Ion migration in mixed halide perovskite (MHP) absorber layers limits the long-term stability of wide-band gap (WBG) solar cells, posing a challenge to commercialization. We address this challenge with an “ionic lockdown” strategy using a vinyl imidazolium-iodine couple, [VIm][I], at the device interface. The iodine counterion effectively occupies surface iodide vacancies, suppressing ion migration. This treatment neutralizes native defects and locks volatile iodide and organic cations, as evidenced by an increase in defect formation energies by ∼0.8 eV and activation energy for ion migration by ∼0.59 eV. We demonstrate this with MAPb(I0.5Br0.5)3, a highly unstable MHP composition. In situ GIWAXS under AM1.5G at 85 °C shows no peak splitting, confirming the photostability. Devices treated with [VIm][I] retain 90% power conversion efficiency (PCE) under continuous illumination and recover 99% PCE in the dark. These results highlight the potential of [VIm][I] for enhancing the stability of WBG cells across different compositions, paving the way for more durable perovskite-based photovoltaic technologies.