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

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.