SOLUTION FOR USE IN FILLING MICROMETER-SIZE CAVITIES

Abstract

Solution for use in filling micrometer-size cavities (10), the solution comprising a first solvent, a first polymer (102) having a first molecular weight, a second polymer (103) having a second molecular weight, luminophores (101) and a surfactant, the second molecular weight being 10 to 50 times greater than the first molecular weight.

Claims

1. A solution, comprising a first solvent, a first polymer having a first molecular weight, a second polymer having a second molecular weight, luminophores and a surfactant, the second molecular weight being 10 to 50 times greater than the first molecular weight.

2. The solution according to claim 1, wherein the first molecular weight is approximately 20 times greater than the second molecular weight.

3. The solution according to claim 1, wherein the first polymer and the second polymer are chosen independently of each other from the group consisting of a poly(meth)acrylate, a polysiloxane, a polycarbonate, a poly(-caprolactone), a polystyrene and copolymers thereof.

4. The solution according to claim 1, wherein the first polymer and/or the second polymer are homopolymers.

5. The solution according to claim 1, wherein the first polymer and the second polymer are of the same chemical nature.

6. The solution according to claim 1, wherein the first polymer and the second polymer are poly(methyl methacrylate).

7. The solution according to claim 1, wherein the solution comprises a second solvent, the boiling point of which is at least 30 C. higher than the temperature of the first solvent.

8. The solution according to claim 6, wherein the first solvent is butanone and the second solvent is anisole.

9. The solution according to claim 1, wherein the luminophores are selected from the group consisting of phosphors and quantum dots.

10. The solution according to claim 1, the luminophores have a largest dimension less than 1 m.

11. The solution according to claim 1, wherein the ratio solvent/dry matter is in the range from 40% to 60% by mass.

12. The solution according to claim 1, wherein the ratio luminophores/dry matter is in the range from 15% to 50% by mass.

13. A method for preparing a composite material, comprising a polymer matrix in which luminophores are dispersed, in micrometer-size cavities, the method comprising the following consecutive steps: providing a substrate comprising micrometer-size cavities, the cavities comprising a bottom, a side wall and an open upper section; depositing a solution according to claim 1, in the cavities (10); and evaporating the solvent or solvents, so as to form a composite material comprising a polymer matrix in which the luminophores are dispersed.

14. The method according to claim 13, wherein the bottom of the cavity comprises GaN.

15. The method according to claim 13, wherein the cavities have an open upper section having a largest dimension in the range from 1 m to 100 m, and a depth in the range from 1 to 50 m.

16. The method according to claim 13, wherein the substrate comprises an emission structure having an active zone suitable for emitting a first visible radiation at a first wavelength, the luminophores of the solution deposited in the cavities being suitable for emitting a second visible radiation at a second wavelength by conversion of the first visible radiation, the second wavelength being greater than the first wavelength.

17. The solution according to claim 1, wherein the first polymer and the second polymer are poly(methyl methacrylate, the first solvent is butanone and the second solvent is anisole.

18. The solution according to claim 1, wherein the luminophores have a largest dimension less than 500 nm.

19. The method according to claim 15, wherein the cavities have an open upper section having a largest dimension in the range from 1 to 20 m, and a depth in the range from 5 to 10 m.

20. The solution according to claim 9, wherein the phosphors are selected from the group consisting of SrSi.sub.2O.sub.2N.sub.2:Eu.sup.2+, -SiAlON, Y.sub.3Al.sub.5O.sub.12:Ce.sup.3+, SrSi.sub.5N.sub.8:Eu.sup.2+, (Ba,Sr).sub.2Si.sub.5N.sub.8:Eu.sup.2+, (Ca,Sr), AlSiN.sub.3:Eu.sup.2+, Sr[LiAlN.sub.4]:Eu.sup.2+, Sr[Mg.sub.3SiN.sub.4]:Eu.sup.2+, Sr.sub.1-x Ca.sub.xS:Eu.sup.2+, K.sub.2SiF.sub.6:Mn.sup.4+, and the quantum dots are selected from the group consisting of ZnS, ZnSe, CdS, CdSe, CdZnSe, CdTe, PbS, InP, CuInS.sub.2, CuGaS.sub.2, CuInSe.sub.2, CuGaSe.sub.2, CuInGaSe.sub.2, CuSe, InSe, and GaSe.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0045] The present invention will be better understood on the basis of the following description and the attached drawings, of which:

[0046] FIG. 1, previously described, is a schematic representation of various forces exerted on a liquid in a tube,

[0047] FIG. 2, previously described, is a scanning electron microscope image of microcavities filled with a solution according to the prior art,

[0048] FIG. 3 is a schematic representation, in cross-section, of a microcavity filled with the solution according to a particular embodiment of the invention,

[0049] FIG. 4 is a schematic representation, in cross-section, of a substrate comprising a plurality of microcavities filled with a solution according to a particular embodiment of the invention,

[0050] FIG. 5 is a schematic representation, in cross-section, of a substrate comprising a plurality of microcavities filled with a solution according to a particular embodiment of the invention,

[0051] FIG. 6 is a scanning electron microscope image, by secondary electrons, of microcavities filled with a solution having a single polymer,

[0052] FIG. 7 is a scanning electron microscope image, by secondary electrons, of microcavities filled with a solution having two polymers, according to a particular embodiment of the invention,

[0053] FIG. 8 is a scanning electron microscope image, by backscattered electrons, of microcavities filled with a solution having a single polymer,

[0054] FIG. 9 is a scanning electron microscope image, by backscattered electrons, of microcavities filled with a solution having two polymers, according to a particular embodiment of the invention,

[0055] The various parts shown in the figures are not necessarily on a uniform scale, in order to make the figures more readable.

[0056] The various possibilities (variants and embodiments) should be understood as not being exclusive from each other and being able to be combined together.

DETAILED DISCLOSURE OF THE SPECIAL EMBODIMENTS

[0057] The solution according to the invention is intended to fill micrometer-size cavities 10. The solution comprises: [0058] luminophores 101, [0059] a surfactant, [0060] a first polymer 102 having a first molecular weight, of low molecular weight, [0061] the second polymer 103 having a second molecular weight, of high molecular weight, [0062] one or more solvents.

[0063] As shown in FIG. 3, after evaporation of the solvent or solvents, the cavities 10 are partially or completely filled by a composite material 100 comprising a polymer matrix 102, 103 in which the luminophores 101 are homogeneously dispersed.

[0064] The luminophores 101:

[0065] The luminophores 101 are in the form of particles. Here, particles means elements of micrometer or nanometre size and having a spherical, cylindrical or ovoid shape.

[0066] In general, the luminophores 101 have a largest dimension less than 1 m, and preferably less than 500 nm.

[0067] The luminophores 101 can be organic fluorophores and/or phosphors and/or quantum dots.

[0068] According to a first advantageous embodiment, the luminophores 101 can be chosen among the phosphors, inorganic luminescent materials, preferably in the form of particles. The green to yellow phosphors are SrSi.sub.2O.sub.2N.sub.2:Eu.sup.2+, -SiAlON, Y.sub.3A.sub.5O.sub.12:Ce.sup.3+, YAG:Ce; the orange and red phosphors can be SrSi.sub.5N.sub.8:Eu.sup.2+, or again nitrides such as (Ba,Sr).sub.2Si.sub.5N.sub.8:Eu.sup.2+, (Ca,Sr), AlSiN.sub.3:Eu.sup.2+, Sr[LiAlN.sub.4]:Eu.sup.2+, Sr[Mg.sub.3SiN.sub.4]:Eu.sup.2+, or a sulfide such as Sr.sub.1-x Ca.sub.x S:Eu.sup.2+, or again a fluoride such as K.sub.2SiF.sub.6:Mn.sup.4+. A mixture of these phosphors can be chosen to fill the cavities 10. The phosphors have a largest dimension in the range from 10 nm to 300 nm, for example of order 100 nm.

[0069] According to a second advantageous embodiment, the luminophores 101 can be quantum dots. The quantum dots are semiconductor nanocrystals. They can emit in the ultra-violet, visible, near infrared and infrared. Certain quantum dots advantageously emit at a very narrow wavelength (typically, the width at half-height of the emission peak is 30 nm). The quantum dots have a largest dimension in the range from 2 nm to 20 nm, and preferably from 1 nm to 10 nm. The quantum dots can be chosen among ZnS, ZnSe, CdS, CdSe, CdZnSe, CdTe, PbS, InP, CuInS.sub.2, CuGaS.sub.2, CuInSe.sub.2, CuGaSe.sub.2, CuInGaSe.sub.2, CuSe, InSe, and GaSe. A mixture of these quantum dots can be chosen to fill the cavities 10. Since their absorption is high and their emission is strong, it is possible to use them in low quantities, in contrast to organic fluorophores which have a very low luminous intensity.

[0070] Preferably, the filling rate of luminophores 101 in the dry matter after deposition is in the range from 15% to 50% by volume.

[0071] The Surfactant:

[0072] The surfactant ensures the dispersion of the luminophores 101 in the solution. The surfactant can partially or completely cover the surface of the luminophore particles 101. A surfactant is chosen which does not modify the optical properties of the luminophores.

[0073] The surfactant is preferably non-ionic, in other words it is preferably not charged. It can be, for example, one of the TEGO-type surfactants marketed by Evonik Resource Efficiency GmbH. A Siloxane-based TEGO for example is chosen, in order to have a device with a high longevity. It is also possible to choose a surfactant from the Triton range marketed by the Dow Chemical Company.

[0074] The Polymers 102, 103:

[0075] The solution comprises a first polymer 102 having a first molecular weight and a second polymer 103 having a second molecular weight, the second molecular weight being 10 to 50 times greater than the first molecular weight, and preferably 15 to 35 times greater than the first molecular weight.

[0076] Here, molecular weight means the number average molecular weight Mn, also called the number average molar mass, and corresponds to the average of molar masses weighted by the number of chains of each length. The molecular weight can, for example, be determined by size-exclusion chromatography with polystyrene standards.

[0077] For example, the polymers originate from a radical polymerisation reaction where the polydispersity index I is Mw/Mn=2.

[0078] In general, the first molecular weight is at least 500 g/mol and at most 50,000 g/mol, and preferably from 10,000 g/mol to 25,000 g/mol.

[0079] In general, the second molecular weight is at least 5000 g/mol and at most 2,500,000 g/mol, and preferably at least 50,000 g/mol and at most 1,250,000 g/mol.

[0080] The first polymer 102 and the second polymer 103, having different molecular weights, and therefore different chain lengths, serve as a matrix to mechanically hold the luminophores 101.

[0081] The high molecular weight polymer 103 will play the role of a texturing agent by freezing and preserving the extended or swollen structure of the polymers. The low molecular weight polymer will shrink in order to adhere on the skeleton of the high molecular weight polymer. The luminophore particles remain trapped in the polymer network and do not migrate. This makes it possible to reduce or eliminate the shrinkage during drying.

[0082] At the time of filling of the cavity with the solution, the large chains act as a skeleton for the polymer structure. These large chains will freeze the luminophore load and the small polymer chains, which leads to a good homogeneity of the solution and therefore to a good distribution of the luminophores within the microcavity. This structuring of the polymers has a positive impact on the control of the shrinkage of the solvent: the framework formed by the large chains will limit and control the shrinkage effects associated with the evaporation of the solvent.

[0083] With such a solution, there is no deformation of the air/solid interface during the evaporation of the solvent.

[0084] In order to have an instantaneous freezing during drying, a second molecular weight is chosen that is from 10 to 50 times greater than the first molecular weight. For example, a second molecular weight is chosen which is 20 times greater than the first molecular weight.

[0085] In order to choose the polymers 102, 103, a person skilled in the art will take into account, in particular, the stability of the solution and its viscosity in order to be able to deposit it by means of currently available deposition techniques.

[0086] The ratio between long chains and short chains makes it possible to control the initial viscosity of the mixture and to formulate a solution with a viscosity that is compatible with, for example, a coating process. The first low molecular weight polymer 102 is preferably in the majority, in other words the first polymer 102 represents more than 50% of the total mass of polymers. The solutions obtained can be deposited by many deposition techniques and are stable over time, which allows them to be deposited in a manner reproducible over time.

[0087] Polymers will also be chosen which have a refractive index that does not disrupt the excitation of the luminophores for the extraction of the emitted light.

[0088] The choice of polymers will therefore also take into account the capacity of the solvent(s)/polymers/surfactant system to disperse the luminophores.

[0089] The polymers 102, 103 are preferably chosen, independently of one another, among polyalkylmethacrylates (such as PMMA), polyarylmethacrylates, polycyclic polymethacrylates (for example of polybornanemethacrylate type), polyalkyl acrylates, polyarylacrylates, polyarylsiloxanes and polyalkylsiloxanes. The polymers can also be chosen among polycarbonates, poly(-caprolactone), and polystyrenes. The polymers are preferably non-substituted polymers. These polymers fulfil the criteria set out above. These polymers have, for example, a C1-C10 carbon chain.

[0090] The polymers 102, 103 can be homopolymers or copolymers of the previously cited polymers.

[0091] It is possible to use polymers 102, 103 of different chemical nature, for example PMMA and polystyrene.

[0092] It is also possible to use polymers 102, 103 of the same chemical nature, in other words of the same family. Preferably, the two polymers 102, 103 are two PMMA of different molecular weights.

[0093] The Solvent(s):

[0094] The solvent is preferably organic.

[0095] In order to have a low viscosity, a solution is advantageously chosen which contains at least one good solvent. This solution may additionally comprise an intermediate solvent.

[0096] Here, good solvent means that it allows the polymer chains to be swollen. Polymer/polymer interactions are disadvantaged. The polymers are well dispersed in the solvent.

[0097] Here, intermediate solvent means that it is indifferent to having polymer/polymer or polymer/solvent interactions.

[0098] Here, poor solvent means that the polymer/polymer interactions are favoured, the polymer chains collapse on themselves and/or precipitate entirely.

[0099] The choice of solvent can be made using the Hansen parameters. The Hansen parameters make it possible to determine a sphere of solubility for each type of polymer. Each solvent also has its own Hansen parameters. It is therefore possible to locate the solvent in the same reference frame as the sphere of the polymer studied. If the solvent is in the sphere, it is a good solvent for the polymer. If the solvent is outside the sphere, then it is a poor solvent of the polymer. These parameters also make it possible to calculate the coordinates of a mixture of solvents.

[0100] By way of illustration, the Hansen parameters of PMMA are [17.7, 9.1, 7.1], those of anisole (also called methoxybenzene) are [17.8, 4.1, 6.7], and those of methyl ethyl ketone (also called butanone or MEK) are [16, 9, 5.1]. The first two are paramount for the choice of the solvent. MEK is a good solvent of PMMA, and anisole is an intermediate solvent.

[0101] In order to limit the interactions between the polymer chains in solution, and therefore to retain a reasonable viscosity, it is also necessary that the difference between the boiling points of the solvents is at least thirty degrees Celsius and that it is preferably the best solvent which is the most volatile.

[0102] For PMMA, a solution can therefore be chosen containing a good solvent such as MEK (boiling point: 89 C.) and an intermediate solvent such as anisole (boiling point: 155.5 C.). MEK, the good solvent, will evaporate first after the deposition. Anisole, being an intermediate solvent, will fix and preserve the structure of the polymers.

[0103] It is also possible to determine the polymers/solvents associations by using the Hildebrand solubility parameter.

[0104] The solubility parameter for PMMA, methyl ethyl ketone and anisole are respectively: 9.3 (cal/cm.sup.3).sup.1/2, 9.43 (cal/cm.sup.3).sup.1/2 and 9.7 (cal/cm.sup.3).sup.1/2. This confirms the results obtained using the Hansen parameter: MEK is a better solvent for PMMA than anisole.

[0105] The viscosity will be determined depending on the wet deposition technique (coating, ink jet, spin coating, for example).

[0106] A solution is chosen which is sufficiently wetting to disperse the luminophores but not too wetting as not to create convex menisci in the microcavities.

[0107] The ratio solvent/dry matter is preferably in the range from 40% to 60% by mass.

[0108] Method for Filling the Cavities 10 with Luminophores 101:

[0109] The method for filling the micrometer-size cavities 10 with luminophores 101 comprises the following consecutive steps: [0110] providing a substrate comprising micrometer-size cavities 10, [0111] depositing a solution as defined above in the cavities 10, [0112] evaporating the solvent or solvents so as to obtain a dry composite material comprising a polymer matrix in which the luminophores 101 are dispersed.

[0113] As shown in FIGS. 4 and 5, the substrate comprises at least one cavity 10 to be filled. Preferably, the substrate comprises a plurality of cavities 10. The cavities 10 are, for example, arranged forming an array with a plurality of rows and a plurality of columns of cavities. The spacing of the cavities in the plane of the substrate is, for example, between approximately 10 m and 100 m, and preferably between 50 m and 100 m.

[0114] The cavities 10 are of micrometer size, in other words they have dimensions ranging from the order of one micrometer to hundreds of micrometers. These are also referred to as microcavities.

[0115] The cavities 10 have a closed lower wall 13 (or bottom), a side wall 12 and an open upper section. The lower wall and the upper section can have identical or different shapes, for example circular, elliptical, square, rectangular shape, etc. The cavities are, for example, tubes of square or circular cross-section.

[0116] The height of the side wall 12 defines the depth of the cavity 10. The cavities 10 have a depth ranging from several micrometers (for example, 1, 2 or 3 m) to 50 m, preferably from 5 m to 10 m.

[0117] The cavities 10 have an upper section having a largest dimension ranging from a few micrometers (for example, 1, 2 or 3 m) to 100 m, and preferably from 1 m to 20 m, for example in the order of 10 m.

[0118] The walls 12 of the cavities 10 can be formed of any suitable material.

[0119] This may be a metallic wall 12, such as copper or aluminium, or a metal oxide wall, for example made of alumina (FIG. 4). Such a wall 12 can be produced by electrochemical deposition, optionally following by an oxidation step. Advantageously, the wall 12 is reflective and non-absorbent.

[0120] The walls 12 can be produced from a temporary material (FIG. 5), in other words after obtaining the composite material 100, in the cavities 10, the wall 12 is removed. It may be a resin, for example. The resin can be removed by chemical means. An array of pads of composite material 100, separated by air, is thus obtained.

[0121] The bottom 13 of the cavity 10 is a light-emitting layer It may be, for example, a layer of GaN.

[0122] The cavities 10 can be produced on a support 11 made, for example, of CdSe or InGaN, or made of an InGaN/GaN bilayer.

[0123] Preferably, the substrate comprises an emission structure having an active zone suitable for emitting a first visible radiation at a first wavelength .sub.1, the luminophores 101 of the solution deposited in the cavities 10 being suitable for emitting a second visible radiation at a second wavelength .sub.2 by conversion of the first visible radiation, the second wavelength .sub.2 being such that .sub.1<.sub.2.

[0124] The solution can be deposited by any liquid dispensing technique. By way of illustration, this may be a deposition by inkjet, spraying, soaking, coating (with a doctor blade or by spin coating deposition), or 3D printing.

[0125] For conversion layers comprising quantum dots, it is possible to have a composite material 100 having a thickness ranging, for example, from 5 to 10 m. The quantum dots have a good absorption. A low quantity is sufficient.

[0126] For conversion layers comprising phosphors, it is possible to have a composite material 100 having a thickness ranging, for example, from 50 m to 100 m.

ILLUSTRATIVE EXAMPLES AND NON-LIMITING EMBODIMENTS

[0127] The following table describes various solutions. The solutions S19, S20, S25 and S26 are solutions according to the invention and comprising two PMMA of different molecular weights. The solutions S17, S18, S23 and S24 had been produced by way of comparison and contain only one PMMA. The surfactant is TEGO Dispers 670, marketed by Evonik Resource Efficiency GmbH.

TABLE-US-00001 S17 S19 S18 S20 S23 S25 S24 S26 PMMA 8.4% 8.4%.sup. 15.2% 15.2%.sup. 8.4% 17.5% 15,000 g/mol 5.9% of 15,000 g/mol 13.7% of 15,000 g/mol 5.9% of 15,000 g/mol 15.6% of 15,000 g/mol 15,000 g/mol 15,000 g/mol 15,000 g/mol 2.5% of 1.5% of 2.5% of 1.4% of 350,000 g/mol 350,000 g/mol 350,000 g/mol 350,000 g/mol Anisole 59% 45.7% 62% 52.9% YAG:Ce 3.5% 26.8% 3.6% 3.6% MEK 23.3% 6% 24.3% 24.3% Surfactant 5.8% 6.3% 1.7% 1.7%

[0128] These solutions are then deposited on substrates having microcavities, by means of a coating technique. The walls of the cavities are covered by a layer of aluminium. The cavities filled with the solutions have been observed by scanning electron microscopy in secondary electron mode (FIGS. 6 and 7) and in backscattered electron mode (FIGS. 8 and 9).

[0129] The cavities filled with the solutions according to the invention (FIGS. 7 and 9) have a homogeneous distribution of luminophores over the entire height of the cavity. This is not the case for the comparative solutions (FIGS. 6 and 8).