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.

Automated summary

The intrinsic phase instability of CsPbI2Br perovskite in solar cells is addressed using BMIMPF6 as an additive to improve phase stability. The study shows that the addition of BMIMPF6 reduces the formation energy of CsPbI2Br, resulting in improved structural stability and charge transfer efficiency. CsPbI2Br solar cells with BMIMPF6 exhibit a PCE of 16.2% and excellent stability under various storage and thermal conditions.