Due to high fluorescence quantum efficiency (PLQY) and narrow half-peak full width (FWHM), all-inorganic CsPbBr3 backlight display sources. Perovskite quantum dots glass, possessing excellent luminescence properties of quantum dots and physical/chemical stability of glass, has attracted wide attention. However, during its preparation process, serious component volatilization would occur since the melting temperature of Cs-Pb-X glass is as high as 1 000 ℃, and it is difficult to control grain size and distribution; therefore, the experimental repeatability becomes a problem. The currently reported CsPbBr3 quantum dot glass is still difficult to achieve a peak wavelength close to the Rec.2020 standard of 525?535 nm high-purity green narrow band luminescence.In this work, based on an in-situ growth method, CsPbBr3 quantum dots-porous glass composite (CPB-PG for short) was prepared by using porous glass as template. Since the growth of quantum dot is limited by the pore channels of the PG, the composition/structure of the composite material has good controllability. Varying the process conditions of heat treatment for phase separation, acid leaching and alkali washing, the pore size distribution of CsPbBr3 quantum dots in CPB-PG can be manipulated. The maximum full width at half maxima (FWHM) of CPB-PG is merely 18 nm, enabling ultra-pure green light emission that is essential for wide-gamut backlight display (the color coordinates get close to the Rec.2020 standard). A white light-emitting diode (LED) prototype device was constructed by coupling green CPB-PG, red K0.42Cs0.58Pb(Br1.5I1.5) quantum dot glass powders and InGaN blue chip; upon driven at 20 mA current, the device yields white light with luminous efficacy of 30 lm/W, correlated color temperature of7?710 K, color rendering index of 75.8, and covering 96% color gamut under the Rec.2020 standard.
Methods The composition of the parent glass is 69SiO2-24B2O3-7Na2O. The prepared parent glass is heat treated at a temperature slightly higher than glass transition temperature for a certain time to produce nanoscale phase separation. The alkali-rich borate phase was dissolved by acid leaching, while the silicon-rich phase was retained, thus forming nanoscale pores. The size of pore size caused by acid leaching can be controlled by adjusting the temperature and time of heat treatment. By further alkali washing out, the silica material accumulated in the rigid pores of the silicon-rich phase skeleton can be removed, so as to expand the volumes of pores.CsPbBr3 quantum dots are not synthesized in advance. CsBr and PbBr2 are dissolved in water, and then add PG powder. Cs+, Pb2+, Brwill be adsorbed in the PG glass channels. After drying the aqueous solution, CsPbBr3 underwent in-situ crystallization in the pore channels to obtain the CPB-PG composite.
Results and discussion XRD analyses demonstrate the precipitation of CsPbBr3 nanocrystals from glass matrix. The crystalline phase is dependent on the feeding amount of CsBr and PbBr2. When the feeding amount gets higher than 0.12 mmol, the Cs4PbBr6 impurity phase will precipitate out. Careful TEM analyses were performed to reveal the microstructural features of the precipitated CsPbBr3 nanocrystals. Interestingly, one can see some twin crystals, i.e., two crystals form a mirror-symmetric orientation relationship along a crystal plane. The formation of twins helps to stabilize the phase structure. The crystal size of CsPbBr3 gradually increases in the following the sequence: CPB-PG1With the increase of crystal size, the luminescence peak redshifts: 514 nm→527 nm→532 nm, which is related to the quantum confinement effect. The half-peak full width (FWHM) of CPB-PG2 sample is the narrowest, merely 18 nm. The luminescent dynamical studies reveal that the luminescent decay curves can be well fitted by the double exponential function of equation, where the fast (τ1) and slow (τ2) lifetime components can be determined. τ1 gets close to the lifetime of colloidal CsPbBr3 quantum dots reported in the literature (5?10 ns), and can be attributed to the exciton recombination fluorescence transition process affected by non-radiative relaxation process. τ2 results in the delay of fluorescence decay, which can be attributed to the trapping effect of shallow trap level. It was found that with the increase of crystal size, τ1 tends to decrease, indicating that the number of defects leading to fluorescence non-radiative relaxation may increase; meanwhile, τ2 increases, indicating that the associated shallow trap level defects also increase. It can be inferred that when CsPbBr3 melted and recrystallized in PG glass with large pore size, the quantum dot components may be more easily volatilized through pores, resulting in more vacancy defects.
Benefiting the narrow FWHM of CPB-PG2, the CIE coordinate of (0.142, 0.782) gets close to the green coordinates defined by the standard Rec. 2020 standard (0.170, 0.797). Upon coupling the glass with blue LED chips, the electroluminescence is very bright.In order to verify the potential application of CPB-PG for w-LED , the CPB-PG2 powders, K0.42Cs0.58Pb(Br1.5I1.5) quantum dots glass powder and silica gel were mixed together, and then coated on the surface of InGaN 450 nm blue LED chip, whereupon w-LED prototype device was constructed. The constructed W-LED yields luminous flux of 1.58 lm, luminous efficiency of 30 lm/W, CCT of 7 710 K, CRI of 75.8, color coordinates of (0.284, 0.358), and covering 96% color gamut under the Rec. 2020.
Conclusions Using porous glass as template, CPB-PG composites were prepared by impregnating porous glass powder in halide salt solution, CsPbBr3 nanocrystals in-situ precipitate out in the pore channels during the succeeding heating and cooling processes.The crystal size of CsPbBr3 can be adjusted by varying the pore size of porous glass under different heat treatment, acid leaching and alkali washing conditions. The prepared CPB-PG composite emits green light in a narrow band with a FWHM of 18 nm,corresponding to a quantum efficiency of 56%. Fluorescence kinetic analysis revealed that there are structural defects in CPB-PG,greatly affecting the fluorescence properties. The constructed w-LED prototype device can cover approximately 96% color gamut under the Rec. 2020 standard, demonstrating a potential application in LCD backlight displays.