We have synthesized a new low bandgap alternating polyfluorene copolymer (PFNAP) based on dioctylfluorene and a donor-acceptor monomer with an electron-withdrawing moiety as a side chain, via a Suzuki polymerization reaction. The resulting copolymer has low optical and electrochemical bandgaps. The optical bandgap and the electrochemical bandgap of PFNAP are 1.82 and 1.89 eV, respectively. The bulk heterojunction polymer solar cells were fabricated with the conjugated polymer as the electron donor and 6.6-phenyl C(61)-butyric acid methyl ester (PCBM) as the electron acceptor. The power conversion efficiencies (PCE) of the solar cells based on PFNAP:PCBM (1:3) and PFNAP:PCBM (1:4) are 0.61% and 0.67%, respectively, under the illumination of AM 1.5 G, 100 mW/cm(2). (C) 2010 Elsevier B.V. All rights reserved.