Well defined sequences of non-uniform and uniform degradation steps are applied on several high quality bifacial p-i-n solar cells with different thicknesses of the intrinsic layer. This procedure allows one to separate interface and bulk effects in the degradation of a-Si:H solar cells, or more precisely to separate the influence of the modification of the internal electric field profile (due mainly to effect near the interface) from that of an increase in the deep defect density in the bulk; according to present data the latter is clearly responsible for the major part of a-Si:H cell degradation.