Phosphor-converted White LEDs Research Articles

Abnormal anti-quenching and controllable multi-transitions of Bi3+ luminescence by temperature in a yellow-emitting LuVO4 :Bi3+ phosphor for UV-converted white LEDs.

Phosphors with an efficient yellow-emitting color play a crucial role in phosphor-converted white LEDs (pc-WLEDs), but popular yellow phosphors such as YAG:Ce or Eu(2+) -doped (oxy)nitrides cannot smoothly meet this seemingly simple requirement due to their strong absorptions in the visible range. Herein, we report a novel yellow-emitting LuVO(4) :Bi(3+) phosphor that can solve this shortcoming. The emission from LuVO(4) :Bi(3+) shows a peak at 576 nm with a quantum efficiency (QE) of up to 68 %, good resistance to thermal quenching (T(50 %) =573 K), and no severe thermal degradation after heating-cooling cycles upon UV excitation. The yellow emission, as verified by X-ray photoelectron spectra (XPS), originates from the ((3)P(0),(3)P(1))→(1) S0 transitions of Bi(3+). Increasing the temperature from 10 to 300 K produces a temperature-dependent energy-transfer process between VO(4)(3-) groups and Bi(3+), and further heating of the samples to 573 K intensifies the emission. However, it subsequently weakens, accompanied by blueshifts of the emission peaks. This abnormal anti-thermal quenching can be ascribed to temperature-dependent energy transfer from VO(4)(3-) groups to Bi(3+), a population redistribution between the excited states of (3)P(0) and (3)P(1) upon thermal stimulation, and discharge of electrons trapped in defects with a trap depth of 359 K. Device fabrication with the as-prepared phosphor LuVO(4) :Bi(3+) has proved that it can act as a good yellow phosphor for pc-WLEDs.

Essential Factor for Determining Optical Output of Phosphor-Converted LEDs

An essential factor of the particle number is exploited for the phosphor excitation in phosphor-converted white LEDs. The particle number can clearly reveal the dependence of the light output flux and the correlated color temperature upon the conventional parameters, thickness, and concentration of the phosphors in a simpler way. In addition, we also find that there might exist an optimal particle number for the maximal luminous light output. An empirical function is then proposed for successfully modeling the relation between the output light and the particle number.

Photoluminescence properties of color-tunable novel Na2Ca4(PO4)2SiO4:Ce3+, Tb3+ near ultraviolet convertible phosphors

A series of Ce3+ and Tb3+ doped color-tunable novel phosphors with both phosphate and silicate hosts, Na2Ca4(PO4)2SiO4, were synthesized by a solid-state process for the first time as far as we know. The photoluminescence properties, energy transfer (ET) among rare earth ions and temperature quenching effect were investigated. Dipole–dipole and dipole–quadrupole interactions were proved to be responsible for ET mechanisms of Ce3+–Ce3+ and Ce3+–Tb3+ in Na2Ca4(PO4)2SiO4, respectively. Furthermore, the emitting color of phosphors can be tuned from blue to green by increasing the dopant concentrations of Tb3+ ions. Finally, temperature-quenching measurement revealed that the as-synthesized phosphor had good thermal characteristics, which indicates that the phosphor can be served as a key material for phosphor-converted white LEDs.

Structure and photoluminescence properties of Na2Y2B2O7:Ce3+,Tb3+ phosphors for solid-state lighting application

Novel phosphors of Na2Y2B2O7:Ce3+ and Na2Y2B2O7:Ce3+,Tb3+ were synthesized by a solid-state process and characterized with X-ray diffraction, photoluminescence (PL), PL excitation (PLE), and fluorescence decay time. The relationship between the observed Ce3+ emission properties and the crystal structure of Na2Y2B2O7 is built and the result shows that Ce3+ ions occupy two non-equivalent Y3+ sites in the host. The codoping of Tb3+ in Na2Y2B2O7:Ce3+ can tune the emitting colour from blue to green and increase the luminescence quantum efficiency to 75.2% by the energy transfer of Ce3+→Tb3+. The energy transfer processes of Ce3+–Ce3+ and Ce3+–Tb3+ are analysed and determined to be electric dipole–dipole and dipole–quadrupole, respectively. The broad PLE spectra of Na2Y2B2O7:Ce3+ and Na2Y2B2O7:Ce3+,Tb3+ are well matched with the emission from near UV LED chips, which indicates that they are promising blue and green phosphors for phosphor-converted white LEDs.

Broadly tuning Bi3+ emission via crystal field modulation in solid solution compounds (Y,Lu,Sc)VO4:Bi for ultraviolet converted white LEDs

For phosphor-converted white LEDs based on UV chips, it is essential to search high efficient phosphors that better feature with broadly tunable emission and particularly have no or less excitation in visible ranges.

Enhancement of optical characteristics of green-emitting single-phase Lu3Al5O12: Ce3+ phosphor using CeO2@B2O3 core–shell for white light generation

High quality and single-phase Lu3Al5O12 phosphor materials are prepared with a solid state reaction method using CeO2@B2O3 core–shell structure. This technique makes it possible to obtain pure phase and allows enhancement of Lu3Al5O12 phosphor. The transmittance eletron microscope image and electron energy loss spectrum of the CeO2@B2O3 core–shell structure particles show that the B2O3 coating is uniform, with a thickness of around 10nm, and that the core–shell structure is consisted of Ce and B elements. The Lu3Al5O12: Ce3+ phosphor using CeO2@B2O3 core–shell structure shows a higher emission intensity than other ones. The phosphor-converted white LEDs were realized with Lu3Al5O12: Ce3+ phosphor, which shows a high CRI value with a luminous efficacy of 104.58lm/W. This phosphor and the synthesis method are expected to be promising candidates for realizing the white LEDs.

Photoluminescence properties of (Sr1−xCax)1−yZrO3:yEu3+ phosphors for near-ultraviolet excited LEDs

Abstract Red-emitting (Sr 1− x Ca x ) 1− y ZrO 3 : y Eu 3+ (0≤ x ≤1.0 and 0.01≤ y ≤0.1) phosphors are synthesized by a modified solid-state reaction. The crystal structure and photoluminescence properties of the (Sr 1− x Ca x ) 1− y ZrO 3 : y Eu 3+ are studied, depending on Ca 2+ and Eu 3+ contents. The emission peak (614 nm) from the 5 D 0 → 7 F 2 transition is much stronger than that (592 nm) from the 5 D 0 → 7 F 1 transition, indicating efficient red emission. The substitution of Ca 2+ for Sr 2+ in Sr 0.95 ZrO 3 :0.05Eu 3+ greatly enhances the excitation and emission intensities. The most efficient red emission is obtained for the Ca 0.95 ZrO 3 :0.05Eu phosphor. The Ca 0.95 ZrO 3 :0.05Eu 3+ phosphor is a strong candidate as a red component for phosphor-converted white LEDs.

Optical Model for Novel Glass-Based Phosphor-Converted White Light-Emitting Diodes

We proposed an optical model for phosphor-converted white LEDs (pc-WLEDs) that utilized Ce:YAG doped glasses as novel phosphor-converted materials. In this model, precise simulation of the chromatic performance of the glass-based pc-WLEDs was conducted. Between optical simulation and experimental measurement, the color difference $\Delta E$ was 1.2%. Meanwhile, the difference of correlated color temperature limited from 1 wt% to 5 wt% phosphor concentration between the simulation and measurement was 184 K. Such a model for glass phosphors will be helpful to design high-power glass-based pc-WLEDs.

Enhancing Angular Color Uniformity of Phosphor-Converted White Light-Emitting Diodes by Phosphor Dip-Transfer Coating

High angular color uniformity (ACU) is strongly required in many illumination applications. In this study, we presented the phosphor dip-transfer coating method to realize high ACU for phosphor-converted white LEDs. The phosphor mixture was coated on top surfaces of LED chips and formed thin and convex phosphor layers by the dip-transfer coating. The optical simulations and experiments were conducted for performance verifications. Both numerical and experimental results show that the present method can obtain very high ACU. The angular correlated color temperature (CCT) deviation reaches 378 K when the average CCT of white LEDs is around 6200 K and it reduces to 63 K at the average CCT of about 4000 K.

Guest Editorial Recent Advances in Solid State Lighting

This Special Issue on Solid State Lighting, comprising the April/May/June 2013 issues of the IEEE/OSA Journal of Display Technology, focuses on the material physics and innovations, device physics and simulations, new device concepts and engineering, and system implementation of high efficiency solid state lighting technologies. The 45 papers of this Special Issue on Solid State Lighting covers both the innovations in inorganic and organic LEDs technologies, as well as the new concepts required for system implementation of the technology. We are very fortunate to have several excellent manuscripts in the key topics areas related to the progress for achieving high internal quantum efficiency and novel approaches for improving light extraction in III-Nitride LEDs, new concept for achieving phosphor-converted white LEDs, and key advances in the important field of organic LEDs. The progress in both organic and inorganic LEDs is very important, and the advances are complementary of one another attributed to the different key applications enabled by these two technologies. This Special Issue on Solid State Lighting covers three separate categories: 1) innovations for addressing the internal quantum efficiency and efficiency-droop in III-Nitride LEDs; 2) novel approaches for addressing light extraction efficiency in III-Nitride LEDs, and enabling circuits and systems for LEDs; and 3) novel approaches for white LEDs, and key advances and review on the material and device concepts in organic LEDs.

Phosphor-converted LED modeling using near-field chromatic luminance data

A new method of creating a source model of a phosphor-converted white LED is demonstrated. It is based on a simple phosphor model, of which some key parameters have been obtained from measuring the near-field chromatic and luminance characteristics of a complete LED. The accuracy of the model is verified by measurements and simulations on an LED with a ball lens as primary optic.

Warm white light generation from a single phased phosphor Sr10[(PO4)5.5(BO4)0.5](BO2):Eu2+, Mn2+, Tb3+ for light emitting diodes

A novel single phased white light emitting phosphor Sr10[(PO4)5.5(BO4)0.5](BO2):Eu2+, Mn2+, Tb3+ was synthesized by solid-state reaction for the first time. The crystal structure, photoluminescence properties and the efficient energy transfer from different Eu2+ sites to Mn2+ and Tb3+ in Sr10[(PO4)5.5(BO4)0.5](BO2) is studied in detail by X-ray diffractometry Rietveld method, luminescence spectra, energy-transfer efficiency and lifetimes. Through effective energy transfer, the wavelength-tunable warm white light can be realized with superior chromaticity coordinates of (0.37, 0.30) and low correlated color temperature (CCT = 3512 K) by coupling the emission bands peaking at 410, 542 and 649 nm attributed to the contribution from Eu2+, Tb3+ and Mn2+, respectively. The results indicate the white phosphor Sr10[(PO4)5.5(BO4)0.5](BO2):Eu2+, Mn2+, Tb3+ can serve as a promising material for phosphor-converted warm white LEDs.

Synthesis and Photoluminescence Properties of Divalent Europium Doped-CaAlSi1+2xN3+2xOx (x = 0–1) Solid Solution Phosphors

The calcium cyanamide (CaCN2) reduction route was applied for the synthesis of the Eu2+ (2 at%)-activated CaAlSi1+2xN3+2xOx (x = 0–1) phosphors, and the impact of oxygen content on photoluminescence properties was investigated. The relationship of crystallographic parameters of the solid solution and oxygen content were also studied. All of the samples showed a broadband excitation covering the range from UV to visible region, revealing that this phosphor was a potential candidate for the phosphor-converted white LEDs. Under the UV/blue excitation, this series phosphor gave the emission peaks varying from 585 to 641 nm. The shifting behavior was dominantly contributed to the second luminescent center formed by the introduction of oxygen into nitride phosphors.

Crystal structure refinement and luminescence properties of Ce3+ singly doped and Ce3+/Mn2+ co-doped KBaY(BO3)2 for n-UV pumped white-light-emitting diodes

A series of novel Ce3+ singly doped and Ce3+/Mn2+ co-doped color-tunable KBaY(BO3)2 phosphors were synthesized by a high-temperature solid-state reaction, and the crystal structures and luminescence properties were investigated in detail. The crystallographic cation sites occupancy of the doping Ce3+ and Mn2+ ions for KBaY(BO3)2: Ce3+, Mn2+ samples were analyzed by the Rietveld refinement method. It was demonstrated that Ce3+ and Mn2+ ions can both enter into the two types of cation sites in the KBaY(BO3)2 crystal lattice, with a slight tendency of occupancy for Ce3+ on Y3+ ion sites and Mn2+ on the Ba2+/K+ ion sites. By controlling Mn2+ ions content with a fixed Ce3+ concentration, the emission color of the phosphor varied from blue (0.160, 0.150) to pink-white (0.348, 0.272) and eventually to orange (0.490, 0.372). The critical energy transfer distance calculated by the concentration quenching theory was 16.52 Å and the energy transfer from Ce3+ to Mn2+ took place via a resonance-type dipole–dipole mechanism. By combining the single-phase KBa0.97Y0.97(BO3)2: 0.01Ce3+, 0.05Mn2+ phosphor with an n-UV 370 nm InGaN chip, a phosphor-converted white LED lamp was successfully fabricated, producing a white light with a warm correlated color temperature of 4710 K and color coordinates of (0.347, 0.307).

Design of a compact modified total internal reflection lens for high angular color uniformity

Total internal reflection (TIR) lenses are optical components that are used to collimate the light or to generate a desired uniform illumination. However, most TIR designs do not pay attention to color uniformity, an increasingly important issue for the quality of lighting, and have a considerable dimension, which also limits their applications. In this study, we proposed an effective design principle of color mixing, and a phosphor-converted white LED module integrated with a compact modified free form TIR component was presented and optimized to achieve compact size and high angular color uniformity (ACU). Optimization results indicated that modified LED packages could achieve a smaller size in vertical height (~0.52) within the same horizontal radius, compared with LEDs integrated with classic TIR lenses. Meanwhile, the enhancement of ACU with an optimum appropriate divergence half angle (65°) reached as high as 84% in terms of normalized standard deviation of yellow-blue ratio from 0.888 to 0.429.

Photoluminescence properties of Ce3+ doped YSiO2N blue-emitting phosphors

Oxonitridosilicate phosphors with compositions of Y1−xCexSiO2N (x=0–0.1) were synthesized by a new synthetic route based on a solid state reaction among YSi, CeSi, SiO2 and Y2O3 compounds at high temperature and high pressure. The photoluminescence properties dependent on Ce concentration and temperature were investigated. Concentration quenching occurred when the doped Ce3+ concentration was more than 3 mol.%. The emission spectra showed red shifts from 430 to 447 nm with the increased Ce concentration from 0.5 mol.% to 10 mol.%. The quenching temperature was estimated as ∼380 K. The chromaticity coordinates of the excitation and emission spectra were stable against the temperature. This study showed these YSiO2N:Ce3+ phosphors the potential applications in the three-RGB phosphor-converted white LEDs.

Effects of YAG: Ce Phosphor Particle Size on Luminous Flux and Angular Color Uniformity of Phosphor-Converted White LEDs

Effects of YAG: Ce phosphor particle size on the luminous flux and angular color uniformity have been studied by using Monte Carlo ray-tracing simulation. A white LED model basing on Cree XLamp is constructed. The phosphor particle size is changed from 2 to 20 μm. Different simulation cases are carried out by changing the phosphor concentration and thickness. Results revealed that the luminous flux and angular color uniformity have complex relationships with the particle size depending on applied phosphor concentration and desired color temperature. A general finding is that an increase of the particle size is beneficial for the improvement of luminous flux but leads to worse angular color uniformity. A 2 μm particle achieves not only the best angular color uniformity but the lowest luminous flux. A particle size larger than 10 μm generates the highest luminous flux at low color temperature when the concentration is around 1.5 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . A particle size between 2 μm and 10 μm shows combined moderate performance when the phosphor concentration is higher than 2 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . To realize the best combined performance, a particle size around 6 μm is suggested.

Yellow-emitting (Ca2Lu1−xCex)(ScMg)Si3O12 phosphor and its application for white LEDs

Luminescence properties of the yellow-emitting (Ca2Lu1−xCex)(ScMg)Si3O12 (CLSM:xCe3+x=0.01–0.15) phosphor are investigated for various Ce3+ concentrations. Different Ce3+ emission sites and energy transfers between them are observed, resulting in a red shift of the emission spectra from 530 to 575nm with increasing x from 0.01 to 0.15. Combining with blue (460nm) InGaN LEDs, CLSM:Ce3+ shows excellent performances for phosphor-converted white LEDs with higher color rendering index Ra of 87.4–87.9 and lower color temperature TC of 5034–5814K, especially for warm pcWLEDs with a high color rendering (Ra>80) and a low color temperature (TC<4000K). Thermal quenching behaviors depending on Ce3+ concentrations and temperatures are discussed.

Will switching to LED outdoor lighting increase sky glow?

As LED sources are increasingly being used for outdoor lighting, concerns are being raised about their impact on man-made sky glow. This paper compares the amount of light scattered back to Earth from a 6500 K phosphor-converted white LED light source to that from a 2050 K high pressure sodium (HPS) light source. Calculations based solely on molecular Rayleigh scattering provide an upper limit of 22% more scatter from the LED source, but are not realistic because the atmosphere has significant scatter from aerosol content. Adding in the effects of aerosols in the atmosphere, as derived from aerosol optical depth measurements and Mie scattering distributions, reduces the wavelength dependency of scattered light to where the LED source has roughly 10–20% more scattered light contributing to sky glow. Scattering ratios (LED:HPS) are calculated for different angles and atmospheric conditions.

On the Color Stability of Phosphor-Converted White LEDs Under DC, PWM, and Bilevel Drive

Most commercial white LEDs are made from nitride-based blue LEDs coated with yttrium aluminium garnet phosphor, which produce spectra that shift in opposite directions under the influences of drive current and junction temperature changes. This property gives rise to different emitted spectra, hence chromaticity properties, when the LED is driven/dimmed by different current waveforms. By using a commercial white LED sample, LUXEON K2, the effects of drive current and junction temperature on the changes of chromaticity coordinates are studied experimentally. The impact of dc, pulse width modulation (PWM), and bilevel current waveform is discussed through a graphical analysis, followed by experimental verification. It is proven that dc offers the best color stability over dimming due to the counteracting influences of drive current and junction temperature variations, whereas an LED constantly suffers from noneliminable chromaticity changes when driven by the PWM. Theoretical explanations are given to justify these cases, and it is found that, for the case of dc drive, an ideal heat sink's thermal resistance can be selected based on a simple equation to minimize the overall chromaticity change over dimming. This paper provides an in-depth discussion on the relations between the chromaticity properties of phosphor-converted (pc) white LEDs and the driving/dimming methods used.