Compositional Dependence of Chemical and Electrical Properties in Cu2ZnSnS4 Thin Films

Photovoltaic-grade thin-film solar cell absorber layers composed of Cu
₂
ZnSnS
₄
are most often prepared under nonstoichiometric compositions, in particular Cu-poor and Zn-rich conditions. The effects of compositional deviations on chemical and electrical properties of this material are under investigation and are expected to have a strong influence on final device performance. Herein, a study of variation in the cationic ratios (Cu/Zn, Cu/Sn) across a broad range and their effect on the properties of thin films is presented. After etching to remove the ZnS phase, the ratio of Cu/Zn is nearly constant between all the films, while there are significant variations in the ratio of Cu/Sn. This is correlated with changes in the chemical and electrical properties of these films, as determined by variable temperature conductivity measurements. The postetching composition and absence of Sn–S secondary phases points to multivalency of Sn in this material, in addition to other potential factors such as differences in grain size and defect formation. The Cu/Sn ratio is shown to strongly affect the electrical properties of films, in particular of the conductivity, grain boundary barriers, and localization length.