Polymer‐Assisted Morphology Regulation Enables 18.3%–Efficiency in <i>o</i> ‐Xylene Processed All‐Small‐Molecule Organic Solar Cells

Abstract All‐small‐molecule organic solar cells (ASM‐OSCs) offer advantages in structure definition and synthesis but suffer from morphology control challenges due to donor‐acceptor miscibility. Herein, a simple yet highly effective morphology regulation strategy is introduced by incorporating a small amount of polymer donor PM6 into the BTR‐Cl: Y6 binary system. The chain structure of PM6 further directs epitaxial growth of the BTR‐Cl phase, driving the formation of a continuous fibrillar network within the donor domains. Consequently, additive‐free and o‐xylene processed devices exhibit superior charge dynamics, significantly boosting short‐circuit current density and fill factor, achieving a champion efficiency of 18.3%—among the highest for non‐halogenated solvent processed small donor‐small acceptor dominated OSCs. Besides, introducing PM1 into the BTR‐Cl: Y6 system also achieved 18.0% PCE. Moreover, the devices also show exceptional thickness insensitivity—retaining 96% of peak efficiency even at 300 nm active‐layer thickness. Leveraging the advantage with non‐halogenated solvents, large‐area OSCs (13.5 cm 2 ) achieving 12.2% PCE are successfully fabricated. Flexible devices delivered 14.9% efficiency and maintained 92% initial PCE after 800 bending cycles, primarily attributed to PM6's long‐chain structure, enhancing the crack‐onset strain of the active layer. This work provides a facile morphology‐regulation strategy for high‐performance small donor‐small acceptor dominated OSCs.