Rigid Double‐Spiro‐Grid Polymer Regulates Donor Fibrillar Pre‐Assembly and Acceptor Interdiffusion in Layer‐by‐Layer Organic Solar Cells

Layer-by-layer (LBL) fabrication of organic solar cells (OSCs) decouples donor and acceptor deposition, which, however, induces difficulties in controlling donor crystallization and acceptor interdiffusion. Here, we introduce NPSG, a fully conjugated double-spiro-grid polymer, to regulate donor fibrillar pre-assembly before acceptor deposition. Morphological results reveal that NPSG shifts D18 assembly from loosely associated aggregates toward pronounced fibrillar pre-assemblies in solution. Molecular dynamics simulations further suggest that the rigid NPSG backbone provides multiple noncovalent interaction sites that favor anisotropic association between D18 chain segments. During film formation, these fibrillar precursors evolve into a coherent and solvent-resistant donor framework, which moderates L8-BO interdiffusion and promotes a vertically graded, double fibrillar heterojunction. The resulting morphology facilitates interfacial charge transfer, improves the balance of electron and hole transport, and suppresses nongeminate recombination. Consequently, the optimized D18+NPSG/L8-BO devices achieve a power conversion efficiency (PCE) of 21.2%, with a certified value of 20.7%. This strategy also improves device reproducibility, processing tolerance, applicability across different donor/acceptor systems, and operational stability under maximum-power-point (MPP) tracking. These findings establish donor-underlayer pre-assembly regulation as an effective strategy for efficient and robust sequentially processed OSCs.