Synthesis of Metastable Phase Wurtzite Copper Zinc Tin Sulfide (WZ-CZTS)Nanocrystals and Their Photovoltaic Applications: a Review
SUN Shuhong1, ZHU Yan1, QING Hongmei1, HU Yongmao2, YANG Bin3
1 Faculty of Materials Science and Engineering,Kunming University of Science and Technology,Kunming 650093; 2 Faculty of Engineering, Dali University, Dali 671003; 3 The National Engineering Laboratory for Vacuum Metallurgy,Kunming University of Science and Technology, Kunming 650093
Abstract: The direct bandgap semiconductor compound copper zinc tin sulphide (Cu2ZnSnS4, CZTS) is an ideal core for the development of green, low-cost, high-efficiency thin-film solar cells due to their high optical absorption coefficient, safe and non-toxic components, rich content as well as excellent photoelectric performance. In the early stage, the research was mainly focused on the application of zinc-tin-tin phase (kesterite-type, KT-CZTS) in CZTS-based thin-film solar cells. With the deepening of the research, a major breakthrough has been made in the the photoelectric conversion efficiency, yet there is still a big gap between its theoretical expected vaule -32.2%. The wurtzite-type (WZ-CZTS) prepared in 2011 exhibited a higher carrier concentration and a lower resistivity than the KT-CZTS and stannite-type (ST-CZTS), therefore it has a great potential for application in photovoltaic devices. However, WZ-CZTS is attributed to a metastable phase, which is easy to transform into a stable structure-KT-CZTS when the external conditions change. Besides, a small deviation of the stoichiometric ratio is inclined to produce binary or ternary compound heterophases, thus reduce the formation of heterophases, and prepare the pure , morphology-controlled WZ-CZTS nanocrystals are crucial as well as apply them in thin film solar cells. In recent years, except for investigating the effects of WZ-CZTS on device performance, researchers have been continuously attempting to search for a suitable preparation methods and related organic solvents, sulfur sources, precursor ratios, and optimized thin film preparation processes. And so far fruitful results have been achieved. The device efficiency is drastically improved while the high carrier concentration advantage of WZ-CZTS is fully utilized. At present, the conversion efficiency of solar cells prepared by using WZ-CZTS as the thin film solar cell absor-ber layer has rapidly increased from the initial 2.44% to 6.0%. Initially the preparation of WZ-CZTS powder mainly adopted hot injection method, while the complex preparation process, expensive equipment and high reaction temperature (≥240 ℃) restricted its widely application. In the past two years, researchers introduced one-pot method to prepare WZ-CZTS powder. This method has a series of merits including low reaction temperature, no need for inert gas protection and simple process flow, thus making it possible for large-scale industrial production of WZ-CZTS powder. At present, the nano-crystal ink coating method is mainly used for preparing WZ-CZTS solar cell absorber layer film, which has a good crystallinity, fewer defects, relatively simple process, low preparation cost and easy realization of large-scale production,and has attracted widespread attention. In this paper, the structural characteristics, preparation process, photoelectric properties and application of WZ-CZTS are reviewed. The effects of solvent, sulfur source and precursor on the phase and morphology are highlighted. In addition, the application of WZ-CZTS in photovoltaic field is discussed. Finally, it summarizes the current challenges and future research directions of WZ-CZTS and looks forward to possible future breakthroughs.
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