Effect of ammonia etching on structural and electrical properties of Cu2ZnSn(S,Se)4 absorbers

The efficiency of Cu2ZnSn(Sx,Se1−x)4 (CZTSSe) solar cells is significantly lower than that of other solar cells such as Cu(In,Ga)Se2 solar cells. This is because the open circuit voltage (Voc) 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 NH4OH etchant. By the NH4OH 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 NH4OH 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. Voc and fill factor are increased by decreasing the concentration of NH4OH, 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 Voc of CZTSSe solar cells are improved by reducing use of NH4OH in CBD-CdS solution.