Double junction photoelectrochemical solar cells based on Cu2ZnSnS4/Cu2ZnSnSe4 thin film as composite photocathode

•A new structure photocathode with CZTS/CZTSe double-layer film was fabricated via a blade coating process.•The film was to convert DSSC into double junction solar cell by the replacement of the Pt film with CZTS/CZTSe film.•CZTS and CZTSe nanoparticles were synthesized by a low-cost solvothermal process to high-efficiency solar cells.•The prepared cell achieves efficiency as high as 8.83% which is similar to conventional Pt-based DSSC (8.49%). A solvothermal method was used to synthesize Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanoparticles. CZTS/CZTSe bilayer films have been fabricated via a layer-by-layer blade coating process on the fluorine dope tin oxide (FTO) substrates. We converted conventional dye-sensitized solar cells (DSSCs) into double junction photoelectrochemical solar cells with the replacement of the Pt-coated counter electrode with the as-prepared films as composite photocathodes. Compared with conventional DSSCs, the cells show an increased short circuit current and power conversion efficiency.