Flexible Alicyclic Ammonium Induced Bifacial Defect Passivation for Efficient and Operationally Stable Perovskite Solar Cells

Abstract Effective defect passivation at both the top and bottom interfaces of the perovskite active layer is crucial for achieving highly efficient perovskite solar cells and enhancing their operational stability. At present, such passivation is implemented via two separate steps, thus increasing the processing complexity. To address this challenge, a bifacial passivator is developed, cyclohexylmethylammonium fluoride (CMAF), which is capable of simultaneously modifying both perovskite interfaces despite being introduced only through a simple spin‐coating onto the SnO 2 layer. The fluoride anions (F − ) effectively passivate defects on the surface of SnO 2 . At the same time, the cyclohexylmethylammonium cations (CMA + ) migrate to the perovskite surface during annealing, leading to a passivation of A‐site vacancies. Furthermore, CMAF enhances perovskite crystallinity and optimizes energy band alignment between the perovskite and hole transport layers. As a result, PSCs treated with CMAF achieve a maximum efficiency of 25.52%, along with superior humidity and long‐term operational stability. Comparative analysis with phenylmethylammonium fluoride (PMAF) reveals that the unique flexible alicyclic structure of CMA⁺ leads to a more diffuse electrostatic potential distribution, which is key to its enhanced mobility within the [PbI 6 ]4 − framework. These findings offer valuable insights for the molecular design of passivation agents for perovskite solar cells.