Atomic-Level Regiospecific Engineering of Asymmetric Acceptors for Efficient Organic Solar Cells With Reduced Energetic Disorder.

To address the intrinsic trade-off between open-circuit voltage and charge transport in symmetric non-fullerene acceptors, we report an asymmetric molecular engineering strategy based on regiospecific monochloro-isomeric substitution. While the α- and β-isomers differ only in chlorine positioning, they exhibit distinct electronic and morphological fingerprints. Systematic investigations reveal that the β-chlorinated isomer, BTP-2Cl-βCl, orchestrates a synergistic elevation of the LUMO level and a refined electrostatic surface potential, enabling a champion efficiency of 20.32% with an exceptional fill factor of 80.93% in binary devices, surpassing both its symmetric parent and α-isomer. By incorporating a dimer third component, the ternary devices achieve a superior PCE of 20.51%. Regiospecific β-substitution steers the blend toward an idealized film-formation kinetic regime and ensures superior energy-level homogeneity. This precise control reconciles nucleation dynamics with crystal growth, fostering a high-purity, bi-continuous interpenetrating network that substantially suppresses non-radiative recombination losses and minimizes energetic disorder. Our work demonstrates that subtle regiospecific modifications in asymmetric designs can serve as a decisive lever to break intrinsic performance barriers, providing a transformative paradigm for next-generation organic photovoltaics through atomic-level structural precision.