Unraveling the Role of Molecular Flexibility in Enhancing Pyridine Based Defect Passivation for Efficient and Stable Perovskite Solar Cells

Abstract Molecular modulation of the perovskite/C 60 interface to reduce defect density plays a decisive role in achieving high‐efficiency and stable inverted perovskite solar cells. However, the impact of substituent flexibility on passivation performance remains insufficiently understood. Here, two structurally analogous pyridine‐based molecules with distinct central substituents 1, 2‐bis(4‐pyridyl) ethane (2PYET) featuring a flexible alkyl chain and 1, 4‐di(4‐pyridyl) benzene (2PYBEN) possessing a rigid phenyl core are designed to elucidate the role of molecular flexibility in perovskite surface passivation. Our study reveals that the flexible central substituent significantly enhances the electron cloud density of the pyridine groups, thereby improving their passivation capability, while simultaneously suppressing molecular aggregation and promoting better interfacial contact. As a result, devices modified with 2PYET achieved a champion power conversion efficiency of 26.03% for 0.09 cm 2 devices and 24.45% for 1.0 cm 2 devices.