Radiation unit with at least one LED

Technical field

0001The invention is based on a lighting unit with at least one LED as a light source according to the preamble of claim 1. It is in particular an LED emitting in the visible or white on the basis of a primarily UV-emitting LED.

State of the art

0002An illumination unit with at least one LED as a light source, which emits white light, for example, is currently predominantly combined by a combination of a Ga (In) N LED emitting in the blue at around 460 nm and a yellow emitting YAG: Ce³⁺-Luminous material realized (US 5 998 925 and EP 862 794). Two different yellow phosphors are often used for good color rendering, as described in WO-A 01/08453. The problem here is that the two phosphors often show different temperature behavior, even if their structure is similar. A well-known example is the Y-garnet (YAG: Ce) doped with Ce luminescent in yellow and the (Y, Gd) garnet which is luminescent in comparison with longer waves. This leads to color fluctuations and changes in color rendering at different operating temperatures.

0003From the publication "On new rare-earth doped M-Si-Al-ON materials" by van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4, chapter 11, a class of fluorescent materials is known which, in abbreviation of its Structure as a sialone <img file="EP1296383A2_D0001.tif" />-Sialones) are called, which can be doped with Ce, Eu, or Tb. With Ce doping, an emission in the range of 515 to 540 nm is achieved with excitation with 365 nm or 254 nm.

Presentation of the invention

0004It is an object of the present invention to provide a lighting unit with at least one LED as a light source according to the preamble of claim 1, which is characterized by high constancy at changing operating temperatures. Another object is to provide a lighting unit which emits white and in particular has a high color rendering and high yield.

0005This object is achieved by the characterizing features of claim 1. Particularly advantageous refinements can be found in the dependent claims.

0006According to the invention, a sialon that emits green and comes from the class of Ce-activated sialons is used as the phosphor for the LED-based lighting unit, the sialon having the formula M_{p / 2}Si_12-pqAl_{p + q}O_qN_16-q: Ce³⁺ obeys, with M = Ca individually or in combination with Sr, with q = 0 to 2.5 and p = 1.5 to 3. Preferably a high value for p is chosen, namely p = 2.5 to 3, and a relative low value for q, namely q = 0 to 1, in particular up to 0.8. Ca is preferably used alone for the cation M.

0007The Ce fraction which replaces part of the cation M should be 0.5 to 15%, preferably 1 to 10%, in particular 2 to 6%, of the M cation, as a result of which a particularly precise choice of the emission wavelength can be made and the light output can also be optimized. An increasing Ce content generally causes the peak emission to shift towards longer wavelengths.

0008Special advantages of this phosphor in connection with an LED-based lighting unit are its high efficiency, its outstanding temperature stability (no sensitivity to changes in the operating temperature) and a surprisingly high quenching temperature for the luminescence, as well as the high color rendering that can be achieved, especially when combined with at least one other phosphor . The quenching temperature, i.e. the temperature at which the luminescence is destroyed due to the heat supplied, is even so high that it was outside the preselected measuring range (maximum 140 ° C).

0009Another advantage of this class of phosphor is that the starting material (in particular Si₃N₄) is in finely divided form. This phosphor therefore no longer has to be ground, which saves one work step and there are no losses in efficiency. Typical average grain sizes of the phosphor are 0.5 to 5 µm. In contrast, conventional phosphors such as YAG: Ce must be ground so that they remain dispersed in the casting resin and do not sink to the bottom. This grinding process often leads to loss of efficiency. Despite the fine grain size of the starting material, the phosphor according to the invention has good absorption.

0010In addition to the generation of a colored light source by excitation by means of UV radiation from an LED, the generation of white light with the aid of this phosphor in particular offers advantages. This is done with a UV-emitting LED as the primary light source using at least three phosphors.

0011White light with good color rendering is generated in particular by the combination of a UV-LED (eg primary emission at 300 to 430 nm, preferably 380 to 420 nm), a green phosphor according to the invention (emission between 495-540 nm) and a blue (emission: 440- 480 nm) and a red-emitting phosphor (emission: 560-620 nm).

0012As a green phosphor, M_{p / 2}Si_12-pqAl_{p + q}O_qN_16-q: Ce³⁺ used. M = Ca individually or in combination with Sr, the Sr content is preferably less than 30 mol%. This green phosphor has excellent thermal stability and shows excellent luminescence behavior at higher temperatures, which are typical for LEDs: up to 80 ° C, it shows no decrease in luminescence within the scope of the measurement accuracy. In comparison, the conventional garnet phosphors show a clearly measurable decrease in luminescence at 80 ° C: it can be 5 to 30%, depending on the selected cation composition in the system (Y, Gd, Lu)₃(Al, Ga)₅O₁₂.

0013The great advantages of Ce sialons are their pronounced stability against hot acids, bases and also their thermal and mechanical stability. Surprisingly, these sialons show excellent temperature quenching behavior of the luminescence of activator ions in these compounds. This makes these compounds strong competitors for luminescent materials, which sometimes shine brighter and / or more efficiently at room temperature, but show luminescence loss due to temperature quenching when used. For example, the blue-green glowing Sr₄Al₁₄O₂₅: Eu²⁺ a quantum efficiency of around 85% at room temperature. At 100 ° C, however, the efficiency has dropped to around 60%. Thiogallate, endowed with Eu²⁺, depending on their cation composition (MGa₂S₄: Eu²⁺) can luminesce in the entire green area, also lose 20% -30%. Yellow luminescent Ce³⁺-Doped grenades lose about 10-30% of the efficiency at room temperature, depending on the proportion of Gd: Y and Al: Ga at high temperature.

0014A white mixture can be produced on the basis of a UV-emitting LED using these Ce-doped sialons together with a blue phosphor, such as BaMgAl₁₀O₁₇: Eu²⁺ (BAM), Ba₅SiO₄(Cl, Br)₆: Eu²⁺, CaLa₂S₄: Ce³⁺ or (Ca, Sr, Ba)₅(PO₄)₃Cl: Eu²⁺ (SCAP). Another ingredient is a red phosphor like (Y, La, Gd, Lu)₂O₂S: Eu³⁺, SrS: Eu²⁺ or Sr₂Si₅N₈: Eu²⁺.

0015The color rendering can be improved if necessary by adding another green phosphor with a shifted emission maximum (for example Eu-doped thiogallates or Sr aluminates). Another possibility is the use of the Ce-doped sialon as the only phosphor to achieve a color-emitting LED.

0016Depending on the Ce³⁺Content is the body color of this material, especially with low oxygen content, almost white over pale green to deep green. Because of the excellent temperature stability and mechanical stability, this Ce-Sialon is well suited as an environmentally friendly green pigment or phosphor for a wide variety of applications. This is especially true when M is replaced by 5 to 10% Ce.

0017In general, the smaller q and the larger p, the higher the quantum efficiency. A phosphor with M = Ca, p = 3 and q = 0 is particularly preferred.

0018The light that the phosphor according to the invention emits under UV light is very unsaturated green, with color coordinates of x ∼ 0.22 / y ∼ 0.41. The luminescence depends on Ce³⁺Content: the emission shifts long-wave with increasing Ce³⁺-Salary.

0019The optical properties, unlike those of the sialons described in the literature, are surprising, at least when the oxygen content is low (below 5 mol% of the nitrogen content, preferably below 2 mol%) and the cation content is high (p = 1.5 to 3). The samples are often white to greenish-white, so they do not absorb in the blue area and luminesce much more short-wave than described in the literature. This means that the phosphor according to the invention is very well suited for UV-LED (or possibly UV applications), and above all that it shows no competing absorption of blue light, so that a relatively long-wave UV primary emission (380 to 420 nm ) can be selected. The longer the wavelength of the UV emission can be selected, the less energy it is and the more gentle it is on the LED, which extends its lifespan. This advantageous behavior is shown in particular by luminescent materials with little or no oxygen, ie up to at most 5 mol%, based on nitrogen, according to the formula M._{p / 2}Si_12-pqAl_{p + q}O_qN_16-q: Ce³⁺, with M = Ca individually or in combination with Sr, in particular with q = 0 to 0.7 and p = 2.5 to 3.

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0020The invention will be explained in more detail below with the aid of several exemplary embodiments. Show it:<dl id="dl0001" compact="compact"><dt>Figure 1</dt><dd>a semiconductor device that serves as a light source (LED) for white light;</dd><dt>Figure 2</dt><dd>a lighting unit with phosphors according to the present invention;</dd><dt>Figure 3 to 5</dt><dd>the emission spectrum and reflection spectrum of various sialon phosphors according to the present invention;</dd><dt>Figure 6</dt><dd>the temperature behavior of Sialon HU18A / 01 and the temperature behavior of thiogallate phosphor in comparison.</dd></dl>

Description of the drawings

0021For use in a white LED together with a GaInN chip, for example, a structure similar to that described in US Pat. No. 5,998,925 is used. The structure of such a light source for white light is shown explicitly in FIG. 1. The light source is a semiconductor component (chip 1) of the InGaN type with a peak emission wavelength of 400 nm with a first and second electrical connection 2, 3, which is embedded in an opaque basic housing 8 in the region of a recess 9. One of the connections 3 is connected to the chip 1 via a bonding wire 14. The recess has a wall 17 which serves as a reflector for the primary radiation of the chip 1. The recess 9 is filled with a casting compound 5, which contains an epoxy casting resin (80 to 90% by weight) and phosphor pigments 6 (less than 15% by weight) as main components. Other small proportions include methyl ether and Aerosil. The phosphor pigments are a mixture of BAM, SrS and sialon pigments.

0022FIG. 2 shows a section of a surface light 20 as a lighting unit. It consists of a common carrier 21 onto which a cuboid outer housing 22 is glued. Its top is provided with a common cover 23. The cuboid housing has cutouts in which individual semiconductor components 24 are accommodated. They are UV emitting diodes with a peak emission of typically 360 nm. The conversion to white light takes place by means of conversion layers which are seated directly in the casting resin of the individual LEDs, as described in FIG. 1, or layers 25 which are applied to all surfaces accessible to UV radiation. These include the inner surfaces of the side walls of the housing, the cover and the base part. The conversion layers 25 consist of three phosphors which emit in the red, green and blue spectral range using the phosphors according to the invention.

0023Some phosphors of the Sialon type are summarized in Tab. 1. These are Ca sialones of the Ca type_1,5Si₉Al₃N₁₆, where a portion of the cation Ca, which is between 2 and 8 mol% of Ca, is replaced by Ce. Typical quantum efficiencies of these phosphors are 50 to 70%, with the peak emission (max. Em.) Shifting from 497 nm with low Ce doping to about 508 nm with high Ce doping. This behavior for oxygen-free sialones is particularly surprising because the literature cited at the beginning clearly shows longer-wave peak emissions. The oxygen content probably has a significant impact here. According to the invention, therefore, an oxygen-free pigment of the Ce-sialon type is also claimed, which forms its peak emission in the range from 495 to 510 nm.

00243 to 5 show the emission and the reflection behavior of different sialons as a function of the wavelength.

00253a shows the emission spectrum of the sialon Ca._1.5Al₃Si₉N₁₆: Ce³⁺ (2.7 mol% of Ce in the cation Ca) (test number HU18A / 01) when excited by 400 nm. The maximum is 499 nm, the mean wavelength is 517 nm. The quantum efficiency of QE of special samples was 61 to 71% , depending on the details of the preparation. The reflection (FIG. 3b) is approximately R400 = 66% at 400 nm and approximately R360 = 57% at 360 nm. This data is also listed in Tab. 1.

0026The synthesis of the sialon HU18A / 01 is described in more detail below as an example.

0027The phosphor powder is produced by a high-temperature solid-state reaction. For this purpose, for example, the high-purity starting materials Ca₃N₂, AIN, and Si₃N₄ mixed together with a molar ratio of 1.44: 3: 9. The grain size of the Si₃N₄ lies with d₅₀ = 1.6 µm with d₁₀ = 0.4 and d₉₀ = 3.9 µm. A small amount of CeO₂ (or, for example, Ce nitride) is added for the purpose of doping and replaces the corresponding molar amount of Ca.₃N₂. For a 2.7 mol% share of Ce, this corresponds to the gross formula (approx_1.46Ce_0,04) Al₃Si₉N₁₆. Due to the addition of oxygen together with the Ce as Ce oxide, the exact gross formula is (Ca_1.46Ce_0,04) Al₃Si₉O_0,045N_15,97 to watch.

0028After the individual components have been mixed well, the powder is at about 1700 ° C for about 2 h in a reducing atmosphere (N.₂/H₂) heated and thus reacts to the above compound.

0029Figure 4 shows the emission spectrum (Figure 4a) of the sialon Ca._1.5Al₃Si₉N₁₆: Ce³⁺ (5%) (test number HU31 / 01) when excited by 400 nm. The maximum is 498 nm, the mean wavelength is 520 nm. The quantum efficiency QE is 53%. The reflection (FIG. 4b) is approximately R400 = 55% at 400 nm and approximately R360 = 48% at 360 nm.

0030Figure 5 shows the emission spectrum (Figure 5a) of the sialon Ca._1.5Al₃Si₉N₁₆: Ce³⁺ (8%) (test number HU32 / 01) when excited by 400 nm. The maximum is 508 nm, the mean wavelength is 523 nm. The quantum efficiency QE is 45%. The reflection (FIG. 5b) is approximately R400 = 48% at 400 nm and approximately R360 = 40% at 360 nm.

0031FIG. 6 shows the quantum efficiency as a function of the temperature for different phosphors. It turns out that the green-emitting thiogallate SrGa known per se₂S₄: Eu (curve that is represented by triangles) has a comparable quantum efficiency (normalized to 100% at room temperature) than a typical Sialon (curve that is represented by diamonds: here HU18A / 01 from Table 1). The value 100% corresponds absolutely to a quantum efficiency of approximately 66%. Surprisingly, the quantum efficiency of the thiogallate deteriorates significantly during operation of the LED under typical temperature loads of 80 ° C to 100 ° C, while the Ce-doped Sialon phosphor maintains its quantum efficiency almost constant. So while a Ce-doped Sialon phosphor only appears to deliver average results (at room temperature) at first glance, its suitability for Lukoleds and other temperature-stressed lighting units is even directly comparable with the standard YAG: Ce used in case of stress.

0032This shows the particular suitability of the Ce-doped sialons for use in luminescence conversion LEDs and in particular in phosphor mixtures together with other temperature-stable phosphors such as SrS. <tables id="tabl0001" num="0001"><table frame="all"><title>Tab. 1</title><tgroup cols="8" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="19.68mm" /><colspec colnum="2" colname="col2" colwidth="19.68mm" /><colspec colnum="3" colname="col3" colwidth="19.68mm" /><colspec colnum="4" colname="col4" colwidth="19.68mm" /><colspec colnum="5" colname="col5" colwidth="19.68mm" /><colspec colnum="6" colname="col6" colwidth="19.68mm" /><colspec colnum="7" colname="col7" colwidth="19.68mm" /><colspec colnum="8" colname="col8" colwidth="19.68mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center">connection</entry><entry namest="col2" nameend="col2" align="center">Short name</entry><entry namest="col3" nameend="col3" align="center">QE</entry><entry namest="col4" nameend="col4" align="center">R360</entry><entry namest="col5" nameend="col5" align="center">R400</entry><entry namest="col6" nameend="col6" align="center">Max. Em.</entry><entry namest="col7" nameend="col7" align="center">x</entry><entry namest="col8" nameend="col8" align="center">y</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Ca1.5Al3Si9N16: Ce3 + (2.7%)</entry><entry namest="col2" nameend="col2" align="left">HU18A / 01</entry><entry namest="col3" nameend="col3" align="center">66</entry><entry namest="col4" nameend="col4" align="center">57</entry><entry namest="col5" nameend="col5" align="center">66</entry><entry namest="col6" nameend="col6" align="center">499</entry><entry namest="col7" nameend="col7" align="left">0,220</entry><entry namest="col8" nameend="col8" align="left">0,396</entry></row><row><entry namest="col1" nameend="col1" align="left">Ca1.5Al3Si9N16: Ce3 + (5%)</entry><entry namest="col2" nameend="col2" align="left">HU31 / 01</entry><entry namest="col3" nameend="col3" align="center">53</entry><entry namest="col4" nameend="col4" align="center">48</entry><entry namest="col5" nameend="col5" align="center">55</entry><entry namest="col6" nameend="col6" align="center">498</entry><entry namest="col7" nameend="col7" align="left">0,237</entry><entry namest="col8" nameend="col8" align="left">0,422</entry></row><row><entry namest="col1" nameend="col1" align="left">Ca1.5Al3Si9N16: Ce3 + (8%)</entry><entry namest="col2" nameend="col2" align="left">HU32 / 01</entry><entry namest="col3" nameend="col3" align="center">45</entry><entry namest="col4" nameend="col4" align="center">40</entry><entry namest="col5" nameend="col5" align="center">48</entry><entry namest="col6" nameend="col6" align="center">508</entry><entry namest="col7" nameend="col7" align="left">0,247</entry><entry namest="col8" nameend="col8" align="left">0,435</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Ca1.5Al3Si9N16: Ce3 + (2.7%)</entry><entry namest="col2" nameend="col2" align="left">HU18 / 01</entry><entry namest="col3" nameend="col3" align="center">61</entry><entry namest="col4" nameend="col4" align="center">55</entry><entry namest="col5" nameend="col5" align="center">67</entry><entry namest="col6" nameend="col6" align="center">497</entry><entry namest="col7" nameend="col7" align="left">0,225</entry><entry namest="col8" nameend="col8" align="left">0,397</entry></row></tbody></tgroup></table></tables>