In this work we have tested in bulk heterojunction solar cells a polymer with a high oxidation potential, Poly-(dithieno[3,2-b:2',3'-d] thiophene)-3-decylthiophene (P1), as electron donor together with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor. We have studied the photovoltaic characteristics of solar cells made with different PI:PCBM blend composition. The incident photon-to-current efficiency spectra reaches EQEs of 40%, evidencing good charge photogeneration. The best power conversion efficiency, eta=0.83 under AM1.5 illumination, is found for solar cells containing 67% in weight of PCBM. As expected for polymers with high oxidation potentials, the open circuit voltage V-oc attains high values (V-oc >= 0.8 V), independently from the photoactive blend composition, but the fill factors FF of the cells are low (FF: 0.3-0.38), evidencing poor charge transport properties. The data here reported suggest that transport features, probably arising from poor polymer hole mobility, are the main limitation for cell performances. (c) 2008 Published by Elsevier B.V.