A novel Si(p)/Mg2Si(i)/Si(n) PV cell architecture is proposed on the basis of numerical simulation and ab intio calculation in the framework of the density functional theory (DFT). First-principles calculations on the basis of the relaxed Mg2Si(11 0)/Si(111) heterojunction suggest that the valence band offset is about -0.35 eV with respect to the valence band maximum (VBM) of Si. The effect and corresponding mechanism of band offsets, doping concentration, layer thickness, as well as defect states on the.performance of solar cells have been studied and discussed in detail. The optimised ideal Si(p)/Mg2Si(i)/Si(n) heterojunction solar cell with a total thickness of 2.15 pm of active materials is predicted to provide a large open-circuit voltage of 0.654 V and a high conversion efficiency up to 22.254%, which is much higher than single junction cells and rivals the performance of crystalline silicon solar cells over 100 times thicker. This work demonstrates great potential to develop Mg2Si based low-cost and environmentally friendly solar cells on the ground of the well-established silicon technology.