An efficiency of 7.17% on a 5x5cm2-sized Cu2ZnSnS4 (CZTS) thin-film submodule was achieved. For fabrication, vacuum-based processes were used to deposit a Mo back electrode and a metal precursor. The CZTS absorber layer was formed through high-temperature annealing with sulfur containing gas. A Cd-based buffer layer and a ZnO window layer were then deposited by chemical bath deposition and metal-organic chemical vapor deposition, respectively. A monolithically integrated structure with seven series connections was introduced by laser and mechanical patterning. Current progress resulted from an improvement in sulfurization, the introduction of annealing and ZnO window tuning. Furthermore, 4.76% efficiency on 5x5cm2-sized Cd-free CZTS submodule with a Zn-based buffer layer had been achieved. Its structure was essentially the same as the Cd-based CZTS submodule except for the buffer layer. Optimization of absorber composition and tuning of annealing conditions for the Zn-based buffer contributed to this achievement. Preliminarily, 4.72% efficiency was obtained on the CZTS submodule with an In-based buffer as an alternative to the Cd-free buffer. In view of the ready availability and environmentallyfriendliness of constituent materials, we are intensively accelerating the development of Cd-free CZTS submodules with higher efficiency.
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