Enhancing performance of Cu2ZnSn(S, Se)4 solar cells via non-uniform gradient and flat bands induced by Cd substitution

Severe carrier recombination at the back (Mo/CZTSSe) and front (CZTSSe/CdS) interfaces is one of the most important reasons hindering the development of open-circuit voltage (VOC) and fill factor (FF) in Cu2ZnSn(S, Se)4 (CZTSSe) solar cells. In this study, we intentionally introduced a non-uniform distribution of Cd impurities into the middle of the absorber layer, designing and fabricating a CZTSSe solar cell with a non-uniform "V"-shaped graded bandgap structure. This structure is aimed at providing a favorable back electric field, reducing carrier recombination at the Mo/CZTSSe interface. The PCE of the CZTSSe solar cell improved from 8.88 % to 10.89 %, significantly enhancing FF and VOC. Additionally, we utilized the solar cell simulation software SCAPS-1D to simulate the position of the minimum point in the V-shaped graded bandgap and combined this with experimental results to explore the effect of Cd doping location on the performance of CZTSSe solar cells. It's worth noting that the non-uniform Cd-doped solar cell displayed exceptional stability, demonstrating an efficiency enhancement from 10.28 % to 10.94 % after being exposed to air for 30 days.