In summary, we discussed how interface properties of a-Si:H/c-Si structures may determine the electronic passivation behavior in conjunction with the solar cell performance. The beneficial combination of a-Si:H(p,n,) and a-Si:H(i) for passivation is explained in terms of the defective interface with a doped layer and the excellent interface with an intrinsic layer. Post annealing treatments have been found to be a good tool to unravel the physical mechanism of passivation. The annealing studies demonstrate the need for a careful assessment of process temperature during c-Si/a-Si:H heterostructure device fabrication. For intrinsic film deposition, the deposition temperature should be sufficiently low to prevent epi-Si growth. For doped layer deposition, care has to be taken not to generate harmful defects already at moderate temperatures. The novel material a-SiO:H demonstrates the advantage of the prevention of harmful epitaxial growth and less optical absorption.