Solution processing is mandatory to continue the development of large scale and low cost organic solar cells. Tungsten oxide (WO3) has shown good performance when used as a hole transporting layer in organic solar cells. Here this oxide has been deposited by a sol-gel technique on top of the active layer in an inverted structure device. This layer has been characterised by XPS and UPS to understand the effect of the stoichiometry and of the energy levels on the device performance. In the optimised structure, an average power conversion efficiency (PCE) of 3.5% has been achieved, which is comparable to devices with vacuum deposited transition metal oxide using poly(3-hexylthiophene):[6,6]-phenyl-C-61 butyric acid methyl ester (P3HT:PCBM) as an active layer. Moreover in our study no annealing is needed after the deposition of the sol-gel processed oxide, which is really promising for the development of industrial roll-to-roll production on flexible substrates. (C) 2013 Elsevier B.V. All rights reserved.