Illumination System Comprising a Radiation Source and a Luminescent Material

Abstract

Illumination system comprising a radiation source and a luminescent material comprising a first phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said first phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III). A light-emitting diode as a radiation source is especially contemplated. The invention also relates to a red to amber yellow emitting europium(III)-activated phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III). Furthermore the invention relates to the use of the phosphor for general illumination, traffic and signage lighting, automotive and for backlighting of liquid crystal displays.

Description

The present invention generally relates to an illumination system comprising a radiation source and a luminescent material comprising a phosphor. The invention also relates to a phosphor for use in such an illumination system.

More particularly, the invention relates to an illumination system for the generation of specific, colored light, including white light, by luminescent down conversion and additive color mixing based on a blue to violet radiation-emitting diode in combination with a luminescent material comprising a phosphor. Today light emitting illumination systems comprising visible colored light emitting diodes as radiation sources are used single or in clusters for all kind of applications where rugged, compact, lightweight, high efficiency, long-life, low voltage sources of white or coloured illumination are needed.

Such applications comprise inter alia illumination of small LCD displays in consumer products such as cellular phones, digital cameras and hand held computers. Pertinent uses include also status indicators on such products as computer monitors, stereo receivers, CD players, VCRs, and the like. Such indicators are also found in systems such as instrument panels in aircraft, trains, ships, cars, etc.

Recently, various attempts have been made to make white light emitting illumination systems by using light-emitting diodes as radiation sources. When generating white light with an arrangement of red, green and blue light emitting diodes, the problem has been that white light of the desired tone cannot be generated owing to variations in the tone, luminance, and other properties of the individual light-emitting diodes.

In order to solve this problem, various illumination system concepts have been developed for converting the color of light-emitted by light-emitting diodes by means of a luminescent material comprising a phosphor so as to provide a visible white light illumination.

Such white light illumination systems have been based in particular either on the trichromatic (RGB) approach, i.e. mixing of three colors: namely red, green and blue, in which case the latter component of the output light may be provided by a phosphor or by the primary emission of the LED; or in a second, simplified solution, on the dichromatic (BY) approach, i.e. mixing of yellow and blue, in which case the yellow secondary component of the output light may be provided by a yellow phosphor and the blue component may be provided by a phosphor or by the primary emission of blue LEDs.

One dichromatic (BY) approach, as disclosed, for example, in U.S. Pat. No. 5,998,925, uses a blue light emitting diode of InGaN semiconductor material combined with Y₃Al₅O₁₂:Ce(YAG-Ce) as a phosphor. The YAG-Ce phosphor is coated on the InGaN LED, and a portion of the blue light emitted from the LED is converted into yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. This system thus emits both blue light emitted from the LED and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands is perceived as white light by an observer with a typical CRI in the middle 70 s and a color temperature Tc, that ranges from about 6000 K to about 8000 K.

Although the method is advantageously simple and readily implemented, it is disadvantageously poor in color rendering at low color temperatures, resulting from a lack of red color content, and it suffers from a color-shifting problem as the operational current increases. Therefore it is not an ideal light source for illumination.

One trichromatic RGB approach for manufacturing a white LED may be implemented by exploiting an ultraviolet-emitting UV LED for excitation of a set of phosphors. In this approach the visible part of the emitting spectrum is completely generated by phosphors. The UV radiation emitted by the LED excites the phosphors to emit red, green and blue light, and these tri-color lights are further mixed into white light. However, moving the pump source into the UV spectral range results in a reduced radiant efficacy because of increased energy losses in the conversion process. Besides, the packaging materials have an ageing problem due to the UV light damages. Therefore this is not a proper way to produce a white illumination source.

Another trichromatic lamp approach for generating white light is disclosed in U.S. Pat. No. 6,686,691. The invention according to U.S. Pat. No. 6,686,691 relates to a tri-color lamp with specific red and green phosphors excitable by a common blue light emitting diode (LED). This arrangement provides a mixing of three light sources: light emitted from the two phosphors and unabsorbed light emitted from the LED. Power fractions of each of the light sources can be varied to achieve good color rendering.

Yet, it is a general concern with phosphor-converted LED lamps using blue to violet emitting LEDs for excitation of phosphors that currently known phosphors have not been developed and optimized for such excitation.

Currently known phosphors were developed and optimized for two major applications: (1) fluorescent lamps that utilize 254 nm UV radiation from a Hg discharge for excitation and (2) CRTs, where the RGB-phosphors are excited by an electron beam.

This leads to a new challenge to be met by phosphors in phosphor- converted LEDs.

It has been found in particular that the electro-optical efficiency of conventional red phosphors, for example Y₂O₃:Eu(III), as mentioned in US2004/0000862, is unsatisfactory in illumination systems, using a LED die? as a radiation source, as these red phosphors only slightly absorb radiation having a wavelength above 300 nm.

Therefore, there is a need to provide an illumination system comprising a novel luminescent material that is excitable by a radiation source with an emission in the blue-violet range and emits in the visible yellow-amber-red range of the electromagnetic spectrum.

Desirable characteristics for illumination systems for general purposes are also a high brightness at economical cost.

Thus the present invention provides an illumination system, comprising a radiation source and a luminescent material comprising a first phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said first phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

According to a first aspect of the invention, a white-light illumination system comprises a blue light emitting diode having a peak emission wavelength in the range of 325 to 495 nm as a radiation source.

An illumination system comprising this phosphor has an improved quantum yield for the blue to violet excitation radiation having a wavelength λ in the range between 325 and 495 nm.

As the phosphor comprising europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), emits in the red-amber-yellow range of the electromagnetic spectrum, the illumination system comprising such a phosphor is capable of providing red to amber to yellow or white light.

An essential factor is that the europium(III)-activated phosphors are narrow-band emitters in the red-amber-yellow wavelength range emitting between 580 and 700 nm range of the electromagnetic spectrum, so that little or no light is generated at wavelengths that are positioned in the visible spectrum away from the desired red, amber, or yellow wavelengths.

Such a narrow-band emission helps to increase the efficacy of the illumination system.

Applications of the invention include inter alia indicators, traffic lighting, street lighting, security lighting and lighting of automated factory, and signal lighting for cars and traffic as well as general illumination. Applications of the invention include colored also security lighting as well as signage lighting for cars and traffic. Another field of applications includes backlighting of liquid crystal displays.

One embodiment of the invention provides a white light illumination system comprising a blue light emitting diode having a peak emission wavelength in the blue-violet range of 400 to 495 nm as a radiation source and a luminescent material comprising a first phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said first phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), and at least one second phosphor.

In particular, the luminescent material may be a phosphor blend comprising a first phosphor comprising europium(III) as an activator in a host lattice selected from the compounds of an oxo-anion species with a cationic metal species, comprising yttrium(III) and gadolinium(III), and a green phosphor.

The emission spectrum of such a luminescent material comprising additional green phosphors has the appropriate wavelengths for obtaining, together with the blue to violet light of the LED and the yellow to red light of the europium-activated oxygen -dominated type phosphor according to the invention, a high-quality white light with good color rendering at the required color temperature.

Such a green phosphor may be selected from the group of terbium(III)-activated phosphor compounds.

It is furthermore preferred that the green Tb(III) -activated phosphor is selected from the group of: (Y_xGd_1−x)BO₃:Tb (0<×<1),LaPO₄:Tb; LaPO₄:Ce,Th; (Y_xGd_1−x)₃Al₅O₁₂:Tb (0<×<1); CeMgAl₁₁0₁₉:Tb; GdMgB₅O₁₀:Ce,Th; (Y_xGd_1−x)BO₃:Tb(0<×<1); (Y_xGd_1−x)₂SiO₅:Tb (0<×<1), Gd₂O₂S:Tb; LaOBr:Tb, and LaOCl:Tb.

An illumination system comprising terbium(III)-activated phosphors as a second phosphor can provide a composite white output light that is well-balanced with respect to color. In particular, the composite white output light has a narrow-band emission in the red color range, in contrast to the broad-band emission of the conventional lamp. This characteristic makes the device ideal for applications in which a high lumen equivalence is required.

According to one embodiment of the invention, the luminescent material comprises the first phosphor combined with a photonic bandgap material to concentrate the primary radiation emitted by the LED in the position of the phosphor in the luminescent material for enhanced absorption.

The luminescent material may comprise the first phosphor, having a grain size d_m1>500 nanometers—also to enhance absorption of the primary radiation.

According to one embodiment of the invention, the luminescent material comprises the first phosphor as a transparent monolithic ceramic microstructure material.

The application of the phosphor as a transparent monolithic ceramic material renders it possible to adjust a much stronger optical absorption, while the material remains transparent to the pump radiation.

Said application of transparent ceramic material as a conversion layer in blue pumped LED lamps has even more advantages:

In high-power LEDs, a significant heating up of the phosphor material, possibly leading to thermal quenching cannot be prevented. Thermal conductivity is better when a transparent ceramic material is used. In addition, a larger volume is heated.

Lower concentrations of luminescent ions can be chosen, preventing or reducing concentration quenching.

Optical functionality can be easily integrated, e.g. in the form of lenses.

According to one embodiment of the invention, the luminescent material may comprise the first phosphor having a grain size d_m1 and the second phosphor having a grain size d_m2<d_m1.

Another aspect of the present invention provides a phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

The luminescent material is excitable by UV-radiation with wavelengths from 200 nm to 400 nm, but is excited with higher efficiency by blue to violet light emitted by a blue light emitting diode having a wavelength around 400 to 495 nm. Thus the luminescent material has ideal characteristics for conversion of the blue light of a nitride semiconductor light-emitting component into white light.

These phosphors are narrow-band emitters whose visible emission is so narrow that the wavelength range of their emission is less than 20 nm, where the visible emission is predominantly located.

Additional important characteristics of the phosphors include 1) resistance to thermal quenching of luminescence at typical device operating temperatures (e.g. 80° C.); 2) lack of interfering reactivity with the encapsulating resins used in the device fabrication; 3) suitable absorptive profiles to minimize dead absorption within the visible spectrum; 4) a temporally stable lumen output over the operating lifetime of the device and; 5) compositionally controlled tuning of the phosphors' excitation and emission properties.

Preferably the molar proportion of the amount of gadolinium in the host lattice is less than 50 mole percent.

The phosphor may comprise in addition a co-activator selected from bismuth(III) and praseodymium(III), which are able to absorb blue light and to transfer the absorbed energy to the europium(III)-activator cations.

Preferably the anionic oxygen-containing species is selected from the group of oxide, oxysulfide, oxyhalides, borates, aluminates, gallates, silicates, germanates, phosphates, arsenate, vanadate, niobate, tantalate, and mixtures thereof

It is also preferred that the phosphor comprises the activator in a molar proportion of 0.001 to 20 mole % relative to the cation in the host lattice and the co-activator in a molar proportion of 0.001 to 2 mole % relative to the cation in the host lattice.

Especially preferred are phosphors selected from the group of: (Y_l-x-yGd_x)₂O₂S:Eu_y, (Y_1-x-yGd_x)VO₄:Eu_y, (Y_1-x-y-zGdx)OCl:Eu_yBi_z, (Y_1-x-yGd_x)(V,P,B)O₄:Eu_y, (Y_1-x-yGd_x)NbO₄:Eu_y, (Y_1-x-yGd_x)TaO₄:Eu_y, and (Y_1-X-Y-zGd_x)₂O₃:Eu_yBi_z, wherein 0<x <1,0<y<0.2 and 0<z<0.02.

These phosphors may have a coating selected from the group of fluorides and orthophosphates of the elemeunts aluminum, scandium, yttrium, lanthanum, gadolinium and lutetium, the oxides of aluminum, yttrium, and lanthanum, and the nitride of aluminum.

The illumination system according to the invention comprises a luminescent material comprising a phosphor capable of absorbing part of the radiation emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

While the use of the present phosphors and luminescent materials is contemplated for a wide array of illumination applications, the present invention is described with particular reference to and finds particular application in illumination systems comprising light-emitting diodes, especially ultraviolet to blue light-emitting diodes as their radiation sources.

The type and amount of the cationic metal species yttrium and gadolinium present in the phosphor compound dictate the physical and/or chemical properties of the compound, while the local bonding environments of europium(III) in the oxygen-dominant host lattice determine the characteristics of its emission and absorption spectra.

Anionic oxygen-containing species (or oxygen-dominant anions) are generally defined as oxygen-containing species having a net negative ionic charge. Host lattices comprising oxygen-dominant anions have the following properties: a) they have a large bandgap so as not to absorb the emitted radiation from the activator and b) they are relatively stiff, so that lattice vibrations, which lead to non-radiative relaxation and decrease efficiency, are not easily excited.

Of particular interest are the oxides, oxysulfides, oxyhalides, and oxo-anions of boron, aluminum, gallium, silicon, germanium, phosphorus, arsenic, vanadium, niobium, and tantalum, and combinations or mixtures thereof.

Typically, said oxo-anions are comprised of individual monomer subunits [A^a+O_xO_y/2]^a-2x-y, wherein A is selected from boron, aluminum, gallium, silicon germanium, phosphorus, arsenic, vanadium, niobium, and tantalum, a is the respective oxidation number thereof, O is oxygen, and x+y is an integer equal to 3 or 4. The subunits may be bound together by conventional covalent oxygen-bridge bonds (i.e., shared electrons).

Oxo-anions are either isolated (finite), or oligomeric, i.e. connected to a limited number of adjacent oxo-anions by oxygen bridges, or alternatively they are directly interconnected through oxygen bonding into infinite chains, sheets, or 3-dimensional framework structures.

The host lattice may contain a single oxo-anion species, a mixture of different oxo-anions, or combination of more than one element selected from boron, aluminum, gallium, silicon germanium, phosphor, arsenic, vanadium in one oxo-anionic species.

By way of example, such combinations may comprise borosilicate, phosphosilicate, phosphovanadate, phosphotantalate, aluminosilicate, and aluminoborate.

In the host lattice of the phosphors according to the invention, the anionic oxygen-containing species are used with specific counter-ions, i.e. cationic metal species comprising yttrium and gadolinium.

The incorporation of gadolinium(III) in the host lattice increases the proportion of covalent bonding and ligand-field splitting. This leads to a shift of excitation (and emission) bands to longer wavelengths in comparison with the basic host lattice comprising only yttrium(III).

A gadolinium-containing host lattice is excellent in acting as a sensitizer host lattice , because both the ground state and excited states lie within the bandgap of about 6 eV of the host lattice. Gadolinium absorbs and emits radiation via 4f-5df transitions, i.e. electronic transitions involving f-orbital energy levels. While f-f transitions are quantum-mechanically forbidden, resulting in weak emission intensities, it is known that Gd(III) strongly absorbs radiation through allowed 4f-5df transitions (via d- orbital/f-orbital mixing) and consequently produces high emission intensities in the UV-B range of the electromagnetic spectrum.

This host lattice typically preferably comprises a major proportion of yttrium, up to about 50 mole percents of gadolinium, and a minor activating proportion, typically about 0.03 to 2 mole percent of the rare earth activator Eu(III), plus possibly a co-activator, selected from bismuth and praseodymium.

Especially useful materials of the phosphor according to the invention are: (Y_1-x-yGd_x)₂O₂S:Eu_y, (Y_1-x-yGd_x)VO₄:Eu_y, (Y_1-x-y-zGd_x)OCl:Eu_yBi_z, (Y_1-x-yGd_x)(V,P,B)O₄:Eu_y, (Y_1-x-yGd_x)NbO₄:Eu_y, (Y_1-x-yGd_x)TaO₄:Eu_y, (Y_1-x-y-zGd_x)₂O₃:Eu_yBi_z, wherein 0<x<1; 0<y<0.2 and 0<z<0.02.

Table 1 discloses color points and lumen equivalencies of a selection of phosphors according to the invention in comparison with two conventional phosphors (in italics).

TABLE 1
			lumen
			equivalency
Chem. Composition	λ [max]	color point x, y	[lm/W]
CaS:Eu	655	0.69, 0.30	90
Sr2Si5N8:Eu	625	0.62, 0.38	180
(Y1−x−yGdx)2O2S:Euy	620	0.66, 0.33	210
(Y1−x−yGdx)VO4:Euy	615	0.65, 0.33	225
(Y1−x−y−zGdx)OCl:EuyBiz	612	0.64, 0.35	230
(Y1−x−yGdx)(V,P,B)O4:Euy	615	0.65, 0.33	225
(Y1−x−yGdx)NbO4:Euy	615	0.64, 0.33	255
(Y1−x−yGdx)TaO4:Euy	615	0.64, 0.35	250
(Y1−x−y−zGdx)2O3:EuyBiz	611	0.65, 0.34	280

Especially oxo-anionic compounds of yttrium and gadolinium are useful hosts for europium (III), because the oxygen ligation of europium(III) substantially impacts its emission and absorption spectra. The limited electronegativity of the oxo-anions of yttrium and gadolinium decreases the degeneration of the electronic states of europium, producing emission and absorption bands which differ substantially from those produced in e.g. halide hosts: they are narrower and have different relative intensities and different positions. In general, the absolute position and width of an emission or absorption band shifts to lower energy as the electronegativity of the surrounding anions decreases.

The emission spectra of the phosphor comprising gadolinium and yttrium in the host lattice resemble that of the phosphor comprising only yttrium. It exhibits a narrow emission band at 580 to 700 nm due to the 4f-4f transitions of Eu(III).

The molar proportion z of Eu(III) is preferably in a range of 0.003<z<0.2.

When the proportion z of Eu(III) is 0.003 or lower, luminance decreases because the number of excited emission centers of photoluminescence due to Eu(III) decreases and, when z is greater than 0.2, density quenching occurs. Concentration quenching refers to the decrease in emission intensity which occurs when the concentration of an activation agent added to increase the luminance of the luminescent material is increased beyond an optimum level.

Replacing some of the europium in a europium-activated phosphor with bismuth as a sensitizer has the effect that the incident energy, photon or excited electron, is absorbed from the discharge of the discharge-maintaining composition by the bismuth, and the activator forms the site where the electron radiatively relaxes.

These europium(III)-activated oxygen-dominant phosphors according to the invention are responsive to more energetic portions of the electromagnetic spectrum than just the visible portion of the spectrum.

In particular, the phosphors according to the invention are excitable by UV-emission lines with wavelengths from 200 to 400 nm, but are also excited with high efficiency by LED light emitted by a blue-violet light emitting component having a wavelength from 400 to 495 nm. Thus the luminescent material has ideal characteristics for converting blue light of nitride semiconductor light-emitting components into white light.

Preferably the europium(III)-activated type phosphors according to the invention may be coated with a thin, uniform protective layer of one or more compounds selected from the group formed by the fluorides and orthophosphates of the elements aluminum, scandium, yttrium, lanthanum, gadolinium, and lutetium, the oxides of aluminum, yttrium, and lanthanum, and the nitride of aluminum.

The protective layer thickness customarily ranges from 0.001 to 0.2 micrometers and is thus so thin that it can be penetrated by the radiation of the radiation source without substantial loss of energy. The coatings of these materials on the phosphor particles may be applied, for example, by deposition from the gas phase or a wet coating process.

To improve their absorption characteristics, these phosphors are preferably used in a grain size distribution in a range of grain size d_m1>500 nm. The grain size is determined by the capabilities of the phosphor to absorb radiation and absorb as well as scatter visible radiation, but also by the necessity to form a phosphor coating that bonds well to the substrate. The latter requirement is met only by very small grains, but the light output is improved in slightly larger grains in a range of grain size d_m1>500 nm.

The invention also relates to an illumination system comprising a radiation source and a luminescent material comprising a first phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said first phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

Any configuration of an illumination system which includes a radiation source and phosphor composition is contemplated in the present invention, preferably with the addition of other well-known phosphors, which may be combined to achieve a specific color or white light when irradiated by a primary UV or blue-violet light as specified above.

Radiation sources such as those found in discharge lamps and luminescent lamps, such as mercury low- and high-pressure discharge lamps, sulfur discharge lamps, and discharge lamps based molecular radiators are contemplated for use as radiation sources with the present inventive phosphor compositions.

Preferred radiation sources include any semiconductor optical radiation emitters and other devices that emit optical radiation in response to electrical excitation. Semiconductor optical radiation emitters include inter alia light-emitting diode LED chips, light-emitting polymers (LEPs), organic light-emitting devices (OLEDs), polymer light-emitting devices (PLEDs), etc.

In a preferred embodiment of the invention, the radiation source is a blue to violet light-emitting diode having an emission with a peak emission wavelength in the range of 325 to 495 nm.

A detailed construction of one embodiment of such an illumination system comprising a radiation source and a luminescent material as shown in FIG. 1 will now be described.

FIG. 1 is a schematic view of a chip type light-emitting diode with a coating comprising the luminescent material. The device comprises a chip type light-emitting diode (LED) 1 as a radiation source. The light-emitting diode chip is positioned in a reflector cup lead frame 2. The chip 1 is connected to a first terminal 6 via a bond wire 7, and directly to a second electric terminal 6. The recess of the reflector cup is filled with a coating material, which contains a luminescent material according to the invention so as to form a coating layer which is embedded in the reflector cup. The phosphors are applied either separately or in a mixture.

The coating material typically comprises a polymer for encapsulating the phosphor or phosphor blend. In this embodiment, the phosphor or phosphor blend should exhibit high stability properties against the encapsulant. Preferably, the polymer is optically clear to prevent any significant light scattering. A variety of polymers are known in the LED industry for making LED lamps.

In one embodiment, the polymer is selected from the group consisting of epoxy and silicone resins. Adding the phosphor mixture to a polymer precursor liquid can achieve the encapsulation. For example, the phosphor mixture may be a granular powder. Introducing phosphor particles into a polymer precursor liquid results in formation of a slurry (i.e. a suspension of particles). Upon polymerization, the phosphor mixture is fixed rigidly in place by the encapsulation. In one embodiment, both the luminescent material and the LED dice? are encapsulated in the polymer.

The transparent coating material may comprise light-diffusing particles, advantageously so-called diffusers. Examples of such diffusers are mineral fillers, in particular CaF₂, TiO₂, SiO₂, CaCO₃, and BaSO₄, or else organic pigments. These materials can be added to the above-mentioned resins in a simple manner.

According to one embodiment of the invention, the coating material is selected from coating materials with a refractive index in the range of the refractive index of the phosphor material.

According to a further preferred embodiment of the invention, the coating layer is a double layer, wherein the first phosphor is provided as a thin-film layer or a layer comprising the first phosphor as nanoparticles. This first layer is provided in contact with the LED chip. Layer thickness should be sufficient to provide efficient absorption. A second layer comprising a green-to-yellow emitting phosphor is provided on top of the first layer.

Another embodiment uses a monolithic transparent ceramic material comprising the first phosphor as a conversion layer in LED lamps. If the first phosphor is provided as a monolithic transparent ceramic material, absorption as well as scattering by the first phosphor material is low.

Optical transmission characteristics from translucent to transparent provide a high light yield in the conversion of the high-energy radiation and assure a high transmissivity of the luminescent emission within the luminescent material

The use of a transparent ceramic material renders it possible to adjust a much stronger optical absorption, while the material remains transparent to the pump radiation.

Such optical transmission characteristics of the phosphor material can be achieved with a high-density ceramic material that has an optimized low residual porosity. In addition to a crystal anisotropy of the optical refractive index due to a non-uniform crystal structure, foreign-phase inclusions as well as grain boundaries and, in particular, pores are disruptive for an optimum transmission of the luminescent emission.

A phosphor material with a transparent monolithic ceramic microstructure may be obtained in a conventional flux process or by a wet chemical method where precursor co-precipitates are formed, followed by a conversion of the precursor precipitates into an oxygen-containing monolithic ceramic material by sintering, preferably under isostatic pressure. The possibility to machine the ceramic monolithic material may improve light extraction and render it possible to create lenses and light-guiding effects.

The absorption of primary radiation by the phosphors according to the invention can be further improved if they are combined with a photonic band gap material.

Photonic band-gap (PBG) materials represent a new class of dielectric materials capable of guiding and manipulating the flow of light on the scale of the wavelength of light. Photonic band-gap materials consist of a periodic arrangement of dielectric elements, e.g. hollow spheres or hollow cylinders, in a dielectric host material with high refractive index with a lattice constant comparable to the wavelength of light.

In analogy to the forbidden energy range—the band-gap—for electrons in a semiconductor, such materials can show a band-gap for the energy spectrum of photons. Thus the surface of such PBG materials can act like a perfect dielectric mirror for impinging light. This renders it possible to capture, i.e. localize light in two and three dimensions with radii of curvature which were previously inaccessible.

According to one embodiment of the invention, photonic band-gap materials consisting of a one-dimensional periodic arrangement of dielectric elements may be used as a PBG layer in a layer stack alternating with phosphor layers. The PBG layer may be a double layer, comprising alternating layers of materials of high and low refractive index, each having a layer thickness q=λ/4, the phosphor layer having a layer thickness of λ, wherein λ is the wavelength of the incident blue to violet primary radiation.

According to another embodiment, the phosphor may be incorporated in a known three-dimensional photonic band-gap material such as opals or inverse opals.

According to a further embodiment, a three-dimensional PBG material comprised by the first phosphor material of the invention is contemplated.

In operation, electrical power is supplied to the LED chip to activate the chip. When activated, the chip emits the primary light, e.g. blue to violet light. A portion of the emitted primary light is completely or partly absorbed by the luminescent material in the coating layer. The luminescent material then emits secondary light, i.e. the converted light having a longer peak wavelength, primarily yellow to amber to red in a sufficiently broad band, specifically with a significant proportion of narrow-band red in response to absorption of the primary light. The remaining unabsorbed portion of the emitted primary light is transmitted through the luminescent laye along with the secondary light. The encapsulation directs the unabsorbed primary light and the secondary light in a general direction as output light. The output light is thus a composite light that is composed of the primary light emitted from the die? and the secondary light emitted from the luminescent layer.

The color temperature or color point (color location in the CIE chromaticity diagram) of the output light of an illumination system according to the invention will vary in dependence on the spectral distributions and intensities of the secondary light in comparison with the primary light.

Firstly, the color temperature or color point of the primary light can be varied by a suitable choice of the light-emitting diode.

Secondly, the color temperature or color point of the secondary light can be varied by a suitable choice of the phosphor in the luminescent material, its particle size and its concentration. Furthermore, these arrangements also advantageously afford the possibility of using phosphor blends in the luminescent material, as a result of which, advantageously, the desired hue can be set even more accurately.

According to one embodiment of the invention, the output light may have a spectral distribution such that it appears to be “white” light.

A white light emitting illumination system according to the invention can advantageously be produced by choosing the luminescent material such that a blue radiation emitted by a blue light emitting diode is converted into complementary wavelength ranges so as to form trichromatic white light according to the RGB concept. In this case, the luminescent material may be a blend of two phosphors, a yellow to red europium(III)-activated compound of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), and a green phosphor.

The red light emitting phosphor may be especially selected from the group of: (Y_1-x-yGd_x)₂O₂S:Eu_y, (Y_1-x-yGd_x)VO₄:Eu_y, (Y_1-x-y-zGd_x)OCl:Eu_yBi_z, (Y_1-x-yGd_x)(V,P,B)O₄:Eu_y, (Y_1-x-yGd_x)NbO₄:Eu_y, (Y_1-x-yGd_x)TaO₄:Eu_y, and (Y_1-x-y-zGd_x)₂O₃:Eu_yBi_z, wherein 0<x<1, 0<y<0.2 and 0<z<0.02.

The green-emitting phosphor is selected from the group of the terbium(III)-activated green phosphors, especially from the group of (Y_xGd_1-x)BO₃:Tb (0<x<1),LaPO₄:Tb; LaPO₄:Ce,Tb; (Y_xGd_1-x)₃Al₅O₁₂:Tb (0<x<1); CeMgAl₁₁O₁₉:Tb; GdMgB₅O₁₀:Ce,Tb; (Y_xGd_1-x)BO₃:Tb(0<x<1); (Y_xGd1-x)₂SiO₅:Tb (0<x<1), Gd₂O₂S:Tb; LaOBr:Tb, and LaOCl:Tb.

The luminescent material may comprise the first phosphor having a grain size dm1 second phosphor having a grain size dm2<dm1.

A second red luminescent material may also be used, in addition, in order to improve the color rendering of this illumination system. The luminescent material may be a blend of two phosphors, a yellow to amber to red europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), and a red phosphor selected from the group of (Ca_1-xSr_x)S:Eu, wherein 0<x<1, and (Sr_1-x-yBa_xCa_y)_2-zSi_5-aAl_aN_8-aO_a:Eu_z, wherein ps 0≦a<5,0<x≦1,0≦y≦1, and 0<z≦0.2.

Particularly good results are achieved with a blue LED whose emission maximum lies at 400 to 495 nm. An optimum has been found to lie at 445 to 465 nm, taking particular account of the excitation spectrum of the europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

A white light emission with an even higher luminous efficacy is possible when red and green phosphors are used together with a blue-emitting LED and a yellow to red emitting europium(III)-activated compound of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

In the example given here, the white-light emitting illumination system according to the invention can particularly preferably be realized by admixing the inorganic luminescent material comprising a mixture of two phosphors with a silicone resin used to produce the luminescence conversion encapsulation or layer. A first phosphor (1) is the red-emitting (Y,Gd)₂O₃:Eu(III), the second phosphor (2) is the red-emitting CaS:Eu, and the third phosphor (3) is a green-emitting phosphor of type (Ce,Tb)MgAl₁₁₁₉.

Part of a blue radiation emitted by a 462 nm InGaN light emitting diode is shifted by the inorganic luminescent material (Y,Gd)₂O₃:Eu(III) into the red spectral region and, consequently, into a wavelength range which is complementarily colored with respect to the color blue. Another part of the blue radiation emitted by the 462 nm InGaN light emitting diode is shifted by the inorganic luminescent material (Ce,Tb)MgAl₁₁O₁₉ into the green spectral region. A human observer perceives the combination of blue primary light and the polychromatic secondary light of the phosphor blend as white light.

The hue (color point in the CIE chromaticity diagram) of the white light thus produced can be varied in this case by a suitable choice of the phosphor mixture and concentration.

FIG. 2 shows the emission spectrum of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED chip, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and a broad-band green phosphor.

FIG. 3 shows the luminous efficacy and CRI of a white light emitting LED with Tc=2500 K (CRI=76). comprising a blue (460 nm) emitting LED chip, (Y,Gd)₂O₃ :Eu(III) as a red-emitting phosphor, and a broad-band green phosphor.

FIG. 4 shows the emission spectrum of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED chip, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and (Ce,Tb)MgAl₁₁O₁₉ as a green phosphor.

FIG. 5 shows the luminous efficacy and CRI of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED chip, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and (Ce,Tb)MgAl₁₁O₁₉ as a green phosphor.

The luminescent material may be a blend of three phosphors, a yellow to red europium(III)-activated compound of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), such as (Y_1-x-yGd_x)₂O₂S:Eu_y, (Y_1-x-yGd_x)VO₄:Eu_y, (Y_1-x-y-zGd_x)OCl:Eu_yBi_z, (Y_1-x-yGd_x)(V,P,B)O₄:Eu_y, (Y_1-x-yGd_x)NbO₄:Eu_y, (Y_1-x-yGd_x)TaO₄:Euy, and (Y_1-x-y-zGd_x)₂0₃:Eu_yBi_z, wherein 0<x<1; 0<y<0.2 and 0<z<0.02, a second red phosphor selected from the group (Ca_1-xSr_x) S:Eu, wherein 0≦x≦1, and (Sr_1-x-yBa_xCa_y)_2-zSi_5-aAl_aN_8-aO_a:Eu_z, wherein 0≦a<5, 0<x≦1; 0≦y≦1 and 0<z≦0.2, and a green Tb(III) -activated phosphor selected from the group of (Y_xGd_1-x)BO₃:Tb (0<x<1),LaPO₄:Tb; LaPO₄:Ce,Tb; (Y_xGd_1-x)₃Al₅O₁₂:Tb (0<x<1); CeMgAl₁₁O₁₉:Tb; GdMgB₅O₁₀:Ce,Tb; (Y_xGd_1-x)BO₃:Tb(0<x<1); (Y_xGd_1-x)₂SiO₅:Tb (0<x<1), Gd₂O₂S:Tb; LaOBr:Tb and LaOCl:Tb.

Useful second green and red phosphors and their optical properties are summarized in the following Table 2.

TABLE 2
Composition	λmax [nm]	Color point x, y
(Ba1−xSrx)2SiO4:Eu	523	0.272, 0.640
SrGa2S4:Eu	535	0.270, 0.686
SrSi2N2O2:Eu	541	0.356, 0.606
SrS:Eu	610	0.627, 0.372
(Sr1−x−yCaxBay)2Si5N8:Eu	615	0.615, 0.384
(Sr1−x−yCaxBay)2Si5-xAlxN8-xOx:Eu	615-650	*
CaS:Eu	655	0.700, 0.303
(Sr1−xCax)S:Eu	610-655	*

In the example given here, the white-light emitting illumination system according to the invention can particularly preferably be realized by admixing the inorganic luminescent material comprising a mixture of two phosphors with a silicone resin used to produce the luminescence conversion encapsulation or layer. A first phosphor (1) is the red-emitting (Y,Gd)₂O₃:Eu(III), the second phosphor (2) is the red-emitting CaS:Eu, and the third phosphor (3) is a green-emitting phosphor of the (Ce,Tb)MgAl₁₁O₁₉ type.

According to a further aspect of the invention, an illumination system that emits output light having a spectral distribution such that it appears to be “yellow to red” light is contemplated.

In one embodiment, a yellow to red light emitting illumination system according to the invention can advantageously be produced by choosing the luminescent material such that a blue radiation emitted by the blue light emitting diode is converted into complementary wavelength ranges so as to form dichromatic yellow to red light. In this case, yellow light is produced by the luminescent material that comprises phosphors including europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

A luminescent material comprising a europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), as a phosphor is particularly well suited as a yellow component for stimulation by a primary UVA or blue radiation source such as, for example, an UVA-emitting LED or blue-emitting LED.

It is possible thereby to implement an illumination system emitting in the yellow to amber to red regions of the electromagnetic spectrum.

In a further embodiment, a yellow-light emitting illumination system according to the invention can advantageously be produced by choosing the luminescent material such that a blue radiation emitted by the blue light emitting diode is converted into complementary wavelength ranges so as to form dichromatic yellow to red light.

In this embodiment, yellow to red light is produced by the luminescent materials that comprise a europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

Particularly good results are achieved with a blue LED whose emission maximum lies at 400 to 480 nm. An optimum was found to lie at 445 to 465 nm, taking particular account of the excitation spectrum of the europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

Part of a blue radiation emitted by a 462 nm InGaN light emitting diode is shifted by the inorganic luminescent material into the yellow to red spectral region and, consequently, into a wavelength range which is complementarily colored with respect to the color blue. A human observer perceives the combination of blue primary light and the excess secondary light of the yellow to red emitting phosphor as yellow to red light.

The color output of the LED-phosphor system is very sensitive to the thickness of the phosphor layer: if the phosphor layer is thick and comprises an excess of a yellow to red europium(III)-activated compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III), then a lesser amount of the blue LED light will penetrate through the thick phosphor layer. The combined LED-phosphor system will then appear yellow to red, because it is dominated by the yellow to red secondary light of the phosphor. Therefore, the thickness of the phosphor layer is a critical variable affecting the color output of the system.

The hue (color point in the CIE chromaticity diagram) of the yellow to red light thereby produced can be varied in this case by a suitable choice of the phosphor mixture and concentration.

A red light emitting illumination system according to the invention can particularly preferably be realized by admixing an excess of the inorganic luminescent material (Y,Gd)₂O₃:Eu(III) with a silicone resin used to produce the luminescence conversion encapsulation or layer. Part of a blue radiation emitted by a 462 nm InGaN light-emitting diode is shifted by the inorganic luminescent material (Y,Gd)₂O₃:Eu(III) into the red spectral region, i.e. into a wavelength range which is complementarily colored with respect to the color blue. A human observer perceives the combination of blue primary light and the excess secondary light of the red-emitting phosphor as red to cyan light.

FIG. 1 is a schematic view of a dichromatic white LED lamp comprising a phosphor of the present invention positioned in a pathway of light emitted by an LED structure.

FIG. 2 shows the emission spectrum of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED chip, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and a broad-band green phosphor.

FIG. 3 shows luminous efficacy and CRI of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED die, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor and a broad-band green phosphor.

FIG. 4 shows the emission spectrum of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED die, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and (Ce,Tb)MgAl₁₁O₁₉ as a green phosphor.

FIG. 5 shows the luminous efficacy and CRI of a white light emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED die, (Y,Gd)₂O₃:Eu(III) as a red-emitting phosphor, and (Ce,Tb)MgAl₁₁O₁₉ as a green phosphor.

Claims

1. Illumination system, comprising a radiation source and a luminescent material comprising a first phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said first phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

2. Illumination system according to claim 1, wherein the radiation source is a light-emitting diode having an emission with a peak emission wavelength in the range of 325 to 495 nm.

3. Illumination system according to claim 1, wherein the luminescent material in addition comprises a second phosphor.

4. Illumination system according to claim 3, wherein the second phosphor is a green phosphor selected from the group of terbium(III)-activated compounds.

5. Illumination system according to claim 4, wherein the second phosphor is selected from the group of YxGd1-x)BO3:Tb (0<x<1),LaPO4:Tb; LaPO4:Ce,Th; (YxGd1-x)3Al5O12:Tb (0<x<1); CeMgAl11O19:Tb; GdMgB5O10:Ce,Th; (YxGd1-x)BO3:Tb(0<x<1); (YxGd1-x)2SiO5:Tb (0<x<1), Gd2O2S:Tb; LaOBr:Tb, and LaOCl:Tb.

6. Illumination system according to claim 1, wherein the luminescent material comprises a first phosphor combined with a photonic bandgap material.

7. Illumination system according to claim 1, wherein the luminescent material comprises a first phosphor having a medium (median?) grain size dm1>500 nanometers.

8. Illumination system according to claim 1, wherein the luminescent material comprises the first phosphor exhibiting a transparent monolithic ceramic microstructure.

9. Illumination system according to claim 1, wherein the luminescent material comprises a first phosphor having a grain size dm1 and a second phosphor having a grain size dm2<dm1.

10. Phosphor capable of absorbing part of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said phosphor comprises europium(III) as an activator in a host lattice selected from the compounds of an anionic oxygen-containing species with a cationic metal species, comprising yttrium(III) and gadolinium(III).

11. Phosphor according to claim 10, wherein the molar proportion of the amount of gadolinium in the host lattice is less than 50 mole percent.

12. Phosphor according to claim 12, comprising in addition a co-activator selected from bismuth(III) and praseodymium(III).

13. Phosphor according to claim 12, wherein the anionic oxygen-containing species is selected from the group of oxide, oxysulfide, oxyhalides, borates, aluminates, gallates, silicates, germanates, phosphates, arsenate, vanadate, niobate, tantalate, and mixtures thereof.

14. Phosphor as claimed in claim 12, which phosphor comprises the activator in a molar proportion of 0.001 to 20 mole % relative to the cation in the host lattice.

15. Phosphor as claimed in claim 12, which phosphor comprises the co-activator in a molar proportion of 0.001 to 2 mole % relative to the cation in the host lattice.

16. Phosphor as claimed in claim 12, selected from the group of: (Y1-x-yGdx)2O2S:Euy, (Y1-x-yGdx)VO4:Euy, (Y1-x-y-zGdx)OCl:EuyBiz, (Y1-x-yGdx)(V,P,B)O4:Euy, (Y1-x-yGdx)NbO4:Euy, (Y1-x-yGdx)TaO4:Euy, and (Y1-x-y-zGdx)2O3:EuyBiz, wherein 0<x<1; 0<y<0.2 and 0<z<0.02.

17. Phosphor according to claim 12, wherein the phosphor is provided with a coating selected from the group of fluorides and orthophosphates of the elements aluminum, scandium, yttrium, lanthanum, gadolinium, and lutetium, the oxides of aluminum, yttrium, and lanthanum, and the nitride of aluminum.

18. Use of a phosphor according to claim 10 for general illumination, traffic and signage lighting, automotive and for backlighting of liquid crystal displays.