Photovoltaic devices of end-functional-group-modified poly 3-(hexylthiophene)/ [6,6]-phenyl-C-61 butyric acid methyl ester (P3HT:PCBM; see figure) are fabricated with thermal annealing. The surface energies between donor and acceptor were matched by varying the end group, which can be used to control vertical and horizontal phase separation in the active layer, leading mixed nanomorphology with optimized phase separation, low series resistance, and high performance for solar cell devices.