Characteristics of Cu2ZnSnSe4 and Cu2ZnSn(Se,S)4 absorber thin films prepared by post selenization and sequential sulfurization of co-evaporated Cu-Zn-Sn precursors

Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 absorber films processed from co-evaporated Cu-Zn-Sn precursor films by post selenization and sequential sulfurization were investigated by varying Cu/(Zn + Sn) ratio and selenization/sufurization temperatures. Structural, electrical and optical properties were largely dependent on the experimental parameters. The selenization process of the Cu-Zn-Sn precursors was successful in producing Cu2ZnSnSe4 phase with dense microstructure when the Cu/(Zn + Sn) ratio of 0.8 was initially used. The absorber led to a photovoltaic cell efficiency of ∼1.04% with a short circuit current of ∼21.9 mA/cm2. Sequential sulfurization of the selenized films resulted in completely-transformed Cu2ZnSn(S,Se)4 phase with a minor secondary phase of Cu2S. Cell performance was not improved after sulfurization primarily due to poor microstructural features that became worse with increasing the Cu/(Zn + Sn) ratio.