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A Dissertation Entitled Development of Back Contacts for Cdte Thin Films Solar Cells by Fadhil Khalaf Dahash Alfadhili Submit A Dissertation entitled Development of Back Contacts for CdTe Thin Films Solar Cells by Fadhil Khalaf Dahash Alfadhili 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. Randy J. Ellingson, Committee Member ___________________________________________ Dr. Robert Collins, Committee Member Dr. Nikolas Podraza, Committee Member Dr. Daniel Georgiev, Committee Member ___________________________________________ Dr. Amanda C. Bryant-Friedrich, Dean College of Graduate Studies The University of Toledo May 2020 Copyright 2020 Fadhil Khalaf Dahash Alfadhili 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 Development of Back Contacts for CdTe Thin Films Solar Cells by Fadhil Khalaf Dahash Alfadhili Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo May 2020 Thin film solar cells based on polycrystalline p-type cadmium telluride (CdTe) represent one of these the most promising photovoltaic (PV) device due to high efficiency and low-cost production. Currently, CdTe solar cells provide the lowest cost electricity generation in utility-scale applications, which is a cost-competitive with the traditional power source, fossil fuel. CdTe thin film PV has attained 22.1 % of power conversion efficiency for small area scale and 18.6 % for modules scale. However, the high efficiency of CdTe devices has been achieved by increasing the photo-generated current by changing the traditional window layer (CdS) of CdTe to a wider bandgap material with better band alignment. The open-circuit voltage (VOC) remains below the theoretical limit due to a barrier at the back of the device due to the deep valence band edge of CdTe (-5.9 eV). Voc can be increased by adding a buffer layer between CdTe and the back electrode to decrease band banding and reducing carrier recombination at the back interface. In this thesis, several materials were investigated as a back-buffer layer, such as single-wall carbon nanotube (SWCNT), zinc telluride (ZnTe), tellurium (Te), and cadmium zinc telluride (CZT) to minimize the bend bending at CdTe/back-buffer layer interface. An alternative method to reduce the carrier recombination at the rear surface, the use of aluminum oxide iii (Al2O3) layer as a passivation layer was also demonstrated. Finally, an effective method of CdCl2 treatment for CZT thin film was investigated. This method shows that zinc (Zn) can be maintained during the heat treatment. iv Acknowledgements First, I would like to express my deepest and sincere gratitude and appreciation to my advisor, Dr. Michael Heben, for his excellent guidance and patience through my academic study. I would also like to thank my committee members, Dr. Randy Ellingson, Dr. Robert Collins, Dr. Nikolas Podraza, and Dr. Daniel Georgiev for their valuable advice and feedback. My special thanks go to Dr. Adam Phillips for his unlimited support and guidance throughout my academic research journey. Also, I would like to thank my colleagues, who helped me throughout this journey. Furthermore, I would like to thank all the faculty members and staff in the Department of Physics and Astronomy and the Wright Center for Photovoltaics Innovation and Commercialization (PVIC). My appreciation and thanks also go to the Higher Committee for Education Development in Iraq (HCED) for sponsoring my studies. Finally, special thanks and deepest gratitude to my wife, Jannah, for understanding the challenges, for her patience and helpfulness, as well as my kids: Zahraa, Narjes, Hussien, and Zainab. Besides, I am thankful to my mother, brothers, and sisters for their support and encouragement. v Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ............................................................................................................ vi Table of Contents .............................................................................................................. vii List of Tables ................................................................................................................... xii List of Figures .................................................................................................................. xiii List of Abbreviations .........................................................................................................xx List of Symbols ............................................................................................................... xxii 1 Introduction and Motivation ...................................................................................1 1.1 Motivation .........................................................................................................1 1.2 Solar Cell Basics ...............................................................................................3 1.3 Solar Cell Parameters ........................................................................................4 1.4 CdTe Solar Cells ...............................................................................................7 1.5 CdTe Device Components .................................................................................7 1.5.1 Substrate and Front Contact ...............................................................8 1.5.2 Absorber layer CdTe ..........................................................................8 1.5.3 Window layer (emitter) .......................................................................9 1.5.4 CdCl2 Activation ................................................................................9 1.5.5 Back Contact ....................................................................................10 1.6 Dissertation Overview .....................................................................................11 vi 2 Use of Single Wall Carbon Nanotube films doped with Triethyloxonium Hexachlorantimonate as a Transparent Back Contact for CdTe Solar Cells .........13 2.1 Introduction .....................................................................................................14 2.2 Experimental Details ........................................................................................16 2.3 Results and Discussion ...................................................................................17 2.3.1 SWCNT on Glass Substrate .............................................................17 2.3.2 SWCNT Film on CdTe ....................................................................18 2.3.3 Devices Performance .......................................................................19 2.3.4 The Differences in SWCNT Doping on Glass and CdTe Substrates21 2.3.5 The Activation Energy of OA Doping .............................................22 2.3.6 OA Interacting with CdTe ...............................................................24 2.4 Conclusion .......................................................................................................26 3 Controlling Band Alignment at the Back Interface of Cadmium Telluride Solar Cells Using ZnTe and Te Buffer Layers ................................................................27 3.1 Introduction ......................................................................................................28 3.2 Experimental Details ........................................................................................29 3.3 Results and Discussion ...................................................................................31 3.3.1 Raman Spectroscopy Analysis ..........................................................31 3.3.2 Device Performances ........................................................................32 3.3.3 Auger Depth Profile ..........................................................................32 3.3.4 Back Barrier Height Analysis ...........................................................34 3.4 Conclusion .......................................................................................................36 4 Potential of Cd1-XZXnTe Thin Film Back Buffer Layer for CdTe Solar Cells ....38 vii 4.1 Introduction ......................................................................................................39 4.2 Experimental Details ........................................................................................40 4.3 Results and Discussion ...................................................................................40 4.3.1 Optical Properties of CZT Film .......................................................40 4.3.2 Electric Properties of CZT Film doped with Cu ..............................41 4.3.3 Device Performance ..........................................................................42 4.3.4 Barrier Height Measurements ...........................................................44 4.4 Conclusion .......................................................................................................46 5 Back Contact Passivation of CdTe Solar Cells by Solution-Processed Oxidized Aluminum…… ......................................................................................................47 5.1 Introduction ......................................................................................................48
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