The device operation principle of amorphous silicon/crystalline silicon heterojunction solar cell is discussed. The band diagram obtained by the computer model developed in the commercial simulator Sentaurus shows that the c-Si surface is inverted at the interface between a-Si and c-Si (heterointerface). A strong inversion gives a strong electric field at the c-Si surface, which in turn facilitates the transport of minority carriers across the heterointerface. A high performance device requires a strongly inverted c-Si surface. Calculations are performed to show that the doping of the doped a-Si layer, the thickness of the intrinsic layer, and the defect state density at the heterointerface all affect the inversion of the crystalline silicon surface. Unlike homojunction devices, the defects in heterojunction devices have a greater role in transport mechanism than in recombination mechanism. The results show that in devices with a large number of defects at the interface, the fill factor degrades with little change in open circuit voltage. This explains why it is relatively easy to obtain VOC's approaching 700 mV with heterojunctions but often with low fill factors.