Development of Earth-abundant CZTS Thin Film Solar Cells with Sulfurization Technique
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Development of Earth-abundant CZTS Thin Film Solar Cells with Sulfurization Technique Hironori Katagiri1,2, Kazuo Jimbo1 and Tsukasa Washio1,2 1 Nagaoka National College of Technology, 888 Nishikatakai, Nagaoka, 940-8532, Japan 2 JST-CREST, 888 Nishikatakai, Nagaoka, 940-8532, Japan ABSTRACT Cu2ZnSnS4 (henceforth CZTS) absorber layers are successfully synthesized by a sulfurization technique of physical vapor deposited precursors. In our previous report, we have clarified that the off-stoichiometry composition of Cu-poor and Zn-rich is desirable to achieve high conversion efficiency. By using CZTS compound target that provide such active composition, we could conduct a simple single sputtering method to prepare CZTS absorber. In our laboratory, a two-stage process of precursor preparation followed by sulfurization is a major fabrication method from the start of this study. We think that this method is suitable for a mass production. An optimization of the sulfurization process is a quite important issue because the active composition was already revealed. In this paper, TG/DTA system available in the H2S atmosphere is introduced to optimize the sulfurization condition. As a result, bump-free CZTS films were prepared successfully and the fluctuation of J-V properties in one substrate was drastically suppressed. INTRODUCTION In 1988, K. Ito and T. Nakazawa of Shinshu University succeeded in fabricating the CZTS thin film by an atomic beam sputtering technique, and clarified that the optical band gap energy was near the optimum value of 1.45 eV as a solar cell absorber layer. In addition, the photovoltaic effect of 165 mV was obtained by making hetero diode with transparent conductive film with Cadmium Tin Oxide (CTO) [1]. In 1996, the authors fabricated successfully a CZTS thin film on a soda-lime glass (SLG) substrate by an electron beam (EB) evaporation followed by a sulfurization technique. As a result, the open-circuit voltage of 400 mV and the conversion efficiency of 0.66% were obtained for the first time by making a thin film solar cell structure of SLG/Mo/CZTS/CdS/AZO [2]. To improve the conversion efficiency, many experiments have been conducted since then. In the late 1990’s, the authors focused on the stoichiometric CZTS films to characterize their properties. As for now, this might be one of the reasons limiting the conversion efficiency as low as 2 %. Table I shows the fabrication processes used and the obtained photovoltaic properties in our early works. In fact, our study after process (4) revealed that in order to obtain the high conversion efficiency cells, CZTS of the off-stoichiometric composition; namely Cu-poor and Zn-rich composition, was preferable to the stoichiometry. To clarify such active composition, a co-sputtering system was utilized in our laboratory. As a result, we determined the active composition as follows: 0.76-0.90 for Cu/(Zn+Sn), 1.1-1.3 for Zn/Sn, 1.8-2.0 for Cu/Sn. And now, we are able to prepare CZTS films using a simple single sputtering followed by a sulfurization technique
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