Structural Homology Solid Additives Enhancing Crystallization Thermodynamics for Constructing Ordered Fibrillar Network Toward 20.22% Efficiency Layer‐by‐Layer Organic Solar Cells

ABSTRACT Precise morphology regulation in layer‐by‐layer (LbL) processed organic solar cells (OSCs) is critical for breaking the 20% efficiency bottleneck. However, conventional solid additives often disrupt intrinsic polymer packing. Herein, we present a “structural homology” strategy by using the native D18 polymer monomer (A1) as a benchmark, and have developed an advanced additive (A2) via precise heteroatom engineering. By substituting A1's fused thiophenes with thiazoles while preserving identical alkylated bridges, A2 achieves absolute lattice compatibility. The incorporation of electron‐withdrawing imine nitrogen atoms endows A2 with a larger dipole moment and specific S···N conformational locks. Acting as an ideal homological template, A2 synergistically modulates crystallization thermodynamics to construct a highly ordered fibrillar network with tightened π‐π stacking. Consequently, A2‐regulated D18/L8‐BO LbL devices exhibit accelerated exciton dissociation and suppressed trap‐assisted recombination, achieving an exceptional power conversion efficiency (PCE) of 20.22%, substantially outperforming the control (19.23%) and A1‐processed (19.75%) devices.