Excited-state Dynamics Of Zero-dimensional Perovskite Of Cs₄PbBr₆ Single Crystal

Halide perovskites have excellent optical and electronic properties such as high carrier mobility,high photoluminescence quantum yield,narrow full width at half-maximum and tunable emission wavelength and have exhibited potential of application in several fields including solar cells,LEDs,photodetectors and lasers.Low-dimensional halide perovskites possess low-dimensional crystal structures,in which the quantum confinement effect can increase the exciton binding energy and improve the luminescent performance of the material.Low-dimensional perovskites not only provide a rich selection of luminescent materials,and their unique crystal structure also serves as a special platform for the research of photophysical properties of perovskites.However,the luminescent mechanism of low-dimensional perovskites remains unclear.Compared with three-dimensional（3D）perovskites,Cs₄PbBr₆ is zero-dimensional（0D）and has high photoluminescence quantum yield,good stability,and its luminescence is not affected by size and aggregation.Cs₄PbBr₆ has shown potential in LED and other luminescent applications,but its luminescent mechanism is still under debate.The excited state dynamics of luminescent materials have important influence on the properties of materials and design of devices,and time-resolved spectroscopy is an effective means to study the dynamics of materials.In this thesis,we synthesized 0D perovskite Cs₄PbBr₆ and 3D perovskite CsPbBr₃crystals,and studied the dynamics of both crystals using DFT calculation,steady-state spectroscopy and time-resolved spectroscopy.The results indicated that the green luminescence stems from Cs₄PbBr₆ itself instead of CsPbBr₃ impurity.The work mainly includes the following two parts:1.Preparation of Cs₄PbBr₆ and CsPbBr₃ crystals and investigation of their steady-state optical properties.In order to elucidate whether the luminescence of Cs₄PbBr₆ originates from the material itself or from the impurities of CsPbBr₃,Cs₄PbBr₆ and CsPbBr₃ crystals were synthesized and were proved to be pure single crystals with good crystallinity through various means.Green fluorescence of Cs₄PbBr₆ crystals indicated that the luminescence did not originate from CsPbBr₃impurities.The energy level of Cs₄PbBr₆ and CsPbBr₃ were obtained through DFT calculation and steady-state spectroscopy,and the band gap and defect energy level of Cs₄PbBr₆ were obtained.2.Excited-state dynamics of Cs₄PbBr₆ and CsPbBr₃ crystals.Three methods of time-correlated single photon counting,streak camera and femtosecond transient absorption spectroscopy were used to obtain the dynamics of the samples on nanosecond,picosecond and femtosecond scale,respectively,and the excited-state dynamics of Cs₄PbBr₆ and CsPbBr₃ were analyzed.The dynamics of Cs₄PbBr₆include exciton trapping to the defect level（212 fs）,exciton relaxation within the defect level（4 ps）and exciton recombination（113 ps and>ns）,while those of CsPbBr₃ were intraband hot-exciton relaxation（925 fs）,exciton trapping to trap state（21 ps）and recovery of excited carriers to the ground state（336 ps and>ns）.