Optical Characterization of Cesium Lead Bromide Perovskites

CsPbBr3 and Cs4PbBr6 perovskite powders have been synthesized through a relativelysimple low-temperature and low-cost method. Nanocrystalline films have also been deposited fromsolutions with four different molar compositions of binary salt precursors. Optical absorption,emission and excitation spectra have been performed in the UV-visible spectral range while X-raydiffraction (XRD) has been recorded to characterize the nanocrystal morphology for the differentmolar compositions. A preferential orientation of crystallites along the (024) crystalline plane hasbeen observed as a function of the different deposition conditions in films growth. All the crystalsshow an absorption edge around 530 nm; Tauc plots of the absorption returned bandgaps rangingfrom 2.29 to 2.35 eV characteristic of CsPbBr3 phase. We attribute the UV absorption band peakedat 324 nm to the fundamental band-to-band transition for Cs4PbBr6. It was observed that the sampleswith the most ordered Cs4PbBr6 crystals exhibited the most intense emission of light, with a brightgreen emission at 520 nm, which are however due to the luminescence of the inclusion of CsPbBr3nanoclusters into the Cs4PbBr6. The latter shows instead an intense UV emission. Differently, thepure CsPbBr3 powder did not show any intense fluorescent emission. The excitation spectra of thegreen fluorescent emission in all samples closely resemble the CsPbBr3 absorption with the peculiardip around 324 nm as expected from density of state calculations reported in the literature.