Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells show great potential due to non‐toxicity and low cost. The quality of CZTSSe absorber suffering from the high‐temperature selenization process is the key to overcoming open‐circuit voltage (VOC) deficit and obtaining high efficiency. In this work, the authors develop a selenization technique to improve the quality of the CZTSSe layer by pre‐evaporation selenization. This method provides saturated selenium vapor at the beginning of the selenization process to promote the crystallization process. The oversaturated selenium source greatly shortens the annealing time at high temperatures. The prepared CZTSSe films have larger grains and fewer voids. The measurement of the space charge limited current shows that the defect density of the CZTSSe absorber is significantly reduced from 4.43 × 1013 cm−3 to 3.46 × 1013 cm−3. Electrical tests show that the devices have better charge separation at the interface, resulting in a charge recombination lifetime, increasing from 133 to 197 µs. The power conversion efficiency of the flexible CZTSSe solar cell reaches 10.24% with VOC and the fill factor enhances to 463 mV and 62%, respectively. The safe non‐toxicity and efficient selenization method is expected to provide a new strategy for high‐efficiency flexible CZTSSe solar cells.

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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.