Synthesis and photophysical properties of regioregular low bandgap copolymers with controlled 5-fluorobenzotriazole orientation for photovoltaic application

Two copolymers P1 and P2, with regioregular and random structures, respectively, were synthesized and their optical and electrochemical properties (both theoretical and experimental) were investigated. The regioregular copolymer P1 showed a lower optical bandgap and a high degree of crystallinity compared to the random P2 copolymer. These copolymers were used as electron donors along with PC71BM as the electron acceptor for the fabrication of solution processed bulk heterojunction solar cells. After the optimization of the weight ratio between the donor and the acceptor and the concentration of the solvent additive i.e. DIO in chloroform, the solar cells based on regioregular P1 exhibit higher power conversion efficiency (7.66%) than the random P2 (5.33%). The enhancement in the power conversion efficiency has been attributed to the increased hole mobility due to the high regioregularity of the conjugated copolymer backbone and effective ordering between the polymer chains. This work brings forth and establishes the importance of copolymers having a regioregular A–D–A–D structure so as to offer significant performance benefits over the random D–A copolymer. This approach is a promising new route to materials for highly efficient polymer solar cells.