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 opencircuit voltage (V-OC) and filling factor (FF), while the decreased PCE is due to decreased V-OC, FF, and J(SC). By using quantitative analysis methods, the increased V-OC and FF are mainly attributed to the increased shunt resistance (R-sh) and decreased reverse saturation current density (J(0)), and the decreased short-circuit current density (J(SC)) is attributed to the increased conduction band offset (CBO). The influence mechanism of Zn doping on R-sh, J(0), and CBO is discussed.