20.31% Efficiency Layer‐by‐Layer Organic Solar Cells Enabled by 3D Side‐Chain Topology‐Driven Dual‐Fiber Interpenetrating Networks

ABSTRACT Constructing robust nanofibrillar networks in layer‐by‐layer (LbL) organic solar cells (OSCs) is challenging since small‐molecule acceptors lack polymer‐like interlocking capabilities. Herein, we propose a topology‐driven strategy using bulky siloxane‐terminated side chains to induce fibrillation. We synthesized asymmetric acceptors BTP‐2Ph and BTP‐3Ph by substituting one alkyl chain of L8‐BO with diphenylmethylsilyl and triphenylsilyl groups, respectively. We reveal a size‐dependent competition between steric hindrance and intermolecular interlocking. The bulkier triphenylsilyl group in BTP‐3Ph provides strong interlocking that overrides steric‐induced crystallinity loss, driving the formation of an interconnected acceptor nanofibrillar network. This creates an ideal dual‐fiber morphology with the D18 donor. Consequently, the D18/BTP‐3Ph device achieves an impressive 20.31% efficiency, significantly outperforming L8‐BO (19.28%). Crucially, this physically interlocked framework kinetically freezes the optimal phase separation, enabling excellent operational stability with 85% initial efficiency retention after 650 h of continuous one‐sun illumination.