Wide bandgap polymer donors with an acceptor1-π-acceptor2-π configuration for efficient polymer solar cells

Abstract

To maximize the photovoltaic performance of polymer solar cells (PSCs), polymer donors and nonfullerene acceptors (NFAs) should be designed with complementarily optical and electronic properties as well as desirable morphological features. Currently, high-performance polymer donors are mainly constructed by alternating copolymerization of electron-rich (D) and electron-deficient (A) units. Here, we demonstrate that acceptor1-π-acceptor2-π (A1-π-A2-π)-type polymer donors constructed by copolymerization of two different A units can achieve comparable photovoltaic properties with those of D-π-A-π-type counterparts. By using the ladder-type 4,10-bis(2-hexyldecyl)-thieno[2ˊ,3ˊ:5,6]pyrido[3,4-g]thieno-[3,2-c]isoquinoline-5,11-dione as A1 unit, difluorobenzotriazole or benzodithiophene-4,8-dione as A2 unit, and thiophene unit as the π bridge, two A1-π-A2-π-type copolymers, P03 and P07, are successfully synthesized. Due to the weakened intramolecular charge-transfer effect, P03 and P07 exhibit wide bandgaps of 1.88 and 1.74 eV, respectively. In combination with the NFA of L8-BO, the P03:L8-BO-based blend shows superior crystallinity and uniform phase-separated morphology thereby leading to a higher power conversion efficiency (PCE) of 15.03 % in comparison with the P07:L8-BO-based blend which shows a PCE of 9.84 %. Notably, the 15.03 % efficiency is by far the highest value for PSCs based on A1-π-A2-π-type polymer donors. These results provide useful guidance for the future design of high-performance donor copolymers for PSCs.