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

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-<i>b</i>′]­porphyrins (QP) or quinoxalino­[2,3-<i>b</i>′]­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 optoelectronic properties of the copolymers. With the π-bridge varying from thiophene to bithiophene, then to terthiophene, the hole mobility of the copolymers increased gradually, and the absorption was broadened in turn. Zn ion in the porphyrin ring also had a significant influence on the physicochemical and photovoltaic properties. Bulk heterojunction solar cells with the polymers as donor and PC₇₁BM as acceptor demonstrated PCEs of 0.97% for P­(C-T-QP), 1.97% for P­(C-BT-QP), 2.53% for P­(C-TT-QP), and 1.45% for P­(C-TT-QP-Zn). All of them are among the highest PCE values of PSCs based on porphyrin polymers. Among the four polymers, although the P­(C-TT-QP-Zn) shows the highest hole mobility and the widest absorption, the corresponding PSC demonstrated the lowest PCE because the morphology of P­(C-TT-QP-Zn)/PC₇₁BM blend film is not beneficial to the exciton dissociation and charge carriers transport. This study provides a new insight toward the design and future development of quinoxalinoporphyrin-based conjugated polymers