A simple method for spontaneous formation of nanoripples on ZnO thin films was developed, and these nanostructured ZnO films were used as hole-blocking layer in inverted organic solar cells. Moreover, the size (height) of nanoripples on ZnO surface could be controlled in the range of several tens of nanometers. Among various ZnO films, surface structures with similar to 70 nm-high nanoripples resulted in the best photovoltaic performance of the organic solar cell consisting of a stack of indium tin oxide/ZnO/ regioregular poly (3-hexyl thiophene), phenyl-C(61)-butyric acid methyl ester/Ag. The power conversion efficiency of inverted organic solar cells consisting of with 70 nm-high ZnO nanoripples (similar to 3.2%) was higher than that of a relatively flat ZnO surface by a factor of similar to 2. Existence of nanoripples on ZnO results in a higher contact area between ZnO and active layer, leading to an enhanced photovoltaic performance. (C) 2011 Elsevier B.V. All rights reserved.