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, Alq₃-PDI and Alq₃-PDI2, are synthesized by flanking a 3D tri(8-hydroxyquinoline)aluminum(III) (Alq₃) core with PDI and a helical PDI dimer (PDI2) to construct high-performance small molecular nonfullerene acceptors (SMAs). The 3D Alq₃ 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 Alq₃-PDI, the extended π-conjugation of Alq₃-PDI2 results in higher-order molecular packing, which improves the absorption and phase separation behavior. Thus, the Alq₃-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 Alq₃ 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 Alq₃-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.