[en] A silicon heterojunction solar cell constructed with sub-stoichiometric molybdenum oxide (MoO_x) carrier-selective layer and crystalline silicon substrate, which possesses a potential to achieve high power conversion efficiency, is investigated by numerical simulation tool AFORS-HET. In this work, MoO_x is chosen as the emitter layer of the silicon heterojunction solar cell to mitigate parasitic light absorption losses. The influences of MoO_x and p-a-Si:H layers with different thicknesses on the performances of the heterojunction solar cells are compared. The surface effect passivation is then used to explain the behavior of open circuit voltage increased with thickness variation, which physical mechanism is characterized by introducing the built-in electric field and minority carrier lifetime. Furthermore, we use the carrier recombination rate, band offset, and built-in electric field to investigate the effect of defect density states in the MoO_x hole selective layer. In this paper, the highest power conversion efficiency of 27.27% is obtained by optimizing the thickness and defect density states of MoO_x layer. (paper)