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To enhance light harvesting in organic photovoltaic devices, we propose the incorporation of a metal ͑aluminum͒ mask film in the system's usual layout. We fabricate devices in a sandwich geometry, where the mask ͑nanoindented with a periodic array of holes of sizes d and spacing s͒ is added between the transparent electrode and the active layer formed by a blend of the semiconducting polymer P3HT and substituted fullerene. Its function is to promote trapping of the incident light into the device's cavity ͑the region corresponding to the active layer͒. For d, we set a value that allows light diffraction through the holes in the relevant absorption range of the polymer. To optimize the mask structure, we consider a very simple model to determine the s leading to trapped fields that are relatively intense and homogeneous within the device. From measurements of the action spectra, we show that, indeed, such architecture can considerably improve the resulting photocurrent efficiencies-one order of magnitude in the best situation studied.