The dependence of solar cell parameters on i-aSi:H (non- or lightly-doped hydrogenated amorphous silicon) layer thickness in an aSi:H/cSi (crystalline silicon) heterojunction solar cell was analyzed using numerical simulation. By considering the quantum confinement effect at interfaces between i-aSi:H and cSi, experimental data which had not been explained by simulation could be successfully interpreted. The mechanism of an open-circuit voltage increase was visually presented by analyzing carrier distributions and quasi-Fermi levels near cSi surfaces and in the i-aSi:H layers. The optimized thicknesses of the i-aSi:H layers in both front and rear junctions were suggested to obtain the maximum conversion efficiency. The influences of the quantum confinement effect on the simulation results were discussed.