Cs‐treatments in Kesterite Thin‐Film Solar Cells for Efficient Perovskite Tandems

Abstract Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells are an attractive choice for a bottom cell of the low‐cost and environmental tandem solar cells with perovskite. However, the progress in developing efficient perovskite/CZTSSe tandem solar cells has been hindered by the lack of high performance of the CZTSSe bottom cell. Here, an efficient CZTSSe bottom cell is demonstrated by adopting a facile and effective CsF treatment process. It is found that the CsF treatment not only facilitates grain growth and improves phase homogeneity by suppressing the detrimental deep‐level defects and secondary phases, but also induces larger band bending and stronger drift force at the P‐N junction. As a result, the carrier extraction/transport can be effectively accelerated, while reducing the interfacial recombination. These combined effects eventually result in a significant performance enhancement from 8.38% to 10.20%. The CsF‐treated CZTSSe solar cell is finally applied to the mechanically‐stacked perovskite/CZTSSe 4‐terminal tandem cell by coupling a semi‐transparent perovskite top cell, which exhibits the highest reported tandem efficiency of 23.01%.