Nanoscale charge transport and local surface potential distribution to probe defect passivation in Ag doped Cu₂ZnSnS₄ absorbing layer

The performance of earth abundant Cu₂ZnSnS₄ (CZTS) material is limited by high deficit of open circuit voltage (V_OC) which is mainly due to the easy formation of Cu_Zn antisite defects. Suppression of Cu_Zn defects is thus inevitably required for further developments in CZTS based solar cells. We studied systematic increase of Ag doping in CZTS thin film and investigated the nanoscale electrical properties using Kelvin probe force microscopy and current sensing atomic force microscopy (CAFM) to probe Cu_Zn defects. Crystallographic analysis indicated the successful partial substitution of Cu⁺ ions by large size Ag⁺ ions. The considerable decrease in grain boundary potential from 66.50 5.44 mV to 13.50 2.61 mV with Ag doping, suggesting the substantial decrease in Cu_Zn defects. Consequently, CAFM measurement confirms the remarkable increment in minority carrier current with Ag doping and their local mobility in CZTS layer. Finally, the lower persistent photoconductivity and fast decay response of photogenerated carriers for Ag doped CZTS photodetector further validate our results. This study provides a fresh approach of controlling deleterious Cu_Zn defects in CZTS by tuning Ag content that may guide researchers to develop next generation high-performance CZTS based solar cells.