In this study, intrinsic a-SiO:H films were prepared by a conventional radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of SiH 4, H 2 and CO 2. Changes in the optical, electrical and structural properties of the a-SiO:H films were investigated systematically by controlling the deposition parameters, mainly the gas ratio of CO 2 to SiH 4, and hydrogen dilution concentration. By introducing the CO 2 gas, three kinds of O-related IR features are found at 780, 980 and 2090 cm -1. With increasing the CO 2/SiH 4 ratio, the absorption at 780 cm -1 strongly coupled to the Si-H and Si-O-Si motions increases noticeably, indicating that the defect density in the films increases because the mode at 780 cm -1 is unique signature of a particularly local geometry related to defects. In order to diminish the defect density level in the films, hydrogen dilution was performed. As the H dilution concentration increases, the defect density is reduced significantly and the photo-conductivity is improved to 10 -4 S/cm. Influences of oxygen incorporation into the Si network and H dilution on the performance of a-SiO:H solar cells are also examined in detail.