Grain Boundary Electrostatic Regulation Suppresses Halide Segregation and Boosts Photovoltage in Wide‐Bandgap Perovskite Solar Cells

Wide‐bandgap (WBG) perovskite solar cells are pivotal for tandem photovoltaics, yet their performance is limited by photoinduced phase instability, primarily caused by halide segregation and interfacial non‐radiative recombination. Herein, we report a post‐deposition passivation strategy to form an ultrathin quasi‐2D capping layer on WBG perovskite films. Using nanoscale characterization via Kelvin probe force microscopy and conductive atomic force microscopy, we directly visualize that the passivation increases the local work function at grain boundaries (GBs) and induces a potential inversion, thereby reconstructing band bending and charge distribution. This electrostatic regulation effectively weakens the driving force for iodide accumulation and migration while strongly suppressing leakage pathways along GBs. Furthermore, the quasi‑2D layer not only chemically passivates undercoordinated defects but also optimizes energy‑level alignment at the perovskite/C 60 interface, promoting charge extraction and inhibiting non‑radiative recombination. Consequently, optimized devices achieve an open‑circuit voltage exceeding 1.30 V and a champion power conversion efficiency of 20.12%, along with significantly enhanced photostability. This work unveils the microscopic mechanism of suppressing halide segregation and voltage loss through GB electrostatic regulation, providing a clear interfacial design principle for developing efficient and stable WBG perovskites for tandem solar cells.