Research On The Luminescence Mechanism Of Zero-dimensional Perovskite Cs₄PbBr₆ Microcrystals

The all-inorganic cesium lead halide perovskite has recently been intensively studied because of their outstanding optoelectronic properties and promising applications in solar cells,fluorescent displays,detectors,and lasers.At present,the research on three-dimensional cesium lead halide perovskite materials(CsPb X₃,X=Cl,Br,I)is relatively successful,and the research on zero-dimensional materials(Cs₄Pb X₆,X=Cl,Br,I)is still in the developing stage.Cs₄PbBr₆ is the most typical zero-dimensional perovskite material that has recently attracted widespread attention due to its narrow bandwidth,high efficiency green photoluminescence and controversial luminescence mechanism.Spectroscopy analysis under extreme conditions such as variable temperature and high pressure has been demonstrated to be helpful for exploring the luminescence mechanism of perovskite materials.For these reasons,the fluorescence spectra of Cs₄PbBr₆ microcrystal under variable temperature and high pressure conditions were investigated in this work,and the results are compared with three-dimensional CsPbBr₃ nanocrystal in detail to explore the luminescence mechanism of zero-dimensional perovskite materials.Firstly,steady-state fluorescence spectra and fluorescence dynamics of Cs₄PbBr₆microcrystals and CsPbBr₃ nanocrystals dispersed in cyclohexane under variable temperature were observed,and the temperature dependences of fluorescence characteristic parameters such as intensity,peak position,full width at half maximum,lifetime and the proportion of attenuation component were obtained.The influence of temperature on lattice structure and luminescence characteristics have been deeply analyzed.The results suggest that the green photoluminescence of Cs₄PbBr₆ is extrinsic luminescence,which is related to lattice defects,and at the same time,it is probably derived from the CsPbBr₃ impurities embedded in the Cs₄PbBr₆ matrix.Next,the steady-state fluorescence spectra and fluorescence kinetics of these two materials under high-pressure conditions were observed,and the pressure dependences of the fluorescence characteristic parameters were obtained.It is found that the variation of typical spectral characteristic parameters with pressure is similar for these two materials and the fluorescence spectra can recover when the pressure was released.The in-situ XRD pattern of the Cs₄PbBr₆ microcrystalline powder sample under high pressure were observed.The similar pressure dependences of the fluorescence parameters of the two materials and the different behavior after pressure relief between the XRD and luminescence confirmed the view that the green photoluminescence of Cs₄PbBr₆ microcrystals is extrinsic luminescence.We also perform theoretical calculations on Cs₄PbBr₆ under high pressure.The calculation results show that the lattice structure of Cs₄PbBr₆ undergoes changes such as lattice compression and unit cell distortion under pressure,and the inflection point of the unit cell parameters with pressure is close to the mutation point of the XRD pattern,which confirms the reliability of the theoretical calculations.The theoretically calculated band gap corresponding to the highest pressure value in the experiment is 3.2 e V,which further proves that the green photoluminescence of Cs₄PbBr₆ crystallites is extrinsic luminescence.The temperature and pressure effects and theoretical calculations show that the photoluminescence of Cs₄PbBr₆ is not intrinsic luminescence,but originated from the impurity of CsPbBr₃ embedded in Cs₄PbBr₆ matrix and lattice defects.