In organic photovoltaic (OPV) cells, the nanoscale morphology of the organic layer is an essential parameter governing their charge-transport properties. Optimization of the vertical composition gradient in the poly(3-hexylthiophene) (P3HT) active layer in an OPV cell with an enhanced efficiency was achieved by using different sonication times of 10, 15, 20, 25, and 30 min. Atomic force microscopy images confirmed that the sonication process increased the roughness of the P3HT layer in a vertically modulated nanoscale morphology. Photoluminescence spectra exhibited a strong peak corresponding to the P3HT particles. The power conversion efficiency of the OPV cells with a vertically modulated P3HT nanostructural layer sonicated at 15 min was enhanced by 1.04% due to an increase in the interfacial region between the acceptor and the donor materials. (C) 2012 Elsevier B. V. All rights reserved.