We investigate the surface-passivation-improvement mechanism in perovskite solar cells by the surface modification of NiO_x hole-selective contacts with 2PACz SAMs. The surface modification of NiO_x with 2PACz improved the solar cells’ open-circuit voltage and short-circuit current density. Photoelectron yield spectroscopy measurements suggest that the 2PACz modification increases the work function of the NiO_x HSCs, thereby enhancing field-effect passivation at the perovskite–HSC interface. Density functional theory calculations and electron spin resonance measurements indicate that an increase in the work function results from a vacuum level shift due to the electric dipole moment of 2PACz molecules and formation of space-charge region at the NiO_x–2PACz interface. The performance improvement is explained as a result of enhanced field-effect passivation caused by the vacuum level shift and thus the increase in the work function.