The stoichiometry value x of WO x , or its oxidation state, is crucial for improving performances of the hole-selective contact heterojunction silicon solar cell. However, it is challenging to tune the films' oxidation state using the well-known evaporation method. In this study, a simulation was performed to analyze the effect of x on short-circuit current (J sc ) loss, attributed to the holeselective contact in the device. Compared to the thickness of WO x layer, x has a more important role in minimizing J sc loss. Based on the simulation, the WO x /c-Si heterojunction solar cells having hole-selective WO x contacts with tuned x to vary its oxidation state were fabricated using reactive magnetron sputtering. The relationships of the open-circuit voltage (V oc ) and J sc with respect to x were similar. The experimentally determined J sc increased from 34.7 to 36.6 mA cm −2 when x was increased from 2.72 to 2.77; this result is consistent with the simulation. Nevertheless, fill factor (FF) reduced with the increase of x, owing to the reduced conductivity of WO x . Both oxidation state and film conductivity must be as high as possible to simultaneously achieve high V oc , J sc , and FF. The lowest x yielded a solar cell efficiency of 13.3%.

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