Recently, the transition metal oxide thin film has been actively investigated for doping-free heterojunction Si solar cells. However, most of the research on characterizing the chemical state and work function of the metal oxide thin film has been conducted on its surface, while there has been little work on the characterization on the subsurface of the metal oxide thin film. Here, we systematically investigate the chemical state and work function of the evaporated nickel oxide (NiOx) thin film on a Si substrate as a function of the depth position. We found that the chemical state of the NiOx thin film is highly affected by the surface chemical reaction. For instance, an air-exposed NiOx surface exists more in nickel hydroxide [Ni(OH)(2)] than in nickel monoxide (NiO). In addition, we discern that NiOx near the Si substrate exists in nickel silicide (NiSix). The changed chemical state of the NiOx thin film creates a high variation in the work function as a function of the depth position in the range of 4.4-5.4 eV. We also investigate the heterojunction Si solar cell with the NiOx thin film. We found that the performance of the heterojunction Si solar cell was determined according to the air exposure on the NiOx thin film inducing an undesirable chemical reaction. The heterojunction Si solar cell with the air-exposed NiOx thin film shows a relatively low efficiency of 11.84% by the reduced work function of the NiOx thin film, while one with the controlled NiOx thin film exhibits an enhanced efficiency of 14.23%.