The plasma enhanced chemical vapor deposition(PECVD) system was used for fabricating the silicon films with different hydrogen dilution ratio(RH) under the high power density, high pressure and low substrate temperature. High-resolution transmission electron microscopy(HRTEM) and Raman spectroscopy indicated that the thin films were nanocrystalline silicon(nc-Si) films which contained nanocrystallites with grain size around3-5 nm. The effects of the RH on the optical band gaps of the nc-Si thin films were studied. The results showed that the optical band gaps of the nc-Si thin films increased with the increased in the RH. An double nc-Si p-layers structure was developed to improve the i/p interface in hydrogenated amorphous silicon(a-Si:H) solar cells. The efficiency of the double nc-Si p-layers cell was improved by17% compared its counterpart of the single nc-Si p-layer cell.