very lucrative advant age of these materials is the ability to tailor their electronic and optical properties by chemical modification for a specific application. For instance, color tuning molecules to emit in the blue, green or red portion of the visible spectrum is easily achieved and can be accomplished with a variety of molecular structures and their derivatives[ 21, 22] such as for iridium or platinum complexes[21-24] as shown in Figure 1-6. In addition, the abs orption coefficient and fluorescence yield of these organic materials can be very high making them suitable for various device applications such as OLEDs, OPVs etc. which will be discussed in the following sections of this chapter. However, in spite of the above advantages, these organic materials have some disadvantages for use in electronic and optoelec tronic devices. As a result of the weak intermolecular interactions, these materials generally have much lower carrier mobilities than inorganic electronic mate rials, typically less than 1 cm2/Vs. In addition, due to the low carrier density in these materials, the electrical conductivity is low and hence devices based on these materials show high resistance, thus limiting the device performance.
25 Another issue is achieving high purity organi c materials, with purity levels close to that of inorganic semiconductors such as s ilicon. Hence, these materials exhibit high trap density due to both impuritie s and structural defects. Finally, most organic devices show degradation in ambient environment limiting their commercialization into the consumer markets. The lifetimes of t hese devices can however be considerably improved using optimized encapsulation techni ques. Recently very long lifetime organic devices have been demonstrated especially for OLEDs. Also there have extensive research efforts to study t he inherent degradation of OLED materials leading to useful insights[26-31]. Hence, it is believed that th rough focused research efforts most of these disadvantages can be overcome leading to successful commercialization of organic semiconductors in OLEDs, OPVs and various other device applications. 1.2 Organic Semiconductor Devices In the last 20 years or so, tremendous progress has been made in various different devices based on these organic semiconductor materials. This shor t section will outline very briefly different types of devices and cu rrent state of the ar t fabricated with these materials. The discus sion is ma inly limited to OLEDs and OPVs. 1.2.1 Organic Light Emitting Diodes (OLEDs) An OLED is basically an LED whose electr oluminescent layer is composed of an organic compound. In most simple cases, an OLED can consist of a single layer of an organic material that is sandwiched betw een two electrodes. When a voltage bias is applied across two electrodes, charges are injected into the organic layers where they recombine to form an exciton that radiativ ely decays to the ground state. A conventional OLED structure has following components: SUBSTRATE To support OLED, generally glass/ plastic
26 ANODE. To inject holes in the device HOLE TRANSPORT LAYER (HTL). To transport the injected holes to the recombination zone where they will recombine with the electrons. ELECTRON TRANSPORT LAYER (ETL). To transport the injected electrons. In conventional OLED structure ETL also serves as an Emitting Layer (EML). CATHODE. To inject electrons in the device. Most OLED devices have a separate EML sandwiched bet ween HTL and ETL. The injected holes and electrons travel thro ugh the transport layers to combine at the interface and produce light. Figure 1-7 shows t he schematic of a device with a separate EML. In addition a Hole Injecting Layer (HIL) is mostly used to facilitate hole injection. The HIL is sandwiched between the anode and the HTL. Apart from these more common layers sometimes other organic layers might be incorporated in the structure to confine carriers and excitons effectively e.g. HOLE BLOCKING LAYER (HBL). Blocking layer used at the EML/ETL interface to block holes from entering ETL so as to avoid any emission from ETL. ELECTRON BLOCKING LAYER (EBL). Blocking layer used at the HTL/EML interface to block electrons from entering HTL so as to avoid any emission from HTL. Either the bottom or top electrode is usually transparent or at least semitransparent to allow for the extraction of light generated within t hese organic layers. The OLED technology is the most mature technology based on organic materials. Full-color displays based on fluorescent and phosphorescent OLEDs and PLEDs are already in commercial production. Figure 1-8 shows the 11 full color OLED TV launched by Sony in 2007. Also OLEDs have already found there way in commercial market as cell phone, mp3 pl ayers and car stereo displays, and it is believed that many more unconventional applications of OLED will be out in the market soon such as flexible, roll-able and bendable displays as shown in Figure 1-9.
27 OLEDs are particularly suited for these applicat ions because of their better readability in sunlight and their low power consumption. Also OLEDs can be used as backlights in flat-panel displays for light weight and low power advantages. OLEDs also demonstrate great potential for solid state lighting applications such as general space illumination, and large-area light-emitti ng elements. OLEDs are wide area light sources and they typically emit less light per unit area than their inorganic counterparts, which are usually point-light sources. Large area OL ED lighting panels and OLED chandelier were demonstrated by J unji Kido of Yamagata University, Japan at SPIE optics and photonics c onference, 2009. Hence, these OLED devices hold lot of promise if we can overcome lifetime limitations. 1.2.2 Organic Photovoltaic Devices (OPVs) Photovoltaic power generation is a one-st ep process in which light energy is converted into electrical energy. A PV device achiev es this energy conversion by absorbing light incident on the cell, resulting in a separation of c harge carriers, which leads to photocurrent in the circuit that does work on an external load. There is a wide variety of choice of organic and inorganic semiconducting materials available to fabricate photovoltaic devices. It is a subject of intense research to apply these available materials to fabric ate highly efficient, long-lifetime solar cell modules with a reasonable figure of merit ($/W). Organic based photovoltaic cells have been the subject of intense industrial and academic research effort over the last few decades. Various approaches used for fabrication of these devices include small molecular based OPVs, polymeric solar cells, and the dye-sensitized solar cell (DSSC). Of the three approaches, DSSC is most efficient. Recently, polymer bulk heterojuncti on solar cells have been receiving a lot of
28 attention due to various boosts in their device efficiency. Recently, a polymer cell with 6% efficiency has been reported by Park et al. Also another recent report by Solarmer Energy broke the 7% barrier for organic PV efficiency . The simplest architecture that may be used for an organic PV device is a planar heterojunction, shown in Fig. 1-10 a) which is similar to OL ED device structure. In the heterostructure device, a thin film of donor organic molecule and a film of electron acceptor are sandwiched between contacts in a planar configuration. Excitons created in the bulk may diffuse to the junction and s eparate. However, plan ar heterojunctions are inherently inefficient; because charge ca rriers have very short diffusion lengths typically few nanometers in organic semico nductors. Hence, planar cells must be fabricated very thin to maximi ze efficiency, but the thinner the cell, the less light it can absorb. This has been overcome by fabricating bulk heterojunctions (BHJs) as shown in Figure 1-10 b). In a BHJ, the electron donor and accept or materials are blended together via co-evaporation in small molecules or spin casting for polymers. Regions of both donor and acceptor materials are thus separated by only several nanometers, hence, these devices show better efficien cy as shown in Fig 1-11. Although BHJ devices show better device performance, th ese devices require close control over materials morphology. Many va riables such as: materials ch oice, solvents (for polymer based BHJ devices), and the donor-acceptor ratio need to be considered to optimize the performance of these devices. 1.2.3 Other Devices Based on Organic Semiconductors Besides O LEDs and OPVs, many other electronic and optoelectronic devices based on organic semiconductors have been demonstrated, such as organic
29 photodetectors[39, 40], organic thin film transis tors[41, 42], light emitting transistors[43, 44], light up conversion devices, organi c lasers[46-48], memo ries[49, 50] and chemical sensors[51, 52]. 1.3 Physics of Organic Semiconductors: Fundamentals and Processes As pointed out in earlier discussion, the nature of bonding in orga nic semiconductors is fundamentally different from their i norganic counterparts. They exhibit van der Waals intermolecular forc es whereas inorganic semiconductors are typically covalently bonded. This results in not only widely different mechanical or thermodynamic properties, but also leads to much weaker delocalization of electronic states among molecules which directly affects the optical and electrical properties of these materials. Since we are studying thes e materials from point of view of their applications in opto-electronic devices, t he discussion would be incomplete without discussing these properties of organic materials. 1.3.1 Optical Properties As a consequence of their weak electronic de localization, these materials differ in two important ways from their inorganic counter parts. The first one is the existence of well defined spin states i.e. singlet states and triplet states. Thes e states have important consequences for the device properties of these material systems as will be discussed below and sets the upper limit for the effici ency of OLEDs. The se cond very important difference is that the excit ons are localized on the molecule, which leads to large binding energy in these molecules which significantly affect the performance of photovoltaic cells and photo-detectors as the binding energy has to be overcome in order to generate a pair of positive and negative charges. Since this thesis is focused
30 on OLEDs, we will keep the discussions limited to properties correlating to their device performance. 18.104.22.168 Electronic pr ocesses in molecules In a molecule there are two possible fate s for electronically excited states to decay. Radiative decay process is a process in which a molecule losses its excitation energy as a photon. This is the underlyi ng phenom enon for light emission in OLEDs. A more common fate is nonradiative decay, in which the energy is lost in form of wither vibration, rotation and transla tion of the neighboring molecule s. There are two different phenomenon involved in radiative decay namely: fluorescence and phosphorescence which are discussed below: Fluorescence Fluorescence is defined as s pontaneous emission of radiation within few nanoseconds of electrical or optical excitation of the molecule. Figure 1-12 shows the steps involved in fluorescence. First we have the initial absorption step leading to formation of an excit ed state. Also the molecule can be excited via electrical energy as in case of OLEDs. This step is us ually followed by relaxation of the excited state and then finally leading to radiative decay via fluorescence. Hence, the absorption and emission spectrum look like that s hown in Fig 1-13 because the fluorescence usually occurs at lower energy (higher wa velength) than the inci dent radiation as a fraction of energy is lost to su rroundings via vibration losses. Phosphorescence Phosphorescence is defined as spontaneous emission of radiation for long periods of times (even hours, but usually seconds or fractions of seconds) of electrical or optical excitation of the molecule. Figure 1-14 shows the steps involved in phosphorescence for a molecule wit h a singlet ground stat e. The first steps are the same as in case of fluorescence, but the presence of a triplet excited state
31 changes everything. The singlet and triplet excited states share a common geometry where their potential energy curves inte rsect. Hence, the molecule may undergo an intersystem crossing here by unpairing two electron spins. Singlettriplet transitions may occur in case of spin-orbit coupling in molecules. We can expect intersystem crossing to be important when a molecule contains a moderately heavy atom, because then the spin-orbit coupling is even larger. If a molecule crosses over into a triplet st ate, it continues to loose some of its energy to its surroundings by steeping down in triplet ladder. At the lowest point its trapped, it cant be converted back to a singlet state as it has a lower energy state now. The molecule can not return to its ground st ate as the transition is spin forbidden. However, the decay is not totally forbidden as the spin orbit coupling breaks the selection rule. The molecules are therefore able to emit weakly and the emission continues long after the excitation. Various types of radiative and non-radiative processes in molecules are usually summarized in a schematic known as Jabl onski diagram which is depicted here in Figure 1-15. 1.3.2 Electrical Properties In this secti on we will discuss some of the fundamentals affecting the electrical characteristics of these materials. The main concepts that are discussed are the models of charge carrier transport in organic se miconductors and the fundamentals of charge and energy transfer through excitons in these molecules. 22.214.171.124 Charge carrier transport The treatment of charge carrier transport in these materials still remains a topic of interest for experimental and theoretical study. The transport mechanism in organic
32 materials falls between two extreme case s, band transport model or polaron hopping transport model. Band transport is usually observed in highly purified molecular crystals (delocalized systems), and t he polaron model is observed in amorphous organic solids in which carriers are localized, and distor t the surrounding lattice as they move. The Band Model. For a delocalized charge carrier in an organic semiconductor, the resulting bands are similar to the case of crystalline, inorganic semiconductors. The weak van der Waals intermolecular forces lead to narrow band-widths (typically a few kT) as compared to inorganic semiconductors Hence, the mobilities in molecular crystals reach values of only up to 1-10 cm2/Vs. When the bandwidth is comparable to the thermal energy, all levels of the band can be populated as opposed to those found in the lower energy tail of the band. The Polaron-Hopping Model. The other extreme case fo r charge carrier transport in organic molecules is the hopping model. This model is usually applied to amorphous organic solids which constitutes a vast ma jority of organic semiconductors. Hopping model leads to much lower mobility values of around 10-3 cm2/Vs or less. The band model is based on the assump tion that the charge carrier mean free paths are much larger than lattice constant. In cases where this condition is not satisfied and charge transport consists of the carrier residing on a lattice site for an extended period of time followed by rapid intermolecular jumps. In such cases, the band model is no longer valid. In these systems, the carriers polariz e the surrounding medium which relaxes to a new equilibrium configuration. The charge carri er, along with the distor tion it creates in the medium, is referred to as a polaron[3, 55]. As a result of the reconfiguration of the lattice, the polaron can be cons idered as residing in a potential well. Transport requires
33 the hopping of the polaron between potential wells, traversing energy barriers with each hop as depicted in Figure 1-16. Hence, in any given material at sufficiently low temperature, the dominant mechanism is na rrow band transport. At higher temperatures it is difficult to distinguish between band-like and hopping transpor t. The materials examined in this thesis are typically am orphous; hence, the ch arge transport is expected to be dominated by hopping between lattice sites. Apart from the intermolecular charge transport model, charge injection effects along with space charge limited conduction and trapping effects need to be taken into account to completely understand and describe the electrical char acteristics of organic semiconductors. 126.96.36.199 Excitons Exc itons are coulomb correlated or bound electron-hole pairs. It is an elementary excitation, or a quasiparticle of a solid. Ex citons provide a means to transport energy without transporting net charge. H ence it carries no electric current. Excitons play an important role in opto-electronic properties of organic materials[56, 57]. Due to the strong tendency of localization of charge carriers in organic semiconductors, excitation leads to creation of excitons instead of free electron and holes as in inorganic semiconductors. Three types of excitons have been observed in crystalline solids as shown in Figure 1-17 namely a Wannier-Mott exciton, a Frenkel exciton and a chargetransfer exciton. In a Frenkel exciton, electron and hole re side on the same molecule when in an excited state. In other words, when a material's dielectric constant is very small, the Coulomb interaction between electron and hol e become very strong and the excitons tend to be much smaller, of the same order as the unit cell (or on the same molecule),
34 so the electron and hole sit on the same molecu le or cell. Frenkel exciton has typical binding energy on the order of 1.0 eV. In inorganic semiconductors, the dielectric constant is generally larger, and as a result, screening tends to reduce the Coulomb interaction between electrons and holes. This results in the formation of a Mott-Wannier exciton which has a radius much larger than the lattice spacing. With increasing intermolecular interactions the excitonic state may delocalize over adjacent molecules, forming a charge transfer (CT) exciton. While Frenkel and CT excitons are present in organic semi conductors, highly delocalized Wannier-Mott excitons with R >> a are commonly found in inorganic semiconductors. Due to the Columbic interaction between the constit uent electron and hole, the Frenkel or CT excitons are tightly bound while Wannier-Mott excitons in inorganic semiconductors are only loosely bound. 188.8.131.52 Energy and charge transfer in molecules The motion of excitons in organic semic onductors can be classified into two categories: 1) migration among same spec ies of molecules, and 2) energy transfer between different species of molecules. Si milar to the transport of charge carriers, exciton migration or diffusion in organi c semiconductors can occur through band transport or via the hopping process. The weak intermolecular interactions also lead to limited exciton mobility in organic semiconductors. Energy transfer between a donor molecule (initial exciton site) and an acceptor molecule (final exciton site) ma y occur via three different rout es: 1) radiative transfer or photon re-absorption, 2) Frs ter energy transfer, and 3) Dexter energy transfer. In radiative transfer, the photon emitted by recombination of the exciton on the donor
35 molecule is reabsorbed by the acceptor mo lecule. This process may occur at long distances, typically more than 100 An over lap in the emission spectrum of the donor molecule and the absorption spectrum of the acceptor mole cule is required. Frster energy transfer also depends upon such a spectral overlap, however, no photon is actually emitted. Rather, the dipole-dipole interaction between the donor and acceptor molecules induces a resonant transition of the donor molecule to the ground state and the acceptor molecule to the excited state. The distance over which Frster energy transfer occurs may be up to 100 shorte r than that for the photon re-absorption process due to the strong distance dependence of the dipole-dipole interaction. Besides radiative and nonradiative reco mbination, Dexter energy transfer occurring as a result of an electron exchan ge mechanism. It requires an overlap of the wave functions of the energy donor and t he energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. 1.4 Summary In this chapter, basics of organic semi conductor and their properties, advantages and disadv antages were covered. Also some of the major optoelectronic devices based on this class of materials were highlighted. The underlying optical and electrical processes affecting the performa nce of these devices were also discussed. However as the thesis title suggests this work is focused on understanding the device physics of OLEDs and the next chapter will cover some of the basics important for understanding organic light emitting diodes. We will highlight some of t he specific applications of OLEDs and also shed light on some of the common standard terminology and definitions commonly used in relation to OLED s. Chapter 2 will also cover a review of history of OLEDs leading to current state of the art devices. We will also discuss how
36 blue OLEDs are an important link in the comm ercialization of OLEDs and what are the factors limiting the performance of these devices. Both chapter 1 and 2 together laid a foundation for rest of this dissertation which focuses on how to optimize all these factors affecting device performance in order to maximize the device performance.
37 Figure 1-1. Formation of and bonds with sp2 hybridization. (a) Schematic representation of sp2 hybridization with and (b) Schematic showing interactions of energy levels in sp2 hybridization. (Reproduced from http://www.orgworld.de/ ) Figure 1-2. Schematic representation of sp2 hybridization leading to formation of delocalized bond which further paves the way for hopping transport in organic semiconductors as illustrated. a) b)
38 Figure 1-3. Atomic orbitals (AOs) (s, p) co mbine to form molecular orbitals (MOs) ( ). Figure 1-4. Molecular structures of some of commonly used small molecules for organic electronic devices
39 Figure 1-5. Structures of some of the polymers widely used in solution processed organic optoelectronic devices. Figure 1-6. CIE coordinates of phosphorescent cyclomated platinum complexes with slight alteration in molecular structure.
40 Figure 1-7. Schematic showing a typical OLED device stack with various layers. Figure 1-8. Sonys 11 OLED television. Reproduced from http://www.theage.com.au/news/articles/s ony-to-ship-first-oled-ultrathintv/2007/10/02/119109108271 0.html .
41 Figure 1-9. A picture of Samsung bendab le OLED display. (Reproduced from http://www.siftwire.com/samsungcreate-a-bendab le-oled-screen.html ). Figure 1-10. Schematic showing two prevalen t device architectures used for solar cells a) planar heterojunction photovoltaic cell and b) bulk heterojunction photovoltaic cell.
42 Figure 1-11. Plots showing comparison of device performance for typical planar and bulk heterojunction photovoltaic cell. (a) Photo J-V characteristics and (b) IPCE of organic PV cells with planar heterojunction and bulk heterojunction devices fabricated with CuPc and AlPcCl (PHJ:planar heterojunction, BHJ: bulk heterojunction) (reproduced with permission from D. Y. Kim, F. So, and Y. Gao, "Aluminum phthalocyanine ch loride/C60 organic photovoltaic cells with high open-circuit voltages," Sol. Energy Mater. Sol. Cells vol. 93, pp. 1688-1691, 2009 ).
43 Figure 1-12. Schematic showing the steps involved in fluorescence. (Adapted from http://www.bris.ac.uk/synaptic/i nfo /imaging/imaging_1.htm ) Figure 1-13. Example of absor ption and emission spectra for an organic material (PFO) showing florescence. Red curve depi cts absorption spectra and blue curve depicts emission spectra. (Adapted from http://www.adsdyes.com/products/ pdf/homopolymers/ADS129 BE_DATA.pdf )
44 Figure 1-14. Schematic sho wing steps involved in phosphorescence. (Adapted from http://www.umich.edu/~protein/AP/rtp.html ) Figure 1-15. Jablonski energy diagram su mmarizing the processes involved in fluorescence and phosphorescence. The respective time scales of all the processes are also indicated. (Adapted from http://www.olympusmicro.com/primer /java/jablonsk i/jabintro/index.html )
45 Figure 1-16. Schematic depicting t he energy diagram for hopping transport. Figure 1-17. Schematic showing various types of excitons in materials (Adapted from V. Bulovic, Lecture notes on Organic Electronics ).
46 CHAPTER 2 ORGANIC LIGHT EMITTING DIODES 2.1 Need for OLEDs 2.1.1 Display Applications As we pointed out in the introduc tory c hapter, organic light emitting diodes is the most mature technology based on organic se miconductors. Various research efforts over the last of couple of decades in both industry and academia have paved the way for the implementation of efficient red, green and blue OLEDs in passive and active matrix displays. Low info rmation content OLED displays fabricated by various manufacturers such as Philips, Pioneer and Samsung have already been commercialized. Recently Sony launched th e first OLED 11 diagonal TV. There are several key reasons for the interest in OLEDs for displays in terms of viewing angle, flexibility, weight, thickness, microsecond response times, and efficiency as we briefly outlined before. The average OL ED thickness is about 100 nm, so display thickness and weight are limited by the substrates. Prototype flexible displays have been already demonstrated by Samsung, CDT and various other manufacturers. Also the response times of OLEDs for both fluor escence and phosphorescence materials are well within the range of video applications. Finally, the most significant advantage of OLED s lies in the fact that its possible to achieve very high internal quantum effici ency (close to 100%) by using organic phosphorescent materials. In contrast, the ef ficiency of an LCD is severely limited by the use of color filters for f iltering the white back light to get red, green, blue components and hence more than half of the avai lable optical power is lost.
47 2.1.2 Lighting Applications Interest in OLED technology to satisf y general white lighting applic ations is increasing. Lighting is one of the major factors in total pr imary energy consumption of electricity. In light of this fact it is ob vious that increasing the efficiency of general lighting will lead to tremendous e nergy savings. This has led to various research efforts being focused on improving the effici ency and lifetime of white OLEDs. OLEDs are attractive for use in energy effi cient lighting as not only they can give very high luminous efficiencies as compared to incandescent and fluorescent lighting but they also use very low operation voltages (<5V). The typical incandescent light bulb operates at about 12-17 lm/W whereas now white OLEDs with close to 100 lm/W efficiency have been demonstrated. Hence, we can see that using OLEDs instead of incandescent bulbs can lead to huge savings in electrical power consumption. Even the compact florescent lamps operate at lower efficiency compared to most white OLED efficiencies being reported these days. 2.2 OLED Lighting: Standard Terms and Definitions 2.2.1 Introduction Before we move forw ard to discuss these OL ED in detail, it is very important to understand the terminology used to describe some of the attributes of visible light which is frequently used in reference to OLEDs. Also we will discuss some of the lighting standards. Towards the end we wi ll discuss OLED device parameters and efficiency measurements for these OLED devices. This information enables one to decipher the plethora of quantitative characte rizations of such devices, and to gain insight into their design and fabrication.
48 2.2.2 Basic Concepts First we will describe some of the terms that are commonly used when discus sing light sources. 184.108.40.206 Luminance flux Luminance flux or power is the perceived powe r of light. It is di fferent from radi ant flux as luminance flux takes into account the varying sensitivity of the human eye to different wavelengths of light (usually refe rred as phototpic response). Radiant flux on the other hand is the measure of total power of light emitted. dVKm)( Here, (luminance flux) is in lumens (lm), is the radiant power in W/nm, is the wavelength in nm, V ( ) is the photopic or scotopic response, and Km is the maximum spectral luminous efficacy, which is 683 lm/W and 1754 lm/W for photopic and scotopic vision, respectively. The SI unit of luminance flux is the lumen (lm). One lumen is defined as the flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. The candela is the SI unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (photopic or scotopic response). Further, the luminance, L, is a photometric measure of the luminous intensity per un it area of light in a given direction. It describes the amount of light that falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). A non-SI term for the same unit is the "nit".
49 220.127.116.11 Lambertian source A lambertian source is a light source whose luminous int ensity in any direction varies as the cosine of the angle between t hat direction and the surface normal of the source. Hence, a lambertian source has sa me luminance regardless of viewing angle. All OLEDs are typically assumed to be lam bertian emitters as the deviations are pretty small. 18.104.22.168 Human eye response Photopic vision is the response of the eye under well-lit conditions. In humans photopic vision allows color perception medi ated by cone cells. The human eye uses three types of cones to sense light in thr ee respective bands of color. The biological pigments of these three diffe rent types of cones have maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (Bluish-Green), 564 nm (YellowishGreen). Their sensitivity ranges overlap to provide vision throughout the visible spectrum. The maximum efficacy is 683 lm/W at a wavelength of 555 nm (green). Apart from cones, the human eye has another class of photodetector cells known as the rods. Rods are extremely sensitive to light and are responsible for scotopic or night vision as they are sensitive in low light conditions (<0.01 cd/m2). The photopic response is active only above luminance le vels of greater than 3 cd/m2. in the intermediate conditions of 0.01 and 3 cd/m2 the eye has a mesopic response and has a mix of scotopic and photopic responsivities. The peak spectral sensitivity of photopic vision is red-shifted by 40 nm from t he peak spectral sensitivity of scotopic vision as shown in Figure 2-1. The photopic spectr al responsivity is used in luminous intensity calculations for all light sources. Figure 2-2 shows the mesopic response which lies somewhere between the photopic and scotopic response.
50 22.214.171.124 Color correlated temperature Color temperature is a characteristic of vi sible light that has important applicat ions with respect to lighting applications. The color temperature of a light source is calculated by comparing its chromaticity with that of an ideal black-body radi ator. The temperature (measured in Kelvin (K)) at which the heated black-body radiator matches the color of the light source is that source's color corre lated temperature. Highe r color temperatures (5000 K or more) denote cool (greenblue) co lors and lower color temperatures (2700 3000 K) refer to warm (yellowred) colors. However, light sources such as fluore scent lamps, LEDs etc emit light by processes other than heating or raising the temperature of a body. Thus the emitted radiation from such sources does not follow a black-body spectrum. Hence, these sources are assigned a correlated color te mperature (CCT). CCT is the color temperature of a black body radiator whic h to human color perception most closely matches the light from the lamp. 126.96.36.199 Color rendering index Color rend ering or color rendition index is a quantitative measure of the ability of a light source to reproduce the colors of various objects as compared to an ideal or natural light source. The color of two li ght sources may appear identical when viewed directly and will therefore have the same CCT. However, the color of the reflected light from an object illuminated by these two sour ces may be significantly different which is measured in terms of CRI. To determine the CRI, the reflection from an object of a light source of a particular correlated color temperature is compared to the reflection from the same object under illumination from a blackbody radiator of the same color temperature. The similarity
51 between the two sources is ranked on a scale of 0 to 100, where a rating of 100 is a perfect match. Sources with a CRI value abov e 80 are considered high quality lighting sources. 188.8.131.52 CIE color coordinates CIE color coordinates are used to describe the chromaticity of any light source. This system was given by Intern ational Commission on Illumination (CIE) in 1931 to describe the color sensation of a light sour ce. The tristimulus values of a color as defined in CIE are the amounts of three primary colors in a three-component additive color model needed to match that test colo r. The tristimulus values CIE 1931 color space are denoted as X, Y, and Z. Two light sources have the same apparent color to an observer when they have the same tristi mulus values, no matter what spectral distributions of light were used to produce them. The CIE has defined a set of three color-matching functions )(),(),( zyx and which are defined as the spectral sensitivity curves of three linear light detectors that yield tristimulus values as X, Y, and Z. The tristimulus values for a color with a spectral power distribution )( I are given in terms of the standard observer by: OdxIX)()( OdyIY)()( OdzIZ)()( where is the wavelength of the equivalent monochromat ic light (measured in nanometers).
52 Since the human eye has three types of co lor sensors, a full plot of all visible colors is a three-dimensional figure. However, the concept of color can be divided into two parts: brightness and chromaticity. The CIE XYZ color space was designed such that the Y parameter was a measure of t he brightness or luminance of a color. The chromaticity of a color was then specified by the two derived paramet ers x and y, two of the three normalized values which are func tions of all three tristimulus values: ZYX X x ZYX Y y yx ZYX Z z 1 The derived color space specified by x, y, and Y is known as the CIE xyY color space and is widely used to specify colors in practice. Figure 2-3 shows the CIE chromaticity diagram. 2.3 Device Parameters and Device Efficiency Measurements Being a relatively mature technology, ther e are already estab lished standards of measurement for OLEDs develop ed in an ad hoc fashion. Ho wever, there are different ways in which the device parameters can be expressed and interpreted. Here we describe the device measuremen t parameter and how they have been used to calculate device efficiency. The discussion will be k ept short as these are based on well established methods in literature. LIV measurements were performed on OLED devices using the LIV setup in our lab as shown in Figure 2-4. The setup c onsisted of a source-meter which sources voltage and measures current a photodiode was used to measure light output, the
53 photodiode was connected to a Pico-ammete r which measured the photocurrent. The photocurrent is then converted to Cd/m2 using already calibrated values. The device area for all the devices in this dissertation is 0.04 cm2. Typically the efficiency of OLEDs is expressed in three diffe rent forms in literature: Current efficiency Power efficiency External quantum efficiency (EQE) 2.3.1 Current Efficiency Luminous efficiency ( L) or current efficiency is defined as the luminance per unit of current flowing through the device. Current efficiency is expressed in candelas per amp. It can be simply calcul ated from the LIV measurements as the ratio of luminance calculated from photocurrent to the current density (current divided by device area) of the device 10*)/( )/(min2 2cmmAJ mcdanceLuL 2.3.2 Power Efficiency Luminous power efficiency or luminosity is the ratio of luminous power emitted in the forward direction to the total electr ical power required to drive the OLED. VI lmosityLuOLED P)(min The lm/W power efficiency takes the Photopic response into account. The wavelength independent, fundamental unit of power effici ency is the wall plug efficiency [W/W] which is defined as the ratio of opt ical power emitted by the device to the electrical power used to drive the device.
54 VI POLED OLED W W/ 2.3.3 External Quantum Efficiency External quantum efficiency is defined as the ratio of number of photons emitted by OLED in the forward viewing direction to the number of electrons injected. While the EQE can also be defined as the total number of photons emitted by the OLE D divided by the number of electrons that definition is not as useful (especially for display devices) and also poses more difficulties in m easurement of device efficiency. As defined above the EQE is simply a ratio and is usually expressed as a percentage by multiplying with 100%. T he current efficiency and EQE are almost equivalent with the exception t hat the current efficiency take s into account the Photopic response of the eye whereas EQE does not. In all the measurements done on devices, in this dissertation, we calculate the efficiency only in the forward viewing directio n. We used a setup similar to described by Forrest et al which is reproduced in Figure 2-5. In this measurement geometry only the photons emitted in the forward direction are measured by the photodiode, hence, this gives us an accurate measurement of the EQE and current efficiency. 2.4 OLEDs: From History to Cu rrent State of the Art OLEDs As discuss ed already OLEDs are the most mature technology based on organic semiconductors, hence, some of their history was already highlighted in section on q I d hc IOLED ext # of photon # of electron
55 organic semiconductors. This section focuse s on the historical development of OLEDs and how OLEDs have evolved over time. 2.4.1 Phosphorescent materials After the initial breakthrough with heterostructure devices significant performance enhancem ent was achieved using the incorporat ion of fluorescent dopants in emitting layers employing the concept of sensit ized luminescence in an organic guest-host system. Use of dopants led to improvisation in device efficiencies as dopants usually have high PL efficiencies. Also, use of dopants led to possibility of making devices with varied color emissions. The next significant step in increasing device efficiencies involved the incorporation of a phosphores cent dopant into the structure. The phosphorescent OLEDs employ a similar devi ce structure as shown in Figure 1-6 wherein the emitting layer will consist of phosphorescent dopant and host system. Light emission in OLEDs depends mainly on two opto-electronic processes: fluorescence and phosphorescence. Phosphoresc ence is different from fluorescence in its origin in forbidden transitions that violat e spin conservation as discussed previously in section 1.2.1. The excitons responsib le for phosphorescence and fluorescence are named as triplets and singlets. According to the quantum mechanics, the spin of hole and electron can be coupled to form four st ates: three triplet states and one singlet. These sub states differ by their relative spin orientations. Acco rding to statistics, all four sub states have equal probability of being occupied. Hence the use of triplet transitions can, in principle, make devices four time s as efficient as the ones just harvesting singlets. However, phosphorescent dopants also required complex device architectures to confines excitons in the EML. This led to the use of blocking layers in devices. Many papers report the use of exciton blocking laye rs to prevent excito ns from diffusing to
56 nearby charge transport layers. Also, usually a number of charge blocking layers such as hole blocking layer and electron blocking layers are used to confine charge carriers in bulk of emitting layer. Hole blocking layers have been around for a long time in OLEDs but the need for electron blocking layers especially in electro-phosphorescent devices has only recently become apparent. 2.4.2 Weak Link in High Efficiency White OLEDs: Blue Since phos phorescent OLEDs came into pi cture, most of the efforts towards fabricating high efficiency white OLEDs use one or more phosphorescent dopants to maximize the efficiency of the device. Wh ite light-emitting OLEDs can be generated by four different approaches as shown in Figure 2-6.. In the first approach a single white emitting stack is used which is usua lly fabricated by doping three monochromatic dopants: red, green and blue in a common hos t. Sometimes a combination of blue and orange dopant is also used to give a whit e emitting layer. Although this device architecture is easy to fabricate, but it does not have good color stability and also there is little room to tune the color without a ffecting device performance. The second approach uses three monochromatic emitting layers stacked on top of each other. The combination of light output from these layers combines to gi ve the final white spectrum for the device. For this device many di fferent host and dopants are involved hence it relies on complex processing methods. Although this device architecture leads to color homogeneity over the active area but it is difficult to control the color over different range of current densities as the recombination zone might move in the device leading to different contributions from the differ ent emitting layers. In the third approach the RGB stack is horizontally patte rned. The main advantage of this approach is that the output spectrum of a horizont al stack can be changed while operating the device when
57 addressing the patterns separately. However, this method relies on expensive printing techniques. For all the above mentioned methods color stability and reliability over long term operation is difficult to be achieved d ue to different lifetime aging rates of the emitters involved. The fourth approach which was used in this project is using a single color emitting (blue) OLED in combinat ion with a down-conver sion layer. The down conversion layer absorbs a fraction of the light emitted from OL ED and re-emits at larger wavelength (red and green). The remitted light from the down conversion layer along with the remaining fraction of light fr om blue OLED constitute a white spectrum. This approach has various advantages su ch as easy fabrication and better color stability as the aging rate is determined by only one emitter. This concept was first demonstrated for fabricating white light-emi tting inorganic devices and is widely used in LEDs. Later on, the same concept was implemented to field of OLEDs by Duggal et al. Hence, we can see that for fabricating high efficiency white PHOLED, we either need three high efficiency monochromatic com ponents: Red, green and blue or at least a high efficiency blue emitting OLED. Very high efficiency in green emitters has been demonstrated in various guest-host combinati ons[24, 71]. One of the most commonly used green emitters is irppy3 which emits at a peak wavelength of 530 nm. Another common green dopant is pq(ir)acac. Very high efficiency has been demonstrated with CBP-ir(pp)y3 and pq(ir)acac-TAZ system reaching cl ose to the theoretical limit of quantum efficiency. However, blue em itting phosphorescent devices, which are essential for achieving high efficiency white OLEDs, were still lagging behind in terms of device efficiency until very recently[72, 73].
58 The focus of this work is to remove this weak link in development of high efficiency white PHOLEDs by fabricating highly effi cient blue phosphorescent OLEDs. We will focus on reviewing current literature in bl ue PHOLEDs in the following section. We will also review briefly factors affecting and limiting the perform ance of these blue phosphorescent devices. We study, examine and optimize each of these factors in the subsequent chapters to get high efficiency in these devices. The end goal of the project was to use these high efficiency PHOLEDs in development of white OLEDs using down conversion phosphors which will be briefly outlined toward the end of the thesis. 2.4.3 Blue Phosphorescent OLEDs Various blue phosphor escent dopants have been used in the literat ure. Figure 2-7 shows some of the most common blue phosphore scent emitters. FIrpic and FIr6 are the most commonly used blue phosphorescent dopant s. FIrpic is a sky blue emitter whereas FIr6 is a deeper blue emitter. This thesis used FIrpic as the emitter in all devices. Hence, from here on we will focus on Firpic only. Firpic was one of the first dopants to be used in blue PHOLEDs. in the firs t report published by Adachi et al 4,4'Bis(carbazol-9-yl)biphenyl (CBP) was used as a host for FIrpic OLEDs. However, with CBP as host endothermic energy transfer fr om guest to host was observed, i.e. there was backwards energy transfer from FIrpic to CBP as the triplet energy of FIrpic (T1= 2.7 eV) is higher than that of CBP (T 1= 2.6 eV) which will be discussed in detail in chapter 3. The external quantum effi ciency obtained with CBP devices (5.7%) was much lower compared to the prevailing e fficiencies for green PHOLEDs. Thereafter, higher triplet energy hosts were introduced for FIrpic OLEDs such as N, N-dicarbazolyl3, 5-benzene (mCP) or 4,4'-Bis(9-carbaz olyl)-2,2'-dimethyl-biphenyl (CDBP) which pushed the efficiency further up to almo st 10%. When we started working on this
59 problem the best reported results we re around 10-12% only whereas green phosphorescent OLEDs were already close to 20% EQE. Hence, it is an important issue to understand and optimize all the factors limiting the performance of these blue PHOLEDs. 2.5 Review of Factors Limi ting Blue OLED Performance The OLED external quantum e fficiency is defined as: phpex ph ext int where ph is the light out-coupling efficiency and int is the internal quantum efficiency of the device. Light extraction efficiency is dependent on the device architecture. Most of the light emitted fr om the OLED is lost in the gl ass substrate, and the ITO and the organic layers due to total internal reflection. Many efforts have been focused on improving the light extraction efficiency of OLED devices by surface roughening, use of micro lenses[79, 80], low index grat ings and microcavity structures. Microcavity structures were used as a light extraction mechanism in this project which will be briefly discussed towards the end of this dissertation. The internal quantum efficiency depends on the following parameters: ex which is the fraction of excitons formed upon absorption of photons (~1 for phosphorescent materials), the charge balance factor and p the efficiency of the radiative decay process. Hence, to maximize the efficiency, all these factors need to be optimized. One of the im portant factors is triplet exciton confinement , and the triplet energy of the host material as well as the nearby charge transport layers should be greater than that of the phosphorescent dopant. Th is confinement prevents backwards energy transfer from the dopant to the host or the transport materials. We
60 will demonstrate in detail, the effect of the triplet energy of the transport layers and the host material on the performance of Iridium (III)bis [(4,6-di-fluorophenyl)pyridinatoN,C2] picolinate (FIrpic) based blue phosphorescent OLEDs, in chapter 3 of this thesis. We also study the effect of different guesthost interactions on the performance of Firpic devices and how does that impact the emissi on mechanism in the device. This effect will be discussed in detail in chapter 4. Another factor which is very important fo r achieving high efficiency is the balance of electrons and holes in the devices[24 ]. Imbalance in charge transport leads to accumulation of carriers at the interfaces th ereby resulting in loss in efficiency and lifetime. Conventionally, this fact or is assumed to be close to one ( 1). However, in most OLED devices charge balance factor is far from its ideal value. Chapter 5 discusses the effect of charge bal ance on device performance in detail. We also study charge balance in devices based on di fferent host materials. Further we point out how we can tweak the charge balance in the device using high mobility or doped charge transport layers. Chapter 6 will focus on the aspect of achieving better charge balance in the devices using ambipolar hos t materials. Chapter 7 uses mixed host approach to get charge balanced high efficien cy devices. By optimization of all the above mentioned factors, we were able to ac hieve high efficiency blue phosphorescent OLEDs with a maximum efficiency of 60 cd/A (about 50 lm/W) which is one of the highest value reported for such devices[86 ]. Chapter 8 presents the conclusion and outlook along with briefly outlining the use of microcavity structures and down conversion phosphors to get hi gh efficiency white devices.
61 Figure 2-1. Photopic and scotopic respons e curves for human eye. (Adapted from http://www.prismalenceuk.com/light_vision ) Figure 2-2. Mesopic response curve for human eye. (Adapted from http://www.prismalenceuk.com/light_vision )
62 Figure 2-3. CIE chromaticity diagram. (Adapt ed from http://www.silicalighting.eu/q-a/qa/the-cie-diagram ) Figure 2-4. A picture of LI V setup used in this study for measurements on OLED devices.
63 Figure 2-5. Experimental geometry used for efficiency measurements and calculations in this dissertation (Reproduced with permissi on from S. R. Forrest, D. D. C. Bradley, and M. E. Thompson, "Measur ing the Efficiency of Organic LightEmitting Devices," Adv. Mater. vol. 15, pp. 1043-1048, 2003 ). Figure 2-6. Different approaches used for producing white light a) single white stack b) RGB layers stacked on top of each other c) patterened RGB pixeld d) downconversion of blue light with yellow phosphors.
64 Figure 2-7. Structure of co mmon blue phosphorescent dopa nts and their peak emission wavelength a) FIrpic (472 nm) ) b) FIr6 (461 nm) c) mer-Ir(Pmb)3 (415 nm). www.lumtec.com.tw [8 9]. a) b) c)
65 CHAPTER 3 EFFECT OF TRIPLET EN ERGY CONFINEMENT ON PERFORM ANCE OF BLUE PHOPHORESCENT OLEDS 3.1 The Problem: Triplet Energy Confinement Most of the small molecule devic es use multilayer heterostructure device architecture. The first OLED devic e consisted of a diamine and Alq3 as emitting layer which also acted as ETL, this laid the f oundation for use of heterostructure devices. As the use of dopants became prevalent[16 ], the device structure was modified to include more layers, each with a specific f unction, such as hole and electron transport layer for providing efficient transport in the device. Emitting layer usually consists of a host material doped with a small concentration of the emitter. The fu nction of the host is to serve as a matrix for the dopant and to pr ovide efficient energy or charge transfer to the emitter. With the dawn of era phosphor escence, the devic e structures became more and more complicated. Phosphoresc ent dopants also required complex device architectures to confines excitons in t he EML. Singlet excitons have short diffusion lengths, on the order of tens to hundreds of Angstroms. Triplet states have much longer lifetimes than their singl et counterparts, allowing them to diffuse >1000 [66, 90]. Thus, it is essential to use device architectu res that will confine the excitons within EML. Triplet exciton confinement in a three layer double heterostructure is possible if the host, HTL and ETL have higher triplet energy than triplet energy of the emitter (as shown in Figure 3-1) but this is not always ac hieved with common OLED materials. In the first phosphorescent PtOEP devices made by Baldo et al the efficiency was about 4.2%. The triplet energi es of PtOEP, NPB and Alq3 are 1.9 eV, 2.3 eV and 2.0 eV, respectively[91, 92]. It is energetically unfavorable, therefore, for excito ns to diffuse into NPB, but the triplet energy of Al q3 being close to that of PtOEP, Alq3 does not block
66 these excitons effectively. Hence Baldo group used BCP as a combined hole and exciton blocker to improve efficiency to 5.6%[67, 90]. When the first phosphorescent blue device using a Firpic:CBP guest host system, they observed loss of efficiency due to backwar ds energy transfer from FIrpic to CBP as triplet energy of CBP is lower than that of FI rpic[67, 74]. Thereafter, higher triplet energy hosts were used like mCP or CDBP whic h show exothermic host-guest energy transfer[75, 76]. Figure 3-2 a) shows the phosphorescence spectra of CBP, mCP and FIrpic reproduced from ref [ 75]. The phosphorescent spectra of the materials which do not phosphoresce at room temp erature, is measur ed at low temperature to obtain their triplet energy levels. The triplet energy of the molecule is calculated from the wavelength at which the onset of phosphorescence takes place. As is clearly evident from Figure 3-2 a) phosphore scence spectra of mCP takes off at a lower wavelength (higher energy) as compared to that of FIrpic and hence it provides good exciton confinement for FIrpic molecules. The e fficiency for two devices made with CBP and mCP as host reproduced from the same ref  are also shown in Figure 3-2 b) to illustrate the effect of tr iplet energy of host clearly. In 2004, Goushi et al published a study s howing triplet exciton confinement by using high triplet energy hole transporting layers such as TAPC, TPD as opposed to commonly used NPB which has lower triplet energy. Figure 3-3 a) shows the phosphorescence spectra for the three hole tr ansporting molecules and Figure 3-3 b) shows the effect of triplet energy of these hole transporting layers on device performance. TAPC has the highest energy at the onset of phosphorescence and the device with TAPC as HTL has the highest efficiency amongst all three.
67 Also recently Tanaka et al[71, 94] have demonstrated very high efficiency phosphorescent devices using high triplet energy transport layers for green PHOLEDs. They have reported device effici encies of 29% EQE at 100 cd/m2 and 26% at 1000Cd/m2, power efficiencies of 133lm/W at 100Cd/m2 and 107lm/W at 1000Cd/m2 (see Figure 3-4 a) & b)). They used high triplet energy materials for both hole and electron transport to confine al l the excitons in the bulk of emitting layer thus achieving one of highest reported efficiency for green phosphorescent OLEDs. This triplet exciton confinement by charge transporting layers has also been demonstrated in polymer PHOLEDs. Specif ically, devices using 1,3-bis[(4-tertbutylphenyl)-1,3,4oxadiazolyl] phenylene (OXD-7, T1=2.7 eV) as an electron transporting layer (ETL) have higher efficien cy than those made with 2,9-dimethyl-4,7diphenyl-1,10phenanthroline (BCP, T1=2.6 eV) as an ETL. Fo r blue PHOLEDs, various high triplet energy host materials have been intr oduced, and it was found that there is a strong correlation between the triplet energy of the host and the device efficiency. For example, there has been work done on the effect of the host triplet energy on the device efficiency using host materials such as 3,5-N,Ndicarbazolebenzene(mCP, T1=2.9 eV) 4,4-bis(9-dicarbazolyl)2,2-dimethyl-biphenyl (CDBP, T1=3.0 eV) and 9-(4tert-butylphenyl)-3,6-bis(triphenyls ilyl) -9H-carbazole (CzSi, T1=3.02 eV). Hence, for designing an efficient phosphorescent device triplet energy of the host and the adjacent transport layers should be greater than that of the emitte r to prevent any backwards energy transfer processes. In the next sections we will show the effect of triplet energy of charge transport laye rs (hole and electron) and host on device performance of blue PHOLED.
68 3.2 Triplet Exciton Confinement w ith Charge Transport Layers In our present study, we have fabricat ed blue phosphorescent devices based on iridium(III) bis[(4,6-difluor ophenyl)-pyridinato-N,C2 ] picolinate (FIrpic) with different hole transporting materials as well as elec tron transporting materials. Using TAPC as the HTL, we achieved a maximum current efficiency of L= (19.8) cd/A, external quantum efficiency ( EQE) of (10.1.5)%, and power efficiency P of (11.6) lm/W, which is approximately 35% higher than that in previous ly reported devices with -NPD as the HTL (maximum EQE=7.5.8%). We attribute this impr ovement to the enhanced electron and exciton confinement in the dev ices as TAPC possesses a lower lowest unoccupied molecular orbital energy (LUMO) and higher triplet energy than those in NPD. On the contrary, the device efficiency wa s not very sensitive to the triplet energies of the hole blocking layers (HBLs) and the resu lts suggest that the el ectron transporting properties of the HBL also play an important role in determining the device efficiency. 3.2.1 Hole Transport Layer The device structures used in this study ar e shown in Fig. 3-5. mCP was selected as a host because of its triplet energy and its high photoluminescence efficiency when it is doped with FIrpic. All devices were fabricated on glass substrates precoated with indium tin oxide (ITO) transparent conducting electrode with a sheet resistance of 20 / sq. Substrates were first cleaned in acetone and isopropanol, and then cleaned by exposure for 15 min to an ultraviolet-ozone ambient. A 20-nm-thick film of poly(3,4ethylenedioxythiophene)polystyrenesulfonic acid (PEDOT:PSS) hole injection layer as spin coated over ITO substrate and baked at 180 C for 15 min. To complete the device fabrication, a 20-nm-thick HTL (NPD, TPD, or TAPC), a 20-nm-thick 3% FIrpic doped mCP emitting layer, a 20-nmthick HBL (BCP ), a 20-nm-thick tris(8-quinolinolato)
69 aluminum (Alq3) electron injection layer, and a lithi um fluoride/ aluminum cathode were deposited sequentially without breaking the vacuum (~1X10 6 Torr).The active area of the device is 2X2 mm2. Devices were encapsulated in nitrogen ambient immediately after deposition using a glass cover slide sealed to the substrate with a UV-curable adhesive. The device efficiencies with different HTLs are shown in Fig. 3-6. The maximum current efficiencies of devices with different HTLs are 14. 3, 17.5, and 19.8 cd/A for NPD, TPD, and TAPC, respectively. The triple t energies of the HTLs used in this study are 2.29, 2.34, and 2.87 eV for NPD, TPD, and TAPC, respectively. Figure 3-6 also shows the triplet energies of different HTLs used in the present experiments along with the triplet energies of Firpic and mCP. Our resu lts indicate that TAPC confines the triplet excitons within the emitting layer most effectively. In devices with TPD and NPD, exciton energy readily transfers from FIrpic to the HTL, resulting in lower device efficiency. It should be noted that current efficiency of TPD dev ice is slightly higher than that of the NPD device, even thoughT1 of TPD is almost similar to T1 of NPD ( T1 =0.05 eV). As outlined in the previous section Gous hi et al also found similar results in their green phosphorescent devices where they observed a significant difference in efficiency despite the small difference in the triplet energies between TPD and NPD. Given that the triplet energies of both TPD and NPD are lower than that of FIrpic, the recombination zone should be kept away fr om the HTL interface. Since TPD has a higher hole mobility ( TPD=~1X10 3 cm2 /V s) than NPD (NPD=~5X10 4 cm2/Vs), it is expected that the TPD device should have an emission zone further away from the HTL
70 interface compared with the NPD device, resu lting in higher efficiency in the TPD device. 3.2.2 Electron Transport Layer An analogous study was done wit h elec tron transport materials for blue phosphorescent OLEDs. The device structure used was similar to that used for HTL study. TAPC was used as the hole transport layer for these devices and three different hole blocking layers: BCP, BPhen and OXD-7. The device structure is shown in Figure 3-7. The current and power efficiencies of several devices with different ETLs are shown in Figure 3-8. The three electron tr ansporting materials studied here are BCP (T1=2.6 eV), BPhen (T1=2.5 eV), and OXD-7 (T1=2.7 eV). OXD-7 has the highest triplet energy but its device e fficiency (21.7 cd/A) is only slig htly higher than that of the other two devices with BCP (19.8 cd/A) and BPhen (21.1 cd/A). In cont rast to the HTL, there is no clear trend in device efficiency in correlation between the triplet energy of the ETL. To understand how ETLs affect PHOLEDs efficiency, current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the above devices with different ETLs were measured and the results are shown in Figure 3-9. Current density and luminance of BPhen device are higher than those of OXD-7 and BCP devices due to higher electron mobility of BPhen (5.2X10 4 cm2/Vs at ~105 V/cm) than that of BCP (5.5X10 6 cm2/Vs at 105 V/cm ) and OXD-7. Current -voltage characteristics of devices with BCP and OXD-7 in Figure 3-8 b) i ndicate that electron mobilities of the two materials are similar. Compared to ETL, the hole mobility of TAPC (1.0X10 2 cm2/Vs at 105 V/cm ) is two to three orders of magnitude higher t han the electron mobilities of the HBLs used
71 in this study. Thus, the devices with BC P and OXD-7 as the HBL are largely hole dominant. Since charge balance also plays an important role in determining the device efficiency, the triplet energy of the ETL is not the only determining factor affecting the device efficiency. Among all ETLs studied her e, BPhen has the highest electron mobility and provides better charge balance; therefore, the device efficiency is not significantly lower than those of the other two devices even though it has lower triplet energy. Based on these results, both high trip let energy and electron mobility are important to achieve high device efficiency. We have investigated the effects of triplet energy and charge transporting properties of electron and hole trans porting materials on FIrpic-doped blue phosphorescent devices. We found that the device efficiency is a strong function of the hole transporting materials used. The device with TAPC HTL shows the highest efficiency due to the high triplet energy. While the triplet energies of TPD and NPD are similar, the TPD device gives higher efficiency compared with the NPD device due to the higher hole mobility in TPD. On the other hand, the device efficiency does not appear to be affected by the triplet energy of the ETL, indicating that the electron mobility of the ETLs plays a dominant ro le in determining the device efficiency. 3.3 Host-Dopant Effect As discuss ed in the previous section, the triplet energy of the host and the charge transport materials[83, 93] used in PHOLEDs, is one of the most important factors determining the performance of PHOLED s. Initially, 4,4'-bis(carbazol-9-yl) biphenyl (CBP) was used as a host for FIrpic OLEDs. With CBP as the host endothermic energy transfer from the guest to the host was observed, as the triplet energy of FIrpic (T1= 2.7 eV) is higher than that of CBP (T1= 2.6 eV).
72 Thereafter, higher triplet energy host materi als such as mCP with triplet energy of 2.9 eV, 4,4'-bis(9-carbazolyl)-2,2'-dimethyl -biphenyl (CDBP) wit h triplet energy of 3.0 eV, and p-bis(triphenylsilyly)benzene (UGH 2) with a triplet ener gy of 3.5 eV were introduced. Figure 3-9 shows the structure of different host materials used in this study and Figure 3-10 shows the device structur e which was similar to the one used for the hole transport layer study. In this study, all four hosts were used to fabricate FIrpic devic es and Figure 3-11 shows the efficiency plots for different host materials. The lowest device efficiency is seen for the devices with CBP host (14.5 cd/A ) which is consistent with the fact that these devices show endothermic energy transfer from CBP to FIrpic. The peak device efficiencies for mCP (20.7 cd /A) and CDBP (19.9 cd/A) host materials are very similar and these results are expected as these mate rials have almost equal triplet energies. The efficiencies for these devic es differ in roll-off which is probably due to the difference in their charge transporting properties. The efficiency was found to be the highest for UGH2 host (31.7 cd/A) which has the highest triplet energy as well as the highest energy band gap which is most efficient in exciton confinement. Even though the efficiency was found to be highest for devices with UGH2 host, this cant be completely attributed to its tr iplet energy. We will shed some more light on this in the next chapter. UGH2 has a diffe rent transport and emission mechanism as will be clear from the next section. H ence, it is unfair to directly compare the triplet energies of these materials without pointing the difference in mechanism of guest-host interactions.
73 3.4 Summary We systematically studied the effect of tr iplet exciton confinement on performance of FIrpic based blue PHOLEDs. In case of ho le transport materials, the device efficiency is signific antly affected by the triplet energy of hole transport materials. However, in case of electron transport materials, the device performance was not strongly correlated to the triplet energy of t he electron transport molecule and both triplet energy and mobility had an impact on device performance. All the electron transport materials used upto this point had lower triple t energy than that of FIrpic hence, a clear understanding of the effect of triplet exci ton confinement with ETL was not achieved. Later on, in this thesis we introduce a high triplet energy ETL material which helped us not only to clearly illustrate the effect of triplet energy of ETL, but also helped us in isolating the effect of mobility and triplet energy of tr ansport layer on device performance. In the study involving the host materials, it was found that not only the triplet energy of the host material but also the interaction betwe en the host matrix and the guest molecules significantly impacts the device characteristics.
74 Figure 3-1. Studying triplet exciton confi nement a) demonstrating exciton loss pathways if the system has poor exciton confinement b) if triplet energy of host and transport materials is higher than that of emitter loss is minimized. Figure 3-2. Illustrating the effect of host tr iplet energy a) Phosphorescence spectra of host materials CBP and mcP compared to that of FIrpic illustrating the difference in the triplet energy of these materials. The triplet energy is measured as the onset of phosphorescenc e. B) Device efficiency with both CBP and mCP illustrating the effect on device performance.(Reproduced with permission from R. J. Holmes, S. R. Forre st, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon, and M. E. Thompson, "Blue organic electrophosphorescence using exothe rmic host--guest energy transfer," Appl. Phys. Lett. vol. 82, pp. 2422-2424, 2003) a ) b )
75 Figure 3-3. Illustrating the effect of triplet energy of HTL on device performance; a) Phosphorescence spectra of HTL mate rials NPD, TPD and TAPC, b) device efficiency for three different HTL ma terials used in green phosphorescent materials. (Reproduced with permission fr om K. Goushi, R. Kwong, J. J. Brown, H. Sasabe, and C. Adachi, "Triplet exciton confinement and unconfinement by adjacent hole-transport layers," J. Appl. Phys. vol. 95, pp. 7798-7802, 2004 )
76 Figure 3-4. Illustrating the effect of triplet energy of ETL. a) Phos phorescence spectra of B3PYMPM (ETL) compared with the spectra of Ir(ppy)3. b) Power efficiency luminance and current efficiencyluminance characteristics for green PHOLED (Reproduced from ref D. T anaka, Y. Agata, T. Takeda, S. Watanabe, and J. Kido, "High luminous efficiency blue organic light-emitting devices using high triplet excited energy materials," Japanese Journal of Applied Physics Part 2-Letters & Express Letters, vol. 46, pp. L117-L119, 2007 )
77 Figure 3-5. Device structure of blue PHOLEDs for studying effect of triplet energy confinement from HTL Figure 3-6. Current efficiency and power efficiency of blue PHOLEDs by changing HTLs.
78 Figure 3-7. Device structure used for studying effect of different electron transport materials (ETMs). Figure 3-8. Comparing different ETLs fo r blue PHOLEDs. a) Current efficiency comparison of blue PHOLEDs with BCP, BPhen, and OXD-7 as a HBL. b) Luminance-current-voltage graph (L-I-V) of blue PHOLEDS with BCP, BPhen, and OXD-7 as a HBL. a) b)
79 Figure 3-9. Device structure used for comparing different hos t materials and to study the guest-host interactions Figure 3-10. Device structure used for compar ing different host materials and to study the guest-host interactions
80 Figure 3-11. Current efficiency of devices fabricated with different host materials.
81 Table 3-1. Table listing the tr iplet energies of various host materials used in this study and their corresponding efficiency comparis on at the same current density of 1.5 mA/cm2. Host Host T1(eV) EQE (%) @1.5 mA/cm 2 CBP 2.5 5.7 mCP 2.9 7.5 CDBP 3.0 10.4 UGH2 3.2 15.0 Current density of 1.5 mA/cm2 was chosen as the peak of all the devices with different host materials coincided at that point.
82 CHAPTER 4 EFFECT OF GUEST-HOST INERACTIONS 4.1 FIrpic Doping Concentrati on in Different Host Systems In most devices, luminescence of phosphorescent dopants is due to Frster energy transfer and high photoluminescenc e quantum efficiencies are achieved with low dopant concentration. For example, bis[(4,6-di-fluorophenyl)-pyridinateN,C2`]picolinate (FIrpic) doped N, N-dicarbazolyl-3, 5-benzen e (mCP) films have very high photoluminescence quantum efficiency at low doping concentration (as shown in Figure 4-1) and the efficiency decreases with increasing doping concentration. Figure 4-1 (reproduced with permission from  ) shows the photolum inescence quantum yield of three common phosphor escent RGB dopants: Ir(ppy)3, Btp2Ir(acac) and FIrpic in CBP host. For FIrpic PL quantum yield measurement in mCP host is also shown to illustrate the effect of triplet energy of host molecule. From th is plot we can see that for all these dopants, the PL efficiency peaks at low doping concentrations and at high doping concentration quenching mechanisms (tripl et-triplet quenching) come into play. FIrpic films maintain still a high enough quantum efficiency of 16% in neat films as the fluorination on the ppy ligand hinders self -quenching interactions. The peak efficiency for Firpic in mCP hosts is seen at low concentration of 1-2%. In previous reports on the mCP-FIrpic host-dopant system low doping concentration (2 wt%) of FIrpic was used to achieve a maximum current efficiency of 20 cd/A[75, 102]. However, in the wide band gap UGH2 host based devices reported in the previous chapter we used 10% FIrpic in the EML and still got higher efficiency as compared to mCP host. Therefore, in order to understand the diffe rences between the mCP and UGH2 host
83 systems, we investigate the effect of FIrp ic doping concentration on charge transport as well as carrier recombination in these two host systems. 4.2 Investigating Guest-Host Interactions Both mCP and UGH2 were used as a host in our present study. Figure 4-2 shows the device structure used in this study along with the energy level alignments for different layers. To understand the effect of doping concentration in both mCP and UGH2, OLEDs were fabricated with differ ent FIrpic doping concentrations. We analyze the effect of doping concentrations on t he current density and efficiency of these devices. OLED devic es were fabricated with FIrpic doping concentrations of 2-3%, 5%, 10% and 20%. LIV measurements were per formed on these devices as described earlier. 4.2.1 mCP Host Figure 4-3 shows the current density-voltage (J-V) characteristics for mCP devices with different FIrpic concentrations. The cu rrent density of the devices with different doping concentrations does not change very si gnificantly. The current density shows slight increase but does not increase signific antly with doping concentration of FIrpic. It is evident from the J-V characteristics that there is no strong correlation between the device current density and the FIrpic doping c oncentration, and the presence of FIrpic does not appear to affect the carrier transport significantly. Thus we conclude that the carrier transport is through the mCP host mole cules and the FIrpic emission occurs via Frster energy transfer from the host to the dopant. 4.2.2 UGH2 Host Figure 4-4 shows the J-V characteristics for UGH2 devices with varying FIrpic concentrations. We first note that the curr ent density of the UGH2 devices with low
84 FIrpic concentration is at least two to thr ee orders of magnitude lo wer than that in the corresponding mCP devices. Furthermore, in contrast with the mCP devices, the current density of the UGH2 devices is a strong function of the dopant concentration, and the current density increases by more than three orders of magni tude as the dopant concentration increases from 2% to 20%. Since UGH2 is a very wide gap material[101, 103], it mostly behaves as an insulator with low conductivity and carrier injection into UGH2 is difficult. With the presence of t he dopant molecules, c harge carriers transport via hoping between FIrpic molecules, resulting in a substantial enhancement in carrier transport in the emitting layer. Since the UGH2 molecules are not participating in carrier transport, the dopant molecules probably ac t as carrier trapping and radiative recombination centers. This point will be further discussed in the section below. 4.2.3 Effect on Device Efficiency To further understand the underlying devic e physics in the two different host systems, we study the dependence of device efficiency as a function of doping concentration. Figure 4-5 shows the peak curr ent efficiency as a function of FIrpic concentration for both mCP and UGH2 devices In previous reports, the maximum electroluminescence efficiency in the FI rpic-mCP dopant-host system[75, 104] was achieved at low doping concentration, and the efficiency decreased with increasing doping concentration due to quenching. Our dev ice efficiency data for mCP devices are in agreement with the published reports that a maximum current efficiency of ~21 cd/A is reached at 2 wt% doping concentration and the efficiency falls sharply with increasing FIrpic concentration. In cont rast, the UGH2 host devices show a very different trend. Here, the current efficiency is 20 cd/A at 2 wt% concentration, similar to that of the FIrpic doped mCP devices, and increases with increasing FIrpic concentration. It
85 reaches a maximum value of 32 cd/A at 10 wt % and decreases slightly to 30 cd/A at a very high concentration of 20 wt%. Our dat a indicate that the increase of device efficiency is coming from two factors. Fi rst, the dopant molecules enhance the charge transport as well as charge balance resultin g in higher efficiency with increasing doping concentration. Second, the dopant molecules also act as carrier trapping centers as well as exciton recombination centers, and hi gher doping concentration leads to more efficient carrier trapping and radiative recombi nation resulting in higher efficiency. It should also be noted that the excitons are confined by the insu lating host effectively and the quenching mechanisms do not come into pl ay until the concentration reaches 20%. Hence, high efficiency can be achieved at very high concentration in a wide band gap host. Based on the data shown in Figure 4-5, we established that 2 wt% doping is the optimum concentration for the mCP devices while 10 wt% FIrpic doping is the optimum concentration for the UGH2 devices in the current device architecture. 4.2.4 Photoluminescence Quantum Yield Measurements (PLQY) As we hav e learnt already that the guest hos t interactions in two different systems mCP and UGH2 lead to very different device e fficiency trends in EL device efficiency. In this section we compare the PL effi ciencies of the two systems with varying concentration of the dopant, in order to understand the underlying reason for this difference. If the PL efficiency measurements results differ fr om the EL efficiency trends, this difference in trends might be arising from the difference in transport interactions. However, if they agree closely, the differ ence between the two systems might be mainly because of the difference in exciton and energy transfer in the two hosts. For PLQY measurements we fabricated 100 nm thick films of the respective hosts with varying dopant concentrations. The me asurements were done similar to those
86 reported for mCP in ref . The system was also calibrated as described by Kawamura et al by using a standard light source and a neat film of tr is(8-quinolinolato) aluminum(III) complex as a standard fluor escent emitter which showed a PLQY of 20%. Figure 4-6 shows the photoluminescence e fficiency trends for FIrpic doped UGH2 thin films. Films were fabricated with 5% 10%, 20%, 30% and 40% of FIrpic loading in UGH2. The trend seen in PL efficiency of these UG H2 films is quite different from that of mCP. The PL efficiency of these films peaks at about 30% whereas for mCP films at such high concentration the PL efficiency h ad already gone down quite a bit. Hence, we can conclude that the differenc e that we see in the two host systems is mainly because of different route for excit on and energy transfer as we see similar trends in EL and PL efficiency. However, in EL devices the maximum efficiency for UGH2 host systems was found to be at 10% whereas in PL measurement the highest efficiency was obtained for 30% doped film which indicates that transpor t in the two systems might also be playing a significant role in determining the opt imum concentration and quenching mechanisms in the device. Infact, we show later on in the dissertation that t he optimum EL device efficiency moves to higher doping concentra tions as the transport in the device is improved. 4.3 Summary In summary, we have studied the effects of different guest-host interactions on the efficiency of FIrpic based PHOLEDs and found t hat the host materi al plays an important role in determining the device current and ef ficiency. The carrier transport in mCP devices is independent of the FIrpic conc entration, and the host is responsible for carrier transport and emission occurs via Frster energy transfer from the host to guest
87 molecules. In contrast, the carrier transport in UGH2 devices is a strong function of the FIrpic concentration, and the dopant molecules are responsible for both carrier transport and radiative recombination. Higher doping concentration leads to enhanced carrier transport and higher lumi nescence efficiency.
88 Figure 4-1. Photoluminescence efficiency for thin films of phosphorescent dopants a) CBP:Ir(ppy)3 b) CBP:Btp2(Ir)acac and c) CBP:FIrpic (solid symbols) and mCP:FIrpic (hollow symbols). (Reproduced with permission from Y. Kawamura, K. Goushi, J. Brooks, and J. J. Brown, "100% phosphorescence quantum efficiency of Ir(III) comple xes in organic semiconductor films," Appl. Phys. Lett. vol. 86, pp. 071104, 2005 ). Figure 4-2. Device structure used for investigating guest-host interactions.
89 Figure 4-3. J-V characteristics for devic es with mCP as host and different doping concentration of FIrpic molecules Figure 4-4. J-V characteristics for devices with UGH2 as host and different doping concentration of FIrpic molecules
90 Figure 4-5. Concentration dependence of dev ice efficiency for devices with mCP and UGH2 as host (lines are used only as a guide to the eye). 051015202530354045 40 45 50 55 60 65 70 75 80 85 FIrpic QY(%)FIrpic concentration Figure 4-6. PL efficiency measurem ent for FIrpic doped UGH2 films.
91 CHAPTER 5 IMPORTANCE OF CHARGE BALANCE IN BLUE PHOSPHORESCENT OLEDS Charge balance is a very important factor in the performance of an OLED. It has already been establis hed that internal quantum e fficiency of the device is directly related to the balance of electrons and holes in the device. For maximizing the efficiency of blue PHOLEDs we need to confine the recombination zone in t he bulk of the EML. Charge imbalance causes the recombination z one to move to the interfaces. Commonly used transport materials such NPD (T1=2.29 eV), BCP (T1=2.5 eV) and Alq3 (T1= 2.0 eV) have lower triplet energy than that of FIrpic (T1=2.7 eV). Hence, if the recombination zone is at the interf ace then the excitons can be quenched by lower triplet energy charge transporting layer as di scussed in chapter 3. Also, charge balance strongly affects the efficiency roll-off in a phosphorescent device. Recombination at the interface leads to increased triplet quenc hing and loss of efficiency at high current densities and roll-off in efficiency. 5.1 Investigating Charge Balance In this chapter we investigate charge bal anc e in FIrpic based OLEDs by fabricating single carrier devices. Figure 5-1 a) shows the structure of our cont rol OLED device and 5-1 b) and c) shows the device structur e for electron-only and hole-only devices respectively. The hole mobility of TAPC (~ 1.0X10-2 cm2/Vs) is four orders of magnitude higher than the electron mobility of BCP (5.5X10-6 cm2/Vs). Hence, it is expected that in our control mCP device shown in Fi gure 5-1 a), the recombination zone will be located at the interface of EML and ETL. Figure 5-2 shows the current density vs. voltage plot for the single carrier (hole and electron only) devices. Fr om this IV data it
92 can be clearly seen that the hole current dens ity is orders of magnitude higher than the electron current density, indicating t hat the device is very hole dominant. As the devices are found to be deficient in electron transport, it is expected that the recombination zone of the device is lo cated at the EM L/ETL interface. In the next section, we probe the recombination zone in t hese devices to verify that our hypothesis about the location of recombination is indeed correct. 5.2 Probing the Location of Recombination Zone To verify the location of recomb ination zone and charge balance, two modifications of our control dual carrier device were fabr icated which have dif ferent location of FIrpic doping in the emitting layer. The left doped device (doped at the HTL/EML interface) as shown in Figur e 5-3 a) has the following structure: ITO/PEDOT/TAPC/mCP:FIrpic (10 nm) /mCP (10 nm)/BCP/LiF/Al and the right doped (doped at the HTL/EML interface) device as shown in Figure 5-3 b) has the following structure: ITO/PEDOT/TAPC/mCP (10 nm)/mCP: FIrpic (10 nm) /BCP/LiF/Al. Since in the mCP host based FIrpic devices, the ch arge carriers are trans ported through the host, the location of doping should not a ffect the carrier transport. Measurements were done on all these different sets of dev ices and their current-luminescence-voltage (LIV) and efficiency data was studied to probe the location of recombination zone in these devices. Figure 5-4 compares the device characteri stics for the control device and the two devices shown in figure 5-3. All three devices show almost identical JV characteristics as shown in Figure 5-4 a) as expected. This ve rifies the fact that the location of dopant in the emitting layer is not affecting the charge transport in the device. However, the luminance of the three devices does not fo llow the same trend. Luminance in the device
93 doped on the surface of HTL/ETL (left doped) devic e is lower by more than an order of magnitude. This shows that in case of left doped device, exciton formation and/or radiative decay is lower than the other two devices. This c an be explained by our earlier hypothesis that the recombination zone is loca ted at the EML/ETL in terface. Because of the difference in charge carrier mobilities in the two transport layers, the electrons might lag behind even further to r each the dopant to cause exciton formation and radiative recombination in case of the left doped devic e. Hence, this leads to the huge difference in efficiency between the right-doped device and the other two. As the dopant is located at the TAPC/mCP interface, fewer electrons are able to reach the doped layer resulting in lower device efficiency as shown in Figur e 5-4 b). One the other hand, the other two devices have much higher efficiency compar ed to the left-doped device, indicating that the recombination zone is lo cated on mCP/BCP interface. 5.3 Importance of Tuning Charge Balance for Blue PHOLEDs From the investigation of charge balance and probing the recombination zone in these FIrpic based blue PHOLEDs, we demonstr ated that these devices are largely hole dominant and the recombination z one in t hese devices is indeed located at the EML/ETL interface as is illustrated in Figur e 5-5 a). However, considering the triplet exciton confinement in this device as shown in Figure 5-5 b), we can expect that this charge imbalance affects the device performance in this case even more severely as the triplet energy of BCP is lower than that of FIrpic unlike TAPC. Hence, if the majority of recombination occurs at EML/BCP interface, there is high probability of these triplet excitons being quenched by the lower triplet energy BCP layer. Hence, the issue of charge imbalance added to the problem of triplet exciton confinement makes this issue
94 severe. To summarize the two main issues affecting the efficiency of FIrpic based blue PHOLEDs: Poor charge balance mainly due to lower electron mobility of electron transport layer as compared to hole m obility of hole transport layer Poor triplet exciton confinement at EML/ETL interface due to lower triplet energy of ETL. To overcome these two issues we firs t and foremost need to use a high triplet energy electron transport material to minimize the loss of triple t excitons from dopant to neighboring charge transport layers. The trip let excitons in phosphorescent devices tend to have long diffusion lengths, even if the charge balance is significantly improved to push the recombination zone away from t he EML/ETL interface, there might still be possibility of quenching of excitons from ET L. However, charge balance is also an equally important issue which we will clearly dem onstrate at the end of this chapter and there are different ways to tune the charge balance in the device some of which will be discussed in following chapters. In this c hapter we will present two approaches to tune the charge transport and balance in OLED devic es: 1) Using doped transport layers and 2) using high mobility elec tron transport materials. 5.4 Tuning Charge Balance with Doped Transport Layers Since the triplet energy is a property of a material and there are few high triplet energy ET L materials available commercially, here we focus on improving transport in the device by doping. We used n-doped and p-doped transport layers to study how the charge balance and device performance is affe cted by these changes in transport. For the p-doped layer we used MeO-TPD doped with 2 mol%F4TCNQ. BPhen doped with 1:1 Lithium was used as n-doped layer. To l ook at the effect of these doped transport layers we fabricated three sets of devices: 1) control device 2) p-i-n device and 3) n-
95 doped device. The device structures for pi-n and n-doped device are shown in Figure 5-6. LIV characteristics were measured for all the three sets of OLED devices. Figure 5-7 shows the current density efficiency char acteristics for these devices. The p-i-n device shows higher efficiency as compared to intrinsic device indicating that improving the transport in the device does improv e the device performance as the current efficiency increases to about 35cd/A from 27 cd/A. however, as our control device was hole dominant to star t with, even though the p-doped layer might help to push down the operation voltage but it might wo rsen the charge balance in the device. To illustrate this fact we fabricated an n-doped device and this device has the highest peak efficiency of all three which agrees with the fact that electron transport is lacking in the device and achieving better charge balance is the key to high efficiency. The n-doped device show 39 cd/A current efficiency which shows a 30% enhancement over the UGH2 control device. However, the peak for the n-doped devic es has shifted to lower current density which is probably due to dynamic nature of charge balance. Charge balance using doped transport layers was fully optimized by using different alkali metal as dopants with our collaborators[108, 109] and is not a part of this di ssertation. Even with this minimal optimization with doped charge transport layers we achieved close to 20% EQE for FIrpic based blue PHOLEDs which is a significant enhancement from the modest 1012% of EQE of other published de vices in the literature. Since in devices with UGH2 host the transport is occurring through the dopant in the emitting layer, it can be argued that when we increase the transport in the OLED device we might need to use a higher FIrpic loading so that transport through EML does not limit the device performanc e. Based on this argument we fabricated another p-i-n
96 device with higher doping concentration of FIrp ic (15%). The efficiency comparison for 10% and 15% doped devices is shown in figure 5-8. The device with 15% FIrpic doping shows higher peak efficiency but again the rolloff for this device is steeper which might be due to triplet-triplet quenching effects. Henc e, we show that we can tune the charge balance in the device by using doped charge tr ansport layers. However, the lower triplet energy of the electron transport layer still remains an issue which will be discussed in the next section where we use a high mobility high triplet energy electron transport layer to achieve even better device performance for these FIrpic devices. 5.5 Tuning Charge Balance with High Tr iplet Energy High Mobility Electron Transport Material Since, the main issue limiting the efficiency of FIrpic devices has been identified as the lower triplet energy of BCP as compar ed to FIrpic leading to triplet exciton quenching. If BCP is replaced with electron transport materials with higher electron mobility and higher triplet energy, char ge balance will be improved and exciton quenching will be reduced resultin g in higher overall device e fficiency. In our present study, we chose tris[3-(3-pyridyl)-mesity l]borane (3TPYMB) as an electron transport material since its electron mobility (~10-5 cm2V-1s-1) is about an order of magnitude higher than that of BCP and it has one of the highest triplet energy (T1=2.98 eV) amongst all electron transport mate rials (ETMs) used in OLEDs. To show the effect of the electron tr ansport due to 3TPYMB, electron-only devices using BCP and 3TPYMB along with 4,7-di phenyl-1,10-phenanthroline (BPhen) was fabricated. Here, BPhen (T1=2.5 eV) was also used sinc e it has triplet energy similar to that of BCP while its electron mobility (10-4 cm2/Vs) is the highest amongst all three ETMs. HOMO (highest occupied molecula r orbital) and LUMO (lowest unoccupied
97 molecular orbital) energy levels, triplet energy and mobility for all the ETMs have been summarized in table 5-1. Figure 5-9 shows t he I-V characteristics of the three electrononly devices and the results show that the BPhen device has the highest current density followed by the 3TPYMB and then the BCP devic es. Those results are consistent with the mobility values of the corres ponding electron transport materials. To demonstrate the effect of charge balance on OLED device performance we fabricated devices with the three different electron transport mate rials. OLED device structure was same as the one shown in Fi gure 5-1 a). Figure 5-10 shows the device efficiencies for all three devices. The BCP device has the lowest e fficiency of all which is in agreement with its lowest mobility and low triplet energy. Although the electron mobility of BPhen is substantially higher t han those of the other two ETMs, due to its low triplet energy, the device efficiency is onl y slightly higher than that of the BCP device. Finally, the 3TPYMB device shows substantially higher efficiency (60 cd/A) compared to the other two devices. Similar e fficiency has also been reported for FIrpic based devices[111, 112]. Our optimized 3TPYM B device shows a peak luminous power efficiency of 50 lm/W. 5.6 Triplet Energy or Mobility? To study the interplay between the tr iplet energy and the charge balance, 3TPYMB was used as the ETL in UGH2 host based FIrpic PHOLEDs. As discussed in the previous section, we hav e demonstrated high efficiency in mCP host FIrpic devices with this ETL. In addition to that Tanaka et al also have previously demonstrated very high efficiency FIrpic devices with this ETL. While high efficiency has been demonstrated with this ETL, it is not clear whether the high triplet energy or the high
98 electron mobility is the dominant reason for its high efficiency. Here, we will show that both high electron mobility and high triplet energy are required for high device efficiency. The OLED devices used in this study have the following structure: ITO/TAPC (40 nm) as a HTL/ mCP (N,N`-dicarbazolyl-3,5benzene) (10 nm) as an electron blocking layer/UGH-2 (1,4-Bis(triphenylsilyl)benzene) la yer doped with 10 wt% FIrpic (20 nm) as an EML/ ETL (40 nm) as a hole blocker. The cathode consisted of a 1 nm thick layer of LiF followed by a 100 nm thick Al. The device structure is shown in Figure 5-11. In the control device (Device A) BCP was used as a hole blocking/electron transport layer. To study the effects of triple t energy and electron transport of the ETL in UGH2 host devices, a device (Device B) was f abricated with 3TPY MB as an ETL. Figure 5-12 a) shows the LIV characte ristics for the two devices using BCP (Device A) and 3TPYMB (Device B) as the ETL. Using 3TPYMB as the ETL there is a 5X increase in the current dens ity of the device at 12 V, indicating the high electron mobility effect of 3TPYMB. Figure 5-12 b) s hows the external quan tum efficiency (EQE) and luminous efficacy comparisons for device A and B. The control device having BCP as an ETL shows 15.3 % EQE (~30 cd/A cu rrent efficiency) and 13 lm/W luminous efficacy, while the device with 3TPYMB as an ETL shows a substantial enhancement in both current and luminous efficiencies (23% EQE, 49 cd/A current efficiency and 31.6 lm/W luminous efficiency). To determine the interplay between the triplet energy and charge balance, we fabricated two other devices with a double-la yer ETL. In these devices the electron transport layer is composed of two layers: ETL1 is adjacent to the EML while ETL2 is adjacent to the cathode. In device C, 3TPYM B is used as ETL1 and BCP is used as
99 ETL2, both having a thickness of 20 nm each. In device D, BCP is used as ETL1 and 3TPYMB is used as ETL2, both having a thick ness of 20 nm each. The energy diagram for device C is shown as in Figure 5-13. De vice C illustrates the effect of electron transport on the device performance as 3TPYMB provides good exciton confinement at the EML/3TPYMB interface while the BCP layer (20 nm) is expected to impede the electron transport in the device. Device D illustrates the triple t energy effect on the device as the BCP layer is in contact with the EML resulting in luminescence quenching at the interface. Figure 5-14 a) shows the LIV characteristi cs of all four devices. Both devices C and D having similar current densities show s lightly higher current density than device A. This is because of the difference in electron transport between BCP and 3TPYMB. The efficiencies of all devices are shown in Figure 5-14 b). Both devices C and D have higher efficiency compared to the control devic e A indicating either higher triplet energy or higher electron mobility leads to enhanc ement in device efficiency. While both devices C and D have about the same peak effici encies, the efficiency roll-off in device D is more steep than that in device C indica ting that a slight imbalance in transport leads to strong exciton quenching at the EML/ ETL interface at high current densities. The fact that device B has higher efficien cy than device C indicates the enhancement in efficiency due to improved charge balance. Charge imbalance in device C lead to high concentration of excitons formed at the inte rface between the EML and ETL, resulting in exciton quenching and lower efficiency compared with device B. Hence we can conclude that both high triplet energy and high electron mobility are necessary to achieve a high efficiency blue PHOLED.
100 5.7 Improving Charge Transport a nd Balance in the Emitting Lay er Based on the experiments conducted with hi gher FIrpic doping concentration in pi-n devices, we learned that when we incr ease the overall charge transport in the device, an increase in the doping concentration gives further boost to device efficiency. A higher doping concentration in the EML probably provides more hopping sites and hence improves the charge transpor t in the emitting layer. Also FIrpic is known to have a better electron transport than hole transport so increasing FIrpic concentration might also aid the charge balance improvement fo r the device. Based on this understanding, the optimized doping concentration might be higher than 10% FIrpic for devices with 3TPYMB as ETL. Hence, we fabricated dev ices with varying doping concentration of FIrpic. OLED devices were fabricated with va rying concentration of FIrpic doped in EML and 3TPYMB was used as an ETL. The device with 20% doping was found to have the highest efficiency. Figure 5-15 shows the e fficiency comparison of device with 10% and 20% doped FIrpic in EML. Efficiency enhanc ement with increased phosphor loading was more evident at low current density levels probably bec ause of ease of injection with higher doping concentrations. Also at higher doping concentration there is still a possibility of triplet-triplet quenching inhibi ting the enhancement in device efficiency at higher current density levels. To further understand how the increased doping conc entration affects the transport in EML single carrier devices were fabricated. El ectron only devices were fabricated with the device stru cture as EML (100 nm)/ 3TPYM B (40 nm)/ LiF (1 nm)/Al. hole only devices were fabric ated with the device structure as TAPC (40 nm)/ mCP (10 nm)/ EML (100 nm)/Au. Both hole and electron side were kept similar to the control OLED device so that the results can be eas ily correlated. A thicker EML was used so
101 that the transport in the single carrier is dominated by that of EM L. Figure 5-16 shows the measured current density vo ltage characteristics for thes e single carrier devices. Figure 5-16 a) shows the J-V characterist ics for electron only devices and Figure 5-16 b) shows the J-V characteristics for hole only devices. As can be seen from both these plots we see and increase in both the elec tron and hole current with increased doping concentration. For the electron current we see more than 12X increase in current density at about 10V whereas for the hole curr ent we see an increase of about 9X in current density, when the doping concentration is increased from 10% to 20%. Hence, we see that in a wide band gap host such as UGH2 doping concen tration affects both the charge transport and balance in the device and hence can be used as important tool tom optimize device performance. 5.8 Summary We investigated the effect of ch arge balanc e on the performance of blue PHOLEDs. To probe the charge balance in the device single carrier devices were fabricated and the devices were found to be largely hole dominant. Also the recombination zone in these devices was found to be located at the EML/ETL interface. Also most of the commonly available el ectron transport materials have low triplet energies which can be a major cause for conc ern as the recombination zone is located at ETL interface. Hence it was concluded that the efficiency of the device is limited by both the triplet energy and mobility of electr on transport material. We found that for achieving high efficiency in PHOLEDs both these properties are very important to maximize the quantum efficiency of the device. Two different approaches were used for tuning the charge transport and balance in blue PHOLEDs: 1) using doped charge transport layers and 2) using high mobility high triplet energy ETL. We also
102 demonstrated that in a wide band gap host, doping concentration of FIrpic can also be used to tune the transport in the emitting layer of the devices. The high triplet energy ETL material (3TPYMB) has high enough triple t energy to block triplet excitons from FIrpic. However, further improvements in material design might be needed to fully optimize the charge balance in the device.
103 Figure 5-1. Device structures for single carri er devices used in this study a) Control OLED device structure b) hole only dev ice structure c) electron only device structure.
104 Figure 5-2. Current density-volt age (J-V) characteristics for single carrier devices shown in Figure 5-1 using TAPC as HTL and BCP as ETL. Figure 5-3. Devices fabricated for probing the recombination zone. a) Device doped only on interface of HTL/EML (left doped), b) Device doped only on interface of EML/ETL (right doped).
105 Figure 5-4. Device probing the location of recombination zone in the device; a) LIV characteristics for devices in Figure 5-3 and control device shown in Figure 51. The filled symbols indicate the current density whereas open symbols indicate luminance of the corresponding device. b) Current efficiency for these devices. Figure 5-5. Illustrating the location of recombinati on zone and triplet exciton confinement for blue PHOLEDs; a) Schematic showing the location of recombination zone in the blue PHOLED and b) schematic showing triplet energies of HTL, dopant and ETL.
106 Figure 5-6. Device structure for UGH2 host control device and p-i-n device. 0.01 0.1 1 100 10 20 30 40 0 10 20 30 40 Current efficiency [cd/A]EQE[%]Current Densit y [ mA/cm2 ] Control device N doped device p-i-n device Figure 5-7. Efficiency comparison for three sets of devices: control device, p-i-n device and n-doped device.
107 1E-30.010.1110100 0 10 20 30 40 50 10% FIrpic 15% FIrpicCurrent efficiency[cd/A]Current density [mA/cm 2 ] Figure 5-8. Comparison of device efficiency fo r two p-i-n devices with different FIrpic doping concentrations: 10% and 15%. 0246810 10-610-510-410-310-210-1100101 BCP BPhen BCP 3TPYMB BPhenCurrent density [mA/cm2]Voltage [V]3TPYMB Figure 5-9. JV characteristics of elec tron only devices with BCP, 3TPYMB and BPhen as the ETL showing highest current density at a given voltage for BPhen, followed by 3TPYMB and lowest for BCP. Inset shows the molecular structure of the three electron transport materials.
108 Figure 5-10. Efficiencies for OLED devices with BCP, 3TPYMB and BPhen as the ETL Figure 5-11. Device structure of blue PHOLED used in this study with UGH2 as the host with BCP or 3TPYMB used as ETL.
109 024681012 1E-5 1E-4 1E-3 0.01 0.1 1 10 100 Device A Device BCurrent Density [mA/cm2]Voltage [V](a)1E-30.010.1110100 0 10 20 30 0 10 20 30 External Quantum Efficiency[%] Power Efficiency [lm/W]Current Density [mA/cm2] Device A Device B (b) Figure 5-12. UGH2 host based OLED devices with BCP and 3TPYMB as ETL. a) LIV characteristics for devices using BCP (Device A) and 3TPYMB (Device B) as the ETL. b) Luminous efficacy and EQ E comparison for device A and B. Figure 5-13. Energy diagram for device C sh owing the energy levels for respective layers.
110 024681012 1E-5 1E-4 1E-3 0.01 0.1 1 10 100 Current Density [mA/cm2]Voltage [V] Device A Device B Device C Device D(a)1E-30.010.1110100 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 Power Efficiency [lm/W] Current Efficiency [Cd/A]Current Density [mA/cm2] Device A Device B Device C Device D (b) Figure 5-14. Comparing device performance for devices A, B, C, and D; a) LIV characteristics of all 4 devices: devi ce A, B, C and D an d b) the current efficiency and luminous efficacy for devices A, B, C and D 1E-30.010.1110100 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Current efficiency [cd/A] Power efficiency [lm/W] Ctdit[A/ 2 ] 10% FIrpic 20% FIrpic Figure 5-15. Efficiency comparison for UG H2 devices with 10% and 20% FIrpic doping concentration.
111 0246810 1E-5 1E-4 1E-3 10% doping 20% dopingCurrent density [mA/cm 2 ]Voltage[V] EML(100 nm)/3TPYMB(40 nm)/ LiF(1 nm)/Al (50 nm) 0246810 1E-5 1E-4 1E-3 0.01 0.1 10% doping 20% dopingCurrent density [mA/cm 2 ]Voltage [V]TAPC(40 nm)/mCP(10 nm)/EML(100 nm)/Au (50 nm) Figure 5-16. Illustrating the ef fect of higher dopant concentra tion on transport in EML; a) J-V characteristics of electron only dev ices with 10% and 20% FIrpic doped in UGH2 b) J-V characteristics of hol e only devices with 10% and 20% FIrpic doped in UGH2. a ) b )
112 Table 5-1. Energy levels, triplet energy and mobility parameters for different electron transport materials used in this study ETL HOMO (eV) LUMO (eV) T1 (eV) Mobility (cm2V-1s-1) BCP[107, 113] 6.5 3.0 2.5 106 BPhen[97, 114] 6.4 3.2 2.5 104 3TPYMB 6.77 3.3 2.98 105
113 CHAPTER 6 ACHIEVING CHARGE BALANCE USIN G AMBIPOLAR HOST MATERIALS 6.1 Charge Balance in EML Charge balance is a key element in achi evin g both the efficiencies and lifetimes necessary to make a viable OLED lighting device. Charge imbalance usually leads to lower quantum efficiency and steep quantum efficiency roll-off[106, 116]. Since most of the commercially available host ma terials mCP UGH2 etc. are not ambipolar, recent efforts have been made to synthesize ambipolar host materials in order to optimize charge balance in phosphor escent organic light emitting devices[111, 117]. The ability of the host material to conduc t both electrons and holes is important for maintaining charge balance in the emissive layer (EML). When the conductance of one of the charge carriers is limited, the charge transport of that ca rrier is provided mainly by the phosphor. Reliance on the emitting dopant for charge transport can be undesirable since it implies high doping levels (high consumption of precious metals such as iridium or platinum that are co mponents of the phosphor escent emitters) and may compromise device stability via electrochemical degradation of the emitter. CBP is a well-known ambipolar host ma terial. Although CBP does show ambipolar transport, it usually crystallizes in thin films, which reduces the OLED lifetime. Additionally, the triplet energy of CBP lies below that of FIrpic. Thus, when utilizing CBP as a host for FIrpic, back energy transfer from the dopant to the host causes loss in the quantum efficiency. In this section, we introduce a phosphine oxide-based, ambipolar host material for blue phosphors, 4(diphenylphos phoryl) -N ,N-diphenylaniline (HM-A1; the structure is
114 shown in the inset of Figure 6-7a)). To date, known phosphine ox ides predominantly transport electrons[119, 120]. The phosphine-o xide-based host used in this study is HM-A1, is designed to also support hole transport by incorporating a triphenylamine moiety onto the otherwise electron-trans porting aromatic phosphine oxide. The advantage of using these phosphine oxides ma terials is that they have high triplet energies that helps prevent the back-energy tr ansfer from the emitter to the host. HMA1 has triplet energy of 2.84 eV even afte r the incorporation of the triphenylamine moiety, which is still greater than the triplet energy of Firpic. Hence, we expect that HMA1 will serve as a better host for FIrpic molecules. Moreover, most of the other host materi als used in the literature are largely unipolar hosts. The host materials which hav e been used so far for FIrpic based devices (CBP, mCP and CDBP) are all carbazole based hosts and hence, transport only holes. This also would further amplify the problem of charge imbalance that we noted in these devices earlier as the EML also plays a role in determining charge balance in blue phosphorescent devices. Hence, in this c hapter we focus on studying devices with ambipolar hosts and investigating charge balance in these systems. 6.2 Designing Concept for Ambipolar Host Hereby we propose a design concept for designing hos t materials with the appropriate energy levels for better char ge injection, charge transport and charge blocking without sacrificing the high triplet energies of the materials. For high efficiency in blue PHOLEDs we need to sati sfy the following parameters: Triplet exciton confinement Confine charge carriers and recomb ination zone in bulk of EML and Charge balance in the device
115 Based on these criteria, the requirements th at we considered are (as illustrated in Figure 6-1): Host and all charge injection/blocking materials should have higher triplet exciton energies than the phosphor, Charge transport levels should be engineered so that neither el ement of the guesthost system behaves as a trap for ei ther sign of charge carrier, the host materials for FIrpic should have a shallower HOMO energy and deeper LUMO for optimizing charge injection and transport into the EML without trapping of charge on the dopant at t he HTL/EML interface, and The electron injecting / hole blocking materials (HBMs) will be designed to have LUMO energy compatible with the LUMO of the HM and HO MO deeper than that of the host. The focus of this work carried out at Pa cific Northwest National Laboratory was to develop a library of ambi polar PO compounds/host mate rials with optimized charge transporting properties. The approach we used to design and develop host materials is to combine a known hole transport moiety (H Tm) with a known electron transport moiety (ETm) via a common aryl bridge as illustrated in Figure 6-2. The goal of this work is intended to address several related questions and establish design rules for host development; (a) can we tune the HOMO/LUMO energies, (b) does the attachment of TPA to PO hinder electron tr ansport, (c) does the attachment of TPA to PO hinder hole transport, (d) do the possible charge transfer st ates reduce the trip let energy. Previously it was demonstrated that choice of the HTm fixes the energy of the HOMO state[120, 121]. The OLED group at PNNL had previo usly evaluated both theoretically and experimentally triphenyl amine (TPA) and N-phenylcarbazole (NPhCBz) as the HTm. The TPA moiety has a shallower HOMO than that of NPhCBz, an d therefore it was considered a better HTm for this study. In this chapter we introduce host materials containing TPA as the hole transporting moiety and PO as the electron transport moiety.
116 We changed the groups around the PO moiety to study the LUMO energy of the resulting molecules. Figure 6-3 shows the st ructure of four diffe rent host molecules under consideration in this study which have different si de group neighboring the PO moiety. In order to understand the suitability of these different materials as host materials and to look at their energy level with respect to injection barriers and charge transport abilities. In order to evaluate that geometry optimizations were performed and HOMO LUMO energy levels were calculated. A ll calculations were performed with the NWChem computational package at the MS CF, EMSL. Molecular orbitals were visualized using Extensible Computat ional Chemistry Environment (ECCE) a component of the Molecular Science Software Suite (MS3) developed at Pacific Northwest National Laboratory Geometry optimizations for these molecules were carried out using the widely used hybrid functional B3LYP. Figure 6-4 shows the predicted energy levels for all the materials under cons ideration. For all molecules studied, the HOMO is localized on the tri phenyl amine moiety. T herefore, the HOMO energy does not vary significantly along the seri es of molecules reported and is virtually independent of the elec tron transporting moiety. LUMO is localized on the aryl groups around the PO moiety, and the LUMO energy is determined by the energy of the lowest energy aryl group on the PO group. Thus we are able to tune the HOMO and LUMO independent of each other to optimize the material for appropriate charge injection and blocking. 6.3 Investigating Charge balance in PO B ased Ambipolar Host Material The utility of organic phosphine oxide (PO) materials as electron transporting host materials for FIrpic has been previously demonstrated by our collaborators at
117 PNNL[123-126] in blue OLEDs. However, t hese PO materials do not oxidize readily and hence have limited hole injection ability. Henc e, using these PO materials as host severely limits the charge balance in the device. In this section focus on the host material A (HM-A1) and evaluate its ability to transport both holes and electrons in FIrpic devices. To illustrate the ambipolar nature we fabricated three sets of devices: HMA1:FIrpic device with HMA1 as ETL (Device set A), HMA1:FIrpic device with variable FIrpic doping concentrations (Device set B) and HMA1: FIrpic device with undoped HMA1 interlayer between the HTL and the EML (Device set C). Except for the first device set wher e HMA was both the host and ETL, 2,8bis(diphenylphosphoryl)dibenz othiophene (PO15) was used as the ETL material. All devices were grown on commercial ITO, and the hole injection/transport layers consisted of sequentially deposited CuPC, NPD and TCTA; LiF and Al were deposited to form the cathode. Type A devices with the HM-A1 ETL thickness of 20 nm and 50 nm gave rise to pure emission from FIrpic, indicati ng that the host material has the ability to transport electrons and that the recombinati on occurs in the EML. The EQE of the devices with 20 nm-thick ETL was 4.4% measured at 13 mA/cm2, whereas upon increasing the ETL thickness to 50 nm, the EQE increased to 10.1% (at 13 mA/cm2). For the carriers to recombine in the EML, HM-A1 has to support electron transport. The observed FIrpic emission indicates that the direct attachm ent of the TPA moiety does not adversely affect the electron transpor t ability that the PO group provides. The HOMO of HM-A1 is relatively shallow to make it an inferior ETL/HBL material. The HOMO level of PO15 is sufficiently deep; t herefore, using PO15 as the ETL/HBL also gives rise to improved EQE. Also, the current density in the series of devices of variable FIrpic concentration did not change significant ly when the doping level changed from 2
118 to 16%, which indicates that the emitter does not play a ma jor role in hole transport (Figure 6-5 a)) as was observed for UGH2 dev ices previously. Also Figure 6-6 shows the EQE dependence on FIrpic doping level fo r PO15 (electron transport host) and HMA1 host. It is clear from Figure 6-5 b) that HM-A1 device efficiency does not have a strong dependence on FIrpic concentration. To further examine the ability of HM-A1 to conduct hole current, devices with an interlayer of HMA between the hole transpo rting layer and the emissive layer were fabricated. In this type of dev ices, the interlayer of HM-A1 was effectively part of the hole-transporting layer. The corresponding device structure was as follows: ITO/10nm CuPc/20nm NPD/ 5nm TCTA / X nm HMA (X = 0, 5, 10, 20, 40 )/ 15nm 5% Firpic:HMA1/ 50 nm PO15/ 1nm LiF/100nm Al. As shown in Figure 6-6, the emission spectra of the devices with and without the hole injection layer are dominated by FIrpic, indicating that the electron and hole recombi nation occurred in the emissive zone. To further investigate the charge balanc e in these ambipolar host system we fabricated unipolar devices that approximate either the hole or the electron transport in a functioning OLED using HM-A1 as the host. The hole-only devices had the following structure: ITO/40 nm TAPC/60 nm HM-A1:5%FIrpic/20 nm Au/100 nm Al. Au contacts were used to prevent electron inject ion from the cathode. Electron only devices had the structure: ITO/20 nm Al/60 nm HM-A1:5%FIrpic/40 nm PO15/1 nm LiF/100 nm Al. Coating the ITO surface wit h aluminum eliminated hole in jection from ITO, and thus ensured electron-only devices. The current density-voltage data for these unipolar devices is shown in Figure 6-7. The J-V data clearly s how that the hole-only and
119 electron-only current densities are comparabl e. This clearly illustrates the ambipolar nature of the HM-A1 host. In contrast to previous reports with phospine oxide based host materials that do not have good hole transport properties, the drive voltage dependence on the phosphor doping level was much weaker for the devic es with HM-A1 as the host material[119, 120]. The weak dependence of the device drive voltage on the doping level is indicative of charge transport via the host rather t han the dopant. The ambipolar conductivity of the HM-A1 host material eliminates t he need for charge transport by the emitter. Figure 6-8 shows the efficiency for the optimized PHOLED devices with HM-A1 host. As compared to a unipolar (PO15) host device at 100 cd/m2, the device efficiency went up by almost 20% (from ~33 lm/W fo r PO15 to ~47 lm/W for HM-A1). 6.4 Evidence for Broad Recombinat ion Zone w ith Ambipolar Host We demonstrated above that hole-electron balance in the device emissive region is an important factor that affects the device efficiency. It was also demonstrated that it is possible to reduce the quantum efficiency ro ll-off by delocalizing the exciton formation region in the emissive layer (EML). Loc ation of the emitting zone at the electron transport layer (ETL) interface in the hole -rich mCP:FIrpic devices suggests that controlling the charge transport within the host layer will allow for tuning of the emission zone distribution[129, 130]. In this section, we studied the location of the emission zone in the devices with phosphine oxide-based ho sts and electron transport materials. We also show that the alteration of a chemical structure of the phosphine oxide host allows for relocation of the emissive region withi n the EML from one heter o-interface to the other by the choice of the host material. The width of the emission zone can also be tuned by varying the ETL material.
120 The chemical structures of the host ma terial 9-(6-(diphenylphosphoryl)pyridin-3yl)-9 H -carbazole (HM-A5) and the ETL material 2,6-bis(4-(diphenylphosphoryl) phenyl)pyridine (BM-A11) used for this study are shown on Figure 6-9. The chemical structure of the host material HM-A5 is different from the previously reported HM-A1 host by having a carb azole hole-transporting unit instead of triphenylamine, and a pyridine bridge bet ween the hole-transporting and electrontransporting parts of the molecule instead of a phenyl bridge. The presence of pyridinecontaining molecular building blocks is k nown to enhance electron transport in OLED materials[132, 133]. This enhancement was indeed observed in HM-A5-based single carrier devices as shown in Figure 6-10. The hole-only devices shown on Figure 610 had the following structure: ITO/150 nm host/20 nm Au/100 nm Al, where host is one of these materials: HM-A1, PO12 or HM-A5. A 20 nm thick layer of gold was placed between al uminum and the organics to prevent electron injection into the organics fr om the cathode. Electron-only devices had the following structure: ITO/20 nm Al/150 nm host/1 nm LiF/100 nm Al. It can be seen from Figure 6-5 that the charge transport in HM-A1 and HM-A5-based devices differs significantly, whereas PO12 represents an intermediate case and shows the most ambipolar character among the three hosts, with preference to electron transport at higher currents. Electron transport strongl y dominates in HM-A5, and its hole conductivity is very limited throughout the ent ire range of current densities. HM-A1 has a preference for hole transport, with the J-V characteristics converging to ambipolar conductance when the current density in creases. As will be shown below, strong
121 domination of one carrier within the host results in a highly localized emission zone in OLEDs. We studied the FIrpic OLEDs with partially doped emissive layers to determine the location of the emission zone in the dev ice depending on the host and ETL materials choice. In the devices with the following stru cture ITO/35 nm T APC/15 nm EML/50 nm ETL/1 nm LiF/100 nm Al the EML consist ed of 5 nm FIrpic-doped regions located at either the ETL interface, in the middle of the EML, or at the hole transport layer (HTL) interface, whereas the rest of the EML consisted of neat films of a host material. Two different phosphine oxide-based electron tr ansporting materials PO15 and BM-A11 were used as ETLs for each EML configurati on. All the devices with partially-doped EMLs were compared to the standard devices that had a uniformly doped 15 nm thick EML. It can be seen from t he upper graph of Figure 6-6 that most of the emission in HM-A1 host-based FIrpic devices comes fr om the EML part adj acent to the ETL interface. For HM-A1 devices, the EQE decreases as the FIrpic-doped zone moves away from the ETL interface for both ETLs. The degree of the light output drop is also dependent on the ETL material. It can be infe rred from the data on the lower graph of Figure 6-11 that the re combination zone is wider for PO15 devices compared to that of BM-A11, which is reflected in a weaker EQE drop when moving the doped region away from the EML/ETL interface. When a PO15 ETL is used, there is less than a two-fold efficiency drop, whereas the EQE of HM-A1: FIrpic devices with the BM-A11 ETL drops from 14% to 2% as the FIrpic-doped zone is moved to the HTL interface. The LUMO energy level value for HM-A1 derived from the solution electrochemist ry data is -2.56
122 eV. The LUMO levels of the ETL materials estimated using the same methodology are 2.86 and -3.11 eV for PO15 and BM-A11, res pectively. These estimated LUMO energy levels suggest that a better alignment of the LUMO levels of the host and ETL that leads to lower charge injection barriers results in wider emission zones. The single-carrier device data on Figure 610 shows that HM-A5 predominantly conducts electrons as opposed to HM-A1. In accordance with this observation, the location of the emission zone in case of HM-A 5 has shifted to the HTL interface. This shift is seen in Figure 6-11. The maximum luminance comes from the devices with the FIrpic-doped part of HM-A5 located next to the TAPC HTL. PO12 host material represents an intermediate case. Based on singl e-carrier device data, it has the most ambipolar character among the three hosts discussed here, which correlates well with the recombination zone position centered and distributed more evenly within the EML. Unlike for HM-A1, no dependence of re combination zone width on ETL was observed in HM-A5 and PO12 devices. Th e independence of t he width of the recombination zone on the ETL choice in ca se of HM-A5 and PO12 is in agreement that most of the emission in HM-A5 and PO12-based OLEDs occurs far away from the host/ETL interface. The LUMO energies of HM-A5 and PO12 are -2.9 eV and -2.62 eV respectively, according to the estimation from solution reduction potentials, and the different EQE of the devices with two diffe rent ETLs within each doping zone on Figure 6-11 follows the LUMO energy level al ignment between the ETL and the host. The location of the emission zone at the EML/ETL interface in the HM-A1-based OLEDs supports the conclusion drawn from the analysis of single-carrier devices that
123 HM-A1 is hole-rich, which is the reason for the exciton formation to occur predominantly at the EML/ETL interface. 6.5 Summary We have presented design criteria for des igning an ambipolar host material. An ambipolar host materi al was successfully designed and charge transport and balance in HM-A1 as host devices was illustrated with the help of OLEDs and ingle carrier devices. In addition to that we demonstrated how a c hemical modification made on a phosphine oxide-based host molecule allowed for t uning the emission zone location within the EML. By substituting a phenyl linkage bet ween hole transporting and electron transporting fragments of the host molecule, we demonstrated a shi ft of the emissive zone in the corresponding blue phosphorescent OLEDs from the EML/ETL interface to the EML/HTL interface. The distribution of the recombination zone depending on the ETL material suggests that the ETLs with be tter electron injection/transport properties result in wider emission regions with in the doped EML. Our analysis emphasizes the importance of charge balance to delocalize the recombination/emission zone for maximizing the EQE/luminance.
124 Figure 6-1. Design criteria used for designing host materials to optimize charge injection and charge carrier blocking in heterostructure blue PHOLEDs. Figure 6-2. Design strategy used for designi ng ambipolar host mate rials by combination of hole transport and electron transport moiety using an aryl linkage. Electron-transporting phosphine oxide Hole-transporting aromatic amine
125 Figure 6-3. Chemical structures of molecules evaluated for host design. Figure 6-4. HOMO and LUMO energy levels for the four evaluated host molecules as computed at B3LYP/6-31G* level.
126 Figure 6-5. Illustrating am bipolar transport in HMA hos t material; a) The J-V characteristics (above) for the devices with HMA1:FIrpic emissive layers that have variable FIrpic concentrations. The device structure is as follows: ITO/10nm CuPc/20nm NPD/5nm TCTA/15 nm HMA: x%FIrpic (x = 2, 4, 8, 12, 16%)/50nm PO15/1nm LiF/100nm Al. b) The EQE dependence on the FIrpic doping levels is shown below for the devices with HMA and PO15 host materials. a ) b )
127 Figure 6-6. EQE as a function of varyi ng thicknesses of an undoped HM-A1 interlayer between the HTL and the EML. The stru cture of the devices is ITO/CuPc/ NPD/TCTA/HM-A1/5% FIrpic: HM-A1/PO 15/LiF/Al. The inset shows the EL spectra for the devices with interlay er thicknesses from 50 to 400 Figure 6-7. J-V characteristics for the hole only (hollow squares) and electron only (solid red squares) devices using the ambipolar host HM-A1. The chemical structure of HM-A1 is shown in the inset.
128 0.010.1110100 0 5 10 15 20 EQE %Current density [mA/cm2]0 10 20 30 40 50 60 Power efficiency [lm/W] Figure 6-8. Efficiency of optimized device wi th HM-A1 as host. Device structure is ITO/TAPC/TCTA/host:Firpic/PO15/LiF/Al N P P O O BM-A11 N N P O HM-A5 Figure 6-9. Chemical structur e of another host synthesized on the same design strategy and another electron transport/hole blocking material used in this study.
129 05101520 10-810-610-410-2100102 HMA5 e HMA1 e PO12 e HMA5 h HMA1 h PO12 hCurrent density, mA/cm2Voltage, V Figure 6-10. Hole-only (hollow symbols) and electron-only (filled symbols) single carrier devices for HM-A5 (triangles), PO12 (circles) and HM-A1 (squares) host materials. The hole-only device stru ctures: ITO/150 nm host/20 nm Au/100 nm Al; Electron-only device structures: ITO/20 nm Al/150 nm host/1 nm LiF/100 nm Al, host = HM-A 1, PO12 or HM-A5.
130 2 4 6 8 10 12 14 16 18 2 4 6 8 10 12 14 16 18 EQE % at 1 mA/cm2 ETL = PO15 EML configuration EQE % at 1 mA/cm2 ETL = BM-A11 HM-A1 PO12 HM-A5 STABC ETL HTL ETL HTL ETL HTL ETL HTL2 4 6 8 10 12 14 16 18 2 4 6 8 10 12 14 16 18 EQE % at 1 mA/cm2 ETL = PO15 EML configuration EQE % at 1 mA/cm2 ETL = BM-A11 HM-A1 PO12 HM-A5 STABC ETL HTL ETL HTL ETL HTL ETL HTL Figure 6-11. Recombination zone study in PO based host materials. External quantum efficiency (EQE %) measured at 1 mA/cm2 of the PO-based Firpic-doped OLEDs depending on the FIrpic doping region location. Reference configuration ST: the standard device structure with 150 -thick uniformly doped EML. Configuration A: HTL/ 10 nm host/5 nm host:5%FIrpic /ETL/LiF/Al; Configuration B: HTL/5nm host/5nm host:5%FIrpic/5nm host/ETL/LiF/Al; Configuration C: HTL/5 nm host:5%FIrpic/10 nm host/ETL/LiF/Al, host = HM-A1, PO12 or HM-A5. The pattern-filled areas on the device EML schemati cs of the upper graph correspond to the FIrpicdoped hosts
131 CHAPTER 7 USING MIXED HOST ARCHITECTURE TO ACHIEVE CHARGE BALANCE 7.1 Introduction In the previ ous chapter, we discussed usi ng ambipolar host materials as a tool to achieve charge balance in the emitting layer of the device. As we pointed before, most commercially available host materials such as mCP and UGH2 are not ambipolar and have poor transport properties, making it difficult to achieve charge balance in a device using such materials as hosts. Mixed host systems have been demonstrated previously by blending of HTL and ETL to form the host of the emissive layer with the intent being to eliminate interfaces and improving device lifetime. Choong et al were the first to use the mixed host layer approach or the bipolar transport laye r approach to improve device lifetime. They used a bipolar layer of NPB and Al q3 doped with methyl quinacridone (mqa) and were able to obtain a 7X lifetime enhancem ent as compared to the conventional heterostructure device. The driving voltage was reduced due to the bipolar characteristics of the mixed EML. After this first mix ed host publication, a number of publications followed making use of the same approach to achieve long lifetime in blue fluorescent devices[134, 135]. A mixed host layer can also lead to bette r charge balance in the EML, which can be optimized easily by varying the ratio of the two transport materials. This is particularly beneficial for PHOLEDs, as it has been demonstrated that efficiency roll-off depends strongly on charge balance for ir idium based phosphorescent devices. Kim et al. demonstrated low roll-off and low ex citon density in the EML by optimizing the ratio of a hole transport material TCTA and an electron transport material 1,3,5-tris(N-
132 phenylbenzimidazole-2-yl)benzene (TPBI) for tris(2-phenylpyridine) iridium [Ir(ppy)3] devices. They also observed reduced drive voltage, leading to high power efficiencies. Figure 7-1 illustrates the concept of mixed host architecture in an OLED. 7.2 Blue PHOLED Devices with Mixed Host Architecture In this study, we used a mixed host approach to achieve charge balance in blue PHOLED devices with FIrpic by co-deposit ing a hole transport material, an electron transport material, and the dopant to form the emitting laye r (EML). This mixed host layer exhibits bipolar transport characteri stics which help improve charge balance and lower oper ation voltage. The mixed host devices also had high external quantum efficiency (EQE) and reduced efficiency roll-off at high current densities. Initially, we fabricated devices wi th an electron transporting host 2,8bis(diphenylphosphoryl)dibenz othiophene (PO15) and hole transporting host di-[4-(N,Nditolyl-amino)-phenyl]cyclohe xane (TAPC) separately. These devices had lower quantum efficiencies even though there was no tr iplet exciton quenching as the triplet energies of both TAPC (T1= 2.9 eV) and PO15 (T1= 3.1 eV) is higher than that of FIrpic. Also the roll-off in these devices was quite steep due to unipolar nat ure of these hosts. We then fabricated devices with a mixture of TAPC and PO15 to create a mixed host system and optimized the ratio of the two trans port materials to achieve charge balance in the device. Using this approach we were able to achieve a high peak efficiency of 52 lm/W at 60 cd/m2. 7.2.1 Unipolar Host Devices TAPC and PO15 were used as the host for FIrpic based devices. TAPC is mainly a hole transporting material and PO15 transpor ts only electrons. As mentioned above, both have high enough triplet energies to provi de good triplet exciton blocking for FIrpic.
133 The device structure used for these devices was TAPC/host:FIrpic(x%)/PO15/LiF/Al. FIrpic concentration was varied in these hosts to study the effect of FIrpic concentration on charge transport and device pe rformance. Figure 7-2 show s the device structure for these unipolar host devices. As seen in Figure 7-3, when TAPC was used as the host, both the EQE and power efficiency increase only slightly with incr easing FIrpic concentration, which might be due to slight increase in electron transport with increasing FIrpic concentration. The operation voltage slightly increases with increas ing concentration of FIrpic (Figure 7-4) showing that, at higher concentration, FIrp ic might be trapping carriers. The EQE of these devices is low even though there is no triplet quenching from TAPC (HTL and EML) or PO15 (ETL). The EL spectrum sh ows pure FIrpic emission. Hence, the lower efficiency observed when using TAPC as the host can be attributed mainly to poor charge balance in the EML. C onsidering that TAPC has a high LUMO of 1.8 eV and is known to have good electron blocking properties, no electron transport is expected through TAPC in the EML. Additionally, TAPC is known to have very high hole mobility, therefore, the recombination zone in these devices is likely to be narrow and at the EML/ETL interface, leading to the steep roll-off seen in the EQE vs. current density inset in Figure 7-3. Previously, we published device data for different FIrpic concentrations in an OLED using PO15 as the host. EQE increased and drive voltage decreased as FIrpic concentration increased because FIrp ic helped facilitate hol e transport. PO15 has a deep HOMO, creating a large energy barri er for hole injection. As such, hole
134 transport is not expected in PO15, meaning t hat the charge carriers in the EML when PO15 is the host were predominantly electrons indicating a lack of charge balance. 7.2.2 Devices with Mixed Host EML In order to address the i ssues noted with the devices with unipolar hosts, we fabricated devices having a bl end of TAPC and PO15 as EML as shown in Figure 7.5. Table 7-1 shows the device par ameters for devices fabricat ed with the same structure using PO15, TAPC, or TAPC-PO15 mixed host as the host in the EML. Devices with a PO15 host have higher operation voltage than devices with a TAPC host, showing that the mobility of electrons in PO15 is worse than the mobility of holes in TAPC. To achieve balanced transport of electrons and holes in the EML, devices were fabricated using a mixed host EML with varying ratios of TAPC and PO15. The optimized mixed host device shows the superior performance of mixed host PHOLED over that of PHOLEDS made using the individual transport layers as hosts, attributable largely to the charge balanc e in the mixed host device. Figure 7-6 shows the dependency of device efficiency on TAPC weight percentage in the mixed host layer. The current efficiency numbers are reported at a current density of 1 mA/cm2. The data shows that when we use eit her a hole transporti ng or a electron transporting host by itself, the devices are not charge balanced because the EML does not have ambipolar transport. By employing the mixed host, we were able to achieve much higher efficiency in comparison to dev ices made with just one of the constituents of the mixed host. Starting from 0% TAPC (neat PO15 as the host), as more TAPC is included in the mixed layer, the charge balance increases leading to an increase in device efficiency. At 55 weight percent TAPC, efficiency reaches a maximum, indicating good charge balance for this mix ed host composition. As the TAPC content
135 increases the operating volt age of the device decreases, likely due to TAPC having a higher hole mobility than the elec tron mobility in PO15. The driv e voltage is also lowest at 55 weight percent TAPC as shown in Fi gure 7-7 which is additional evidence that the optimum charge balance for these mate rials in this device structure has been reached. To illustrate the effect of using a mix ed host emitting layer in OLED devices, we plot the current densityluminance and voltage characteristics for devices with TAPC, PO15 as host and compare them to the optimized mixed host device as shown in Figure 7-8. From this plot, it is evident that the device with mixed host EML has lower turn on voltage as compared to both the unipolar host devices. The cu rrent density of the mixed host device is actually higher than both T APC and PO15 host devices almost across the entire voltage range. This implies that usi ng a bipolar emitting layer improves the transport and balance of hole and electron charge carriers in the device leading to lowering of operation voltage and increase in overall curr ent density of the device. Moreover, even though the current density of the mixed host is st ill fairly close to that of the TAPC host (since TAPC has higher mobility, it dominates the transport characteristics and operation voltage); the luminance of the device is much higher in magnitude as compared to bot h TAPC and PO15 host devices. Again, this clearly illustrates the effect of charge balance on the device performance. Figure 7-9 shows the device efficiency as a function of brightness for the optimized TAPC: PO15 mixed host device. The device has a peak current efficiency of about 50 cd/A and a peak power efficiency of about 59 lm/W due to the low operation voltage. Impressively, the efficiency is qui te flat in the region 30-3000 cd/m2. To illustrate that
136 this low efficiency roll-off is a result of better charge balance in the mixed host device, the efficiency of devices with 5% FIrpic in di fferent hosts i.e. TAPC host, PO15 host and TAPC (55%):PO15 (45%), is plotted in Figure 7-10. The roll-off is worst in the case of devices that use only PO15 as the host as the charge carrier transport is probably deviates most from the balance condition. T he device that uses TAPC as host has slightly better roll-off than PO15. TAPC showed little dependence on FIrpic concentration, indicating possible electron transport in the TAPC host and explaining why the roll-off for TAPC host devices is not as bad as PO15. The mixed host devices have the least amount of efficiency roll-off, corroborating the fact that charge balance reduces the efficiency roll-off in phosphorescent devices. Hence, using the mixed host architecture not only we gain in device efficiency but we also achieve a flatter efficiency response from the device. Hence, it is possible to achieve devices with high efficiencies at hi gh brightness with this mixed host approach. Also, mixed host approach eliminates two major interfaces as compared to the baseline heterostructure device and hence, makes it possible to achieve high efficiency devices with better device lifetime. 7.3 Summary In this secti on we demonstrated hi gh efficiency in FIrpic based blue phosphorescent devices with low efficiency roll-off using a mixed host system. Devices were fabricated with varying concentration of hole transport material TAPC and electron transport material PO15 to optimize the c harge balance. The optimum composition of the EML was found to be as TAPC (55%), PO15 (40%), FIrpic (5%) by weight. At this optimum layer composition both EQE and operation voltage we re optimized leading to a high peak power efficiency of 59 lm/W. Even at 1000 cd/m2, the device has a current
137 efficiency of 53 cd/A and a power efficien cy of 49 lm/W, which is one of the highest efficiencies reported for FIrpic based devices without any outcouplin g techniques. This demonstrates that achieving charge balance in the device is the key to achieve high efficiencies at high brightness values and that mixed host devices are a viable means to do so.
138 Figure 7-1. Device structure for heterostr ucture and mixed host devices. a) Schematic illustrating a heterostructure OLED in which a narrow recombination zone is located on the interface of EML and ET L. b) Schematic illustrating a mixed host OLED in which EML is a blend of HTL and ETL material with dopant. The recombination zone profile is broad and located in bulk of EML. Figure 7-2. Device structure used for f abricating unipolar host devices where EML consisted of either TAPC or PO15 as host doped with FIrpic.
139 01 02 0 0 4 8 12 16 20 EQEEQE %% FIrpic0 5 10 15 20 25 30 1E-30.010.1110100 0 5 10 15 External quantum efficiency [%]Current density [mA/cm2] lm/W Power efficiency [lm/W] Figure 7-3. Efficiency characteristics fo r devices fabricated with TAPC as host doped with varying concentration of FIrpic emi tter. Inset shows the external quantum efficiency versus current density for one particular device illustrating the roll off in the device. 01 02 0 4.0 4.1 4.2 4.3 4.4 4.5 Voltage@13 mA/cm2% FIrpic V Figure 7-4. Dependence of oper ation voltage on doping concentration of FIrpic in devices with TAPC as host.
140 Figure 7-5. Schematic illustrating the device architecture of devices with a mixed host EML. 020406080100 0 10 20 30 40 50 60 @1 mA/cm 2Current efficiency [cd/A]TAPC wt% in EML Figure 7-6. Dependence of device efficiency on TAPC concentration in emitting layer.
141 020406080100 4.0 4.2 4.4 4.6 4.8 5.0 5.2 Voltage@13mA/cm2TAPC Weight % V Figure 7-7. Plot showing t he trend of operation voltage of mixed host devices with varying concentration of TAPC in the emitting layer. 012345678 10-710-610-510-410-310-210-1100101102103 Mixed host TAPC host PO15 hostVoltage [V]Current density [mA/cm2] 100101102103104105106107 Luminance [cd/m2] Figure 7-8. Plot showing the comparis on of current densityluminancevoltage characteristics for devices with TAPC, PO15 as host and the optimized mixed host device.
142 10100100010000 0 10 20 30 40 50 60 Luminance [cd/m2]Current efficiency [cd/A]0 10 20 30 40 50 60 Power efficiency [lm/W] Figure 7-9. Plot showing the representative current and power efficiency characteristics for the optimized mixed host device. 110100100010000 0 10 20 30 40 50 60 Mixed host TAPC host PO15 hostCurrent efficiency [cd/A]Luminance [cd/m2] Figure 7-10. Plot showing the comparison of device efficiency as a function of brightness for the TAPC host, PO15 host and optimized mixed host devices.
143 Table 7-1. Device parameters for devices fabricated with the sa me structure using PO15, TAPC, or TAPC-PO15 mix ed host as the host in the EML Host V@13 mA/cm2 (V) Power efficiency (lm/W) TAPC 4.27 16.33 PO15 5.11 8.21 TAPC 55%:PO15 (45%) 4.23 28.48
144 CHAPTER 8 CONCLUSIONS AND FUTURE WORK 8.1 Summary The quest for energy efficient lighting has led to various research efforts being focused on alternative energy effi cient light sources such as OLEDs. OLEDs have emerged as a low cost and commercially viable lighting alternative. However, there are still some issues which need optimization before their potential can be realized to the maximum. Also there is huge market for OLEDs for display applications for both small and large area applications. An OLED scree n can also be used for backlighting in displays. For all these applications a st able high efficiency OLED is needed. For fabricating high efficiency white devices, a highly efficient blue PHOLED is needed. In this project we systematically studi ed the factors limiting the performance of blue phosphorescent OLEDs and by optimizat ion of these factors we significantly enhanced the performance of blue PHOLEDs. Based on our study, there are several materials and device parameters affecting the device performance such as triplet energy and mobility of the materials. T he effect of these factors on the device performance of blue PHOLEDs is as described below. 8.1.1 Triplet Exciton Confinement For phosphorescent materials, tri plet ex citon confinement is a very important factor affecting the device performance. Tr iplet excitons have much longer diffusion lengths. Hence, it is possible to have lu minescence quenching if any of the charge transporting materials or the host have lower tr iplet energy than that of phosphorescent dopant. In order to maximize the radiative decay we need to have good triplet exciton confinement for the phosphorescent emitter. This confinement prevents backwards
145 energy transfer from the dopant to the host or the transport materials. In chapter 3 we focused on studying the effect of this trip let exciton confinement on performance of these blue PHOLEDs. We systematically studied the effect of triple t energy of HTL, ETL and host material on the performance of FIrp ic based devices and found that the triplet exciton confinement is an important factor for optimizi ng the device performance. The lower triplet energy of most of the electron transport materials was found to be a limiting factor. For the host materials not only the triplet energy of the material is an important consideration; the guest-host interactions affecting the mechanisms of charge and energy transfer also play an important par t in determining device performance which were discussed in chapter 4. 8.1.2 Charge Balance in Blue PHOLEDs The other important factor which we focus on is the charge balanc e in the devices. Imbalance in charge transport leads to accumulation of carriers at the interfaces thereby resulting in loss in efficiency and lifetime. Typically the electron mobility of electron transport materials is lower than that of the hole transport materials used in blue PHOLED devices. Hence, these devices ar e largely hole dominant. This imbalance in charge transport affects the location of the re combination zone in the device. Since the devices are hole dominant, the recombinat ion zone is located on the EML/ETL interface. Also as we just pointed out the triplet energy of commonly used ETLs is lower than that of blue dopant. Hence, this char ge imbalance also affects the luminescence quenching and hence the device performance.
146 8.1.3 High Efficiency Blue PHOLEDs Based on the study of triplet exciton confinement and charge balance it was concluded that there are two main issues liming the device performance Poor triplet exciton confi nement at EML/ETL interface and Poor charge balance due to lower mob ility of the electron transport layer Based on this conclusion we proposed f our different approaches to tune the charge balance in these devices and optimize dev ice performance to get highly efficient blue PHOLEDs. The different approaches used were as follows: Use of doped transport layers to enhance charge transport and balance in the device was discussed in chapter 5 (Peak efficiency achieved 45 cd/A). Using a high mobility high triplet energy material 3TPYMB using which device efficiency of 60 cd/A (50 lm/W )as demonstrated in chapter 5. Using ambipolar host materials for charge balance in EML, with this approach device efficiency of 50 cd/A is achieved. (chapter 6) Mixed host emitting layer to tune charge bal ance in device. Devices based on this approach had the highest power efficiency of 59 lm/W (50 cd/A). (chapter 7) Also it was demonstrated in chapter 5 t hat both high mobility and high triplet energy are required to achieve high effici ency in these blue phosphorescent devices. Hence, an understanding of the device physics is very important in order to achieve high efficiency for OLED devices. Its important to optimize and explore device architectures to use the current library of materials to get better device performance and it also an important issue to focus on design and development of new materials with better suited properties. 8.2 Light Extraction One of the very important fa ctors in determining the ext e rnal quantum efficiency is the light extraction efficiency. Light extr action efficiency is dependent on the device
147 architecture. Most of the light emitted from the OLED is lost in the glass substrate, the ITO electrode, and the organic layers due to total internal reflection. Many efforts have been focused on improving the OLED li ght extraction efficiency by surface roughening, use of micro lenses[79, 80], low index gratings use of high index glass substrates and microc avity structures etc. We also addressed this light extraction iss ue by integrating micro cavities with our high efficiency blue PHOLED. It is found t hat blue microcavity PHOLEDs show the current efficiency enhancement of X1.3 (2 3.7 > 30.9 cd/A) and the power efficiency enhancement of X1.72 (10.8 > 19.1 lm/W). Blue PHOLED devices with two and four layers of quarter wave stacks (QWS) have been fabricated on the glass substrate. The device stack (thicknesses of t he dielectric layers) was opti mized to maximize the microcavity effect at 475nm. The optimized dev ice has the following structure: glass substrate/SiO2 (79nm) /TiO2 (48nm) or SiO2 (79nm)/TiO2 (48nm)/SiO2 (79nm)/TiO2 (48nm)/ITO (50nm)/PEDOT (25nm) /TAPC(20nm) /FIrpic: mCP (20nm) /BCP (40nm)/LiF (1nm)/aluminum (100nm). As the number of layers of quarter wave stacks increase the spectrum becomes more and more narrow. For our devices the spectrum of 4QWS device has a narrower FWHM and has more blue component in the spectrum than those of the 2QWS and noncavity devices. Hence, the 4QWS device shows lower efficiency as compared to a device using 2QWS substrate because of the dependence of luminance on photopic response. As the spectrum has more blue color, its luminance decreases even if the output flux remains the same. Hence, t here is a trade-off relationship between blue color optimization and power efficiency. Since t he ultimate goal of this project, was to
148 use this device for fabricating white OLED s using down conversion phosphors, more saturated blue emitting device (4QWS device) is more appropriate since absorption spectrum region of yellow phos phor is 450~500nm wavelength. Hence, the blue PHOLEDs f abricated here were further studied to optimize them for use with micro cavity substrates. It was found these microcavity substrates redistributed the optical modes of the devices both spectrally and spatially. Hence, it was possible to optimize the cavity strengt h and length to minimize of the ITO/organic mode and maximize of the extracted mode leading to enhancement of the light outcoupling efficiency. Micro-cavity OLEDs wit h well optimized cavity strength and length improves the light intensity at normal angl e as well as all integrated angle. Using microcavities with FIrpic based blue PHOLEDs EQE enhancement of 11% (2QWS) and 41% (4QWS) was achieved. 8.3 Down Conversion Power efficiency of blue microcavity OLED s is the key to achieve efficient white OLEDs wit h down conversion. As discussed above the efficiency of devices presented in this dissertation was further enhanced by using microcavity substrate OLEDs to achieve the maximum power efficiency. Finally, down-conversion phosphors were integrated with the blue micro-cavity PH OLEDs and power efficiency of 67.2 ~ 73.5 lm/W (1.64 ~ 1.79X) has been achieved wi th a combination of yellow and red phosphors. Macrolens was used for further enhancement of power efficiency using enhancing light out-coupling. The enhancement of power efficiency due to the macrolens was about 45% leading to very high efficiency white PHOLEDs of over 100 lm/W power efficiency.
149 Hence, we successfully demonstrate that by optimizing the blue PHOLED device to get high efficiency by tuning various dev ice and light extraction parameters, it is possible to successfully remove the weak li nk in development of white PHOLEDs. We successfully demonstrated blue PHOLED devi ces of over 50 lm/W efficiency which paved the way for our 100 lm/W white device. 8.4 Future Work Over the last two decades progress in the field of organic electronics has been quite rapid. Both device efficiency and lifetim e have come a long way from the early on efforts culminating today with the commerc ialization of various organic displays in consumer products. However, there are still various areas in which research can be focused on in this field and some of them will be highlighted here (both in relation to this work and otherwise). At the end of this work we demonstrated blue PHOLEDs with 60 cd/A (27% EQE) without light extraction. As J. J. Kim pr esented in SPIE Optics and Photonics held in San Diego earlier this year the theoretical limit for the EQE of these devices is about 2930%. Hence, the efficiency achieved in this work is quite close to the theoretical limit and not much room for fu rther device optimization (without use of light extraction techniques) is available. Hence, in relation to this work I think furthe r research efforts for blue phosphorescent devices should be focused on light extraction techniques to extract the ITO/organic mode and the substrate mode. One of the main issue concerning t hese blue PHOLEDs which still remains a challenge is the lifetime of these blue PHOLEDs. Although in this work no lifetime studies were done specifically on FIrpic devices it is a well known fact that most of these devices have poor lifetim e as compared to that required for commercial scale
150 display or lighting applications. There is an urgent need to do a through lifetime study for these devices in relation to both ma terial degradation and ex ploring alternative device architectures. Kondakov et al[26-28] have done some exemplary work exploring the degradation in similar devices and there is a need to do a similar study for blue PHOLEDs. Although the mixed host approach usually leads to a significant enhancement in device lifetime, device arch itecture and material design should be focused specially on the lifetime issue along with maintaining a high enough device efficiency for commercial applications. In addition to the lifetime issue, one of the main hurdles in the large scale commercialization of OLEDs is to fully real ize their low cost potential. Hence, going forward we need to focus on developing high efficiency solution processed devices. Also the OLED driver circuit issues need to be resolved to achieve low cost, better OLED displays. If the cost of OLED displays and lighting products can be reduced markedly, then the lifetime issue also becomes less important as t he idea of cheap use and throw products can be realized. Apart from blue PHOLEDs, ther e are lots of other interest ing areas in this field of organic electronics and it would be impossible to list of them here so we will just list a few in this section. Using triplet materials fo r photovoltaic materials is a very interesting subject of ongoing research. Also fabric ation of tandem devices with conductive interconnection units is also an interesting problem. For solution processed devices, the effect of morphology of thes e thin films and how that af fects the device performance especially in organic photovoltaic cells is a subject of ongoing research.
151 Although in this section we have discuss ed only few two potential future research directions for organic electronics, there are numerous other interesting and challenging scientific and research problems. Even a fter decades of the dawn of era of organic electronics it continues to be an extens ively researched area both for fundamental science and technological applications and will definitely continue to do so in the foreseeable future.
152 LIST OF REFERENCES  W. Shockley, "The path to the conc eption of the junction transistor," IEEE Trans. Electron Devices vol. 23, pp. 597-620, 1976.  M. Pope and C. E. Swenberg, Electronic processes in organic crystals and polymers 2ed: Oxford University Press, 1999.  E. A. Silinsh and V. Capek, Organic Molecular Crystals: Interacton Localization, and Transport Phenomena 1ed: American Institut e of Physics, 1997.  W. Brtting, Physics of Organic Semiconductors Wiley-VCH, 2005.  N. Karl, "Organic semiconductors," in Festkrperprobleme 14 1974, pp. 261290.  H. Klauk, Organic Electronics: Materials, Manufacturing, and Applications WileyVCH 2006.  M. Pope, H. P. Kallmann, and P. M agnante, "Electroluminescence in Organic Crystals," J. Chem. Phys. vol. 38, pp. 2042-2043, 1963.  W. Helfrich and W. G. Schneider, "R ecombination Radiation in Anthracene Crystals," Phys. Rev. Lett. vol. 14, pp. 229, 1965.  C. K. Chiang, C. R. Fincher, Y. W. Park A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, "Electrical Conductivity in Doped Polyacetylene," Phys. Rev. Lett. vol. 39, pp. 1098, 1977.  T. A. Skotheim, Handbook of Conducting Polymers M. Dekker, New York, 1986.  C. W. Tang, "Two-lay er organic photovoltaic cell," Appl. Phys. Lett. vol. 48, pp. 183-185, 1986.  H. Koezuka, A. Tsumur a, and T. Ando, "Field-effect transistor with polythiophene thin film," Syn. Met. vol. 18, pp. 699-704, 1987.  J. H. Burroughes, C. A. Jones, and R. H. Friend, "New semiconductor device physics in polymer diodes and transistors," Nature vol. 335, pp. 137-141, 1988.  G. Horowitz, D. Fichou, X. Peng, Z. Xu, and F. Garnier, "A fi eld-effect transistor based on conjugated alpha-sexithienyl," Solid State Comm. vol. 72, pp. 381-384, 1989.  C. W. Tang and S. A. VanSlyke, "Organic electroluminescent diodes," Appl. Phys. Lett. vol. 51, pp. 913-915, 1987.  C. W. Tang, S. A. VanSlyke, and C. H. Chen, "Electroluminescence of doped organic thin films," J. Appl. Phys. vol. 65, pp. 3610-3616, 1989.
153  J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Ho lmes, "Light-emitting diodes based on conjugated polymers," Nature vol. 347, pp. 539-541, 1990.  H. Klauk, M. Halik, U. Zschiesc hang, F. Eder, G. Schmid, and C. Dehm, "Pentacene organic transistors and ring oscillators on glass and on flexible polymeric substrates," Appl. Phys. Lett. vol. 82, pp. 4175-4177, 2003.  S. Uchida, J. Xue, B. P. Rand, and S. R. Forrest, "Organic small molecule solar cells with a homogeneously mixed copper phthalocyanine: C[sub 60] active layer," Appl. Phys. Lett., vol. 84, pp. 4218-4220, 2004.  Y. Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, "Stacked pentacene layer organic thin-film transistors with improved characteristics," Elec. Dev. Lett., IEEE, vol. 18, pp. 606-608, 1997.  J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau, and M. E. Thompson, "Synthesis and Characterization of Phosphorescent Cyclometalated Platinum Complexes," Inorg. Chem. vol. 41, pp. 3055-3066, 2002.  S. Lamansky, P. Djurovic h, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, and M. E. Thompson, "Highly Phosphorescent BisCyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes," J. Am. Chem. Soc. vol. 123, pp. 4304-4312, 2001.  S. Lamansky, P. I. Djurovich, F. Abdel-Razzaq, S. Ga ron, D. L. Murphy, and M. E. Thompson, "Cyclometalated Ir comp lexes in polymer organic light-emitting devices," J. Appl. Phys. vol. 92, pp. 1570-1575, 2002.  C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, "Nearly 100% internal phosphorescence efficiency in an organic light-emitting device," J. Appl. Phys. vol. 90, pp. 5048-5051, 2001.  G. He, C. Rothe, S. Mu rano, A. Werner, O. Zeika, and J. Birnstock, "White stacked OLED with 38 lm/W and 100,000hour lifetime at 1000 cd/m[sup 2] for display and lighting applications," J. Soc. Info. Disp., vol. 17, pp. 159-165, 2009.  D. Y. Kondakov, J. R. Sandifer, C. W. Tang, and R. H. Young, "33.1: Invited Paper: OLED Aging Mechanisms: from Fl uorescence Quenchers to Nonradiative Recombination Centers," SID Symp. Digest Tech. Paper vol. 34, pp. 1068-1071, 2003.  D. Y. Kondakov, W. C. Lenhart, and W. F. Nichols, "Operational degradation of organic light-emitting diodes: Me chanism and identification of chemical products," J. Appl. Phys. vol. 101, pp. 024512-7, 2007.
154  D. Y. Kondakov, J. R. Sandifer, C. W. Tang, and R. H. Young, "Nonradiative recombination centers and electrical aging of organic light-emitting diodes: Direct connection between accumulation of trapped charge and luminance loss," J. Appl. Phys. vol. 93, pp. 1108-1119, 2003.  H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M. Hor, and G. Xu, "Degradation Mechanism of Small Molecule-Based Organic Light-Emitting Devices," Science vol. 283, pp. 1900-1902, 1999.  H. Aziz and G. Xu, "A degradation mechanism of or ganic light-emitting devices," Syn. Met. vol. 80, pp. 7-10, 1996.  L. Ke, C. Soo-Jin, K. Zhang, and N. Yakovlev, "Degradation and failure of organic light-emitting devices," Appl. Phys. Lett. vol. 80, pp. 2195-2197, 2002.  http://www.theage.com.au/news/articles/sony-to-ship-first-oled-ultrathintv/2007/10/02/119109 1082710.html, September 2009.  http://www.siftwire.com/samsun g-create-a-bendable-oled-screen.html, September 2009.  J. Kido, "High Performance OLEDs for General Lighting," presented at SPIE Optics and Photonics,, San Diego, 2009.  J. Nelson, The Physics of Solar Cells : Imperial College Press, 2003.  S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, and A. J. Heeger, "Bulk het erojunction solar cells with internal quantum efficiency approaching 100%," Nat. Photon. vol. 3, pp. 297-302, 2009.  "Solarmer Sets new worl d record for 7.6% efficient OPV, OSA Direct Newsletter, Vol. 7, Issue 40 October 26,2009."  D. Y. Kim, F. So, and Y. Gao, "Aluminum phthalo cyanine chloride/C60 organic photovoltaic cells with high open-circuit voltages," Sol. Energy Mater. Sol. Cells vol. 93, pp. 1688-1691, 2009.  P. Peumans, V. Bulovic, and S. R. Forrest, "Efficient, high-bandwidth organic multilayer photodetectors," Appl. Phys. Lett. vol. 76, pp. 3855-3857, 2000.  P. Peumans, A. Yakimo v, and S. R. Forrest, "Small molecular weight organic thin-film photodetectors and solar cells," J. Appl. Phys. vol. 93, pp. 3693-3723, 2003.  M. Halik, H. Klauk, U. Zschieschang, T. Kriem, G. Schmid, W. Radlik, and K. Wussow, "Fully patterned all-organic thin film transistors," Appl. Phys. Lett., vol. 81, pp. 289-291, 2002.
155  C. R. Newman, C. D. Frisbie, D. A. da Silva Fil ho, J.-L. Bredas, P. C. Ewbank, and K. R. Mann, "Introduction to Organic Th in Film Transistors and Design of nChannel Organic Semiconductors," Chem Mater. vol. 16, pp. 4436-4451, 2004.  C. Rost, S. Karg, W. Riess, M. A. Loi, M. Murgia, and M. Muccini, "Ambipolar light-emitting organic field-effect transistor," Appl. Phys. Lett. vol. 85, pp. 16131615, 2004.  K. Kudo, S. Tanaka, M. Iizuka, and M. Nakamura, "F abrication and device characterization of organic li ght emitting transistors," Thin Solid Films vol. 438439, pp. 330-333, 2003.  M. Chikamatsu, Y. Ic hino, N. Takada, M. Yoshida, T. Kamata, and K. Yase, "Light up-conversion from near-infrared to blue using a photoresponsive organic light-emitting device," Appl. Phys. Lett. vol. 81, pp. 769-771, 2002.  V. G. Kozlov, V. Bulovic, P. E. Bu rrows, and S. R. Forrest, "Laser action in organic semiconductor waveguide and doub le-heterostructure devices," Nature vol. 389, pp. 362-364, 1997.  N. Tessler, G. J. Dent on, and R. H. Friend, "Lasi ng from conjugat ed-polymer microcavities," Nature vol. 382, pp. 695-697, 1996.  M. Berggren, A. Dodabalap ur, R. E. Slusher, and Z. Bao, "Light amplification in organic thin films using cascade energy transfer," Nature vol. 389, pp. 466-469, 1997.  S. Moller, C. Perlov, W. Jackson, C. Taussig, and S. R. Forrest, "A polymer/semiconductor write-once read-many-times memory," Nature vol. 426, pp. 166-169, 2003.  L. Ma, S. Pyo, J. Ouyang, Q. Xu, and Y. Yang, "Nonvolatile electrical bistability of organic/metal-nanocluster/organic system," Appl. Phys. Lett. vol. 82, pp. 14191421, 2003.  B. Crone, A. Dodaba lapur, A. Gelperin, L. Torsi, H. E. Katz, A. J. Lovinger, and Z. Bao, "Electronic sensing of vapors with organic transistors," Appl. Phys. Lett. vol. 78, pp. 2229-2231, 2001.  J. A. Covington, J. W. Gardner, D. Briand, and N. F. de Rooij, "A polymer gate FET sensor array for det ecting organic vapours," Sens. and Actu. B. Chem. vol. 77, pp. 155-162, 2001.  P. Atkins and J. d. Paula, Physical Chemistry 7th ed: W. H. Freeman, 2001.  http://www.olympusmicro.com/primer /java/jablonsk i/jabintro/index.html, September 2009.
156  J. D. Wright, Molecular Crystals 2nd ed: Cambridge University Press, 1995.  P. Atkins and R. Friedman, Molecular Quantum Mechanics 4 ed. Oxford: Oxford University Press, 2005.  C. Kittel, Introduction to Solid State Physics 7 ed. New York: John Wiley and Sons, Inc.  http://www.orgworld.de/, September 2009.  http://www.bris.ac.uk/synapt ic/info/imaging/imaging_1. htm, September 2009.  http://www.adsdyes.com/products/pdf/homopolymers/ADS129BE_ DATA.pdf," September 2009.  http://www.umich.edu/~protein/AP/rtp.html, Septemer 2009.  V. Bulovic, "Lecture notes on Organic Electronics."  J.-S. Kim, P. K. H. Ho, N. C. Greenham, and R. H. Friend, "Electroluminescence emission pattern of organi c light-emitting diodes: Implications for device efficiency calculations," J. Appl. Phys. vol. 88, pp. 1073-1081, 2000.  R. D. Zakia and L. Stroebel, Focal Encyclopedia of Photography 3 ed: Focal Press, 1996.  S. R. Forrest, D. D. C. Bradley, and M. E. Thompson, "Measuring the Efficiency of Organic Light-Emitting Devices," Adv. Mater. vol. 15, pp. 1043-1048, 2003.  M. A. Baldo, D. F. O' Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, "Highly efficient phosphorescent emission from organic electroluminescent devices," Nature vol. 395, pp. 151-154, 1998.  V. I. Adamovich, S. R. Cordero, P. I. Djurovich, A. Tamayo, M. E. Thompson, B. W. D'Andrade, and S. R. Forrest, "New charge-carrier blocking materials for high efficiency OLEDs," Org. Electron. vol. 4, pp. 77-87, 2003.  B. C. Krummacher, V.-E. Choong, M. K. Mathai, S. A. Choulis, F. So, F. Jermann, T. Fiedler, and M. Zachau, "Highl y efficient white organic light-emitting diode," Appl. Phys. Lett. vol. 88, pp. 113506, 2006.  P. Schlotter, R. Schmidt, and J. Schneider, "Luminescence conversion of blue light emitting diodes," Appl. Phys. A: Mater. Sci. & Proc. vol. 64, pp. 417-418, 1997.  A. R. Duggal, J. J. Shi ang, C. M. Heller, and D. F. Foust, "Organic light-emitting devices for illumination quality white light," Appl. Phys. Lett. vol. 80, pp. 34703472, 2002.
157  D. Tanaka, H. Sasabe, Y.-J. Li, S.-J Su, T. Takeda, and J. Kido, "Ultra High Efficiency Green Organic Li ght-Emitting Devices Jap. J. Appl. Phys. vol. 46, pp. L10-L12, 2007.  D. Tanaka, Y. Agata, T. Takeda, S. Watanabe, and J. Kido, "High luminous efficiency blue organic light-emitting devic es using high triplet excited energy materials," Japanese Journal of Applied Physics Part 2-Letters & Express Letters vol. 46, pp. L117-L119, 2007.  N. Chopra, J. Lee, Y. Zheng, S.-H. Eo m, J. Xue, and F. So "High efficiency blue phosphorescent organic lig ht-emitting device," Applied Physics Letters vol. 93, pp. 143307-3, 2008.  C. Adachi, R. Kwong, C, P. Djurovich, V. Adam ovich, M. Baldo, A M. Thompson, E, and S. Forrest, R "Endothe rmic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials," vol. 79: AIP, 2001, pp. 2082-2084.  R. J. Holmes, S. R. Forrest, Y. J. T ung, R. C. Kwong, J. J. Brown, S. Garon, and M. E. Thompson, "Blue organic electr ophosphorescence using exothermic host-guest energy transfer," Appl. Phys. Lett., vol. 82, pp. 2422-2424, 2003.  S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, and F. Sato, "Confinement of triplet energy on phosphorescent molecules for highly-efficient organic blue-lightemitting devices," Applied Physics Letters, vol. 83, pp. 569-571, 2003.  J. Lee, N. Chopra, and F. So, "Cavity effects on li ght extraction in organic light emitting devices," Applied Physics Letters, vol. 92, pp. 033303, 2008.  R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G. H. Dohler, B. Dutta, M. Kuijk, G. Borghs, and P. Heremans, "Impact of texture-enhanced transmission on high-efficiency surfac e-textured light-emitting diodes," Appl. Phys. Lett. vol. 79, pp. 2315-2317, 2001.  N. Toshitaka, T. Naoto, J. Nori yuki, and F. Hironaka, "Thin-film waveguiding mode light extraction in organic electrolum inescent device using high refractive index substrate," J. Appl. Phys. vol. 97, pp. 054505, 2005.  Y. Sun and S. R. Forrest, "Or ganic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography," J. Appl. Phys. vol. 100, pp. 073106, 2006.  Y. Sun and S. R. Forrest, "Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids," Nat Photon vol. 2, pp. 483-487, 2008.  C.-L. Lin, H.-W. Lin, and C.-C. Wu, "E xamining microcavity organic light-emitting devices having two metal mirrors," Applied Physics Letters vol. 87, pp. 021101, 2005.
158  J. Lee, N. Chopra, S.-H Eom, Y. Zheng, J. Xue, F. So, and J. Shi, "Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices," Appl. Phys. Lett. vol. 93, pp. 123306, 2008.  J.-H. Lee, T.-C. Lin, C. -C. Liao, and F. H. Yang, "S tudy on organic light-emitting device with more balanced charge tr ansport," presented at SPIE, 2005.  V.-E. Choong, S. Shi, J. Curless, C.-L. Shieh, H. C. Lee, F. So, J. Shen, and J. Yang, "Organic light-emitting diodes with a bipolar transport layer," Appl. Phys. Lett., vol. 75, pp. 172-174, 1999.  N. Chopra, J. Lee, Y. Zheng, S.-H. Eo m, J. Xue, and F. So "Effect of charge balance on high efficiency blue phosphores cent organic light emitting diodes," Accepted for publication, ACS Applied Materials & Interfaces 2009.  http://www.prismalenceuk.com/light_vision, September 2009.  http://www.silicalighting.eu/ q-a/q-a/the-cie-diagram/, S eptember 2009.  www.lumtec.com.tw, September 2009.  D. F. O. Brien, M. A. Baldo, M. E. Thompson, and S. R. Forrest, "Improved energy transfer in electrophosphorescent devices," Appl. Phys. Lett. vol. 74, pp. 442-444, 1999.  M. A. Baldo and S. R. Forre st, "Transient analysis of organic electrophosphorescence: I. Transient anal ysis of triplet energy transfer," Physical Review B vol. 62, pp. 10958, 2000.  M. A. Baldo, C. Adachi, and S. R. Forrest, "Transient analysis of organic electrophosphorescence. II. Transient analysis of triple t-triplet annihilation," Phys. Rev. B vol. 62, pp. 10967, 2000.  K. Goushi, R. Kwong, J. J. Brown, H. Sasabe, and C. Adachi, "Triplet exciton confinement and unconfinement by adjacent hole-transport layers," J. Appl. Phys. vol. 95, pp. 7798-7802, 2004.  J. Kido, I. Nobuhiro, L. Yanjun, Y. Agata, and H. Shimizu, "Application of Electron-transporting Wide Energy Gap Mate rial for Organic Electroluminescent Devices," 2005.  M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. C. Wu, F. C. Fang, Y. L. Liao, K. T. Wong, and C. I. Wu, "Highly efficient organic blue electrophosphorescent devices based on 3,6-bis(triphenylsilyl)carbazole as the host material," Adv. Mater. vol. 18, pp. 1216-+, 2006.
159  Y. Kawamura, K. Goushi, J. Brooks, and J. J. Brown, "100% phosphorescence quantum efficiency of Ir(III) comple xes in organic semiconductor films," Appl. Phys. Lett. vol. 86, pp. 071104, 2005.  Q. Xin, W. L. Li, W. M. Su, T. L. Li, Z. S. Su, B. Chu, and B. Li, "Emission mechanism in organic light-emitting devices comprising a europium complex as emitter and an electron trans porting material as host," J. Appl. Phys. vol. 101, pp. 044512, 2007.  S. Naka, H. Okada, H. Onnagawa, and T. Tsutsui, High electron mobility in bathophenanthroline," Appl. Phys. Lett. vol. 76, pp. 197-199, 2000.  M. K. Fung, Z. Xie, S. T. Lee, L. S. Hung, J. Shi, and Y. Q. Li, "An Efficient Pure Blue Organic Light-Emitting Device with Low Driving Voltages," Adv. Mater., vol. 14, pp. 1317-1321, 2002.  P. Strohriegl and J. V. Grazulevicius, "Charge-Transporting Molecular Glasses," Advanced Materials, vol. 14, pp. 1439-1452, 2002.  X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest, and M. E. Thompson, "Ultrahigh Energy Gap Hosts in D eep Blue Organic Electrophosphorescent Devices," Chem. Mater. vol. 16, pp. 4743-4747, 2004.  L. Jaewon, C. Neetu, E. Sang-Hyun, Z. Ying, X. Jiangeng, S. Franky, and S. Jianmin, "Effects of trip let energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices," Applied Physics Letters vol. 93, pp. 123306, 2008.  R. J. Holmes, B. W. D. Andrade, S. R. Forrest, X. Ren, J. Li, and M. E. Thompson, "Efficient, deep-blue organi c electrophosphorescence by guest charge trapping," Appl. Phys. Lett. vol. 83, pp. 3818-3820, 2003.  K. Yuichiro, G. Kenichi, B. Ja son, and J. B. Julie, "100% phosphorescence quantum efficiency of Ir(III) comple xes in organic semiconductor films," Applied Physics Letters, vol. 86, pp. 071104, 2005.  Y. Kawamura, H. Sasabe, and C. Adachi, "Simple Accurate System for Measuring Absolute Photoluminescenc e Quantum Efficiency in Organic SolidState Thin Films," Jap. J. Appl. Phys. vol. 43, pp. 7729-7730, 2004.  N. C. Giebink and S. R. Forrest, "Quantum efficiency ro ll-off at high brightness in fluorescent and phosphorescent organic light emitting diodes," Phys. Rev.B (Cond. Matter and Mater. Phys.) vol. 77, pp. 235215, 2008.  Y. Q. Li, M. K. Fung, Z. Xie, S. T. Lee, L. S. Hung, and J. Sh i, "An Efficient Pure Blue Organic Light-Emitting Device with Low Driving Voltages," Adv. Mater., vol. 14, pp. 1317-1321, 2002.
160  S.-H. Eom, Y. Zheng, N. Chopra, J. Lee, F. So, and J. Xue, "Low voltage and very high efficiency deep-blue phosphores cent organic light-emitting devices," Appl. Phys. Lett. vol. 93, pp. 133309, 2008.  S.-H. Eom, Y. Zheng, E. Wrzesniewski J. Lee, N. Chopra, F. So, and J. Xue, "Effect of electron injection and transport materials on efficiency of deep-blue phosphorescent organic light-emitting devices," Org. Electron. vol. 10, pp. 686691, 2009.  D. Tanaka, T. Takeda, T. Chiba, S. Watanabe, and J. Kido, "Novel Electrontransport Material Containing Boron Atom with a High Triplet Excited Energy Level," Chemistry Letters vol. 36, pp. 262-263, 2007.  S.-J. Su, H. Sasabe, T. Takeda, and J. Kido, "Pyridine-Containing Bipolar Host Materials for Highly Efficient Blue Phosphorescent OLEDs," Chemistry of Materials vol. 20, pp. 1691-1693, 2008.  H. Sasabe, E. Gonmori, T. Chiba, Y.-J. Li, D. Tanaka S.-J. Su, T. Takeda, Y.-J. Pu, K.-i. Nakayama, and J. Kido, "WideEnergy-Gap Electron-Transport Materials Containing 3,5-Dipyridylphenyl Moieties fo r an Ultra High Efficiency Blue Organic Light-Emitting Device," Chem Mater. vol. 20, pp. 5951-5953, 2008.  M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, "Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer," Appl. Phys. Lett. vol. 79, pp. 156-158, 2001.  G. T. Lei, L. D. Wang, L. Duan, J. H. Wang, and Y. Qiu, "Highly efficient blue electrophosphorescent devices with a novel host material," Syn. Met. vol. 144, pp. 249-252, 2004.  K. S. Yook and J. Y. Lee, "High efficiency and low efficiency roll off in white phosphorescent organic light-emitting di odes by managing host structures," Appl. Phys. Lett. vol. 92, pp. 193308-3, 2008.  S. H. Kim, J. Jang, K. S. Yook, and J. Y. Lee, "Stable efficiency roll-off in phosphorescent organic lig ht-emitting diodes," Appl. Phys. Lett., vol. 92, pp. 023513-3, 2008.  F.-M. Hsu, C.-H. Chien, P.-I. Shih and C.-F. Shu, "Phosphine-Oxide-Containing Bipolar Host Material for Blue Electrophosphorescent Devices," Chem Mater. vol. 21, pp. 1017-1022, 2009.  S.-C. Chang, G. He, F.-C. Chen, T.-F. Guo, and Y. Yang, "Degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes," Appl. Phys. Lett. vol. 79, pp. 2088-2090, 2001.
161  P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak, P. Djurovich, and M. E. Thompson, "Ultraviolet electrolumi nescence and blue-gr een phosphorescence using an organic diphosphine oxide charge transporting layer," vol. 88: AIP, 2006, pp. 183503.  A. B. Padmaperuma, L. S. S apochak, and P. E. Burrows, "New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2,7-Bis(diphenylphosphine oxide)-9,9dimethylfluorene," Chem Mater. vol. 18, pp. 2389-2396, 2006.  L. S. Sapochak, A. B. Padmaperuma, X. Cai, J. L. Male, and P. E. Burrows, "Inductive Effects of Diphenylphosphoryl Moieties on Carbazole Host Materials: Design Rules for Blue Electrophosphore scent Organic Light-Emitting Devices J. Phys. Chem. C vol. 112, pp. 7989-7996, 2008.  A. D. Becke, "Density-functional thermochemistry. III. The role of exact exchange," J. Chem. Phys. vol. 98, pp. 5648-5652, 1993.  X. Cai, A. B. Padmaperuma, L. S. Sapochak, P. A. Vecchi, and P. E. Burrows, "Electron and hole transport in a wide ba ndgap organic phosphine oxide for blue electrophosphorescence," Appl. Phys. Lett. vol. 92, pp. 083308, 2008.  P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak, P. Djurovich, and M. E. Thompson, "Ultraviolet electrolumi nescence and blue-gr een phosphorescence using an organic diphosphine oxid e charge transporting layer," Appl. Phys. Lett., vol. 88, pp. 183503, 2006.  A. B. Padmaperuma, L. S. Sapoc hak, and P. E. Burrows, "A New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2, 7Bis(diphenylphosphine oxide),9dimethylfluorene," Chem. Mater. pp. Medium: X, 2006.  L. S. Sapochak, A. B. Padmaperuma, P. A. Vecchi, H. Qiao, and P. E. Burrows, "Design strategies for achieving high triplet energy electron transporting host materials for blue electrophosphorescence," 2006.  Z. Q. Gao, M. Luo, X. H. Sun, H. L. Ta m, M. S. Wong, B. X. Mi, P. F. Xia, K. W. Cheah, and C. H. Chen, "New Host Cont aining Bipolar Carrier Transport Moiety for High-Efficiency Electrophosphorescence at Low Voltages," Adv. Mater., vol. 21, pp. 688-692, 2009.  X. Cai, A. B. Padmaperuma, L. S. Sapochak, P. A. Vecchi, and P. E. Burrows, "Electron and hole transport in a wide ba ndgap organic phosphine oxide for blue electrophosphorescence," Appl. Phys. Lett. vol. 92, pp. 083308-3, 2008.  N. Chopra, J. Lee, Y. Zheng, S.-H. Eom, J. Xue, and F. So, "Effect of the Charge Balance on High-Efficiency Blue-Phosphor escent Organic Light-Emitting Diodes," ACS Appl. Mater. Int. vol. 1, pp. 1169-1172, 2009.
162  S. H. Kim, J. Jang, and J. Y. Lee, "High efficien cy phosphorescent organic lightemitting diodes using carbazole-ty pe triplet exciton blocking layer," Appl. Phys. Lett., vol. 90, pp. 223505-3, 2007.  E. Polikarpov, J. S. Swensen, N. Chopra, F. So, and A. B. Padmaperuma, "An ambipolar phosphine oxide-based hos t for high power efficiency blue phosphorescent organic light emitting devices," Appl. Phys. Lett. vol. 94, pp. 223304-3, 2009.  G. Hughes and M. R. Bryce, "Elect ron-transporting materials for organic electroluminescent and elec trophosphorescent devices," J. Mat. Chem. vol. 15, pp. 94-107, 2005.  S. Shi-Jian, C. Takayuki, T. Taka shi, and K. Junji, "P yridine-Containing Triphenylbenzene Derivatives with High El ectron Mobility for Highly Efficient Phosphorescent OLEDs," Adv. Mater. vol. 20, pp. 2125-2130, 2008.  S. W. Liu, C. A. Huang, J. H. Lee, K. H. Yang, C. C. Chen, and Y. Chang, "Blue mixed host organic light emitting devices," Thin Solid Films vol. 453-454, pp. 312-315, 2004.  J.-H. Lee, W. C. I., S.-W. Liu, C. -A. Huang, and Y. Chang, "Mixed host organic light-emitting devices with low driving voltage and long lifetime," Appl. Phys. Lett. vol. 86, pp. 103506, 2005.  S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, and K. Leo, "White organic light-emitting diodes with fluorescent tube efficiency," Nature vol. 459, pp. 234-238, 2009.  J.-J. Kim and S.-Y. Kim, "Theoretical analysis of the optical outcoupling efficiency in organic light emitting diodes," pres ented at SPIE Optics and Photonics, San Diego, 2009.  V.-E. Choong, S. Shi, J. Curless, and F. So, "Bipol ar transport organic light emitting diodes with enhanced reliability by LiF doping," Appl. Phys. Lett. vol. 76, pp. 958-960, 2000.
163 BIOGRAPHICAL SKETCH Neetu Chopra was born and raised in De lhi, India. She graduated from High school in the year 2001 with a dream to make something of herself. With this dream in her eyes, she applied to various engineering colleges in Indi a. She joined Delhi College of Engineering soon afterward as a polymer science and chemical technology major. While at Delhi College of Engineering, she interned at Na tional Physical Laboratory, Delhi India which was her first tryst with ma terials science research. She worked on two major projects one project was focused on synthesis of silicon carbide nanofibers using sol-gel technique and the other project was focused on OLEDs. The experience of working in an advanced research environment r eally helped her to find her own niche. During the senior year of her undergraduat e studies, she was faced with a difficult choice, one which would shape her future car eer. She had the option of taking up a job or going for graduate studies. Graduate studies essentially meant leaving the comfort of her home in Delhi in pursuit of a school which offered best in terms of research opportunity and a research mentor. Being academically oriented, she was convinced that further education in her field of interest would be mo st rewarding for her long term career goals though having multiple enticing job offers closer to home did not make the decision easier. She applied and was admitted to the graduat e program in materials science and engineering at the Univ ersity of Florida in 2005. During the course of her graduate studies she had the opportunity to work with various organic devices such as OLEDs, organic solar cells, devices using down conversion and upconversion of ener gy to operate. She learnt to use different types of equipment and characterization techniques to fabricate devices and study materials and device properties and their correlation with each other.
164 She also had an opportunity to work at Pacific Northwest National Laboratory, Richland, WA, in the spring of 2009, which she immensely enjoyed. Not only was she able to collaborate with exceptional researcher s from different backgrounds but she also was able to interact and live with all intern s from different backgrounds and nationalities which was a great learning experience in itself. Upon completion of this degree, Neetu hopes to continue to research actively in the area of Organic Electroni cs. Right now she is trying to choose between a few postdoctoral opportunities that have been pres ented to her. She feels that god has been kind to her always; and, even though destinations might seem distant but the journey so far has been very rewarding.