Temperature-Dependent Ultrafast Spectroscopy Study Of All-Inorganic Perovskite Materials

The development of materials science is the foundation of high-tech development and an important manifestation of a country's industrial technological strength.Humans have never stopped exploring new materials.The perovskite material(ABX₃)is a new type of material with unique physical and chemical properties,ranging from conductors to insulators,and is widely used in information technology,electrochemical physics and life sciences.Especially since 2012,blowout research results have appeared one after another,after breakthroughs in the application of perovskite materials in solar cells.From traditional three-dimensional perovskites to emerging two-dimensional perovskites,to one-dimensional nanowires and zero-dimensional quantum dots,perovskite materials with different structures have good applications in the field of optoelectronic devices.There are hundreds of known perovskite materials.Among them,the halide perovskite materials,which have emerged in recent years,have excellent photophysical properties,including easy-to-adjust optical band gaps,high photoluminescence quantum yields,and much high charge generation and transfer efficiency.Due to the advantages of high absorption coefficient,high defect resistance,high fluorescence quantum efficiency(up to 95%or more),narrow half-width(12-42nm),and tunable full visible spectrum(400-700 nm),it becomes to be a promising active material and has shown good application prospects in the field of electronics,such as solar cells,light-emitting diodes(LED),displays,photodetectors,and lasers.The main contents of this paper are as follows:1.The intrinsic structure and spectral properties of CSPb I₃ nanocrystals have been studied in this paper.By means of transmission electron microscopy(TEM),it was found that the Cs Pb I₃ nanocrystals prepared by our group own stable structure and uniform size.2.In this paper,the evolution of the energy state structure and radiation properties of Cs Pb I₃ nanocrystals with the decreasing of temperature was studied by means of steady-state absorption spectroscopy,steady-state fluorescence spectroscopy and time-correlated single photon counting(TCSPC)technique.It was found that the electronic state transition(from ES-I to ES-II)of Cs Pb I₃ nanocrystals occurred at 210K.The bandgap in ES-I and ES-II widens with the temperature,but the trend of widening in ES-II is obviously less than that in ES-I.The absorption cross section(?)at different temperatures is calculated,and the high photon flux can cause phonon reabsorption and delay phonon emission.It is proved that the cooling process of hot carriers is assigned to the bottleneck effect of hot phonon.3.In this paper,by employing the temperature-dependent transient absorption spectroscopy technology,we study the carrier characteristics of the all-inorganic perovskite material Cs Pb I₃ nanocrystals under special environmental conditions.The disappearance of shallow defect states caused by lattice shrinkage in ES-I reduces the trapping of hot carriers,which further affects the carrier dynamics.The average number of photons in each nanocrystal(<N>)at different temperatures was calculated by the absorption cross-section(?),which confirmed that the thermal effect generated by the increase of<N>in ES-II is stronger than that in ES-I.We compared the cooling process of hot carriers and the heating effect of carriers in different electronic energy states,and found that the thermal effect originated from the increasing of<N>in ES-II is stronger than that in ES-I.We attribute these reasons to the reduction of the band gap,the changing of E_LO,the disappearance of shallow defects,and the energy exchange with environmental molecules(such as a small amount of N₂ molecules).