Post hydrogen treatment effects of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p-layer of p/i/n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated by measuring-the thickness, electrical, and optical properties of the film before and after hydrogen treatment. It was found that the boron-doped a-SiC:H film mas simultaneously etched and passivated by the treatment. The performance of the cell with a hydrogen treated p-layer was improved by similar to 7% due to an increase in open circuit voltage (V-oc) and fill factor (F.F.), compared to that of the untreated cell, although the p-layer thickness was nearly identical in both cases, The increase in V-oc and F.F, could be explained by an increase in the built-in potential due to a decrease in the film activation energy. This electrical property improvement was well explained by the passivation effect of a SiH2/SiH ratio decrease of the film calculated by FTIR spectra. These film charges by post hydrogen treatment are considered to occur in the bulk of the boron-doped a-SiC:H film.