Coated narrow band red-emitting fluorosilicates for semiconductor LEDS

Abstract

The invention provides a lighting unit comprising a light source, configured to generate light source light and a particulate luminescent material, configured to convert at least part of the light source light into luminescent material light, wherein the light source comprises a light emitting diode (LED), wherein the particulate luminescent material comprises particles comprising cores, said cores comprising a phosphor comprising M.sub.xM.sub.2-2xAX.sub.6 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 tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, and wherein the particles further comprise a metal phosphate based coating, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si and Al.

Claims

1. A lighting unit comprising; a light source, configured to generate light source light; and a particulate luminescent material configured to convert at least part of the light source light into luminescent material light, wherein the light source comprises a light emitting diode (LED), wherein the particulate luminescent material comprises particles comprising cores, said cores comprising a phosphor comprising M.sub.xM.sub.2-2xAX.sub.6 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 tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, and wherein the particles further comprise a metal phosphate based coating, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si, and Al.

2. The lighting unit according to claim 1, wherein the metal phosphate based coating comprises an aluminum phosphate coating.

3. The lighting unit according to claim 1, wherein the particulate luminescent material is obtainable by contacting phosphor particles with a liquid comprising a precursor of the metal phosphate based coating, and wherein said liquid is obtainable by mixing an alcohol comprising liquid, a metal salt that is soluble in the alcohol comprising liquid, and a phosphate source, retrieving the thus treated phosphor particles, and drying the thus obtained treated phosphor particles to provide the particulate luminescent material.

4. The lighting unit according to claim 3, wherein the phosphate source comprises P.sub.2O.sub.5.

5. The lighting unit according to claim 1, wherein M.sub.xM.sub.2-2xAX.sub.6 comprises K.sub.2SiF.sub.6.

6. The lighting unit according to claim 1, wherein the light source is configured to generate blue light.

7. A lighting unit comprising; a light source, configured to generate light source light; and a particulate luminescent material configured to convert at least part of the light source light into luminescent material light, wherein the light source comprises a light emitting diode (LED), wherein the particulate luminescent material comprises particles comprising cores, said cores comprising a phosphor comprising M.sub.xM.sub.2-2xAX.sub.6 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 tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, and wherein the particles further comprise a metal phosphate based coating, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si, and Al, wherein the particulate luminescent material further comprises one or more other phosphors selected from the group consisting of a divalent europium containing nitride luminescent material, a divalent europium containing oxynitride luminescent material, a trivalent cerium containing garnet, and a trivalent cerium containing oxynitride.

8. A method for the preparation of a particulate luminescent material which comprises particles comprising cores and a metal phosphate coating, wherein the cores comprise a phosphor comprising M.sub.xM.sub.2-2xAX.sub.6 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 tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si, and Al, and wherein the method comprises: contacting phosphor particles with a liquid comprising a precursor of the metal phosphate based coating, mixing an alcohol comprising liquid, a metal salt that is soluble in the alcohol comprising liquid, and a phosphate source, to obtain said liquid, retrieving the treated phosphor particles, and drying the obtained treated phosphor particles to provide the particulate luminescent material.

9. The method according to claim 8, wherein the metal of the precursor of the metal phosphate based coating comprises aluminum.

10. The method according to claim 8, wherein the phosphate source comprises P.sub.2O.sub.5, and wherein the alcohol is a C2-C4 alcohol.

11. The method according to claim 8, wherein M.sub.xM.sub.2-2xAX.sub.6 comprises K.sub.2SiF.sub.6.

12. The method according to claim 8, wherein the phosphor particles are obtainable by a method comprising mixing a soluble salt of alkaline cation, a soluble salt of tetravalent manganese precursor, a tetravalent cation source, in an aqueous solution of an inorganic acid at least comprising HF, precipitating the phosphor, and drying the phosphor thus obtained, wherein the drying or any other optional later heat treatment process of the phosphor is performed at a temperature below 200 C.

13. The method according to claim 8, wherein the alcohol comprises a C2-C4 alcohol.

14. The method according to claim 8, wherein the particulate luminescent material further comprises one or more other phosphors selected from the group consisting of a divalent europium containing nitride luminescent material, a divalent europium containing oxynitride luminescent material, a trivalent cerium containing garnet, and a trivalent cerium containing oxynitride.

15. A particulate luminescent material which comprises particles comprising cores and a metal phosphate coating, wherein the cores comprise a phosphor comprising M.sub.xM.sub.2-2xAX.sub.6 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 tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si and, Al.

16. The particulate luminescent material according to claim 15, wherein M.sub.xM.sub.2-2xAX.sub.6 comprises K.sub.2SiF.sub.6 and wherein the metal phosphate based coating comprises an aluminum phosphate coating.

17. The particulate luminescent material according to claim 15, wherein the particulate luminescent material further comprises one or more other phosphors selected from the group consisting of a divalent europium containing nitride luminescent material, a divalent europium containing oxynitride luminescent material, a trivalent cerium containing garnet, and a trivalent cerium containing oxynitride.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

(2) FIGS. 1a-1c schematically depict some embodiments of the lighting unit; the drawings are not necessarily on scale;

(3) FIG. 2 shows emission (right y-axis) and reflection (left y-axis) spectra of Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP);

(4) FIG. 3 shows conductivity measurements, with on the y-axis the special conductivity, normalized to 1, and on the x-axis the time in seconds of Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP) in deonized water;

(5) FIG. 4 shows the quantum efficiency (QE) as function of the time t in days Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP) in an accelerated stress test (85 C. and 85% humidity); and

(6) FIG. 5 very schematically depicts the luminescent material 20.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) FIG. 1a schematically depicts an embodiment of the lighting unit, indicated with reference 100, of the invention. The lighting unit comprises a light source 10, which is in this schematic drawing a LED (light emitting diode). In this embodiment, on top of the light source 10, here on the (light exit) surface 15, thus downstream of the light source 10, a luminescent material 20 is provided. This luminescent material 20 comprises phosphor as described herein, indicated with reference 40 (see also FIG. 5). By way of example, the lighting unit 100 further comprises, for instance for light extraction properties, a (transmissive) dome 61. This is an embodiment of a transmissive optical element 60, which is in this embodiment arranged downstream of the light source 10 and also downstream of the light conversion layer 20. The light source 10 provides light source light 11 (not indicated in the drawing), which is at least partly converted by the light conversion layer 20 into luminescent material light 51. The light emanating from the lighting unit is indicated with reference 101, and contains at least this luminescent material light 51, but optionally, dependent upon the absorption of luminescent material 50 also light source light 11. In an embodiment, the lighting unit light 101 may have a CCT of 5000 K or lower. However, also a higher CCT may be possible. The CCT may be tuned by tuning the amount of the luminescent material 20, including the optional presence of other phosphors 40 that the herein indicated hexafluorosilicate.

(8) FIG. 1b schematically depicts another embodiment, without dome, but with an optional coating 62. This coating 62 is a further example of a transmissive optical element 60. Note that the coating 62 may in an embodiment be one or more of a polymeric layer, a silicone layer, or an epoxy layer. Alternatively or additionally a coating of silicon dioxide and/or silicon nitride may be applied.

(9) In both schematically depicted embodiment of FIGS. 1a-1b, the luminescent material 20 is in physical contact with the light source 10, or at least its light exit surface (i.e. surface 15), such as the die of a LED. In FIG. 1c, however, the luminescent material 20 is arranged remote from the light source 10. In this embodiment, the luminescent material 20 is configured upstream of a transmissive (i.e. light transmissive) support 30, such as an exit window. The surface of the support 30, to which the light conversion layer 20 is applied, is indicated with reference 65. Note that the luminescent material 20 may also be arranged downstream of the support 30, or at both sides of the support luminescent material 20 may be applied. The distance between the luminescent material 20 and the light source (especially its light exit surface 15) is indicated with reference dl, and may be in the range of 0.1 mm-10 cm. Note that in the configuration of FIG. 1c, in principle also more than one light source 10 may be applied.

(10) FIG. 2 shows emission (right y-axis) and reflection (left y-axis) spectra of Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP). As can be seen, luminescence does not substantially change (the emission spectra overlap) and reflection in the blue region, for these examples, only show a very slight decrease. This may be improved by changing the layer thickness, the dopant concentration, and also particle size. The indication I on the right y-axis refers to the photoluminescence intensity, normalized to 1. R refers to reflectance, also normalized to 1. As can be seen, the luminesce of the phosphor can be considered narrow band luminescence, as the luminescence substantially consists of line emission (and not of band emission, as is the case for most Eu.sup.2 and Ce.sup.3+ phosphors used in the field (and indicated above)).

(11) FIG. 3 shows conductivity measurements, with on the y-axis the special conductivity, normalized to 1, and on the x-axis the time in seconds of Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP) in deonized water. The coated sample has a substantial better behaviour.

(12) FIG. 4 shows the quantum efficiency (QE) as function of the time t in days Mn-doped K.sub.2SiF.sub.6 non-coated and coated (the latter is indicated with -ALP) in an accelerated stress test (85 C. and 85% humidity). Again, the coated sample has a substantial better behaviour.

(13) FIG. 5 very schematically depicts the luminescent material 20. It may substantially consist of particles 200 with cores 201 comprising phosphor or phosphor material, indicated with reference 40, and a coating (shell) 202 comprising the herein described aluminum phosphate material. The reference d indicates the dimensions of the core of the particle, especially diameter, and dl indicates the thickness of the shell or coating.

EXPERIMENTAL

(14) The novel core-shell phosphor disclosed herein is obtained in two steps. Firstly, the Mn-doped potassium hexafluorosilicate is prepared as co-precipitates at room temperature from aqueous HF solution containing the Mn-dopant. For the preparation of Mn.sup.4+-doped K.sub.2SiF.sub.6 stoichiometric amounts of the starting materials KHF.sub.2, and KMnO.sub.4 are dissolved in aqueous HF. Subsequently, a stoichiometric amount of SiO.sub.2 is added to the aqueous HF solution. The concentration of Mn.sup.4 in the aqueous HF solution was 8 mole %. The precipitates were filtered, washed repeatedly with 2-propanole, and then dried at 100 C. in vacuum.

(15) Subsequently, the protected shell of the Mn-doped K.sub.2SiF.sub.6 is prepared by suspending the core powder in a mixture of ethanolic Al(NO.sub.3).sub.3*9H.sub.2O and P.sub.2O.sub.5 with a mole ratio of K.sub.2SiF.sub.6:Al:P=1:0.06:0.06. The solvent is evaporated during stirring and elevated temperatures (approx. 80 C.). Finally, the powder is heated at 200 C. for 1 hour resulting in a partially hydrolysed alcoholates of esters.

(16) The photoluminescence spectra (emission spectra, FIG. 2) of such core-shell Mn-doped hexafluorosilicates reveal an emission in the red region from about 600 to 660 nm. The main emission peak is located at approx. 631 nm. The lumen equivalent of the shown spectrum is approx. 198 lm/W. The reflection in the green and yellow spectral range is at least R>0.92 which results in a very low absorption of green- and yellow-emitted phosphors used for warm white applications. Moreover, the self-absorption of the invented core-shell phosphor is low due to a reflection of at least 0.95 and higher in the spectral range from 600-660 nm.

(17) X-ray photoelectron spectroscopy (XPS) measurements reveal a significant drop of the core elements K, Si, and F, and an increase of the shell elements Al, P, O, and C, after applying the shell onto the core phosphor with the procedure mentioned above.

(18) TABLE-US-00001 K.sub.2SiF.sub.6 [at. %] K.sub.2SiF.sub.6AlP [at %] XPS Elements K2p 23.86 10.49 Si2s + 2p 9.82 7.43 F1s 65.32 27.67 O1s 1.00 33.68 Al2p 0.00 4.06 P2p 0.00 4.50 C1s 0.00 11.77 Optical Characteristics QE 0.81 0.78 x/y 0.691/0.308 0.692/0.307 LE (lm/W) 198 198

(19) Below, an example is given for the preparation of the K,Rb variant of the hexafluorosilicate. Coating may be applied as indicate above.

VARIATIONS

(20) Some coated KSiF with different Al:P ratios (Al:P=2:1, 1:1, 1:0.5, and 1:0.25) were made which all gave good coatings. The results shown above were with an Al:P ratio of 1:1.

(21) The mixed alkali metal hexafluorosilicate phosphors described herein may be obtained as co-precipitates at room temperature from aqueous HF solution containing the Mn-dopant. For the preparation of Mn.sup.4+-doped KRbSiF.sub.6 stoichiometric amounts of the starting materials RbF, KHF.sub.2, and KMnO.sub.4 are dissolved in aqueous HF. Subsequently, a stoichiometric amount of SiO.sub.2 is added to the aqueous HF solution. The concentration of Mn.sup.7+ in the aqueous HF solution was 8 mole %. The precipitates were filtered, washed repeatedly with 2-propanole, and then dried at RT in vacuum.

(22) Additionally, it is possible that a variety of other starting materials may be used to produce the inventive hexafluorosilicate phosphor via co-precipitation from aqueous solution (e.g. rubidium/potassium nitrate, rubidium/potassium chloride).

(23) The precipitated sample was indexed as hexagonal lattice from their X-ray powder pattern (using Cu-K radiation). After heating at 300 C., the sample transforms to a cubic lattice as found in the XRD data base.