A novel concept of D-A-type copolymers based on fluorene polymer incorporated with 3,6-carbazole unit enhances the device performance for organic photovoltaic cells. P(F45C5-DTBT), incorporating 5 mol % 3,6-carbazole into P(2,7F-DTBT), shows an almost 2-fold improvement (5.1%) in power conversion efficiency relative to P(2,7F-DTBT) (2.6%). This results is ascribed to the good balance between electron and hole mobilities in the devices (mu(e)/mu(h) similar to 1.8 for P(F45C5-DTBT) vs 152 for P(2,7F-DTBT)), and the formation of a nanoscale morphology in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM).