Revealing the Effect of Halogenation Strategy on the Regulation of Crystallization Kinetics and Molecular Packing for High-Performance Organic Solar Cells

Halogenation of non-fused ring electron acceptors (NFREAs) plays an important role in regulating their optoelectronic properties. However, the underlying mechanisms and their impact on the performance of organic solar cells (OSCs) have remained unclear. Herein, a series of halogenated NFREAs incorporating F, Cl, and Br, are prepared to study their effect on crystallization kinetics, phase separation, molecular packing, and charge transport. Among various halogenation strategies, chlorination minimizes the Coulomb attractive energy between donor and acceptor, thereby facilitating exciton dissociation. In situ UV–vis absorption tests reveal that chlorinated acceptors exhibit a longer crystallization time, effectively suppressing excessive molecular aggregation and enhancing overall crystallinity. Additionally, chlorinated acceptors exhibit a longer exciton diffusion length, which promotes exciton dissociation while mitigating charge recombination in the devices. Consequently, two chlorinated NFREAs, TCN-Cl, and PCN-Cl, yield an impressive power conversion efficiency (PCE) of 14.85% and 15.30%, respectively, when blended with PM6 and J52 donors. These values represent the highest reported PCEs to date for NFREAs with A-π-A’-π-A and A-π-D-π-A structures. The study elucidates the crucial role of chlorination in extending exciton diffusion length and crystallization time. These effects significantly benefit phase separation within the active layers, enhance charge separation, and suppress recombination for achieving high-efficiency OSCs.