An efficiency of 6.3% with an In-based buffer layer and 5.8% with a Zn-based buffer layer 5×5cm2-sized Cu2ZnSnS4 (CZTS) thin-film submodules are achieved. These are the vacuum-processes-based sulfur-only CZTS submodules without toxic Cd-based buffer layer. For the CZTS submodules with the In-based buffer layer, we conducted annealing of absorber before chemical bath deposition and found that larger degree of oxidation contributes to the achieved high efficiency. For the CZTS submodule with the Zn-based buffer layer, we adjusted Zn/Sn ratio in absorber surface and confirmed that the lower Zn/Sn ratio in absorber surface contributes to the realized high efficiency. Both improvements on the In- and the Zn-based buffers are from increase in Voc and FF, which clearly shows that their junction quality is improved. We are intensively accelerating the development of environmentally-friendly Cd-free CZTS submodules with higher efficiency and believe that further achievement will be reported at the conference.
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