The efficiency of Cu 2 ZnSn(S x ,Se 1− x ) 4 (CZTSSe) solar cells is significantly lower than that of other solar cells such as Cu(In,Ga)Se 2 solar cells. This is because the open circuit voltage ( V oc ) of CZTSSe solar cells is significantly low as compared to theoretical value. Thus, we focus on the improvement of the hetero junction quality by a cleaning process using NH 4 OH etchant. By the NH 4 OH etching, the decrease in photoluminescence intensity of CZTSSe absorber is observed, implying that the defects are generated by the etching near the surface of CZTSSe absorbers. Energy dispersive X-ray spectroscopy revealed that the cations such as Cu, Zn and Sn are dissolved out by the etching. Therefore, the NH 4 OH etching is not adequate to clean the surface of CZTSSe absorbers. Based on the above result, we optimized the condition of chemical bath deposition (CBD) of CdS buffer layers. V oc and fill factor are increased by decreasing the concentration of NH 4 OH, thereby improving efficiency. This is considered that the defects in space-charge region of CZTSSe solar cells are decreased by optimizing the solution for CBD-CdS. In conclusion, the V oc of CZTSSe solar cells are improved by reducing use of NH 4 OH in CBD-CdS solution.
