Structure-Activity Relationship between Crystallinity and Carrier Transport of Two-Dimensional Donor Units in Organic Solar Cells.

Benzo[1,2-b:4,5-b']dithiophene (BDT) and its derivatives have made important contributions to constructing high-performance polymers. However, it is difficult to clarify the real role of donor units due to the interference of strong electronegativity and crystallinity of acceptor units in the D-A copolymer. Here, we design a cyclohexane-substituted dithieno[3,2-f:2',3'-h]quinoxaline (DTQ)-based acceptor unit with successfully destroyed crystallinity and charge transport. Three donor-dominated materials PQH-BTF, PQH-BTCl, and PQH-BFCl are obtained. It is found that the materials exhibit obvious differences after destroying the crystallization and charge transport of the acceptor unit, and the real role of different two-dimensional donor units in designed polymers is confirmed. The backbone BDF exhibits much stronger intermolecular interactions compared to BDT, while the side chain ThF demonstrates a higher crystallization capacity than that of ThCl. More interestingly, it can be inferred that the molecular backbone is likely to construct miscible-phase crystallization (D-A crystal) while the side chain tends to demonstrate a capacity for pure-phase crystallization (D-D crystal) in a 2D donor system. Different crystallization leads to different exciton transport: pure-phase crystallization is conducive to the reduction of trap-assisted recombination, while miscible crystallization is beneficial to the reduction of bimolecular recombination. This work can help to choose donor units more accurately when preparing D-A copolymers.