Antimony selenide (Sb2Se3) thin films are light-absorbing materials used for low-cost, highly efficient thin-film solar cells. However, because it is deposited onto a comparatively rough substrate, stacking growth of a Sb2Se3 thin film negatively impacts formation of the heterojunction. For this study, Sb2Se3 thin-film solar cells were fabricated in a FTO/CdS/CeO2/Sb2Se3/Au configuration. An ultrathin layer of cerium dioxide (CeO2) was deposited onto the CdS surface by evaporation, and the thickness of the CeO2 interlayer was optimized to improve the photoelectric properties of the solar cells. Compared to the CdS surface in conventional Sb2Se3 thin-film solar cells (FTO/CdS/Sb2Se3/Au), the surface of the CdS buffer in our solar cells was smoothed by the CeO2 layer, which induced columnar growth of the Sb2Se3 grains. Furthermore, diffusion of Ce4+ into Sb2Se3 enhanced its crystallinity and improved the evenness of the Sb2Se3 thin-film surface. The photo-conversion efficiency (PCE) of a solar cell with a 2-nm thick CeO2 interlayer reached 5.14%.