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A Dissertation Entitled Solution Processed High Efficiency Thin Film A Dissertation entitled Solution Processed High Efficiency Thin Film Solar Cells: from Copper Indium Chalcogenides to Methylammonium Lead Halides by Zhaoning Song Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics ________________________________________ Dr. Michael J. Heben, Committee Chair ________________________________________ Dr. Robert W. Collins, Committee Member ________________________________________ Dr. Randy J. Ellingson, Committee Member ________________________________________ Dr. Terry P. Bigioni, Committee Member ________________________________________ Dr. Lawrence S. Anderson-Huang, Committee Member ________________________________________ Dr. Amanda Bryant-Friedrich, Dean College of Graduate Studies The University of Toledo August 2016 Copyright 2016, Zhaoning Song This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Solution Processed High Efficiency Thin Film Solar Cells: from Copper Indium Chalcogenides to Methylammonium Lead Halides by Zhaoning Song Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo August 2016 Photovoltaics (PV) is increasingly recognized as an important component of renewable energy sources after the rapid progress in the last decade due to increasing energy demand and reducing manufacturing costs. Despite the enormous growth of the PV market, the present solar technologies that are dominated by crystalline silicon are still limited by the relatively more expensive cost of electricity compared with power generation in the conventional fossil fuel plants. Consequently, there is an urgent need to increase the performance and reduce the manufacturing costs of solar cells. While the commercial thin film solar cells (CdTe and CuInGaSe2) have already demonstrated high efficiencies, the current fabrication processes heavily rely on intensive capital investment on expensive vacuum-based techniques. To reduce solar module costs, solution-processing techniques have been proposed as a promising route towards low cost, high throughput, large scale manufacturing of high efficiency thin film solar cells. In this thesis, we investigate the solution-processing of copper indium chalcogenides and methylammonium lead halides materials and their applications as high efficiency photovoltaic cells. iii In the first approach, we develop an ultrasonic spray deposition system to prepare the CuIn(S,Se)2 thin films. Spray deposition is a controllable, scalable, and high throughput process that is suitable for industrial manufacturing. Here we first explore the Cu-In-S films prepared by an aqueous precursor ink. By controlling the precursor composition, we fabricate PV devices consisting of the n-type In2S3 window and p-type CuInS2 absorber layers and demonstrate 2% efficiency in the preliminary devices. After replacing the aqueous ink by a hydrazine-based precursor solution and incorporating a selenization process, we are able to fabricate high quality CuIn(S,Se)2 thin film solar cells in both conventional substrate and the backwall superstrate configurations. The efficiency of 7.2% has been achieved in the sprayed CuIn(S,Se)2 devices in the substrate configuration. In the second approach, we investigate solution-processing of the inorganic-organic hybrid metal halide perovskites. We study the impact of reaction temperature and precursor composition on the formation of perovskite materials and propose a pseudo binary phase diagram to guide the processing of the materials. We develop a laser beam induced current (LBIC) technique to spatially resolve the photocurrent collection in the solution-processed devices. Processing defects and impurities phases have been identified as the origins of lower current generation. On the basis of these results, we apply advanced processing techniques in device processing and obtain the champion perovskite device with a 16% efficiency. Additionally, we image the photocurrent generated in the sub-cells of the Si/perovskite tandem devices. The result can be used to improve the design the device structure. Finally, to study the stability of perovskite solar cells, we investigate the spatial and temporal evolution of photocurrent collection across the devices and observe the partially reversible phase transition of perovskite in humid air. iv I dedicate my dissertation to my wife Yachun Zhang and my parents. Their love gives me the strength to advance and makes it all worthwhile. Acknowledgements First, I would like to express my sincere gratitude to my advisor Dr. Michael Heben for his wise advice and guidance during my research and study at The University of Toledo. I especially want to thank him for giving me the freedom to pursue my research interests and supporting me throughout the years. I would also like to thank Dr. Robert Collins, Dr. Randy Ellingson, Dr. Terry Bigioni, and Dr. Lawrence Anderson for serving on my committee and providing valuable feedback and suggestions. I am grateful to Dr. Adam Phillips for his countless efforts and contributions to my research. I also appreciate the help from our research group members and alumni, the faculty members, staff, and fellows of the Department of Physics and Astronomy and the Wright Center for Photovoltaics Innovation and Commercialization (PVIC). Additionally, I would also like to thank our collaborators, Dr. Antonio Abate, Dr. Ullrich Steiner, Jérémie Werner, Dr. Bjöern Niesen, Dr. Stefaan De Wolf, Dr. Yanfa Yan, Ilke Celik, Dr. Defne Apul, Dr. Paul Roland, Corey Grice, and Niraj Shrestha for making contributions to my research work. Finally, my special thanks also go to my friends Yao Xie, Xinxuan Tan, Yue Yu, Chuanxiao Xiao, Dr. Tingting Shi, Dr. Nanke Jiang, Dr. Zhu Wang, Dr. Rui Yang, Dr. Changyong Chen, and Dr. Zhi Ren whose friendships have sustained me. v Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ..............................................................................................................v Table of Contents ............................................................................................................... vi List of Tables ................................................................................................................... xi List of Figures ................................................................................................................... xii List of Abbreviations .........................................................................................................xx List of Symbols ............................................................................................................... xxii 1 Introduction ….. .......................................................................................................1 1.1 Background and Motivation ..............................................................................1 1.2 Focus of the Dissertation ...................................................................................7 1.3 Dissertation Overview .......................................................................................9 2 Device Fabrication and Characterization Methods ................................................13 2.1 Materials Preparation and Device Fabrication .................................................13 2.1.1 Ultrasonic Spray Deposition .............................................................13 2.1.2 Fabrication of CuInSe2 Based Solar Cells ........................................15 2.1.3 Fabrication of CH3NH3PbI3 Based Solar Cells .................................17 2.2 Materials and Device Characterization ............................................................18 2.2.1 Material Characterization Methods ...................................................18 2.2.2 Device Characterization Methods .....................................................18 2.2.3 Laser Beam Induced Current (LBIC) ...........................................................22 vi 3 Spray Pyrolysis of Copper Indium Sulfide Thin Film Solar Cells ........................26 3.1 Introduction and Motivation ............................................................................26 3.1.1 History of CuInS2 Based Solar Cells................................................26 3.1.2 Structural Properties of CuInS2 Based Materials ..............................28 3.2 Cu-In-S Thin Films Prepared by Spray Pyrolysis ...........................................31 3.2.1 Mechanism of Spray Pyrolysis of Cu-In-S Thin Films ....................31 3.2.2 Spray Pyrolysis of Cu2-xS Thin Films ...............................................33 3.2.3 Spray Pyrolysis of CuInS2 and In2S3 thin films ................................35 3.3 Spray Pyrolysis of CuInS2/In2S3 Thin Film Solar Cells ..................................40 3.4 Factors Limited the Performance of the CuInS2 Solar Cells ...........................44 3.5 Conclusion .......................................................................................................46 4 Hydrazine Based Copper Indium Selenide Thin Film Solar Cells ........................48 4.1 Introduction and Motivation ............................................................................48 4.2
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