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

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.