The concentration profiles of B, P, Zn, and O in the active intrinsic (i) νc-Si:H layer and across the interfaces in p-i-n and n-i-p structures have been measured with SIMS. For the ZnO/νc-p/νc-i sequences, an apparent B and O profile extends over several hundred nanometers into the i-layer, and high levels of Zn can be found well above the ZnO substrate layer. These profiles are not affected by annealing at the deposition temperature. Much lower impurity concentrations are measured for n-i-p deposition sequences, or when the p-layer or the i-layer is amorphous. Ruling out diffusion and intermixing processes, evidence for the presence of pinholes in the material is presented, which explain most of the experimental findings.