Ultrathin silicon oxide (a-SiO_x) and titanium nitride (TiN) composite film with high electron concentration has been prepared by wet chemical oxidation and direct current magnetron sputtering, and its passivation effect on the crystalline silicon surface in semiconductor-quasi-insulator-semiconductor (SQIS) photovoltaic devices is investigated. The experimental results show that the power conversion efficiency of the photovoltaic device is relatively increased by 20.72% compared with that of the aluminum back field electrode. The reason for the increase of open-circuit voltage is analyzed by combining the minority carrier lifetime measurement and AFORS-HET simulation software. Ultrathin a-SiO_x/TiN and n-Si have a valence band off of 2.28 eV, which can slow down the recombination of holes on the rear interface and reduce the recombination rate to 1/10 of the original value, thereby effectively increases the open-circuit voltage and achieves the purpose of improving the power conversion efficiency of heterojunction devices. Therefore, the rear passivation contact composite material with simple process and low cost is beneficial for SQIS photovoltaic devices.