Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells
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Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells: Integration Strategy by Rational Design of Materials, Process, and Devices Dissertation by Rabab R. Bahabry In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy of Science King Abdullah University of Science and Technology Thuwal, Kingdom of Saudi Arabia November, 2017 2 EXAMINATION COMMITTEE PAGE The dissertation/thesis of Rabab R. Bahabry is approved by the examination committee. Committee Chairperson: Prof. Muhammad Mustafa Hussain Committee Co-Chair: Prof. Udo Schwingenschlögl Committee Members: Prof. Osman Bakr, Prof. Santosh (External) 3 © November, 2017 Rabab R. Bahabry All Rights Reserved 4 ABSTRACT Towards Cost-Effective Crystalline Silicon Based Flexible Solar Cells: Integration Strategy by Rational Design of Materials, Process, and Devices Rabab R. Bahabry The solar cells market has an annual growth of more than 30 percent over the past 15 years. At the same time, the cost of the solar modules diminished to meet both of the rapid global demand and the technological improvements. In particular for the crystalline silicon solar cells, the workhorse of this technology. The objective of this doctoral thesis is enhancing the efficiency of c-Si solar cells while exploring the cost reduction via innovative techniques. Contact metallization and ultra-flexible wafer based c-Si solar cells are the main areas under investigation. First, Silicon-based solar cells typically utilize screen printed Silver (Ag) metal contacts which affect the optimal electrical performance. To date, metal silicide-based ohmic contacts are occasionally used for the front contact grid lines. In this work, investigation of the microstructure and the electrical characteristics of nickel monosilicide (NiSi) ohmic contacts on the rear side of c-Si solar cells has been carried out. Significant enhancement in the fill factor leading to increasing the total power conversion efficiency is observed. Second, advanced classes of modern application require a new generation of 5 versatile solar cells showcasing extreme mechanical resilience. However, silicon is a brittle material with a fracture strains <1%. Highly flexible Si-based solar cells are available in the form thin films which seem to be disadvantageous over thick Si solar cells due to the reduction of the optical absorption with less active Si material. Here, a complementary metal oxide semiconductor (CMOS) technology based integration strategy is designed where corrugation architecture to enable an ultra-flexible solar cell module from bulk mono-crystalline silicon solar wafer with 17% efficiency. This periodic corrugated array benefits from an interchangeable solar cell segmentation scheme which preserves the active silicon thickness and achieves flexibility via interdigitated back contacts. These cells can reversibly withstand high mechanical stress as the screen-printed metals have fracture strain >15%. Furthermore, the integration of the cells is demonstrated on curved surfaces for a fully functional system. Finally, the developed flexing approach is used to fabricate three-dimensional dome-shaped cells to reduce the optical coupling losses without the use of the expensive solar tracking/tilting systems. 6 ACKNOWLEDGEMENTS All praise is to Allah the almighty, the most merciful and the most beneficent. My sincerest thanks are given to my research advisor and mentor, Prof. Muhammed Mustafa Hussain for giving me the opportunity of pursuing an amazing research area. His support, guidance and valuable suggestions have contributed significantly to the completion of this endeavor. Next, I would like to give my deepest appreciation and gratitude to Prof. Udo Schwingenschlögl, Prof. Osman Baker and Prof. Santosh Kurinec for taking the time to serve in my defense committee. I want to extend my special thanks to the college of the integrated nanotechnology laboratory (INL), especially to Mr. Melvin Cruz and friends: Amani Almuslem, Maha Nour, Joanna Nassar, Sohail Shaikh and Sherjeel Khan. Furthermore, I wish to express my deep thanks to my friend and sister Arwa Kutbee for her kindness, support and for sharing many nice and difficult moments of our Ph.D. Journey. I highly appreciate the efforts of the staff of the Advanced Nanofabrication, Imaging and Characterization Facilities of KAUST, especially Mr. Ibrahim Mohd, Mr. Ahad Syed, Mr. Nimer Wehbi, Mr. Elhadj M. Diallo, Mrs. Nini Wei and Mr. Ulrich Buttner for their incredible support. Moreover, I would like to thank Dr. Abdulrahman ElLabban from KAUST solar cell center. Furthermore, I owe my deepest gratitude to my dear husband Khaled Alamoudi, my caring mom Khadija Qouta, adorable kids Raghad, Shahad, Jouri, Ahmed and Mouhammad and my thoughtful siblings: Rayid, Rudhab, Rihab and Reem for their 7 continuous encouragement, patience, and support. Their high confidence in me was always a vast impulsion in my academic life. Thank you all for your support and never- ending love; this achievement in every bit is yours as much as it is mine. 8 TABLE OF CONTENTS EXAMINATION COMMITTEE PAGE ......................................................................................... 2 ABSTRACT ............................................................................................................................... 4 ACKNOWLEDGEMENTS .......................................................................................................... 6 Table of Contents ................................................................................................................... 8 LIST OF ABBREVIATIONS ...................................................................................................... 12 LIST OF SYMBOLS ................................................................................................................. 14 LIST OF ILLUSTRATION ......................................................................................................... 15 LIST OF TABLES ..................................................................................................................... 22 1- Introduction ..................................................................................................................... 24 1.1 Global energy outlook ................................................................................. 24 1.2 Energy harvesters for IOT ........................................................................... 26 1.3 Classification of Solar Cells .......................................................................... 28 1.4 Solar cells Physics ........................................................................................ 30 1.5 Structure and Principle ............................................................................... 30 1.6 Current Technology Challenges .................................................................. 34 1.7 Dissertation Outline and chapters overview .............................................. 35 2- Low-Cost Nickel Silicide Rear Ohmic Contact for c-Si solar cells ..................................... 38 9 2.1 Introduction ................................................................................................ 41 2.2 Experimental ............................................................................................... 45 2.2.1 Nickel-mono silicide (NiSi) optimization: ............................................................... 45 2.2.2 Screen printed Ag removal .................................................................................... 48 2.2.3 SIMS depth profiling of NiSi/Cu rear contact ......................................................... 49 2.3 Current-Voltage curves of NiSi/Cu and Al rear contacts ............................ 51 2.4 Series resistance determination (Rs) methods ........................................... 58 2.4.1 Comparison of dark and one sun JV-curves ........................................................... 58 2.4.2 Comparison of two JV-curves measured at one and 0.7 sun illumination intensities ............................................................................................................................... 59 2.5 Fabrication process of in-house c-Si solar cell ............................................ 62 2.5.1 J-V curves of rigid c-Si solar cells using NiSi/Cu contacts ....................................... 64 2.5.2 Flexible c-Si solar cell (in-house fabrication) .......................................................... 66 2.6 NiSi/Cu rear contact for thin c-Si ................................................................ 68 2.6.1 Experimental .......................................................................................................... 68 2.6.2 I/V measurements For the NiSi/Cu contacts on the soft polished samples .......... 70 2.7 Conclusion ................................................................................................... 72 3- Ultra-Flexible High-Efficiency Mono-crystalline Silicon Solar Cell ................................... 73 3.1 Introduction ................................................................................................ 74 10 3.2 Results and Discussion ................................................................................ 76 3.2.1 Ultra-Flexible c-Si solar cell .................................................................................... 76 3.2.2 Solar cell efficiency................................................................................................