The study focuses on the influence of the hydrogenated amorphous silicon carbide (a-SiC:H) buffer layer in hydrogenated amorphous silicon (a-Si:H) single-junction and tandem thin-film solar cells. By increasing the undoped a-SiC:H buffer layer thickness from 6nm to 12nm, the JSC in single-junction cell was significantly improved, and the efficiency was increased by 4.5%. The buffer layer also effectively improves the efficiency of the a-Si:H/a-Si:H tandem cells by 7% as a result of the increase in open-circuit voltage (VOC) and short-circuit current (JSC). Although the bottom cell absorbs less short-wavelength photons, the wider-bandgap doped and buffer layers were still necessary for improving the cell efficiency. Presumably, this is because these wider-bandgap layers allow more photons to reach the bottom cell. Also, they can reduce interface recombination.