Meticulous Design of High‐Polarity Interface Material for FACsPbI3 Perovskite Solar Cells with Efficiency of 26.47%

Designing new interface materials with the multifunctions of upper film crystallization control, interfacial defects passivation, and interfacial energy level regulation is crucial for developing efficient and stable perovskite solar cells (PSCs). Herein, a high polarity interfacial material, 2‐cyano‐N,N,N‐trimethylammonium bromide (CNCB), was synthesized to engineer the buried interface between SnO2 and perovskite of the PSCs. Comprehensive theoretical and experimental investigations demonstrate that CNCB interacts with perovskite precursor (PbI2 and FAI) to regulate crystallization kinetics, yielding perovskite films with preferred orientation and reduced defects. Simultaneously, CNCB chemically interacts with both SnO2 and perovskite surfaces, effectively passivating oxygen vacancies in SnO2 and undercoordinated Pb2+ defects at the perovskite buried surface. Furthermore, the high dipole moment of CNCB induces beneficial interfacial polarization, optimizing energy level alignment and suppressing non‐radiative recombination. The CNCB‐modified FACsPbI3 PSCs achieve a champion power conversion efficiency of 26.47% with exceptional operational stability, retaining 87.14% of their initial efficiency after 1000 hours of continuous 1‐sun illumination. This work establishes a molecular design paradigm for multifunctional interfacial materials in perovskite optoelectronics, highlighting the synergistic roles of crystallization control, defect passivation, and dipole engineering in high‐performance devices.