Sterically Engineered Molecular Alloying for Kinetic Regulation of Acceptor Crystallization Achieves 20.73% Efficiency in Ternary Organic Solar Cells

ABSTRACT Precise control over crystallization kinetics and multi‐scale morphological evolution is crucial for achieving high‐performance organic solar cells (OSCs). Herein, we report a molecular alloying strategy by introducing a sterically hindered acceptor, L8‐Ph, into the L8‐BO matrix to construct a homogeneous alloy phase and regulate acceptor crystallization behavior. In situ kinetic analysis reveals that L8‐Ph increases the nucleation barrier and effectively modulates the crystallization process, prolonging the Stage II crystallization duration from 0.17 s in the binary system to 0.23 s in the ternary blend. The moderated crystallization suppresses uncontrolled aggregation while preserving sufficient molecular order, leading to a refined fibrillar network with enhanced crystalline coherence and a more balanced vertical phase separation. These morphological improvements promote charge transport and reduce bimolecular recombination and trap‐assisted losses. Consequently, the D18:L8‐BO:L8‐Ph ternary device delivers a power conversion efficiency (PCE) of 20.73%, outperforming the D18:L8‐BO binary control device with PCE of 19.77%, with a high fill factor of 82.95%. This work establishes sterically hindered acceptor alloying as an effective route to regulate non‐equilibrium crystallization and guide morphology evolution toward high‐efficiency organic photovoltaic devices.