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Low Cost High Efficiency Screen Printed Solar Cells LOW COST HIGH EFFICIENCY SCREEN PRINTED SOLAR CELLS ON CZ AND EPITAXIAL SILICON A Dissertation Presented to The Academic Faculty by Chia-Wei Chen In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Electrical and Computer Engineering Georgia Institute of Technology May 2016 Copyright © 2016 by Chia-Wei Chen LOW COST HIGH EFFICIENCY SCREEN PRINTED SOLAR CELLS ON CZ AND EPITAXIAL SILICON Approved by: Dr. Ajeet Rohatgi, Advisor Dr. Gee-Kung Chang School of Electrical and Computer School of Electrical and Computer Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Thomas K. Gaylord Dr. Shreyes N. Melkote School of Electrical and Computer School of Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Bernard Kippelen School of Electrical and Computer Engineering Georgia Institute of Technology Date Approved: March 7, 2016 This thesis work is dedicated to my parents, Qing-Yan Chen and Men-lan Shaw, my sister Chia-Yu (Christy) Chen, and my beautiful fiancée Yu-Jung (Ruby) Lin, for their encouragement, support and, love. ACKNOWLEDGEMENTS First of all, I would like to express my gratitude to God for all the guidance in this journey. I am always encouraged after going to Him every time when trouble comes. Why are you downcast, O my soul? Why so disturbed within me? Put your hope in God, for I will yet praise him, my Savior and my God. -Psalm 42:5 I would like to express my sincere gratitude to my advisor Dr. Rohatgi for support- ing me, discussing with me and guiding me in the research. Especially, I appreciate the countless hours he spent with me, asking me questions, correcting my papers, and showing me the presentation skills. I would like to thank my committee members: Dr. Thomas K. Gaylord, Dr. Bernard Kippelen, Dr. Gee-Kung Chang, and Dr. Shreyes N. Melkote for serving as my committee members. I am truly grateful for all the current and former members at UCEP (University Center of Excellence for Photovoltaics Research and Education), including Dr. Ian Cooper, Brian Rounsaville, Aditya Kapoor, Sean Riordan, Kenneth Trotman, Kevin Ellis, Dr. Rishi Ramanathan, Dr. Arnab Das, Dr. Moon-Hee Kang, Dr. Kyung Sun Ryu, Eunhwan Cho, Steven Ning, Keeya Madani, Elizabeth Lori Chang, Alan Zhang, Devin Carter, and Keenan Jones. I would like to thank Dr. Jiun-Hong Lai for being my mentor, not only in school but also in life, in the early days of the studies. Special thanks go to Dr. Abasifreke Ebong for the valuable technical discussion in my first paper, to Ajay Upadhyaya for all the supports and discussion in all kinds of experiments, to Vijaykumar Upadhyaya for all the printing works, to iv Francesco Zimbardi for all the help in the clean room, to Keith Tate for showing me how to fabricate the first solar cell in my life, to Dr. Young-Woo Ok for all the technical discussion before and after meetings, to Carla East for administrative issues, to James Keane for all the physical vapor deposition works, to Dr. Yuguo Tao for all the collaborative work in n-type passivated contact cells, to Dr. John Renshaw for teaching me the Sentaurus numerical modeling, to Andrew Marcus Tam and Ying- Yuan (Peter) Huang for learning the modeling so hard, and to Malka Kadish and Ivan Pham for assisting me with the experimental works. I would also like to thank the members in Suniva, including Dr. Atul Gupta, Dr. Adam Payne, Dr. Vijay Yelundur, Dr. Yi Wang, Vinodh Chandrasekaran, Preston Davis, and Liladhar Dahal for their technical supports and valuable discussions in ion implantation and screen-printed processes. Special thanks to Dr. Martin Hermle and Dr. Jan Benick at Fraunhofer ISE for valuable discussions in Sentaurus modeling of passivated contact, to T. S. Ravi and Dr. Ruiying Hao at Crystal Solar Inc. for the epitaxially grown Si wafers, to Marc Finot at Skyline Solar Inc. for the collaborative works in concentrator Si solar cell, to Xudong Chen at nScrypt Inc. for the collaborative works in direct printing technology, and to all the members at Bread of Life Christian Church in Atlanta and all my friends at Taiwanese Student Association for helping me and my families through difficulties. The research in this thesis was supported by the U.S. Department of Energy. v TABLE OF CONTENTS ACKNOWLEDGEMENTS .......................... iv LIST OF TABLES ............................... xii LIST OF FIGURES .............................. xiv LIST OF SYMBOLS AND ABBREVIATIONS ............. xix SUMMARY .................................... xxii I INTRODUCTION ............................. 1 1.1 Statement of the Problem . 1 1.2 Specific Research Objectives . 5 1.2.1 Task 1: Development of High Efficiency Screen-printed Low- Medium Concentrator Si Solar Cell . 5 1.2.2 Task 2: Development of Screen-printed Thin Epi-Si Solar Cell using Epitaxial Wafer Equivalent Structure . 6 1.2.3 Task 3: Development of Screen-printed Thin Epi-Si Solar Cell using Porous Si Layer Transfer Process . 8 1.2.4 Task 4: Development of Screen-printed Epi-Si Solar Cell on Free-standing Epi-Si Kerfless Wafer . 8 1.2.5 Task 5: Development of Advanced High Efficiency Large Area Screen-printed Solar Cell on Direct Kerfless Epitaxially Grown Mono-Crystalline Si Wafer . 9 1.2.6 Task 6: Modeling the Potential of Next Generation Screen- printed N-type Front Junction Cz Si Solar Cells with Tunnel Oxide Passivated Back Contact . 10 II PHYSICS OF SOLAR CELL ...................... 12 2.1 Basic Solar Cell Operation . 12 2.2 Losses Mechanisms in Solar Cell . 13 2.2.1 Shading Loss . 13 2.2.2 Reflection Loss . 14 2.2.3 Incomplete Absorption . 15 vi 2.2.4 Parasitic Absorption in Anti-Reflection Coatings, Metal and Si 16 2.2.5 Resistive Losses . 17 2.2.6 Recombination . 18 2.3 Understanding and Determination of Various Saturation Current Den- sity Components in a Solar Cell . 24 2.3.1 Determination of Front Metal and Passivated Field Contri- bution (Joe:metal and Joe:field) to Emitter Saturation Current Density . 24 2.3.2 Determination of Bulk Recombination Contribution (Job:bulk) to Base Saturation Current Density . 27 2.3.3 Determination of Back Metal and Passivated Field Contribu- 0 0 tion (Job:metal and Job:field) to Base Saturation Current Density 28 2.4 Sentaurus 2D Modeling to Match a Solar Cell . 28 III LITERATURE SURVEY ........................ 31 3.1 Screen-printed Low-medium Concentrator Si Solar Cell . 31 3.2 Thin Epi-Si Solar Cell on Low-Cost Substrate using Epitaxial Wafer Equivalent Structure . 32 3.3 Thin Epi-Si Solar Cell without Substrate using Porous Si Layer Trans- fer Process . 34 3.4 Free-standing Thick Epi-Si Kerfless Wafer . 36 3.5 Epi-Si Solar Cell with Built-in Junctions . 37 3.6 Tunnel Oxide Passivated Contact Solar Cell . 39 IV TASK 1: DEVELOPMENT OF HIGH EFFICIENCY SCREEN- PRINTED LOW-MEDIUM CONCENTRATOR SI SOLAR CELLS 41 4.1 Development of an Analytical Model to Calculate Cell Efficiency un- der Various Concentrations . 42 4.2 Determination of One-sun Voc, Jsc, Rs, Rsh and n-factor for Different Finger Spacings . 43 4.2.1 Use of the Analytical Model to Calculate Highest Achievable Efficiency at Various Concentration from a Given Cell Tech- nology and Structure . 46 4.3 Concentrator Cell Fabrication to Validate the Model and Methodology 48 vii 4.4 Modeling and Experimental Validation of the Impact of Finger Length on the Highest Achievable Efficiency Curve . 49 4.5 Demonstration of High Efficiency ≥20% Efficient Direct Paste-printed Concentrator Si Solar Cell . 52 4.6 Roadmap to ≥21% Efficient Screen-printed Concentrator Si Solar Cell 53 4.7 Summary . 56 V STRATEGY AND VARIOUS CELL STRUCTURES INVESTI- GATED ON EPITAXIALLY GROWN SI SUBSTRATE ..... 57 VI TASK 2: DEVELOPMENT OF SCREEN-PRINTED THIN EPI- SI SOLAR CELL USING EPITAXIAL WAFER EQUIVALENT STRUCTURE ............................... 60 6.1 Porous Si Back Reflector Design and Implementation . 60 6.1.1 Refractive Index Requirement for Efficient Back Reflector . 60 6.1.2 Refractive Index as a Function of Porosity of PSI . 61 6.1.3 PSI Formation to Achieve the Porosity and Refractive Index Targets . 62 6.1.4 Scattering Factor of PSI and Si Interface . 63 6.2 Process Flow . 64 6.2.1 Epitaxial Si Layer Deposition by Chemical Vapor Deposition (CVD) . 64 6.2.2 Cell Processing . 65 6.3 Modeling and Analysis of EpiWE Cell with PSI between the Epi-Si Layer and Si Substrate . 66 6.3.1 Comparison between the Epi-Si Cells with and without PSI Back Reflector . 66 6.3.2 PC1D Modeling of Si cells with Different Active Epi-Si Layer Thickness . 68 6.4 Summary . 70 VII TASK 3: DEVELOPMENT OF SCREEN-PRINTED THIN EPI- SI SOLAR CELL USING POROUS SI LAYER TRANSFER PRO- CESS ..................................... 71 7.1 Process Flow for Screen-printed Thin Epi-Si Cells using PSI Layer Transfer Process . 72 viii 7.1.1 Wafer Preparation . 72 7.1.2 Front Cell Processing . 72 7.1.3 Semi-module Processes . 74 7.1.4 Back Cell Processing . 74 7.1.5 Module Assembly . 75 7.2 Yield Improvement and Texturing Optimization . 75 7.3 Laser Fired Contact . 79 7.4 Optimization of Built-in Boron BSF Design and Bulk Lifetime . 83 7.5 Epi-Si Cells with Different Bulk Thickness . 87 7.6 Summary . 87 VIIITASK 4: DEVELOPMENT OF SCREEN-PRINTED EPI-SI SO- LAR CELL ON FREE-STANDING EPI-SI KERFLESS WAFER 89 8.1 Fabrication and Comparison of P-type PERC cells on Epi-Si and Commercial Grade Cz Si wafers . 89 8.2 Extraction of Bulk Lifetime in the Finished p-type PERC Cells by Measurements and Modeling .
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