Amorphous silicon enables the fabrication of very high-efficiency crystalline-silicon-based solar cells due to its combination of excellent passivation of the crystalline silicon surface and permeability to electrical charges. Yet, amongst other limitations, the passivation it provides degrades upon high-temperature processes, limiting possible post-deposition fabrication possibilities (e.g. forcing the use of low-temperature silver pastes). We previously evidenced the superior temperature stability of low-carbon-content intrinsic amorphous silicon carbide (a-SiCx:H) passivating layers to sidestep this issue, and investigate here in more details the reason for the improved temperature stability. The passivation from intrinsic a-SiCx:H layers is shown to first improved upon annealing, and then degrade past 350 °C. The initial passivation can be improved and the degradation postponed by capping the a-SiCx:H layer by an a-Si:H film. We compare here the passivation provided by stacks of a-Si:H and a-SiCx:H, and investigate the hydrogen bonding and content of these films.