White phosphor converted LED with stable flux output versus temperature

Abstract

The invention provides a lighting device comprising a solid state light source and a ceramic body, wherein the solid state light source is configured to provide blue light source light to the ceramic body, wherein the ceramic body comprises a ceramic material configured to wavelength convert part of the blue light source light into yellow converter light, to provide white lighting device light comprising said blue light source light and said yellow converter light, said white lighting device light having a color point selected from the range of 0.18u0.25 and 0.42v0.54, and wherein the ceramic material comprises a (Y.sub.(1-y-q-z),Gd.sub.y,Lu.sub.q, Ce.sub.z).sub.3(Al.sub.(1-x),Ga.sub.x).sub.5O.sub.12 ceramic material, with 0x0.6, 0y0.5, 0q<1 and 0.001z0.06.

Claims

1. A process including the production of a ceramic body, wherein the ceramic body comprises a (Y(.sub.1-y-q-z),Gd.sub.y,Lu.sub.q,Ce.sub.z).sub.3(Al(.sub.1-x),Ga.sub.x).sub.5O.sub.12 ceramic material, with 0x0.6, 0y0.5, 0q<1, and 0.001z0.06, the process comprising: a sintering stage comprising sintering a mixture of starting materials at a temperature in the range of 1500-2000 C. in a first atmosphere to provide a sintered body; and an annealing stage, following the sintering stage, comprising an annealing at 1000-1600 C. in a second atmosphere to provide said ceramic body, the first atmosphere differing from the second atmosphere, the first atmosphere comprising a neutral or oxidizing atmosphere and the second atmosphere comprising a reducing atmosphere.

2. The process according to claim 1, wherein the starting materials comprise SiO.sub.2, and wherein the sintering stage comprises a hot pressing.

3. The process according to claim 1, wherein the starting materials comprise SiO.sub.2.

4. The process according to claim 1, wherein the ceramic body is configured to wavelength convert part of a blue light source light into yellow converter light, to provide a white lighting device light comprising said blue light source light and said yellow converter light, said white lighting device light having a color point selected from the range of 0.18u0.25 and 0.42v0.54, and the ceramic material comprising a (Y(.sub.1-y-q -z),Gd.sub.y,Lu.sub.q,Ce.sub.z).sub.3(Al.sub.(1-x),Ga.sub.x).sub.5O.sub.12 ceramic material, with 0 x0.6, 0y0.5 , 0q<1, and 0. 001z0.06.

5. The process according to claim 4 further comprising optically coupling the ceramic material to a solid state blue light source.

6. The process according to claim 5, wherein 0.0x0.5, 0.05y0.2, and 0.0015z0.03, and the blue light source light has a dominant wavelength selected from the range of 430-450 nm.

7. The process according to claim 5, wherein the blue light source comprises a light emitting surface configured at a distance of equal to or less than 1 mm from the ceramic body.

8. The process according to claim 7, wherein the blue light source comprises a light emitting surface in physical contact with the ceramic body.

9. The process according to claim 7, wherein the blue light source comprises a plurality of blue light solid state emitting devices, wherein the ceramic body is an elongated ceramic body having a first face and a second face defining a length of the elongated ceramic body, the elongated ceramic body comprising one or more radiation input faces and a radiation exit window, the second face comprising said radiation exit window, the plurality of solid state emitting devices being configured to provide blue light source light to the one or more radiation input faces.

10. The process according to claim 9, further comprising: providing an optical reflector configured downstream of the first face and configured to reflect light back into the elongated ceramic body, wherein the radiation exit window is configured perpendicular to the one or more radiation input faces.

11. The process according to claim 9, wherein the elongated ceramic body comprises a geometrical concentration factor, defined as the ratio of the area of the radiation input faces and the area of the radiation exit window, of at least 2, the process further comprising: providing a collimator configured downstream of the radiation exit window and configured to collimate the converter light.

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) FIG. 1 shows the emission spectrum of a phosphor converted blue LED. Color point is: u=0.204, v=0.478 (CIE 1976). The blue peak wavelength is 439 nm;

(3) FIG. 2 shows a normalized emitted flux (relative intensity (RI)) of a typical white LED as a function of socket temperature operated at 1 A, constructed of a blue LED in combination with a yellow emitting YAG phosphor. The solid line displays the temperature dependence of the flux of a state of the art white LED; the dashed line displays the temperature dependence of the flux of lighting device according to current invention;

(4) FIGS. 3a-3c schematically show embodiments of phosphor converted white LED devices (pcLEDs) and an automotive lamp, respectively;

(5) FIGS. 4a-4c schematically depict some further aspects of the invention;

(6) FIG. 5 schematically depicts some aspects of an embodiment of the processing of the ceramic body and optional further stages; and

(7) FIGS. 6a-6b show some thermo luminescence data.

(8) The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(9) For some white LED applications, e.g. automotive forward lighting, a stable flux output at different temperatures is required. Blue InGaN LEDs reduce output power with increasing temperature. This invention describes the combination of a blue LED with a yellow emitting phosphor with stable white flux output with varying LED and phosphor temperature, by using a Lumiramic converter which increases QE as a function of temperature (at high blue power input).

(10) In automotive forward lighting white LEDs are used, made of a blue emitting InGaN LED (430-460 nm peak emission) combined with a yellow emitting garnet phosphor (Y,Gd).sub.3Al.sub.5O.sub.12 activated with Ce. FIG. 1 shows a typical white emission spectrum. Here the garnet phosphor consists of a Lumiramic converter with a Ce conc. of 0.24% and a Gd concentration of 13% (i.e. (Y.sub.0.8676Gd.sub.0.13Ce.sub.0.0024).sub.3Al.sub.5O.sub.12. FIG. 1 shows the emission spectrum of a phosphor converted blue LED. Color point is: u=0.204, v=0.478 (CIE 1976). The blue peak wavelength is 439 nm.

(11) Although Blue emitting AlInGaN LEDs have an excellent external efficiency as a function of temperature, blue power at constant current decreases with increasing temperatures for current LED devices. For white LEDs the decrease in output power translates into a decrease of emitted white flux. For application of YAG phosphors, depending on phosphor composition the effect is amplified by so-called thermal quenching of the phosphor, describing the decrease of quantum efficiency with increasing temperature.

(12) Amongst others, the current invention provides a white phosphor converted LED with a color point especially of 0.2<u<0.21 and 0.45<v<0.5 (with the CIE1976 color coordinates u and v). This may be achieved by combining a blue InGaN LEDs with a garnet phosphor of the composition of (Y.sub.(1-y-q-z),Gd.sub.y,Lu.sub.q, Ce.sub.z).sub.3(Al.sub.(1-x),Ga.sub.x).sub.5O.sub.12, with 0x0.6, 0y0.5, 0q1, and 0.001z0.06, even more especially in an embodiment x>0, such as 0<x0.5, and within yet a further embodiment 0.05y0.2, and 0.0015z0.03. In order to achieve the desired functionality the material is sintered in oxygen for 8 hours at 1750 C. and after cooling down at room temperature the material is anneal in reducing atmosphere, e.g. forming gas (N.sub.2/H.sub.2) at 1100 C.<1450 C. for a longer period in time, depending on temperature. As a result of this preparation process with a material of the above composition a material is formed that contains a large number of oxygen vacancies. If such a material is illuminated at ambient temperature with uv-light (<370 nm), surprisingly a bright thermo luminescence will be observed when the phosphor temperature is raised above room temperature.

(13) Ceramics have been prepared by reactive sintering of a mixture of yttrium oxide (Y.sub.2O.sub.3), gadolinium oxide (Gd.sub.2O.sub.3), cerium oxide (CeO.sub.2) and aluminum oxide (Al.sub.2O.sub.3) and a SiO.sub.2 containing fluxing agent. Green body preparation can be by uniaxial pressing, slip casting, injection molding, extrusion, tape-casting or other ceramic green body forming techniques. Green bodies were sintered in air or oxygen at temperatures between 1400 C. and 1700 C. dependent on the basic composition (Gd concentration, Al excess or shortage) and the concentration of SiO.sub.2-containing sinter aid. Samples were annealed in reductive atmosphere (H.sub.2, N.sub.2/H.sub.2 of various H.sub.2 concentrations) to create lattice defects.

(14) FIG. 2 shows a normalized emitted flux of a typical white LED as a function of socket temperature operated at 1A, constructed of a blue LED in combination with a yellow emitting YAG phosphor in form of a ceramic body. The solid line displays the temperature dependence of the flux of a state of the art white LED; the dashed line displays the temperature dependence of the flux of lighting device according to current invention. The measurements were done with a short pulse of operation (20 ms) for each temperature. It is clear that the device according to the invention has superior thermal stability.

(15) FIGS. 3a-3c schematically show embodiments of phosphor converted white LED devices pcLEDs and an automotive lamp, respectively.

(16) FIG. 3a schematically depicts an embodiment of a lighting device 1 comprising a solid state light source 10 and a ceramic body 100, wherein the solid state light source 10 is configured to provide blue light source light 11 to the ceramic body 100. The ceramic body 100 comprises a ceramic material 120 configured to wavelength convert part of the blue light source light 11 into yellow converter light 101, to provide white lighting device light 2 comprising said blue light source light 11 and said yellow converter light 101. Especially, said white lighting device light 2 has a color point selected from the range 0.18u0.25 and 0.42v0.54. Further, the ceramic material 120 comprises a (Y.sub.(1-y-q-z),Gd.sub.y,Lu.sub.q, Ce.sub.z).sub.3(Al.sub.(1-x),Ga.sub.x).sub.5O.sub.12 ceramic material, with 0x0.6, 0y0.5, 0q<1, and 0.001z0.06. Especially, in an embodiment x>0, such as 0<x0.5, like x is at least 0.05. In yet a further embodiment 0.05y0.2, and 0.0015z0.03. The light source comprises a light emitting surface 12, here a LED die, having a surface area AL configured at a distance d of equal to or less than 1 mm from the ceramic body 100. In the herein schematically depicted embodiments, the light source 10 comprises said light emitting surface 12 being in physical contact with the ceramic body 100, i.e. d=0 mm. Reference 111 indicates a radiation input face. Further the ceramic body 100 comprises a radiation exit window 112, downstream of the former. The radiation input surface 111 has a surface area A and the radiation exit window 112 has a surface area E. especially for automotive applications wherein 0.8A/AL1.2 and wherein 1E/AL1.5.

(17) Hence, the ceramic body 100, described herein, is especially applied in a transmissive configuration.

(18) FIG. 3b schematically depicts an embodiment of a typical configuration of a phosphor converted LED as used for applications for imaging systems, or other applications. A blue emitting die is covered with a yellow emitting phosphor. In order to keep the luminescent area small, the phosphor layer has a size fitting the blue LED area closely, and the sides of LED and phosphor, which is a Lumiramic converter in this case, are covered with a white side coat (reflector 201), which usually consists of a white powder suspended in silicone, effectively reflecting blue and yellow light. Reference 202 indicates a package or PCB.

(19) FIG. 3c schematically depicts an automotive lamp 300, such as a headlight, comprising the lighting device 1 as described herein, with downstream thereof a collimator 24.

(20) A light emitting device according to the invention may be used in applications including but not being limited to a lamp, a light module, a luminaire, a spot light, a flash light, a projector, a (digital) projection device, automotive lighting such as e.g. a headlight or a taillight of a motor vehicle, arena lighting, theater lighting and architectural lighting.

(21) Light sources which are part of the embodiments according to the invention as set forth below, may be adapted for, in operation, emitting light with a first spectral distribution. This light is subsequently coupled into a light guide or waveguide; here the ceramic body. The light guide or waveguide may convert the light of the first spectral distribution to another spectral distribution and guides the light to an exit surface.

(22) An embodiment of the lighting device as defined herein is schematically depicted in FIG. 4a. FIG. 4a schematically depicts a lighting device 1 comprising a plurality of solid state light sources 10 and an elongated ceramic body 100 having a first face 141 and a second face 142 defining a length L of the elongated ceramic body 100. The elongated ceramic body 100 comprises one or more radiation input faces 111, here by way of example two oppositely arranged faces, indicated with references 143 and 144 (which define e.g. the width W). Further the ceramic body 100 comprises a radiation exit window 112, wherein the second face 142 comprises said radiation exit window 112. The entire second face 142 may be used or configured as radiation exit window. The plurality of solid state light sources 10 are configured to provide (blue) light source light 11 to the one or more radiation input faces 111. As indicated above, they especially are configured to provide to at least one of the radiation input faces 111 a blue power W.sub.opt of in average especially, but not exclusively, at least 0.067 Watt/mm.sup.2.

(23) The elongated ceramic body 100 comprises a ceramic material 120 configured to wavelength convert at least part of the (blue) light source light 11 into converter light 101, such as at least one or more of green and red converter light 101. As indicated above the ceramic material 120 comprises an A.sub.3B.sub.5O.sub.12LCe.sup.3+ ceramic material, wherein A comprises e.g. one or more of yttrium (Y), gadolinium (Gd) and lutetium (Lu), and wherein B comprises e.g. aluminum (Al), (however) especially as defined above. References 20 and 21 indicate an optical filter and a reflector, respectively. The former may reduce e.g. non-green light when green light is desired or may reduce non-red light when red light is desired. The latter may be used to reflect light back into the ceramic body or waveguide, thereby improving the efficiency. Note that more reflectors than the schematically depicted reflector may be used.

(24) The light sources may in principle be any type of point light source, but is in an embodiment a solid state light source such as a Light Emitting Diode (LED), a Laser Diode or Organic Light Emitting Diode (OLED), a plurality of LEDs or Laser Diodes or OLEDs or an array of LEDs or Laser Diodes or OLEDs, or a combination of any of these. The LED may in principle be an LED of any color, or a combination of these, but is in an embodiment a blue light source producing light source light in the UV and/or blue color-range which is defined as a wavelength range of between 380 nm and 490 nm. In another embodiment, the light source is an UV or violet light source, i.e. emitting in a wavelength range of below 420 nm. In case of a plurality or an array of LEDs or Laser Diodes or OLEDs, the LEDs or Laser Diodes or OLEDs may in principle be LEDs or Laser Diodes or OLEDs of two or more different colors, such as, but not limited to, UV, blue, green, yellow or red.

(25) FIGS. 4a-4b schematically depict similar embodiments of the lighting device. Further, the lighting device may include further optical elements, either separate from the waveguide and/or integrated in the waveguide, like e.g. a light concentrating element, such as a compound parabolic light concentrating element (CPC). The lighting devices 1 in FIG. 4b further comprises a collimator 24, such as a CPC.

(26) FIG. 4c schematically depicts some embodiments of possible ceramic bodies as waveguides or luminescent concentrators. The faces are indicated with references 141-146. The first variant, a plate-like or beam-like ceramic body has the faces 141-146. Light sources, which are not shown, may be arranged at one or more of the faces 143-146. The second variant is a tubular rod, with first and second faces 141 and 142, and a circumferential face 143. Light sources, not shown, may be arranged at one or more positions around the ceramic body. Such ceramic body will have a (substantially) circular or round cross-section. The third variant is substantially a combination of the two former variants, with two curved and two flat side faces. The variants shown in FIG. 4c are not limitative. More shapes are possible; i.e. for instance referred to WO2006/054203, which is incorporated herein by reference. The ceramic bodies, which are used as light guides, generally may be rod shaped or bar shaped light guides comprising a height H, a width W, and a length L extending in mutually perpendicular directions and are in embodiments transparent, or transparent and luminescent. The light is guided generally in the length L direction. The height H is in embodiments <10 mm, in other embodiments <5mm, in yet other embodiments <2 mm. The width W is in embodiments <10 mm, in other embodiments <5 mm, in yet embodiments <2 mm. The length L is in embodiments larger than the width W and the height H, in other embodiments at least 2 times the width W or 2 times the height H, in yet other embodiments at least 3 times the width W or 3 times the height H. Hence, the aspect ratio (of length/width) is especially larger than 1, such as equal to or larger than 2. Unless indicated otherwise, the term aspect ratio refers to the ratio length/width.

(27) The aspect ratio of the height H:width W is typically 1:1 (for e.g. general light source applications) or 1:2, 1:3 or 1:4 (for e.g. special light source applications such as headlamps) or 4:3, 16:10, 16:9 or 256:135 (for e.g. display applications). The light guides generally comprise a light input surface and a light exit surface which are not arranged in parallel planes, and in embodiments the light input surface is perpendicular to the light exit surface. In order to achieve a high brightness, concentrated, light output, the area of light exit surface may be smaller than the area of the light input surface. The light exit surface can have any shape, but is in an embodiment shaped as a square, rectangle, round, oval, triangle, pentagon, or hexagon.

(28) FIG. 5 schematically depicts some aspects of an embodiment of the processing of the ceramic body and optional further stages. Stage I indicates providing a combination of starting materials and green body formation. Stage II indicates a sintering stage. Stage III indicates an annealing stage, and stage IV indicates a subsequent processing stage, like polishing, grinding, dicing and building a device. Note that optionally between the indicated stages, other actions or stages may be included, such as a cooling stage between the sintering stage and the annealing stage, etc. etc.

(29) FIG. 6a show the glow curves of the standard as-sintered Y,GdAG material (see also below)_before (lower curve) and after annealing (higher curve) in forming gas. The glow curves are generated by illuminating the material with exciting light (here 360 nm wavelength). After the illumination phase the lamp is switched off and the emitted light is detected while the sample temperature is continuously increased with a linear temperature ramp. Here the temperature increase rate was 84 K/min. While the as-sintered glow curve hardly shows any thermo luminescence, the annealed sample exhibits high intensity TL, with two peaks located at different temperatures, indicating the existence of at least two different trap states in the material. FIG. 6b shows a comparison of glow curves of different garnet materials after annealing. The stoichiometry of the samples is given below:

(30) TABLE-US-00001 Gd (%, not Lu (%, not including Ce) including Ce) Ce (%) SiO.sub.2 (ppm) (Y, Gd)AG 12.6 0 0.22 1000 YAG 0 0 0.25 1500 (Lu, Y)AG 0 50 0.25 2000

(31) The integrated thermo luminescence light, which may be directly proportional to the defect density in the material, was measured between 25-240 C. at a given heating rate and a given UV exposure, with identical settings before and after annealing. It was found that the ratio of thermo luminescence after anneal to thermo luminescence before anneal is in the range of >10, such as at least 12.

(32) The term substantially herein, such as in substantially all light or in substantially consists, will be understood by the person skilled in the art. The term substantially may also include embodiments with entirely, completely, all, etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term substantially may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term comprise includes also embodiments wherein the term comprises means consists of. The term and/or especially relates to one or more of the items mentioned before and after and/or. For instance, a phrase item 1 and/or item 2 and similar phrases may relate to one or more of item 1 and item 2. The term comprising may in an embodiment refer to consisting of but may in another embodiment also refer to containing at least the defined species and optionally one or more other species.

(33) Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

(34) The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

(35) It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb to comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article a or an preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(36) The invention further applies to a device comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

(37) The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.