Poly(quarterthiophene-alt-benzothiadiazole), PQT12oBT and PQT12BT, were synthesized through the Stille coupling reaction. The UV-visible absorption spectrum of PQT12oBT showed peaks at 535 nm; moreover, it exhibited a higher molar absorption coefficient (epsilon=44,000 M-1 cm(-1) at 535 nm) than PQT12BT (3300 M-1 cm(-1)) in chloroform solution. The optical band gap of PQT12oBT was calculated 1.74 eV in solid state. The HOMO and LUMO energy levels of PQT12oBT were -5.18 and -3.44 eV, respectively. From the results of X-ray diffraction measurements, the lamellar d-spacing of PQT12oBT in out-of-plane direction was determined to be 21.6 angstrom, and the pi-pi stacking distance between layers was found to be 4.09 angstrom, with a slight edge-on orientation. Bulk heterojunction-type polymer solar cells were fabricated. With a 1:1 ratio of PQT12oBT and PC71BM, the values of open circuit voltage (V-oc), short circuit current density (J(sc)), fill factor (FF), and power conversion efficiency (PCE) were found to be 0.77 V, 8.9 mA cm(-1), 62.4%, and 4.2%, respectively. In addition, PCE was increased up to 4.4% by the addition of 1-bromonaphthalene (1-BrNT) to the active layer. (C) 2013 Elsevier B.V. All rights reserved.