The emitter of amorphous/crystalline silicon heterojunction (HJ) solar cells is normally very thin. Consequently, the metal used as a front contact can produce a partial or even total depletion of this layer. As a result, the diffusion potential of the p-n junction deviates from its maximum value. In this paper, we report the results concerning HJ in which either metal dots (Au, Al), semitransparent metal layers, or indium tin oxide (ITO) dots or layers were used as front contact on the same HJ structure, namely (p)a-Si:H / (i)a-Si:H / (n)c-Si / Al. We show that, for thin p-layers, the dark and light J-V characteristics of HJ solar cells depend on the material used as front contact. In particular, we found that the dark saturation current increases if a low work function material is used. This increase is interpreted in terms of p-layer depletion, and is shown to directly influence the J-V characteristics under illumination, producing a reduction of the open circuit voltage of solar cells.