Flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells have wide application prospects. N-type window layers and contact among the layers have an important effect on the properties of flexible CZTSSe solar cells. Here, we present a modified structure for flexible CZTSSe solar cells with window layers (CdS/ITO) instead of the traditional window layers (CdS/ZnO/ITO) for higher performance and lower cost. The flexible CZTSSe device realizes 9.2% efficiency with an improved fill factor (FF) from 57.7 to 63.6%. Systematic physical measurements show that the increase in FF comes from a significant decrease in series resistance (R s ). To enhance the antireflective function of the window layers, we introduce MgF 2 antireflection coatings on the surfaces of solar cells. The average J sc of devices is significantly enhanced 6% due to the antireflective effect. A champion device achieves an efficiency of 9.7% with the highest FF of 64.0% in the flexible CZTSSe solar cells. The present investigation will provide a new strategy for the development of flexible CZTSSe solar cells.

Flexible CZTSSe solar cells have attracted much attention due to their earth-abundant elements, high stability, and wide application prospects. However, the environmental problems caused by the high toxicity of the Cd in the buffer layers restrict the development of flexible CZTSSe solar cells. Herein, we develop a Cd-free flexible CZTSSe/ZnO solar cell. The influences of the ZnO films on device performances are investigated. The light absorption capacity of flexible CZTSSe solar cells is enhanced due to the removal of the CdS layer. The optimal thickness of the ZnO buffer layers and the appropriate annealing temperature of the CZTSSe/ZnO are 100 nm and 200 °C. Ultimately, the optimum flexible CZTSSe/ZnO device achieves an efficiency of 5.0%, which is the highest efficiency for flexible CZTSSe/ZnO solar cells. The systematic characterizations indicate that the flexible CZTSSe/ZnO solar cells based on the optimal conditions achieved quality heterojunction, low defect density and better charge transfer capability. This work provides a new strategy for the development of the environmentally friendly and low-cost flexible CZTSSe solar cells.

The buffer layer plays a critical role in high‐performance flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cell. The conventional CdS buffer layer causes the photoelectric performance loss and the pollution of toxic Cd. Herein, the ultrathin CdS and Zn0.8Sn0.2O (ZTO) buffer layer engineering is proposed to reduce interface recombination and Cd content. An ultrathin CdS layer acts as the interface passivation layer to protect the CZTSSe layer and passivate its surface defects. To solve the problems of decreased carrier collection capacity caused by thinning the CdS layer, the ZTO layer with low resistivity and high carrier concentration is obtained by doping Sn into the ZnO layer. The systematic study indicates that the ultrathin CdS and ZTO buffer layers‐based solar cells realize high‐quality interface and band matching. Ultimately, 9.3% efficiency of the ultrathin CdS and ZTO‐based solar cell with 448 mV open‐circuit voltage is obtained, which is higher than that of the standard CdS buffer layer‐based device (8.5% with 419 mV). The study provides a new idea for achieving efficient flexible CZTSSe solar cells through heterojunction interface management.