An Indacenodithienothiophene‐Based Wide Bandgap Small Molecule Guest for Efficient and Stable Ternary Organic Solar Cells

Abstract The strategic and logical development of the third component (guest materials) plays a pivotal and intricate role in improving the efficiency and stability of ternary organic solar cells (OSCs). In this study, a novel guest material with a wide bandgap, named IDTR, is designed, synthesized, and incorporated as the third component. IDTR exhibits complementary absorption characteristics and cascade band alignment with the PM6:Y6 binary system. Morphological analysis reveals that the introduction of IDTR results in strong crystallinity, good miscibility, and proper vertical phase distribution, thereby realizing heightened and balanced charge transport behavior. Remarkably, the novel ternary OSCs have exhibited a significant enhancement in photovoltaic performance. Consequently, open‐circuit voltage (V_OC), short‐circuit current (J_SC), and fill factor (FF) have all witnessed substantial improvements with a remarkable power conversion efficiency (PCE) of 18.94% when L8‐BO replaced Y6. Beyond the pronounced improvement in photovoltaic performance, superior device stability with a T₈₀ approaching 400 h is successfully achieved. This achievement is attributed to the synergistic interplay of IDTR, providing robust support for the overall enhancement of performance. These findings offer crucial guidance and reference for the design and development of efficient and stable OSCs.