Selenization Reaction Driving Force Regulation for High‐Quality Absorber Enabling Li&Na Co‐Doped Flexible CZTSSe Solar Cells With 12.42% Efficiency

ABSTRACT High‐quality kesterite Cu ₂ ZnSn(S,Se) ₄ (CZTSSe) absorbers are very important for efficient flexible solar cells. In this paper, we develop a tailored selenization process leveraging the bendability of Mo foil substrates to improve the quality of the CZTSSe absorbers. This method enhances the driving force of the selenization reaction, resulting in denser CZTSSe absorbers with significantly improved crystallinity. Consequently, the quality of the CZTSSe/CdS heterojunction is obviously improved, raising the fill factor (FF) of the device from 64.76% to 70.62%. In addition, carrier recombination at the CZTSSe/CdS interface is effectively suppressed, and the back‐contact barrier height is decreased by 13.16%, promoting charge carrier transport across the device. The optimized flexible CZTSSe solar cell achieves a champion power conversion efficiency (PCE) of 12.42% with an open‐circuit voltage ( V _OC ) of 0.55 V. Notably, performance uniformity is also improved, with a 1.22 cm ² flexible device reaching 9.59% PCE. This work will provide a novel strategy for improving flexible CZTSSe solar cell performance.