Linker-Mediated Trade-Off Between Acceptor Aggregation and Donor-Acceptor Mixing Enables Organic Solar Cells With Efficiencies Exceeding 20.5.

Small-molecule acceptors (SMAs) bearing steric substituents suppress excessive aggregation-induced nonradiative energy loss (ΔEnrad), but this strategy often compromises charge transport by disrupting π-orbital overlap and blend morphology. This trade-off remains a key limitation to further improving photovoltaic efficiency. Here, we probe this dilemma through linkage-topology engineering by incorporating highly crystalline and luminescent 3,6-dichlorocarbazole units into SMA backbones. Through systematic linker variation, we tune acceptor self-aggregation and donor/acceptor intermolecular interactions, thereby mitigating this intrinsic trade-off. By modulating molecular linkage mode from a direct C─N connection (CHC-1) to a flexible methylene bridge (CHC-2) and a rigid carbonyl bridge (CHC-3), we achieve orthogonal control over acceptor aggregation and donor/acceptor interfacial coupling. Notably, carbonyl-bridged CHC-3 shows the weakest acceptor self-association, but the strongest donor/acceptor coupling with PM6. This combination suppresses charge-transfer-state nonradiative recombination and optimizes film-forming kinetics, leading to an improved vertical composition gradient and a refined surface morphology. Consequently, CHC-3-based binary and ternary devices achieve PCEs of 18.79% and 20.58%, respectively, with a markedly reduced ΔEnrad of 0.204 eV. This work establishes linkage-topology engineering as an effective strategy for balancing luminescence efficiency, charge transport, and morphology in sterically modified SMAs.