The hole mobility and power conversion efficiency of bulk heterojunction solar cells based on P3HT-type donor polymers and the soluble fullerene derivative [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) as an acceptor both show a strong sensitivity to the introduction of interchain branches into the P3HT backbones. Branched B-P3HT copolymers display a distinctly decreased hole mobility and reduced solar cell power conversion efficiency with increasing amount of interchain 3.3'-bithiophene branches within the polythiophene macromolecules. The results illustrate the primary importance of proper solid state packing towards optimum charge transport and solar cell performance.