Co-crystallization of Fibrillar Polymer Donors for Efficient Ternary Organic Solar Cells

Ternary strategy is identified as an effective method to fine-tune the optoelectronic properties of the photoactive layer of organic solar cells (OSCs) toward high power conversion efficiency (PCE). Although numerous high-performance ternary OSCs have been established, the underlying fundamentals of ternary OSCs are still not fully understood, and a general rule for the ternary system design is highly appreciated. In this contribution, we demonstrate the construction of high-performance ternary OSCs via the co-crystallization of conjugated host donor PM6 and guest donor D18-Cl. Due to the similar chemical structure, larger planarity, and lower surface energy of D18-Cl, it locates in the PM6 phase and organizes with PM6 during solution casting, leading to the formation of co-crystallized fibrillar donor phase with enhanced structural order and charge transport channels. As a result, a series of binary host systems, including PM6:BTP-4F-C6-16, PM6:Y6, and PM6:Y7-BO obtained enhanced efficiency with the presence of D18-Cl as the guest donor, achieving a superior PCE of 18.5% in the PM6:D18-Cl:BTP-4F-C6-16 ternary OSCs. This work is the first demonstration of co-crystallized polymer donor fibrils to boost the charge transport and power conversion of ternary OSCs.