CHARGES AND EXCITONS IN ORGANIC AND PERVOSKITE LIGHT EMITTING MATERIALS AND DEVICES BY JUNWEI XU A Dissertation Submitted to the Graduate Faculty of WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Physics May, 2018 Winston-Salem, North Carolina Approved By: David L. Carroll, Ph.D., Advisor Scott M.Geyer, Ph.D., Chair Martin Gulthold, Ph.D. K. Burak Üçer, Ph.D. Richard T. Williams, Ph.D. Dedication and Acknowledgements Most of all, I would like to thank my advisor, Dr. David Carroll, a talented and passionate scientist who took me into Wake Forest University back in 2013. For that, I am very thankful for the fresh new opportunity he offered that I never thought I would have. During my thesis work, I thank him for appreciating my research strengths and patiently encouraging me to improve in my weaker areas. His strong support of my own ideas and research directions, and confidence in my abilities were benefits not all thesis students enjoy (but should). I am also very much indebted to Dr. Richard Williams, who is very patient and thoughtful to guide me to understand these exciting findings of perovskite materials. My time working on optical measurement in ultrafast laser lab was extremely positive and fun. I also would like to thank the members of my dissertation committee, Dr. Burak Üçer, Dr. Scott Geyer, and Dr. Martin Gulthold, not only for their time and patience, but for their intellectual contributions to my development as an independent researcher. To my lab buddies: Yonghua Chen, Corey Hewitt, Jiwen Liu, Gregory Smith, Yuan Li, Wenxiao Huang, Chaochao Dun, Hyunsu Lee, Yue Cui, Drew Onken, Yang Guo, Gabriel Marcus, Linqi Shao, Stephen McCarthy, Tianxiao Shen and so many others I can’t list here. Thanks for all the fun, great company, discussion and helps every day in lab and office. Without their strong opinions and sharp insights, my research project might not have ever come to fruition. I cannot adequately express how thankful I am. Finally, but not least, I want to thank my parents. I thank my father for encouraging me to be an independent thinker, and having confidence and courage to go II after new things that may sound crazy. I thank my mother for teaching me to face the complex world and try to leave the world just a little better than when came into it, and how my dream should a worthy part of that pursuit. And my most heartfelt thanks to my wife, Peiyun. Dedicated to the most intellectually exciting and memorable 5 years in my life. 2013-2018. III Table of Contents List of Illustrations and Tables………………….………………………………………VII Table of Abbreviations………………………………………………………………..XXII Abstract…………………………………………………………………………...….XXIV Chapter 1 Basics of Radiative Emission in Semiconductors .............................................. 1 Spontaneous Emission .................................................................................................... 2 Charge Carrier Transport ................................................................................................ 4 Exciton Generation and Recombination ......................................................................... 9 The Intersystem Crossing (ISC) Model in Singlet-Triplet Dynamics .......................... 12 Exciton Oscillator strength ........................................................................................... 13 Lead Halide Perovskite Emitter .................................................................................... 15 Chapter 2 Charge Balance in AC- Field Driven Organic Light Sources .......................... 18 Introduction of Alternating Current Driven Organic Electroluminescence (AC-OEL) Device ........................................................................................................................... 18 Materials and Characterization ..................................................................................... 21 Carrier Gate Structure in AC-OEL Device ................................................................... 22 Gate Carrier Valve Mechanism .................................................................................... 28 No Gate vs Semiconductor Gate vs Insulator Gate ...................................................... 33 Chapter Summary ......................................................................................................... 38 Chapter 3 Tailoring Spin Mixtures in Color-Tunable AC-OEL Devices ......................... 39 Spin Remixing in Electroluminescence ........................................................................ 39 Materials and Methods .................................................................................................. 42 Fluorescence-Phosphorescence (F-P) Hybrid Emission Layer .................................... 45 IV Maxwell Magnetic Field Coupling ............................................................................... 47 Frequency Controlled Color Tunability ........................................................................ 51 Ion Enhanced Color Tunability ..................................................................................... 60 Energy Transfer Mechanism of Magnetic Dependent ISC ........................................... 63 Chapter Summary ......................................................................................................... 68 Chapter 4 Solution Possessing Organic Small Molecular Emitter ................................... 70 Solution Based AC-OEL Devices ................................................................................. 70 Solubility of Alq3 in Different Solvents ....................................................................... 72 Morpholgy of Spun-cast Alq3 Thin Film and Wash-out Effect ................................... 75 Resonant Effect and Carrier Gate Mechanism in Alq3 Device .................................... 77 Methods and Characterizations ..................................................................................... 84 Chapter 5 Flexible, Stable, White Electroluminescent Devices Under High Frequency . 86 Challenges for Achieving Flexibility ............................................................................ 87 Construction of Metal/Nanowires Composite Electrode .............................................. 90 Bending Test on Nano-Network Composite Cathode .................................................. 97 Iterative Bending Tests on Devices with Al/MWCNTs/Al Electrode ........................ 103 Flexible Carrier Gate in White AC-EL ....................................................................... 104 N-type Polymeric Gate Structure and Application ..................................................... 106 Chapter Summary ....................................................................................................... 120 Chapter 6 Imbedded Nanocrystals of CsPbBr3 in Cs4PbBr6 Matrix in the Application of LED ................................................................................................................................. 121 Discovery of CsPbBr3-Cs4PbBr6 nanocompsite ......................................................... 121 Methods and Measurement ......................................................................................... 123 V Solution growth and properties of Cs-Pb-Br perovskite thin films. ........................... 126 CsPbBr3 and Cs4PbBr6 DMSO-washed powder samples. .......................................... 130 Morphology of Thin Films Comprised of CsPbBr3/Cs4PbBr6 Nano-inclusion Grains. .................................................................................................................................... 132 Light Emitting Diodes (LEDs) using CsPbBr3 nanocrystals in Cs4PbBr6 host. ......... 134 Optical response and kinetics of pure CsPbBr3 and the CsPbBr3/Cs4PbBr6 nano- inclusion composite under high-intensity photoexcitation. ........................................ 139 PL quantum efficiency and decay time dependence on size of the Cs4PbBr6 host micro- grain. ........................................................................................................................... 145 Enhanced Oscillator Strength of CsPbBr3 Imbedded Nanocrystals Relative to Extended CsPbBr3. ...................................................................................................................... 147 Chapter 7 Cs4PbBr6 Single Crystal with CsPbBr3 Nano-Inclusions ............................... 154 Growth of Cs4PbBr6 Single Crystal imbedded with CsPbBr3 Nanoinclusions .......... 155 Compositional analysis of the nano-composite .......................................................... 159 Exction Kinetics and Recombination Analysis .......................................................... 161 CsPbBr3 Phase Evolution in Host Matrix ................................................................... 168 Reference………………………………………………………………………...……..174 Curriculum Vitae……………………………………………………………………….206 VI List of Illustrations and Tables Figure 1-1 | Different lighting technologies. (a) Incandescent bulb; (Image courtesy of thorinside via Flickr) (b) Gas discharge tube; (Image courtesy of Tatalux Lighting) (c) Light emitting diode; (Image courtesy of Adafruit) (d) Organic light emitting device. (Image courtesy of LG Display) ......................................................................................... 1 Figure 1-2 | Scheme of electron hole recombination resulting