The heterojunction with intrinsic thin-layer (HIT) solar cell has reached a record conversion efficiency of 24.7% on a 98 mu m wafer recently. But the physical mechanism of this solar cell is not understood clearly. In this work, the roles of Fermi level of doped aSi:H and band offsets at the a-Si:H/c-Si interface in HIT solar cell were studied through computer simulation. With the increasing of the doping concentration in the emitter and back surface field, the Fermi levels get closer to the band edge and more defects are produced. This simulation shows that for over 25% conversion efficiency solar cell, the value of E-f-E-v in a-Si:H(p) layer should be less than 250 meV, E-f of a-Si:H(n) layer should be as close as possible to the conduction band edge. Furthermore, the band discontinuities at the front a-Si:H/c-Si interface could lead to a sharp decline of the fill factor when the valence band offset is larger than 0.55 eV. However, the conduction band offset makes little impacts on the solar cell performance unless the bandgap of a-Si:H is small than 1.62 eV. As a result, the highest efficiency of 25.21% is obtained by the optimized parameters. (C) 2013 Elsevier Ltd. All rights reserved.