A good valency electron controllability has been found in hydrogenated amorphous silicon carbide prepared by the plasma decomposition of [SiH4 + CH4] gas mixture. Utilizing this a-SiC: H layer as window in an a-SiC: H/a-Si: H heterojunction structure, effects of a-SiC: H on the photovoltaic performance have been investigated. A series of systematic experimental data of photovoltaic performance with parameters of methane fraction, optical gap to window side material and p-layer thickness are presented. Remarkable improvements in both short circuit current density and open circuit voltage have been seen in an a-SiC:H/a-Si: H heterojunction cell. This experimental fact implies that not only a wide gap window effect but also voltage factor due to potential profile steepened with wide dap a-SiC: H contribute to the photovoltaic performance of the a-SiC: H/a-Si: H heterojunction solar cell. It has been shown that a-SiC: H might be a useful new material for the development of low cost high efficiency solar cells.