In this paper the physical mechanism of the S-shaped J-V characteristics of (p) a-Si:H/(n) c-Si heterojunction solar cell at low working temperatures, low impurity concentrations in the a-Si:H layer, high valence band offsets or high interface defect densities is studied by heterojunction interface analysis and AMPS simulations. The results show that the barrier at the amorphous/crystalline interface hinders the collection of photogenerated holes. A high hole accumulation at the interface, in combination, causes a shift of the depletion region from the c-Si into the a-Si:H. This leads to the electric field decreasing, and the enhanced recombination inside the c-Si depletion region causes a significant current loss. It results in the S-shaped J-V characteristics.