Synergistic In/Cd Co-doping strategy for optimizing band alignment achieves 10.23 % efficient ZnO-free Cu2ZnSn(S,Se)4 solar cells

The band alignment of Cu₂ZnSn(S,Se)₄ (CZTSSe)/CdS(In) in novel CZTSSe solar cells without a ZnO window layer has been initially investigated in our previous studies. However, the In content of CdS(In) diffused into the absorber is low, and the ability to regulate the band arrangement is limited. In this work, we employ In-ion-doped absorbers to adjust defects. However, this approach leads to a significant loss in short-circuit current density (Jsc). To address this, Cd is introduced to enhance Jsc through grain growth promotion and morphological improvement, thereby offsetting the Jsc loss induced by In doping. The co-doping of In and Cd reduces the conduction band offset (CBO) from 0.28 eV to 0.21 eV, thereby inhibiting carrier recombination and enhancing transport efficiency. Additionally, the concentration of bulk defects is reduced by an order of magnitude. As a result, the fill factor (FF) increased from 56.97 % to 60.08 %, Jsc increased from 37.10 mA/cm² to 38.66 mA/cm², and open-circuit voltage (Voc) increased from 431.64 mV to 440.48 mV. Finally, the power conversion efficiency (PCE) of CZTSSe solar cell is enhanced to 10.23 %.