An efficiency of 2.01% on a 5×5cm2-sized Cu2ZnSnS4 (CZTS) thin-film submodule is achieved. This is the first submodule by Solar Frontier that does not use In, Se and Cd. For fabrication, vacuum-based processes are used to deposit the Mo back electrode and the metal precursor. The CZTS absorber layer is formed through high-temperature annealing with sulfur containing gas. The Zn-based buffer layer and the ZnO window layer are then deposited by chemical bath deposition and metal-organic chemical vapor deposition, respectively. A monolithically integrated structure with eight series connections is introduced. The uniformity of device performance is characterized by photoluminescence mapping and electroluminescence imaging. The results showed that there is some non-uniformity in our Zn-based buffer submodules compared to our Cd-based buffer submodules. Additionally, it becomes clear that a reduction of series resistance is one of the key issues for further improvement. Preliminarily, 6.21% efficiency on a 5×5cm2-sized Cu2ZnSnS4 submodule with the Cd-based buffer is obtained when we make efforts to decrease the series resistance. Cd-free submodules are also under optimization.
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