Synthesis And Performance Of Manganese-Based Metal Halide Phosphors For LED Applications

Lead-based metal halide luminescent materials have broad application prospects in the field of liquid crystal backlight display（LCD）and solid-state lighting due to their high photoluminescence quantum yield（PLQY）,narrow full width at half maximum（FWHM）and simple preparation method.Unfortunately,the poor stability of lead-based metal halides and the toxicity of lead are the biggest obstacles to their large-scale commercial application.Hence,the development of new metal halide materials with environmentally friendly,high efficiency and excellent stability has become an urgent need.Recently,Mn²⁺-based metal halide materials have become ideal alternatives to lead-based metal halides due to their unique d-d transition,low toxicity and facile synthesis process.However,Mn²⁺-based metal halide phosphors still face the challenges of poor fluorescence performance and unsatisfactory stability,as well as low performance of the fabricated light-emitting diode（LED）devices,which limit their practical application in the field of optoelectronics.Therefore,obtaining efficient and stable Mn²⁺-based metal halide phosphors is of great significance to promote the development of ultra-high-definition and wide-gamut display as well as high-quality lighting.Therefore,a series of studies on the chemical synthesis,fluorescence performance and stability regulation of Mn²⁺-based metal halide green and red phosphors were carried out,revealing the intrinsic influences of defect passivation and crystal field intensity regulation on their emission performances.Moreover,the potential applications of Mn²⁺metal halide green and red phosphors in LCD display and lightings are explored.The main research contents of this paper are as follows:（1）To obtain Mn²⁺-based metal halide phosphor with efficient green emission,[（CH₃）₄N]₂MnX₄（X=Cl,Br,I）phosphors were prepared by large-scale mechanochemical method.The effects of precursor ratio and halogen substitution on the composition,structure and fluorescence properties of Mn²⁺-based metal halide green phosphors were systematically studied.The obtained[（CH₃）₄N]₂MnX₄ phosphors have an isolated[MnX₄]^2-tetrahedral structure.Thanks to the synergistic strategy of organic halide compensation and Br/Cl substitution,the crystal field strength was effectively reduced and the formation of halogen vacancies was hindered.The PLQY of[（CH₃）₄N]₂MnBr₄phosphors under the excitation of ultraviolet light is significantly increased from 20.93%to 97.96%.In addition,the thermal stability and storage stability of[（CH₃）₄N]₂MnBr₄phosphors were investigated.The[（CH₃）₄N]₂MnBr₄ phosphors are applied to fabricate the green LED device,which shows high luminous efficiency（LE）of 172.09 lm/W,and good operating stability(T₅₀ of 176 h).Meanwhile,[（CH₃）₄N]₂MnBr₄ phosphors and commercial K₂Si F₆:Mn⁴⁺red phosphors are encapsulated into a white LED device,which have a good LE of 107.02 lm/W and wide color gamut of 107.9%NTSC.（2）In order to solve the problem of weak blue excitation of Mn²⁺based metal bromide green phosphor,the organic precursor chain length regulation strategy is used to adjust the distance of the adjacent[MnBr₄]^2-tetrahedron in the[(C_xH_2x+1)₄N]₂MnBr₄ structure.The effect of organic precursor chain length on the structure and fluorescence performance of green phosphor was systematically studied.The obtained[（C₂H₅）₄N]₂MnBr₄ phosphors show the emission peak of 518 nm and FWHM of 43 nm.When the shortest distance of the adjacent[MnBr₄]^2-tetrahedron is 9.454?,the exciton transition between the adjacent[MnBr₄]^2-tetrahedron can be effectively inhibited,resulting in the boosting of PLQY from82.09%to 98.70%under blue light excitation.Moreover,the stability and blue light absorption efficiency（from 44.0%to 65.0%）of[（C₂H₅）₄N]₂MnBr₄ green phosphors are successfully improved via Zn²⁺doping strategy.In addition,the operation stability of the high-efficiency green LED device based Zn²⁺doped[（C₂H₅）₄N]₂MnBr₄ green phosphors is also improved,which the operation time of maintaining initial efficiency of 100%increases from 89.78 h to 157.55 h.A high-efficiency and wide color gamut WLED fabricated by[（C₂H₅）₄N]₂Mn_0.90Zn_0.10Br₄ and K₂Si F₆:Mn⁴⁺phosphors exhibits a highly LE of 136.40 lm/W and color gamut of 112.8%NTSC,which proved that Mn²⁺-based metal bromide green phosphors possess a broad application prospect in the field of high-definition display.（3）In order to improve the fluorescence performance of Mn²⁺based metal halide red phosphors and realize the construction of Mn²⁺based WLED devices with high efficacy and wide color gamut,the efficient[（CH₃）₄N]MnCl₃ red phosphors were prepared by a simple precipitation method.The effects of HCl addition,precursor ratio and reaction temperature on the composition,structure and fluorescence properties of[（CH₃）₄N]MnCl₃phosphors were systematically studied.Thanks to the formation of[MnCl₆]^4-octahedron,the prepared phosphors present red emission with a peak position of about 629 nm and FWHM of about 75 nm.The appropriate HCl addition amount is conducive to improving the crystallinity of phosphors and providing a chlorine-rich environment,which can effectively reduce the formation of chlorine defects,leading to the increase of PLQY to70.77%.In addition,the[（CH₃）₄N]MnCl₃ phosphors have good thermal stability,which can retain 53.27%of the fluorescence intensity at room temperature at 400 K.A monochromatic LED device is fabricated by packaging[（CH₃）₄N]MnCl₃ phosphors onto a blue LED chip,which shows an excellent operating stability（maintaining 98.29%of the initial LE after continuous operation for 224 h）.In addition,WLED devices are fabricated by packaging red phosphors and[（C₂H₅）₄N]₂Mn_0.90Zn_0.10Br₄ or[（CH₃）₄N]₂MnBr₄ green phosphors onto blue chips,respectively,which show the maximum LE of 122.42 lm/W,the widest color gamut of 105.1%NTSC and an excellent operating stability(T₅₀ of 740.3h),confirming that the combination of Mn²⁺-based metal halide red emitters and green emitters based on the same cation in the A position is beneficial to obtaining efficient and wide color gamut WLED devices.（4）To solve the problem of poor stability of organic-inorganic hybrid manganese-based metal halides red phosphors,all-inorganic Rb₂MnBr₄·2H₂O crystals were prepared by solution evaporation method.The crystal structure and fluorescence properties of Rb₂MnBr₄·2H₂O were revealed.However,Rb₂MnBr₄·2H₂O exhibits a weak violet emission due to the strong electron-phonon coupling of water molecules.In order to improve the fluorescence performance,a thermally induced partial detachment strategy of water molecules is developed.And the effects of temperature and stoichiometric ratio of precursors on structure and fluorescence properties of phosphors were discussed.When the temperature is 160℃,the obtained Rb₂MnBr₄·0.9H₂O shows the strongest fluorescence emission intensity,which have an emission wavelength of 653 nm,PLQY of62.8%and FWHM of 107 nm.Furthermore,the fluorescence enhancement mechanism caused by the detachment of water molecules was studied by temperature dependent PL and density functional theory calculations,etc.Thanks to the all-inorganic structural framework,Rb₂MnBr₄·0.9H₂O exhibits excellent thermal stability,which can maintain92.68%of room temperature fluorescence intensity at 440 K.Finally,a warm WLED device is successfully constructed by packaging Rb₂MnBr₄·0.9H₂O and commercial green powder onto a blue LED chip,which have a high color rendering index of 91.5,a correlated color temperature of 5238 K,and an LE of 84.84 lm/W,indicating the preparation of rubidium manganese bromide has broad application prospects in the fields of lighting.