Structural distortion induced broad emission in vacancy-ordered halide triple perovskites

Structural distortion in halide perovskites is important to tune the optical properties of the materials. The octahedra formed by metal cations and halide anions in these classes of materials remain symmetric; however, the introduction of asymmetry provides enormous opportunities to improve the photoluminescence emission and excited-state lifetimes for their application in white light emitters. In this work, we have systematically introduced asymmetry in vacancy-ordered halide triple perovskite materials Cs₃M₂X₉ (M = Bi³⁺, Sb³⁺; X = Cl^-, Br^-, I^-) by mixing trivalent sites in three different halide compounds. The Raman and FT-far-IR measurements were used to investigate the distortion introduced in these materials. The distortion is shown to (i) enhance self-trapped excitonic emission, which is broad and intense leading to emission in the complete visible region and (ii) improve excited-state lifetimes. This strategy to create distortion and its proven ability to improve light emission will find application in light-emitting diodes.