Synergistic Modulation of Molecular Orientation and Interfacial Compatibility via Pyrimidine‐Induced Weak Interactions for Efficient Organic Solar Cells

Precise regulation of molecular orientation and optimization of interfacial compatibility between the Ag/active layer and cathode interface layers (CILs) are pivotal for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, two bay ‐modified perylene diimide (PDI) derivatives, PDINN‐PeOO and PDINN‐PemOO, are synthesized by introducing pyridine‐ester and pyrimidine‐ester groups via facile coupling. Compared with pyridine‐functionalized PDINN‐PeOO, pyrimidine‐based PDINN‐PemOO possesses stronger electron‐withdrawing capability. Meanwhile, PDINN‐PemOO can establish dual synergistic weak interactions, namely hydrogen bonding and π – π stacking, thereby inducing a dominant face‐on molecular orientation with suppressed edge‐on packing. More importantly, the optimized stacking configuration further enables PDINN‐PemOO to achieve bidirectional interfacial optimization. At the Ag/CIL interface, pyrimidine‐derived can reduce the work function of Ag, and the enhanced intramolecular charge transfer simultaneously promotes self‐doping. At the active layer/CIL interface, dual weak interactions strengthen molecular binding, reduce interfacial energy, thus suppressing charge recombination. Such bidirectional interfacial modulation dramatically improves stability with a T 80 of 1400 h under N 2 . Consequently, the PDINN‐PemOO‐based device attains an outstanding PCE of 20.06% in the PM6: D18: L8‐BO system. This breakthrough unravels the intrinsic mechanism by which hydrogen bonds and π – π stacking co‐regulate molecular orientation and interfacial compatibility, laying a robust foundation for efficient, stable OSCs.