Thermally evaporated Se, including the molecules of Se2, Se5, Se6, Se7, and Se8 in the vapor at 550–600 °C, is widely used for the selenization of CZTSSe films. However, active small‐molecule Se2 tends to form large clusters of Se atoms at saturation vapor pressure, resulting in the large Se deficiency and poor crystallization of CZTSSe films. To regulate the time for Se to reach saturation vapor pressure and promote the role of active small‐molecule Se, the evaporation area of Se is controlled. Then the corresponding grain growth mechanism of CZTSSe films and device efficiency are systematically investigated. The results demonstrate that the appropriate evaporation area of Se can not only optimize the time for Se vapor to reach saturation, promote the rapid reaction crystallization between the precursor films and active Se2, but also inhibit the nucleation at the CZTSSe/Mo interface, thus making the CZTSSe films show a perfect top‐down grain growth mode. By preliminary optimization, when the evaporation area of Se is 245 mm2, a large‐grain spanning monolayer CZTSSe thin film is obtained; meanwhile, a device with a high efficiency of 12.39% is achieved. This study provides a new selenization strategy for improving the crystallinity of CZTSSe thin films.

As a promising absorber material for thin film solar cells, the reported hightest power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) is still far from the detailed balance limit efficiency due...