FIELD OF THE INVENTION

The invention relates to a luminescent material and to a luminescent material composition comprising such luminescent material. The invention further relates to a device comprising such luminescent material.

BACKGROUND

Trivalent chromium comprising luminescent materials are known in the art. X. Xu, et al., Chemical Engineering Journal 383 (2020) 123108 describe that a novel broadband near-infrared (NIR) phosphor LiInSi2O6:Cr3+(LIS:Cr3+) is successfully synthesized via solid-state reaction. Its crystal structure, photoluminescence properties as well as the application in NIR phosphor-converted LEDs (pc-LEDs) are explored. The LIS:Cr31phosphor has two absorptions bands centered at ˜460 and 670 nm, respectively. Under 460 nm excitation, it shows the broadband NIR emission peaking at ˜840 nm with a full width at half maximum (FWHM) of ˜143 nm. The optimum Cr3+concentration is found to be ˜6 mol %, whereas the intensity decrease at further increased Cr3+content is attributed to the radiative energy transfer process. The LIS:0.06Cr3+phosphor has a quantum yield of ˜75% and can retain ˜77% of the initial intensity at 150° C.

Highly efficient, thermally stable and broad NIR emissions of LIS:Cr31phosphors are caused by the Cr3+preferential occupancy in [InO6] octahedron with weak crystal field but relatively strong rigidity. The NIR pc-LED is fabricated by combining the LIS:Cr31phosphor with the blue LED chip, which has the NIR output power of ˜51.6 mW and the photoelectric efficiency of ˜17.8% at 100 mA drive current. The results suggest that LIS:Cr31phosphors have great potential for applications in broadband NIR pc-LEDs.

SUMMARY OF THE INVENTION

Semiconductor light emitting diodes and laser diodes (collectively referred to herein as “LEDs”) are among the most efficient light sources currently available. The emission spectrum of an LED typically exhibits a single narrow peak at a wavelength determined by the structure of the device and by the composition of the semiconductor materials from which it is constructed. By suitable choice of device structure and material system, LEDs may be designed to operate at ultraviolet, visible, or infrared wavelengths.

LEDs may be combined with one or more wavelength converting materials (generally referred to herein as “phosphors”) that absorb light emitted by the LED and in response emit light of a longer wavelength. For such phosphor-converted LEDs (“pcLEDs”), the fraction of the light emitted by the LED that is absorbed by the phosphors depends on the amount of phosphor material in the optical path of the light emitted by the LED, for example on the concentration of phosphor material in a phosphor layer disposed on or around the LED and the thickness of the layer.

Phosphor-converted LEDs may be designed so that all of the light emitted by the LED is absorbed by one or more phosphors, in which case the emission from the pcLED is entirely from the phosphors. In such cases the phosphor may be selected, for example, to emit light in a narrow spectral region that is not efficiently generated directly by an LED. Alternatively, pcLEDs may be designed so that only a portion of the light emitted by the LED is absorbed by the phosphors, in which case the emission from the pcLED is a mixture of light emitted by the LED and light emitted by the phosphors. By suitable choice of LED, phosphors, and phosphor composition, such a pcLED may be designed to emit, for example, white light having a desired color temperature and desired color-rendering properties.

It appears desirable to provide a luminescent material (herein also indicated as “phosphor”) having a relative broad band emission. Prior art phosphors do not always have this feature. Further, it is desirable that the luminescent material is thermally and chemically stable. However, some phosphors of the prior art are known to have a relatively low quenching temperature and/or to be relatively chemically unstable, such as towards moisture and/or air. Hence, amongst others, it is an object of the present invention to overcome or substantially ameliorate one or more of the disadvantages of prior art, or at least to provide a useful alternative.

Amongst others, a luminescent material is proposed, especially of the pyroxene type, according to the formula E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO:Crx, such as one or more of LiSc1-x-yLuySi2O6:Crx, LiSc1-x-y(Lu,Al)ySi2O6:Crx, and Li1-wSc1-x-wMg2wSi2O6:Crx, wherein Cr3+substitutes for the octahedrally coordinated Sc and/or M atoms, such as Lu and/or Al, and emits in the 700-1100 nm spectral range. Another example of such luminescent material is LiSc1-xSi2O6:Crx. In case M is chosen from the group consisting of Al and Ga (shorter average bond lengths to O compared to Sc), it is expected that the emission of Cr3+is shifted to shorter wavelengths while the emission should be shifted to longer wavelengths if M is chosen from the group consisting of In and Lu (longer average bond lengths to O compared to Sc). The emission band shape and position can thus be adjusted by changing the Sc and M atom ratio. Part of the Li and Sc+M atoms can be replaced by A atoms, such as Mg. Mg incorporation may lead to a red shift and broadening of the emission band. For larger Mg concentrations such as w≥0.5 a shift of the emission band maximum to shorter wavelengths is taking place that is caused by a structural change from the clinopyroxene to the orthopyroxene structure type. The same behavior is expected for A=Zn due to the chemical similarity of Zn and Mg. The incorporation of Ni as A atom may lead to additional emission in the 1100-1700 nm spectral range which is caused by Ni(II) on the octahedral pyroxene lattice sites.

Hence, in an aspect the invention provide a luminescent material (herein also indicated as “first luminescent material) comprising E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO:Crx. In embodiments, E may comprise one or more of Li, Na, and K. Further, in embodiments M may comprise one or more of Al, Ga, In, Tm, Yb, and Lu. Yet further, in embodiments Z may comprise one or more of Ti, Zr, and Hf In yet further embodiments, A may comprise one or more of Mg, Zn, and Ni. In embodiments, 0<x≤0.25. Further, in embodiments 0≤y≤0.75. Also, in yet further embodiments 0≤z≤2. Yet, in further embodiments 0≤u≤1. In embodiments, 0≤w≤1. Especially, in embodiments x+y+u+w≤1. Further, especially z+u≤2. Herein, the luminescent material E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx, is also indicated as the “luminescent material according to the formula” (i.e. complying with E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx), and similar phrases.

Compared to other broad band long wavelength/infrared phosphors known in the art, these phosphors appear to be relatively stable, also against acids and/or have a relatively high quenching temperature. Further, the quantum efficiency (QE) appears surprisingly high. Comparable phosphors may have quantum efficiencies below about 30%, whereas luminescent materials according to the above formula can well have quantum efficiencies of over 40%, even over 60%, such as even over 70%, like in specific embodiments even at least 80% (see also below). Further, the luminescence of the herein described materials may be relatively broad (large full width half maximum (FWHM)). Further, it appears that in embodiments the luminescent material may even have spectral power (even) in the 1050-1100 nm wavelength range. In embodiments, the luminescent material may especially be combined with a shorter wavelength emitting IR phosphor and a blue emitting pump LED to provide a broadband NIR pcLED that emits in about the 600-1100 nm wavelength range with a relatively high efficiency (see also below).

Below, some embodiments are described.

The luminescent material comprises Cr3+as broad band emitter. Especially, in the herein described luminescent materials, trivalent chromium is excitable in the blue, such as at one or more wavelengths selected from the wavelength range of 440-495 nm. Alternatively or additionally, in the herein described luminescent materials, trivalent chromium is excitable in the UV, such as at one or more wavelengths selected from the wavelength range of 380-440 nm. Hence, in embodiments the luminescent material may be excitable with first light, such as blue and/or UV radiation, especially in embodiments with blue light.

Further, it appears that the herein described luminescent materials may have centroid wavelength selected from the wavelength range of 850-900 nm, especially selected from the wavelength range of 860-890 nm. However, other (centroid) wavelengths are not excluded. The term “centroid wavelength” is known in the art, and may especially refer to the wavelength value where half of the light energy is at shorter and half the energy is at longer wavelengths; the value is stated in nanometers (nm). It is a spectral average of the intensity over the wavelength (Σλ*Iλ/(ΣI); i.e. the integration of the intensity over the emission band normalized to the integrated intensity). Centroid wavelength and full width half maximum (FWHM) are, as usual, determined at room temperature (especially 20° C.) of the respective luminescent material. Further, it appears that the herein described luminescent materials may have full width half maximum (FWHM) selected from the range of about 140-200 nm, such as selected from the range of about 150-190 nm. FWHM values of 170 and high quantum efficiencies may be obtained with the luminescent materials described herein. In embodiments, the luminescent material may emit light having a peak wavelength in a range of 700 nm to 1100 nm.

As indicated above, the luminescent material especially comprises E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx. As will be indicated below, more than one luminescent material may be applied. Further, the term “luminescent material” may also refer to a plurality of different luminescent materials complying with the formula E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx. Hence, in embodiments the term luminescent material may refer to a combination of materials, such as a composition of materials, or a multi-layer of materials, essentially consisting of two or more materials each complying with the formula (E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx) as described herein.

Here below, this luminescent material is further explained in relation to a single luminescent material according to the formula E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6.Crx. However, as is clear from the above, different embodiments of the luminescent material may be combined.

Such luminescent material (according to the formula) may especially comprise one or more of Sc, Al, Ga, In, Tm, Yb, and Lu. Trivalent chromium (Cr3+) may replace a part of the total number of atoms of the available one or more of Sc, Al, Ga, In, Tm, Yb, and Lu, especially at least part of Sc. In embodiments 0<x≤0.25. In view of the QE, the x-value may especially be equal to or below about 0.1. Hence, in specific embodiments 0.01≤x≤0.1. Especially, x is at least about 0.025 in view of output of the luminescent material. In yet further specific embodiments, x is selected from the range of 0.04-0.07.

Especially, the luminescent material comprises from the group of Sc, Al, Ga, In, Tm, Yb, and Lu, at least one or more of Sc and Lu. Alternatively or additionally, Al may be available. Hence, in specific embodiments the luminescent material comprises from the group of Sc, Al, Ga, In, Tm, Yb, and Lu, at least one or more of Sc, Lu and Al. Hence, in specific embodiments, from the group of Sc, Al, Ga, In, Tm, Yb, and Lu, essentially only one or more of Sc, Al and Lu are available. Especially, at least Sc may be available.

Good results were obtained in embodiments with Sc and in other embodiments with Sc+Lu. Hence, especially the luminescent material according to the formula comprises one or more of Sc and Lu. Further, in embodiments the number of atoms Sc+Lu>Al+Ga+In+Tm+Yb. Here, this latter formula refers to both embodiments where only Sc may be available and embodiments where Sc+Lu is available. Especially, in embodiments (Al+Ga+In+Tm+Yb)/(Sc+Lu)<1.3, more especially (Al+Ga+In+Tm+Yb)/(Sc+Lu)<0.9, even more especially (Al+Ga+In+Tm+Yb)/(Sc+Lu)<0.8. Yet further, in (alternative) specific embodiments (Al+Ga+In+Tm+Yb+Lu)/(Sc)<3.2, even more especially (Al+Ga+In+Tm+Yb+Lu)/(Sc)<1.5, yet even more especially (Al+Ga+In+Tm+Yb+Lu)/(Sc)<0.5.

Hence, as is clear from the above, M may or may not be available (i.e. y may not be zero or may be zero), but good results are obtained when M comprises at least Lu. As indicated above, in embodiments M comprises one or more of Al, Ga, In, Tm, Yb, and Lu. Further, good results were obtained with 0≤y≤0.75, especially in embodiments 0≤y≤0.50, such as even more especially 0≤y≤0.2. Hence, in embodiments y=0, and in other embodiments y>0.

Therefore, in specific embodiments M may be available, especially (though in embodiments not exclusively) as Lu. In such embodiments, especially 0.02≤y≤0.2, even more especially 0.05≤y≤0.1. With such ranges, good QEs may be obtained. Therefore, in specific embodiments M comprises Lu, even more especially M essentially consists of Lu, and 0.02≤y≤0.2.

The element E may or may not be available. Hence, in embodiments 0≤w≤1. Especially, in embodiments E comprises one or more of Li, Na, and K. When E is available, good results were obtained when E at least comprises Li. Especially, when E is available, E essentially consists of Li. Hence, in embodiments E at least comprises Li. Especially, in embodiments (Na+K)/Li<0.5, even more especially (Na+K)/Li<0.1, yet even more especially (Na+K)/Li<0.05. Good results were obtained when w=0 or when w≠0 then w<1, such as w≤0.5 (see also below in relation to A). Therefore, in specific embodiments E is available and essentially only consists of Li.

The luminescent material according to the formula may comprise one or more of Si, Ti, Zr, and Hf (and Ge, see also below). Or, in other words, luminescent material according to the formula may comprise one or more of Si and Z. As indicated above, Z comprises one or more of Ti, Zr, and Hf. Z may or may not be available. Hence, in embodiments 0≤u≤1. When u>0, Z is available. Good results were obtained when Z at least comprises Zr. Hence, in embodiments Z at least comprises Zr. Further, in embodiments 0≤u≤0.25. Larger values of u may be less desirable in view of e.g. QE and/or may be more difficult to form. Especially, when Z is available, Z may essentially consist of Zr and 0≤u≤0.25.

Further, as indicated above one or more of Si and Ge may be available. Therefore, in embodiments 0≤z≤2. In view of the quantum efficiency, it appears that in general the luminescent materials with a larger Si atom content than Ge content are better. Hence, in embodiments 0≤z≤0.05. In yet more specific embodiment, z=0. In embodiments, z+u≤2. When z=0 and u=0, the luminescent material according to the formula is essentially a silicate.

Especially, in embodiments (Ti+Zr+Hf)/Si<0.2, even more especially (Ti+Zr+Hf)/Si<0.1, yet even more especially (Ti+Zr+Hf)/Si<0.05. Note that Ti, Zr, Hf may be incorporated on the octahedral sites for Sc and Ge may be incorporated on the tetrahedral sites for Si. As indicated above, in specific embodiments in embodiments 0≤u≤0.25.

Part of the above-mentioned Sc and/or M atoms may be replaced by E+A atoms. As indicated above, in embodiments E comprises one or more of Li, Na, and K. Further, in embodiments A comprises one or more of Mg, Zn, and Ni. A may thus be available in the luminescent material according to the formula or may not be available. Hence, in embodiments w=0 and in other embodiments w>0. Especially, in embodiments 0≤w≤0.5. Larger values of w may be less desirable in view of emission peak position and spectral width but may show a cost advantage over materials with lower w. Good results were obtained when A at least comprises Mg. Therefore in embodiments A at least comprises Mg. Hence, in further specific embodiments, wherein A is available, A may essentially consist of Mg. Hence, in specific embodiments A is available and may essentially consist of Mg, with even more especially 0≤w≤0.5 (see further also below).

Mg incorporation may lead to a red shift and broadening of the emission band. For larger Mg concentrations, such as w≥0.5, a shift of the emission band maximum to shorter wavelengths is taking place that may be caused by a structural change from the clinopyroxene to the orthopyroxene structure type. Especially interesting for the application of these phosphors appear composition according to the formula with w values in the range 0.1≤w≤0.4, because some structural disorder is being observed that surprisingly turned out to be beneficial for the application (broadened emission spectrum and high quantum efficiency). The structural disorder can be analyzed by means of X-ray diffraction. While the crystallographic b and c axis shorten with increasing w while the average distance between the layers of chains of edge-sharing octahedra lying in the planes are getting larger. The layer spacing corresponds to aorthopyroxene/4 or sin(β) aclinopyroxene/2. The degree of disorder can be influenced by the processing parameters such as firing temperature, firing time or for example heating and cooling rates. A lower firing temperature can for example be used if a fluxing agent is being added to the firing mixture.

XRD results indicate that powder samples with compositions in-between those of the boundary phases LiScSi2O6(clinopyroxene structure type) and Li0.5Sc0.5MgSi2O6(orthopyroxene structure types) show stacking disorder along the [100] direction with sequences belonging to either the clinopyroxene or orthopyroxene structure type.

The disorder leads to an increase of the average layer thickness. While b and c axis shorten with higher w parameter, the intralayer distance increases. With aorthopyroxene=2 sin β*aclinopyroxenethe change of average cell dimensions as function of Mg content is given in Table 1 below.

TABLE 1wa0/4 = sin(β * am/2 [Å]b [Å]c [Å]04.5888.9265.3520.14.5948.9125.3420.254.5948.8955.3230.354.5868.8715.3090.54.5858.8345.302

Further, in embodiments (in view of stoichiometry) x+y+u+w≤1. Yet further, x+y+u+w>0, as already x may be larger than 0.

Referring to E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6.Crx, it appeared that Mg may (advantageously) broaden the spectrum, (essentially) without lowering QE. Further, it appeared that a partial replacement of Li and Sc by Mg is still processable. Ni2+may show (SWIR) emission >1600 nm when incorporated on the octahedral site. Zn2+might be used instead of Mg2+(or in addition to Mg2+). However, Mg may lead to a higher QE than Zn. Li variants seem to be more stable than K or Na variants. Hence, (Na+K)<Li (see also above). Especially, (Na+K)/Li<0.1, more especially (Na+K)/Li=0. Li variants may also provide a higher QE than Na and/or K variants. Availability of Lu may have an advantageous effect on the QE and or efficiency compared to inclusion of In. Ge may be used to tune the emission characteristics, but the content of Ge may be relatively low in view of QE. Especially, Ge<Si, even more especially Ge/Si<0.05, yet even more especially Ge/Si=0.

In specific embodiments of the luminescent material, E at least comprises Li; M at least comprises Lu and/or A at least comprises Mg; 0.01≤x≤0.1; 0≤y≤0.2; and 0≤z≤0.5. Such luminescent materials appear to provide relatively high QEs. Further, band widths and centroid wavelengths of such luminescent materials are useful. Also the stability against chemical attack such as mineral acids or hot water vapor appeared to be relatively high and thermal quenching of emission appeared to be relatively low (i.e. high quench temperature).

As indicated above, especially good results were obtained with embodiments wherein the luminescent material comprises one or more of LiSc1-x-yLuySi2O6:Crx, LiSc1-x-y(Lu,Al)ySi2O6:Crx, and Li1-wSc1-x-wMg2wSi2O6:Crx. Other combinations of luminescent materials according to the formula may also be possible.

Herein, terms like “Lu,Al”, and similar terms, such as “Al, Ga, Sc” indicate that one or more of the indicate elements may be selected. Hence, “Lu,Al” may refer to Lu, to Al, or to both Lu and Al. Similarly this may apply to similar terms.

Amongst others, the present invention provides novel pyroxene type phosphors that are characterized by broad spectra in the IR spectral range that enable the production of phosphor converted LEDs comprising these phosphors and show superior emission properties for e.g. IR spectroscopy and hyperspectral imaging applications. Further, amongst others the present invention provides chemically stable pyroxene type phosphors that can e.g. be washed with diluted mineral acids to increase performance and improve reliability. To this end—amongst others—Li containing pyroxene type phosphors are being disclosed.

In yet a further aspect, the invention also provides a luminescent material composition, including one or more luminescent materials according to the herein described formula as well as one or more other luminescent materials (“second luminescent material”). Especially, the luminescent materials are chosen such, that both types of luminescent material are both excitable at least one wavelength, such at least both being excitable at one or more wavelengths in the blue wavelength range.

As indicated above, the term “luminescent material” may in specific embodiments also refer to a combination of two or more luminescent material according to the formula indicated above. When using different luminescent materials, the spectral power distribution of the luminescence can be broadened. This may be useful for applications (see also below). As indicated above, in embodiments the luminescent material may be provided as a combination of materials, such as a composition of materials, or a multi-layer of materials, essentially consisting of two or more materials each complying with the formula (E1-wSc1-x-y-u-wMyZuA2wSi2-z-uGezAluO6:Crx) as described herein.

Alternatively or additionally, a second luminescent material may be applied (not conform the formula). Also in this way the spectral power distribution of the luminescence can be broadened, such as to shorter wavelengths (e.g. red) and/or to larger wavelengths (IR). Hence in an aspect the invention also provides a luminescent material composition comprising (a) the luminescent material (“first luminescent material”) as defined herein and (b) a second luminescent material. Especially, the luminescent material is excitable with first light, such as blue and/or UV radiation. Further, especially the luminescent material is configured to provide first luminescence upon irradiation with the first light. Especially, in embodiments the second luminescent material is configured to provide second luminescence (also) upon irradiation with the first light. Further, especially in embodiments the luminescent material and the second luminescent material are configured to provide first and second luminescence in one or more of the red and infrared wavelength ranges. Yet further, in specific embodiments the first and second luminescence have different centroid wavelengths (λ1,λ2). Hence, the spectral power distribution of the emission of such luminescent material composition may be broadened relative to the spectral power distribution of the emission of a single luminescent material according to the formula. Hence, in specific embodiments the invention also provides a luminescent material composition comprising (a) the luminescent material according to any one of the preceding claims and (b) a second luminescent material; wherein the luminescent material is excitable with first light, wherein the luminescent material is configured to provide first luminescence upon irradiation with the first light, wherein the second luminescent material is configured to provide second luminescence upon irradiation with the first light, wherein the luminescent material and the second luminescent material are configured to provide first and second luminescence in one or more of the red and infrared wavelength ranges, and wherein the first and second luminescence have different centroid wavelengths (λ1,λ2).

The different luminescent materials may not only be provided as composition, but may also be provided as multi-layer, such as two or more layers, of which at least two have different composition, and of which one of these at least two may comprise the first luminescent material and less or no second luminescent material, and of which another one of these at least two may comprise the second luminescent material and less or no first luminescent material. Further, different luminescent materials may also be provided at different locations in a light device (see further below).

Further, in specific embodiments the first luminescent material and second luminescent materials are chosen such that the spectral power distributions of the emissions in the red and/or infrared (spectrally) overlap. Further, especially the emission of the second luminescent material (in the red and/or infrared) is a broad band emission, such as an emission having a FWHM of at least 100 nm.

In embodiments, the (first) luminescent material may also be indicated as “NIR phosphor”, as the (first) luminescent material may especially be configured to have one or more emission wavelengths in the range of 700-1100 nm. Further, the second luminescent material may be indicated as “second phosphor”. The second phosphor may be configured to have one or more emission wavelengths in the red and/or NIR. The term “NIR” may especially refer to the 700-1100 nm wavelength range. Hence, the term NIR may also refer to some red hues.

Especially, the second luminescent material may be a luminescent material that emits at least in the NIR, such as at one or more wavelengths selected from the wavelength range of 620-780 nm, such as especially selected from the wavelength range of 620-750 nm.

A suitable luminescent material may be RE3Ga5-x-yAxSiO14:Cry(RE=La, Nd, Gd, Yb, Tm; A=Al, Sc), wherein 0≤x≤1 and 0.005≤y≤0.1. These luminescent materials show strong emission in the wavelength range >900 nm and can thus enhance the spectral power distribution at longer wavelengths in the 900-1100 nm range. Alternatively or additionally, a suitable luminescent material may be Gd3-xRExSc2-y-zLnyGa3-wAlwO12:Crz(Ln=Lu, Y, Yb, Tm; RE=La, Nd), wherein 0<x≤3; 0≤y≤1.5; 0≤z≤0.3; and 0≤w≤2. These luminescence materials show strong emission in the wavelength range <800 nm and can thus enhance the spectral power distribution in the 700-850 nm wavelength range. Alternatively or additionally, a suitable luminescent material may be AAEM1-xF6:Crx(A=Li, Cu; AE=Sr, Ca; M=Al, Ga, Sc), wherein 0.005≤x≤0.2. Alternatively or additionally, a suitable luminescent material may be A2-x(WO4)3:Crx(A=Al, Ga, Sc, Lu, Yb), wherein 0.003<x≤0.5. Alternatively or additionally, a suitable luminescent material may be Sc1-x-yAyMO:Crx, wherein MO=BO3, or MO=P3O9, or MO=(BP3O12)0.5, or MO=(SiP5O19)0.34, with A=Lu, In, Yb, Tm, Y, Ga, Al, wherein 0<x≤0.25, 0≤y≤1.

Alternatively or additionally, a suitable luminescent material may be M2-xSi5-yAlyOyN8-y:Eux(M=Ba, Sr, Ca), wherein 0<x≤0.05, 0≤y≤0.1. Alternatively or additionally, a suitable luminescent material may be M1-xSiAlN3:Eux(M=Sr, Ca), wherein 0<x≤0.03. Alternatively or additionally, a suitable luminescent material may be M1-xLiAl3N4:Eux(M=Ba, Sr, Ca), wherein 0<x≤0.02.These red emitting materials can further excite the Cr(III) activator ions in the first luminescent materials and therefore improve luminescence conversion efficiency. They also can broaden the composed phosphor emission spectrum in the 620-780 nm range.

Hence, in embodiments the second luminescent material may comprise one or more of: (i) RE3Ga5-x-yAxSiO14:Cry(RE=La, Nd, Gd, Yb, Tm; A=Al, Sc), wherein 0≤x≤1 and 0.005≤y≤0.1; (ii) Gd3-xRExSc2-y-zLnyGa3-wAlwO12:Crz(Ln=Lu, Y, Yb, Tm; RE=La, Nd, Lu), wherein 0≤x≤3; 0≤y≤1.5; 0≤z≤0.3; and 0≤w≤2; (iii) AAEM1-xF6:Crx(A=Li, Cu; AE=Sr, Ca; M=Al, Ga, Sc), wherein 0.005≤x≤0.2; (iv) A2-x(WO4)3:Crx(A=Al, Ga, Sc, Lu, Yb), wherein 0.003<x≤0.5; (v) Sc1-x-yAyMO:Crx, wherein MO=BO3, or MO=P3O9, or MO=(BP3O12)0.5, or MO=(SiP5O19)0.34, with A=Lu, In, Yb, Tm, Y, Ga, Al, wherein 0<x≤0.75, 0≤y≤1; (vi) M2-xSi5-yAlyOyN8-y:Eux(M=Ba, Sr, Ca), wherein 0<x≤0.05, 0≤y≤0.1; (vii) M1-xSiAlN3:Eux(M=Sr, Ca), wherein 0<x≤0.03; and (viii) M1-xLiAl3N4:Eux(M=Ba, Sr, Ca), wherein 0<x≤0.02.

An especially suitable material appears to be a Gd and/or Sc based garnet with chromium. This material has a relatively high thermal quenching and a relatively high quantum efficiency. Further, the spectral power distribution of the emission may (only) partly overlap with the spectral power distribution of the emission of the first luminescent material as described herein. Hence, in embodiments the second luminescent material comprises Gd3-xRExSc2-y-zLnyGa3-wAlwO12:Crz(Ln=Lu, Y, Yb, Tm; RE=La, Nd, Lu), wherein 0≤x≤3; 0≤y≤1.5; 0≤z≤0.3; and 0≤w≤2. Especially 0<x≤1 and RE=La, Lu, 0≤y≤0.5 and Ln=Lu, 0.04≤z≤0.16 and 0≤w≤1. Even more especially is 0.05<x≤0.2 and RE=Lu, 0.1≤y≤0.3 and Ln=Lu, 0.06≤z≤0.12 and 0≤w≤0.04.

The luminescent material and/or luminescent material composition may be used to generate light in the red and/or infrared. To this end, a device may be provided comprising a light source (herein also indicated as first light source), wherein the (first) luminescent is configured to convert at least part of the first light, generated by the first light source, into first luminescence. As indicated above, the first luminescence may have centroid wavelength selected from the wavelength range of 850-900 nm, especially selected from the wavelength range of 860-890 nm. Further, the first luminescence may have full width half maximum (FWHM) selected from the range of about 140-200 nm. In embodiments, the luminescent material may emit light having a peak wavelength in a range of 700 nm to 1100 nm. Especially, the light source comprises a solid state light source, like a LED (light emitting device).

Hence, in yet a further aspect the invention provides a device comprising (a) a first light source configured to generate first light, and (b) the luminescent material as defined herein, wherein the luminescent material is configured to convert at least part of the first light in first luminescence. In specific embodiments, the luminescent material may be configured to essentially convert all the first light source light (at least partly) into first luminescence. In such embodiments, there may essentially not remain any first light.

Of course, also the luminescent material composition may be applied. In such embodiments, the first light source may be used to excite the first luminescent material and the second luminescent material. Alternatively or additionally, the excitation spectrum of the second luminescent material may better match with another light source. Hence, optionally a second light source may be applied configured to excite at least the second luminescent material. In such embodiments, the second light source may be configured to generate second light that has another spectral power distribution than the first light of the first light source.

Note that the term “first light source” may also refer to a plurality of first light sources (which may all essentially be of the same type, such as especially of the same bin). Note that the term “second light source” may also refer to a plurality of different first light sources (which may be of different types, such as of different bins).

Especially, the light source(s) indicated herein are solid state light sources, such as LEDs.

Hence, in embodiments the device may comprise (i) the second luminescent material as; and (ii) optionally a second light source configured to generate second light; wherein the second luminescent material is configured to convert one or more of (a) part of the second light and (b) at least part of the optional second light into second luminescence.

Especially good results may be obtained when the first luminescent material comprises one or more of LiSc1-x-yLuySi2O6:Crx, LiSc1-x-y(Lu,Al)ySi2O6:Crx, and Li1-wSc1-x-wMg2wSi2O6:Crx, and when the second luminescent material comprises Gd3-xRExSc2-y-zLnyGa3-wAlwO12:Crz(Ln=Lu, Y, Yb, Tm; RE=La, Nd), wherein 0≤x≤3; 0≤y≤1.5; 0≤z≤0.3; and 0≤w≤2.

Further, the term “second luminescent material” may also refer to a plurality of different second luminescent materials (e.g. emitting at different wavelengths).

The device may be used to irradiate a material or a product. The red and/or infrared light may be used for different purposes. A purpose may be to detect specific species, which may absorb and/or convert into radiation the luminescence of the first luminescent material (and the optional second luminescent material). For instance, in embodiments the device may be used to irradiate human tissue such as for detecting tumor tissue. Further, in embodiments the device may be used to detect e.g. foodstuff ingredients such as sugars, carbohydrates, proteins or fat, etc. Therefore, in specific embodiments the device may also comprise a sensor. Hence, the invention also provides in embodiments the device as described herein further comprising an optical sensor configured to detect radiation in one or more of the red and infrared wavelength ranges. Sensing may be done in a transmissive mode or in a reflective mode.

In specific embodiments, the device may be used for, or may be configured for, hyperspectral imaging.

Hence, in an aspect the invention also provides a wavelength converting structure comprising the first luminescent material and a first light course. Hence, in embodiments, the wavelength converting structure further comprising a light source emitting a first light, the wavelength converting structure disposed in a path of the first light, wherein the NIR phosphor absorbs the first light and emits a second light, the second light having a wavelength range of 700 nm to 1100 nm.

In embodiments, the wavelength converting structure further comprising a second phosphor material, wherein the second phosphor material includes at least one of a green phosphor, a red phosphor, and an IR phosphor.

Instead of the term “luminescent material” also the term “phosphor material” may be applied. The phosphor may comprise a combination of two or more of the herein described phosphors. Two phosphors having the same general formula, but having a different compositions. This may lead to different spectral power distributions.

