Halogenated Rubber Enables 19.78%‐Efficiency Organic Solar Cells With Enhanced Ductility via Halogen‐Mediated Non‐Covalent Interactions

Abstract The pursuit of 20% power conversion efficiencies (PCEs) in organic solar cells (OSCs) has been remarkably successful, yet their intrinsic mechanical fragility remains a fundamental constraint for flexible applications. In this work, this challenge is addressed by incorporating butyl rubber (IIR) and its halogenated derivatives—chlorobutyl rubber (CIIR) and bromobutyl rubber (BIIR)—as additives into the high‐performance D18:L8‐BO system. Through this approach, comprehensive structure‐property relationships between rubber characteristics and device performance is elucidated. At an optimal loading of 5 wt%, CIIR‐modified devices achieve an outstanding balance between mechanical resilience and electronic performance. The chlorine‐mediated intermolecular interactions enhance molecular packing, yielding a champion PCE of 19.78%. In contrast, BIIR disrupts molecular ordering due to steric hindrance from its larger bromine atoms, leading to reduced efficiency. Similarly, IIR exhibits detrimental effects owing to the absence of halogen‐driven interactions. Mechanically, all rubber additives improve the active layer's flexibility, with 30 wt% CIIR device achieves the largest crack‐onset‐strain (COS) of 17.7%.