Two low band gap (LBG) copolymers PCz-DTBTT and PICz-DTBTT, featuring a modified 2,1,3-benzothiadiazole-thiophene-cored unit 2,5-di(4-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene (DTBTT) as acceptor and electron-rich building blocks carbazole (Cz) or indolo[3,2-b]carbazole (ICz) as donor, were synthesized via Suzuki crossing-coupling method. Both copolymers show good solubility in common organic solvents and thermal stabilities. The optical study exhibits a broad absorption in the range of 300-700 nm with the optical band gap () of about 1.78 eV, while the electrochemical cyclic voltammetry (CV) measurement shows the relatively deeper highest occupied molecular orbital (HOMO) energy level (about -5.45 eV). Photovoltaic devices based on the blends of PCz-DTBTT/PC71BM (w:w; 1:2) and PICz-DTBTT/PC71BM (w:w; 1:2) with devices configuration as ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al, show the power conversion efficiency (PCE) up to 2.07 and 2.21 %, with the open circuit voltage (V (oc)) of 0.75 and 0.80 V, short circuit current density (J (sc)) of 6.47 and 6.40 mA cm(-2), fill factor (FF) of 42.6 and 43.2 % under an AM1.5 simulator (100 mW cm(-2)), respectively. It is apparent that DTBTT-based devices exhibit the broad response range, covering from 300 to 720 nm.