Understanding the Origin of Ultrasharp Sub-bandgap Luminescence from Zero-Dimensional Inorganic Perovskite Cs₄PbBr₆

Inorganic zero-dimensional perovskites, such as Cs₄PbBr₆, offer an underexplored opportunity to achieve efficient exciton formation and radiative recombination. In particular, the origin of sub-bandgap green emission from Cs₄PbBr₆ is not well understood. Herein, we develop a sequential deposition approach to growing highly smooth Cs₄PbBr₆ films with low rms roughness of 8.15 nm by thermal evaporation. We find that the films have an excitonic absorption edge at 3.9 eV, but exhibit sub-bandgap photoluminescence at 2.4 eV, with a photoluminescence quantum yield as high as 55 ± 2%. We analyze the origin of this sub-bandgap photoluminescence through in-depth transmission electron microscopy, selective area electron diffraction, energy-dispersive X-ray spectrometry, photothermal deflection spectroscopy, and photoluminescence. From 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 as the active layer in light-emitting diodes, achieving an improved external quantum efficiency of 0.36%, a significant improvement over the CsPbBr₃ control devices with EQEs up to 0.0062%.