Unlocking Synergistic Passivation in Perovskite Solar Cells via Molecular Design of Electron‐Donation and Steric Hindrance

ABSTRACT Interfacial defects remain a critical bottleneck for the efficiency and stability of perovskite solar cells (PSCs), as conventional passivation molecules often struggle to concurrently provide strong passivation and uniform coverage. To resolve this dilemma, we propose a synergistic intramolecular design strategy that integrates electron‐donation and steric engineering. Using 1,3,5‐triazine (TZ) and its methylated derivative 2,4,6‐trimethyl‐1,3,5‐triazine (TMZ) as a model system, we elucidate that the methyl groups synergistically enhance the electron density of Lewis‐basic nitrogen atoms via hyperconjugation for robust defect coordination, while suppressing detrimental π–π stacking to facilitate a uniform coverage atop perovskite film. Consequently, TMZ‐treated PSCs achieve a champion power conversion efficiency of 26.07%, and retain over 90% of their initial efficiency after 1400 h of continuous operation. This work establishes a universal “electronic‐steric synergy” principle for designing high‐performance and ultra‐stable perovskite interfaces.