WO2006059260A1

WO2006059260A1 - Illumination system comprising a radiation source and a fluorescent material

Info

Publication number: WO2006059260A1
Application number: PCT/IB2005/053880
Authority: WO
Inventors: Peter Schmidt; Thomas JÜSTEL; Volker Bachmann; Cornelis Reinder Ronda
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2004-12-03
Filing date: 2005-11-23
Publication date: 2006-06-08
Anticipated expiration: 2007-06-03
Also published as: CN101072843A; TW200628590A; JP2008521994A; EP1819799A1

Abstract

An illumination system, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is a yellow red-emitting ytterbium(II)-activated oxonitridosilicate of general formula (Sr1-x-y-zCaxBay)aSibAlcNdOe:Ybz, wherein 0 ≤ x ≤ l; 0 ≤ y ≤ l; 0.001 < z < 0.2; 0 < a < 2; 0 < b ≤ 2; 0 < c ≤ 2; 0 < d < 7; 0 < e < 2 can provide light sources having high luminosity and color-rendering index, especially in conjunction with a light emitting diode as a radiation source. The red to yellow -emitting ytterbium(II)-activated oxonitridosilicate of general formula (Sr1-x-y-zCaxBay)aSibAlcNdOe:Ybz, wherein 0 ≤ x ≤ l; 0 ≤ y ≤ l; 0.001 < z < 0.2; 0 < a ≤ 2; 0 < b ≤ 2; 0 < c < 2; 0 < d < 7; 0 < e < 2 is efficiently excitable by primary radiation in the near UV-to-blue range of the electromagnetic spectrum.

Description

TITLE OF THE INVENTION

Illumination system comprising a radiation source and a fluorescent material BACKGROUND OF THE INVENTION

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

More particularly, the invention relates to an illumination system and fluorescent material comprising a phosphor for the generation of specific, colored light, including white light, by luminescent down conversion and additive color mixing based an a ultraviolet or blue radiation emitting radiation source. A light- emitting diode as a radiation source is especially contemplated.

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, there has been such a problem that white light of the desired tone cannot be generated due to variations in the tone, luminance and other factors of the light emitting diodes. In order to solve these problems, there have been previously developed various illumination systems, which convert the color of light emitting diodes by means of a fluorescent material comprising a phosphor to provide a visible white light illumination.

Previous white light illumination systems have been based in particular either on the trichromatic (RGB) approach, i.e. on mixing three colors, namely red, green and blue, in which case the components of the output light may be provided by a phosphor and/or by the primary emission of the LED or in a second, simplified solution, on the dichromatic (B Y) approach, mixing yellow and blue colors, 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 a blue LED.

In particular, the dichromatic approach as disclosed e.g. U.S. Patent 5,998,925 uses a blue light emitting diode of InGaN based semiconductor material combined with an Y₃Al₅O₁₂ :Ce (YAG-Ce³⁺) phosphor. The YAG-Ce ³⁺phosphor is coated on the InGaN LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. Thus, this system emits both blue light, emitted from the LED, and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands are perceived as white light by an observer with a typical CRI in the middle 70ties and a color temperature Tc, that ranges from about 6000 K to about 8000 K. A concern with the LED according to US 5,998,925 is that the "white" output light has an undesirable color balance for true color rendition.

For true color rendition the figure of merit is the color-rendering index (CRI). Measuring the color-rendering index is a relative measurement of how the color rendition of an illumination system compares to that of a black body radiator. The CRI equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by a black body radiator.

True color rendition is of importance as colors in general have the role of providing various information of the visual environment for humans. Colors have a particularly great role for the visual information received by car drivers of cars driving on roads or in tunnels. For example, on roads and in tunnels, which are illuminated by lamps of low CRI, it is difficult to distinguish white and yellow lane marking on the road surface.

Also an important thing in color recognition is that the red of a surface color be recognized as red. Because red, in particular, is coded for important meanings such as danger, prohibition, stop and fire fighting. Therefore important point in improving the visual environment from the viewpoint of safety is an illumination that enhances red surfaces.

In case the (BY)- based light source of dichromatic radiation type described previously is used in such a situation, there occurs such a problem that the probability of recognizing red which is an important color for the indication of danger is reduced due to the lack of spectrum in the red region of the visible light spectrum (647-700 nm range). The red deficiency in the output white light causes illuminated red objects to appear less intense in color than they would under a white light having a well-balanced color characteristic.

BRIEF SUMMARY OF THE INVENTION Therefore, there is a need to provide new phosphors that are excitable by a radiation source of the near UV-to-blue range and emit in the visible yellow to red range.

Desirable characteristics for illumination systems for general purposes are also high brightness at economical cost. Thus the present invention provides an illumination system, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e: Yb_z, wherein O≤x≤l; O≤y≤l; 0.001 < z < 0.2; 0 < a < 2;0<b≤2;0<c≤2;0<d≤7;0<e≤2.

An illumination system according to the present invention can provide a composite white output light that is well balanced with respect to color. In particular, the composite white output light has a greater amount of emission in the red color range than the conventional illumination system. This characteristic makes the device ideal for applications in which a true color rendition is required.

Such applications of the invention include inter alias traffic lighting, street lighting, security lighting and lighting of automated factory, and signal lighting for cars and traffic. Especially contemplated as a radiation source is a light emitting diode.

According to a first aspect of the invention a white light illumination system is provided that comprises a blue-light emitting diode having a peak emission wavelength in the range of 420 to 480 nm as a radiation source and a fluorescent material comprising at least one phosphor, that is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0 < x < 1; O≤y≤l; 0.001 ≤z≤0.2; 0<a≤2; 0<b ≤2; 0<c≤ 2; 0<d≤7; 0<e ≤2. Such illumination system will provide white light in operation. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light. An essential factor is that the yellow to red phosphors of the ytterbium(II)- activated oxonitridosilicate type are so broad-banded that they also have a sufficient proportion of the emission throughout the whole spectral region.

According to one embodiment of the first aspect the invention provides a white light illumination system comprising a blue-light emitting diode having a peak emission wavelength in the range of 460 to 480 nm as a radiation source and a fluorescent material comprising a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0≤x≤l;0≤y≤l; 0.001 < z < 0.2;

0<a≤2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2andat least one second phosphor.

When the fluorescent material comprises a phosphor blend of a phosphor of the ytterbium(II)-activated oxonitridosilicate type and at least one second phosphor the color rendition of the white light illumination system according to the invention may be further improved.

In particular, the fluorescent material of this embodiment may be a phosphor blend, comprising a ytterbium(II)-activated oxonitridosilicate of general formula (Sri_-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e: Yb_z, wherein O≤x≤l; O≤y≤l; 0.001 < z < 0.2;

0<a≤2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2andared phosphor.

Such red phosphor may be selected from the group of Eu(II)-activated phosphors, selected from the group (Ca_1-xSr_x) S:Eu, wherein O≤x≤l and (Sr _1-x-yBa_xCa_y )_2-zSi_5- _aAl_aN_8-aO_a:Eu_z wherein 0≤a<5, 0<x≤l, O≤y≤l and 0<z≤l. Otherwise the fluorescent material may be a phosphor blend, comprising ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y- _zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein

O≤x≤l;O≤y≤l;0.001 ≤z≤0.2;0<a≤2;0<b≤2;0<c≤2;0<d

≤ 7; 0 < e ≤ 2 and a yellow-to-green phosphor. Such yellow-to-green phosphor may be selected from the group comprising (Baj_χSr_x)2 Siθ4: Eu, wherein O≤x≤l, SrGa2S4

:Eu, SrSi2N2θ2:Eu, Ln₃Al₅O₁₂ICe, wherein Ln comprises lanthanum and all lanthanide metals, and Y₃Al₅O₁₂ :Ce. The emission spectrum of such a fluorescent material comprising additional phosphors has the appropriate wavelengths to obtain together with the blue light of the LED and the yellow to red light of the ytterbium(II)-activated oxonitridosilicate type phosphor according to the invention a high quality white light with good color rendering at the required color temperature.

According to another embodiment of the invention there is provided a white light illumination system, wherein the radiation source is selected from the light emitting diodes having an emission with a peak emission wavelength in the UV-range of 200 to 420 nm and the fluorescent material comprises at least one phosphor, that is a ytterbium(II)-activated oxonitridosilicate of general formula

(Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O≤x≤l; O≤y≤l; 0.01 < z < 0.2; 0 < a < 2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2anda second phosphor.

In particular, the fluorescent material according to this embodiment may comprise a white light emitting phosphor blend, comprising a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein

O≤x≤l; O≤y≤l; 0.01 ≤z≤0.2; 0<a≤2; 0<b ≤2; 0<c≤2; 0<d≤7; 0<e ≤2 and a blue phosphor.

Such blue phosphor may be selected from the group comprising BaMgAl₁₀0_17:Eu, Ba₅SiO₄(Cl₅Br)₆ : Eu, CaLn₂S_4:Ce, wherein Ln comprises lanthanum and all lanthanide metals and (Sr₅Ba₅Ca) ₅(PO₄)₃Cl:Eu.

A second aspect of the present invention provides an illumination system emitting red to yellow light. Applications of the invention include security lighting as well as signal lighting for cars and traffic.

Especially contemplated is a yellow to red light illumination system, wherein the radiation source is selected from the blue light emitting diodes having an emission with a peak emission wavelength in the range of 400 to 490 nm and the fluorescent material comprises at least one phosphor, that is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0 ≤ x ≤ 1; O≤y≤l; 0.001 ≤z≤0.2; 0<a≤2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2. Also contemplated is a yellow to red light illumination system, wherein the radiation source is selected from the light emitting diodes having an emission with a peak emission wavelength in the UV-range of 200 to 400 nm and the fluorescent material comprises at least one phosphor that is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O≤x≤ljO≤y≤l; 0.001 < z < 0.2; 0<a≤2;0<b≤2;0<c<2; 0<d≤7;0<e<2. Another aspect of the present invention provides a phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said phosphor is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O≤x≤l; O≤y≤l; 0.001 < z < 0.2; 0 < a < 2; 0<b≤2;0<c≤2;0<d<7;0<e<2.

The fluorescent material is excitable by UV-A emission, which has such wavelengths as from 200 nm to400 nm, but is excited with higher efficiency by blue light emitted by a blue light emitting diode having a wavelength around 400 to 490 nm. Thus the fluorescent material has ideal characteristics for conversion of blue light of nitride semiconductor light emitting component into white light.

These phosphors are broadband emitters wherein the visible emission is so broad that there is no 80 nm wavelength range where the visible emission is predominantly located. These ytterbium(II)-activated oxonitridosilicate phosphors emit a broad band in the red to yellow spectral range of the visible spectrum with very high efficiency. Total conversion efficiency can be up to 90 %.

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 luminous output over the operating lifetime of the device and; 5) compositionally controlled tuning of the phosphors excitation and emission properties.

These ytterbium(II)-activated oxonitridosilicate type phosphors may also include europium(II) and other cations including mixtures of cations as co-activators. In particular, the invention relates to specific phosphor composition

Sr₂Si₂N₂θ₂:Yb_z, wherein 0.001 < z < 0.2, which exhibit a high quantum efficiency of 80 - 90 %, high absorbance in the range from 200 nm to 500 nm of 60-80%, an emission spectrum with a peak wave length of about 615 to 625 nm and low loss, below

10% of the luminescent lumen output due to thermal quenching from room temperature to 100 °C

Specific phosphor composition Sr₂Si₂N₂θ₂:Yb_z, wherein 0.001 < z < 0.2 is especially valuable as phosphor in white light emitting phosphor converted LEDs with low color temperature and improved color rendering.

Co-doping of Sr₂Si₂N₂U₂: Yb_z, wherein 0.01 < z < 0.2 with europium(II) yields a luminescent material with an emission spectrum comprising two peak wavelength areas at 542 and 620 nm. These phosphors may have 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.

DETAILED DESCRIPTION OF THE INVENTION The present invention focuses an a ytterbium(II)-activated oxonitridosilicate as a phosphor in any configuration of an illumination system containing a radiation source, including, but not limited to discharge lamps, fluorescent lamps, LEDs, LDs and X-ray tubes. As used herein, the term "radiation" encompasses preferably radiation in the UV and visible regions of the electromagnetic spectrum. While the use of the present phosphor is contemplated for a wide array of illumination, the present invention is described with particular reference to and finds particular application to light emitting diodes, especially UV- and blue-light-emitting diodes.

The fluorescent material according to the invention comprises as an ytterbium(II)-activated oxonitridosilicate. The phosphor conforms to the general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O ≤ x ≤ l; O ≤ y ≤ l;

0.001 < z < 0.2; 0 < a ≤ 2; 0 < b ≤ 2; 0 < c ≤ 2; 0 < d ≤ 7; 0 < e ≤ 2. This class of phosphor material is based an activated luminescence of a substituted oxonitridosilicate. The phosphor of general (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0 < x

≤l; O≤y≤l; 0.001 ≤z≤0.2;0<a≤2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2 comprises a host lattice with the main components of silicon, nitrogen and oxygen in layers of (Si2N2θ2)^2~~ units, that consist of SiON₃-tetrahedrons, which are linked to form a two-dimensional framework. The N atom bridges three Si atoms, while the O atom is bound terminally to the Si atom. Al-O-units may substitute Si-N-units. Six oxygen atoms coordinate the earth alkaline cations of strontium, calcium and barium as well as ytterbium and eventually a co-activator in a distorted trigonal prismatic manner.

Fig. 4 shows the crystal structure of the basic host lattice

wherein the strontium cations may be replaced by ytterbium(II)-cations.

The host lattice for those materials may be five element (two cations) oxonitridosilicate such as ytterbium(II)-activated oxonitridosilicate Sr2Si2N2θ2:Yb, for example, or may comprise more that five elements such as ytterbium(II)-activated calcium-strontium- oxonitridosilicate (Sr,Ca)2Si2N2θ2:Yb for example.

Especially, within the basic host lattice substitution of divalent earth alkaline metal ions of calcium, strontium and barium by divalent rare earth metals such as europium(II) is possible. The proportion z of ytterbium(II) is preferably in a range of 0.00 K z <

0.2.

When the proportion z of ytterbium(II) is 0.001 or lower, luminance decreases because the number of excited emission centers of photoluminescence due to ytterbium(II)-cations decreases and, when the z is greater than 0.2, density quenching occurs. Density quenching refers to the decrease in emission intensity, which occurs when the concentration of an activation agent added to increase the luminance of the fluorescent material is increased beyond an optimum level.

Replacing some of the ytterbium in a ytterbium-activated oxonitridosilicate by europium(II) as a co-activator has the effect, that the europium produces secondary emission that is concentrated in the red region of the visible spectrum, instead of a typical broadband secondary emission from ytterbium(II)- activated oxonitridosilicate phosphor that is generally centered in the orange to yellow region of the visible spectrum.

These ytterbium(II)-activated oxonitridosilicate phosphors are responsive to more energetic portions of the electromagnetic spectrum than the visible portion of the spectrum. In particular, the phosphors according to the invention are especially excitable by a radiation source providing UV-emission with such wavelengths as 200 to 400 nm, such as an UV-LED, but is excited with higher efficiency by a radiation source providing blue having a wavelength from 400 to 490 nm, such as an blue-emitting LED. Thus the fluorescent material has ideal characteristics for converting blue light of nitride semiconductor light emitting diodes into white light.

The method for producing an ytterbium(II)-activated oxonitridosilicate phosphor of the present invention is not particularly restricted, and it can be produced by firing a mixture of starting materials, which provides an ytterbium(II)-activated oxonitridosilicate fluorescent material. For example, one of the preferable compound represented by SrSi₂NiOiIYb²⁺ is produced by the method where ytterbium oxide, strontium carbonate and silicon nitride as the starting materials are weighed and compounded to give a molar ratio of by SrSi₂N₂O₂:Yb2% and then be fired.

Starting materials having a high purity of 99.9% or more and in the form of fine particle having an average particle size of 1 μm or less can be preferably used.

In the first place, the staring materials (i.e., alkaline earth carbonates, ytterbium compounds such as the oxide, and a silicon-nitrogen compound such as silicon diimide or silicon nitride) are well mixed by a dry and/or wet process utilizing any of various known mixing method such as ball mills, V-shaped mixers, stirrers and the like. The obtained mixture is placed in a heat-resistance container such as an alumina crucible and a tungsten boat, and then fired in an electric furnace. A preferred temperature for the firing ranges from 1,200 to 1,500 degree C.

The firing atmosphere is not particularly restricted, and for example, it is preferable to conduct firing in a reducing atmosphere such as an atmosphere comprising inert gas such as nitrogen and argon and the like, and hydrogen in a proportion of 0.1 to 10 volume%. The firing period is determined upon various conditions such as the amount of the mixture charged in the container, the firing temperature and the temperature at which the product is taken out of the furnace, but generally in the range of2 to 4 hours. Fluorescent material obtained by the above-mentioned method may be ground by using, for example, a ball mill, jet mill and the like. Moreover, washing and classification may be conducted. For enhancing the cristallinity of the resulting granular phosphor re-firing is suggested.

The resulting luminescent material is then ground, washed with water and ethanol, dried and sieved. A yellow powder is obtained, which efficiently luminescence at 621 nm under UV and blue excitation. The color point is at x = 0.578 and y = 0.418. The lumen equivalent is 266 lm/W.

After firing, the powders were characterized by powder X-ray diffraction (Cu, Kα-line), which showed that all compounds had formed. Fig. 2 shows the X-ray diffraction data of SrSi₂N₂O₂, Fig. 3 the X-ray diffraction data of SrSi₂N₂O₂: Yb²⁺. Each phosphor of the ytterbium(II)-activated oxonitridosilicate type emits a yellow to red fluorescence when excited by radiation of the UVA or blue range of the electromagnetic spectrum.

In FIG. 5 of the drawings accompanying this specification, the excitation, emission and reflection spectra of SrSi₂N₂O₂: Yb²⁺ are given.

From the excitation spectra, it is also clear that ytterbium(II)-activated oxonitridosilicate phosphor SrSi₂N₂O₂: Yb²⁺ can be excited efficiently with radiation of wavelength between 254 nm and 460 nm.

When excited with radiation of wavelength 365 nm, SrSi₂N₂O₂: Yb²⁺ is found to give a broadband emission, which peak wavelength between 615 and 625 nm tailing out to 700 nm. Preferably the ytterbium(II)-activated oxonitridosilicate 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 gm and, thus, is 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 can be applied, for example, by deposition from the gas phase a wet-coating process. The invention also concerns an illumination system comprising a radiation source and a fluorescent material comprising at least one ytterbium(II)- activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBay)_aSibAl_cNdO_e:Ybz, wherein O ≤ x ≤ lj O ≤ y ≤ l; 0.001 < z < 0.2; 0 < a < 2; 0 < b ≤ 2; 0 < c ≤ 2; 0 < d < 7; 0 < e < 2.

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

Moreover, light emitting components such as those found in discharge lamps and fluorescent lamps, such as mercury low and high pressure discharge lamps, sulfur discharge lamps, and discharge lamps based an molecular radiators are also contemplated for use as radiation sources with the present inventive phosphor compositions.

In a preferred embodiment of the invention the radiation source is a light-emitting diode (LED).

Any configuration of an illumination system which includes a light emitting diode and a ytterbium(II) activated oxonitridosilicate phosphor composition is contemplated in the present invention, preferably with addition of other well-known phosphors, which can be combined to achieve a specific color or white light when irradiated by a LED emitting primary UV or blue light as specified above.

A detailed construction of one embodiment of such illumination system comprising a radiation source and a fluorescent material shown in Fig.1 will now be described.

FIG. 1 shows a schematic view of a chip type light emitting diode with a coating comprising the fluorescent material. The device comprises chip type light emitting diode (LED) 1 as a radiation source. The light-emitting diode dice is positioned in a reflector cup lead frame 2. The dice 1 is connected via a bond wire 7 to a first terminal 6, and directly to a second electric terminal 6. The recess of the reflector cup is filled with a coating material that contains a fluorescent material according to the invention to form a coating layer that is embedded in the reflector cup. The phosphors 3, 4 are applied either separately or in a mixture. The coating material typically comprises a polymer 5 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 significant light scattering. A variety of polymers are known in the LED industry for making LED illumination systems. In one embodiment, the polymer is selected from the group consisting of epoxy and silicone resins. Adding the phosphor mixture to a liquid that is a polymer precursor can perform encapsulation. For example, the phosphor mixture can be a granular powder. Introducing phosphor particles into 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 fluorescent 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, Tiθ2, Siθ2, CaCθ3 or BaSθ4 or any else organic pigments. These materials can be added in a simple manner to the above-mentioned resins. In operation, electrical power is supplied to the dice to activate the dice.

When activated, the dice emits the primary light, e.g. blue light. A portion of the emitted primary light is completely or partially absorbed by the fluorescent material in the coating layer. The fluorescent material then emits secondary light, i.e., the converted light having a longer peak wavelength, primarily yellow in a sufficiently broadband (specifically with a significant proportion of red) in response to absorption of the primary light. The remaining unabsorbed portion of the emitted primary light is transmitted through the fluorescent layer, along with the secondary light. The encapsulation directs the unabsorbed primary light and the secondary light in a general direction as output light. Thus, the output light is a composite light that is composed of the primary light emitted from the die and the secondary light emitted from the fluorescent layer.

The color temperature or color point of the output light of an illumination system according to the invention will vary depending upon the spectral distributions and intensities of the secondary light in comparison to 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 aspect of the invention the output light of the illumination system may have a spectral distribution such that it appears to be "white" light.

In a first embodiment, 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, to form dichromatic white light.

In this case, yellow light is produced by means of the luminescent materials that comprise an ytterbium(II)-activated oxonitridosilicate phosphor. Also a second fluorescent material can be used, in addition, in order to improve the color rendition of this illumination system. Particularly good results are achieved with a blue LED whose emission maximum lies at 400 to 490 nm. An optimum has been found to lie at 445 to 468 nm, taking particular account of the excitation spectrum of the ytterbium(II)-activated oxonitridosilicate.

A white-light emitting illumination system according to the invention can particularly preferably be realized by admixing the inorganic luminescent material SrSi₂NiOiIYb²⁺ with a silicon resin used to produce the luminescence conversion encapsulation or layer. Part of a blue radiation emitted by a 462nm InGaN light emitting diode is shifted by the inorganic luminescent material SrSi₂N₂O₂IYb²⁺ into the orange 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 secondary light of the yellow-emitting phosphor as white light. FIG. 6 shows the emission spectra of such illumination system comprising blue emitting InGaN die with primary emission at 462 nm and SrSi₂N₂O₂:Yb²⁺ as the fluorescent material, which together form an overall spectrum which conveys a white color sensation of high quality.

When compared with the spectral distribution of the white output light generated by the prior art LED the apparent difference in the spectral distribution is the shift of the peak wavelength which is in the red region of the visible spectrum. Thus, the white output light generated by the illumination system has a significant additional amount of red color, as compared to the output light generated by the prior art LED.

In a second embodiment, 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 the blue light emitting diode is converted into complementary wavelength ranges, to form polychromatic white light. In this case, yellow light is produced by means of the luminescent materials that comprise a blend of phosphors including ytterbium(II)-activated oxonitridosilicate phosphor and a second phosphor.

Yielding white light emission with even high color rendering is possible by using additional red and green broad band emitter phosphors covering the whole spectral range together with a blue-emitting LED and a yellow to red emitting ytterbium(II)-activated oxonitridosilicate phosphor.

Useful second phosphors and their optical properties are summarized in the following table 2.

Table 2:

The luminescent materials may be a blend of two phosphors, a yellow to red ytterbium(II) activated oxonitridosilicate phosphor and a red phosphor selected from the group (Ca_1-xSr_x) S: Eu, wherein 0 < x < 1 and

(Sr _1-x-yBa_xCa_y )₂Si_5-aAl_aN_8-aO_a:Eu wherein 0 < a < 5, 0 < x ≤ l and O ≤ y ≤ l.

The luminescent materials may be a blend of two phosphors, e.g. a yellow to red ytterbium(II) activated oxonitridosilicate phosphor and a green phosphor selected from the group comprising (Baj_χSr_x)2 Siθ4: Eu, wherein 0 < x < 1, SrGa2S4 :Eu and SrSi2N2U2:Eu.

The luminescent materials may be a blend of three phosphors, e.g. a yellow to red ytterbium(II) activated oxonitridosilicate phosphor, a red phosphor selected from the group (Ca_1-xSr_x) S:Eu, wherein 0 < x < 1 and (Sr _1-x-yBa_xCa_y )₂Si_5- _aAl_aN_8-aO_a:Eu wherein 0 < a < 5, 0 < x < land O ≤ y ≤ l and a yellow to green phosphor selected from the group comprising (Baj_xSr_x)2 SiC^: Eu, wherein 0 ≤ x ≤ 1, SrGa2S4 :Eu and SrSi2N2U2:Eu.

A white-light emitting illumination system according to the invention can particularly preferably be realized by admixing the inorganic luminescent material comprising a mixture of three phosphors with a silicon resin used to produce the luminescence conversion encapsulation or layer. A first phosphor (1) is the yellow- emitting oxonitridosilicate SrSi₂N₂O₂IYb²⁺, the second phosphor (2) is the red-emitting CaS: Eu, and the third (3) is a green-emitting phosphor of type SrSi2N2U2:Eu.

Part of a blue radiation emitted by a 462nm InGaN light emitting diode is shifted by the inorganic luminescent material SrSi₂N₂O₂: Yb²⁺ into the yellow spectral region and, consequently, into a wavelength range which is complementarily colored with respect to the color blue. Another part of blue radiation emitted by a 462nm InGaN light emitting diode is shifted by the inorganic luminescent material CaS: Eu into the red spectral region. Still another part of blue radiation emitted by a 462nm InGaN light emitting diode is shifted by the inorganic luminescent material SrSi2N2θ2:Eu 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 thereby produced can in this case be varied by a suitable choice of the phosphors in respect of mixture and concentration.

In a third embodiment, a white-light emitting illumination system according to the invention can advantageously be produced by choosing the luminescent material such that a UV radiation emitted by the UV light emitting diode is converted into complementary wavelength ranges, to form dichromatic white light. In this case, the yellow and blue light is produced by means of the luminescent materials. Yellow light is produced by means of the luminescent materials that comprise an ytterbium(II)-activated oxonitridosilicate phosphor. Blue light is produced by means of the luminescent materials that comprise a blue phosphor selected from the group comprising BaMgAl₁₀0_17:Eu, Ba₅SiO₄(Cl₅Br)₆ : Eu, CaLn₂S^Ce and (Sr₅Ba₅Ca) ₅(PO₄)₃Cl:Eu. Particularly good results are achieved in conjunction with a UVA light emitting diode, whose emission maximum lies at 200 to 400 nm. An optimum has been found to lie at 365 nm, taking particular account of the excitation spectrum of the ytterbium(II)-activated oxonitridosilicate.

In a fourth embodiment, a white-light emitting illumination system according to the invention can advantageously be produced by choosing the luminescent material such that UV radiation emitted by a UV emitting diode is converted into complementary wavelength ranges, to form polychromatic white light e.g. by additive color triads, for example blue, green and red.

In this case, the yellow to red, the green and blue light is produced by means of the luminescent materials.

Also a second red fluorescent material can be used, in addition, in order to improve the color rendition of this illumination system.

Yielding white light emission with even high color rendering is possible by using blue and green broad band emitter phosphors covering the whole spectral range together with a UV emitting LED and a yellow to red emitting ytterbium(II)- activated oxonitridosilicate phosphor.

The luminescent materials may be a blend of three phosphors, a yellow to red ytterbium(II) activated oxonitridosilicate phosphor, a blue phosphor selected from the group comprising BaMgAl₁₀0_17:Eu, Ba₅SiO₄(Cl₅Br)₆ :Eu, CaLn₂S^Ce and (Sr₅Ba₅Ca) ₅(PO₄)₃Cl:Eu and a yellow to green phosphor selected from the group comprising (Baj_xSr_x)2 SiO^ Eu₅ wherein 0 < x < I₅ SrGa2S4 :Eu and SrSi2N2θ2:Eu.

According to 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.

Fluorescent material comprising ytterbium(II) activated oxonitridosilicate as phosphor is particularly well suited as a yellow to red 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 red regions of the electromagnetic spectrum.

In a fifth 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, to form dichromatic yellow to red light.

In this case, yellow to red light is produced by means of the luminescent materials that comprise an ytterbium(II)-activated oxonitridosilicate phosphor.

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 ytterbium(II) activated oxonitridosilicate phosphor, 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 in this case be varied by a suitable choice of the phosphor in respect of mixture and concentration.

Particularly good results are achieved with a blue LED whose emission maximum lies at 400 to 480 nm. An optimum has been found to lie at 445 to 465 nm, taking particular account of the excitation spectrum of the oxonitridosilicate.

A yellow-to orange light emitting illumination system according to the invention can particularly preferably be realized by admixing an excess of the inorganic luminescent material SrSi2N2θ2:Yb²⁺ with a silicon resin used to produce the luminescence conversion encapsulation or layer. The blue radiation emitted by a 462nm InGaN light emitting diode is shifted by the inorganic luminescent material SrSi₂NiOiIYb²⁺ into the yellow to orange 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 orange -emitting phosphor as yellow to range light.

In a sixth 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 UV radiation emitted by the UV emitting diode is converted entirely into monochromatic yellow to red light. In this case, the yellow to red light is produced by means of the luminescent materials.

A yellow-light emitting illumination system according to the invention can particularly preferably be realized by admixing the inorganic luminescent material SrSi₂N₂OiIYb²⁺ with a silicon resin used to produce the luminescence conversion encapsulation or layer. Part of a UV radiation emitted by a UV emitting diode is shifted by the inorganic luminescent material SrSi₂N₂O₂:Yb²⁺ into the orange spectral region. A human observer perceives the combination of UVA primary radiation and the secondary light of the orange-emitting phosphor as yellow to orange light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows 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 XRD pattern of SrSi₂N₂Oi measured by Cu Ka radiation.

FIG. 3 shows the XRD pattern of SrSi₂N₂O₂: Yb²⁺ measured by Cu Ka radiation. FIG. 4 shows the layered structure of the host lattice SrSi₂N₂O₂.

FIG. 5 shows excitation, emission and reflection spectra of SrSi₂N₂O₂=Yb²⁺.

FIG. 6 shows the spectral radiance of an illumination system comprising a blue LED and SrSi₂N₂O₂:Yb²⁺ as fluorescent material.

Claims

CLAIMS:

1. Illumination system, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said at least one phosphor is an ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0 < x < 1 ; 0 < y < 1 ; 0.001 < z < 0.2; 0<a≤2;0<b≤2;0<c<2;0<d<7;0<e<2.

2. Illumination system according to claim 1, wherein the radiation source is a light emitting diode.

3. Illumination system according to claim 1, wherein the radiation source is selected from the blue light emitting diodes having an emission with a peak emission wavelength in the range of 400 to 490 nm and wherein the fluorescent material comprising a ytterbium(II)-activated oxonitridosilicate of general formula (Sri_-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e: Yb_z, wherein 0≤x≤l;0≤y≤l; 0.001 < z < 0.2; 0 < a < 2;0<b≤2;0<c≤2;0<d<7;0<e<2.

4. Illumination system according to claim 1, wherein the radiation source is selected from the light emitting diodes having an emission with a peak emission wavelength in the range of 400 to 490 nm and the fluorescent material comprises an ytterbium(II)-activated oxonitridosilicate of general formula

(Sri_-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein 0≤x≤l;0≤y≤l; 0.001 < z < 0.2; 0 < a < 2;0<b≤2;0<c≤2;0<d<7;0<e<2anda second phosphor.

5. Illumination system according to claim 4, wherein the second phosphor is a 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≤l,0≤y≤land 0 < z < 0.09.

6. Illumination system according to claim 4, wherein the second phosphor is a yellow to green phosphor selected from the group comprising (Ba_1-X Sr_x)2 Siθ4:Eu, wherein 0 < x < 1, SrGa2S4 :Eu, SrSi2N2θ2:Eu, Ln₃Al₅O₁₂)Ce, wherein Ln comprises lanthanum and all lanthanide metals, and Y₃Al₅O₁₂ :Ce

7. Illumination system according to claim 1, wherein the radiation source is selected from the light emitting diodes having an emission with a peak emission wavelength in the UV range of 200 to 400 nm and wherein the fluorescent material comprises a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O≤x≤ljO≤y≤l; 0.001 < z < 0.2; 0 < a < 2;0<b≤2;0<c≤2;0<d≤7;0<e≤2.

8. Illumination system according to claim 1, wherein the radiation source is selected from the light emitting diodes having an emission with a peak emission wavelength in the UV-range of 200 to 400 nm and wherein the fluorescent material comprises a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e: Yb_z, wherein 0<x≤l;0≤y<l; 0.01 < z < 0.2; 0 < a ≤ 2; 0<b≤2;0<c≤2;0<d≤7;0<e≤2anda second phosphor.

9. Illumination system according to claim 8, wherein the second phosphor is a blue phosphor selected from the group of BaMgAl₁₀0_17:Eu, Ba₅SiO₄(Cl₅Br)₆ : Eu, CaLn₂S_4:Ce, (Sr₅Ba₅Ca) ₅(PO₄)₃Cl:Eu and LaSi₃N₅:Ce.

10. Illumination system according to claim 8, wherein the second phosphor is a 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 < 5.00 <x≤l,O≤y≤landO<z≤ 0.09.

11. Illumination system according to claim 8, wherein the second phosphor is a yellow to green phosphor selected from the group comprising (Ba_1-X Sr_x)2 SiO^Eu, wherein 0 < x < 1, SrGa2S4 :Eu, SrSi2N2θ2:Eu, Ln₃Al₅O₁₂ICe, wherein Ln comprises lanthanum and all lanthanide metals, and Y₃Al₅O₁₂ :Ce

12. Phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said phosphor is a ytterbium(II)-activated oxonitridosilicate of general formula (Sr_1-x-y-zCa_xBa_y)_aSi_bAl_cN_dO_e:Yb_z, wherein O≤x≤ljO≤y≤l; 0.001 < z < 0.2; 0 < a < 2;0<b≤2;0<c≤2;0<d<7;0<e<2.

13. Phosphor according to claim 12, wherein said phosphor additionally comprises a co-activator.

14. Phosphor according to claim 12, wherein said phosphor is an ytterbium(II)-activated oxonitridosilicate of general formula SrSi₂N₂O₂IYb²⁺.

15. Phosphor according to claim 12, wherein the phosphor has 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.