In this work, we report the application of a sol–gel derived ZnO thin film as a buffer layer for high efficiency inverted polymer solar cells. ZnO films are widely used in such devices because they have a relatively high electron mobility, high transparency and environmental stability. The ZnO precursor was prepared by dissolving zinc acetate and ethanolamine in the 2-methoxyethanol. ZnO thin films were then deposited on indium tin oxide (ITO)/glass substrates by spin coating the above solution. Inverted polymer solar cells with the configuration ITO/ZnO/photoactive layer/MoOx/Ag were realized in order to investigate the performance of ZnO thin film. The photoactive layer is a blend of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM) (1:1.5 w/w). We made a comparative study of the photovoltaic behavior of devices with ZnO films deposited using different sol–gel recipes. In particular, we varied the zinc acetate/ethanolamine molar ratio to have ZnO films with different trace amounts of starting materials. In addition we also prepared ZnO films annealed at 200 °C for different times (from 5′ to 60′) in order to evaluate this effect on the trace amount removal.