Synthesis and photovoltaic performance of N-dioctylmethyl- 2,7-carbazole-alt-5,7.bis（thiophen.2-yl）-2,3-biphenylthieno[3,4.b] pyrazine copolymeric derivatives appending various donor units in phenyl moieties

A series of N-dioctylmethyl-2,7-carbazole-alt-5,7-bis（thiophen-2-yl）-2,3-biphenylthieno[3,4-b]pyrazine （PCz-3ThPz-Ph） co- polymeric derivatives appending various donor units in two phenyl rings, namely, PCz-3ThPz-PhTh, PCz-3ThPz-PhF1, PCz-3ThPz-PhCz, and PCz-3ThPz-PhTpa were synthesized and characterized. The effect of these appending donor units, e.g., thiophene （Th）, fluorene （F1）, carbazole （Cz）, and triarylamine （Tpa）, was investigated on dispersible, optical, electrochemical, and photovoltaic properties for their polymers. The copolymers of PCz-3ThPz-PhCz and PCz-3ThPz-PhTpa containing Cz or Tpa units exhibited higher short-circuit current density （Jsc） and power conversion efficiency （PCE） in their bulk heterojunction polymeric solar cells. The highest PCE of 1.66% and Jsc of 7.16 mA cm-2 were obtained in a device with the PCz-3ThPz-PhCz/PC61BM blend under AM 1.5 G irradiation （100 mW cm 2）; these values are 1.78 and 1.59 times higher than the corresponding values for the PCz-3ThPz-Ph-based device. When PC61BM was placed by PC71BM, the PCz-3ThPz-PhCz-based device displayed an enhanced PCE of 2.98% and a Jsc of 10.88 mA cm-2. This work demonstrated that appending additional hole-transporting units of Cz and Tpa into the side-chain of a polymer with a D-A backbone can significantly enhance the photovoltaic performance of their resultant polymers.