Enhancing Interfacial Stability of FAPbI ₃ Perovskite Solar Cells by Introducing 2D/3D Heterojunction Based on Functional Imidazolium Cations

Interfacial engineering has been widely used to reduce interfacial non‐radiative recombination and enhance the performance of perovskite solar cells (PSCs), but stability issue of interfacial passivation itself has been seldom explored. Herein, we design two halide imidazolium‐based molecules with halogen octyl chains (X8C‐mImX, X = Br, I) for n‐i‐p typed PSCs. It is found that 2D/3D heterojunction structure based on substituted imidazolium cations is formed at the perovskite/spiro‐OMeTAD interface, which effectively passivates interface defects. Meanwhile, free Br − and I − ions can passivate iodide‐vacancy‐related defects at the interfaces, while Br − ions can further contribute to defect passivation in the bulk. With the synergistic effects of the mixed I8C‐mImI/Br8C‐mImBr (I/Br‐mIm), small‐area PSC device presents 26.33% efficiency with negligible hysteresis. Typically, the 2D phase (I/Br‐mIm) 2 PbI 4 , exhibits excellent thermal stability to guarantee the stability of the interface passivation. Accordingly, modified devices exhibit better thermal stability by maintaining 92% of initial efficiency after 1200 h at 85°C. The devices also retain 90% of initial efficiency after 1000 h continuous illumination with a bias voltage close to maximum power point (MPP), indicating its good operational stability. This work provides a feasible way to enhance interface stability and overall device stability by selecting suitable post‐treatment materials.