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Spectroscopic Studies of Exciton Electronic Structure and Charge Recombination in Solution Processed Semiconductors for Photovoltaics Mark E

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Spectroscopic Studies of Exciton Electronic Structure and Charge Recombination in Solution Processed Semiconductors for Photovoltaics Mark E Spectroscopic Studies of Exciton Electronic Structure and Charge Recombination in Solution Processed Semiconductors for Photovoltaics Mark E. Ziffer A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2018 Reading Committee: David S. Ginger, Chair David J. Masiello Munira Khalil Program Authorized to Offer Degree: Chemistry © Copyright 2018 Mark E. Ziffer ii Abstract University of Washington Spectroscopic Studies of Exciton Electronic Structure and Charge Recombination in Solution Processed Semiconductors for Photovoltaics Mark E. Ziffer Chair of the Supervisory Committee: Professor David S. Ginger Department of Chemistry Understanding the fundamental photophysical processes that occur in solution processed semiconductors is the key to realizing their full potential in optoelectronic devices and uncovering new design strategies for materials. In this thesis we study the fundamental photophysics of promising classes of solution processed semiconductor systems for photovoltaics: hybrid organic- inorganic perovskites and π-conjugated polymers, along with blends of π-conjugated polymers and small molecules. In hybrid perovskites, we will gain an understanding of the initial photoexcitations that occur upon absorption of light at the semiconductor band edge using electroabsorption spectroscopy. Through detailed analysis of the electric field modulated optical response we will determine the fundamental nature of excitons that form upon band edge photoexcitation, including their binding energy, their ionization under an electric field, and their tendency to scatter into free charge continuum states at room temperature. In π-conjugated polymers, we will study how chemical interactions in the solid state dictate the electronic properties of excitons that form upon photoexcitation. We will use a variety of spectroscopic techniques to gain a deep understanding of the through-bond (intrachain) and through-space (interchain) chromophore interactions that dictate how excitons delocalize in aggregates of polymers with different chemical structure. Finally, in blends of π-conjugated polymers and small iii molecules, we will use time resolved spectroscopy methods to study the pathways that photogenerated charges take to recombine to the ground state. We will find that radiative recombination pathways can be accessed by photogenerated free charges in an efficient photovoltaic blend, which has largely been overlooked in the field of organic photovoltaics and is likely a key to improving the design of more efficient materials systems. We hope that the conclusions drawn from the studies provided in this thesis will help contribute to the fundamental understanding of the photophysics in these important emerging photovoltaic systems, and hopefully, provide inspiration for new avenues of investigation. iv Table of Contents Abstract .......................................................................................................................................... iii List of Figures .............................................................................................................................. viii Dedication ...................................................................................................................................... xi Acknowledgments......................................................................................................................... xii Chapter 1: Introduction ................................................................................................................... 1 Section 1.1 Materials Systems Studied ....................................................................................... 2 Section 1.2 Fundamentals of Excited States in Semiconductors ................................................ 4 Section 1.3 Charge Recombination in Semiconductors ............................................................ 10 References ................................................................................................................................. 12 Chapter 2: Electroabsorption Spectroscopy Measurements of the Exciton Binding Energy, Electron-Hole Reduced Effective Mass, and Band Gap in the Perovskite CH3NH3PbI3 ............. 15 Section 2.1 Overview ................................................................................................................ 15 Section 2.2 Introduction ............................................................................................................ 15 Section 2.3 Low-field FKA response of CH3NH3PbI3 at room temperature ............................ 17 Section 2.4. Field ionization of the Wannier exciton at low temperature ................................ 22 Section 2.5. Conclusion ............................................................................................................ 23 Appendix A ............................................................................................................................... 24 References ................................................................................................................................. 25 Chapter 3: H- and J- Aggregate Characteristics and Exciton Polarizability in π-Conjugated Polymers ....................................................................................................................................... 31 v Section 3.1 Overview ................................................................................................................ 31 Section 3.2 Introduction ............................................................................................................ 31 Section 3.3 Chemical structures of P1 and P2 .......................................................................... 33 Section 3.4 Characterization of Aggregation in Dilute Solution .............................................. 34 Section 3.5 Characterizing Solid State Interactions with Spectroscopic Ellipsometry ............ 36 Section 3.6 Characterizing H- and J- Like Coupling with Photoluminescence Spectroscopy . 37 Section 3.7 Electroabsorption Spectroscopy ............................................................................ 41 Section 3.8 Conclusion ............................................................................................................. 44 Appendix B ............................................................................................................................... 45 Acknowledgments .................................................................................................................... 45 References ................................................................................................................................. 46 Chapter 4: Long-lived, Non-Geminate, Radiative Recombination of Photogenerated Charges in a Polymer:Small-Molecule Acceptor Photovoltaic Blend ............................................................... 50 Section 4.1 Overview ................................................................................................................ 50 Section 4.2 Introduction ............................................................................................................ 50 Section 4.3 Molecular properties of PCE10 and FIDTT-2PDI ................................................ 53 Section 4.4 Low non-radiative loss and efficient charge generation in PCE10:FIDTT-2PDI . 54 Section 4.5 Photoluminescence from Radiative Nongeminate Charge Recombination ........... 56 Section 4.6 Molecular and Microstructural Aspects of Low Voltage Loss .............................. 61 Section 4.7 Conclusion ............................................................................................................. 63 Appendix C ............................................................................................................................... 65 Acknowledgments .................................................................................................................... 65 References ................................................................................................................................. 66 vi Conclusion .................................................................................................................................... 72 Appendix A Supporting Information for Chapter 2 ...................................................................... 75 Experimental Methods .............................................................................................................. 75 Supplementary Information ...................................................................................................... 78 Supplementary References ..................................................................................................... 110 Appendix B: Supporting Information for Chapter 3 ................................................................... 113 Experimental Methods ............................................................................................................ 113 Supplementary Information .................................................................................................... 115 Supplementary References ..................................................................................................... 125 Appendix C: Supporting Information for Chapter 4 ..................................................................
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