Effects of π-Conjugated Bridges on Photovoltaic Properties of Donor-π-Acceptor Conjugated Copolymers

Abstract

A series of conjugated donor (D)-π-acceptor (A) copolymers, P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), based on benzodithiophene (BDT) donor unit and benzothiadiazole (BT) acceptor unit with different π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridges between the BDT donor unit and BT acceptor unit are furan (F) in P(BDT-F-BT), thiophene (T) in P(BDT-T-BT) and thieno[3,2-b]thiophene (TT) in P(BDT-TT-BT). It was found that the π-bridges significantly affect the molecular architecture and optoelectronic properties of the copolymers. With the π-bridge varied from furan to thiophene, then to thieno[3,2-b]thiophene, the shape of the molecular chains changed from z-shaped to almost straight line gradually. Band gaps of P(BDT-F-BT), P(BDT-T-BT) and P(BDT-TT-BT) were tuned from 1.96 to 1.82 to 1.78 eV with HOMO levels up-shifted from −5.44 to −5.35 to −5.21 eV, respectively. Bulk heterojunction solar cells with the polymers as donor and PC71BM as acceptor demonstrated power conversion efficiency varied from 2.81% for P(BDT-F-BT) to 3.72% for P(BDT-T-BT) and to 4.93% for P(BDT-TT-BT). Compared to furan and thiophene, thieno[3,2-b]thiophene π-bridge in the copolymers shows superior photovoltaic performance. The results indicate that the photovoltaic performance of some high efficiency D–A copolymers reported in literatures could be improved further by inserting suitable π-bridges.