The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by blend composition and thermal annealing conditions. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) diffraction measurements show that in the considered blends, ordering of P3HT plays a key role in understanding the PV-performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. Annealing however improves the crystallinity, explaining the observed spectral broadening and is also resulting in a higher mobility of the holes in P3HT. (c) 2006 Elsevier B.V. All rights reserved.