Efficient Cd-Free Flexible CZTSSe Solar Cells with Quality Interfaces by Using the Zn1–xSnxO Buffer Layer

The severe carrier recombination at the CZTSSe/CdS heterojunction interface and high toxicity of Cd limit the further development of flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells. Here, we apply a suitable and environmentally friendly Zn1–xSnxO (ZTO) buffer layer to suppress the interface recombination in the flexible solar cell for the first time. The ZTO buffer layers with good qualities have been successfully obtained by varying the Sn/(Zn + Sn) value. The solar cell has obtained an efficiency of 8.7%, which is the highest efficiency reported for a Cd-free flexible CZTSSe solar cell so far. The optimal device has more ideal diode parameter values, indicating that the interface quality is significantly improved. The VOC-T characteristic demonstrates that the interfacial recombination of flexible ZTO devices has been greatly reduced. The theoretical simulations show that the flexible CZTSSe devices with optimal ZTO layers achieve good band alignment to reduce the interface recombination, elucidating the intrinsic link between the band structure and the device performance. This investigation provides a solution to achieve environmentally friendly and efficient flexible CZTSSe solar cells by designing the interface engineering of the absorber/buffer heterojunction in solar cells.