Silicon heterojunction solar cells can achieve high conversion efficiency with a simple structure. In this study, we investigate the passivation characteristics of VOx thin films as a hole-selective contact layer using ALD (atomic layer deposition). Passivation characteristics improve with iVoc (implied open-circuit voltage) of 662 mV and minority carrier lifetime of 73.9 μs after post-deposition annealing (PDA) at 100 °C. The improved values are mainly attributed to a decrease in carbon during the VOx thin film process after PDA. However, once it is annealed at temperatures above 250 °C the properties are rapidly degraded. X-ray photoelectron spectroscopy is used to analyze the chemical states of the VOx thin film. As the annealing temperature increases, it shows more formation of SiOx at the interface increases. The ratio of V5+ to V4+, which is the oxidation states of vanadium oxide thin films, are 6:4 for both as-deposition and annealing at 100 °C, and 5:5 for annealing at 300 °C. The lower the carbon content of the ALD VOx film and the higher the V5+ ratio, the better the passivation characteristics.