Photovoltaic Performance Improvement of D-A Copolymers Containing Bithiazole Acceptor Unit by Using Bithiophene Bridges

Two bithiophene-bridged D-A copolymers, PDTSBTBTz and PBDTBTBTz, based on bithiazole acceptor unit and dithienosiole (DTS) or benzodithiophene (BDT) donor unit, were synthesized by the Pd-catalyzed Stille-coupling reaction. The two copolymers exhibit good thermal stability, strong absorption in the visible region, and relatively lower HOMO energy level at ca. -5.10 eV. The hole mobilities of PDTSBTBTz and PBDTBTBTz measured by SCLC method are 1.85 x 10(-3) and 1.77 x 10(-3) cm(2)/(V s), respectively. Power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PDTSBTBTz: PC70BM (1:1, w/w) was 3.82% with J(sc) = 8.68 mA/cm(2), V-oc = 0.72 V, and FF = 0.611, under the illumination of AM1.5, 100 mW/cm(2). In contrast, the PCE of the PSC based on PBDTBTBTz:PC70BM (1:1, w/w) reached 4.46% with J(sc) = 9.01 mA/cm(2), V-oc = 0.82 V, and FF = 0.603. These results indicate that bithiophene-bridged D A copolymers are promising photovoltaic donor materials for the application in PSCs.