A series of low bandgap conjugated polymers consisting of benzothiadiazole alternating with dithienothiophene (DTT) or dithienopyrrole (DTP) unit with or without 3-alkylthiophene bridge have been synthesized. Effect of the fused rings and 3-alkylthiophene bridge on the thermal, optical, electrochemical, charge transport, and photovoltaic properties of these polymers have been investigated. These polymers show broad absorption extending from 300 to 1000 nm with optical bandgaps as low as 1.2 eV; the details of which can be varied either by incorporating 3-alkylthiophene bridge or by replacing DTT with DTP. The LUMO levels (-2.9 to -3.3 eV) are essentially unaffected by the specific choice of donor moiety, whereas the HOMO levels (-4.6 to -5.6 eV) are more sensitive to the choice of donor. The DTT and DTP polymers with 3-alkylthiophene bridge were found to exhibit hole mobilities of 8 x 10(-5) and 3 x 10(-2) cm(2) V-1 s(-1), respectively, in top-contact organic field-effect transistors. Power conversion efficiencies in the range 0.17-0.43% were obtained under simulated AM 1.5, 100 mW cm(-2) irradiation for polymer solar cells using the DTT and DTP-based polymers with 3-alkylthiophene bridge as donor and fullerene derivatives as acceptor. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5498-5508, 2009