Recent studies on amorphous-crystalline silicon p-i-n heterojunctions (HIT, Heterojunction with Intrinsic Thin layer) have indicated the potential for these materials to be used as low cost, high efficiency solar cells. Moreover, the HIT structure is also expected to be suitably applied to various semiconductor junctions for micro and optoelectronic devices. Despite this potential, the exact operation of these cells is not yet fully understood and an S-shaped J-V characteristic curve is observed under certain conditions, leading to a poor collection efficiency. We present in this paper some results, obtained using the computer simulator ASCA, about the internal electric configuration of a p a-Si:H/a-Si:H/n c-Si heterostructure under different illumination conditions. The obtained results are related to the corresponding simulated J-V characteristic curve and compared with our experimental results, in order to explain the S-shaped J-V curve featured by these device. The band discontinuities at the amorphous-crystalline interface are shown to be responsible for such a behavior. We find that the conduction band offset is the most limiting parameter for the optimal collection of photogenerated carriers.