This work describes the preparation of a doped layer-free hydrogenated amorphous silicon (a-Si:H) thin film solar cell consisting of a vanadium pentoxide (V2O5-x) window layer, an intrinsic a-Si:H absorber layer, and a lithium fluoride (LiF)/aluminum (Al) back electrode. The large difference between the work functions of the V2O5-x layer and the LiF/Al electrode permitted photogenerated carriers in the i-a-Si:H absorber layer to be effectively separated and collected. The effects of the V2O5-x layer thickness and the oxidation states on the photovoltaic performance were investigated in detail. X-ray photoelectron spectroscopy analysis confirmed that the major species of the sputtered V2O5-x thin films were V5+ and V4+. Optimization of the V2O5-x window layer yielded a power conversion efficiency of 7.04%, which was comparable to the power conversion efficiency of a typical a-Si:H solar cell (7.09%). (C) 2013 AIP Publishing LLC.