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High Performance Inverted Pyramidal Texture for Silicon Photovoltaics

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High Performance Inverted Pyramidal Texture for Silicon Photovoltaics High Performance Inverted Pyramidal Texture for Silicon Photovoltaics by Kitty Kumar A thesis submitted in conformity with the requirements for the degree of Doctorate of Philosophy Department of Materials Science and Engineering University of Toronto © Copyright by Kitty Kumar, 2013 High Performance Inverted Pyramidal Texture for Silicon Photovoltaics Kitty Kumar Doctorate of Philosophy Department of Materials Science and Engineering University of Toronto 2013 Abstract An inverted pyramidal grating texture is known to reduce both surface reflection and to promote light trapping in crystalline silicon (c-Si) solar cells. However, these textures are not used in commercial solar cells mainly because of high fabrication costs, limited scalability of conventional fabrication techniques to thin wafers, and insufficient knowledge of the optimum grating parameters for silicon of different thicknesses. These issues become even more important as industry makes a transition to thinner Si wafers to reduce device cost. The objective of this this thesis is to address all of these issues. Firstly, a new process for inverted pyramidal texturing of c-Si has been developed that is compatible with thin silicon wafers and foils. Secondly, a theoretical study of the optimum inverted pyramidal grating parameters has been done for a wide range of Si wafer thicknesses. Finally, the optical performance of the optimal textures has been experimentally verified. The laser assisted texturing method produces high quality inverted pyramids in a non- cleanroom environment and is potentially scalable to mass production. Because of the contactless fabrication, the method can be used to texture fragile, ultra-thin Si foils. This ii approach also offers precise control of the patterned areas, which can exclude areas for front surface contacts on PV devices. The wave-optical study of the size dependent performance of inverted pyramidal textures identifies a 1000 nm period as being universally optimal for silicon thicknesses ranging from 2 – 400 m. As a point of comparison, inverted pyramidal textures were also fabricated by electron beam lithography. The measured reflectances show that textures with micron scale periodicity outperform a submicron periodic texture, in agreement with trends predicted by the wave-optical simulations. Finally, a novel phenomenon of internal structuring within an optically transparent thin film is described that was discovered in the course of optimizing the laser processing parameters for making apertures in the hard mask layer for PV surface texturization. This phenomenon shows promise for varied applications such as marking of surfaces, and the production of buried channels within a thin film for lab on chip architectures. iii To my Parents Dr. Jonas Salk said “Good parents give their children Roots and Wings. Roots to know where home is, Wings to fly away and exercise what's been taught them.” I am lucky to have such great parents. Thank you Mom and Dad for Everything! iv Acknowledgements First and foremost, I would like to sincerely thank my supervisors, Professors Jun Nogami, Nazir P. Kherani and Peter R. Herman, for giving me this wonderful opportunity to conduct my graduate studies in their world-class teams. I am grateful for their constant guidance, feedback and support throughout my doctoral study. I would like to thank Kenneth K.C. Li for sharing his knowledge and experience in laser processing. Without his knowledge and assistance in laser processing this work would not have been possible. I would also like to express my gratitude towards Dr. Jianzhao Li for ICCD experiments and Ali Khalatpour for simulation work. I am also grateful to the University of Toronto, the Department of Materials Science and Engineering, Hatch Ltd., Natural Sciences and Engineering Research Council of Canada, and the Ontario Research Fund – Research Excellence for their financial support. v Table of Contents ABSTRACT .........................................................................................................................................................II ACKNOWLEDGEMENTS ................................................................................................................................ V TABLE OF CONTENTS .................................................................................................................................. VI LIST OF TABLES ............................................................................................................................................ VII LIST OF FIGURES ........................................................................................................................................ VIII LIST OF APPENDIX FIGURES .................................................................................................................. XIV CHAPTER 1 INTRODUCTION ......................................................................................................................... 1 1.1 SOLAR ENERGY ............................................................................................................................................. 1 1.2 CRYSTALLINE SILICON SOLAR CELLS ........................................................................................................... 4 1.3 SURFACE TEXTURING ................................................................................................................................... 5 1.4 GRATED SURFACE TEXTURES FOR LIGHT-TRAPPING .................................................................................... 7 1.5 RESEARCH OBJECTIVES ................................................................................................................................ 9 1.6 THESIS OUTLINE ......................................................................................................................................... 10 CHAPTER 1 REFERENCES .................................................................................................................................. 12 CHAPTER 2 QUANTIZED LASER PROCESSING OF THIN DIELECTRIC FILMS ON SILICON ... 16 2.1 INTRODUCTION ........................................................................................................................................... 16 2.1.1 Laser Processing of Transparent Dielectric Films ............................................................................. 17 2.2 FEMTOSECOND LASER-MATERIAL INTERACTION IN TRANSPARENT DIELECTRIC FILMS ............................. 18 2.2.1 Theoretical Calculation of Electron Density inside the Film.............................................................. 19 2.3 QUANTIZED INTERNAL STRUCTURING AND EJECTION OF A TRANSPARENT FILM ........................................ 24 2.3.1 Film Morphology as a function of Laser Exposure ............................................................................ 25 2.4 VERIFICATION OF QUANTIZED FILM EJECTION WITH AN INTENSIFIED CCD CAMERA ................................. 28 2.5 LARGE AREA PROCESSING OF FILMS .......................................................................................................... 31 2.6 SUMMARY ................................................................................................................................................... 34 2.7 DISCUSSION ................................................................................................................................................ 35 2.8 EXPERIMENTAL METHODS .......................................................................................................................... 35 CHAPTER 2 REFERENCES .................................................................................................................................. 38 CHAPTER 3 FEMTOSECOND LASER DIRECT HARD MASK WRITING FOR SELECTIVE FACILE MICRON-SCALE INVERTED-PYRAMID PATTERNING OF SILICON ................................ 41 3.1 INTRODUCTION ........................................................................................................................................... 41 3.2 LASER PATTERNING OF A HARD MASK ........................................................................................................ 42 3.3 TEXTURING METHOD .................................................................................................................................. 43 3.4 OPTIMIZATION OF SINGLE PULSE LASER EXPOSURE AND HARD MASK THICKNESS FOR HIGH RESOLUTION WRITING ........................................................................................................................................................... 47 3.5 HARD MASK WRITING WITH MULTIPLE PULSE EXPOSURE ......................................................................... 51 3.6 FACILE PATTERNING FOR LARGE AREA INVERTED PYRAMIDAL TEXTURE, V-GROOVES AND RESERVOIRS53 3.7 DISCUSSION ................................................................................................................................................ 54 3.8 CONCLUSIONS AND OUTLOOK ..................................................................................................................... 56 CHAPTER 3 REFERENCES .................................................................................................................................. 57 CHAPTER 4 WAVE-OPTICAL STUDY OF INVERTED PYRAMIDAL TEXTURE ON THICK SILICON WAFERS AND ULTRA-THIN FOILS .........................................................................................
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