Pressure‐Modulated Molecular Stacking Strategy Extends Exciton Diffusion Length for Thick‐Film (300 nm) Organic Photovoltaics Exceeding 19% Efficiency

Abstract Thick‐film (>300 nm) organic solar cells (OSCs) have attracted increasing attention in recent years due to their compatibility with large‐scale industrial production. However, the inherently short exciton diffusion length ( L D ) of organic semiconductors severely restricts exciton diffusion to the interface in the larger donor/acceptor (D/A) domains, thereby impeding the photovoltaic performance, especially open circuit voltage and fill factor for the commercialized thick‐film OSCs. Herein, a pressure‐modulated molecular stacking (PMMS) strategy is employed to enhance crystallization and regulate fluid confinement depth (the grating depth of imprinted PM6) by controlling the imprinting pressure, thereby optimizing D/A inter‐penetration with favorable vertical phase separation morphology. This strategy can significantly extend L D (from ≈ 26.5 to ≈ 40.3 nm) to facilitate efficient exciton diffusion and carrier transport by enhancing ordered molecular stacking. Consequently, the best devices achieve one of the highest power conversion efficiencies (PCE)s of 20.20% (100 nm) and 19.27% (300 nm, certified as 18.88%), respectively, while the large‐area module (16.94 cm 2 ) exhibits an impressive PCE of 17.01% for D18/BTP‐eC9:L8‐BO ternary system via blade‐coating technology. This work provides a valuable approach to extending L D by constructing favorable vertical phase separation morphology for achieving large‐scale high‐efficiency thick‐film OSCs.