A new carbazole-based electron accepting unit, 5-(2,7-dibromo-9H-carbazol-9-yl)benzo[a]phenazine (CBP), was newly designed and synthesized as the acceptor part of donor-acceptor type low band-gap polymers for polymer solar cells. The CBP was copolymerized with electron donating monomers such as benzo[1,2-b:4,5-b]dithiophene (BDT) or 4,8-bis(2-octyl-2-thienyl)-benzo[1,2-b:4,5-b]dithiophene (BDTT) through Stille cross-coupling polymerization, and produced two alternating copolymers, PBDT-CBP and PBDTT-CBP. An alternating copolymer (PBDT-CBZ) consisted of 2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole (CBZ) and BDT units was also synthesized for comparison. PBDT-CBZ showed the maximum absorption at 430 nm and did not show absorption at wavelengths longer than 513 nm. However, CBP containing polymers (PBDT-CBP and PBDTT-CBP) showed a broad absorption between 300 and 850 nm due to the intramolecular charge transfer interaction between the electron donating and accepting blocks in the polymeric backbone. Bulk heterojunction photovoltaic devices were fabricated using the synthesized polymers as electron donors and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as electron acceptor. One of these devices showed a power conversion efficiency of 2.33%, with an open-circuit voltage of 0.81 V, a short-circuit current of 6.97 mA/cm2, and a fill factor (FF) of 0.41 under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW/cm2). (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013, 51, 2354-2365