Synergistic Optimization for Exciton Delocalization and Morphology in Ternary Organic Solar Cells Enabled by a Low‐Nucleation‐Barrier Guest Acceptor

Abstract Strengthening delocalized singlet exciton (DSE) is an effective strategy to improve open‐circuit voltage (V oc ) and short‐circuit current density ( J sc ) in organic solar cells (OSCs); however, practical methods to achieve this are still severely limited. In this work, the novel guest acceptor J‐OC is introduced to modulate DSE formation as well as bulk heterojunction morphology within the PM6:BTP‐eC9 host system. J‐OC's low nucleation barrier enables it to act as a seed crystal, effectively accelerating nucleation and optimizing crystallization kinetics. This process significantly enhances crystallinity and crystal perfection, facilitating DSE formation and contributing to improved V oc . Furthermore, GIWAXS and morphology analyses reveal J‐OC's multifunctional role in enhancing component miscibility, optimizing exciton distribution, and promoting a fiber‐like morphology. Consequently, ternary devices based on PM6:BTP‐eC9:J‐OC achieved an outstanding power conversion efficiency (PCE) of 20.02%, with simultaneous increases in V oc , J sc , and fill factor. This performance surpasses binary devices based on PM6:BTP‐eC9 (PCE = 19.08%) and PM6:J‐OC (PCE = 17.05%). This work demonstrates the synergistic effects of employing a low‐nucleation‐barrier guest acceptor in a ternary strategy to concurrently optimize DSE formation and morphology.