Multifunctional Halogenated Third Components Enable Additive‐Free Organic Solar Cells with 20.63% Efficiency via Regulating Molecular Stacking and Suppressing Exciton‐Vibration Coupling

Achieving simultaneous morphology optimization and energy‐loss suppression in additive‐free organic solar cells (OSCs) remains a major challenge. Here, we report a halogenated third‐component strategy to regulate molecular stacking and exciton‐vibration coupling in additive‐free D18:L8‐BO blends. We show that halogen‐dependent modulation effectively tailors intermolecular interactions, molecular packing, and photoinduced dynamics, leading to markedly different photovoltaic behaviors. Among them, the fluorinated third component, eC9‐4F, delivers the most effective regulation, enabling stronger electrostatic modulation and a more favorable packing configuration with enhanced π – π stacking coherence. Such optimized local packing suppresses exciton‐vibration coupling, reduces vibrational dissipation and energy loss, prolongs exciton diffusion, and improves charge generation and transport. As a result, the additive‐free D18:L8‐BO:eC9‐4F device achieves a power conversion efficiency (PCE) of 20.63% (certified as 20.07%), while flexible devices fabricated on PEN substrates deliver a PCE of 19.34% (certified as 18.65%). These results highlight the strong potential of halogenated third‐component engineering for simultaneously advancing efficiency, mechanical flexibility, and practical applicability in additive‐free OSCs. Our work reveals the multiscale interplay among molecular stacking, energetic disorder, vibrational dissipation, and charge dynamics, and establishes halogenated third‐component engineering as an effective route toward low‐loss, high‐efficiency additive‐free OSCs.