Luminescent converter, lighting device, lamp and luminaire

Abstract

Luminescent converter (100) comprising a matrix (410) and luminescent particles (110,120) dispersed in the matrix. The matrix comprises a polymeric material. The luminescent particles comprise a core (130) of a first luminescent material (160) and a shell (140) of a second luminescent material (170). The core is for at least 70% enclosed by the shell. The first luminescent material comprises a manganese 4+-activated hexafluorosilicate phosphor and the second luminescent material comprises a cerium activated aluminum garnet phosphor.

Description

[0001] Luminescent converter, lighting device, lamp and luminaire

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a luminescent converter, a lighting device comprising the converter, and a lamp or luminaire comprising the lighting device.

[0004] BACKGROUND OF THE INVENTION

[0005] Phosphor-converted white light-emitting diodes (generally referred to as LEDs) are becoming increasingly popular for general lighting. These types of white light emitting LEDs include one or more photoluminescence materials, typically inorganic phosphor materials, which absorb a portion of the excitation light, most conveniently blue light emitted by the LED, and re-emit light of a different color at longer wavelength. Manganese 4+ activated alkaline / alkaline earth fluoride phosphors in the narrow-band ASF:Mn4+class, generally referred to as ASF:Mn phosphors, such as K2SiFe:Mn4+, generally referred to as KSF:Mn, are desirable as a red phosphor for lighting applications.

[0006] Yet, a disadvantage of ASF:Mn phosphors is their too fast reduction or loss of their photoluminescence emission, i.e. quantum efficiency, of the phosphor because of their sensitivity for water or moisture, which causes damage to the dopant manganese 4+ Furthermore, the reaction of the fluoride-based compound, with water (or moisture) can generate a very corrosive hydrofluoric acid that can react with LED packaging material thereby leading to component failure. Hence, there is a need to protect the ASF:Mn phosphors, such as KSF:Mn, against exposure to water and moisture present in the ambient atmosphere. Current ways of protection of KSF:Mn particles against ambient atmosphere, i.e. against moisture and / or water, occur either via coating the KSF:Mn particles with Mn-free KSF, or with various inorganic materials like e.g. silicone, AI2O3, or with a polymer material. Alternatively, in a stacked layer structure on a LED die, protection of the KSF:Mn layer is attained in that said KSF:Mn layer is enclosed by, i.e. sandwiched between, the LED-die and a green emitting garnet: Ce3+class phosphor, generally referred to as garnet: Ce phosphors, i.e. an (Y 3-xCex)AlsOi2 generally referred to as YAG:Ce phosphor layer. SUMMARY OF THE INVENTION

[0007] It is an object of the invention to improve the level of the performance of the luminescence properties of the known ASF:Mn phosphors during lifetime. These and other objects may be achieved by a luminescent converter in accordance with the independent claim. Embodiments of the luminescent converter according to the present invention are defined in the dependent claims. Hence, according to an aspect of the present invention, there is provided a luminescent converter comprising a matrix and luminescent particles dispersed in the matrix, wherein the matrix comprises a polymeric material; wherein the luminescent particles comprises a core of a first luminescent material and a shell of a second luminescent material; wherein the core is for at least 70% enclosed by the shell, such as for at least 80%, preferably for at least 90%, more preferably for at least 95%, most preferably fully enclosed (i.e. 100%); wherein the first luminescent material comprises an ASF:Mn-activated phosphor; and wherein the second luminescent material comprises a garnet: Ce activated phosphor. Or expressed shortly, the luminescent converter comprises in a matrix dispersed particles and / or flakes of an ASF:Mn activated phosphor individually protected by a garnet: Ce activated phosphor. The ASF:Mn phosphor thus is protected against water and moisture by the gametCe phosphor, such as YAG:Ce or (Lu3-xCex)AlsOi2 generally referred to as LuAG:Ce, similar to the protection provided by a polymer, inorganic material, or nonphosphor coating. Yet, said protection by the garnet: Ce phosphor provides additional advantages, as will be explained below at the hand of the representatives examples YAG:Ce and KSF:Mn.

[0008] KSF:Mn and YAG:Ce, for example, are often used in combination with high power blue emitting LEDs, having the majority of its emission peaking at about 450 nm, to provide high brightness white light. The excitation spectra of YAG:Ce and KSF:Mn both peak at about 450 nm. The luminescence emission of the KSF:Mn is a parity- and spin- forbidden d^d transition, leading to decay times of the KSF:Mn luminescence being in the order of about 10 ms at room temperature. The result of this relatively (very) long decay time is that both saturation and degradation of the KSF:Mn phosphor occur relatively fast when KSF:Mn is directly exposed to relatively high excitation power, i.e. an intensity of excitation power of about 40W / cm2 and more.

[0009] YAG:Ce on the other hand has a spin parity-allowed d— >f transition of both excitation by about 450 nm and emission of about 530 nm. This leads to a decay time for YAG:Ce of about 30 ns, i.e. about 300000 times faster than the decay time of KSF:Mn. Saturation and / or degradation of the YAG:Ce therefore does hardly or not occur at excitation power intensities up to 1000 W / cm2. Hence, enclosing the KSF:Mn with YAG:Ce protects the KSF:Mn against too high intensity excitation power by high power blue emitting LEDs, and hence against saturation and relatively quick degradation of the manganese 4+ activated phosphor.

[0010] Furthermore, there is no / hardly absorption by the KSF:Mn of the emitted "green" radiation of YAG:Ce and also no / hardly absorption by the YAG:Ce of the emitted "red" radiation of KSF:Mn. By choosing the right amount of absorption by YAG:Ce and KSF:Mn of the "blue" radiation as emitted by the LED die, an ideal combination of red, green, and blue emission (RGB emission) is obtained that renders white light. By selecting the amounts of blue radiation being absorbed (and hence subsequently being converted) by YAG:Ce, respectively KSF:Mn (and hence also the amount of blue radiation not being absorbed but being emitted) the (correlated) color temperature of the RGB emission can be set. The amount of radiation absorbed by YAG:Ce and KSF:Mn can be set by variations in layer / shell / core thicknesses, the concentration of particles / flakes in the polymeric / ceramic binder, the concentration of the luminescent particles in the matrix, and / or the concentration of activator ions Ce3+ and Mn4+ in respectively YAG:Ce and KSF:Mn.

[0011] As is clear from the above, the luminescent materials in the luminescent particles comprises two general classes of phosphor material, i.e. a gametiCe class phosphors and a narrow-band ASF:Mn class phosphors. These classes can be generally defined / described as follows:

[0012] - Garnet: Ce class:

[0013] Luminescent material of the type A3BsOi2:Ce3+, wherein A in embodiments comprises one or more of Y, La, Gd, Tb and Lu, especially (at least) one or more of Y, Gd, Tb and Lu, and wherein B in embodiments comprises one or more of Al, Ga, In and Sc. Especially, A may comprise one or more of Y, Gd and Lu, such as especially one or more of Y and Lu. Especially, B may comprise one or more of Al and Ga, more especially at least Al, such as essentially entirely Al. Hence, especially suitable luminescent materials are cerium comprising garnet materials. Embodiments of garnets especially include A3B5O12 garnets, wherein A comprises at least yttrium or lutetium and wherein B comprises at least aluminum. Such garnets may be doped with cerium (Ce), with praseodymium (Pr) or a combination of cerium and praseodymium; especially however with Ce. Especially, B may comprise aluminum (Al); however, in addition to aluminum, B may also partly comprise gallium (Ga) and / or scandium (Sc) and / or indium (In), especially up to about 20% of B, more especially up to about 10 % of B (i.e. the B ions essentially consist of 90 or more mole % of Al and 10 or less mole % of one or more of Ga, Sc and In); B may especially comprise up to about 10% gallium. In another variant, B and O may at least partly be replaced by Si and N. The element A may especially be selected from the group consisting of yttrium (Y), gadolinium (Gd), terbium (Tb) and lutetium (Lu). Further, Gd and / or Tb are especially only present up to an amount of about 20% of A. In a specific embodiment, the garnet luminescent material comprises (Y i-xLux)3BsOi2:Ce, wherein x is equal to or larger than 0 and equal to or smaller than 1. The term “:Ce”, indicates that part of the metal ions (i.e. in the garnets: part of the “A” ions) in the luminescent material is replaced by Ce. For instance, in the case of (Y i- xLux)3AlsOi2:Ce, part of Y and / or Lu is replaced by Ce. This is known to the person skilled in the art. Ce will replace A in general for not more than 10%; in general, the Ce concentration will be in the range of 0.1 to 4%, especially 0.1 to 2% (relative to A). Assuming 1% Ce and 10% Y, the full correct formula could be (Yo.iLuo.89Ceo.oi)3A150i2. Ce in garnets is (essentially) only in the trivalent state, as is known to the person skilled in the art.

[0014] - ASF:Mn class:

[0015] Luminescent material of the type M’xM2-2xAX6 doped with tetravalent manganese, wherein M’ comprises an alkaline earth cation, M comprises an alkaline cation, and x is in the range of 0-1, wherein A comprises a tetraval ent cation, for instance comprising one or more of silicon and titanium, wherein X comprises a monovalent anion, at least comprising fluorine.

[0016] Relevant alkaline cations (M) are sodium (Na), potassium (K) and rubidium (Rb). Optionally, also lithium and / or cesium may be applied. In a preferred embodiment, M comprises at least potassium. In yet another embodiment, M comprises at least rubidium. The phrase “wherein M comprises at least potassium” indicates for instance that of all M cations in a mole M’xM2-2xAX6 , a fraction comprises K+ and an optionally remaining fraction comprises one or more other monovalent (alkaline) cations (see also below). In another preferred embodiment, M comprises at least potassium and rubidium. Optionally, the M’xM2-2xAX6 luminescent material has the hexagonal phase. In yet another embodiment, the M’xM2-2xAX6 luminescent material has the cubic phase. For x=0, the composition is M2AX6.

[0017] Relevant alkaline earth cations (M’) are magnesium (Mg), strontium (Sr), calcium (Ca) and barium (Ba), especially one or more of Sr and Ba. The term “tetravalent manganese” refers to Mn4+. This is a well-known luminescent ion. In the formula as indicated above, part of the tetravalent cation A (such as Si) is being replaced by manganese. Hence, M’XM2-2XAX6 doped with tetraval ent manganese may also be indicated as M’XM2-2XAI- mMnmXe. The mole percentage of manganese, i.e. the percentage it replaces the tetravalent cation A will in general be in the range of 0.1-15 %, especially 1-12 %, i.e. m is in the range of 0.001-0.15, especially in the range of 0.01-0.12.

[0018] As indicated above, X relates to a monovalent anion, but at least comprises fluorine. Other monovalent anions that may optionally be present may be selected from the group consisting of chlorine (Cl), bromine (Br), and iodine (I).

[0019] In an embodiment, M’xM2-2xAX6 comprises K2SiF6 (indicated herein also as KSF system).

[0020] As indicated above, in another preferred embodiment, M’xM2-2xAX6 comprises KRbSiFe (herein also indicated as K,Rb system). As indicated above, part of silicon is replaced by manganese (i.e. the formula may also be described as K2Sii-mMnmF6 or KRbSii- mMnmFe, with m as indicated above, or as KRbSiFe:Mn and K2SiFe:Mn, respectively). As manganese replaces part of a host lattice ion and has a specific function, it is also indicated as “dopant” or “activator”. Hence, the hexafluorosilicate is doped or activated with manganese (Mn4+).

[0021] In specific embodiments, the first luminescent material may comprise K2SiFe:Mn4+. Alternatively or additionally, in embodiments the first luminescent material may comprise (K,Rb)2SiF6:Mn4+. Alternatively or additionally, in embodiments the first luminescent material may comprise K2TiFe:Mn4+. In embodiments, the first luminescent material may comprise K2(Si,Ti)Fe:Mn4+. As can be derived from the above, “Si,Ti” may indicate one or more of Si and Ti.

[0022] The luminescent converter may have the feature that the ASF:Mn phosphor upon excitation with blue light (i.e. light having a peak emission at about 450 nm) generates first converted light with a peak emission wavelength in a wavelength range from 625 nm to 635 nm, such as 627 nm to 633 nm or 629 nm to 632 nm, and a full-width-half-maximum equal to or smaller than 30 nm, and wherein the gametCe phosphor upon excitation with said blue light generates second converted light with a peak emission wavelength in a wavelength range from 510 nm - 580 nm, such as 510 nm to 540 nm or 540 nm to 580 nm, and a full- width-half-maximum (FWHM) equal to or larger than 80 nm, such as >= 90 nm, >= 95 nm or >= 100 nm. The emission of the ASF:Mn phosphor at about 630 nm with such a small FWHM is perceived as saturated red and the human eye sensitivity is still quite high in this wavelength range, which is beneficial for lighting applications. A combination of the ASF:Mn phosphor with green-yellow emitting gametCe phosphors, having its peak emission at about 530 nm upon excitation with said blue light, are very attractive and desirable in general lighting applications for providing efficient white lighting with a high general Color Rendering Index (CRI).

[0023] The luminescent converter may have the feature that the luminescent particles have a spherical-like shape and wherein the core is fully enclosed by the shell. In such particles the protection of the ASF:Mn phosphor against both water / moisture and intense excitation radiation is further improved. Typically, the garnet: Ce phosphor and the ASF:Mn phosphor particles have sizes in the range of about 10 micrometer or less, preferably 5 micrometer or less, down to about 0.5 micrometer, enabling to embed the particles in relatively thin layers of the matrix of polymeric material, for example, in polymer layers having a thickness in the range of 25 to 250 micrometer or 25 to 150 micrometer. The matrix typically comprises luminescent materials embedded in a polymer material.

[0024] The binder may comprise an inorganic binder e.g. a sol-gel or glass based material or an organic binder e.g. poly methyl methacrylate (PMMA), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC) polymer matrix. The binder may be a silicon based polymer. The silicone based polymer may be obtained from a material from the group of: Tetramethoxysilane, Tetraethoxysilane, trimethoxy(methyl)silane, and triethoxy(methyl)silane.

[0025] Spherical-like particles can for example be manufactured with a sol-gel process which is a wet-chemical process.

[0026] The luminescent converter may have the feature that the shell has an average radial thickness and the core has an average diameter, wherein the average diameter of the core is larger than the average radial thickness of the shell, preferably the average diameter of the core is at least 1.5 times as large as the average radial thickness of the shell. Thus a minimum ratio between the ASF:Mn phosphor and the gametiCe phosphor is ensured. Thus a convenient working range is obtained for attaining a white emission spectrum by the LED die-phosphor converter combination through variation in cerium and / or manganese dopant concentration in respectively the gametiCe phosphor and the ASF:Mn phosphor.

[0027] The luminescent converter may have the feature that the effective (average) diameter of the luminescent particles is in a range from 2-300 micrometer, such as 5 to 150 micrometer, 10 to 100 micrometer, or 30 to 70 micrometer, i.e. have an equivalent sphere size in the range of 3-300 micrometer, such as 5 to 150 micrometer, 10 to 100 micrometer, or 30 to 70 micrometer. Particles are three-dimensional objects, and unless they are perfect spheres, for example emulsions or bubbles, they cannot be fully described by a single dimension such as a radius or diameter. For the sake of simplification, it is often convenient to define the particle size using the concept of equivalent spheres. It is important to realize that different measurement techniques use different equivalent sphere models and therefore will not necessarily give exactly the same result for the particle diameter. The equivalent sphere concept works very well for regular shaped particles, wherein many particle sizing techniques are based on a simple one-dimensional sphere equivalent measuring concept. However, it may not always be appropriate for irregular shaped particles, such as needles or plates, where the size in at least one dimension can differ significantly from that of the other dimensions. Yet, in the context of this invention the particle size is defined by the diameter of an equivalent sphere, generally referred to as effective diameter, having the same property as the actual particle, such as having the same volume or mass, for example.

[0028] The luminescent converter may have the feature that the luminescent particles have a flake-like shape and comprise a stack of layers stacked along a height H of the flakeshape, with height H being the height of the side-walls of the flake, said stack comprising a first outer layer, a second outer layer and an intermediate layer, wherein the core comprises the intermediate layer comprising the ASF:Mn phosphor and wherein the shell comprises the first outer layer and the second outer layer comprising the garnet: Ce phosphor. The flakes have the advantage over spherical-like particles that their manufacturing is somewhat simpler. For example, the flakes can be manufactured by stacking proposed layers on top of each other using a deposition technique such as doctor blading, spin coating. Subsequently, phosphor flakes can be produced by using grinding, ball milling or cutting technologies. The staked of layers may also be sintered. In the flakes, the ASF:Mn phosphor is not fully enclosed by the gametCe phosphor, yet the ASF:Mn phosphor still for the fast major part is protected against water / moisture and intense blue excitation radiation by the garnet: Ce phosphor.

[0029] By choosing process parameters in the deposition technique and in the subsequent grinding process, respectively the (individual) layer thicknesses of the stacked layers and the size / dimensions of the flakes can be controlled. Thus, the luminescent converter may have the feature that the flake-like shape further has a length L and a width W, the flake shape having a first aspect ratio L / H of at least 5, and having a second aspect ratio W / H of at least 5, preferably at least 10, more preferably at least 15 most preferably at least 20. Furthermore, the luminescent converter may have the feature that L is in a range from 50- 1000 micrometer such as 100-700 micrometer or 250-500 micrometer, H is in a range from 5-100 micrometer such as 7-70 micrometer or 10-40 micrometer, W is in a range from 50- 1000 micrometer such as 100-700 micrometer or 250-500 micrometer. At these thicknesses of the stacked layers and / or dimensions of the flakes it is attained that a ratio between the ASF:Mn phosphor that is protected by the gametCe phosphor and unprotected ASF:Mn phosphor of the side walls, is large enough for the luminescent converter to sufficiently perform during lifetime.

[0030] The luminescent converter may have the feature that the thickness of the intermediate layer is larger than the thickness of the first outer layer and larger than the thickness of the second outer layer, preferably the intermediate layer is at least 1.5 times larger than the thickness of both the first outer layer and the second outer layer. The layer of the first luminescent material is sandwiched between the two layers of second luminescent material. To obtain a good ratio in the amount of emission by the core and the shell in a flake-shaped particle typically the thickness of the first luminescent material in the shell preferably is higher than the thickness of the second luminescent material in the core to obtain about the same volumes of first and second luminescent materials. More preferably the intermediate layer is about two times the thickness of both the first and second outer layer.

[0031] The luminescent converter may have the feature that the first luminescent material is a KSF:Mn phosphor and the second luminescent material is YAG:Ce and / or LuAG:Ce. From the known ASF:Mn phosphors and garnet: Ce phosphors these are the most commonly used phosphors. These phosphors have the advantage that they are relatively stable and that their manufacturing process and lifetime properties are best known. Yet, suitable alternative ASF:Mn phosphors are selected from the group consisting of: K2GeF6:Mn4+, K2TiF6:Mn4+, K2SnF6:Mn4+, Na2TiF6:Mn4+, Na2ZrF6:Mn4+, Cs2SiF6:Mn4+, Cs2TiF6:Mn4+, Rb2SiF6:Mn4+, and Rb2TiF6:Mn4+.

[0032] The luminescent converter may have the feature that the first luminescent material in the core has a first concentration, Cl, the second luminescent material in the shell has a second concentration, C2, and C2 < Cl, preferably C2 <= 0.8*Cl, more preferably C2 <= 0.5*Cl. Both the core and the shell typically need not to consist of only luminescent material, but may comprise luminescent material embedded in a ceramic or organic binder. The concentration of the first luminescent material in the binder may be different from the concentration of the second luminescent material in the binder. The binder for the first luminescent material may be different from or may be the same as the binder for the second luminescent material. Typically the volume of the shell tends to be different from, i.e. much larger than, the volume of the core of a particle, This similarly applies for a flake, wherein the layer of the first luminescent material is sandwiched between the two layers of second luminescent material. To obtain a good ratio in the amount of emission by the core and the shell typically the concentration of the first luminescent material in the shell preferably is higher than the concentration of the second luminescent material in the core to compensate said difference in volume. Still, Cl and C2 may each be 100%, yet typically may be in the range of 20%-99% in volume.

[0033] The luminescent converter may have the feature that the weight ratio in the matrix of the luminescent particles to the polymeric material is in a range 2% to 10%, for example in a range of 2% to 6% or 2.5% to 5%. The effect obtained by this range that a desired ration between conversion of the spectrum wavelength emitted by the LED and the transmission of said spectrum through the matrix is realized.

[0034] The luminescent converter may have the feature that the matrix has a first sublayer and a second sub-layer, wherein the concentration of the luminescent particles in the first sub-layer is at least two or at least three times higher than in the second sub-layer. The luminescent particles are embedded in a (polymer) matrix. The luminescent particles were added to the polymer matrix when this still had a low viscosity (i.e. being relatively fluid) enabling the luminescent particles (having a higher specific density than the specific density of the matrix) to slowly settle towards the LED die. Said settling of the luminescent particles involves that two sublayers in the "solidified" sublayers in the polymer matrix are formed. The two sublayers comprise a first sublayer which comprises a relatively high concentration of flakes, and a second sublayer on top of the first sublayer which comprises hardly / no flakes. Typically, the concentration of luminescent particles in the first sublayer is at least three times higher than the concentration of luminescent particles in the second sublayer.

[0035] The invention further relates to a lighting device configured to provide in operation device light and comprising a LED light source and a luminescent converter according to the invention, wherein the LED light source emits LED light source light having a peak emission wavelength in a range from 420 nm to 490 nm; wherein the luminescent particles are configured such that (i) the shell is configured to partly convert the LED light source light into second converted light and to partly transmit the LED light source light and (ii) the core is configured to partly convert the LED light source light into first converted light; and wherein the device light comprises the first converted light, the second converted light and optionally part of the LED light source light. The lighting device could, for example, be a LED package comprising a LED die mounted in a (white) light reflective cup, which is filled with a polymer matrix in which luminescent particles are embedded. Alternatively, or additionally, the lighting device could, for example, be a LED filament, a Chip On Board LED strip (COB LED strip), or a strip with surface mounted device LEDs (SMD LEDs). The invention still further relates to a lamp or a luminaire comprising the lighting device according to the invention. The luminescent converter is very suitable for application in lighting devices, lamps and luminaires as these typically are used for providing white light for general illumination.

[0036] BRIEF DESCRIPTION OF THE DRAWINGS

[0037] This and other aspects of the present invention will now be described in more detail, with reference to the appended, schematic drawings showing embodiments of the present invention, in which:

[0038] Fig. la shows the excitation spectrum and the emission spectrum of YAG:Ce;

[0039] Fig. lb shows the excitation spectrum and the emission spectrum of KSF:Mn;

[0040] Fig. 1c shows an example of an emission spectrum of a blue light emitting LED;

[0041] Fig. Id shows a white light emission spectrum a combination of a blue light emitting LED and a luminescent converter according to the invention;

[0042] Fig. 2a shows a cross sectional view of a luminescent converter according to the invention embodied as a spherical-like shaped particle.

[0043] Fig. 2b shows a perspective view of a luminescent converter according to the invention embodied as a flake-shaped particle;

[0044] Fig. 3 shows a LED package comprising a luminescent converter according to the invention;

[0045] Fig. 4 shows a LED package comprising a matrix with sub-layers;

[0046] Fig. 5a shows a lamp according to the invention;

[0047] Fig. 5b shows a luminaire according to the invention.

[0048] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] Fig. la shows the excitation spectrum 10 and the emission spectrum 20 of Y2.9A150i2:Ceo.i (referred to as YAG:Ce) at room temperature. As visible, the excitation spectrum 10 of YAG:Ce peaks at about 450 nm and upon excitation by 450 nm radiation, the emission spectrum 20 of YAG:Ce is for the fast major part in the green-yellow part of the spectrum and peaks at about 560 nm. The YAG:Ce emission spectrum 20 has a FWHM of at least 80 nm, i.e. in the figure the FWHM of the emission spectrum 20 of YAG:Ce is about 110 nm. As further visible there is no absorption by YAG:Ce in the 525-700 nm wavelength range, hence YAG:Ce essentially does not overlap with the emission spectrum 40 of KSF:Mn (shown in fig. lb).

[0050] Fig. lb shows the excitation spectrum 30 and the emission spectrum 40 of KSF:Mn. Like the excitation spectrum 10 ofYAG:Ce, the excitation spectrum 30 of KSF:Mn is for the fast major part in the blue wavelength range and peaks at about 450 nm. Upon excitation with 450 nm radiation, the emission of the KSF:Mn phosphor is mainly at about 630 nm with a small FWHM. The emission of KSF:Mn is perceived as saturated red and the human eye sensitivity is still quite high in this wavelength range, which is beneficial for lighting applications. A combination of the narrow-band red emission spectrum 40 of KSF:Mn with the green-yellow emission spectrum 20 of YAG:Ce (having its peak emission at about 530 nm upon excitation with 450 nm radiation, see Fig. la), is very attractive and desirable in general lighting applications for providing efficient white lighting with a high general Color Rendering Index (CRI). As visible there is essentially no absorption by KSF:Mn in the 500-600 nm wavelength range, hence the absorption of (visible) light by KSF:Mn essentially does not overlap with the emission spectrum 20 of YAG:Ce. The excitation and emission characteristics of both YAG:Ce and KSF:Mn are such that they form an ideal combination with a blue LED, peaking at about 450 nm, for generating white light in an efficient manner and having a relatively good CRI.

[0051] Fig. 1c shows an example of a relatively narrow-band emission spectrum of a blue LED having an LED light source light emission spectrum 60 essentially only in the blue wavelength range peaking at about 450 nm and with a FWHM of about 40 nm. The LED is a high-brightness InGaN-based blue LED. The LED structures were grown by Metalorganic Chemical Vapor Deposition on sapphire substrates. All the structures consisted of an undoped GaN layer followed by a Si-doped n-GaN contact layer, undoped InGaN / GaN MQW active region, and a Mg-doped p-GaN contact layer.

[0052] FIG. Id shows an example of a combined emission spectrum 80, of a blue LED peaking at about 450 nm, Lu2.95AlsOi2:Ceo.o5 (LuAG:Ce) peaking at about 530 nm and KSF:Mn. The combined emission spectrum 80, being the emission spectrum 80 of the device light, lamp light and / or luminaire light, provides white light with a color temperature Tc of about 3450 K, with a color rendering index (CRI) of about 90, and with a luminous efficiency of radiation about 300 Im / W.

[0053] Fig. 2a schematically shows a cross sectional view of a core-shell type, spherical-like shaped particle 110 suitable for use in a luminescent converter according to the invention. The spherical -like particle 110 comprises as a core 130 a first luminescent material 160 of KSF:Mn and as a shell 140 a second luminescent material 170 of YAG:Ce. The core 130 has an average core diameter CD and the shell 140 has an average radial thickness SRT, here the average core diameter CD is about three times the radial thickness SRT of the shell 140. The average size of the spherical particles 110 is in the range of 3 to 200 micrometer. The core 130 is fully enclosed by the shell 140, hence the KSF:Mn 160 is fully protected by the YAG:Ce 170 of the shell 140 against water / moisture and additionally protected against the blue light excitation of about 450 nm of too high intensity. When embedded in a matrix comprising a polymeric material (see figs. 3, 4), these core-shell type, spherical-like shaped particles 110 form a luminescent converter, which, for example, may be applied in a LED package according to the invention (see figs. 3 and 4).

[0054] Fig. 2b schematically shows a perspective view of a flake-like shaped particle 120 (generally referred to as flake) suitable for use in a luminescent converter according to the invention. The flake 120 comprises a layer stack 180 of layers stacked along a height H of the flake 120, i.e. a side-wall 120a of the flake 120 has the height H. Said layer stack 180 comprises as a shell 140 a first outer layer 140a and a second outer layer 140b and as a core 130 an intermediate layer 130a. The intermediate layer 130a comprises the ASF:Mn phosphor 160 embedded in a binder 150, and the first 140a and second outer layer 140b comprise the gametCe phosphor 170, here Lu2..99AlsOi2:Ceo.oi (LuAG:Ce), embedded in the same binder 150. Thus, except for the portion of the core 130 at the side wall 120a of the flake 120, the core 130 which comprises the ASF:Mn phosphor is largely, i.e. for at least 70% and here for about 90%, enclosed by the shell 140 comprising the gametCe phosphor. The flake 120 further has a length L and a width W in directions perpendicular to each other and perpendicular to height H, with the flake 120 having a first aspect ratio L / H of at least 5, and having a second aspect ratio W / H of at least 5. Typically L is in a range from 50-1000 micrometer, H is in a range from 5-100 micrometer, W is in a range from 50-1000 micrometer. The thickness (Ti) of the intermediate layer 130a is larger than the thickness (Tol, To2) of both the first 140a outer layer and the second outer layer 140b, preferably the intermediate layer 130a is at least 1.5 times larger than the thickness of both the first outer layer 140a and the second outer layer 140b.

[0055] Fig. 3 shows a schematic drawing of a LED package 500 comprising a luminescent converter 100 according to a first embodiment of the invention. The LED package 500 comprises a LED die 300 configured to emit in operation blue LED light source light having a peak wavelength at about 450 nm. The LED die 300 is mounted in a (white) light reflective cup 400, which is filled with a matrix 410 comprising a polymeric material in which luminescent particles 110, here particles with a spherical-like shape, are embedded in a homogeneous manner. The spherical-like particles 110 have a similar structure as described in relation to Fig. 2a, yet its shell 140 comprising LuAG:Ce 170 as the gametCe phosphor and its core 130 comprising KSF:Mn 160 as the ASF:Mn phosphor. The KSF:Mn phosphor 160 and LuAG:Ce phosphor 170 in the luminescent particles 110 are configured to partly convert the LED light source light into respectively first and second converted light.

[0056] Fig. 4 shows a schematic drawing of a LED package 500 comprising a luminescent converter 100 according to a second embodiment of the invention. The LED package 500 comprises a LED die 300 configured to emit in operation blue LED light source light having a peak wavelength at about 450 nm. The LED die 300 is mounted in a (white) light reflective cup 400, which is filled with a matrix 410 of polymeric material in which luminescent particles 120, here particles with a flake-like shape (generally referred to as flakes 120), are embedded. The reflector cup has a bottom surface 400a at which the LED die 300 is arranged and an open top face 400b opposite the bottom surface 400a. The flakes 120 have a similar structure as described in relation to Fig. 2b, yet its shell comprising the YAG:Ce phosphor and its core comprising the KSF:Mn phosphor. The luminescent flakes 120 were added to the polymer matrix 410 when this still had a low viscosity (i.e. being relatively fluid) enabling the flakes 120 (having a higher specific density than the specific density of the matrix) to slowly "swirling down" (under the influence of gravity) towards the LED die 300. Because of the leafing properties of the flakes 120, the flakes 120 orient themselves during swirling such that the height H of the flakes 120 extend in a direction more or less perpendicular to the bottom surface 400a and / or the top face 400b of the reflector cup 400. The orientation of the flakes 120 thus attained has the additional advantage that the exposed portion of the intermediate layer of KSF:Mn at the side-wall 120a (see fig. 2b) of the flakes 120 is not / hardly directly exposed to the high intensity of the blue radiation emitted by the LED die 300. Furthermore, said swirling involves that two sublayers 410a, 410b in the "solidified" sublayers in the matrix 410 of polymeric material are formed. The two sublayers 410a, 410b comprise a first sublayer 410a approximate to the bottom surface 400a (and remote from the top face 400b) and which comprises a relatively high concentration of flakes 120, and a second sublayer 410b (on top of the first sublayer 410a) remote from the bottom surface 400a (and approximate to the top face 400b) which comprises hardly / no flakes 120. As shown in the figure, the concentration of flakes 120 in the first sublayer 410a is about five times higher than the concentration of flakes 120 in the second sublayer 410b. Fig. 5a shows schematic view of a lamp 1000 according to the invention. The lamp 1000 is a LED filament lamp comprising a light transmissive envelope 1010 provided with lamp cap 1020 having electrical contacts 1030,1040 for electrically connecting to a power source, and optionally a sensor and / or controller (not shown). A lighting device 1080 comprising a plurality of LED filaments 1060 mounted on a frame-stem combination 1050 is arranged in a light transmissive envelope 1010, the LED filaments 1060 are mechanically and electrically connected to the lamp cap 1020 via the frame-stem combination 1050. Each LED filament 1060 comprises an elongated luminescent converter 100 according to the invention. The elongated luminescent converter 100 comprising, for example, luminescent particles of the type as described in Fig. 2a, which are embedded in a light transmissive matrix of polymeric material.

[0057] Fig. 5b shows a schematic, cross-sectional of a luminaire 2000 according to the invention. The luminaire 2000 comprises a housing 2010 accommodating a visible light reflective reflector 2020 and lighting device 2030 comprising two LED packages 500 of Fig. 4b, said LED packages 500 are arranged on a PCB 2040 at a bottom 2020a of the reflector 2020. The reflector 2020 has a light exit window 2020b opposite the bottom 2020a, the light exit window 2020b being provided with a cover 2050. Said cover 2050 may be an optical element to (further) shape / collimate the light issued by the LED packages 500 during operation into a shaped luminaire light beam along an optical axis OA of the reflector 2020. After (partial) collimation of the device light 2100 by the reflector 2020, the device light 2100 is exiting the luminaire 2000 through the light exit window 2020b as luminaire light (having, for example, the combined spectrum 80 as shown in fig. Id). The lighting device 2030 of the luminaire 2000 further comprises a driver / controller 2060 to control the light output by the LED packages 500, the LED packages 500 may be individually controlled or controlled as a group. The control may include, for example, an occupancy or ambient light sensor for automatic control, or a remotely controlled sensor for receiving control signals from a (handheld) remote control device.

Claims

CLAIMS1. Luminescent converter (100) comprising a matrix (410) and luminescent particles (110,120) dispersed in the matrix, wherein the matrix comprises a polymeric material; wherein the luminescent particles comprises a core (130) of a first luminescent material (160) and a shell (140) of a second luminescent material (170); wherein the core is for at least 70% enclosed by the shell; wherein the first luminescent material comprises a manganese 4+-activated hexafluorosilicate phosphor (160); wherein the second luminescent material comprises a cerium activated aluminum garnet phosphor (170), and wherein the luminescent particles (120) have a flake-like shape and comprise a layer stack (180) of layers (130a, 140a, 140b) stacked along aheight H of the flake (120), said layer stack comprising a first outer layer (140a), a second outer layer (140b) and an intermediate layer (130a), wherein the core (130) comprises the intermediate layer comprising the manganese 4+-activated hexafluorosilicate phosphor (160) and wherein the shell (140) comprises the first outer layer and the second outer layer comprising the cerium activated aluminum garnet phosphor (170).

2. The luminescent converter (100) of claim 1, wherein the manganese 4+- activated hexafluorosilicate phosphor(160) generates first converted light with a peak emission wavelength in a wavelength range from 625 nm to 635 nm and a full-width-half- maximum equal to or smaller than 30 nm, and wherein the cerium activated aluminum garnet phosphor (170) generates second converted light with a peak emission wavelength in a wavelength range from 510 - 580 nm and a full-width-half-maximum equal to or larger than 80 nm.

3. The luminescent converter (100) according to claim 1 or 2, wherein the flakes (120) with the flake-like shape further has a length L and a width W, the flake shape having afirst aspect ratio L / H of at least 5, and having a second aspect ratio W / H of at least 5, wherein W and L extend in directions perpendicular to each other and perpendicular to height H.

4. The luminescent converter (100) according to claim 3, wherein L is in a range from 50-1000 micrometer, H is in a range from 5-100 micrometer, and W is in a range from 50-1000 micrometer.

5. The luminescent converter (100) according to any one of the preceding claims, wherein the thickness (Ti) of the intermediate layer (130a) measured in a direction parallel to height H is larger than the thickness (Tol, To2) of both the first outer layer (140a) and the second outer layer (140b), preferably the intermediate layer is at least 1.5 times larger than the thickness of both the first outer layer and the second outer layer.

6. The luminescent converter (100) according to any one of the preceding claims, wherein: the first luminescent material (160) is a K2SiFe:Mn4+, activated phosphor (160); and the second luminescent material (170) is a (¥3-xCex)AlsOi2 and / or a (Lus- xCex)AlsOi2, phosphor (170).

7. The luminescent converter (100) according to any one of the preceding claims, wherein: the first luminescent material (160) in the core (130) has a first volume concentration (Cl); the second luminescent material (170) in the shell (140) has a second volume concentration (C2); andC2<C1.

8. The luminescent converter (100) according to any one of the preceding claims, wherein the weight ratio of the luminescent particles (110,120) to the polymeric material of the matrix (410) is in a range 2% to 10%.

9. The luminescent converter (100) according to any one of the preceding claims, wherein the matrix (410) has a first sub-layer (410a) and a second sub-layer (410b), whereinthe concentration of the luminescent particles (110,120) in the first sub-layer is at least three times higher than the concentration of luminescent particles in the second sub-layer.

10. A lighting device (1080,2030) configured to provide in operation device light (2100) and comprising a LED light source (130) and a luminescent converter (100) according to any one of the preceding claims, wherein the LED light source emits LED light source light having a peak emission wavelength in a range from 420 nm to 490 nm; wherein the luminescent particles (110,120) are configured such that (i) the shell (140) is configured to partly convert the LED light source light into second converted light and to partly transmit the LED light source light and (ii) the core (130) is configured to partly convert the LED light source light into first converted light; and wherein the device light comprises the first converted light, the second converted light and optionally part of the LED light source light.

11. A lamp (1000) or a luminaire (2000) comprising the lighting device (1080,2030) according to claim 10.

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