For HIT (heterojunction with intrinsic thin-layer) solar cell with Al back surface field on p-type Si substrate, the impacts of substrate resistivity on the solar cell performance were investigated by utilizing AFORS-HET software as a numerical computer simulation tool. The results show that the optimized substrate resistivity (R(op)) to obtain the maximal solar cell efficiency is relative to the bulk defect density, such as oxygen defect density (D(od)), in the substrate and the interface defect density (D(it)) on the interface of amorphous/crystalline Si heterojunction. The larger D(od) or D(it) is, the higher R(op) is. The effect of D(it) is more obvious. R(op) is about 0.5 Omega cm for D(it) = 1.0 x 10(11)/cm(2), but is higher than 1.0 Omega cm for D(it) = 1.0 x 10(12)/cm(2). In order to obtain very excellent solar cell performance, Si substrate, with the resistivity of 0.5 Omega cm, D(od) lower than 1.0 x 10(10)/cm(3), and D(it) lower than 1.0 x 10(11)/cm(2), is preferred, which is different to the traditional opinion that 1.0 Omega cm resistivity is the best. (C) 2008 Elsevier Ltd. All rights reserved.