Geometric Complementarity and Electrostatic Fluctuation Control in Dual-Asymmetric Acceptors for Efficient Organic Solar Cells.

Abstract

Asymmetric molecular design has emerged as an effective strategy for developing high-performance acceptors for organic solar cells (OSCs). Here, we introduce a dual-asymmetric topology strategy, in which the stereochemical configurations of both the central core and terminal groups are co-engineered. Using this approach, we synthesized an isomeric pair, Th2Cl-a-2Cl and Th2Cl-b-2Cl and benchmarked them against the singly asymmetric a-CH-Th2Cl and Th2Cl-4Cl. We find that the distinctive steric conformation of Th2Cl-a-2Cl promotes a geometrically complementary, interlocked packing motif within the crystal lattice. This not only reinforces the three-dimensional network connectivity but, more importantly, gives rise to a more uniform local electrostatic environment at the molecular-skeleton scale. As a result, Th2Cl-a-2Cl exhibits weaker and more balanced environment-induced electrostatic fluctuations along the backbone, which helps reduce local electrostatic perturbations during charge transport. Consequently, PM6:Th2Cl-a-2Cl devices deliver a champion PCE of 20.18% with a high fill factor (FF) of 80.88%, while significantly suppressing the non-radiative recombination loss (ΔE₃) to 0.190 eV. These results establish geometric complementarity and electrostatic homogeneity, achieved through topological engineering, as an effective route to improve the efficiency of OSCs.