Tungsten oxide (WOx) can be incorporated into amorphous/crystalline silicon heterojunction solar cells as hole contact and for interface modification between p-type amorphous silicon and indium tin oxide. This paper aims at understanding the influence of tungsten oxides properties on silicon heterojunction solar cells. Using in-system photoelectron spectroscopy on thermally evaporated WOx layers, it was verified that WOx with a stoichiometry close to WO3 features a work function close to 6 eV and is therefore suitable as hole contact on silicon. Additionally the oxygen vacancy concentration in WOx was measured using photoelectron spectroscopy. High oxygen vacancy concentrations in WOx lead to a low band bending in the WOx/silicon-junction. Furthermore solar cells were fabricated using the same WOx, and the band bending in these cells is correlated with their fill factors (FF) and open circuit voltages (VOC). Combining these results, the following picture arises: positively charged oxygen vacancies raise the Fermi-level in WOx and reduce the band bending at the WOx/silicon-junction. This, in turn, leads to reduced VOC and FF. Thus, when incorporating WOx into silicon solar cells it is important to minimize the oxygen vacancy density in WOx. Therefore deposition methods, enabling adjustment of the WOx stoichiometry are preferable.