High Efficiency Blue Phosphorescent Organic Light Emitting Diodes
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HIGH EFFICIENCY BLUE PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODES By NEETU CHOPRA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009 1 © 2009 Neetu Chopra 2 To my Family and Sushant 3 ACKNOWLEDGMENTS A dissertation is almost never a solitary effort and neither is this one. As Ludwig Wittgenstein wisely said “knowledge in the end is based on acknowledgement”. Hence, writing this dissertation would be meaningless 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 stimulating discussions with Dr. So and my group members Jaewon Lee, Kaushik Roy Choudhury, Doyoung Kim, Dongwoo song, Cephas Small, Alok Gupta, Galileo Sarasqueta, Jegadesan Subbiah, Mike Hartel, Mikail Shaikh, Song Chen, Pieter De Somer, Verena Giese, Daniel S. Duncan, Jiyon Song, Fredrick Steffy, Jesse Manders, Nikhil Bhandari and their contribution to these pages cant be acknowledged enough. I am especially thankful to Dr. Jiangeng 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 DOE 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 sincere thanks to the OLED team managers Dan Gaspar and Mark Gross at Pacific Northwest 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 work would have possible without the help of James S. Swensen, Eugene Polikarpov, Lelia Cosimbescu, Charles C. Bonham, Liang 4 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, Namita, 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 alien land wondering if I will ever fit in and I found my place right from the start. For giving me this comfort, a large number of friends played a very big role and I would like to thank everyone Purushottam, Vibahva, Richa, Aniruddh, Arul, Praneetha, Karamjit, 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 times. I probably would have never made it to Gainesville, if it was not for the 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 encouragement, I would not be obtaining this degree. Lastly, I would like to thank my family, mummy, papa and didi for their belief in me through all these years, for always encouraging me to pursue my dreams, for enduring 5 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. 6 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES .......................................................................................................... 10 LIST OF FIGURES ........................................................................................................ 11 LIST OF ABBREVIATIONS ........................................................................................... 15 ABSTRACT ................................................................................................................... 17 CHAPTER 1 INTRODUCTION: BRIEF REVIEW OF ORGANIC ELECTRONIC MATERIALS .... 19 1.1 Organic Semiconductors ................................................................................... 19 1.1.1 Interest in Organic Semiconductors ........................................................ 21 1.1.2 Organic Semiconductors: Two General Classes ..................................... 22 1.1.3 Why Organic Semiconductors: Advantages and Disadvantages ............. 23 1.2 Organic Semiconductor Devices ....................................................................... 25 1.2.1 Organic Light Emitting Diodes (OLEDs) .................................................. 25 1.2.2 Organic Photovoltaic Devices (OPVs) ..................................................... 27 1.2.3 Other Devices Based on Organic Semiconductors ................................. 28 1.3 Physics of Organic Semiconductors: Fundamentals and Processes ................ 29 1.3.1 Optical Properties .................................................................................... 29 1.3.1.1 Electronic processes in molecules ................................................. 30 1.3.2 Electrical Properties ................................................................................. 31 1.3.2.1 Charge carrier transport ................................................................. 31 1.3.2.2 Excitons ......................................................................................... 33 1.3.2.3 Energy and charge transfer in molecules ....................................... 34 1.4 Summary .......................................................................................................... 35 2 ORGANIC LIGHT EMITTING DIODES ................................................................... 46 2.1 Need for OLEDs ............................................................................................... 46 2.1.1 Display Applications ................................................................................ 46 2.1.2 Lighting Applications ................................................................................ 47 2.2 OLED Lighting: Standard Terms and Definitions .............................................. 47 2.2.1 Introduction .............................................................................................. 47 2.2.2 Basic Concepts ....................................................................................... 48 2.2.2.1 Luminance flux ............................................................................... 48 2.2.2.2 Lambertian source ......................................................................... 49 2.2.2.3 Human eye response ..................................................................... 49 2.2.2.4 Color correlated temperature ......................................................... 50 2.2.2.5 Color rendering index ..................................................................... 50 7 2.2.2.6 CIE color coordinates ..................................................................... 51 2.3 Device Parameters and Device Efficiency Measurements ................................ 52 2.3.1 Current Efficiency .................................................................................... 53 2.3.2 Power Efficiency ...................................................................................... 53 2.3.3 External Quantum Efficiency ................................................................... 54 2.4 OLEDs: From History to Current State of the Art OLEDs .................................. 54 2.4.1 Phosphorescent materials ....................................................................... 55 2.4.2 Weak Link in High Efficiency White OLEDs: Blue ................................... 56 2.4.3 Blue Phosphorescent OLEDs .................................................................. 58 2.5 Review of Factors Limiting Blue OLED Performance ........................................ 59 3 EFFECT OF TRIPLET ENERGY CONFINEMENT ON PERFORMANCE OF BLUE PHOPHORESCENT OLEDS ........................................................................ 65 3.1 The Problem: Triplet Energy Confinement ........................................................ 65 3.2 Triplet Exciton Confinement with Charge Transport Layers .............................. 68 3.2.1 Hole Transport Layer ............................................................................... 68 3.2.2 Electron Transport Layer ......................................................................... 70 3.3 Host-Dopant Effect............................................................................................ 71 3.4 Summary .......................................................................................................... 73 4 EFFECT OF GUEST-HOST INERACTIONS .......................................................... 82 4.1 FIrpic