Light trapping is a key issue in improving the efficiency of thin-film Si solar cells, and using a back reflector material plays a critical role in improving a cell's light-trapping efficiency. In this study, we developed n-type microcrystalline silicon oxide (n-μc-SiOx) films that are suitable for use as back reflectors in thin-film silicon solar cells. They exhibit a lower refractive index and lower absorption spectra, especially at long wavelengths of >700 nm, than conventional ZnO:Al materials, which are beneficial for this application. The n-μc-SiOx films were prepared by the PECVD (plasma-enhanced chemical vapor deposition) method and applied to the fabrication of back reflectors in μc-Si:H solar cells. We also characterized the changes in cell performance with respect to the refractive index, conductivity, and thickness of the n-μc-SiOx back reflectors. The novel back reflector boosts the total current density by up to 3.0% with the help of the enhanced long-wavelength response. It also improves open circuit voltage (Voc) and fill factor (FF), which may be attributed to the reduced shunt current caused by the anisotropic electrical characteristics of the n-μc-SiOx layer. Finally, we could achieve a conversion efficiency for the hydrogenated microcrystalline silicon (μc-Si:H) solar cells of up to 9.3% (Voc: 0.501 V, Jsc: 27.4 mA/cm2, FF: 0.68) using the n-μc-SiOx back reflector. ► We suggest new application of n-type microcrystalline silicon oxide (n-μc-SiOx). ► The n-μc-SiOx is applied as new back reflector material for thin-film Si solar cell. ► The n-μc-SiOx exhibits superior optical properties to conventional ZnO:Al. ► The new back reflector boosts not only the Jsc but also the Voc and the FF. ► The thickness of n-μc-SiOx back reflector is optimized for higher performance.