Preparation And Optical Properties Of High Stable Perovskite Quantum Dots/Phosphor Composites

Widespread application of YAG:Ce-based white light-emitting diodes（WLEDs）is being challenged for their poor color rendering index（CRI）and high correlated color temperature（CCT）due to the lack of red emission.Organic-inorganic hybrid perovskite quantum dots（MAPb X₃ QDs,X=Cl,Br,I or mixture of them）,presenting high photoluminescence quantum yield,tunable emission color and simple fabrication process have been recognized as promising red-emitting component for warm WLEDs.However,additional protection process for QDs always needed due to their poor stability,which complicates the fabrication procedures of WLEDs.Besides,nano-sized QDs and micron YAG:Ce powder display different sinking behavior in silicone,which also hard to ensure the uniform distribution of the two components and subsequently stable optical characteristics of the device.Herein,a one-pot anchoring strategy is implemented to realize the enhanced thermal and photo-stabilities of in-situ generated red-emitting MAPb Br_0.5I_2.5 QDs and simultaneously strong interaction between the QDs and YAG:Ce to form QDs@A-YAG composite,which greatly simplifies the processes,and also avoids the uneven distribution of phosphor and QDs in silicone.In this paper,the surface modification of YAG:Ce phosphors was firstly studied.Then,QDs were in situ generated on the surface of YAG:Ce to form QDs@A-YAG composite materials.Optical properties and stability of the composites were investigated;finally,WLED was fabricated by combining the composite material with blue chip,and the influence of the QDs concentration of the composite on the optical properties of WLED device was studied.The main research contents are as follows:（1）The surface modification of YAG:Ce phosphors was studied,including the surface amino functionalization and silica coating of YAG:Ce phosphors.We found that the absorption spectrum,emission spectrum,quantum yield and fluorescence lifetime of YAG:Ce phosphor after amino functionalization were not significantly different from pristine phosphor,which indicated that the optical properties of YAG:Ce phosphors are not affected by amino functionalization.It is confirmed by SEM that the silica coating on the surface of YAG:Ce phosphors is successful.The peak position of the emission spectrum after coating is the same as that of pristine YAG:Ce,and the intensity is reduced by 5%.The quantum yield and fluorescence lifetime are almost the same before and after coating.This also shows that the silica coating has little effect on the luminescence properties of YAG:Ce phosphors.（2）The composites were formed by in-situ generation of perovskite quantum dots（QDs@A-YAG）on the surface of amino-functionalized YAG:Ce phosphors by a non-uniform nucleation method.We found that most of the QDs had diameters in the range of15–20 nm,and the hemispherical or spherical QDs were uniformly dispersed on the A-YAG surface.At the excitation wavelength of 405 nm,the simultaneous emission of yellow and red light was achieved in the QDs@A-YAG composite,and the quantum yield of the composite increased from 46.5%to 54.8%with the increase of phosphors.QDs@A-YAG composite has a shorter PL lifetime than A-YAG,which also confirms the fluorescence resonance energy transfer from A-YAG to QDs.Perovskite quantum dots were also in situ generated on the surface of silica sphere-coated YAG:Ce phosphor（S-YAG）by means of non-uniform nucleation.Under excitation at 405 nm,the QDs@S-YAG composite also achieved simultaneous emission of yellow and red light.（3）The thermal stability and photostability of QDs@A-YAG and QDs-YAG were studied.Compared with QDs-YAG,most of the QDs in QDs@A-YAG were anchored on the surface of A-YAG.The movement and regrowth of QDs into large crystals is inhibited.The relative PL intensity of QDs in QDs-YAG dropped sharply to below 20%after 20 h of irradiation.In contrast,QDs in QDs@A-YAG exhibited 72%PL intensity retention after 20 h.This also strongly supports that amino groups can stabilize QDs and further prevent their aggregation under light.For QDs@A-YAG,the emission peaks of QDs are blue-shifted by about 15 nm from 40°C to 80°C,and this blue-shift can be attributed to the thermally induced growth of underdeveloped QDs grains physically adsorbed on A-YAG.,thus avoiding the aggregation of fully grown QDs anchored on the surface of A-YAG,on the contrary,for QDs-YAG,the emission peak of QDs is red-shifted by about 5nm from 40℃to 80°C,in general,QDs particles at high temperature Easily aggregated,resulting in a red-shift of the PL peak wavelength.Finally,the photostability of the in-situ synthesized QDs@S-YAG composites was also investigated.The spectra of QDs@S-YAG and YAG after 40 h were almost identical,and this decrease may be due to the interaction between Si O₂ and QDs.As a result,QDs are prone to photoquenching.Finally,QDs@A-YAG was used to fabricate white LEDs.We found that the CRI increased from78 for YAG based WLED to 92 for QDs@A-YAG based WLED,and the CCT decreased from 5899 K to 4755 K.The QDs@A-YAG based WLED also showed excellent stability when the current continued to increase.