Ionic Lockdown:\nSealing Migration Channels across\nDevice Interfaces in Mixed Halide Perovskite Solar Cells

Ion migration in mixed halide perovskite (MHP) absorber\nlayers\nlimits the long-term stability of wide-band gap (WBG) solar cells,\nposing a challenge to commercialization. We address this challenge\nwith an “ionic lockdown” strategy using a vinyl imidazolium–iodine\ncouple, [VIm][I], at the device interface. The iodine counterion effectively\noccupies surface iodide vacancies, suppressing ion migration. This\ntreatment neutralizes native defects and locks volatile iodide and\norganic cations, as evidenced by an increase in defect formation energies\nby ∼0.8 eV and activation energy for ion migration by ∼0.59\neV. We demonstrate this with MAPb(I<sub>0.5</sub>Br<sub>0.5</sub>)<sub>3</sub>, a highly unstable MHP composition. In situ GIWAXS under\nAM1.5G at 85 °C shows no peak splitting, confirming the photostability.\nDevices treated with [VIm][I] retain 90% power conversion efficiency\n(PCE) under continuous illumination and recover 99% PCE in the dark.\nThese results highlight the potential of [VIm][I] for enhancing the\nstability of WBG cells across different compositions, paving the way\nfor more durable perovskite-based photovoltaic technologies.