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| Home > In process > Electronic band structure and radiation- and photostimulated processes scintillation materials based оn perovskites |
| Home > In process > Electronic band structure and radiation- and photostimulated processes scintillation materials based оn perovskites |
| Book/Dissertation / PhD Thesis | PUBDB-2026-02581 |
2025
ISBN: 0000-0003-2485-3473
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Abstract: In this thesis, a comprehensive investigation of the electronic energy structureand associated radiation- and photostimulated processes in a wide class of perovskite-based scintillation materials is presented. The research encompasses the study ofinorganic crystals Cs₂MCl₆ (M = Hf, Zr), CsPbX₃ (X = Cl, Br, I), as well as organic-inorganic perovskites with the general formula MAPbX₃ (X = Cl, Br, I).The crystal structure of inorganic Cs₂HfCl₆ and Cs₂ZrCl₆ perovskites wasinvestigated. On the basis of a powder X-ray diffraction, it has been found that bothperovskites possess the cubic symmetry (Fm3m space group), which remains stablewithin a wide temperature range (9–300 K) without structural phase transitions. Thelattice parameters and their temperature evolution were refined by the Rietveld method,revealing a smooth reduction in lattice parameters upon cooling without any anomalies.Amplitude spectra of scintillation response for Cs₂HfCl₆ and Cs₂ZrCl₆ crystalswere investigated at room temperature under gamma-irradiation from a ¹³⁷Cs sourceand alpha particle irradiation from a ²⁴¹Am source. It was found that the scintillationlight yield of Cs₂HfCl₆ is 24800 photons/MeV with an energy resolution of 5.3%, whileCs₂ZrCl₆ exhibits 33900 photons/MeV with an energy resolution of 4.5%. The alpha-to-beta ratios were determined to be 0.39 and 0.35, respectively. For the first time, thenegative thermal quenching of scintillation emission was detected in theaforementioned crystals, manifested as a significant increase in scintillation pulseintensity within the temperature range of 125 – 150 K. This effect is a manifestation ofnegative thermal quenching explained by thermal activation of trapped carriers. A8model of the emission center is proposed that consistently explains the observedchanges of emission intensity with temperature in the studied crystals. Themeasurements of scintillation decay curves revealed complex kinetics due to delayedrecombination processes. In Cs₂HfCl₆ crystal, the decay time constants and the relativecontributions of the amplitude of different components exhibit only minor changesover the entire temperature range, whereas in Cs₂ZrCl₆ all decay time constants changeby an order of magnitude, and the amplitude contributions of the short and fastcomponents show pronounced changes above 150 K.А comprehensive investigations of the crystal structure, electronic structure andoptical properties of CsPbBr₃ and CH₃NH₃PbBr₃ perovskites was carried out using thecomputer modeling on the basis of the first-principles calculations within the densityfunctional theory (DFT) are presented. It was established that these materials exhibitthree temperature-dependent phases: cubic, tetragonal and orthorhombic. Thegeometric lattice parameters were optimized, and variations in structural characteristicswere analyzed depending on the applied approximations of exchange-correlationpotentials (GGA-PBE, GGA-PBEsol, GGA-PBE+U, GGA-PBEsol+U) were analyzed.To gain deeper insight into the bonding characteristics in the studied crystal phases, aMulliken population analysis was performed for all ions in the unit cell.It was found that CsPbBr₃ and CH₃NH₃PbBr₃ perovskites possess the directbandgaps in all three phases. Calculations of the electronic structure revealed asignificant effect of a spin-orbit coupling on the bandgap value due to the presence ofheavy Pb atoms. The effectiveness of the GGA-PBEsol+U approach in accuratelydescribing the electronic structure and achieving good agreement with experimentaldata has been demonstrated. Additionally, analysis of the X-ray luminescence spectrarevealed a strong correlation between the excitonic peak energy below 70 K and thebandgap values of the low-temperature orthorhombic phase, as calculated using theGGA(PBE)+U and GGA(PBEsol)+U functionals. The total and partial density of statesanalyses indicated that all three crystalline phases of the studied perovskites exhibit asimilar distribution of the electronic states: the valence band maximum is primarily9composed of bromine p-orbitals and lead s-orbitals, while the conduction bandminimum mainly consists of lead p-orbitals.Optical properties of the perovskites, particularly absorption coefficients anddielectric functions, were studied, confirming the high potential of CsPbBr₃ andCH₃NH₃PbBr₃ for applications in the optoelectronic devices, solar cells, and LEDs overa wide temperature range. Based on the obtained results, these materials are consideredpromising candidates for use as sensitive elements in high-efficiency radiationdetectors.First-principles calculations within the density functional theory (DFT) wereconducted to study the electronic energy structure of MAPbCl₃, and the primaryfeatures of its reflectivity spectra were interpreted within the broad energy range from3 to 10 eV. The clear peaks corresponding to the excitonic transitions around 3.22 eVand 3.94 eV near the absorption edge, along with additional optical transitions at higherenergies, were identified, indicating a complex electronic structure of the material. Thecalculated total and partial density of states revealed a contribution pattern similar tothat of MAPbBr₃, with halogen p-orbitals and lead s- and p-orbitals forming the valenceand conduction bands, respectively.The temperature dependence of the radiative decay dynamics of excitons underhigh-energy radiation excitation was studied by measuring the X-ray luminescencespectra and scintillation decay times of MAPbCl₃ crystals. Taking into account thecomplex nature of the obtained spectra at low temperatures, the photoluminescencespectra were additionally measured for a more accurate interpretation. The emissionfrom free and bound excitons in the range of 385–430 nm was observed, along with asignificant thermal quenching of luminescence at temperatures above 100 K. Toaccurately characterize the decay processes, the measured curves were subjected todeconvolution analysis, which allowed us to isolate and recover the intrinsicluminescence decay characteristics by accounting the known instrumental responsefunction. At cryogenic temperatures, MAPbCl₃ crystals exhibit extremely fast non-exponential decay (<1 ns). These findings underscore the need for further investigation10into the optoelectronic properties of MAPbCl₃ to enhance its characteristics forpractical applications.The study presents the results of low-temperature scintillation and structuralproperties of CsPbCl₃ single crystals. Analysis of the structural parameters obtainedvia X-ray diffraction confirmed the orthorhombic perovskite structure at cryogenictemperatures, and no indications of impurity phases and monoclinic distortions wereobserved in the examined sample. Temperature-dependent studies of luminescence andits decay kinetics under X-ray excitation revealed that, at low temperatures, the crystalexhibits intense narrow-band emission at 420 nm with very fast decay kinetics. Thisemission, characterized by strong thermal quenching, is associated with radiativerecombination of trapped excitons. The decay time constants of the fast, medium, andslow emission components at 10 K are 0.1 ns, 1 ns, and 11 ns, respectively. Themeasured scintillation light yield for CsPbCl₃ is 140±15% in respect of LYSO-Cecrystal or 19000±2000 photons/MeV at 10 K under 14 keV X-ray excitation. Thescintillation efficiency of CsPbCl₃ remains approximately constant below ~50 K anddecreases upon further heating. Due to the low amplitude of the slow component,CsPbCl₃ demonstrates an exceptionally fast response to excitation, making it apromising material for use in fast scintillation detectors operating at cryogenictemperatures.Due to the crystalline acentricity leading to arising of the bulk photovoltaic (PV)effect, ferroelectrics are considered as the promising candidates for solar cellapplications. This study demonstrated that upon illumination with intense ultravioletlight, CH₃NH₃PbI₃ single crystals undergo the transition from their pristineantiferroelectric state to a ferroelectric one, with the magnitude of the photoresponsesignificantly depending on the duration of prior UV exposure, while the effect ofvarying the intensity of such irradiation is negligible. Consequently, the bulkphotovoltaic effect, typical for ferroelectrics, would be realized. The photovoltagevalues obtained in this study, related to the polarization state of the sample, indicate asubstantial contribution of BPVE. The theoretically maximal power conversion11efficiency achieved was found to be among the highest reported for the single crystalsof this ferroic class. The study also proposes potential pathways to enhance energyconversion efficiency in prospective solar cells based on such materials.
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