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 × 10 –3 and 1.77 × 10 –3 cm 2 /(V s), respectively. Power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PDTSBTBTz: PC 70 BM (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:PC 70 BM (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.