Critical role of Zn/Sn ratio to enhance Cu-Zn-Sn-S solar cell efficiency by suppressing detrimental Cu2-xS secondary phase

The co-existing secondary phases, in off-stoichiometric Cu-Zn-Sn-S (CZTS) solar cells, are one of the main causes of performance gap between earth abundant CZTS and matured Cu-In-Ga-Se (CIGS) solar cells technology. Highly metallic Cu-S based secondary phases provide shunting paths to the current flow and critically affect the device performance. Several chemical etchants have been developed and used to remove conductive Cu2-xS phase from CZTS absorber surface before device fabrication. Present work is mainly focused to tune the Z/Sn composition ratio in co-sputtered CZTS active layer to suppress the formation of Cu2-xS phase during the growth itself to eliminate the additional step of etching in device fabrication. Significant reduction of Cu2-xS secondary phase was observed with decreased Zn/Sn ratio, which resulted in more than 3 times efficiency enhancement for the final composition. The fabricated solar cell in the stacking order of Ag/ITO/i-ZnO/CdS/CZTS/Mo/SLG with optimum composition showed an efficiency of 6.11% without any antireflection coating. Controlling the Zrit/Sn ratio resulted in an effective and virtuous solution to avoid the detrimental secondary phases like Cu2-xS which conventionally required a chemical etching treatment.