Aromatic Connectivity Governs Aggregation and Morphology in Ternary Organic Solar Cells

ABSTRACT Inadequate control over aggregation and morphology evolution remains a major constraint in ternary organic solar cells (TOSCs). To address this, two small‐molecule donors, C1 and C2, were designed with identical backbones but distinct aromatic connecting units: a phenyl linkage in C1 and a thiophene linkage in C2. It is revealed that steric torsion is utilized by the phenyl linkage in C1 to suppress excessive self‐aggregation and induce balanced, mixed‐orientation molecular packing. In contrast, the thiophene linkage in C2 promotes a coplanar backbone with tighter, predominantly face‐on π–π stacking, leading to over‐crystallization. Consequently, C1 facilitates finer interpenetrating networks with redistributed π–π interactions, supporting efficient in‐plane charge transport. Benefiting from this optimized morphology and favorable energy alignment, the PM6:Y6:C1 device achieves an outstanding Fill Factor (FF) of 77.90% and a Power Conversion Efficiency (PCE) of 17.98%, significantly outperforming binary devices and the C2‐based ternary devices. This work establishes aromatic connecting unit modulation as a pivotal strategy for precisely controlling aggregation and nanoscale morphology in high‐performance TOSCs.