The Effect of Strain on Vibrations in Cubic CsPbI3 Perovskite

Cubic CsPbI3 is an inorganic halide perovskite that is being explored for highly efficient and stable solar cells. Studies into lead perovskite materials have grown exponentially over the last decade due to their rapid increase in power conversion efficiency growing from 3.8% in 2009 to 25.2% in 2020.This project is a computational study using Density Functional Theory (DFT) aimed at determining how the vibrations change with strain. We found the optimized structure using different functionals (PBE, LDA, and PBEsol) to be in good agreement with experiment, and found that the band structures differed mainly by a constant vertical shift. Phonon calculations were carried out for the unstrained structure, and imaginary frequencies and structural instabilities were investigated for the cubic structure of CsPbI3. We then computed how phonon frequencies changed because of strain. Due to the symmetry, there are Raman-active modes only when Cs is off-centered. Consequently, we investigated how IR-active frequencies changed due to strain, which can allow measurement of strain with IR spectroscopy. Ultimately, this project will provide more insight into perovskite materials that can be used to further increase efficiency in solar panels.