Fundamental aspects of the influence of the nanomorphology in phase-separated conjugated polymer/fullerene bulk heterojunction blends are presented. A variety of experimental techniques were combined to resolve the structure of fullerene and polymer domains on the nanometer scale. As predicted theoretically, it is experimentally identified that the phase-separated domain size as well as the percolation of both hole- and electron-conducting phases is crucial to improve the power conversion efficiency of organic solar cell devices. Among the experimental techniques we applied atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), scanning and transmission electron microscopy (SEM, TEM). New insights are presented about the conformation and distribution of the conjugated polymer within the photoactive layer, which has a major impact on the device performance. (c) 2005 Elsevier B.V. All rights reserved.