ADVANCED METALLISATION METHODS FOR MONOCRYSTALLINE SILICON WAFER SOLAR CELLS

ADVANCED METALLISATION METHODS FOR MONOCRYSTALLINE SILICON WAFER SOLAR CELLS ANKIT KHANNA (M.Tech., Indian Institute of Technology, Varanasi) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILIOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2015 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. Ankit KHANNA 15th July 2015 Acknowledgements I would like to take this opportunity to thank several people and organisations who have helped me during my PhD candidature. First of all, I would like to express my sincere gratitude to my supervisor Prof. Armin Aberle for giving me the opportunity to pursue a PhD in silicon photovoltaics at the Solar Energy Research Institute of Singapore (SERIS) as well as for his expert guidance throughout my PhD candidature. I am indebted to my co-supervisor, Dr. Thomas Mueller for his role as my day-to-day scientific advisor. Thank you Thomas for your support, ideas and guidance. Thank you also for organising funding for my research attachment and travel to conferences. I would also like to thank the Department of Electrical and Computer Engineering (ECE) at the National University of Singapore (NUS) for a PhD scholarship. I am thankful to Dr. Bram Hoex, Dr Prabir Kanti Basu, Dr. Rolf Stangl and Dr. Johnson Wong for their contributions to my work: Bram for his role in mentoring PhD students at SERIS through the PhD students meeting and other periodic interactions, Basu for helping me understand various industrial aspects of silicon wafer solar cells, Rolf for introducing me to advanced solar cell modelling, and Johnson for several enlightening discussions. I carried out the latter part of my PhD research at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) as part of a research attachment. I would like to acknowledge funding for the research attachment provided by the National Research Foundation, Prime Minister’s Office, Singapore under its Clean Energy Research Programme (CERP Award No. NRF2010EWT-CERP001-022). I am grateful to i Dr. Stefan Glunz and Dr. Markus Glatthaar for hosting me at Fraunhofer ISE for a research attachment. I am also very thankful to Christian Schmiga for supervising my work at Fraunhofer ISE. I would like to thank several work colleagues for their contributions to my work and for creating a cordial work environment. Thank you Vinodh Shanmugam, Dr. Zhi Peng Ling, Dr. Shubham Duttagupta, Dr. Ge Jia, Naomi Nandakumar, Kishan Shetty, Avishek Kumar, Ranjani Sridharan, Jai Prakash Singh, Dr. Jiaying Ye, Dr. Ziv Hameiri, Pooja Chaturvedi, Philipp Steutzel, Jessen Cunnusamy, Dr. Licheng Liu, Samuel Raj, Ann Roberts, Jason Avancena, Edwin Carmona, Allan Ferdinand (affable colleagues I met at SERIS); Aleksander Filipovic, Dr. Andre Kalio, KurtUlrich Ritzau, Mathias Kamp, Dr. Jonas Bartsch, Elisabeth Schäffer, Arthur Kremer, Steve Olweya, Sebastian Binder, Aline Gautrein, Dr. Michael Rauer, Annika Tuschinsky, Katja Krüger, Karin Zimmermann, Gisela Cimmioti, Rainer Neubauer, Felix Schätzle, Antonio Leimenstoll, Michael Linse, Marc Retzlaff, Tim Niewelt, Daniele Palaferri, Ino Geisemeyer, Heiko Steinkemper, Torge Behrendt, Maik Simon, Julian Schrof, Siddharth Modi (affable colleagues I met at Fraunhofer ISE); and Dr. Martin Heinrich (an affable colleague at both SERIS and Fraunhofer ISE). I am also thankful that many of you were generous with your time outside of work and that we became good friends. Last but not least, a “shout-out” also to three close friends, outside of work, in Singapore and Germany: my beautiful and warm-hearted girlfriend Thi Ha Nguyen, my humble banker friend Veesam Nagarjuna, and my workaholic friend Dr. (med) Constantin Anastasopoulos who epitomises that a very busy work life and a fulfilling social life can co-exist. ii Contents Abstract . vi List of tables viii List of figures ix List of symbols xiii List of abbreviations xvi Introduction .1 1.1 Thesis motivation 1.2 Scientific and technical issues addressed in this thesis . 1.3 Thesis outline Background and literature review .8 2.1 Operating principles of silicon wafer solar cells . 2.2 Current-voltage characteristics . 11 2.3 Fabrication 12 2.3.1 Homojunction silicon wafer solar cells 12 2.3.2 Heterojunction silicon wafer solar cells . 15 2.4 Characterisation techniques 19 2.4.1 Current-voltage measurement 19 2.4.2 Spectral response 20 2.4.3 Suns-𝑉𝑜𝑐 . 21 2.4.4 Contact resistance measurement 22 2.4.5 Scanning electron microscopy . 25 2.4.6 Energy dispersive X-ray spectroscopy . 26 2.4.7 Statistical surface profiling 27 2.4.8 Electrochemical capacitance-voltage profiling 29 2.4.9 Photoluminescence and electroluminescence 30 2.5 Theoretical metal-semiconductor contact models . 31 2.6 Metallisation technologies for silicon wafer solar cells 37 2.6.1 Metal evaporation 37 2.6.2 Metal printing . 38 2.6.3 Electrochemical metallisation 41 iii Development of a fill factor loss analysis method for silicon wafer solar cells . 43 3.1 Introduction . 43 3.2 Fill factor loss analysis method . 46 3.2.1 Upper limit of fill factor . 46 3.2.2 Quantification of loss mechanisms 47 3.3 Error analysis 48 3.4 Examples of application of the method . 53 3.4.1 Inline-diffused p-type silicon wafer cell 53 3.4.2 Heterojunction silicon wafer solar cell 59 3.5 Discussion of injection-dependent effects 62 3.6 Chapter summary 63 Investigation of the influence of random pyramid surface texture on silver screen-printed contact formation for homojunction silicon wafer solar cells . 65 4.1 Introduction . 65 4.2 Experiment 67 4.3 Statistical characterisation of pyramid height distributions 71 4.4 Investigation of contact microstructure . 73 4.4.1 Contact microstructure overview . 73 4.4.2 Influence of texture height/pyramid density 76 4.4.3 Influence of pyramidal texture uniformity . 80 4.5 Guidelines to tailor pyramid height distributions 84 4.6 Chapter summary 85 Experimental analysis of silver light-induced plating for homojunction silicon wafer solar cells . 87 5.1 Introduction . 87 5.2 Contacting lightly doped silicon regions 88 5.3 Influence of silver light-induced plating on contact resistance . 90 5.3.1 Phosphorus-diffused silicon . 90 5.3.2 Phosphorus ion-implanted silicon 93 5.4 Solar cell results 95 5.5 Chapter summary 99 iv Experimental analysis of silver screen printing for heterojunction silicon wafer solar cells 100 6.1 Introduction . 100 6.2 Characterisation of TCO contacting pastes . 103 6.2.1 Contact formation to AZO layers . 103 6.2.2 Annealing behaviour of TCO contacting pastes 105 6.3 Calculated paste-related series resistance components . 108 6.4 Solar cell results 109 6.4.1 Loss mechanisms of fabricated cells 111 6.5 Chapter summary 113 Experimental analysis of copper electroplating for heterojunction silicon wafer solar cells 115 7.1 Introduction . 115 7.2 Transparent conductive oxide layer masking 116 7.2.1 Screen-printed masking 116 7.2.2 Laser-patterned masking 124 7.3 Solar cell results 128 7.4 Chapter summary 132 Conclusions, contributions and future research 134 8.1 Conclusions . 134 8.2 Author’s original contributions . 136 8.3 Proposed future research . 138 8.3.1 Extended fill factor loss analysis 138 8.3.2 Further development of large-area heterojunction cells . 138 8.3.3 Reliability of copper metallised heterojunction cells . 139 Appendix . 141 References . 143 List of publications 158 v Abstract This thesis focuses on the application and characterisation of screen printing and electrochemical (plating) technologies for silicon wafer solar cell metallisation. The metallisation technologies are applied to silicon homojunction and amorphous silicon/crystalline silicon heterojunction solar cells fabricated on monocrystalline silicon wafers. For silicon wafer solar cells it is extremely important to achieve high fill factors to maximize the power generation capabilities of the cell. Metallisation processes have a significant influence on a solar cell’s fill factor. Therefore, a method is developed to quantify fill factor losses due to ohmic and recombination loss mechanisms. The method is demonstrated on both silicon homojunction and heterojunction solar cells. Industrial monocrystalline silicon wafer solar cells are alkaline textured on at least the illuminated surface and the resultant random-pyramid surface texture has a significant influence on metal contact formation. A comprehensive experimental study is carried out to investigate this influence for screen-printed silver contacts to phosphorusdiffused silicon. The study involves correlating statistics of pyramid height distribution to the cells’ electrical parameters and the microstructure of the metallised interfaces. Based on the study, guidelines are developed to optimise silver screenprinted contacts to phosphorus-diffused silicon by tailoring the surface texture. Silver screen-printed contact formation to phosphorus-doped silicon regions requires higher doping than the corresponding requirement for evaporated metallisation. However, highly doped illuminated regions are undesirable for silicon wafer solar cells as they limit the cells’ short-circuit current and open-circuit voltage. To overcome this limitation, silver light-induced plating is applied to improve screen- vi and behavior of passivation layers," Journal of The Electrochemical Society, vol. 137, pp. 3612-3626, 1990. [16] D. L. King and M. E. 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Aberle, "Electrical and microstructural analysis of contact formation on lightly doped phosphorus emitters using thick-film Ag screen printing pastes," IEEE Journal of Photovoltaics, vol. 4, pp. 168-174, 2014. 3. A. Khanna, P. K. Basu, A. Filipovic, V. Shanmugam, C. Schmiga, A. G. Aberle, and T. Mueller, "Influence of random pyramid surface texture on silver screen-printed contact formation for monocrystalline silicon wafer solar cells," Solar Energy Materials and Solar Cells, vol. 132, pp. 589-596, 2015. 4. P. K. Basu, A. Khanna, and Z. Hameiri, "The effect of front pyramid heights on the efficiency of homogeneously textured inline-diffused screen-printed monocrystalline silicon wafer solar cells," Renewable Energy, vol. 78, pp. 590598, 2015. 5. A. Khanna, K. U. Ritzau, M. Kamp, A. Filipovic, C. Schmiga, M. Glatthaar, A. G. Aberle, and T. Mueller, “Screen-printed masking of transparent conductive oxide layers for copper plating of silicon heterojunction cell plating applications,” Applied Surface Science, vol. 349, pp. 880-886, 2015. 6. A. Khanna, R. A. Stangl, J. Wong, S. Duttagupta, A. G. Aberle, T. Mueller, “An extended fill factor loss analysis method for silicon wafer solar cells considering injection-dependent recombination,” in preparation, 2015. International conference papers 1. A. Khanna, J. Cunnusamy, F. Lin, A. G. Aberle, and T. Mueller, "Substantial improvement in the front contact resistance of screen-printed silicon wafer solar cells after a brief light-induced silver plating step," in Proc. 27th European Photovoltaic Solar Energy Conference, Frankfurt, 2012, pp. 1696-1699. 2. Z. P. Ling, F. Ma, S. Duttagupta, M. Tang, J. Ge, A. Khanna, T. Mueller, A. G. Aberle, R. A. Stangl, “Three-dimensional numerical analysis of hybrid heterojunction silicon wafer solar cells with front-side locally diffused emitter and 158 rear-side heterojunction BSF point contacts,” in Proc. 28th European Photovoltaic Solar Energy Conference, Paris, 2013, pp. 800-805. 3. A. Khanna, Z. P. Ling, V. Shanmugam, M. B. Boreland, I. Hayashi, D. Kirk, H. Akimoto, A. G. Aberle, and T. Mueller, "Screen printed metallisation for silicon heterojunction cells," in Proc. 28th European Photovoltaic Solar Energy Conference, Paris, 2013, pp. 1336-1339. 4. V. Shanmugam, J. Cunnusamy, A. Khanna, M. B. Boreland, and T. Mueller, "Optimisation of screen-printed metallisation for industrial high-efficiency silicon wafer solar cells," Energy Procedia, vol. 33, pp. 64-69, 2013. 5. R. A, Stangl, A. Khanna, J. Wong, S. Duttagupta, F. Ma, Z. Qiu, B. Hoex, A. G. Aberle, “Accurate performance prediction and loss analysis of silicon solar cells considering nonlinear recombination,” in Proc. 29th European Photovoltaic Solar Energy Conference, Amsterdam, 2014, pp. 446-450. Patent application 1. T. Mueller, A. Khanna, J. Wong, A. Karpour, F. Zheng, A. G. Aberle. Method of fabricating a solar cell. Patent Cooperation Treaty (PCT) application PCT/ SG2013/000088, 2013. 159 [...]... metals like silver (Ag)] Therefore, the focus of this PhD research is on the application and characterisation of advanced metallisation methods which are potentially suitable for cost-effective highefficiency mono-Si solar cells 3 1.2 Scientific and technical issues addressed in this thesis This thesis investigates advanced metallisation methods for mono-Si solar cells Two metallisation technologies are... annealing is evaluated for silicon heterojunction cell metallisation Screen-printed large-area silicon heterojunction cell results are presented and the loss mechanisms of these cells are discussed Substituting silver screen printing by copper plating for Si wafer solar cell metallisation can lead to significant cost savings Copper plating is especially applicable to silicon heterojunction cells because the... on a solar cell’s fill factor It is therefore necessary to identify various fill factor loss mechanisms to evaluate solar cell metallisation processes A new fill factor loss analysis method is developed in this thesis for Si wafer solar cells 2 Industrial mono-Si solar cells are random-pyramid textured on the illuminated surface and the resultant surface texture influences screen-printed contact formation... ion-implanted n+ Si regions is studied Screen printing for homojunction Si wafer solar cells typically includes a hightemperature (~800 °C) firing step for contact formation to heavily doped Si regions However, HET solar cells have two unique metallisation requirements: (1) the metal needs to contact a TCO layer and (2) the annealing step for contact formation/metal sintering needs to be restricted to... to such requirements and applied to HET cells 5 In order to metallise HET cells using metal plating, the transparent conductive oxide layer (TCO) for such cells needs to be masked before the plating step Therefore, TCO masking methods for HET cell plating using standard PV processing tools are examined 6 Copper (Cu) electroplating is investigated for HET cell metallisation with a view to reduce material... silicon (c-Si) The bandgap of c-Si is well suited for terrestrial PV applications (see section 2.1 for more details) Furthermore, Si is a cheap, abundant, environmentally benign material, and an excellent knowledge bank for Si devices has been built up by the microelectronics industry As a result Si is also the dominant material for industrial PV technologies, with Si wafer solar cells accounting for. .. reduction with regards to Si wafer solar cells requires either (1) advanced and cost-effective manufacturing technologies which improve the solar cell’s energy conversion efficiency or (2) the reduction of material costs associated with cell fabrication (example: reduction in the usage of expensive materials for solar cell processing or through the use of thinner Si wafers) Metallisation technologies... ion-implanted silicon regions A substantial reduction in the contact resistance of screen-printed silver contacts to both phosphorus-diffused and phosphorus ion-implanted silicon regions is observed after a brief silver light-induced plating step Screen printing for homojunction silicon wafer solar cells typically includes a hightemperature (800-900°C peak temperature) firing step for contact formation... Chapter 6 6 Substituting Ag screen printing by Cu plating for Si wafer solar cell metallisation can lead to significant cost savings Cu plating is especially applicable to HET cells because TCO layers in HET cells prevent cell degrading Cu diffusion into the Si wafer However, TCO layers need to be masked during plating TCO masking and Cu plating for HET cells is investigated in Chapter 7 Chapter 8 concludes... sides 𝑉𝑜𝑐 and 𝐼 𝑠𝑐 Substituting for 𝐹𝐹 in Eq (2.1), 𝜂 can be written in terms of 𝑉𝑜𝑐 , 𝐼 𝑠𝑐 and 𝐹𝐹: 𝜂= 𝑉𝑜𝑐 𝐼 𝑠𝑐 𝐹𝐹 1000 𝑊𝑚−2 × 𝐴 (2.3) 2.3 Fabrication The scope of this thesis covers metallisation methods applicable to homojunction Si solar cells and amorphous Si (a-Si:H)/c-Si heterojunction Si solar cells, both fabricated on mono-Si wafers The fabrication of both of these solar cell types is briefly . principles of silicon wafer solar cells 8 2.2 Current-voltage characteristics 11 2.3 Fabrication 12 2.3.1 Homojunction silicon wafer solar cells 12 2.3.2 Heterojunction silicon wafer solar cells. technologies for silicon wafer solar cell metallisation. The metallisation technologies are applied to silicon homojunction and amorphous silicon/ crystalline silicon heterojunction solar cells fabricated. ADVANCED METALLISATION METHODS FOR MONOCRYSTALLINE SILICON WAFER SOLAR CELLS ANKIT KHANNA (M.Tech., Indian Institute of Technology, Varanasi) A THESIS SUBMITTED FOR THE