Novel Cu2znsns4 Nanocrystals: a Promising Approach for High
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Novel Cu2ZnSnS4 Nanocrystals: A Promising Approach for High- Performance and Low-Cost Cu2ZnSnS4 Photovoltaics Xu Liu A THESIS IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY School of Photovoltaic and Renewable Energy Engineering Faculty of Engineering August 2017 THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: LIU First name: XU Other name/s: Abbreviation for degree as given in the University calendar: PHD School: School of Photovoltaic and Renewable Energy Engineering Faculty: Faculty of Engineering Title: Novel Cu2ZnSnS4 Nanocrystals: A Promising Approach for High-Performance and Low-Cost Cu2ZnSnS4 Photovoltaics Abstract 350 words maximum: Cu2ZnSnS4 (CZTS) nanocrystals can be prepared as the ink for fabrication of CZTS thin film and corresponding solar cells. Nanocrystal ink allows for the preparation of low-cost thin film semiconductors at low temperatures and at high production rates and yields. However, great difficulties in achieving nearly micron-sized large grains from nano-sized crystals using sulfurization treatment challenge the use of CZTS nanocrystals for pure-sulfide CZTS solar cells. This thesis aims at overcoming this grain growth problem, achieving high quality CZTS absorber layers for solar cells, and revealing the grain growth mechanism during sulfurization. In this work, CZTS large grains are successfully obtained by an efficient phase-transition-driven grain growth strategy. Metastable wurtzite CZTS nanocrystals are chose and synthesized to make precursor coatings instead of stable kesterite CZTS nanocrystals. Three necessary factors containing precursor’s composition, sulfurization annealing condition, and incorporation of extrinsic dopant are firstly exploited. The annealed CZTS absorber layer has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. The preliminary photovoltaic cells demonstrate 4.83% efficiency without anti- reflection coatings. To enhance phase and compositional controllability as well as reduce the thickness of fine-grained layers, a solution-based-doping method is devised for tuning Na content and SnS powder is introduced into the sulfurization treatment. The results demonstrate that Na amount and the use of SnS powder provide leverage with which to improve the microstructure and compositional distribution of the final absorber. By employing this leverage to optimize CZTS absorbers prepared from wurtzite nanocrystals, the thickness of the notorious fine-grained layer is significantly reduced and the energy conversion efficiency of solar cells is increased to 6.0% in the absence of an antireflection coating layer due to the improvement of absorbers’ quality. By classifying samples in terms of temperature and time series, we systematically study the formation pathway from nanometer-sized crystals in a wurtzite phase to micrometer-sized grains in a kesterite phase. An in-depth understanding of the spatial grain growth and composition and microstructure evolution during the transformation course is clearly revealed, paving the way of delivering better device performance by this solution-based nanocrystal method in future. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). ……………………………………………… ……………………………………..……… ... ….……….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: i COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.’ Signed…………………………………… Date………………………………………… AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ ORIGINALITY STATEMENT ii ‘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 in style, presentation and linguistic expression is acknowledged.’ Signed…………………………………… Date………………………………………… iii Scientia Manu et Mente ——— UNSW Motto 百种弊病，皆从懒生 ———曾国藩 Guofan Zeng Hundreds of ills, all from the lazy iv ACKNOWLEDGEMENTS I would like to express my gratitude to the many people who have helped and supported me during the past three-and-a-half years of research. I would like to thank my supervisor Dr. Xiaojing Hao and joint-supervisor Scientia Prof. Martin Green for their support, inspiration, and valuable advice for this thesis. I would like to greatly thank Xiaojing for her constant confidence on me and her good research platform. I would like to thank A/ Prof. John Stride for sharing his lab resources in School of Chemistry and professional advices on chemical experiment. Thanks also to Dr.Robert Patterson and Nicholas Shaw for lab induction, lab maintenance, and advice on equipment maintenance. Thanks to the administration and laboratory support team in SPREE for making the centre friendly and safe for conducting research. Thanks to Dr. Yu Wang, Dr Charlie Kong, Katie Levick, Yin Yao and Sean Lim from UNSW Analytical Centre for their training on XRD, FIB, TEM, SEM, and AFM measurements. I would like to thank Dr. Jialiang Huang, Dr. Fangyang Liu, Dr. Ziheng Liu, Aobo Pu, Dr. Ning Song, Dr. Rui Sheng, Kaiwen Sun, Fangzhou Zhou, Zhilong Zhang, Yuanfang Zhang for their valuable help on experiment and characterizations. i Finally, I would like to thank my family for their love and support. Thanks to my parents for their trust on me. Love you. Thanks to the people who make me strong and indomitable. ii ABSTRACT Cu2ZnSnS4 (CZTS) nanocrystals can be prepared as the ink for fabrication of CZTS thin film and corresponding solar cells. Nanocrystal ink allows for the preparation of low- cost thin film semiconductors at low temperatures and at high production rates and yields. However, great difficulties in achieving nearly micron-sized large grains from nano-sized crystals using sulfurization treatment challenge the use of CZTS nanocrystals for pure-sulfide CZTS solar cells. This thesis aims at overcoming this grain growth problem, achieving high quality CZTS absorber layers for solar cells, and revealing the grain growth mechanism during sulfurization. In this work, CZTS large grains are successfully obtained by an efficient phase- transition-driven grain growth strategy. Metastable wurtzite CZTS nanocrystals are chose and synthesized to make precursor coatings instead of stable kesterite CZTS nanocrystals. Three necessary factors containing precursor’s composition, sulfurization annealing condition, and incorporation of extrinsic dopant are firstly exploited. The annealed CZTS absorber layer has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. The preliminary photovoltaic cells demonstrate 4.83% efficiency without anti-reflection coatings. iii To enhance phase and compositional controllability as well as reduce the thickness of fine-grained layers, a solution-based-doping method is devised for tuning Na content and SnS powder in introduced into the sulfurization treatment. The results demonstrate that