The photovoltaic properties of three fluorene-thiophene-based donor-acceptor copolymers with low band gap and reasonably high hole mobility were studied in copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction photovoltaic cells. The copolymers were poly[2,7-(9,9'-dihexylfluorene)-alt-2,3-dimethyl-5,7-dithien-2-yl-quinoxaline] (PFDDTQ) (band gap = 1.94 eV: mobility = 2.83 x 10(-5) cm(2) V(-1) s(-1)), poly[2,7-(9,9'-dihexylfluorene)-alt-4,7-dithien-2-yl -2,1,3-benzothiadiazole] (PFDTBT) (band gap = 1.82 eV; mobility=4.71 x 10(-5) cm(2) V(-1) s(-1)) and poly[2,7-(9,9'-dihexylfluorene)-alt-2,3-dimethyl-5,7-dithien-2-yl-thieno [3,4-b] pyrazine] (PFDDTTP) (band gap = 1.68 eV; mobility = 1.18 x 10(-4) cm(2) V(-1) s(-1)). The order in the short-circuit current density and power-conversion efficiency of the photovoltaic cells was PFDTBT>PFDDTQ>PFDDTTP, which contradicted the order in the band gap and mobility. The short-circuit current density and power-conversion efficiency (PCE) coincided instead with the order in the mobility of the copolyrner/PCBM blend, where the mobility was increased for PFDTBT and PFDDTQ owing to the charge transfer with PCBM, but was decreased for PFDDTTP due to phase separation resulting from the strong intermolecular interactions of PFDDTTP. With its high blended mobility and low band gap, PFDTBT achieved a PCE of 1.1%. (C) 2009 Elsevier B.V. All rights reserved.