A series of low-band-gap (LBG) donor-accepor conjugated main-chain copolymers (P1-P4) containing planar 2,7-carbazole as electron donors and bithiazole units (4,4'-dihexyl-2,2'-bithiazole and 4,4'-dihexyl-5,5'-di(thiophen-2-yl)-2,2'-bithiazole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with optical band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. Moreover, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (V-oc) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C-61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C-71-butyric acid methyl ester (PC71BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC71BM (1:1.5 w/w) showed the best power conversion efficiency value of 1.01% with a short circuit current density (J(sc)) of 4.83 mA/cm(2), a fill factor (FF) of 35%, and V-oc -0.60 V under 100 mW/cm(2) of AM 1.5 white-light illumination. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5479-5489, 2010