Conformation-Regulated Thiazole Solid Additive for Manipulating Molecular Stacking Behavior Enables over 20% Efficiency in Bilayer Organic Solar Cells

Abstract The additive strategy is an effective approach for optimizing morphology of the active layer in organic solar cells (OSCs). To elucidate the impact of distinct molecular conformations of additives on morphology regulation, this work investigates two novel brominated thiazole-based solid additives, namely, 5,5′-dibromo-2,2′-bithiazole (DBBTz) with a linked-ring structure as well as 2,5-dibromothiazolo[5,4-d] thiazole (DBTTz) with a fused-ring structure, and incorporates the additive into D18/L8-BO devices. Both contain a thiazole heterocycle and Br atom, which regulate acceptor molecules via S···N interactions and halogen bonds to optimize film morphology. Thus, D18/L8-BO:DBBTz- and D18/L8-BO:DBTTz-based OSCs achieve power conversion efficiencies (PCEs) of 19.72% and 20.04%, respectively, significantly higher than that of control devices (18.51%). Compared with DBBTz, the rigid planar characteristic of DBTTz further enhances π−π stacking and improves film crystallinity, establishing superior charge transport pathways, thus achieving an outstanding efficiency exceeding 20%. Moreover, the D18/L8-BO:DBTTz-based OSC retains more than 95% of its initial efficiency after storage for 1000 h. This study proposes new insights into the molecular design of additives and offers a feasible approach for realizing highly efficient and stable bilayer OSCs, contributing to the sustainable development and utilization of solar energy sources.