Studies On Stability Of Cesium Lead Halide Perovskite Quantum Dots And Their Application In Light-Emitting Devices

In recent years,all-inorganic cesium lead halide perovskite（Cs Pb X₃）quantum dots have attracted the great attention of researchers as a star material in the optoelectronic field and achieved a series of excellent research results.Cs Pb X₃ quantum dots（QDs）show bright development prospects and potential commercial value in solid-state lighting,anti-counterfeiting encryption,and display fields because of their excellent optoelectronic properties（high photoluminescence efficiency,adjustable luminous wavelength,high color purity,and high defect tolerance）.As a kind of ionic crystal,perovskite quantum dots have low formation energy,which is beneficial for the mutual transformation of multiple phase structures under external stimulation.However,at present,the fluorescent intelligent materials of perovskite quantum dots based on this characteristic are relatively scarce,which is needed to be further developed and applied in the field of anti-counterfeiting encryption.In the field of photoluminescent light-emitting diodes and electroluminescent light-emitting diodes,perovskite quantum dots must have stable fluorescence properties.Because of the fragile stability of perovskite quantum dots,they are easy to be damaged by high temperature,light,water,and oxygen in the environment,resulting in the decline of fluorescence performance and a decrease in device lifetime.Therefore,how to further improve the stability of quantum dots and solve the problem of carrier transport is the key to its commercialization.Given the above problems,the research contents of this paper are as follows:（1）A space-confined strategy combined with a super-hydrophobic mechanism was developed to realize the reversible switching of luminescent properties by absorbing the Cs₄Pb Br₆ QDs into a super-hydrophobic polydivinylbenzene（PDB）porous framework.Under the protection of PDB,the non-luminescent Cs₄Pb Br₆ and luminescent Cs Pb Br₃transform reversibly by alternate water treatment and removal of water to form a water-responsive fluorescent smart material,which is accompanied by the alternating extraction and reinsertion of Cs Br.The composites can stably reproduce the fluorescence reversible phenomenon after 9 cycles and maintain 55%of the initial photoluminescence quantum yields after soaking in water for 95 days.The super-hydrophobic porous structure of PDB not only provides the confined space for dissolved Cs Br but also inhibits the overflow of trapped Cs Br.By investigating the effect of hydrophilicity and hydrophobicity of protective matrix on reversible switching of luminescent properties,it is further confirmed that the super-hydrophobicity of the hierarchical porous matrix is the key factor to ensure the stable reversible phase transition between Cs₄Pb Br₆ and Cs Pb Br₃.By changing the proportion of halogen,it is verified that there is a similar reversible transformation phenomenon in blue and red light emission materials（Cs₄Pb X₆@PDB,X=Cl,Br,and I）.Furthermore,the Cs₄Pb Br₆@PDB in waterborne luminescent inks by screen printing opens a way in the field of anti-counterfeiting owing to the excellent stability of encryption and decryption.（2）A selective heterogeneous nucleation–growth strategy was proposed to prepare aminated hierarchical three-dimensional layered double hydroxide（A3D-LDH）/Cs Pb Br₃composites with excellent luminescent properties and high stability.By anchoring the Cs Pb Br₃ QDs on the A3D-LDH,the A3D-LDH/Cs Pb Br₃ composites exhibit high solid photo-luminescence quantum yield（82.35%）and enhanced stability against the adverse environment（humidity air,irradiation,and heat）.The composites maintain 46.7%and51.5%of the initial photoluminescence intensity after 50 days in air and irradiation for 85hours,respectively.The size of anchored QDs could be controlled by changing the reaction temperature and ligand concentration,and similar composites with other halogen components could be synthesized by the same preparation method（A3D-LDH/Cs Pb X₃,X=Cl,Br and I）.The stability mechanism of the A3D-LDH/Cs Pb Br₃ composites is mainly originated from the strong interaction between the amino group on the A3D-LDH matrix and the Pb²⁺in the Cs Pb Br₃ QDs,which gives the quantum dot anchoring effect.The A3D-LDH with hierarchical flower structure has a larger specific surface area,which provides abundant anchoring sites and isolates the QDs from each other.The above mechanisms effectively reduce the movement and aggregation of QDs and passivate the surface defects of QDs.An efficient white light-emitting diode（LED）was fabricated by introducing A3D-LDH/Cs Pb Br₃ composites as green phosphors with color coordinates（0.349,0.349）.The white LEDs display a luminous efficiency of 71.1 lm/W,and the corresponding value of the color rendering index（CRI）is 71.0.This LED with good white light emission performance and excellent spectral stability offers a promising candidate for solid-state lighting and display.（3）The blue Cs Pb Br₃ quantum dots with the strong quantum confinement effect were prepared by the supersaturated precipitation principle at room temperature using methyl methacrylate as an anti-solvent.The in situ ligand exchange strategy is used to passivate the surface defects of Cs Pb Br₃ quantum dots,reduce the non-radiation recombination process,and improve the fluorescence quantum yield and optical and thermal stability of QDs.At 70℃,the treated QDs still maintain 46%of the initial photoluminescence intensity and the emission peak keep stable at 459 nm during the heating process.Cooperating with short-chain conjugated ligands,the carrier transport capacity of QDs is enhanced.Based on these QDs,pure blue light-emitting electroluminescent LED was successfully constructed,and its structure is ITO/PEDOT:PSS/PTAA/PEABr/QDs/TPBi/Li F/Al.The LED has pure blue light emission of 463 nm with the color coordinate（0.130,0.070）.The optimized device exhibits a maximum EQE of 1.80%,a maximum luminance of 327.7 cd/m² and its turn-on voltage is 3.20 V,which opens up a new idea for the application of Cs Pb Br₃ quantum dots in the field of the efficient blue LED.