Efficiency Improvement of Flexible Cu₂ZnSn(S,Se)₄ Solar Cells by Window Layer Interface Engineering

Flexible Cu2ZnSn(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 (Rs). To enhance the antireflective function of the window layers, we introduce MgF2 antireflection coatings on the surfaces of solar cells. The average Jsc 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.