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.