Photovoltaic cells based on boron-doped p-type amorphous carbon on n-Si have been synthesized by rf plasma-enhanced CVD using trimethylboron (TMB). In order to identify the optimal doping condition, various TMB/CH4 flow ratios have been investigated. A p-type a-C:H/n-Si heterojunction solar cells with a conversion efficiency as high as 0.007 % have been fabricated. The spectral response of cell photocurrents in the short wavelength region below 700 nm is determined to be due to the optical absorption of the boron-doped a-C:H film based on the optical gap measured by UV-visible spectroscopy. The photovoltaic properties of the a-C:H based solar cell structures are discussed with the dark and illuminated current density-voltage characteristics (efficiency, short-circuit current, open-circuit voltage, fill factor and series resistance) as well as optical properties of boron-doped a-C:H film characterized by UV-visible spectroscopy.