We report substantial improvements in efficiency and thermal stability of the poly(3-hexylthiophene):[6.6]-phenyl-C-61-butyric acid methyl ester (PCBM) (P3HT:PCBM) bulk heterojunction (BHJ) system by adding a diblock copolymer P3HT-b-P(St(89)BAz(11))-C-60 (P3HT=poly (3-hexylthiophene), St=styrene, BAz=benzyl aside) as a compatibilizer. Small amounts of the diblock copolymer alter the interfacial morphology between the P3HT and PCBM components, resulting in a noticeable difference in phase segregation of the BHJ films, as evident in atomic force microscopy images. The best performance is observed in the P3HT:PCBM cell with 10% P3HT-b-P(St(89)BAz(11))-C-60, which exhibits substantially improved power conversion efficiency and thermal stability compared to the P3HT:PCBM control device. (C) 2012 Elsevier B.V. All rights reserved.