Boosting efficiency to 13.07% in flexible Cu2ZnSn(S,Se)4 solar cells via heterojunction regulation of defects and stress
Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are promising for lightweight and mechanically pliable photovoltaics, yet their performance is limited by severe non-radiative recombination and residual stress. Here, we report a sealed constant temperature (SCT) annealing strategy that simultaneously optimizes the CZTSSe/CdS heterojunction and alleviates stress. Under uniform mild thermal conditions (85°C, 5 h), SCT annealing promotes gradient diffusion of Cd2+ into the absorber, partially substituting Zn2+, which optimizes band alignment, passivates interface defects, and suppresses near-interface CuZn defects. This reduces open-circuit voltage loss and improves fill factor. The flexible device achieves a power conversion efficiency of 13.07%, a significant improvement over the reference (12.1%). The SCT strategy also enhances mechanical flexibility by reducing residual stress. Our findings provide a controllable route to advance both efficiency and flexibility of flexible CZTSSe solar cells.