A low band gap donor-acceptor (D-A) copolymer PTBTBDT, namely, poly(2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-alt-4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b'] dithiophene), was designed and synthesized via a Pd-catalyzed Stille polycondensation reaction. The polymer was characterized using H-1 NMR spectroscopy, UV-vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). PTBTBDT has good solubility in common organic solvents, good thermal stability, broad absorption, low band gap and exhibits not only high hole mobility but also moderate photovoltaic properties. PTBTBDT displays broad absorption in the wavelength range from 300 nm to 630 nm, and its HOMO and LUMO energy levels were calculated to be -4.98 eV and -3.34 eV, respectively. Bulk heterojunction solar cells were fabricated using PTBTBDT as the electron donor and PC70BM as the acceptor. The device exhibits a power conversion efficiency of 2.12% with a current density of 5.45 mA cm(-2), an open-circuit voltage of 0.72 V, and a fill factor of 54% under the illumination of AM 1.5 G, 100 mW cm(-2). Under similar device fabrication conditions, the PTBTBDT based device showed considerably improved efficiency among its previously synthesized counterparts, i.e. PBDTDTBTz and PBDTBTz based devices, which have 1.7% and 1.4% efficiencies, respectively. The hole mobility of the PTBTBDT : PC70BM (1 : 2 w/w) blend reached up to 1.47 x 10(-3) cm(2) V-1 s(-1) as calculated by the space-charge-limited current (SCLC) method. By side-chain engineering, this study demonstrates a good example of tuning the absorption range, energy level, charge transport, and photovoltaic properties of polymers.