Understanding the\nOrigin of Ultrasharp Sub-bandgap\nLuminescence from Zero-Dimensional Inorganic Perovskite Cs₄PbBr₆

Abstract

Inorganic zero-dimensional perovskites, such as Cs₄PbBr₆, offer an underexplored opportunity to achieve\nefficient\nexciton formation and radiative recombination. In particular, the\norigin of sub-bandgap green emission from Cs₄PbBr₆ is not well understood. Herein, we develop a sequential deposition\napproach to growing highly smooth Cs₄PbBr₆ films\nwith low rms roughness of 8.15 nm by thermal evaporation. We\nfind that the films have an excitonic absorption edge at 3.9 eV, but\nexhibit sub-bandgap photoluminescence at 2.4 eV, with a photoluminescence\nquantum yield as high as 55 ± 2%. We analyze the origin of this\nsub-bandgap photoluminescence through in-depth transmission electron\nmicroscopy, selective area electron diffraction, energy-dispersive\nX-ray spectrometry, photothermal deflection spectroscopy, and photoluminescence.\nFrom these measurements, we find that the Cs₄PbBr₆ contains residual CsPbBr₃, and the wider bandgap Cs₄PbBr₆ confines the excitons in the CsPbBr₃, enabling high photoluminescence quantum yields. We use this material\nas the active layer in light-emitting diodes, achieving an improved\nexternal quantum efficiency of 0.36%, a significant improvement\nover the CsPbBr₃ control devices with EQEs up to 0.0062%.