Effect of surface sulfurization on the solar conversion efficiency of Cu-Zn-Sn (CZT) alloy film, which was prepared by DC-magnetron sputtering with a single Cu-Zn-Sn target, followed by a H2S-sulfurization-afterH(2)Se-selenization (SAS) process to convert the alloy film to a Cu2ZnSn(S, Se)(4) (CZTSSe) absorber, was investigated. Compared with a conventional Cu2ZnSnSe4 (CZTSe) solar cell selenized in H2Se, the conversion efficiency of CZTSSe one prepared in the SAS process was increased from 2.1% to 7.5% (increased by 257%). Conductive atomic force microscopy exhibited that the SAS process effectively reduced the leakage current near the grain boundary, while the external quantum efficiency measurement clearly demonstrated that the SAS process increased the energy bandgap by similar to 0.13 eV and significantly reduced the surface defects in the shorter wavelength region. Raman and temperature-dependent photoluminescence spectroscopies elucidated that the SAS process also led to secondary phases (e.g., SnS, ZnS and Cu2SnS3) and structural defects (e.g., Zn-sn), which are considered to be a key to achieve a highly efficient CZTSSe solar cell in the future.