Polymer‐Assisted Morphology Regulation Enables 18.3%–Efficiency in o‐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.