Hydrogen-argon mixed dilution has been applied for the deposition of boron-doped nanocrystalline silicon carbide (nc-SiCx) thin films. The variations of structural, compositional, electrical and optical properties with the varying H2/Ar ratio are systemically investigated through various characterizations. It is shown that by using H2-Ar mixed dilution for deposition, B-doped nc-SiCx thin film possessing both wide optical band gap (∼2.22 eV) and high conductivity (∼1.9 S cm-1) can be obtained at the H2/Ar flow ratio of 360/140. In addition, the B-doped nc-SiCx thin films are fabricated as the window layers of a-Si thin film solar cells, and the highest conversion efficiency (8.13%) is obtained when applying the window layer with the largest optical band gap energy.