An alternating narrow bandgap conjugated copolymer (PICZ-DTBT, E-g = 1.83 eV) derived from 5,11-di(9-heptadecanyl)indolo[3,2-b]carbazole and 4,7-di(thieno[3,2-b]thien-2-yl)-2,1,3-benzothiadiazole (DTBT), was prepared by the palladium-catalyzed Suzuki coupling reaction. The resultant polymer absorbs light from 350-690 nm, exhibits two absorbance peaks at around 420 and 570 nm and has good solution processibility and thermal stability. The highest occupied molecular orbital (HOMO) energy level and lowest unoccupied molecular orbital (LUMO) level of the copolymer determined by cyclic voltammetry were about -5.18 and -3.35 eV, respectively. Prototype bulk heterojunction photovoltaic cells from solid-state composite films based on PICZ-DTBT and [6,6]-phenyl-C-71 butyric acid methyl ester (PC71BM), show power conversion efficiencies up to 2.4% under 80 mW . cm(-2) illumination (AM1.5) with an open-circuit voltage of V-oc = 0.75 V, a short current density of J(sc) = 6.02 mA . cm(-2), and a fill factor of 42%. This indicates that the copolymer PICZ-DTBT is a viable electron donor material for polymeric solar cells.