Facile Tailor on the Surface of Mo Foil Toward High‐Efficient Flexible CZTSSe Solar Cells

Abstract Flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells is attracting much attention because of their enormous application prospects. However, current flexible CZTSSe solar cells with Mo foil as substrate still suffer from severe back interface problems due to the complexity of substrate surfaces. Herein, a facile approach to tailor the surface of the flexible substrate and modify the back interface between CZTSSe and Mo foil is proposed. The study discloses that a simple polishing can not only improve the wettability of the precursor solution on the substrate unexpectedly and thus improve the quality of the CZTSSe film, but also increase the mechanical stability of the absorber layer grown on Mo foil. The subsequent UV‐ozone treatment helps to form a favorite MoO 3 layer for efficient CZTSSe devices. Surprisingly, the quasi‐ohmic contact is formed between CZTSSe/Mo foil by such combined treatments and thus promoting the carrier collection. Consequently, the efficiency of the flexible CZTSSe solar cell is significantly improved from 4.94% to 10.32% without anti‐reflection layer. The bending durability of the cell fabricated on the treated Mo foil is increased greatly. This work discloses that back contact interface is very important for the carrier collection and thus the highly efficient flexible thin film solar cells.