Morphology Optimization Through Additive‐Mediated Synergistic D/A Molecular Assembly and Crystallization Boosts Efficiency of Organic Solar Cells Exceeding 20%

ABSTRACT Solid additive engineering represents a pivotal strategy for optimizing the morphology of the active layer in organic solar cells (OSCs). However, the markedly divergent physicochemical properties between polymer donors and small‐molecule acceptors often cause asymmetric film evolution. Here, we propose a synergistic molecular assembly strategy employing the volatile solid additive 2,3,5‐trichloropyridine (TCPy), which forms strong noncovalent interactions with both donor D18 and acceptor L8‐BO. TCPy effectively restructures the disordered aggregation state of D18, inducing the formation of ordered J ‐aggregates, while concurrently guiding the nucleation and crystallization of L8‐BO, leading to a significant enhancement in charge mobility. In donor‐acceptor blend systems, TCPy retards film‐drying kinetics and orchestrates the molecular assembly process. These combined effects yield an optimized morphology characterized by a reinforced dual‐fiber interpenetrating network, denser molecular packing, and a more uniform vertical composition distribution. Consequently, OSCs based on the D18:L8‐BO system processed with TCPy achieve a power conversion efficiency (PCE) of 20.13%, a marked improvement over the 18.49% PCE of the control devices. The effectiveness of this strategy is further validated across multiple material systems, including D18:Y6, PM6:BTP‐ec9, and PM6:L8‐BO. This work provides a universal approach for the precise morphological control of high‐performance OSCs through solid additives.