Synthesis and Photovoltaic Properties of Copolymers Based on Bithiophene and Bithiazole

Three simple structured D-A copolymers, PBTBTz-1, PBTBTz-2, and PBTBTz-3, containing bithiophene (BT) donor unit and bithiazole (BTz) acceptor unit with different alkyl chain length were synthesized by the Pd-catalyzed Stille-coupling method. The copolymers were characterized by thermogravimetric analysis, UV-vis absorption, electrochemical cyclic voltammetry, and photovoltaic measurements. The results indicate that the introduction of BTz unit to the polythiophene main chain effectively decreases highest occupied molecular orbital levels of the copolymers and increases the open circuit voltage (V-oc) of polymer solar cells (PSCs) based on the copolymers as donor, and the alkyl chain length influences the photovoltaic properties of the polymers significantly. The PSCs based on PBTBTz-2 and PBTBTz-3 show higher Voc up to 0.77 and 0.81 V, respectively. The power conversion efficiency of the PSC based on PBTBTz-2: PC70BM 1:1(w/w) reached 2.58% with short circuit current of 8.70 mA/cm(2), under the illumination of AM1.5, 100 mW/cm(2). (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2746-2754, 2011