Optimum precursor stacking sequence of Cu₂ZnSnSe₄ thin film solar cell in selenium atmosphere

In this study, Cu₂ZnSnSe₄ (CZTSe) films were prepared via evaporation and selenization. Metallic precursors were first deposited upon a fluorine-doped tin oxide (FTO) substrate via evaporation, followed by selenization in a selenium–argon atmosphere at a maximum temperature of 500 °C for a duration of 30 min. Selenization is always accompanied by interdiffusion between the metallic precursor layers. However, in conventional stacked precursor structures, various secondary phases are formed in the film owing to insufficient interdiffusion between the precursor metals. To address this problem, we constructed CZTSe films using multi-stacked metallic layer precursors (without changing the total thickness of each type of metal or using metal selenides as precursors). Consequently, the secondary phase formation was effectively suppressed, the quality of the CZTSe film was improved, and its performance as a bifacial photovoltaic device was enhanced. The optimal efficiency with front illumination was 4.23 %; bifacial illumination further increased the efficiency to 4.45 %.