The conduction mechanism and the molecular dynamics on the paradigmatic bulk heterojunction formed by poly(3-hexylthiophene) (P3HT) and phenyl-C-71-butyric acid methyl ester (PC71BM) blends have been characterized by dielectric spectroscopy. The results show that hexyl lateral chains of the polymer exhibit a high frequency local motion, similar to a polyethylene-like relaxation. By dielectric spectroscopy the dependence with temperature of the conductivity has also been studied. The results in the low temperature region provide a method to estimate the energy offsets of the system due to the presence of the C-71 derivative. In the high temperature region, the temperature dependence of the conductivity can be described by the Gaussian disorder model, and we have observed that the energetic disorder in this region does not depend upon the content of PC71BM in the blends.