Two new low-bandgap block copolymers derived from dithienylbenzothiadiazole (DTBT) and different electron-rich functional groups (dioctoxyl benzene and N-octyl-diphenylamine), poly(1,4-dioctoxyl-2,5-divinylbenzene-co-4,7-dithiophene-2′-yl-2,1,3-benzothiadiazole) (PPV-DTBT), poly(3,8-divinyl-N-octyl-diphenylamine-co-4,7-dithiophene-2′-yl-2,1,3-benzothiadiazole) (PDPAV-DTBT), were synthesized by Heck cross-coupling polymerization. PPV-DTBT and PDPAV-DTBT are easily soluble in common organic solvents such as o-dichlorobenzene and chloroform. DSC and TGA results indicate that these copolymers possess good thermal stabilities. PPV-DTBT and PDPAV-DTBT films exhibit broad absorption bands at 300–765 nm (with an optical bandgap of 1.62 eV) and 300–733 nm (with an optical bandgap of 1.69 eV), respectively. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PPV-DTBT were estimated by cyclic voltammetry to be −5.43 and −3.74 eV, respectively, and the HOMO and LUMO of PDPAV-DTBT were −5.37 and −3.7 eV, respectively. Preliminary photovoltaic cells based on the composite structure of ITO/PEDOT: PSS/PPV-DTBT:PCBM (1: 2, w/w)/Al showed an open-circuit voltage of 0.75 V, a power conversion efficiency of 0.6%, and a short circuit current of 1.7 mA · cm−2.