Optimizing the Ratio of Sn4+ and Sn2+ in Cu2ZnSn(S,Se)4 Precursor Solution via Air Environment for Highly Efficient Solar Cells

The use of different Sn valence states (such as Sn 4+ and Sn 2+ ) in the Cu 2 ZnSn(S,Se) 4 (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl 2 ·2H 2 O with anhydrous SnCl 4 can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl 4 ·5H 2 O powder is used instead of anhydrous SnCl 4 in air environment, and the proportion of Sn 4+ and Sn 2+ precursor solutions is further systematically studied. When the ratio of Sn 4+ to Sn 2+ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep‐level defects. In particular, the concentration of Cu Zn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When J SC is maintained, V OC and FF are considerably improved. Finally, CZTSSe thin‐film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn 4+ to Sn 2+ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin‐film solar cells is demonstrated.