Light conversion device with coating comprising emitter-type and barrier-type quantum dots

Abstract

A light conversion device (40) comprises a transparent substrate (34) and a coating (14) provided on a first surface (34.1) of the substrate (34), the coating (14) including quantum dots (16, 18) configured to convert at least a part of the light emitted by the light source (12) to at least a longer wavelength, wherein the coating (14) comprises an ordered arrangement of emitter-type quantum dots (16) and barrier-type quantum dots (18), wherein each emitter-type quantum dot (16) is interposed in at least one direction between two adjacent barrier-type quantum dots (18), a bandgap (Eg1) of each emitter-type quantum dot (16) being smaller than a bandgap (Eg2) of the adjacent barrier-type quantum dots (18). The light conversion device (40) can be part of a light emitting device.

Description

[0001] Light conversion device with coating comprising emitter-type and barriertype quantum dots

[0002] Description

[0003] The present invention relates to a light conversion device, comprising a transparent substrate and a coating provided on a first surface of the substrate, the coating including quantum dots configured to convert at least a part of light of a predetermined waveband to at least a longer wavelength.

[0004] As explained later on in more detail, the substrate of the light conversion device can be, can be part of or can comprise a light source for emitting light at least in the predetermined waveband, in particular a mono-band semiconductor light source such an LED or a laser diode. As an alternative, the light conversion device can be provided separately from the light source.

[0005] LEDs do not generate truly monochromatic light but light within a narrow wavelength range. Therefore, in this application, a waveband is to be understood as referring to a wavelength range of no more than 50 nm, preferably of no more than 20 nm and more preferable of no more than 8 nm. In particular, it can refer to light with an emission spectrum having a full width at half maximum of not more than 8 nm to 20 nm centered around a particular wavelength.

[0006] The idea to use quantum dots for photon down-conversion is known e.g. for QLED displays, wherein, for each pixel, a part of the blue light produced by a primary LED is converted to red and green light by suitable quantum dots.

[0007] Furthermore, this technique can also be used for general lighting applications to generate dual band or multiband light based on a short-wavelength primary LED. An advantage of using quantum dots as compared to e.g. a conventional phosphor coating for photon down conversion is that the resulting light spectrum can be widely tuned by changing the size and thus the energy gap of the quantum dots.

[0008] In the prior art, the quantum dots provided on top of the LED are usually embedded in an unordered manner in a kind of slurry with an encapsulating material, e.g. on the basis of silicone.

[0009] In order to improve stability and quantum yield, a particular kind of quantum dots comprising a core-shell structure can be used, wherein a shell material is used to surround the core of the quantum dots, the shell material having a wider band gap than the core material. However, creating a shell with the desired material can be difficult.

[0010] Against this background, the invention proposes a light conversion device with the features of claim 1 . Particular embodiments of the invention are described in the dependent claims.

[0011] According to the invention, the coating comprises an ordered arrangement, preferably an ordered arrangement in three dimensions, of emitter-type quantum dots and barrier-type quantum dots, wherein each emitter-type quantum dot is interposed in at least one direction between two adjacent barrier-type quantum dots, a bandgap of each emitter-type quantum dot being smaller than a bandgap of the adjacent barrier-type quantum dots.

[0012] The emitter type quantum dots, e.g. made from CdSe, can thus be embedded in a matrix made from barrier-type quantum dots with desired material properties, e.g. from AIN, in order to construct quantum wells for photon-down conversion in a very controlled and easy manner.

[0013] Apart from taking part in constructing the quantum wells for photon downconversion, the barrier-type quantum dots can help to dissipate heat from the emitter-type quantum dots and to act as a protection against e.g. humidity or external light in order to prevent or reduce photobleaching.

[0014] The bandgap of the barrier-type quantum dots can be not smaller than about 3.2eV, and the bandgap of the emitter-type quantum dots can be not larger than about 2.75eV.

[0015] The coating can be provided directly on the first surface of the substrate, or an adhesive layer can be provided between the first surface and the coating. The adhesive layer can comprise e.g. an epoxy or a commercial optical glue. Furthermore, it can be provided that the coating is additionally embedded in a thermoplastic matrix, e.g. PMMA, MMA or a suitable photopolymer, or that an additional layer made from a thermoplastic material is provided on top of and / or below the coating.

[0016] The ordered arrangement can be a single layer of barrier-type and emitter-type quantum dots in an ordered arrangement in two dimensions, or it can be an arrangement of several stacked layers of barrier-type and emitter-type quantum dots.

[0017] In particular, the emitter-type quantum dots can form layers interposed between layers made from barrier type quantum dots in order to form a layered (multi) quantum well structure.

[0018] However, in order to improve heat dissipation from the emitter-type quantum dots, it can also be provided that each emitter-type quantum dot is interposed in two or more directions between two barrier-type quantum dots. Preferably, each emittertype quantum dot is completely surrounded in all directions by barrier-type quantum dots.

[0019] For ease of manufacture, the barrier-type quantum dots can all be made from the same material, e.g. from AIN. In order to generate a desired spectrum for a given application, each of the emitter-type quantum dots can belong to a predetermined number of groups, the quantum dots of each group all having the same bandgap. This can be achieved by using different materials and / or differently sized emitter-type quantum dots. The different groups correspond to different wavebands of light emitted from the device.

[0020] As one of the desired functions of the barrier-type quantum dots is to dissipate heat from the emitter-type quantum dots, the material of the barrier-type quantum dots preferably has a higher thermal conductivity than the material or materials of the emitter-type quantum dots.

[0021] At least some of, preferably all the barrier-type quantum dots can comprise or be made from e.g. a lll-V compound semiconductor material such as AIN, BN and GaN; or a group IV-IV compound semiconductor material such as SiC.

[0022] At least some of, preferably all of the emitter-type quantum dots can comprise or be made from a ll-VI compound semiconductor material such as CdSe, CdS, PbS, PbSe, ZnS and ZnSe; or a lll-V compound semiconductor material such as InP, GaP or GaS; or a perovskite material; or a 2D material such as graphene buckyballs; or CulnS2.

[0023] As these types of quantum dots are commercially available, the emitter-type quantum dots can be or can comprise core-shell-quantum dots, wherein the band gap of the core of a core-shell-quantum dot is smaller than the bandgap of the shell, and the bandgap of the shell of the core-shell-quantum dot is smaller than the bandgap of the barrier-type quantum dots.

[0024] The core-shell-quantum dots preferably comprise at least one of the following material combinations: CdSe / CdS, CdSe / ZnS, ZnS / ZnSe, InP / ZnS and InP / ZnSe.

[0025] In order to better direct the emitted light or to enhance the surface area of the device, the coating and / or a surface part of the light source or of the substrate can be formed into an arrangement of pillars preferably forming microwires. This arrangement can in particular be regular.

[0026] The applicant also claims protection for a light emitting device comprising a light conversion device according to the invention and its embodiments as described above.

[0027] According to a first alternative, the substrate of the light conversion device can be, can be part of, or can comprise a light source for emitting light in the predetermined waveband, wherein the coating of the light conversion device is provided on a light emitting surface of the light source as the first surface of the substrate.

[0028] The light source preferably is a semiconductor light source, more preferably even a mono-band semiconductor light source such as an LED or a Laser diode, most preferably an LED for generating blue and / or violet light.

[0029] According to a second alternative of the light emitting device, the light source can be formed separately from the light conversion device, so that the light emitting device comprises the light source as described above and the light conversion device according to the invention as described above.

[0030] In particular, the light emitting device can further comprise a support structure supporting the light conversion device and creating a defined gap between the light emitting surface of the light source and a second surface of the substrate of the light conversion device, the second surface being opposed to the first surface on which the coating is provided.

[0031] In the following, the present invention is explained in more detail with reference to some preferred embodiments of the invention as illustrated in the accompanying figures. Fig. 1 is a schematic illustration of a device according to a first embodiment of the invention together with two enlarged details of the coating and the schematic illustration of the band structure of a part of the coating.

[0032] Fig. 2 is a schematic illustration of a device according to a second embodiment of the invention together with two enlarged details of the coating and the schematic illustration of the band structure of a part of the coating.

[0033] Fig. 3 illustrates the composition of a part of the coating of a device according to ta third embodiment of the invention.

[0034] Fig. 4 is a schematic illustration of a device according to a fourth embodiment of the invention

[0035] Fig. 5 is a schematic illustration of a device according to a fifth embodiment of the invention, and

[0036] Figs. 6 is a schematic illustration of a device according to a sixth embodiment of the invention.

[0037] Figs. 7 and 8 are schematic illustrations of two further embodiments of the invention.

[0038] Corresponding features of different embodiments are denoted by the same reference signs in the figures illustrating these embodiments. Furthermore, for clarity reasons, if several identical features are present in one figure, some but not necessarily all of them are denoted by reference signs. In a similar manner, not all features are necessarily denoted by reference signs in each figure, but mainly those reference signs necessary or helpful for explaining the respective figure are included.

[0039] It is to be noted that all figures are highly simplified and schematic illustrations that are used mainly to illustrate some principle ideas of the invention and that are in particular not drawn to scale, unless indicated otherwise. The figures are in particular not meant to give any information regarding the thickness or thickness ratios of the illustrated layers or components in any way.

[0040] Fig. 1 shows a light emitting device 10 according to a first embodiment of the invention, the device 10 comprising a light source 12 in the form of a monoband LED 13 generating light in a first waveband A1 , e.g. violet light. In this embodiment, the light source 12 acts as the substrate 34 of the illustrated light conversion device 40

[0041] According to the invention, a coating 14 is provided on a light emitting surface 12.1 of the light source 12 being the first surface 34.1 of the substrate 34.

[0042] As illustrated in the two detail enlargements of Fig. 1 , the coating 14 comprises a layered structure comprising alternating barrier layers 22 (crosshatched) made from barrier-type quantum dots 18 and emitter layers 20 (unfilled) made from barrier-type quantum dots 16.

[0043] Each of the six illustrated emitter layers 20 in the coating 14 of Fig. 1 is formed from barrier-type quantum dots 16 belonging to a different group of emitter quantum dots adapted to generate light in six different wavebands A2-A7, e.g. dark blue, light blue, yellow, green, orange and red light as suggested by the different arrows above the device.

[0044] In this manner, each emitter-type quantum dot 16 is interposed in at least one direction (vertical in Fig. 1 ) between two barrier-type quantum dots 18, so that taking into account the different groups of emitter-type quantum dots, a multiple quantum well structure is constructed from the two different types of quantum dots, for generating a broad band spectrum based on the violet light of the primary LED 13, the spectrum comprising wavebands A1 -A7.

[0045] It is noted that of course several emitter layers, e.g. 1 to 500 emitter layers, can be provided for each group, each emitter layer interposed between two barrier layers. As illustrated on the right side of Fig. 1 , the energy gap Eg2 of the barrier-type quantum dots 18 is larger than the energy gap Eg1 of the emitter-type quantum dots 16 for all the different groups of emitter-type quantum dots.

[0046] The second embodiment illustrated in Fig. 2 mainly differs from the first embodiment in that the emitter layers 20 are not exclusively formed from emittertype quantum dots but comprise a mixture of emitter-type and barrier-type quantum dots arranged in a regular pattern so that each emitter-type quantum dot 16 is interposed in two or three (here horizontal) directions between two barriertype quantum dots 18.

[0047] In this manner heat can be more efficiently dissipated from the emitter type quantum dots 16 via the barrier type quantum dots 18 to a suitable heat sink of the device (not illustrated).

[0048] As illustrated in Fig. 3, the emitter-type quantum dots 16 can be core-shell-type quantum dots comprising a core 16c with the energy gap Eg1 and a shell 16s having an energy Egs that lies between Eg1 and the energy gap Eg2 of the barrier-type quantum dots 18.

[0049] Fig. 4 illustrates a further embodiment of the invention. The device 10 of Fig. 1 comprises a violet LED 13 generating light with a wavelength of about 405 nm as the light source 12.

[0050] Before applying the coating 14, the substrate (LED 13) is dehydrated for 30 minutes at 110°C under vacuum.

[0051] Emitter type quantum dots 16, here CdSe / CdS core-shell quantum dots with a size resulting in a bandgap corresponding to an emission wavelength of 450nm, mixed with PMMA, are provided in a first bottle, and barrier-type quantum dots 18, here AIN quantum dots, mixed with PMMA, are provided in a second bottle. Then, the first barrier layer 22.1 , the emitter layer 20 and the second barrier layer 22.2 are subsequently deposited on the substrate (light source 12) e.g. by spin coating, dip coating or inkjet printing. It is noted that the regular arrangement can result from self-organization as known from the prior art.

[0052] After depositing a layer and before depositing the next one, each layer is heated in a post-bake step at a temperature slightly below 100°C under vacuum for about 10 minutes.

[0053] In this manner a dual band light source generating light in the violet and blue range (405nm and 450nm) can be manufactured.

[0054] Figures 5 and 6 illustrate two further embodiments of the invention. In Fig. 5, after depositing the coating 14, the coating 14 and preferably also a part of the light emitting device 12 positioned underneath is or are formed into a regular arrangement of pillars 30 forming microwires in order to guide the emitted light, i.e. by conventional lithography and etching techniques.

[0055] In Fig. 6, a surface part of the light emitting device 12 is provided in a similar manner with an arrangement of pillars 30 (which can also be irregular) in order to increase the surface area, before the coating is 14 is deposited.

[0056] The embodiments of Figs. 7 and 8 mainly differ from the embodiments of the previous figures in that in Figs. 7 and 8 the light conversion device 40 according to an embodiment of the invention is formed separately from the light source 12.

[0057] In both embodiments, the coating 14 is provided on a first surface 34.1 of a substrate 34 which can be made from a suitable material that is transparent at least for light in that waveband or those wavebands generated by the light source 12, e.g. from glass, PVC, PMMA, an optical glue or a similar material. As illustrated in Figs. 7 and 8, an optional adhesive layer 32 can be provided between the light emitting surface 12.1 of the light source 12 and the substrate 34 and / or between the substrate 34 and the coating 14.

[0058] In the example of Fig. 7, the light conversion device 40 is directly fixed to the light source 12 by means of the adhesive layer 32 (which could also be omitted).

[0059] In the example of Fig. 8, the light conversion device 40 is supported above the light emitting surface 12.1 of the light source by means of a support structure 38 which is only shown in a very schematic manner and which can be made from any suitable material such a metal, plastic, etc.

[0060] In this manner, a defined gap 36 or airgap is created. The distance between the light emitting surface 12.1 of the light source and a back surface or second surface 34.2 of the substrate 34, i.e. the height h of the airgap, can be from about 0.05 mm to about 10 mm. The phonon down conversion process in this kind of device is also referred to as “remote pumping”.

[0061] It is noted that in particular in the case of Fig. 8, the light conversion device 40 can be provided, distributed and sold separately from the light source 12, and it is contemplated that existing mono-band LEDs could be retro-fitted with a light conversion device 40 according to the invention in order to create a dual-band or multi-band light source.

Claims

Claims1 . Light conversion device (40), comprising a transparent substrate (34) and a coating (14) provided on a first surface (34.1 ) of the substrate (34), the coating (14) including quantum dots (16, 18) configured to convert light of a predetermined waveband to at least a longer wavelength, characterized in that the coating (14) comprises an ordered arrangement of emitter-type quantum dots (16) and barrier-type quantum dots (18), wherein each emitter-type quantum dot (16) is interposed in at least one direction between two adjacent barrier-type quantum dots (18), a bandgap (Eg1 ) of each emitter-type quantum dot (16) being smaller than a bandgap (Eg2) of the adjacent barrier-type quantum dots (18).

2. Light conversion device (40) according to any of the preceding claims, wherein the emitter-type quantum dots (16) form emitter layers (20) interposed between barrier layers (22) made from barrier-type quantum dots (18).

3. Light conversion device (40) according to any of the preceding claims, wherein each emitter-type quantum dot (16) is interposed in two or three directions between two barrier-type quantum dots (18).

4. Light conversion device (40) according to any of the preceding claims, wherein the barrier-type quantum dots (18) are all made from the same material.

5. Light conversion device (40) according to any of the preceding claims, wherein the emitter-type quantum dots (18) belong to a predetermined number of groups, the quantum dots (18) of each group all having the same bandgap (Eg2).

6. Light conversion device (40) according to any of the preceding claims, wherein the material of the barrier-type quantum dots (18) has a higher thermal conductivity than the material or materials of the emitter-type quantum dots (16).

7. Light conversion device (40) according to any of the preceding claims, wherein at least some of the barrier-type quantum dots (18) comprise or are made from a lll-V compound semiconductor material such as AIN, BN and GaN; or a group IV semiconductor material such as SiC.

8. Light conversion device (40) according to any of the preceding claims, wherein at least some of the emitter-type quantum dots (16) comprise or are made from a ll-VI compound semiconductor material such as CdSe, CdS, PbS, PbSe, ZnS and ZnSe; a lll-V compound semiconductor material such as InP, GaP or GaS; a perovskite material; a 2D material such as graphene buckyballs; or CulnS2.

9. Light conversion device (40) according to any of the preceding claims, wherein the emitter-type quantum dots (16) are or comprise core-shell- quantum dots, wherein the band gap (Eg1 ) of the core (16c) of a core-shell- quantum dot is smaller than the bandgap (Egs) of the shell (16s), and the bandgap (Egs) of the shell (16s) is smaller than the bandgap of the barriertype quantum dots (18), and wherein the core-shell-quantum dots preferably comprise at least one of the following material combinations: CdSe / CdS, CdSe / ZnS, ZnS / ZnSe, InP / ZnS, InP / ZnSe.

10. Light conversion device (40) according to any of the preceding claims, wherein the coating (14) and / or a surface part (12s) of the light source (12) is formed into an arrangement of pillars (30), preferably forming microwires.11 . Light emitting device (10), comprising a light conversion device (40) according to any of claims 1 to 10, wherein the substrate (34) of the light conversion device (40) is, is part of or comprises a light source (12) foremitting light at least in the predetermined waveband, the light source preferably being a mono-band semiconductor light source such as an LED (13) or a laser diode, the coating (14) being provided on a light emitting surface (12.1 ) of the light source (12),12. Light emitting device (10), comprising a light conversion device (40) according to any of claims 1 to 10 and a light source (12) for emitting light at least in the predetermined waveband, the light source preferably being a mono-band semiconductor light source such as an LED (13) or a laser diode, wherein the light conversion device (40) is provided on or above a light emitting surface (12.1 ) of the light source (12).

13. Light emitting device (10) according to claim 12, furthermore comprising a support structure (38) supporting the light conversion device (40) and creating a defined gap (36) between the light emitting surface (12.1 ) of the light source and a second surface (34.2) of the substrate (34) of the light conversion device (40).

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