Wide-gap (highly transparent), hydrogenated amorphous silicon oxide (a-SiOx:H) layers are investigated for heterojunction solar cell applications: Intrinsic a-SiOx:H(i) films are formed in order to prove their applicability for surface passivating buffer layers sandwiched between the crystalline silicon (c-Si) and the doped amorphous layer used for the formation of the emitter and the back-surface-field in heterojunction cells. The a-SiOx:H films are processed by high frequency (70 MHz) plasma decomposition using silane (SiH4), hydrogen (H2), and carbon dioxide (CO2) at the low deposition temperature of 155 °C. Quasi-steady-state photoconductance and transient photoconductance lifetime measurements have been carried out to determine the passivation quality of the intrinsic a-SiOx:H deposited on c-Si of different doping types and levels. A variation of the applied thickness of the grown a-SiOx:H films determines the impact on the performance of heterojunction solar cells. It will be demonstrated that excellent effective lifetimes as high as 4.7 ms on 1 Ωcm n-type float-zone (FZ) material (corresponding to a surface recombination velocity of 2.3 cm/s) and 14.2 ms on 130 Ωcm p-type FZ material (corresponding to a surface recombination velocity of 0.52 cm/s) can be achieved by surface passivation using our a-SiOx:H films. To validate the capability of the intrinsic and doped a-SiOx:H films separately, heterojunction solar cells consisting of (front to back) a-Si:H(p+)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+), a-Si:H(p+)/a-SiOx:H(i)/c-Si(n)/a-SiOx:H(i)/a-Si:H(n+), and the reversed doping sequence have been analyzed. By incorporating a-SiOx:H(i) to the a-Si:H(p)/c-Si structure we find a drastic increase of the open circuit voltage (up to 655 mV for p-type substrates and 695 mV - - for n-type substrates) and accordingly, a higher conversion efficiency than obtained with standard a-Si(i).