Effect of Oligothiophene π-Bridge Length on the Photovoltaic Properties of D–A Copolymers Based on Carbazole and Quinoxalinoporphyrin

Abstract

A series of low-bandgap donor–acceptor (D–A) copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and quinoxalino[2,3-b′]porphyrins (QP) or quinoxalino[2,3-b′]porphyrinatozinc(QP-Zn) as an electron-deficient unit with different length of oligothiophene π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge between the C donor unit and the QP acceptor unit is thiophene (T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and opt...