The effect of thermal annealing on the optical, structural and the dielectric properties of P3HT:PC70BM blended films were investigated. By means of atomic force microscopy, we observed the morphology evolution of the annealed P3HT:PC70BM nanocomposites. Raman spectroscopy showed a substantial ordering in the polymer film after annealing. The absorption spectra of the annealed P3HT:PC70BM films were improved and red shifted than un-annealed samples. The results indicate that the P3HT in the nanocomposite becomes an ordered structure with annealing. The ordered P3HT facilitates the charge transport. From the photoluminescence measurements, the formation of polymer crystallites was observed upon annealing. Thus, the device efficiency reaches 2.2% after annealing at 150 degrees C. Impedance spectroscopy shows the classical complex plan curves; the low frequency is related to the effective lifetime of charge carriers and the high frequency corresponds to the diffusion time of these carriers. Global mobilities are in the range 3.8-4.6 x 10(-3) cm(2) V-1 s-i. (C) 2016 Elsevier Ltd. All rights reserved.