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Novel Uses of Titanium Dioxide for Silicon Solar Cells Doctor of Philosophy Bryce Sydney Richards

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Novel Uses of Titanium Dioxide for Silicon Solar Cells Doctor of Philosophy Bryce Sydney Richards Novel Uses of Titanium Dioxide for Silicon Solar Cells A thesis submitted as partial fulfillment of the requirement for the Degree of Doctor of Philosophy by Bryce Sydney Richards at the Centre for Photovoltaic Engineering and the School of Electrical Engineering University of New South Wales Sydney 2052 New South Wales Australia April 2002 CENTRE FOR PHOTOVOLTAIC ENGINEERING UNSW Certificate of Originality I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by any other person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgment is made in the text. I also declare that the intellectual content of this thesis is the product of my own work, even though I may have received assistance from others on style, presentation and language expression. Bryce Richards Richards, Bryce Sydney Novel Uses of Titanium Dioxide for Silicon Solar Cells PhD Thesis Centre for Photovoltaic Engineering The University of New South Wales Sydney, NSW 2052, Australia Copyright c 2002 all rights reserved ISBN 0 7334 1971 2 Abstract Titanium dioxide (TiO2) thin films have a long history in silicon photovoltaics (PV) as antireflection (AR) coatings due to their excellent optical properties and low deposition cost. This work explores several novel areas where TiO2 thin films could be use to enhance silicon (Si) solar cell performance while reducing device fabrication costs. Amorphous, anatase and rutile TiO2 thin films are deposited using ultrasonic spray- deposition (USD) and chemical vapour deposition (CVD) systems, both designed and con- structed by the author. Initial experiments confirmed that no degradation in the bulk minority carrier lifetime (τbulk) occurred during high-temperature processing, although the stability of the USD-deposited TiO2 films was dependent on the furnace ambient. A major disadvantage of TiO2 AR coatings is that they afford little surface passivation. In this work, a novel method of achieving excellent surface passivation on TiO2-coated silicon wafers is presented. This involved growing a 6 nm-thick SiO2 layer at the TiO2:Si interface by oxidising the wafer after TiO2 film deposition. The increase in surface passivation afforded by the interfacial SiO2 layer results in a decrease in the emitter dark saturation current density −14 2 (J0e) by nearly two orders of magnitude to 4.7 − 7.7 × 10 A/cm . This demonstrates the compatibility of the TiO2/SiO2 stack with high-efficiency solar cells designs. By varying the film deposition and annealing conditions, TiO2 refractive indices in the rangeof1.726 − 2.633 (at λ = 600 nm) could be achieved. Subsequently, a double-layer antireflection (DLAR) coating was designed comprised of low and high TiO2 refractive index material. The best experimental weighted average reflectance (Rw) achieved was 6.5% on a planar silicon wafer in air. TiO2 DLAR coatings are ideally suited to multicrystalline silicon (mc-Si) wafers, which do not respond well to chemical texturing. Modelling performed for a glass and ethyl vinyl acetate (EVA) encapsulated buried-contact solar cell indicated that a TiO2 DLAR coating afforded a 7% increase in the short circuit current density, when compared to a standard, commercially-deposited TiO2 single-layer AR coating. Finally, it is demonstrated that chemical reactions with phosphorus prevent TiO2 from acting as a successful phosphorus diffusion barrier or dopant source. The applicability of TiO2 thin films to various silicon solar cell structures is discussed. Acknowledgements Many people contributed to the success of this work and my survival throughout. First, and foremost, I need to thank (yes, thank!) my partner, Andrea Sch¨afer, for leading me along the path to the PhD. Somehow witnessing all the good and the bad moments during her PhD, ended up creating a positive image for me! Andrea also provided invaluable guidance and tips along the way, and created many shortcuts through the bureaucracy for me. I will never forget your assistance Andrea, and am deeply indebted to you. Vielen Dank, and may our love only grow stronger. Another big Danke, goes out to our daughter, Moana Sch¨afer,who witnessed just over half of my efforts. Thanks for keeping my feet firmly planted on the ground and not letting me drift too far off into “PhD land”! Thank you for all our fun times, and my apologies for the times when my patience wasn’t sufficient to see your needs. Naturally, I would like to thank the input from my supervisors of the years: to Stuart Wenham (UNSW) for his enthusiasm and encouragement; to Christiana Honsberg (UNSW and Georgia Institute of Technology, U.S.A.) for her moral and monetary support; to Francesca Ferrazza (Eurosolare S.p.A.) for the opportunities to see the “real” side of photovoltaics and for being a true friend; and to Jeff Cotter for his valuable advice during the latter stage of the thesis. Several members brought their own special personalities to the Centre and made it a fun and challenging work place. These people include Keith McIntosh, Hamid Mehrvarz, Holger Neuhaus, Alex Slade, Bernhard Vogl, Rob Bardos, Matt Boreland, Martin Bruahart and Tom Puzzer. Thanks for all the ethical, moral and technical conversations. Thanks too for the great computer support, Laurie! I would like to thank other people who assisted with TiO2 thin film characterisation: Dr. Tom Puzzer (UNSW) for SEM/AFM training; Prof. Robert Lamb (UNSW) and Dr. Matt Boreland (Toyota Technical Institute, Japan) for XPS analysis; Prof. David Jamieson (Univ. of Melbourne) for RBS analysis; Sally Rowlands and Prof. Trevor Redgrave (both Univ. of Western Australia) for training and access to the variable-angle spectroscopic ellipsometer (VASE); Dr. Alistair Sproul (UNSW), author of the forthcoming book “Ellipsometry for Dummies”; and to my father, Dr. Ray Richards (Lower Hutt, New Zealand), for his assistance in bringing me up to speed on thermochemistry analysis. 2 I am grateful for the guidance in my career provided by Dr. Andrea Sch¨afer,Prof. Mark Wain- wright, Prof. Stuart Wenham and Prof. Martin Green. The financial support provided by the Faculty of Engineering, the School of Electrical Engineering and the Centre for Photovoltaic Engineering was greatly appreciated. Publications Resulting from this Thesis (to date) B.S. Richards (2004) Comparison of Dielectric Coatings for Buried-Contact Solar Cells: A Review, Progress in Photovoltaics 12 (in press). B.S. Richards, S.R. Richards, M.B. Boreland, D.N. Jamieson (2004) High Temperature Processing of TiO2 Thin Films for Application in Silicon Solar Cells, Journal of Vacuum Science and Technology A, 22(2): 339-348. B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, C.B. Honsberg (2004) Reducing the Production Costs of Buried-Contact Solar Cells using Titanium Dioxide Thin Films, Solar Energy, 76(1-3): 269-276. B.S. Richards (2003) Single-Material TiO2 Double-Layer Antireflection Coatings, Solar Energy Materials and Solar Cells, 79(3), 369-390. B.S. Richards, S.F. Rowlands, C.B. Honsberg, J.E. Cotter (2003) TiO2 DLAR Coatings for Planar Silicon Solar Cells, Progress in Photovoltaics, 11(1), 27-32. B.S. Richards, J.E. Cotter and C.B. Honsberg (2002) Enhancing the surface passivation of TiO2 coated silicon wafers, Appl. Phys. Letters, 80(7), 1123-1125. B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, and C.B. Honsberg (2001) Reducing the production costs of buried-contact solar cells using titanium dioxide thin films, Intl. Solar Energy Society Solar World Congress, 26-30 November, Adelaide. B.S. Richards, J.E. Cotter, C.B. Honsberg and S.R. Wenham (2000) Novel Uses of TiO2 Films in Crystalline Silicon Solar Cells, 28th IEEE Photovoltaic Specialists Conference, Alaska, 375-378. C.B. Honsberg, J.E. Cotter, K.R. McIntosh, S. Pritchard, B.S. Richards and S.R. Wenham, (1999), Design strategies for commercial solar cells using the buried contact technology, IEEE Trans. Electron Devices, 46(10), 1984-92. B.S. Richards, J.E. Cotter, F. Ferrazza, C.B. Honsberg and S.R. Wenham (1998) Lowering the cost of commercial silicon solar cells, Proc. of the Environmental Engineering Research Event 1998, Avoca Beach, New South Wales, 303-308. J.E. Cotter, B.S. Richards, F. Ferrazza, C.B. Honsberg, T.W. Leong, H.R. Mehrvarz, G.A. Naik and S.R. Wenham (1998) Design of a simplified emitter structure for buried contact solar cells, 2nd World Conference Photovoltaic Energy Conversion, Vienna, 1511-1514. Contents 1 Introduction 9 1.1MotivationforthisWork............................. 9 1.2 Australia’s Solar Energy Resource ........................ 11 1.3 Brief Theory of Solar Cell Operation ...................... 12 1.4 Commercially Produced Silicon Solar Cells ................... 14 1.4.1 Screen-Printed Solar Cells ........................ 14 1.4.2 Buried-Contact Solar Cells ........................ 15 1.4.3 Buried-Contact Solar Cell Fabrication Sequence ............ 16 1.4.4 Simplified Buried-Contact Solar Cell .................. 16 1.5 Multicrystalline Silicon .............................. 18 1.5.1 Issues with Multicrystalline Silicon ................... 19 1.6 Why use Titanium Dioxide? ........................... 20 1.6.1 TiO2 Thin Films in Photovoltaics .................... 21 1.7 Thesis Overview and Goal ............................ 25 2 Common Properties of TiO2 Thin Films 29 2.1Introduction.................................... 29 2.2PhysicalProperties................................ 30 2.2.1 The Amorphous − Anatase − Rutile Phase Transformations.............................. 30 2.2.2 The Effect of Impurities on the Anatase − Rutile Phase Transformation 32 2.2.3 Substrate Type .............................. 33 3 4 CONTENTS 2.2.4 Film Defects ................................ 34 2.2.5 Film Density ............................... 34 2.2.6 Non-Stoichiometric TiO2−x Thin Films ................. 36 2.3OpticalProperties................................ 37 2.3.1 Refractive Index, Extinction Coefficient and Scattering................................. 37 2.3.2 TiO2 Thin Film vs.
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