By annealing bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) (RR-PHT) and 1-(3-methoxycarbonylpropyl)-1-phenyl-[6,6] C(61) (PCBM) the quantum efficiency as well as charge carrier mobilities increases, while the bimolecular recombination coefficient decreases. We clarify the capacitance and conductivity proper-ties of both annealed and non-annealed cells using the charge carrier extraction by linearly increasing voltage (CELIV) technique. We found that the capacitance in the annealed samples is a strong function of temperature, while the non-annealed samples show only the geometrical capacitance. The results can be explained by the demixing of the PCBM and RR-PHT and formation of high conductivity regions in the blend after the annealing process. (c) 2007 Elsevier B.V. All rights reserved.