Composite films of P3HT/PCBM (poly[3-hexylthiophene-2,5-diyl]/[6,6]-phenyl C-61 butyric acid methyl ester) are widely used as active layer in plastic solar cells. It was shown that the annealing of the active layer leads to increased optical absorption and therefore to improved performance of solar cells. However the origin of the improved optical absorption is not clear yet. In this work, structural and optical properties of such films were studied by X-ray diffraction and spectroscopic ellipsometry, respectively. The influence of the annealing temperature on film crystallinity and on optical absorption was studied. A direct correlation between optical absorption and film crystallinity was demonstrated, i.e. increased crystallinity leads to an increased optical absorption. The highest crystallinity and the highest optical absorption were observed for films annealed at 125 degrees C. (c) 2005 Elsevier B.V All rights reserved.