Doping Behavior of Zn in CdS and Its Effect on the Power Conversion Efficiency of the Cu2ZnSn(S, Se)4 Solar Cell

High carrier recombination at the Cu2ZnSn (S, Se)4(CZTSSe)/CdS interface is the critical issue that results in the low power conversion efficiency (PCE) of CZTSSe solar cells. To reduce the recombination by optimizing the CZTSSe/CdS interfacial structure, we fabricated a Zn doped CdS (ZnxCd1 – xS) thin film with x of 0–0.32 and a CZTSSe solar cell with the ZnxCd1 – xS as the buffer layer. It is found that Zn substitutes for Cd in the x range of 0–0.26 and that some of the Zn substitutes for Cd and another Zn locates in the interstitial site of the CdS lattice in the x range of 0.26–0.32, which make the lattice mismatch between CZTSSe and ZnxCd1 – xS decrease in x of 0–0.26 and increase in x of 0.26–0.32. The conduction band offset at the CZTSSe/ZnxCd1 – xS interface is demonstrated by XPS to be a positive ″spike″-like type and increases from 0.11 to 0.43 eV as x increases from 0 to 0.32. PCE is increased from 5.00 to 7.73% by optimizing x. The increased PCE is attributed to increased open-circuit voltage (VOC) and filling factor (FF), while the decreased PCE is due to decreased VOC, FF, and JSC. By using quantitative analysis methods, the increased VOC and FF are mainly attributed to the increased shunt resistance (Rsh) and decreased reverse saturation current density (J0), and the decreased short-circuit current density (JSC) is attributed to the increased conduction band offset (CBO). The influence mechanism of Zn doping on Rsh, J0, and CBO is discussed.