Highly stable QDS-composites for solid state lighting and the method of making them through initiator-free polymerization

Abstract

The invention provides a lighting device comprising (i) a light source configured to generate light source light, and (ii) a light converter configured to convert at least part of the light source light into visible converter light, wherein the light converter comprises a polymeric host material with light converter nanoparticles embedded in the polymeric host material, wherein the polymeric host material is based on radical polymerizable monomers, and wherein the polymeric host material contains equal to or less then 5 ppm radical initiator based material relative to the total weight of the polymeric host material.

Claims

1. A device comprising: a light source for generating light source light, a light converter for converting at least part of the light source light into visible converter light, the light converter comprising: a polymeric host material; light converter nanoparticles embedded in the polymeric host material; wherein: the polymeric host material is based on radical polymerizable monomers, and the polymeric host material contains equal to or less than 5 ppm radical initiator based material relative to the total weight of the polymeric host material; and first and second substrates, wherein the first and second substrates sandwich the polymeric host material and the light converter nanoparticles.

2. The device of claim 1 wherein the first and second substrates are glass plates.

3. The device of claim 1 wherein the first and second substrates are barrier films.

4. The device of claim 1 wherein the first substrate is a glass plate and the second substrate is a barrier film.

5. The device of claim 1 wherein the first substrate is a hard metal oxide coating.

6. The device of claim 1 wherein the first and second substrates are encapsulated by a perimeter encapsulation.

7. The device of claim 1 wherein the first substrate is a multi-layer film.

8. The device of claim 1 wherein the first and second substrates are oxygen non-permeable.

9. The device of claim 1 further comprising glue disposed on edges of the light converter.

10. The device of claim 1 wherein the first substrate comprises a container in which the polymeric host material and the light converter nanoparticles are disposed, and the second substrate comprises a cover glued or welded to edges of the container.

11. A device comprising: a light source for generating light source light, a light converter for converting at least part of the light source light into visible converter light, the light converter comprising: a polymeric host material; and light converter nanoparticles embedded in the polymeric host material; wherein: the polymeric host material is based on radical polymerizable monomers, the polymeric host material contains equal to or less than 5 ppm radical initiator based material relative to the total weight of the polymeric host material, and a portion of the light converter is reflective.

12. The device of claim 11 wherein the portion of the light converter that is reflective forms a wall of a light mixing chamber.

13. The device of claim 12 wherein the light mixing chamber comprises an exit window, wherein the exit window is a portion of the light converter that is transmissive.

14. The device of claim 11 wherein the light source light is blue and the visible converter light is one of yellow, green, and red.

15. The device of claim 11 wherein substantially all of the light source light is converted by the light converter, such that light exiting the device is substantially all visible converter light.

16. A device comprising: a light source for generating light source light, a light converter for converting at least part of the light source light into visible converter light, the light converter comprising: a polymeric host material; and light converter nanoparticles embedded in the polymeric host material; wherein: the polymeric host material is based on radical polymerizable monomers, and the polymeric host material contains equal to or less than 5 ppm radical initiator based material relative to the total weight of the polymeric host material; and particles not having nanoparticle character disposed in a path of one of the light source light and the visible converter light.

17. The device of claim 16 wherein the particles not having nanoparticle character are micron sized particulate inorganic luminescent material.

18. The device of claim 16 wherein the particles not having nanoparticle character are reflective Ti02 particles.

19. The device of claim 16 wherein the light converter is enclosed by an encapsulation, wherein the encapsulation is configured to reduce exposure of the light converter to 02.

20. The device of claim 16 wherein the polymeric host material is selected from the group consisting of a poly vinyl polymer, a poly acrylate polymer and a thiol-ene polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

(2) FIGS. 1a-1c schematically depict some aspects of the device(s) of the invention;

(3) FIG. 2 schematically depicts an embodiment of the method of the invention.

(4) The drawings are not necessarily on scale.

(5) FIGS. 3-5 show some experimental results.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) FIG. 1a schematically depicts a lighting device 1 comprising a light source 10 configured to generate light source light 11 and a light converter 100 configured to convert at least part of the light source light 11 into visible converter light 121. Here schematically only one light source 10 is depicted. However, more than one light source 10 may be present.

(7) The light converter has an upstream side 101, which is at least partly directed to the light source 10, and a downstream side, which faces away from the light source 10 (in this transmissive configuration).

(8) The light converter 100 comprises a polymeric host material 110 with light converter nanoparticles 120 embedded in the polymeric host material 110. These can be dots, rods, a combination thereof, etc. (see also above). The light converter nanoparticles 120 generate upon excitation by the light source light 11 visible converter light (and optionally also non-visible radiation, like IR radiation). At least part form the converter light 121 escapes from the downstream side 102 as lighting device light 5. This lighting device light 5, of which at least part is in the visible, at least contains part of the convert light 121, and may optionally also contain some remaining light source light 11.

(9) FIG. 1a schematically depicts the lighting device in operation.

(10) FIG. 1b schematically depicts another embodiment, wherein the light converter 100 is encapsulated. An encapsulation 400 encloses the light converter; this encapsulation may substantially block oxygen (and/or H.sub.2O) transporter from the atmosphere to the light converter. This may add to the stability of the light converter nanoparticles 120 (and the polymeric host). The combination of light converter 100 and encapsulation 400 is herein also indicated as light converter unit 1100.

(11) FIG. 1c schematically depicts one of the applications of the lighting unit 1, here in a liquid crystal display device 2, which comprises a back lighting unit 200 which comprises one or more lighting units 1 (here, one lighting unit is schematically depicted), as well as a LCD panel 300, which can be backlighted with the lighting device light 5 of the lighting unit(s) 100 of the back lighting unit 200.

(12) FIG. 2 schematically depicts an embodiment of the method of the invention, which may comprise providing a mixture comprising radical polymerizable monomers 109 and light converter nanoparticles 120 (and optionally radical initiator; not depicted). The mixture may be provided in a container 401, having walls 404. Part of these walls may be transmissive for light source light and/or converter light (see also FIG. 1b).

(13) Preferably, after providing, or even already during this mixture, the oxygen amount in the atmosphere over the mixture is kept low, for instance by reducing the partial pressure. This may be done by evacuation and/or introduction of an inert gas. This is indicated with the symbol O.sub.2. Further, also especially water vapor may be removed (at the same time and/or in the same way; this is indicated with the symbol H.sub.2O).

(14) Thereafter, polymerization may take place, i.e. polymerizing the radical polymerizable monomers 109 under low oxygen conditions, preferably under an inert atmosphere, thereby providing the polymeric host material 110 with light converter nanoparticles 120 embedded in the polymeric host material 110. Especially, polymerization is started by irradiating the radical polymerizable monomers (indicated by the symbol +hv).

(15) After polymerization, the thus obtained light converter 100 may be entirely encapsulated, such as with a closure 402 configured to close a container opening 405. Optionally, such closure may be welded or glued or otherwise be connected to the container walls 404 in a sealing connection. In this way, the light converter unit 1100 is obtained (see also FIG. 1b), which comprises the thus obtained light converter 100 enclosed by an encapsulation, especially an oxygen non-permeable encapsulation 402,404. One or more parts of this encapsulation 402,404 may be transmissive for light source light and/or converter light (see also FIG. 1b).

(16) Hence, the invention provides a polymer composite obtained by free radical polymerization comprising luminescent light converter nanoparticles containing low concentration of photo initiator. Especially, the concentration of the initiator is less than 1 ppm and more preferentially less than 1 ppb. In fact, a photo curable monomer may be applied without photo initiator. Especially, polymerization takes place under low oxygen conditions, preferably under an inert atmosphere, and with low or zero radical initiator content.

(17) In an embodiment, the polymer is a poly vinyl, polyacrylate or a thiolene system. In an embodiment, the composite comprises a cross-linked network; the cross links are chemical cross links. Especially, the composite may be sealed from atmosphere. This encapsulation may be a hermetic encapsulation. In an embodiment, the encapsulation is thin film packaging.

(18) The quantum dots may have the quantum yield of 60% or above at room temperature. The concentration of light converter nanoparticles is preferentially less than 20%. Optionally, the quantum dots comprise ligands which can copolymerize with the polymer.

(19) The invention further provides a method of producing the composite involving making a monomeric mixture comprising light converter nanoparticles removing oxygen from the system and then placing the mixture in a confinement and initiating polymerization photo chemically upon irradiation with high energetic rays such as UV, X-rays, gamma rays, electrons.

(20) Optionally, the composite may be used in combination with one or more light converting phosphors for producing white light for illumination. The composite can be used in lighting device for backlighting for LCD.

(21) The light converter nanoparticle emission may especially be at least in the red part of the visible spectrum (especially peak position between 610-620 nm).

EXPERIMENTAL

(22) A plurality of systems was made, under different conditions. Here below, one example is given.

(23) A mixture containing 5 wt. % QDs (CdSe with ZnS shell) and acrylate monomer were produced (with various amounts of photo radical initiator). The mixture was then placed in an environment with low O.sub.2 and H.sub.2O concentration<5 ppm to remove oxygen.

(24) Subsequently the mixture was placed between glass plates and exposed to UV radiation (>=1 w/cm.sup.2) 365 nm for initiation of polymerization to obtain a solid polymer containing luminescent QDs. Samples were produced in the presence and absence of photo initiator.

(25) The samples were then tested by irradiating them with laser light emitting at 450 nm (0.4 W/cm.sup.2) at 100? C. and measuring the intensity of the luminescence from quantum dots in Nitrogen atmosphere. The results are shown in FIGS. 3-5, with on the y-axis relative intensity in arbitrary units).

(26) The sample with 1 wt. % initiator (FIG. 3) had a substantial lower stability than the sample with 0.1 wt. % photo initiator (FIG. 4); which was also not very stable. The sample without photo initiator was very stable (FIG. 5).

(27) Photoluminescence stability measurements were performed for at least 500 hours (continuous irradiation with the 450 nm light).