Pressure Induced Emergence of Visible Luminescence in Lead Free Halide Perovskite Cs₃Bi₂Br₉: Effect of Structural Distortion

In recent years, halide perovskites of the formula A₃B₂X₉ (A:Cs, Rb; B:Sb or Bi; and X:halogen) have been in research forefront due to their potential applications in optoelectronic devices and solar cells. We have carried out high pressure x-ray diffraction, Raman spectroscopy and photoluminescence measurements on a model Pb-free solar cell material Cs₃Bi₂Br₉ halide perovskite, which is nonluminescent under ambient conditions. The sample starts showing photoluminescence above 1.4 GPa, due to an isostructural transition to a distorted unit cell. Further enhancement in intensity with pressure is found to be driven by increase in distortion of BiBr₆ octahedra. Electronic band structure calculations show the sample in the high pressure phase to be an indirect band gap semiconductor. The photoluminescence peak shows a kink at higher energy and a broad asymmetric peak at lower energies due to the recombination of free excitons and self trapped excitons, respectively.