Patent Application
20120146078
The invention is based on a conversion LED according to the preamble of claim 1. Such conversion LEDs are in particular suitable for general lighting.
A conversion LED is known from U.S. Pat. No. 6,649,946, which to obtain a white LED uses a blue chip together with Sr2Si5N8:Eu, wherein YAG:Ce is also used as an additional luminescent substance to improve color reproduction. However, only a few efficient LEDs can be realized in this way.
A conversion LED is known from U.S. Pat. No. 7,297,293 which to obtain a white LED uses a blue chip together with (Sr,Ca)2Si5N8:Eu, wherein YAG:Ce and similar luminescent substances with partial replacement of Y by Gd or partial replacement of Al by Ga is also used as an additional luminescent substance to improve color reproduction. However, only a few efficient LEDs can be realized in this way.
A conversion LED is known from EP-A 1 669 429 which uses a blue chip together with special (Sr,Ba)2Si5N8:Eu luminescent substance to obtain a white LED, wherein Lu-AG:Ce as well as similar luminescent substances which are co-doped with Ce and Pr are also used as additional luminescent substances to improve color reproduction.
The object of this invention is to provide a high efficiency conversion LED, wherein the conversion LED in particular achieves a high useful life.
This object is achieved by the characterizing features of claim 1.
Particularly advantageous embodiments are to be found in the dependent claims.
According to the inventive a high efficiency conversion LED is now provided. Not all luminescent substances are stable in LEDs operated at high currents, here in particular at least 250 mA, preferably at least 300 mA, known as high performance LEDs. In particular this problem applies to nitride or oxinitride luminescent substances such as nitride silicate M2Si5N8:Eu. Many such luminescent substances, in particular M2Si5N8:D nitride with D as an activator, suffer significant conversion losses during operation in an LED. In a stress test with up to 700 mA continuous current, white LEDs with such luminescent substances over a short period of time (typically 1000 hours) lose up to 50% of their conversion efficiency. This results in marked instability of the color location.
White LEDs are constantly gaining in significance in general lighting. In particular, the demand for warm white LEDs with low color temperatures, preferably in the 2900 to 3500 K range, in particular 2900 to 3100 K, and for good color reproduction, in particular Ra is at least 93, preferably 96, and at the same time for high efficiency. As a rule these targets are achieved by combining a blue LED with yellow and red luminescent substances. The spectra of all these solutions have a region in the blue-green spectral range in which little radiation is emitted (blue-green gap), resulting in poor color reproduction. To compensate very long-wave blue LEDs are usually used (approx. 460 nm). On the part of chip technology, however, it is advantageous to use LEDs of shorter chip wavelengths as these are significantly more efficient. Wavelengths (peak) of between 430 to 455 nm, in particular 435 to 445 nm are desirable.
If the blue-green portion of the overall range is essentially determined solely by the blue LED, as is the case with previous combinations of long-wave blue LED and yellow as well as red luminescent substances, this results in the overall CRI of the white LED being heavily dependent on the chip wavelength used. For technical reasons, however, a relatively broad range of chip wavelengths must be used in practice, resulting in major fluctuations in the CRI. Furthermore, the luminescent substances must be highly stable with regard to chemical influences, for example, oxygen, humidity, interactions with encapsulation materials, as well as to radiation. In order to ensure a stable color location as the system temperature rises, in addition luminescent substances with very slight temperature slaking characteristics are required.
The most efficient warm white solutions to date are based on a combination of a yellow garnet luminescent substance such as YAG:Ce or YAGaG:Ce, which contains both Al and Ga, and a nitride silicate such as (Ba,Sr,Ca)2Si5N8:Eu. In order to achieve sufficiently good color reproduction, the use of very long-wave blue LEDs (approx. 455 to 465 nm) is necessary here, system efficiency being significantly restricted as a result, however. If shorter chip wavelengths of 430 to 450 nm, preferably up to 445 nm, are used with the previous luminescent substances, however, color reproduction is poor, in particular in the blue-green spectral range. Furthermore, the heavy dependence of the CRI on the blue wavelength results in significant fluctuations of the CRI within the product. The stability of the previous solution in the LED is barely sufficient. In the case of high currents, here in particular at least 250 mA, preferably at least 300 mA, particularly preferably at least 350 mA, it is critical as the thermal load continues to rise.
The new solution consists of a combination of a green to green-yellow emitting garnet luminescent substance and a short-wave, narrow band orange-red emitting nitride silicate luminescent substance. Compared with the previously used yellow (YAG) or green-yellow (YAGaG) garnet, the green garnet luminescent substance has a strongly green-shifted emission, at the same time optimum excitation is strongly short wave-shifted. This green shift of the garnet results in a significant reduction of the blue-green gap in the white spectrum.
Due to these properties significantly shorter wave LEDs (approx. 435 nm to 445 nm peak wavelength instead of 455 nm in the previous solution) can be used and at the same time a CRI of the white LED greater than 80 can be achieved. As a result of the special spectral properties of the newly developed luminescent substance mixture, in addition the CRI remains roughly constant over a broad range of blue LED wavelengths, thus ensuring even color quality within an “LED bin”. In addition the newly developed combination of these luminescent substances is distinguished by very high chemical and photochemical stability as well as very slight temperature slaking characteristics.
Decisive progress now consists of a simultaneous improvement of several properties key from the perspective of application having been achieved, namely with regard to aging stability, efficiency, usable chip wavelength range and temperature stability of the luminescent substances. The difference between this new solution and the already known warm white solutions with low color temperatures, preferably in the range 2900 to 3500 K, in particular 2900 to 3100 K is:
Essential features of the invention in the form of a numbered list are:
Hereinafter the invention is explained in detail on the basis of several exemplary embodiments. The figures show:
The new LuAGaG luminescent substance is perfectly stable within the context of measurement errors (square measuring points) while an orthosilicate reveals clear symptoms of ageing under comparable conditions (round measuring points).
The color reproduction of the warm white LED with the new yellow-green with orange-red luminescent substance mixture according to the inventive is practically independent from the LED wavelength used. A shift in the blue wavelength of 9 nm only results in a CRI loss of 1 point. The counter-example of the previous mixture already loses 5 points where there is a difference of 7 nm in blue wavelength (see Table 1). In order to reduce the CRI loss to 1 point, the addition of a third luminescent substance is necessary, which influences efficiency and color steering negatively.
Surprisingly the new green LuAGaG garnet behaves in a significantly different manner to the comparative luminescent substances. It has a strong green shift with a declining excitation wavelength. The comparative luminescent substances remain approximately constant. The emission spectra of the three luminescent substances are shown in comparison in the blue wavelength range between 430 and 470 nm of interest for LED applications.
The curves of
The use of a lutetium garnet which at most contains Y as an admixture of up to 30 mol.-%, has a significantly positive influence on color reproduction overall as a result of the altered shape of the emission spectrum. The use of Y garnets does not result in such high color reproduction values as can be obtained with Lu garnet. Details of various mixtures can be found in Tab. 2.
As an essential component, Gd is completely unsuitable and should, just like Tb or La, only be added to the component A at the most in small amounts of up to 5 mol.-% for fine tuning. In comparison, a Y fraction of up to approx. 30%, preferably with a fraction of 10 to 25%, provides a good addition to Lu. The cause is the relatively similar ionic radius of Lu and Y. However, higher values of Y would shift the emission of the luminescent substance back into a range which would interfere with the desired performance of the overall system. Compared with yttrium garnets of a similar luminescent substance emission wavelength (sample VGL 1 to VGL 4), and surprisingly even in similarly dominant luminescent substance emission wavelengths (sample VGL 3 and VGL 4), significantly higher color reproduction values Ra8 are produced in samples 1 to 3, see Table 2. As a result of this and as a result of the good excitability of short wavelengths, for the first time highly efficient shortwave blue LEDs can be used for conversion LEDs.
In principle, the use of the luminescent substance mixture for dispersion, as a thin film, etc. directly on the LED or also as known, on a separate carrier upstream of the LED is possible.
The term luminescent substance of the type nitride silicate M2Si5N8:Eu also contains modifications of the simple nitride silicate in which Si can partially be replaced by Al and/or B and where N can be partially replaced by 0 and/or C so that through the replacement charge neutrality is ensured. Such modified nitride silicates are known per se, see for example EP-A 2 058 382. Formally such a nitride silicate can be described as M2X5Y8:D, with M=(Ba,Sr) and X=(Si,A,B) and Y=(N,O,C) and D=Eu alone or with co-doping.
Tab. 3 shows various garnets from the A3B5O12:Ce system with A selected from (Lu,Y). It is demonstrated that for A=Lu through to A=70% Lu, remainder Y good values can be obtained. At the same time the ratio between Al and Ga must be carefully selected for component B. The Ga fraction should be between 10 and 40 mol.-%, in particular 10 to 25%. Table 7 shows various A3B5O12:Ce (Lu, Y) garnets, where the concentration of the activator Ce is 2% respectively of A and A=Lu, Y (the fraction of Lu is specified, remainder is Y) and B=Al, Ga (the fraction Ga is specified, remainder is Al). Pure LuAG:Ce or YAG:Ce is unsuitable. Likewise, the addition of Pr is extremely detrimental to the efficiency of the luminescent substance and should be avoided if possible.
Tab. 4 shows pure LuAGAG luminescent substances with gradually increased Ga fraction. These table values, including those of the other tables, always relate in principle to a pure reference excitation at 460 nm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 037 732.8 | Aug 2009 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP10/61674 | 8/11/2010 | WO | 00 | 2/17/2012 |
