AbstractThe intrinsic phase instability of CsPbI₂Br perovskite hinders its development and application in solar cells. Herein, we adopt 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆), a hydrophobic ionic liquid (IL), to improve the phase stability of CsPbI₂Br. Density functional theory calculation reveals that the formation energy of CsPbI₂Br with BMIMPF₆ is reduced, thereby restraining the [PbX₆]⁴⁻ octahedral tilting. The reduced structural distortion and relaxed lattice strain result in improved phase stability of CsPbI₂Br. In addition, the interfacial dipole moment increases after introducing BMIMPF₆, which facilitates the charge transfer. Consequently, the CsPbI₂Br solar cells with BMIMPF₆ deliver a power conversion efficiency (PCE) of 16.2% along with excellent stability. The unencapsulated devices with BMIMPF₆ maintain 98.9%, 88.6%, and 99.5% of the initial PCEs after 1200 h storage in N₂, 1000 h storage in air (30% relative humidity), and 200 thermal cycles (25–100°C), respectively. image