Self-Trapped Exciton In Halide Double Perovskites:Mechanism,Manipulation And Applications

The luminescent materials are widely used in the field of lighting,display,medical treatment,etc.,which are closely related to our daily life.Different luminescent materials have different luminescent mechanism and characteristics,which correspond to their respective advantages in application fields.Therefore,a deep understanding of the luminescence mechanism and characteristics of materials is beneficial for finding the application fields with special competitiveness.In materials with strong electron-phonon coupling strength,the photoexcited exciton easily causes lattice distortion,forming the self-trapped exciton（STE）with strong quantum confinement.Compared with free exciton（FE）luminescence,the luminescence mechanism,property control,and application of STE are still in the initial stage,which call for in-depth study.In this thesis,we take the lead in introducing the concept of STE into inorganic double perovskites,deepening the understanding of their STE luminescence mechanism.Through electronic dimensionality control and defect passivation,we have obtained a white STE emission with high photoluminescent quantum yield（PLQY）.Furthermore,by taking advantage of its broadband spectrum and negligible self-absorption characteristics,we have demonstrated their competitive application in white light-emitting diodes（LED）and X-ray scintillators,pointing out the way for the further development of STE luminescent materials.The main contents of this thesis are as follows:1)The characterization of the basic properties of Cs₂AgInCl₆.Single crystals are the best object for basic property study.We first obtained centimeter-sized,low trap density[（8.6±1.9）×10⁸ cm^-3],high absolute transparency（75%）,high crystalline[the half peak width of（222）XRD peak around 180 arcsec]Cs₂AgInCl₆ single crystal by an optimized hydrothermal method.Subsequently,we characterized its stability,dielectric constant,carrier mobility,defect density,photoresponse,etc.,revealing its good carrier transport property and relatively high electronic dimensionality,which laid a foundation for the understanding of its luminescent properties.2)The study on the luminescence mechanism of Cs₂AgInCl₆.Based on the Cs₂AgInCl₆single crystal,we found that the Ag-Cl bond of Cs₂AgInCl₆ has strong Raman activity with large deformation,which tends to capture electrons or holes in the excited state and generate lattice deformation,thus forming STE.Further experimental results and theoretical calculation by our collaborator professor Yan Yanfa exhibited that,Ag⁺captured the photoexcited hole with a high effective mass in the excited state due to strong electron-phonon coupling,leading to the Jahn Teller distortion of[Ag Cl₆]octahedron and forming the STE by further capturing a free electron.In this process,two energy losses were generated.Namely,self-trapping energyand lattice deformation energy,resulting in a large stokes shift.Meanwhile,the strong electron phonon coupling leads to the broadening of the photoluminescence spectrum.The theoretical calculation further revealed that the self-traping time of STE in Cs₂AgInCl₆ is about 238 fs,and the electron and hole wave function radii in excitons were significantly different.The understanding of the STE luminescent mechanism laid a foundation for the control of STE luminescence.3)The optimization of PLQY in Cs₂AgInCl₆ and its application in white LED.The PLQY of Cs₂AgInCl₆ was extremely low（<0.1%）due to the parity forbidden transition and relatively high electronic dimensionality.Based on previous understanding of the luminescence mechanism,we specifically proposed Na alloying and trace Bi doping and prepared a white phosphor with high PLQY（86±5%）by hydrothermal method.The key reasons for PLQY enhancement were listed below:i)Na and Ag formed a complete alloy phase in Cs₂AgInCl₆,breaking parity forbidden transition of Cs₂AgInCl₆;the introduction of Na formed the[Na Cl₆]octahedron,which divided the[Ag Cl₆]octahedron,thus the electronic dimensionality was reduced and the wave function overlap between electrons and holes was increased.These two factors significantly increased the probability of radiative recombination.ii)The doping of trace Bi³⁺further reduced the defect density and inhibited the non-radiative recombination.In the end,we combined optimized Cs₂Ag_0.60Na_0.40In Cl₆:Bi³⁺phosphors with an ultraviolet LED,obtaining warm white LED with good stability,exhibiting its application potential in the field of green white light lighting.4)The characterization of Cs₂Ag In_1-xBi_xCl₆ and its application in X-ray scintillator.Taking advantage of the negligible self-absorption characteristic in STE emission,we continued to explore another advantageous application-X-ray scintillator.Utilizing heavy elements Bi alloy,we significantly improved the X-ray scintillation performance of Cs₂AgInCl₆:i)In and Bi formed a complete alloy phase in Cs₂AgInCl₆,breaking parity forbidden transition of Cs₂AgInCl₆;ii)since the X-ray absorption coefficient is directly proportional to the fourth power of atomic number(5(5₀0)0)0)0)0)),the introduction of Bi enhanced the X-ray absorption coefficient of Cs₂AgInCl₆.Finally,the optimized Cs₂Ag In_0.70Bi_0.30Cl₆exhibited good linear response at X-ray dose from 300 n Gy s^-1 to 16.7μGy s^-1.The scintillator yield was comparable with current commercial product LYSO(Lu_1.8Y_0.2Si O₅:Ce³⁺),and the lowest detection limit reached 1.09μGy s^-1,which was 1/5 of the diagnostic requirement(5.5μGy s^-1),demonstrating its potential application in high-performance X-ray scintillation detection.