The thermal transition behaviour, optical and structural properties of spin-coated P3HT:C-60 blended films with different C-60 ratios were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), Fourier transform infrared absorption (FT-IR) spectroscopy and Raman spectroscopy. DSC analysis showed that the P3HT:C-60 blends have quite different thermal characteristics. The absorption spectra of the annealed P3HT:C-60 (1:1 wt%) films becomes enhanced and red shifted. This feature is evident in the photoluminescence measurements where the formation of polymer crystallites upon annealing is observed. Raman spectroscopy showed a substantial ordering in the polymer film during annealing. It was found that the performance of a P3HT:C-60 (1:1 wt%) device was dramatically improved by annealing. (C) 2009 Elsevier B.V. All rights reserved.