We present here a study of the recombination at the hetero-interface of solar cells based on amorphous silicon / crystalline silicon (a-Si:H/c-Si). The volume defects in the amorphous silicon are modeled with the defect-pool model and we study whether the surface defects in c-Si at the a-Si:H/c-Si interface can be considered as a projection of the defects in a-Si:H close to the surface. We study the impact of the defect-pool model parameters on the surface defect density and on the effective lifetime. We show that the calculation of interface defects from the defect-pool model is compatible with experimental results only if the width of the valence band tail is decreased when the thickness of the buffer layer is increased.