Abstract Intrinsic amorphous silicon oxide (a-Si 1 − x O x :H) buffer layers were deposited on both sides of crystalline silicon (c-Si) wafers using plasma-enhanced chemical vapor deposition (PECVD) technique. The input gas flow ratio of carbon dioxide (CO 2 ) to silane (SiH 4 ) was varied in a wide range to study the passivation and structural properties of the a-Si 1 − x O x :H buffer layers. In this work, when the a-Si 1 − x O x :H layer was quite thick (> 15 nm), an extremely high effective lifetime of ~ 10 ms was achieved on the n-type float-zone c-Si (~ 3 Ω-cm, ~ 280 μm) at moderate CO 2 /SiH 4 flow ratios, resulting in an exceptionally low surface recombination velocity ( 2 /SiH 4 flow ratio was either rather low ( 0.47), the surface passivation quality would deteriorate significantly. In addition, a certain amount of epitaxial phase (epi-Si) was observed in some excellent buffer layers made at the moderate CO 2 /SiH 4 ratios. Moreover, it was found that the epi-Si content could be gradually suppressed by slightly increasing the CO 2 /SiH 4 ratio without affecting passivation quality. When the a-Si 1 − x O x :H buffer layer thickness was kept at only a few nanometers as required by silicon heterojunction (SHJ) solar cells, the PECVD optimum condition (CO 2 /SiH 4 ratio) for buffer layers was revealed by applying the a-Si 1 − x O x :H buffer layers directly in a practical SHJ solar cell. We found that when the a-Si 1 − x O x :H buffer layer containing a certain amount (~ 22%) of epi-Si was employed at the back-surface-field side of the solar cell, a high open-circuit voltage ( V OC ) and a high fill factor (FF) were obtained at the same time. By contrast, at the emitter side of the solar cell, only the buffer layer without any epi-Si can be used to provide high-quality surface passivation for an excellent SHJ solar cell.