Two-dimensional X-ray diffraction was employed, to provide a complete understanding of the energy efficiency improvement of organic photovoltaics by AC electric field alignment. Two distinguishable poly (3-hexylthiophene) (P3HT) nanostructures were found in the form of two separated layers, one of which is highly oriented and the other randomly distributed. The finding helps to analyze the crystallite arrangements not only in the interface in the bulk heterojunction, but also those closer to the substrate. The highly oriented P3HT layer, although located near the substrate, was found to enhance the device efficiency, by increasing the short circuit current and decreasing the series resistance. (C) 2012 Elsevier B.V. All rights reserved.