Papers by Keyword: White LEDs

Abstract: A novel red-emitting K₂SiF₆:Mn⁴⁺ phosphors with different Mn⁴⁺ doping mole fraction were synthesized by co-precipitation method. X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR) and fluorescence spectrometer were used to characterize the properties of K₂SiF₆:Mn⁴⁺ phosphors. As-prepared K₂SiF₆:Mn⁴⁺ phosphors have cubic crystal structure. Under excitation at 450 nm, an intense red emission peak around 632 nm corresponding to the ²E_g-⁴A₂ transition of Mn⁴⁺ was observed for 8.37 mol% K₂SiF₆:Mn⁴⁺ phosphors and was used as the optimum doping concentration. The excellent luminescent properties make the phosphor K₂SiF₆:Mn⁴⁺ a candidate for applications in InGaN-YAG:Ce³⁺ type LEDs for high color rendering. “A warm” white light LED with an efficiency of 147 lm/w and a color rendering index of 87.4 at a color temperature of 2864K has been obtained by fabricating YAG:Ce³⁺ with K₂SiF₆:Mn⁴⁺ on an InGaN chip.

Abstract: New luminescent oxynitridosilicates doped with Eu²⁺ and Ce³⁺ have been synthesized by solid state reaction method at 1450oC. The host has the general formula La_1-ySr_1-xSiO₃N: (yCe³⁺, xEu²⁺), which presents greenish-yellow or orange photoluminescence under UV/blue excitation. Remarkable long wavelength emissions centered above 565 nm were observed for the Ce³⁺ activated oxynitridosilicates, with compositions LaSrSiO₃N:0.02Ce³⁺, and LaSrSiO₃N:(0.02Ce³⁺, 0.02Eu²⁺) for 598 nm due to a significant amount of energy transfer from Ce³⁺ ions to Eu²⁺ ions. By combining the title LaSrSiO₃N:(Ce³⁺, Eu²⁺) phosphors with the 450 nm InGaN chip, a broad range of white light with correlated color temperatures of ~3500 K and color-rendering index of 92 can be generated, demonstrating their promising practical applicability to the warm-white light-emitting diodes.

Abstract: YAG:Ce phosphors were synthesized by co-precipitation method. The photoluminescence intensity of samples sintered by the two-step route increases by about 10%, and with 0.5 wt.% NH4Cl added as flux, surface morphologies and size distribution are improved. The white LEDs were fabricated using as-obtained phosphors. It shows natural white light with chromaticity coordinates of (0.3083, 0.3163), color temperature (Tc) of 6886 K, the average CRI of 75, and the luminous efficacy of 85 ml/W. Therefore, The as-obtained YAG:Ce phosphors could be regard to have potential application in white LED devices.

Abstract: A series of yellow-emitting phosphors based on a silicate host matrix, Ca3-xSi2O7: xEu2+, were prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu2+ substitution of Ca2+ does not change the structure of Ca3Si2O7 host. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, and Eu elements. The Ca3-xSi2O7: xEu2+ phosphors can be excited at wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. The phosphor for has the strongest excitation and emission intensity, and the energy transfer style between Eu2+ ions is quadrupole-quadrupole interaction for higher concentration Eu2+ doped Ca3Si2O7 phosphor. The Ca3-xSi2O7: xEu2+ phosphors can be used as candidates for white LEDs.

Abstract: Ce³⁺ and Tb³⁺ co-doped Strontium Pyroborate phosphors were synthesized by the solid-state method. The luminescent properties of phosphors were investigated by using the photoluminescence emission and excitation spectra. Under the excitation of near ultraviolet (n-UV) light, Ce³⁺ and Tb³⁺ co-doped Strontium Pyroborate phosphors exhibited blue emission corresponding to the f-d transition of Ce³⁺ ions and green emission bands corresponding to the f-f transition of Tb³⁺ ions, respectively. In the Ce3+ and Tb3+ co-activated samples the Ce³⁺ ions strongly sensitize the luminescence of the Tb ions. The Ce³⁺ and Tb³⁺ co-doped Strontium Pyroborate phosphor could be considered as one of double emission phosphor for n-UV excited white light emitting diodes.

Abstract: A novel red-emitting NaSrPO₄:Eu³⁺ phosphor was prepared by a conventional solid-state reaction. The luminescence properties and temperature dependence of ₄:Eu³⁺ were investigated. The phosphor exhibited red luminescence under the near-UV excitation of 395 nm, corresponding to the ⁵F₀→⁷L₆ transition of Eu³⁺ ions, which matched well with the excitation wavelength of near-UV LED chips. The electronic transition due to ⁵D₀→⁷F₂ transition of Eu³⁺ ions is stronger than the magnetic dipole transition due to ⁵D₀→⁷F₁ of Eu³⁺ ions. Furthermore, Eu³⁺-doped NaSrPO₄ phosphor shows higher thermally stable luminescence comparable tocommercially available Y₃Al₅O₁₂:Ce³⁺ phosphor. All the investigated suggested that this newly developed phosphor could find applications in white light devise utilizing InGaN-based excitation in the near-UV chips.

Abstract: Basic carbonate monospheres of various lanthanide combinations are successfully synthesized by the urea-based homogeneous precipitation technique, which are then converted into well dispersed phosphor particles that emit diverse colours. Sequential precipitation is commonly observed for these mixed cation systems, calling for adequate annealing of the basic carbonate precursors to attain cation homogenization in the final oxide particles and thus better luminescence, through eliminating localized concentration quenching of luminescence. It is shown that, owing to their excellent dispersion and uniform size, the phosphor spheres are readily assembled into close-packed luminescent films, allowing their wide applications in white LEDs, plasma display panels (PDPs), and field emission displays (FEDs).

Abstract: White LEDs can show different correlated color temperature (abbreviated as CCT) by adjusting the luminous spectrum peak distribution and relative strength. Recent research results indicate that spectral luminous efficiency function of mesopic vision Vmes(λ) and equivalent brightness are associated with luminous spectra of light sources. A series of white LEDs with different CCT are tested and calculated, it shows white LED with higher CCT has higher mesopic vision equivalent brightness, although the brightness are identical when the tester corrected by traditional photopic vision spectral luminous efficiency V(λ). From the equivalent brightness curve fitting, the curve can be described as a polynomial function, for it excellent correlation, the more value of equivalent brightness could be calculated without complex derivation.

Abstract: A series of LiY(WO ₄)(MoO ₄):Eu ³⁺ red phosphors have been synthesized by solid state reactions at high temperatures. The effect of Eu ³⁺ doping concentration, calcination temperature, reaction time and ratio of W/Mo on the optical properties of the red phosphors are studied and optimized. The phase impurity and spectroscopic properties are characterized by X-Ray Diffraction (XRD) and Photo-Luminescence (PL) spectra respectively. It is found that all the samples can be excited efficiently both by near ultraviolet light (395 nm) and blue light (464.5 nm), which match well with the emission of GaN based LED chips. The luminescent intensity maximizes for the sample of Eu ³⁺ 5 mol % in doping concentration (replacing Y) and W:Mo=1:1 in mole with calcinated of 750°C. To improve the dispersion and change the morphology of the phosphor powders, the sample is post-processed through ultrasonic dispersing method and the luminescent intensity of the post-treated sample increases by nearly 20%.

Abstract: A novel blue phosphor based on phosphate host matrix, MgAl(PO₄)O doped with Eu²⁺ was prepared by solid state reaction. The phosphor invariably emits blue luminescence with a peak wavelength at around 450 nm under ultraviolet excitation. The excitation spectrum showed that the phosphor can be efficiently excited by UV-visible light from 250 nm to 420 nm, which is coupled well with the emission of UV LED (350-410 nm). The results indicated that this novel phosphor may become the candidate of blue radiation-converting phosphor for white light emitting diodes in the further.