Impact of sodium on the secondary phases and current pathway in Cu₂(Zn,Sn)Se₄ thin film solar cell

In this study, we investigated the influence of Na content on secondary phases and current pathway in Cu₂(Zn,Sn)Se₄ (CZTSe) thin film solar cells with the following structure: Ti/Mo:Na/Mo/CZTSe/CdS/i-ZnO/ZnO:Al/Al. The application of Na-doped Mo target as a source of sodium. Experimental results demonstrate that increasing the Na content leads to an increase in the quantity of secondary phase SnSe₂ on the surface of the absorber layer; however, it did not appear to affect the secondary phases of Cu₂SnSe₃ (CTSe) or ZnSe. Excessive quantities of Na were shown to have an adverse effect on device efficiency. Our results using conductive atomic force microscopy (C-AFM) revealed that an increase in the quantity of secondary phase SnSe₂ can shift the current pathway on the surface of CZTSe from CZTSe grain boundaries (GBs) to the SnSe₂ grains. The role of secondary phase SnSe₂ of the CZTSe acted as a channel for the current flow, which results in high leakage current and low device efficiency.