Aromatic Connectivity Governs Aggregation and Morphology in Ternary Organic Solar Cells.

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.