Development of rare metal-free CZTS-based thin film solar cells

Cu₂ZnSnS₄ (CZTS) is one of the promising materials for low cost thin film solar cells, because of a suitable band gap energy around 1.5 eV, and the large absorption coefficient over 10⁴ cm^-1 In addition, all constituents of this film are abundant on the crust of the earth, and they are not toxic. In 1996, we reported a fabrication process of CZTS films by sulfurization of E-B evaporated precursors. We formed a new type of thin film solar cells constructed with SLG/Mo/CZTS/CdS/ZnO:Al/Al and achieved the conversion efficiency of 0.66 % for the first time. Conversion efficiency was gradually increased by the modification of the fabrication process. This consists of the optimization of the back contact and the window layer as well as the development of the high quality CZTS absorber. In order to produce the high quality CZTS absorber, the stacking order of precursor was investigated. Then, using the multi-period precursor, we fabricated the CZTS absorber with preferable morphology. From theses results, we confirmed that the film morphology had large influence to the conversion efficiency as well as the composition ratio. After that, we introduced the three-phased co-sputtering system with annealing chamber. Nowadays, the conversion efficiency increased to over 6.7% by optimizing many and many parameters in this system. Quite recently, using CZTS compound target, we achieved over 6.4 % efficiency with CZTS films prepared by the single sputtering followed by sulfurization.