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 Cu2-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 Cu2-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 Cu2-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.
All Science Journal Classification (ASJC) codes
- Mechanics of Materials
- Mechanical Engineering
- Metals and Alloys
- Materials Chemistry