Preparation And Photophysical Properties Of Metal Halide Perovskites

Metal halide perovskites have attracted extensive research and applications due to their excellent photoelectric properties.As a potential optoelectronic material,it has been used in many fields such as light-emitting diodes（LEDs）,solar cells,photodetectors,sensing and photocatalysis.Among them,Pb-based metal halide perovskites are the most attractive due to their high carrier mobility,low exciton binding energy,high defect tolerance,tunable band gap,high quantum yield and high optical absorption coefficient.However,two inherent defects of Pb-based metal halide perovskite materials have hindered their further practical applications.First,the toxicity of Pb endangers human health.Another problem is that the poor stability limits the practical use of the device.To address these issues,Pb can be substituted by composition control to avoid its potential toxicity.However,the obtained photoelectric properties of the lead-free metal halide perovskites are still not comparable to the Pb-based metal halide perovskites.Therefore,ion doping as an effective strategy to improve the photoelectric properties of metal halide perovskites has been widely used in the modification of lead-free metal halide perovskites.Based on this,several lead-free metal halides with high stability are designed in this dissertation,and the luminescence properties are improved by appropriate ion doping.The intrinsic photophysical properties are revealed by combining experimental characterization and theoretical analysis,and the relationship between structural transition and photophysical properties is explored.The main work includes the following five parts:（1）Synthesis and luminescence regulation of Cd-based metal halide perovskites with high environmental stabilityA series of Cd-based metal halide perovskites with high stability are prepared by solvothermal method,including tetragonal Cs₇Cd₃Br₁₃,two-dimension（2D）Ruddlesden-Popper（RP）Cs₂Cd Cl₂Br₂ and Cs₃Cd₂Cl₇.The broadband luminescence properties of Cd-based perovskites are effectively enhanced by appropriate concentration（0.1%）Sb³⁺ion doping,and tunable luminescence from cyan to orange（517-625 nm）is achieved.The temperature-dependent photoluminescence（PL）spectra show that the strong electron-phonon coupling in the Sb:Cs₇Cd₃Br₁₃ lattice is the main reason for the broadband orange emission with large Stokes shift.Combining with experimental characterization and theoretical calculations,it is revealed that the broadband emission of Sb:Cs₇Cd₃Br₁₃ is mainly attributed to the ³P₁-¹S₀transition of Sb³⁺ions.Further theoretical calculations of the formation energies of Cd tetrahedra and octahedra in Cs₇Cd₃Br₁₃ structures with Sb polyhedra have been carried out to determine the preferential substitution between the octahedra.At the same time,the formation energies of Cs₇Cd₃Br₁₃,Cs₂Cd Cl₂Br₂,and Cs₃Cd₂Cl₇ were used to analyze the relationship between structure formation and reaction conditions.Finally,its potential in practical applications is explored through stability and LED performance tests.（2）Synthesis of environmentally friendly Te:Cs₂Zr Cl₆ and improvement of humidity stability of A-site Rb alloyingA series of lead-free vacancy-ordered metal halide perovskite x%Te:Cs₂Zr Cl₆（x=0-100）have been successfully synthesized by solvothermal method.A broadband yellow-green emission with photoluminescence quantum yield（PLQY）of 49.0%was achieved under 390 nm excitation by appropriate（1%）Te⁴⁺doping.The experimental results show that the luminescence comes from the triplet（³P₁-¹S₀）self-trapped exciton（STE）emission of Te⁴⁺ions,and the singlet STE emission of Te⁴⁺ions is found by high-energy light excitation.Further A-site alloying and X-site co-halogenation results show that X-site co-halogenation does not have a positive effect on PL intensity and stability,while A-site Rb alloying can effectively improves PL intensity and stability.The obtained samples have high PLQY and excellent stability,and have promising application prospects in the fields of solid-state lighting and backlight display.（3）Controllable regulation of structure and luminescence of Sb-doped In-based metal halide perovskitesThe preparation of three-dimension（3D）Cs₂KIn Cl₆ and zero-dimension（0D）（Cs/K）₂In Cl₅（H₂O）metal halide perovskites were achieved by solvent method through component engineering.In addition,a reversible crystal structure transition between 3D and 0D is achieved by subsequent addition of precursors.In addition,the cyan emission of 14%Sb:Cs₂KIn Cl₆（PLQY=99.2%）and yellow emission of 14%Sb:（Cs/K）₂In Cl₅（H₂O）（PLQY=81.7%）can be achieved by doping with appropriate concentration of Sb³⁺ions,and the switching between these two structures is realized by component engineering control,resulting in a novel cyan/yellow reversible luminescence switch.The temperature-dependent PL spectra show that the exciton binding energy and electron-phonon coupling in different structures are closely related to the structure,which systematically reveals the relationship between structure and luminescence mechanism.Finally,the stability test and LED application show the potential of its practical application.（4）Reversible optical properties of(C₈H₂₀N)₂Mn Cl₄ caused by thermal structural transformationZero-dimensional Mn-based organic-inorganic metal halide(C₈H₂₀N)₂Mn Cl₄ single crystals are synthesized by a simple slow evaporation method,which exhibits a strong green emission under ultraviolet excitation with a peak at 520nm and a full width at half maxima（FWHM）of 55 nm,which can be attributed to the ⁴T₁-⁶A₁ transition of Mn²⁺ions.Large Mn-Mn distance effectively inhibits the migration of excitation energy between adjacent Mn²⁺emission centers,thus achieving up to a PLQY of 87.7%at lower energy excitation（450 nm）.It is found that the optical properties can be reversibly changed at 220/240 K.Temperature-dependent X-ray diffraction（XRD）spectra confirmed that the optical property transitions are associated with a thermally induced structural phase transition.This work provides a promising strategy for the construction of multifunctional optoelectronic materials and provides new insights for understanding the convertible photophysical properties of perovskite isomers.（5）Single component white light emission and humidity sensitive(C₁₉H₄₂N)₂Sb Cl₅ powderSb-based organic-inorganic hybrid metal halide(C₁₉H₄₂N)₂Sb Cl₅ powders are prepared by a simple cooling crystallization method.It is found that the(C₁₉H₄₂N)₂Sb Cl₅ powders has obvious excitation-dependent luminescence,showing a double-peak warm white emission under 300 nm excitation and a single-peak yellow emission centered at 625 nm at 365 nm,respectively.Further study found that the PL emission of(C₁₉H₄₂N)₂Sb Cl₅ powder showed a red shift after being placed in the environment for a certain time.The converted sample also has a double-peak emission at 300 nm which is closer to white light,and a single-peak red emission centered at 670 nm at 365 nm.The XRD results confirmed the potential structural transformation of the compound.The PLQYs of yellow and red emission can be up to 78.3%and 48.9%,respectively.Among them,the double-peak emission of the compound comes from the STE emission of the singlet ¹P₁ and triplet ³P₁ of Sb³⁺ions,respectively,so that the single-component white luminescence can be achieved.Further Raman and infrared radiation（IR）spectra proved that the red shift of PL emission caused by this obvious structural distortion can be attributed to the effect of H₂O in the structure.This water-sensitive compound not only has potential application value in the field of single-component white light-emitting diode（WLED）,but also has broad application prospects in the fields of humidity detection and anti-counterfeiting.In summary,a series of all-inorganic metal halide perovskites and organic-inorganic hybrid metal halides with high PLQY,good stability,simple preparation and environmental friendliness are prepared for the toxicity and stability of Pb-based metal halide perovskites,which provided ideas for overcoming the inherent defects of Pb-based metal halide perovskites.