Efficient Flexible CZTSSe Solar Cells with Suppressed Back Interfacial Recombination by In Situ Pre‐Selenization of Mo Foil

ABSTRACT Flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells have garnered significant attention due to their broad application prospects. However, devices fabricated on Mo foil substrates still face critical challenges at the back interface, primarily caused by severe interfacial reactions during crystallization. Herein, we propose a pre‐selenization strategy for the Mo foil to suppress back interfacial recombination and improve the efficiency. This treatment facilitates the in situ formation of a uniform and dense MoSe 2 thin layer on the Mo foil, which effectively inhibits the diffusion of S from the CZTSSe absorber into the substrate during subsequent high‐temperature selenization. As a result, the back interface achieves well‐matched energy band alignments with reduced contact barrier by 30%, boosting carrier collection. The pre‐formed MoSe 2 layer improves precursor solution wettability to the Mo foil substrate and induces the formation of a uniform and compact CZTSSe layer. The pre‐selenization suppresses series resistance, leakage current, and carrier recombination, increasing the fill factor from 58.5% to 64.5% of flexible CZTSSe solar cells. Consequently, the device efficiency achieves an enhancement from 9.8% to 11.1% (without anti‐reflection coatings). The Mo foil pre‐selenized strategy offers a promising approach to address back interface issues in flexible kesterite solar cells.