Luminophore combination, conversion element, and optoelectronic device

Abstract

A phosphor combination may include a first phosphor and a second phosphor. The second phosphor may be a red-emitting quantum dot phosphor. The phosphor combination may optionally include a third phosphor that is a red-emitting phosphor with the formula (MB) (TA)3-2x(TC)1+2xO4-4xN4x:E. A conversion element may include the phosphor combination. An optoelectronic device may include the phosphor combination and a radiation-emitting semiconductor chip.

Claims

1. A phosphor combination comprising: a first phosphor; a second phosphor, wherein the second phosphor is a red-emitting quantum dot phosphor; and a third phosphor that is a red-emitting phosphor, wherein the third phosphor has the formula
(MB)(TA).sub.3−2x(TC).sub.1+2xO.sub.4−4xN.sub.4x:E wherein: TA is selected from a group of monovalent metals comprising Li, Na, Cu, Ag, and combinations thereof; MB is selected from a group of divalent metals comprising Mg, Ca, Sr, Ba, Zn, and combinations thereof; TC is selected from a group of trivalent metals comprising B, Al, Ga, In, Y, Fe, Cr, Sc, rare earth metals, and combinations thereof; E is selected from a group comprising Eu, Mn, Ce, Yb, and combinations thereof; and 0<x<0.875.

2. The phosphor combination as claimed in claim 1, further comprising a third phosphor that is a red-emitting phosphor.

3. The phosphor combination as claimed in claim 1, wherein the first phosphor is a green-emitting phosphor.

4. The phosphor combination as claimed in claim 1, wherein the first phosphor is not a quantum dot phosphor.

5. The phosphor combination as claimed in claim 1, wherein the first phosphor comprises particles having a mean particle diameter ranging from 1 μm to 1000 μm.

6. The phosphor combination as claimed in claim 1, wherein the red-emitting quantum dot phosphor having a mean particle diameter ranging from 1 nm to 300 nm.

7. The phosphor combination as claimed in claim 6, wherein the red-emitting quantum dot phosphor comprises at least one of the semiconductor materials selected from the group comprising: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgTe, HgSe, GaP, GaAs, GaSb, AlP, AlAs, AlSb, InP, InAs, InSb, SiC, InN, AN, solid solutions thereof, or combinations thereof.

8. The phosphor combination as claimed in claim 1, wherein the red-emitting quantum dot phosphor has a core-shell structure.

9. The phosphor combination as claimed in claim 8, wherein the red-emitting quantum dot phosphor comprises a core having a mean diameter ranging from 1 to 200 nm.

10. The phosphor combination as claimed in claim 1, wherein the proportion of the red-emitting quantum dot phosphor relative to the total amount of phosphor in the phosphor combination comprises at most 60 percent by weight.

11. The phosphor combination as claimed in claim 1, wherein the third phosphor crystallizes in the tetragonal space group P4.sub.2/m.

12. The phosphor combination as claimed in claim 1, wherein x=0.5, such that the third phosphor has the formula (MB)Li.sub.2Al.sub.2O.sub.2N.sub.2:E, wherein MB is selected from a group of divalent metals comprising Mg, Ca, Sr, Ba, Zn, or combinations thereof, and wherein E is selected from a group comprising Eu, Mn, Ce, Yb, and combinations thereof.

13. The phosphor combination as claimed in claim 1, wherein the third phosphor has the formula SrLi.sub.2Al.sub.2O.sub.2N.sub.2:Eu.sup.2+.

14. The phosphor combination as claimed in claim 1, wherein the proportion of the third phosphor relative to the total amount of phosphor in the phosphor combination is at least 10 percent by weight.

15. The phosphor combination as claimed in claim 1, further comprising at least one further phosphor.

16. The phosphor combination as claimed in claim 1, wherein 0.45<x<0.55.

17. A conversion element comprising the phosphor combination as claimed in claim 1.

18. An optoelectronic device comprising: a radiation-emitting semiconductor chip configured to emit electromagnetic radiation in a first wavelength range; and the phosphor combination as claimed in claim 1.

19. The optoelectronic device as claimed in claim 18, further comprising a conversion element arranged on the radiation-emitting semiconductor chip and/or a potting situated on the radiation-emitting semiconductor chip; wherein the phosphor combination is present in the conversion element or in the potting.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the phosphor combination described herein is explained in more detail in conjunction with non-limiting embodiments and the associated figures.

(2) Elements which are identical, of identical type or act identically are provided with the same reference signs in the figures. The figures and the size relationships of the elements illustrated in the figures among one another should not be regarded as to scale. Rather, individual elements may be illustrated with an exaggerated size in order to enable better illustration and/or in order to afford a better understanding.

(3) FIG. 1A shows a schematic side view of a phosphor combination 10, comprising a first phosphor 1 and a second phosphor 2, which is a red-emitting quantum dot phosphor. In a non-limiting aspect, a third phosphor 3 is additionally present, for example the red-emitting phosphor SrLi.sub.2Al.sub.2O.sub.2N.sub.2:Eu. As illustrated in FIG. 1A, the phosphor combination 10 is a phosphor mixture 10.

(4) FIG. 1B shows a schematic side view of a phosphor combination 10, the phosphors not being mixed together in the case of FIG. 1B. Although the phosphor combination is a phosphor mixture, it is also possible for the phosphors to be present in a manner not mixed together.

(5) FIG. 2 shows a schematic side view of a conversion element 20, which conversion element comprises the phosphor combination 10. In non-limiting aspect, the conversion element comprises the phosphor combination as a phosphor mixture. The conversion element can comprise a matrix material 4, into which the phosphors are embedded.

(6) FIGS. 3A and 3B show in each case a schematic side view of embodiments of optoelectronic devices 30. They each comprise a semiconductor chip 50, in the beam path of which the phosphor combination is situated. In the case of FIG. 3A, a conversion element 20 comprising the phosphor combination is arranged on the semiconductor chip. FIG. 3B shows an optoelectronic device 30 comprising a semiconductor chip 50 and a potting 40. The potting material, which can be e.g. a silicone or a resin, contains the phosphor combination. The optoelectronic devices can additionally comprise a housing 60.

(7) FIG. 4 schematically shows an exemplary construction of the second phosphor. In one non-limiting embodiment of the phosphor combination, the quantum dot phosphor can have a core-shell structure comprising a core 2a and a shell 2b. For example, the core 2a contains or consists of CdSe and the shell 2b contains or consists of CdS.

(8) FIG. 5A shows the absorption properties (A) and emission properties (E) of a conventional red-emitting phosphor. The absorption ranges from the blue range of the spectrum through to the phosphor's inherent red emission. The inventors have recognized that such wide absorption results in the efficiency losses to a considerable extent.

(9) FIG. 5B shows the absorption properties (A) and emission properties (E) of a red-emitting quantum dot phosphor (QD). Quantum dot phosphors absorb almost exclusively in the desired blue range of the spectrum. The absorption and emission ranges are thus separate from one another. Undesired double conversion can thus largely be avoided.

(10) FIG. 6A shows the emission spectrum of a light-emitting diode comprising a blue semiconductor chip and a conventional phosphor combination. The blue semiconductor chip emits blue light. The blue light is partly absorbed by a first phosphor and emitted as green light (G). The blue light is furthermore partly absorbed by a second phosphor and emitted as red light (R). Furthermore, part of the green light is likewise absorbed by the second phosphor and emitted into red light (R). An undesired two-stage conversion (or double conversion) is involved here. Since the quantum efficiency (QE) is approximately 90% for each of the conversion steps, the quantum efficiency in the case of a double conversion is in total only 90%*90%=81%. The double conversion thus results in losses for the quantum efficiency. Moreover, a considerable part of the first, green-emitting phosphor contributes only to the production of red light, but not to the generation of green light. This proportion of the first phosphor acting as “sacrificial phosphor” is thus dispensable for the desired spectrum. However, its presence results in additional scattering losses. The white spectrum (W) emitted overall is thus obtained only with considerable losses concerning the total electrical efficiency.

(11) By contrast, FIG. 6B shows the emission spectrum (W QD) of a light-emitting diode comprising a phosphor combination. Here, too, the blue light from the semiconductor chip is absorbed by a conventional green phosphor. However, since the second phosphor is not a conventional red-emitting phosphor having wide absorption, but rather a red-emitting quantum dot phosphor (R QD) having narrow absorption, double conversion hardly takes place. Undesired losses for the quantum efficiency (QE) owing to double conversion can thus be significantly reduced. Moreover, an in some instances considerable amount of the first, green phosphor can be saved since less green phosphor (G) acts as sacrificial phosphor. The saving in some instances can be more than 30%, e.g. 35%. Consequently, scattering no longer takes place at this saved amount of first phosphor. Moreover, the quantum dots of the second phosphor also effect hardly any scattering. In total, therefore, the scattering that takes place is also significantly less than in the case of conventional phosphor combinations. The phosphor combination can optionally also contain a third phosphor as well. FIG. 6B shows a third phosphor, which likewise emits red (R), but is not a quantum dot phosphor. In this way, only a limited amount of the quantum dot phosphor (R QD) is required, which makes it easier to fulfil the RoHS regulations, while at the same time a highly efficient optoelectronic device is obtained.

(12) FIG. 7 explains computationally how the relative efficiency gains arise in the case of optoelectronic devices which use the phosphor combination.

(13) FIG. 8A summarizes measurement results on an LED and a reference LED and demonstrates the extent to which gains in the luminous efficiency are attributable to the improved conversion efficiency and to the improved spectral efficiency.

(14) FIG. 8B explains what phosphor combinations are used of the LEDs used in the measurements in FIG. 8A. The reference LED comprises a conventional phosphor combination comprising two conventional red-emitting nitride phosphors besides a green-emitting garnet phosphor. By contrast, the LED contains a phosphor combination comprising 1% by weight of a red-emitting quantum dot phosphor. Thus, a part of the conventional red-emitting phosphor is replaced by a red-emitting quantum dot phosphor. Since the red emission thereof is converted directly from the blue semiconductor chip emission—rather than in part also by way of photons of the green phosphor as in the conventional case—in total significantly less of the green phosphor is required. This allows the proportion of green-emitting garnet phosphor to be reduced by 35%, which results in significantly fewer scattering losses. The percent by weight indications for the phosphors in FIG. 8B relate in each case to the sum of the total weight of the phosphors and the matrix material. The phosphor combinations additionally satisfy high standards with regard to the color quality. A color temperature of approximately 3000 K, a CRI value of more than 90 in each case and an R9 value of more than 50 in each case are achieved. The LED makes it possible to achieve these high color qualities with at the same time an increase in efficiency.

(15) FIG. 9 shows the simulated emission spectra of a number of individual phosphors. The blue emission of a semiconductor chip is shown. Furthermore, the green emission of the LuAGaG:Ce phosphor is shown, which phosphor can be used as first phosphor of a phosphor combination. The red emission of a conventional Sr(Sr,Ca)Si.sub.2Al.sub.2N.sub.6:Eu.sup.2+ phosphor is additionally shown. Finally, the emission of an Sr[Al.sub.2Li.sub.2O.sub.2N.sub.2]:Eu.sup.2+ phosphor exhibiting narrowband emission is shown, which phosphor can be used as third phosphor of an phosphor combination. Moreover, the particularly narrowband emission of a CdS/CdSe quantum dot phosphor is shown, which quantum dot phosphor can serve as second phosphor of a phosphor combination.

(16) FIG. 10 shows simulations of white light LED emission spectra based on various phosphor combinations (examples 1-3 and comparative examples 1 and 2).

(17) FIG. 11 shows a table that summarizes the composition of the phosphor combinations. The table compares the respective phosphor combinations with regard to the color coordinates Cx, Cy, the color temperature (CCT), the color rendering index (CRI value), the R9 value (reference color 9, red rendering), and also the spectral efficiency (LER). It is clearly evident from the simulations that all phosphor combinations comprising a first green-emitting LuAGaG:Ce.sup.3+ phosphor and a red-emitting quantum dot phosphor as second phosphor combine a particularly high spectral efficiency (LER) with particularly good color rendering properties (examples 1-3). While example 1 has the best efficiency, example 2 represents the best combination of efficiency and environmental acceptability. Although example 1 exhibits the best efficiency, it does not accord with the RoHS regulations. On account of the presence of Sr[Al.sub.2Li.sub.2O.sub.2N.sub.2]:Eu.sup.2+ as third phosphor, example 2 makes it possible to fulfil the RoHS regulations with at the same time excellent color rendering and efficiency properties.

DETAILED DESCRIPTION

(18) Exemplary embodiments can also be combined with one another, even if such combinations are not shown explicitly in the figures. Furthermore, the exemplary embodiments described in connection with the figures can have additional or alternative features in accordance with the description in the general part.

(19) The way in which phosphor mixtures can be provided is indicated below. For producing the phosphor mixture, firstly the first and second and optionally the third phosphor are provided:

(20) As first phosphor, consideration is given to arbitrary conventional phosphors, such as green-emitting phosphors, such as, for instance, the abovementioned phosphors Si.sub.6−zAl.sub.zO.sub.zN.sub.8−z:RE, Y.sub.3(Al.sub.1−xGa.sub.x).sub.5O.sub.12:Ce, (Gd,Y).sub.3(Al.sub.1−xGa.sub.x).sub.5O.sub.12:Ce, (Tb,Y).sub.3(Al.sub.1−xGa.sub.x).sub.5O.sub.12:Ce, Lu.sub.3(Al.sub.1−xGa.sub.x).sub.5O.sub.12:Ce or (Lu,Y).sub.3(Al.sub.1−xGa.sub.x).sub.5O.sub.12:Ce. The production of these phosphors is known to the person skilled in the art. Moreover, they are commercially available.

(21) For the synthesis of red-emitting quantum dot phosphors, a large number of different syntheses are known from the prior art. Moreover, a variety of red-emitting quantum dot phosphors are commercially available.

(22) The method for producing the third phosphor is explained below:

(23) The third phosphor can be produced by means of solid-state reaction. To that end, the starting materials of the third phosphor can be mixed. By way of example, strontium nitride (Sr.sub.3N.sub.2), aluminum nitride (AlN), aluminum oxide (Al.sub.2O.sub.3), lithium nitride (Li.sub.3N) and europium oxide (Eu.sub.2O.sub.3) can be used for producing SrLi.sub.2Al.sub.2O.sub.2N.sub.2:Eu. The starting materials are mixed in a corresponding ratio with one another. The starting materials can be introduced into a nickel crucible, for example. Afterward, the mixture can be heated to a temperature of between 700° C. and 1000° C., such as 800° C. In addition, the heating can take place in a forming gas flow, the temperatures being maintained for 1 to 400 hours. The proportion of hydrogen (H.sub.2) in nitrogen (N.sub.2) can be 7.5%, for example. The heating and cooling rates can be 250° C. per hour, for example.

(24) As an alternative to the method described above, the third phosphor can also be produced by means of a solid-state synthesis in a welded-shut tantalum ampoule. To that end, the starting materials, such as for example in the case of the third phosphor SrLi.sub.2Al.sub.2N.sub.2O.sub.2:Eu, Sr.sub.3Al.sub.2O.sub.6, Li(Flux), LiN.sub.3 and Eu.sub.2O.sub.3 can be mixed in a corresponding mixture ratio with one another and can be introduced into a tantalum ampoule. By way of example, heating from room temperature to 800° C. is carried out, and the temperature is then maintained for 100 hours, for example, wherein afterward the system is cooled to room temperature again and the third phosphor has been produced. The starting materials of the third phosphor are present as powder, for example. After the heating step, a cooling process can take place, the mixture being cooled to room temperature. Room temperature is understood to mean, in particular, a temperature of 20° C. or 25° C. The synthesis is carried out at moderate temperatures and is therefore very energy-efficient. The requirements made of the furnace used, for example, are thus low. The starting materials are commercially available in a cost-effective manner and are not toxic. The phosphor mixture finally results from a combination of the abovementioned first and second phosphors and optionally the third phosphor. By way of example, powders of the phosphors can be mixed together. By way of example, the phosphors can also be introduced in each case into a matrix material and be dispersed therein. However, it is also possible to introduce each of the phosphors into a dedicated matrix material. The phosphor mixture, which should be understood to mean a combination of the phosphors, arises in this case from a combination of the different matrix materials comprising the respective phosphors.

(25) The phosphor combination according to the invention can be obtained for example by mixing powders of the abovementioned phosphors with or without an additional matrix material.

(26) The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments. Rather, the invention encompasses any novel feature and also any combination of features, which in particular includes any combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

LIST OF REFERENCE SIGNS

(27) 1 First phosphor 2 Second phosphor=quantum dot phosphor 2a Core 2b Shell 3 Third phosphor 4 Matrix material 10 Phosphor combination 20 Conversion element 30 Optoelectronic device 40 Potting 50 Semiconductor chip 60 Housing A Absorption E Emission QD Quantum dot phosphor (“quantum dot”) W White light G Green phosphor R Red phosphor QE Quantum efficiency