The surface composition in spin-coated films of polyfluorene:fullerene blends was determined quantitatively by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. By comparing partial and total electron yield spectra, we found vertical compositional differences in the surface region. Furthermore, the orientation of the polymer chains was investigated by variable-angle NEXAFS. Blend films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] with [6,6]-phenyl-C61-butyric acid methyl ester in two different blend ratios were studied. Results showed polymer enrichment of the surfaces for films with a polymer:fullerene weight ratio of 20:80 and of 50:50, spin-coated from both chlorobenzene and chloroform solutions. The angular dependence of the NEXAFS spectra of the pure polymer films showed a preferential plane-on orientation, which was slightly stronger in the subsurface region than at the surface. In blend films, this orientational preference was less pronounced and the difference between surface and subsurface vanished. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013
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