Efficient (Cu1−xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils

Cation substitution plays a crucial role in improving the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this work, we report a significant efficiency enhancement of flexible CZTSSe solar cells on Mo foils by partial substitution of Cu+ with Ag+. It is found that the band gap (Eg) of (Cu1−xAgx)2ZnSn(S,Se)4 (CAZTSSe) thin films can be adjusted by doping with Ag with x from 0 to 6%, and the minimum Eg is achieved with x = 5%. We also found that Ag doping can obviously increase the average grain size of the CAZTSSe absorber from 0.4 to 1.1 μm. Additionally, the depletion width (Wd) at the heterojunction interface of CAZTSSe/CdS is found to be improved. As a result, the open-circuit voltage deficit (Voc,def) is gradually decreased, and the band tailing is suppressed. Benefiting from the enhanced open-circuit voltage (Voc), the power conversion efficiency (PCE) is successfully enhanced from 4.34% (x = 0) to 6.24% (x = 4%), and the Voc,def decreases from 915 to 848 mV.