Achieving a High Fill Factor and Stability in Perylene Diimide-Based Polymer Solar Cells Using the Molecular Lock Effect between 4,4′-Bipyridine and a Tri(8-hydroxyquinoline)aluminum(III) Core

Two novel perylene diimide (PDI)-based derivatives, Alq(3)-PDI and Alq(3)-PDI2, are synthesized by flanking a 3D tri(8-hydroxyquinoline)aluminum(III) (Alq(3)) core with PDI and a helical PDI dimer (PDI2) to construct high-performance small molecular nonfullerene acceptors (SMAs). The 3D Alq(3) core significantly suppresses the molecular aggregation of the resulting SMAs, leading to a well-mixed blend with a PTTEA donor polymer and weak phase separation. Compared with Alq(3)-PDI, the extended pi-conjugation of Alq(3)-PDI2 results in higher-order molecular packing, which improves the absorption and phase separation behavior. Thus, the Alq(3)-PDI2 devices have higher J(sc) and FF values and better device performance, which are further enhanced by a small amount of 4,4 '-bipyridine (Bipy) as an additive. The coordination between Bipy and the Alq(3) core promotes molecular packing and phase separation, which lower charge recombination and enhanced charge collection in the resulting devices. Therefore, a largely improved J(sc) of 15.74 mA cm(-2) and very high FF of 71.27% are obtained in the Alq(3)-PDI2 devices, resulting in a power conversion efficiency of 9.54%, which is the best value reported for PDI-based polymer solar cells. The coordination can also serve as a molecular lock, which prevents molecular motion and thus improves device stability.