WO2009101578A1 - Light emitting device comprising a ceramic material with line emitter activators and an interference filter - Google Patents

Light emitting device comprising a ceramic material with line emitter activators and an interference filter Download PDF

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Publication number
WO2009101578A1
WO2009101578A1 PCT/IB2009/050545 IB2009050545W WO2009101578A1 WO 2009101578 A1 WO2009101578 A1 WO 2009101578A1 IB 2009050545 W IB2009050545 W IB 2009050545W WO 2009101578 A1 WO2009101578 A1 WO 2009101578A1
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systems
ceramic material
light emitting
emitting device
mol
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PCT/IB2009/050545
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French (fr)
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Cornelis R. Ronda
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Philips Intellectual Property & Standards Gmbh
Koninklijke Philips Electronics N. V.
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Publication of WO2009101578A1 publication Critical patent/WO2009101578A1/en

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    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
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    • C09K11/77—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals
    • C09K11/7766—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals containing two or more rare earth metals
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    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
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    • H01L33/50—Wavelength conversion elements
    • H01L33/501—Wavelength conversion elements characterised by the materials, e.g. binder
    • H01L33/502—Wavelength conversion materials

Definitions

  • the invention relates to light emitting devices, especially to LEDs.
  • This invention relates to light emitting devices, especially to LEDs such as phosphor converted light emitting diodes (pcLEDs). It also concerns the application of these pcLEDs in light sources for LCD backlighting, automotive lighting, projection and signaling purposes.
  • LEDs such as phosphor converted light emitting diodes (pcLEDs). It also concerns the application of these pcLEDs in light sources for LCD backlighting, automotive lighting, projection and signaling purposes.
  • pcLEDs are either white pcLEDs for illumination purposes or so-called full conversion pcLEDs.
  • the latter ones are applied as green and red light sources, whereby the color point is a result of the additive color mixing of the blue radiation emitted by the semiconductor, which was not absorbed by the phosphor and the green to red emission of the luminescent screen, excited by the blue LED light.
  • light emitting device comprising at least one essentially ceramic material and at least one interference filter in optical contact with said ceramic material, whereby said ceramic material comprises at least one of Er 3+ , Tb 3+ , Ho 3+ and Mn 4+ as activator for emitting light.
  • essentially means especially that > 90 %, preferably > 95 % and most preferred > 99 % of the material has the desired structure and/or composition.
  • some additives such as binders or fluxes may be present in the ceramic material. These additives may be incorporated fully or in part into the final material, which then may also be a composite of several chemically different species and particularly include such species known to the art as fluxes and/or binders.
  • Suitable fluxes and/or binders include alkaline earth - or alkaline - metal oxides (e.g. MgO) and fluorides, SiC>2 and the like.
  • ceramic material in the sense of the present invention means and/or includes especially a crystalline or polycrystalline compact material or composite material with a controlled amount of pores or which is pore free.
  • polycrystalline material in the sense of the present invention means and/or includes especially a material with a volume density larger than 90 percent of the main constituent, consisting of more than 80 percent of single crystal domains, with each domain being larger than 0.5 ⁇ m in diameter and may have different crystallo graphic orientations. The single crystal domains may be connected by amorphous or glassy material or by additional crystalline constituents.
  • optical contact in the sense of the present invention means and/or includes especially devices in which the distance between the interference filter and the luminescence layer is that small, that no interference patterns are formed between interference filter and luminescence layer. This also reduces the physical dimensions of the device and undesired leaking of light.
  • interference filter especially means and/or includes an optical component consisting of a plurality of layers with alternating high and low indices of refraction.
  • the ceramic material allows the use within a broad temperature range
  • the doping level of the activators inside the ceramic material is > 0.01% (mol:mol) and ⁇ 25% (mol:mol), preferably > 0.1% (mol:mol)and ⁇ 5% (mol:mol), most preferred > 0.5% (mol:mol) and ⁇ 1,5% (mol:mol).
  • mol:mol denotes, that the doping level of the activators in mol is compared to the ceramic material in mol.
  • the at least one interference filter comprises one filter with a multitude of sub-layers.
  • the number of sub-layers is > 10 and ⁇ 30, whereby the average thickness of each sub-layer is > 50 nm and ⁇ 400 nm, preferably > 100 nm and ⁇ 300 nm
  • the material(s) of the at least one interference layer are preferably chosen so that the at least one interference layer comprises an alternating layered structure with a difference in refractive index.
  • the at least one interference filter comprises a material selected out of the group TiO 2 /SiO 2 , Si(VZnO, Al 2 (VTiO 2 , Al 2 O 3 /ZnO, HfO 2 /SiO 2 , HfO 2 / Al 2 O 3 , Y 2 O 3 /TiO 2 , ZrO 2 /TiO 2 or mixtures thereof.
  • the at least one interference filter comprises several layers which are essentially made out of resins, which may optionally comprise nanoparticles of a material with a high refractive index (preferred materials include Ti ⁇ 2 and ZnO).
  • the ceramic material comprises a material selected out of the group comprising - Ln 2 O 3 : Er, with
  • Ln being selected out of the group comprising Sc, Y, Gd, Lu and mixtures thereof
  • Ln 2 O 3 Ho
  • Ln 2 O 3 Tb
  • M ⁇ being chosen out of the group comprising Si, Ge, Ti, Zr, Hf and mixtures thereof, or mixtures thereof.
  • the ceramic material comprises essentially out of this material.
  • additives may also be present in the bulk compositions.
  • These additives particularly include such species known to the art as fluxes.
  • Suitable fluxes include alkaline earth - or alkaline - metal oxides and fluorides,
  • the index of refraction of the at least one ceramic material is >1.4, preferably >1.7, most preferred
  • the index of refraction may be changed voluntarily with the composition of the ceramic material for many application so that an "index matching" of the ceramic material with the further component of the light emitting device is possible in many cases.
  • the photothermal stability of the ceramic material is >80% to ⁇ 100% after exposure of the ceramic material for 1000 hrs at 200 0 C with a light power density of 10W/cm 2 and an average photon energy of 2.75 eV.
  • photothermal stability in the sense of the present invention especially means and/or includes the conservation of the specific absorption property under simultaneous application of heat and high intensity excitation, i.e. a photothermal stability of 100% indicates that the material is virtually unaffected by the simultaneous irradiation and heat up.
  • the photothermal stability of the ceramic material is >82.5% to ⁇ 95%, preferably >85% to ⁇ 97%, after exposure of the ceramic material for 1000 hrs at 200 0 C with a light power density of 10W/cm 2 and an average photon energy of 2.75 eV.
  • the thermal conductivity of the ceramic material at room temperature is > 0.005 W Cm 1 K "1 to ⁇ 0.75 W cm 1 K "1
  • the ceramic material shows a transparency for normal incidence in air of >10 % to ⁇ 85 % for light in the wavelength range from > 650 nm to ⁇ 800 nm.
  • the transparency for normal incidence is in air of >20 % to ⁇ 80 % for light in the wavelength range from > 550 nm to ⁇ 1000 nm, more preferred >30 % to ⁇ 75 % and most preferred > 40% to ⁇ 70% for a light in the wavelength range from > 650 nm to ⁇ 800 nm.
  • transparency in the sense of the present invention means especially that > 10% preferably >20%, more preferred >30%, most preferred >40% and ⁇ 85% of the incident light of a wavelength, which cannot be absorbed by the material, is transmitted through the sample for normal incidence in air.
  • This wavelength is preferably in the range of > 570 nm and ⁇ 800 nm or > 510 nm and ⁇ 570 nm, for red and green ceramic materials, respectively.
  • the ceramic material has a density of >95% and ⁇ 101% of the theoretical density.
  • the ceramic material has a density of >97% and ⁇ 100% of the theoretical density.
  • the densities lower than 100% according to the described preferred embodiment of the present invention are preferably obtained by sintering of the material to a stage where still pores are present in the ceramic matrix. Most preferred are densities in the range >98.0% and ⁇ 99.8% with total pore volumes in the ceramic matrix within the >0.2 - ⁇ 2% range.
  • a preferred mean pore diameter is in the >400 - ⁇ 2500 nm range.
  • the present invention furthermore relates to a method of producing a material for a ceramic material for a light emitting device according to the present invention comprising a sintering step.
  • sintering step in the sense of the present invention means especially densif ⁇ cation of a precursor powder under the influence of heat, which may be combined with the application of uniaxial or isostatic pressure, without reaching the liquid state of the main constituents of the sintered material.
  • the sintering step is without additional pressure, preferably in reducing or inert atmosphere.
  • method furthermore comprises the step of pressing the precursor material(s) to > 50% to ⁇ 70 %, according to an embodiment of the present invention, > 55% to ⁇ 60 % of its theoretical density before sintering. It has been shown in practice that this improves the sintering steps for most materials as described with the present invention.
  • the method of producing a material for a light emitting device according to the present invention comprises the following steps:
  • a first pressing step preferably a uniaxial pressing step at >10 kN using a suitable powder compacting tool with a mould in the desired shape (e.g. rod- or pellet-shape) and/ or a cold isostatic pressing step preferably at >3000 bar to ⁇ 3500 bar.
  • a suitable powder compacting tool with a mould in the desired shape (e.g. rod- or pellet-shape) and/ or a cold isostatic pressing step preferably at >3000 bar to ⁇ 3500 bar.
  • a hot pressing step preferably a hot isostatic pressing step preferably at >100 bar to ⁇ 2500 bar and preferably at a temperature of >500 0 C to ⁇ 2000 0 C and/or a hot uniaxial pressing step preferably at >100 bar to ⁇ 2500 bar and preferably at a temperature of >500 0 C to ⁇ 2000 0 C.
  • the present invention furthermore relates to a light emitting device, especially a LED comprising a ceramic material of the present invention.
  • a compound and/or a ceramic material according to the present invention may be of use in a broad variety of systems and/or applications, amongst them one or more of the following: - Office lighting systems household application systems shop lighting systems, home lighting systems, accent lighting systems, - spot lighting systems, theatre lighting systems, fiber-optics application systems, projection systems, self-lit display systems, - pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, and - decorative lighting systems portable systems automotive applications green house lighting systems advertisement lighting systems
  • Y2O3:Ho is prepared by mix and fire techniques.
  • the oxides are mixed in alcohol and dried at 100 0 C. Heating is performed at 1500 0 C during 3 hours.
  • the Er- concentration is 1 mole % (relative to Y).
  • the synthesized powder is ball-milled for 24 h using ZrO2 balls in isopropanol as liquid medium during 24 hours. After drying, the material is heat treated for 8 h in air at 400 0 C.
  • precipitation techniques leading to much smaller primary phosphor particles, can be used, to prevent time consuming milling steps.
  • the prepared powder is sintered to transparent ceramic by a hot-pressing process at 1500 0 C for 4 h with a pressure of 30 MPa in an Ar atmosphere.
  • This step can also be performed with the addition of a flux, like LiF in concentrations preferably between 0.01 - 5%.
  • a sample with dimensions IxIxO.5mm 3 is cut from the ingot, polished at all sides and placed on a LED chip with lxlmm 2 size, emitting at 440 nm.
  • the thickness of the sample is chosen in such a manner that almost no blue light is leaking through (less than 2%). Switching on the LED results in green light.
  • Comparative example I As a comparative example, a LED comprising a powder layer of
  • Y2O3 :Ho (grain size 2-8 ⁇ m) with the same thickness and Ho3+ concentration as in the inventive example is prepared.
  • a sample of 1x1x0.5 mm 3 is applied to a blue emitting LED (emission 440 nm). Thickness and Ce 3+ concentration were adjusted to obtain a transmission of blue light through the ceramics of about 2%. Switching on the LED results in yellow light.
  • K 2 TiF 6 doped with Mn 4+ (5 mole%) is prepared by dissolving the hydroxides in diluted hydrofluoric acid (30-50 % HF) in Hydrofluoric acid resistant vessels, e.g. made out of Teflon, during heating and stirring.
  • the luminescent material is isolated by evaporation of the reaction mixture to receive a dry powder.
  • the phosphor is milled as described above.
  • a ceramic layer is prepared by hot pressing for 24 hours at 100 Mpa at a temperature of 600 0 C.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Ceramic Engineering (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Structural Engineering (AREA)
  • Inorganic Chemistry (AREA)
  • Luminescent Compositions (AREA)
  • Led Device Packages (AREA)

Abstract

The invention relates to an improved light emitting device comprising a ceramic with line-emitting activators in optical contact with an interference filter. This light emitting device has a greatly increased light efficiency towards a preferred light- emitting direction.

Description

LIGHT EMITTING DEVICE COMPRISING A CERAMIC MATERIAL WITH LINE EMITTER ACTIVATORS AND AN INTERFERENCE FILTER
FIELD OF THE INVENTION
The invention relates to light emitting devices, especially to LEDs.
BACKGROUND OF THE INVENTION
This invention relates to light emitting devices, especially to LEDs such as phosphor converted light emitting diodes (pcLEDs). It also concerns the application of these pcLEDs in light sources for LCD backlighting, automotive lighting, projection and signaling purposes.
Presently applied pcLEDs are either white pcLEDs for illumination purposes or so-called full conversion pcLEDs. The latter ones are applied as green and red light sources, whereby the color point is a result of the additive color mixing of the blue radiation emitted by the semiconductor, which was not absorbed by the phosphor and the green to red emission of the luminescent screen, excited by the blue LED light.
This is a consequence of the difficulty to completely suppress leakage of blue radiation through the luminescent screen without sacrificing overall radiant efficiency of the light source. For this purpose, ceramic materials have been suggested, e.g. in the
WO2007/039849, which is hereby incorporated by reference.
However, for some applications the luminescence of the LEDs of the prior art cannot be regarded as satisfactory. Therefore there is the need for light emitting devices, which are capable of providing satisfyingly increased luminescence profiles.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a light emitting deviced, which may be used within a wide range of applications and especially provides a satisfyingly increased luminescence profile.
This object is solved by a light emitting device according to claim 1 of the present invention. Accordingly, light emitting device is provided, comprising at least one essentially ceramic material and at least one interference filter in optical contact with said ceramic material, whereby said ceramic material comprises at least one of Er3+, Tb3+, Ho3+ and Mn4+ as activator for emitting light.
The term "essentially" means especially that > 90 %, preferably > 95 % and most preferred > 99 % of the material has the desired structure and/or composition. It should be noted that (depending on the manufacturing procedure) some additives, such as binders or fluxes may be present in the ceramic material. These additives may be incorporated fully or in part into the final material, which then may also be a composite of several chemically different species and particularly include such species known to the art as fluxes and/or binders. Suitable fluxes and/or binders include alkaline earth - or alkaline - metal oxides (e.g. MgO) and fluorides, SiC>2 and the like.
The term "ceramic material" in the sense of the present invention means and/or includes especially a crystalline or polycrystalline compact material or composite material with a controlled amount of pores or which is pore free. The term "polycrystalline material" in the sense of the present invention means and/or includes especially a material with a volume density larger than 90 percent of the main constituent, consisting of more than 80 percent of single crystal domains, with each domain being larger than 0.5 μm in diameter and may have different crystallo graphic orientations. The single crystal domains may be connected by amorphous or glassy material or by additional crystalline constituents.
The term "optical contact" in the sense of the present invention means and/or includes especially devices in which the distance between the interference filter and the luminescence layer is that small, that no interference patterns are formed between interference filter and luminescence layer. This also reduces the physical dimensions of the device and undesired leaking of light. The term "interference filter" especially means and/or includes an optical component consisting of a plurality of layers with alternating high and low indices of refraction.
The use of such a light emitting device has shown for a wide range of applications within the present invention to have at least one of the following advantages:
Due to synergistic effects between the ceramic, comprising the described activators on the one side and the interference filter on the other side, a great increase of luminescence towards a preferred direction can be found. - The ceramic material allows the use within a broad temperature range
No shift in emission is observed (as was observed in many applications involving broad band emitters).
According to a preferred embodiment of the present invention, the doping level of the activators inside the ceramic material is > 0.01% (mol:mol) and < 25% (mol:mol), preferably > 0.1% (mol:mol)and < 5% (mol:mol), most preferred > 0.5% (mol:mol) and < 1,5% (mol:mol). Here the term "mol:mol" denotes, that the doping level of the activators in mol is compared to the ceramic material in mol.
According to a preferred embodiment of the present invention, the at least one interference filter comprises one filter with a multitude of sub-layers. Preferably the number of sub-layers is > 10 and < 30, whereby the average thickness of each sub-layer is > 50 nm and < 400 nm, preferably > 100 nm and < 300 nm
The material(s) of the at least one interference layer are preferably chosen so that the at least one interference layer comprises an alternating layered structure with a difference in refractive index. According to a preferred embodiment of the present invention, the at least one interference filter comprises a material selected out of the group TiO2/SiO2, Si(VZnO, Al2(VTiO2, Al2O3/ZnO, HfO2/SiO2, HfO2/ Al2O3, Y2O3/TiO2, ZrO2/TiO2 or mixtures thereof.
Alternatively and/or additionally, the at least one interference filter comprises several layers which are essentially made out of resins, which may optionally comprise nanoparticles of a material with a high refractive index (preferred materials include Tiθ2 and ZnO).
According to a preferred embodiment of the present invention, the ceramic material comprises a material selected out of the group comprising - Ln2O3: Er, with
Ln being selected out of the group comprising Sc, Y, Gd, Lu and mixtures thereof, Ln2O3: Ho, Ln2O3: Tb, and - M^M11F6IMn with M1 being chosen out of the group comprising Li, K, Na or mixtures thereof Mπ being chosen out of the group comprising Si, Ge, Ti, Zr, Hf and mixtures thereof, or mixtures thereof.
Preferably the ceramic material comprises essentially out of this material.
However, in some applications, trace amounts of additives may also be present in the bulk compositions. These additives particularly include such species known to the art as fluxes. Suitable fluxes include alkaline earth - or alkaline - metal oxides and fluorides,
SiO2 and the like and mixtures thereof. According to a preferred embodiment of the present invention, the index of refraction of the at least one ceramic material is >1.4, preferably >1.7, most preferred
>1.8.
This has been shown to be advantageous for many applications within the present invention. It has surprisingly been found that the index of refraction may be changed voluntarily with the composition of the ceramic material for many application so that an "index matching" of the ceramic material with the further component of the light emitting device is possible in many cases.
According to a preferred embodiment of the present invention, the photothermal stability of the ceramic material is >80% to <100% after exposure of the ceramic material for 1000 hrs at 2000C with a light power density of 10W/cm2 and an average photon energy of 2.75 eV.
The term "photothermal stability" in the sense of the present invention especially means and/or includes the conservation of the specific absorption property under simultaneous application of heat and high intensity excitation, i.e. a photothermal stability of 100% indicates that the material is virtually unaffected by the simultaneous irradiation and heat up.
According to a preferred embodiment of the present invention, the photothermal stability of the ceramic material is >82.5% to <95%, preferably >85% to <97%, after exposure of the ceramic material for 1000 hrs at 2000C with a light power density of 10W/cm2 and an average photon energy of 2.75 eV.
According to a preferred embodiment of the present invention, the thermal conductivity of the ceramic material at room temperature is > 0.005 W Cm 1K"1 to < 0.75 W cm 1K"1
According to one embodiment of the present invention, the ceramic material shows a transparency for normal incidence in air of >10 % to <85 % for light in the wavelength range from > 650 nm to < 800 nm.
Preferably, the transparency for normal incidence is in air of >20 % to < 80 % for light in the wavelength range from > 550 nm to < 1000 nm, more preferred >30 % to <75 % and most preferred > 40% to < 70% for a light in the wavelength range from > 650 nm to < 800 nm.
The term "transparency" in the sense of the present invention means especially that > 10% preferably >20%, more preferred >30%, most preferred >40% and <85% of the incident light of a wavelength, which cannot be absorbed by the material, is transmitted through the sample for normal incidence in air. This wavelength is preferably in the range of > 570 nm and <800 nm or > 510 nm and <570 nm, for red and green ceramic materials, respectively.
According to a preferred embodiment of the present invention, the ceramic material has a density of >95% and < 101% of the theoretical density.
According to a preferred embodiment of the present invention, the ceramic material has a density of >97% and < 100% of the theoretical density. The densities lower than 100% according to the described preferred embodiment of the present invention are preferably obtained by sintering of the material to a stage where still pores are present in the ceramic matrix. Most preferred are densities in the range >98.0% and <99.8% with total pore volumes in the ceramic matrix within the >0.2 - <2% range. A preferred mean pore diameter is in the >400 - < 2500 nm range.
The present invention furthermore relates to a method of producing a material for a ceramic material for a light emitting device according to the present invention comprising a sintering step. The term "sintering step" in the sense of the present invention means especially densifϊcation of a precursor powder under the influence of heat, which may be combined with the application of uniaxial or isostatic pressure, without reaching the liquid state of the main constituents of the sintered material.
According to an embodiment of the present invention, the sintering step is without additional pressure, preferably in reducing or inert atmosphere.
According to an embodiment of the present invention, method furthermore comprises the step of pressing the precursor material(s) to > 50% to < 70 %, according to an embodiment of the present invention, > 55% to < 60 % of its theoretical density before sintering. It has been shown in practice that this improves the sintering steps for most materials as described with the present invention.
According to an embodiment of the present invention, the method of producing a material for a light emitting device according to the present invention comprises the following steps:
(a) Mixing the precursor materials for the material (b) optional firing of the precursor materials, preferably at a temperature of
>500°C to < 15000C to remove volatile materials
(c) optional grinding and washing
(d) a first pressing step, preferably a uniaxial pressing step at >10 kN using a suitable powder compacting tool with a mould in the desired shape (e.g. rod- or pellet-shape) and/ or a cold isostatic pressing step preferably at >3000 bar to < 3500 bar.
(e) a sintering step at >500 0C to < 17000C without additional pressure
(f) a hot pressing step, preferably a hot isostatic pressing step preferably at >100 bar to < 2500 bar and preferably at a temperature of >500 0C to < 20000C and/or a hot uniaxial pressing step preferably at >100 bar to < 2500 bar and preferably at a temperature of >500 0C to < 20000C.
(g) optionally a post annealing step at >500°C to < 17000C in inert atmosphere or air.
The present invention furthermore relates to a light emitting device, especially a LED comprising a ceramic material of the present invention.
A compound and/or a ceramic material according to the present invention may be of use in a broad variety of systems and/or applications, amongst them one or more of the following: - Office lighting systems household application systems shop lighting systems, home lighting systems, accent lighting systems, - spot lighting systems, theatre lighting systems, fiber-optics application systems, projection systems, self-lit display systems, - pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, and - decorative lighting systems portable systems automotive applications green house lighting systems advertisement lighting systems
The aforementioned components, as well as the claimed components and the components to be used in accordance with the invention in the described embodiments, are not subject to any special exceptions with respect to their size, shape, compound selection and technical concept such that the selection criteria known in the pertinent field can be applied without limitations.
DETAILED DESCRIPTION OF EMBODIMENTS
Additional details, features, characteristics and advantages of the object of the invention are disclosed in the sub claims and the following description of the examples, which —in an exemplary fashion— show several embodiments and examples of inventive light emitting devices.
Example I
Preparation of a LED according to a first inventive example: Y2O3:Ho is prepared by mix and fire techniques. The oxides are mixed in alcohol and dried at 1000C. Heating is performed at 1500 0C during 3 hours. The Er- concentration is 1 mole % (relative to Y). The synthesized powder is ball-milled for 24 h using ZrO2 balls in isopropanol as liquid medium during 24 hours. After drying, the material is heat treated for 8 h in air at 400 0C. Alternatively, also precipitation techniques, leading to much smaller primary phosphor particles, can be used, to prevent time consuming milling steps. The prepared powder is sintered to transparent ceramic by a hot-pressing process at 1500 0C for 4 h with a pressure of 30 MPa in an Ar atmosphere. This step can also be performed with the addition of a flux, like LiF in concentrations preferably between 0.01 - 5%.
A sample with dimensions IxIxO.5mm3 is cut from the ingot, polished at all sides and placed on a LED chip with lxlmm2 size, emitting at 440 nm. The thickness of the sample is chosen in such a manner that almost no blue light is leaking through (less than 2%). Switching on the LED results in green light.
Application of an interference filter (consisting of 20 layers of alternating SiC>2 and TiC^) on top of the transparent ceramic phosphor layer results in an increase in light intensity in the forward direction of more than a factor of 1.7.
Comparative example I : As a comparative example, a LED comprising a powder layer of
Y2O3 :Ho (grain size 2-8 μm) with the same thickness and Ho3+ concentration as in the inventive example is prepared.
However, an application of the LED according to the comparative example I did not result in efficient generation of green light. Almost no blue and green light leaves the powder layer in the forward direction, most of the light is reflected back to the LED and lost by scattering into undesired directions and absorption in LED and phosphor layer.
Comparative Example II
As a further comparative Example, a ceramic phosphor material consisting of YAG:Ce (broad band emission) in a manner analogous to the inventive example I.
A sample of 1x1x0.5 mm3 is applied to a blue emitting LED (emission 440 nm). Thickness and Ce3+ concentration were adjusted to obtain a transmission of blue light through the ceramics of about 2%. Switching on the LED results in yellow light.
Application of an interference filter (identical to the one described above) on top of the transparent ceramic phosphor layer results in an increase in light intensity in the forward direction of about a factor of 1.4.
This is far less than the optical gain of the inventive example I, making the inventive LEDs especially useful for devices in which a luminance gain in the forward direction is desired and/or advantageous. Surprisingly, it was also observed that the emission spectrum of Ce3+ in YAG was partially blue shifted when introducing the interference filter, approximately by 10 nm.
This effect could not be observed with any of the LEDs according to the present invention which is considered being an additional advantage of the present invention.
Inventive example II
K2TiF6 doped with Mn4+(5 mole%) is prepared by dissolving the hydroxides in diluted hydrofluoric acid (30-50 % HF) in Hydrofluoric acid resistant vessels, e.g. made out of Teflon, during heating and stirring. The luminescent material is isolated by evaporation of the reaction mixture to receive a dry powder. The phosphor is milled as described above. A ceramic layer is prepared by hot pressing for 24 hours at 100 Mpa at a temperature of 6000C.
Application of an interference filter (identical to the one described above) on top of the transparent ceramic phosphor layer results in an increase in light intensity in the forward direction of about a factor of 1.6.
The particular combinations of elements and features in the above detailed embodiments are exemplary only; the interchanging and substitution of these teachings with other teachings in this and the patents/applications incorporated by reference are also expressly contemplated. As those skilled in the art will recognize, variations, modifications, and other implementations of what is described herein can occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention's scope is defined in the following claims and the equivalents thereto. Furthermore, reference signs used in the description and claims do not limit the scope of the invention as claimed.

Claims

CLAIMS:
1. Light emitting device, comprising at least one essentially ceramic material and at least one interference filter in optical contact with said ceramic material, whereby said ceramic material comprises at least one OfEr3+, Tb3+, Ho3+ and Mn4+ as activator for emitting light.
2. The light emitting device of claim 1, whereby the doping level of the activators inside the ceramic material is > 0.01% (mol:mol) and < 25% (mol:mol).
3. The light emitting device of any of the claims 1 to 2, whereby the ceramic material comprises a material selected out of the group comprising
Ln2O3: Er, with Ln being selected out of the group comprising Sc, Y, Gd, Lu and mixtures thereof, Ln2O3: Ho, Ln2O3: Tb, and M1M11F6 :Mn with M1 being chosen out of the group comprising Li, K, Na or mixtures thereof
Mπ being chosen out of the group comprising Si, Ge, Ti, Zr, Hf and mixtures thereof, or mixtures thereof.
4. The light emitting device of any of the claims 1 to 3, whereby the index of refraction of the at least one ceramic material is >1.4. The light emitting device of any of the claims 1 to 4, whereby the photothermal stability of the ceramic material is >80% to <100% after exposure of the ceramic material for 1000 hrs at 2000C with a light power density of 10W/cm2 and an average photon energy of 2.75 eV.
5. The light emitting device of any of the claims 1 to 5, whereby the thermal conductivity of the ceramic material is > 0.005 W Cm 1K"1 to < 75W Cm 1K"1
6. The light emitting device of any of the claims 1 to 6, whereby the ceramic material has a density of >95% and < 101% of the theoretical density.
7. A system comprising a light emitting device according to any of the claims 1 to 7, the system being used in one or more of the following applications:
- Office lighting systems household application systems shop lighting systems, home lighting systems, accent lighting systems, - spot lighting systems, theatre lighting systems, fiber-optics application systems, projection systems, self-lit display systems, - pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, and - decorative lighting systems portable systems automotive applications green house lighting systems Advertisement systems
PCT/IB2009/050545 2008-02-12 2009-02-10 Light emitting device comprising a ceramic material with line emitter activators and an interference filter WO2009101578A1 (en)

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