AMPS simulator, which was developed by Pennsylvania State University, has been used to simulate photovoltaic performances of nc-Si:H/ c-Si solar cells. It is shown that interface states are essential factors prominently influencing open circuit voltages (VOC) and fill factors (FF) of these structured solar cells. Short circuit current density (JSC) or spectral response seems more sensitive to the thickness of intrinsic a-Si:H buffer layers inserted into n+-nc-Si:H layer and p-c-Si substrates. Impacts of bandgap offset on solar cell performances have also been analyzed. As ΔEC increases, degradation of VOC and FF owing to interface states are dramatically recovered. This implies that the interface state cannot merely be regarded as carrier recombination centres, and impacts of interfacial layer on devices need further investigation. Theoretical maximum efficiency of up to 3117%(AM15, 100mW/cm2, 040—11μm) has been obtained with BSF structure, idealized light-trapping effect(RF=0, RB=1) and no interface states.