A comparative investigation of secondary phases and MoSe₂ in Cu₂ZnSnSe₄ solar cells: Effect of Zn/Sn ratio

Our objective in this study was to elucidate the influence of the Zn/Sn ratio (ZTR) in the absorber layer on the formation of secondary phases, leakage current pathways, and the thickness of the MoSe₂ layer in Cu₂ZnSnSe₄ (CZTSe) solar cells. Experiment results revealed that under Zn-rich conditions (ZTR ≥1.11), the bottom of the absorber layer contains small quantities of Cu_2-XSe and SnSe₂ secondary phases. Under Sn-rich conditions (ZTR ≤0.75), large quantities of SnSe, SnSe₂, and Cu₂SnSe₃ secondary phases appeared on the surface of the absorber layer. Conductive atomic force microscopy (C-AFM) revealed that the presence of secondary phases caused leakage current over large areas at the surface of the absorber layer. We determined that Cu_2-XSe is involved in leakage current at CZTSe grain boundaries, whereas SnSe₂ or Cu₂SnSe₃ tend to promote leakage current in the grains (or near grain boundaries). A higher ZTR resulted in a thinner MoSe₂ layer due to large quantities of Cu and Na at the bottom of Zn-rich ZTR specimens, which promoted the formation of Cu_2-xSe and Na₂Se phases. These phases competed with the Mo to react with the Se, which decreased the amount of the MoSe₂ formed from Se and Mo, resulting in a thinner MoSe₂ layer. The highest conversion efficiency (5.2%) was obtained from a Zn-rich specimen (ZTR1.43). In contrast, the Sn-rich specimens (ZTR0.75 and ZTR0.63) presented conversion efficiency of 0%. The influence of ZTR on device performance can be attributed to widespread leakage current due to excessive quantities of SnSe₂ and Cu₂SnSe₃ secondary phases on the surface of the absorber layer. An overly thick MoSe₂ layer was also shown to result in excessive series resistance.