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Improved Carrier Selectivity of Diffused Silicon Wafer Solar Cells

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Improved Carrier Selectivity of Diffused Silicon Wafer Solar Cells IMPROVED CARRIER SELECTIVITY OF DIFFUSED SILICON WAFER SOLAR CELLS Alexander To A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Photovoltaic and Renewable Energy Engineering Faculty of Engineering December 2017 ORIGINALITY STATEMENT 'I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or p g in style, resentation and linguisti� sion is acknowled ed.' � ............. Date .....z :s./J/.1.�............... ............................ v “No. No out. No back. Only in. Stay in. On in. Still... Ever tried. Ever failed. No matter. Try again. Fail again. Fail better.” - Samuel Beckett, Worstword Ho vi vii ABSTRACT The majority of commercial solar cells are fabricated on crystalline silicon wafers with diffused homojunctions. This is forecast to continue into the near future. This thesis explores how improving the carrier selectivity of homojunction solar cells can achieve higher conversion efficiencies, by reducing the losses at the contacted and non-contacted surfaces. The carrier conductivity at the metal-silicon contact was investigated for both heavily doped n+ and p+ silicon. For diffused n+ electron collectors, it was shown that thin layers (<5 nm) of aluminium oxide (AlOx) deposited between the silicon nitride (SiNx) and screen-printed silver paste can improve contact resistivity, whereas thicker layers increased the contact resistivity. Through simulation and cell fabrication, this effect was shown to be beneficial for solar cells limited by contact resistance, whereas the adverse effects of thicker layers were mitigated by changing the paste formulation. For diffused p+ hole collectors, a method of electroless plating nickel seed layers to boron diffused silicon was developed which demonstrated contact resistivity’s < 1 mΩ.cm2 for lightly diffused p+ silicon. However, it was found that the contact recombination and contact resistivity was higher for the electroless nickel plated contacts in comparison to aluminium evaporated references. Regarding the non-metallised, heavily doped silicon surfaces, improved understanding of the recombination rate at diffused inverted surfaces was achieved through modelling of the injection level dependent lifetime behaviour with technology computer-aided-design (TCAD) simulations. This resulted in the development of a novel contactless method of extracting the interface charge (Q) and surface recombination velocity parameters from passivated and diffused silicon surfaces. This addresses a current limitation of existing interface characterisation techniques, and allows the study of the electronic properties of these highly relevant surfaces. This method was demonstrated on a range of dielectric passivated n+ and p+ surfaces, and the extracted Q values were comparable to results obtained using conventional techniques. Finally, the findings were implemented in TCAD simulations of state-of-the-art homojunction interdigitated back contact (IBC) solar cells, demonstrating the technology potential. This work highlights novel technologies and methods to improve the next generation of diffused silicon wafer solar cells. viii ix ACKNOWLEDGEMENTS Firstly, to my supervisors Dr. Bram Hoex and Dr. Alison Lennon, thank you for your continued support and mentorship. I have been extremely lucky to have had such dedicated, intelligent and passionate mentors. Bram, I have really enjoyed working with you, and learnt so much in the process. Thank you for pushing me to produce high quality research, making time for my concerns, the amazingly fast turn-around on proof reads, and for guiding my development as a researcher. Alison, since supervising my undergraduate thesis (almost 6 years ago now!), I am truly grateful for everything you’ve taught me over that time. I have learnt so much from your extensive knowledge and strong example. Thank you for supporting me, and advocating on my behalf. I am incredibly fortunate to have had the support of so many wonderful colleagues throughout my thesis. Firstly, Dr Craig Johnson, it has been a pleasure working with you, thank you for sharing your perspective, thoughts and wisdom. To Dr Michael Pollard, I’ve learnt a lot from your meticulous nature, knowledge and hard work ethic. Thank you for persevering with me through those exhaustive afterhours photolithography sessions! To Dr Ma Fajun, I’m so fortunate to have been able to work with a true expert such as yourself, thank you for all your help with the Sentaurus lifetime simulations. To Dr John Rodriguez (Nate), I could never fully acknowledge all that I have gained from our friendship throughout my studies—thanks for everything. Nitin, thank you for always being there at the important moments, with the right advice and guidance—my work is better in no small part as a result of our fruitful discussions. Jack Colwell, I hassled you the most because you are the best. Thanks for the countless measurements and other processes performed on my behalf. Dr. Tasmiat Rahman at the University of Southampton, thanks for all your help with the Sentaurus simulations, and for extending the opportunity to collaborate on the IBC cells project, it’s really been a lot of fun working with you. To Rasmus Davidsen at DTU, I truly appreciate your kindness, generosity and friendship throughout this whole process, supporting this work in so many ways. To Nicholas Grant at Warwick University, thanks for your help with the super-acid measurements, and for your invaluable contributions to the process development within this work. Alessandro and Sofia, thank you for your assistance with the almost impossible volume of processing we had to get through! To Dr. Beniamino Iandolo, thank you for imaging my samples on such short notice. x Of course, I’d like to thank the whole LDOT team. Thank you for keeping the labs and equipment running. In particular, I’d like to acknowledge Kian, for his tireless efforts helping me through all the process and equipment issues I’ve experienced along the way. To Dr. Linda Macks and her wonderful team at ANFF; if you hadn’t done such a wonderful job keeping ANFF running this thesis would not have been possible. Thank you to the SIRF Operations team, in particular Kyung, Ly Mai and Nino—thank you for all those countless depositions! I’d also like to acknowledge Australian Renewable Energy Agency (ARENA), who support the PV industry and provided my Ph.D. scholarship. I am truly grateful to so many of my friends for their continued support, understanding and encouragement over the years. Firstly, to my colleagues and friends, thank you for making SPREE such an open, collaborative and fun place to work—you are all deadest legends. Thanks to all my past and present colleagues in the 1st Gen. group, from whom I’ve learnt so much—in particular Mal, Ziv, Mattias, Cath, Ali, Dave, Udo, Kyung, Alex Li, Jack An, Alex Han, Ned, Dan Chung, Rhett, Carlos, Rob D, Yu Yao and Doris. Natasha, thanks for being such an amazing support and friend. To all my fellow students in Bram’s group, thank you for all your support and friendship. To the Hippies, thanks for being such an amazing group of supportive friends. In particular, thank you Richie, Ben and Izzy for being there for me when I needed you guys most. To my dear friend Soph, you’ve gone above and beyond the ‘call of duty’ and I really appreciate it, thank you. To my friends Jean, Sam, Amir, Tan, Scott, Ajay, and Bryan, you’ve all supported, inspired and motivated me along the way. You’ve helped me focus, persevere and see this through, thanks for everything. Fr. Ross Jones and Ailsa Gillett, thank you for continued guidance, and for being such amazing mentors and role models. Special thanks to Sally, for her support, patience, and understanding. Sally, you’ve been amazing through all of this, and I could not have done it without you. You’ve always been my first support and fiercest critic, thank you. Finally, to my parents, thank you for your constant support, love and guidance. Thanks for giving me the platform to accomplish all that I have, and the freedom to follow my interests and passions. I could never repay all that you have done for me, thank you. xi CONTENTS INTRODUCTION ........................................................................................................ 1 1.1 THESIS OBJECTIVES .................................................................................................. 4 1.2 THESIS OUTLINE ....................................................................................................... 7 LITERATURE REVIEW CARRIER SELECTIVITY, SEMICONDUCTOR SURFACES AND CONTACTS ................................................................................... 10 2.1 INTRODUCTION: SOLAR CELL CARRIER SEPARATION AND SELECTIVITY .................. 10 2.1.1 Carrier selectivity for diffused solar cells ..................................................... 13 2.2 SEMICONDUCTOR SURFACES .................................................................................
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