©Copyright by Devendra Khatiwada 2019 All Rights Reserved Fabrication and Optimization of Single-Junction GaAs Thin Film Solar Cells on Epi-ready Flexible Metal Tapes for Low-cost Photovoltaics A Dissertation Presented to the Faculty of the Department of Mechanical Engineering University of Houston In Partial Fulfillment for the Requirements of the Degree Doctor of Philosophy in Materials Science and Engineering by Devendra Khatiwada August 2019 Fabrication and Optimization of Single-Junction GaAs Thin Film Solar Cells on Epi-ready Flexible Metal Tapes for Low-cost Photovoltaics Devendra Khatiwada Approved: Chair of the Committee Venkat Selvamanickam, Professor, Mechanical Engineering Committee Members: Haleh Ardebili, Associate Professor, Mechanical Engineering James K.Meen, Research Associate Professor, Chemistry Jae -Hyun Ryou, Associate Professor, Mechanical Engineering M e c Alamgir Karim, Associate Professor, h Material Science and Engineering a n i c Suresh K.Khator, Associate Dean, Alamgir Karim, Professor and a Cullen College of Engineering Director, Material Science and l Engineering E n g ii i n e e r Acknowledgements Firstly I would like to acknowledge my thesis advisor professor Venkat Selvamanickam for his persistent help, guidance and offering me opportunities to involve in various interesting project. The last four years has been an outstanding experience to work with him. His patience in research, scientific knowledge, hard work and technical skills always inspired me to continuously work in different projects. I am very lucky to have him as my advisor. It was a great pleasure to work with the excellent members in our research group. Special thanks to Dr.Pavel Dutta, Dr.Monika Rathi, Sicong Son, Carlos Favela, Sahil Sharma, Sara Pouladi for their great help during my research work. Special thanks to Sicong Son who was always enthusiastic to help me at any time. I appreciate help and guidance from Dr.Jae-Hyuan Ryou in each step of my research work. I would like to thank my committee member Dr.Alamgir Karim, Dr. James K.Meen and Dr.Haleh Ardebili. Thanks to Dr.Alamgir Karim for providing all the support needed from the department. I would specially thank to my family member for their immense support. My lovely wife, Yojana Ghimire has been extremely supportive in all the circumstances and through this entire process. Without her it would have been impossible to complete my goal of research. v Fabrication and Optimization of Single-Junction GaAs Thin Film Solar Cells on Epi-ready Flexible Metal Tapes for Low-cost Photovoltaics An Abstract of a Dissertation Presented to the Faculty of the Department of Mechanical Engineering University of Houston In Partial Fulfillment for the Requirements of the Degree Doctor of Philosophy in Materials Science and Engineering by Devendra Khatiwada August 2019 vi Abstract Properties like high efficiency, flexibility and light weight, resistance to UV radiation and moisture and a low temperature coefficient make gallium arsenide (GaAs) more favorable than the ubiquitously-used silicon for solar cells. In spite of their high efficiency, GaAs solar cells have found limited use in various application due to high cost of the GaAs or Ge wafer used. In an approach to produce low-cost GaAs solar cells, we have developed a technology to grow epitaxial semiconductor thin films on low-cost flexible epi-ready metal tapes that can replace the expensive wafers. The template layers on the epi-ready metal tapes are grown via a roll-to-roll process using Ion Beam Assisted Deposition (IBAD). Metal organic chemical vapor deposition (MOCVD) is used to epitaxially grow GaAs solar cells structure on ‘single-crystalline-like’ germanium film on epi-ready metal foils. These epitaxial GaAs films exhibit excellent crystalline alignment with high carrier mobility, excellent crystalline alignment and optoelectronic properties. These grown thin films were processed via photo-lithography, etching and contact deposition to fabricate single-junction (1J) GaAs solar cell devices. The fabricated solar cell underwent a process of cap layer removal (passivation) and anti-reflection coating. Efficiency greater than 6% with open circuit voltage (VOC) of 566mV, fill factor (FF) of 68%, short circuit 2 current density (JSC) of 17.4mA/cm was obtained. Steps were taken to improve the quality of p-n junction by improving the quality of Ge template and incorporating an intrinsic layer with p-i-n solar cell structures. A 2 device efficiency of 11.5 % with VOC of 566mV, FF of 68%, JSC of 17.4mA/cm was vii obtained at 1 Sun on improved Ge template, using CVD germanium instead of sputtered germanium used before. Solar cells fabricated with the improved p-i-n structure showed a −2 device efficiency of 13.2% at 1 sun with VOC of 650 mV, JSC of 28 mA cm , and FF of 72 %. These thin film GaAs photovoltaics, with further improvement in quality, can potentially lead to light-weight, inexpensive and scalable solar cell manufacturing. viii Table of Contents Acknowledgement……………………………………...……………………..…...….…..v Abstract………………………………………………………………….……………….vii Table of Contents………………………………...………………………..………….…..ix List of Figures……………………………………..………………....................…...…..xiv List of Tables……………………………………………...……………………...……..xix List of Abbreviation……………………………………………..…...…………….........xxi Chapter 1 Introduction ........................................................................................................ 1 1.1 Introduction to Solar cells ............................................................................. 1 1.1.1 Background............................................................................................. 1 1.2 Photovoltaics (PVs) ....................................................................................... 1 1.2.1 Semiconductors as photovoltaic materials ............................................. 3 1.3 The p-n junction in semiconductors .............................................................. 6 1.3.1 Mechanism and characteristic curve for p-n junctions under bias ......... 9 Chapter 2 Solar Cells: Structures and Characterizations .................................................. 13 2.1 Solar or Photovoltaic Cell ........................................................................... 13 2.2 Solar Cell Structure ..................................................................................... 15 2.3 Solar Cell Characteristics ............................................................................ 18 2.3.1 Equivalent circuit for solar cells ........................................................... 19 2.4 Solar cell parameters used for measurements ............................................. 21 ix 2.4.1 Short circuit current density (Jsc) .......................................................... 22 2.4.2 Open circuit voltage (Voc) .................................................................... 23 2.4.3 Leakage current (Io) ............................................................................. 24 2.4.4 Maximum power point (Pmax) ............................................................... 25 2.4.5 Fill Factor (FF) ..................................................................................... 25 2.4.6 Efficiency ............................................................................................. 25 2.4.6.1 Air Mass (AM) .............................................................................. 26 2.5 Series and shunt resistance in solar cell ...................................................... 27 2.6 Loss mechanism in solar cells ..................................................................... 29 2.6.1 Optical Loss .......................................................................................... 29 2.6.1.1 Absorption loss ............................................................................. 29 2.6.1.2 Reflection Loss ............................................................................. 31 2.6.2 Recombination loss .............................................................................. 33 2.6.2.1 Direct recombination .................................................................... 34 2.6.2.2 Auger recombination .................................................................... 34 2.6.2.3 Trap-assisted recombination ......................................................... 35 2.7 Conclusion ................................................................................................... 35 Chapter 3 III-V semiconductor material and solar cells fabrication and characterization 36 3.1 Introduction ................................................................................................. 36 3.2 Growth Methods of III-V Semiconductor ................................................... 37 x 3.2.1 Liquid-encapsulated Czochralski (LEC) Method ................................. 37 3.2.2 Float Zone and Bridgman ..................................................................... 38 3.2.3 Liquid Phase Epitaxy (LPE) ................................................................. 38 3.2.4 Molecular Beam Epitaxy ...................................................................... 39 3.2.5 Metal Organic Chemical Vapor Deposition (MOCVD) ...................... 40 3.3 III-V Single-junction Gallium Arsenide Solar Cells Structure and Design 42 3.4 III-V Multi -junction Solar Cell Structure and Design ............................... 44 3.5 Characterization tools for characterizing III-V materials and