Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has attracted considerable attention as a non-toxic and earth-abundant solar cell material. During selenization of CZTSSe film at high temperature, the reaction between CZTSSe and Mo is one of the main reasons that result in unfavorable absorber and interface quality, which leads to large open circuit voltage deficit (VOC-def) and low fill factor (FF). Herein, a WO3 intermediate layer introduced at the back interface can effectually inhibit the unfavorable interface reaction between absorber and back electrode in the preliminary selenization progress; thus high-quality crystals are obtained. Through this back interface engineering, the traditional problems of phase segregation, voids in the absorber and over thick Mo(S,Se)2 at the back interface can be well solved, which greatly lessens the recombination in the bulk and at the interface. The increased minority carrier diffusion length, decreased barrier height at back interface contact and reduced deep acceptor defects give rise to systematic improvement in VOC and FF, finally a 12.66% conversion efficiency for CZTSSe solar cell has been achieved. This work provides a simple way to fabricate highly efficient solar cells and promotes a deeper understanding of the function of intermediate layer at back interface in kesterite-based solar cells. © 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences