Efficient Planar Heterojunction Organic Solar Cell with Enhanced Crystallization and Diffusivity of Acceptor

In the field of organic solar cells (OSCs), planar heterojunctions (PHJs) have received less attention. This study demonstrates that enhancing the crystallization and diffusivity of nonfullerene small-molecule acceptors (NF-SMAs) through sequential deposition significantly optimizes the morphology of PHJ OSCs, driving notable performance enhancements. An additive 1,2,4,5-tetrabromobenzene (TBrB) is employed during sequential deposition, enabling such desirable morphological control in OSCs. <i>In situ</i> UV-vis absorption spectroscopy reveals that TBrB selectively induces rapid aggregation of NF-SMAs, L8-BO, within subseconds. Structural analysis confirms that TBrB promotes the formation of a 3D "honeycomb" structure of L8-BO. Simultaneously, TBrB enhances L8-BO diffusivity into the D18 layer, resulting in a widened and well-intermixed region. These morphological optimizations improve the charge transfer efficiency and reduce bimolecular recombination, achieving a peak power conversion efficiency (PCE) of 19.25%. This study underscores the critical role of fine-tuning solidification processes in sequential deposition to optimize the morphology and performance of OSCs.