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Definitions

• the present invention is directed to novel light emitting device comprising a stack of luminescent layers, especially a pcLED comprising a combination of a green emitting luminescent ceramic material and an orange emitting luminescent layer.
• Phosphors comprising silicates, phosphates (for example, apatite) and aluminates as host materials, with transition metals or rare earth metals added as activating materials to the host materials, are widely known.
• phosphates for example, apatite
• aluminates as host materials, with transition metals or rare earth metals added as activating materials to the host materials.
• transition metals or rare earth metals added as activating materials to the host materials
• Today ' s warm white phosphor converted LEDs are produced e.g. by combining a YAG phosphor with a red phosphor powder layer. Variations in phosphor layer thickness currently lead to variation of the white LED color point away from the Planckian locus, which in general is an aim.
• An advantage of layered phosphor combinations compared to a powder mixture of two phosphor layers is a reduced interaction of the phosphors, i.e. absorption of emitted light of the green phosphor by the red emitting phosphor. Due to variations in the red phosphor layer thickness the resulting change of color moves away from the Planckian locus and the resulting emission color is no longer white.
• LEDs having a optimized luminous efficiency and color rendering which are usable within a wide range of applications.
• a light emitting device comprising a blue emitting LED, a first phosphor layer exited at the emitting wavelength of the LED, which ceramic phosphor has an emission wavelength within a range of between 500 nm and 560 nm, and a second phosphor layer having a color point u' in the range between 0.24 and 0.35 and a peak emission of the phosphor layer of ⁇ > 600nm.
• u ' is defined by the definitions of the uniform color space CIE 1976. According to CIE 1976 and CIE Standard S 014-5/E:2009 u is defined as: ,
• the phosphor having a color point u' in the range between 0.24 and 0.35 and a peak emission of the phosphor layer of ⁇ > 600nm comprises a material according to the general formula
• M is selected from the group consisting of Ca, Sr or mixtures thereof; and whereby
• the term "essentially” means especially that > 95 %, preferably > 97 % and most preferred > 99 % wt-%.
• LEDs may be built which show improved lighting features, especially color rendering in warm white phosphor converted LEDs.
• the material has been found to have a broad band emission and an appropriate red emission especially suited for white pcLEDs.
• the material has been found to be capable to compensate color variations caused by varying layer thickness of red phosphor layers in pcLEDs.
• x is >0.0001 and ⁇ 0.005, preferably >0.0002 and ⁇ 0.0005. This has been found to be advantageous for many applications, since when x is too high, the color point of the material is not in the range to compensate the color variations caused by the varying thickness red phosphor layers in pcLEDs. If x is too low, the color rendering properties deteriorate.
• the luminescent material is a ceramic body.
• ceramic in the sense of the present invention means and/or includes especially a crystalline or polycrystalline compact material or composite material with a controlled amount of pores or which is pore free.
• y is >0.001 and ⁇ 0.04, preferably >0.002 and ⁇ 0.005, and more preferred >0.02 and ⁇ 0.03.
• Ce is deemed to act as a second emitting material. Ce doping makes the material emitting in a green to orange color range, while Eu doping makes the material emitting in a deep red color range. Surprisingly, doping of the material with both rare-earth elements makes the material emitting in the inventive way.
• z is >0.05 and ⁇ 0.25, preferably >0.1 and ⁇ 0.15.
• the oxygen content in the material is deemed to be a result of impurities of the basic material. However, if the contend of oxygen in the material is too high, the red component of the emission will be too low. This makes the material emitting in a yellow range, rather than emitting in the intended color range.
• the content of Ca in M is >80%
• the content of Sr in M is >80%
• the material has a color point u' in a CIE chromaticity diagram in the range of between 0.24 and 0.35. This has been found to be advantageous for many applications, since this color point is appropriate to compensate color variations of red phosphor layers in pcLEDs.
• the peak emission of the second phosphor layer ⁇ is > 600nm, preferably ⁇ is > 610 nm. This has been found to be advantageous for many applications, since it enables an adequate color rendering CRI > 75.
• the blue emitting LED may have a peak emission wavelength within the range of between 400 nm and 480 nm. This has been found to be advantageous for many applications, since such an emission wavelength excites the ceramic green emitting phosphors as well as the phosphor layer having a color point u' in the range between 0.24 and 0.35 and a peak emission of the phosphor layer of ⁇ > 600nm in a way, that an optimized luminous efficiency and color rendering can be achieved.
• the first phosphor layer is a sintered polycrystalline ceramic body.
• the ceramic body may have a layer thickness within the range of between 80 ⁇ and 300 ⁇ .
• the first phosphor layer is a ceramic body, like e.g. a lumiramicTM.
• the phosphor layer having a color point u' in the range between 0.24 and 0.35 and a peak emission of the phosphor layer of ⁇ > 600nm is a mixture of different phosphor materials, which mixture leads to a phosphor layer having the mentioned properties with respect to the color point and the peak emission wavelength of the emission spectra.
• the mixture comprises a first luminescent material of the general formula Lu 3 Al 5 0i 2 :Ce or SrSi 2 0 2 N 2 :Eu and a second luminescent material which may be a conventional red phosphor material, like e.g.
• the present invention furthermore relates to a light emitting material, especially a LED, comprising at least one material as described above.
• a light emitting device may be of use in a broad variety of systems and/or applications, amongst them one or more of the following:
• Fig. 1 shows CIE 1976 color coordinates of an of a state of the art pcLED comprising a red phosphor and a pcLED comprising a luminescent material according to an embodiment of the invention
• Fig. 2 shows CIE 1976 color coordinates with color points for a blue LED, a green luminescent ceramic (lumiramicTM) and an orange luminescent material according to an embodiment of the invention
• Fig. 3 shows a comparison of an emission spectrum of a luminescent material according to an embodiment of the invention and of a red phosphor according to the state of the art.
• Fig. 4 shows the emission spectra of another embodiment according to the invention.
• Fig. 1 shows CIE 1976 color coordinates of a state of the art pcLED comprising a red phosphor and a pcLED comprising a luminescent material according to an embodiment of the invention having the generic formula CaSiAlN3_ z O z :Eu 2+ x Ce 2+ y , whereby x is 0.004, y is 0.01 and z is 0.06.
• Fig. 1 represents the variation of the color points of a state of the art combination of a lumiramic with a red phosphor at increasing thickness of the red phosphor layer (thickness increasing from left to right). As it can be seen the color point strongly depends on the layer thickness of the red phosphor layer.
• the color point of the luminescent material according to the invention is varying almost only along the Planckian with varying thickness of the red/orange phosphor layer. This is due to the broad band emission of the inventive luminescent material, which enables good color rendering CRI >75 for white LEDs.
• Fig. 2 shows CIE 1976 color coordinates with color points for a blue LED, a green luminescent ceramic (LumiramicTM) and an orange luminescent material according to an embodiment of the invention.
• a luminescent material range phosphor according to the invention is selected and combined in a pcLED-device with an green luminescent ceramic
• the white color points vary along the Planckian with varying thickness of the red /orange phosphor layer.
• the use of a luminescent material according to the invention therefore makes the production of white pcLEDs on basis of a blue emitting LED much more reliable since the effect of the phosphor layer thickness on the color point is significantly reduced.
• Fig, 3 shows an emission spectrum of a luminescent material (CECAS) according to the generic formula CaSiAlN3- z O z :Eu 2+ x Ce 2+ y , whereby x is 0.004,y is 0.0 land z is 0.06 for 450 nm excitation with an emission maximum at about 625 nm and an FWHM (full width half maximum) of more than 120 nm.
• a broad emission enables to compensate layer thickness variation of the phosphor layer over a wide range.
• the emission spectra of a conventional red phosphor is shown.
• Fig. 4 shows the emission spectra of another embodiment according to the invention.
• the first green phosphor is a luminescent material according to the general formula Lu 3 Al 5 0i 2 :Ce
• the second green phosphor is a luminescent material according to the general formula SrSi 2 0 2 N2:Eu.
• the green phosphors are mixed as powders, individually, with a red phosphor powder in a ratio of 1 : 2 by weight. 16.2 % vol.-% of the mixed phosphor powders were added to a silicone.
• the spectra were measured using a layer with a thickness of the silicon -phosphor-mixture of about 25 ⁇ . As one can see in the spectra, the resulting phosphor mixtures have a ⁇ > 600nm.

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• 2011
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