Cu 2 ZnSnSe 4 Thin‐Film Solar Cells by Thermal Co‐evaporation with 11.6% Efficiency and Improved Minority Carrier Diffusion Length

11.6%-efficiency Cu2ZnSnSe4 (CZTSe) thin-film solar cells are fabricated via a thermal co-evaporation method. The CZTSe thin film with improved microstructure exhibits a minority carrier diffusion length over 2 μm, resulting in efficient photogenerated carrier collection in the device. A comparative study of photoluminescence in pure selenide and pure sulfide devices shows reduced band-tailing for the pure selenide phase.