Inhibiting octahedral tilting for stable CsPbI2Br solar cells

The 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 N-2, 1000 h storage in air (30% relative humidity), and 200 thermal cycles (25-100 degrees C), respectively.