WO2012164284A2

WO2012164284A2 - Semiconductor nanoparticle - based materials for use in light emitting diodes, optoelectronic displays and the like

Info

Publication number: WO2012164284A2
Application number: PCT/GB2012/051218
Authority: WO
Inventors: Imad Naasani; Hao PANG; Siobhan DANIELS; Emma Fitzgerald; Mark Christopher Mccairn; David Webb; James Harris
Original assignee: Nanoco Technologies Ltd
Current assignee: Nanoco Technologies Ltd
Priority date: 2011-05-31
Filing date: 2012-05-31
Publication date: 2012-12-06
Anticipated expiration: 2013-11-30
Also published as: US20130105839A1; IL247806A0; CA2836057A1; US9082941B2; CN103597622A; AU2012264430A1; US20150270455A1; GB201109054D0; KR20140024404A; JP2014523634A; IL229643A0; EP2715810A2; IL229643A; WO2012164284A3

Abstract

The present invention relates to a formulation incorporating nanoparticles, particularly quantum dot (QD) nanoparticles, into an optically clear medium (resin) to be used as a phosphor material in lighting and display applications, and as a down converting phosphor material in LEDs (light emitting diodes). The resin is compatible with QDs to allow high performance and stability of QD-based LEDs, lighting and display applications.

Description

SEMICONDUCTOR NANOPARTICLE BASED MATERIALS FOR USE IN LIGHT

EMITTING DIODES, OPTOELECTRONIC DISPLAYS AND THE LIKE

The present invention relates to semiconductor nanoparticle - based materials for use in light emitting devices, such as, but not limited to, light emitting diodes (LEDs) and optoelectronic displays. Particularly, but not exclusively, the present invention relates to resin formulations for use in the fabrication of quantum dot (QD)-based light emitting devices, methods for producing said formulations, processes for fabricating such devices employing said formulations and the devices thus formed.

Light-emitting diodes (LEDs) are becoming more important to modern day life and it is envisaged that they will become one of the major applications in many forms of lighting such as automobile lights, traffic signals, general lighting, liquid crystal display (LCD) backlighting and display screens. Currently, LED devices are made from inorganic solid-state compound semiconductors, such as AIGaAs (red), AIGalnP (orange-yellow-green), and AIGalnN (green-blue), however, using a mixture of the available solid-state compound semiconductors, solid-state LEDs which emit white light cannot be produced. Moreover, it is difficult to produce "pure" colours by mixing solid-state LEDs of different frequencies. Therefore, currently the main method of colour mixing to produce a required colour, including white, is to use a combination of phosphorescent materials which are placed on top of the solid- state LED whereby the light from the LED (the "primary light") is absorbed by the phosphorescent material and then re-emitted at a different frequency (the "secondary light"), i.e. the phosphorescent materials down convert the primary light to the secondary light. Moreover, the use of white LEDs produced by phosphor down-conversion leads to lower cost and simpler device fabrication than a combination of solid-state red-green-blue LEDs.

Current phosphorescent materials used in down converting applications absorb UV or mainly blue light and converts it to longer wavelengths, with most phosphors currently using trivalent rare-earth doped oxides or halophosphates. White emission can be obtained by blending phosphors which emit in the blue, green and red regions with that of a blue or UV emitting solid-state device, i.e. a blue light emitting LED plus a green phosphor such as, SrGa₂S₄:Eu₂ ⁺, and a red phosphor such as, SrSiEu₂ ⁺ or a UV light emitting LED plus a yellow phosphor such as,

l Sr₂P₂0₇:Eu₂ ⁺;Mu₂ ⁺, and a blue-green phosphor. White LEDs can also be made by combining a blue LED with a yellow phosphor, however, colour control and colour rendering is poor when using this methodology due to lack of tunability of the LEDs and the phosphor. Moreover, conventional LED phosphor technology uses down converting materials that have poor colour rendering (i.e. colour rendering index (CRI) < 75).

There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions in the order of 2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties which can be exploited in many commercial applications such as optical and electronic devices and other applications that ranging from biological labelling, photovoltaics, catalysis, biological imaging, LEDs, general space lighting and electroluminescent displays amongst many new and emerging applications.

The most studied of semiconductor materials have been the chalcogenides 11- VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. Reproducible methods for the large scale production of these materials have been developed from "bottom up" techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles, using "wet" chemical procedures.

Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor being, with many materials including semiconductor nanoparticles, that there is a change in the electronic properties of the material with size, moreover, because of quantum confinement effects the band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an 'electron in a box' giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the "electron and hole", produced by the absorption of electromagnetic radiation, a photon, with energy greater then the first excitonic transition, are closer together than they would be in the corresponding macrocrystalline material, moreover the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition of the nanoparticle material. Thus, QDs have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter. Core semiconductor nanoparticles, which consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface which can lead to non-radiative electron- hole recombinations. One method to eliminate defects and dangling bonds on the inorganic surface of the QD is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material epitaxially on the surface of the core particle, to produce a "core-shell" particle. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is a ZnS shell grown on the surface of a CdSe core. Another approach is to prepare a core-multi shell structure where the "electron-hole" pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a quantum dot- quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS which is then over grown by a monolayer of CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer. To add further stability to QDs and help to confine the electron-hole pair one of the most common approaches is by epitaxially growing a compositionally graded alloy layer on the core this can help to alleviate strain that could otherwise led to defects. Moreover for a CdSe core in order to improve structural stability and quantum yield, rather growing a shell of ZnS directly on the core a graded alloy layer of Cdi_-xZn_xSei_-yS_y can be used. This has been found to greatly enhance the photoluminescence emission of the QDs. Doping QDs with atomic impurities is an efficient way also of manipulating the emission and absorption properties of the nanoparticle. Procedures for doping of wide band gap materials, such as zinc selenide and zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), have been developed. Doping with different luminescence activators in a semiconducting nanocrystal can tune the photoluminescence and electroluminescence at energies even lower than the band gap of the bulk material whereas the quantum size effect can tune the excitation energy with the size of the QDs without having a significant change in the energy of the activator related emission.

An object of the present invention is to obviate or mitigate one or more of the problems with current methods for fabricating semiconductor nanoparticle - based light emitting devices. A first aspect of the present invention provides a formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table, said nanoparticles being incorporated into an optically transparent poly(meth)acrylate encapsulation medium.

Aspects of the present invention relate to the incorporation of fluorescent semiconductor nanoparticles (e.g. quantum dots (QDs)) into an optically clear and chemically stable medium, which may be referred to herein as a "resin" - this term may encompass any suitable host material in which the semiconductor nanoparticles are incorporated. The present invention provides formulations or resins incoporporating the nanoparticles alone (the nanoparticles being embedded directly in the encapsulation medium or resin), nanoparticles contained in or associated with beads or bead-like architectures, or combinations thereof. The formulation according to the present invention allows QDs particularly, CFQDs, in particular lll-V QDs, to be used as a phosphor material with high performance, minimal aggregation, and preserved quantum yield in the final light-emitting device. A second aspect of the present invention provides a formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

A third aspect of the present invention provides a formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a multivalent crosslinking compound reacted in the presence of a photoinitiator.

In the first and second aspects of the present invention said poly(meth)acrylate may be any suitable (meth)acrylate-based polymer. It preferably incorporates a medium- to-long carbon chain, such as a C₈-C₂₀ carbon chain, more preferably a Ci₂-Ci₈ carbon chain. It is preferred that the poly(meth)acrylate is selected from the group consisting of polylauryl (meth)acrylate, polystearyl (meth)acrylate, polytrimethylsilyl (meth)acrylate, polytrimethylsilyloxyalkyl (meth)acrylate (e.g. 2- (Trimethylsilyloxy)ethyl methacrylate), polyglycidyl (meth)acrylate, methyl (meth)acrylate and combinations thereof. Each of the aforementioned acylates may be substitued or unsubstituted with one or more chemical groups, such as an alkyl group, for example a methyl, ethyl or propyl group. Structures of the monomers from which these polymers can be obtained are set out below. It is particularly preferred that said poly(meth)acrylate is polylaurylmethacrylate.

Laurylmethacrylate Laurylacrylate

Stearylmethacrylate Stearylacrylate

2-(Trimethylsiloxy)ethylmethacrylate Trimethylsilylmethacrylate

In the first aspect of the present invention it is preferred that said poly(meth)acrylate encapsulation medium is derived from a (meth)acrylate monomer, preferably laurylmethacrylate, and a trivalent crosslinking compound.

In each of the above-defined aspects of the present invention any suitable multivalent crosslinking agent may be used provided it is compatible with the (meth)acrylate monomer(s) being used and the photoinitiator. A trivalent crosslinking compound is preferred, such as trimethylolpropanetrimethacrylate. The structures of trimethylolpropanetrimethacrylate and another preferred trivalent crosslinking compound are set out below.

1 ,3,5-Triacryloylhexahydro-1 ,3,5-triazine Trimethylolpropanetrimethacrylate In the first aspect of the present invention the monomer and crosslinking compound are preferably reacted in the presence of a photoinitiator. In this preferred embodiment of the first aspect and in the second and third aspects, any suitable photoinitiator may be used provided it is compatible with the (meth)acrylate monomer(s) and crosslinking compound being used. A preferred type of photoinitiator is a bis-acylphosphine oxide (BAPO) photoinitiator, such as bis(2,4,6- trimethylbenzoyl)-phenylphosphineoxide, which is marketed as Irgacure 819^®.

The photoinitiator (e.g., Irgacure 819^®) is preferably dissolved in the crosslinker (e.g., trimethylolpropanetrimethacrylate). This may then be added to the (meth)acrylate monomer(s) to provide an encapsulant precursor mixture. An aliquot of the mixture is then added to a sample of the desired semiconductor nanoparticles (e.g. cadmium-free QDs, such as lll-V (e.g. InP) QDs). The nanoparticle-containing mixture can then be used to fabricate the desired light emitting device. By way of example, an appropriate volume of the nanoparticle-containing mixture may be deposited into a cup of an LED or used to make a phosphor sheet using any appropriate technique for generating a nanoparticle film (e.g. inkjet printing, casting, doctor blade, roller coating, screen printing etc). The filled LED or printed display device may then be irradiated to provide a cured, optically transparent matrix that incorporates the desired type(s) of nanoparticles.

The formulation according to any aspect of the present invention may include one or more additives to aid the preparation of the formulation, processibility of the formulation and/or to enhance the performance of the final device. Additives may include one or more from the following group: polymerization enhancers (e.g. benzophenones, BF₃); wave-guiding materials (e.g. fumed silica and its derivatives, polymethylmethacrylate (PMMA)); agents for increasing viscosity (e.g. fumed silica, hydrophobic polymers, polylaurylmethacrylate (PLMA), dextrin palmitate); agents for enhancing light transmittance; and agents for improving the solubility of the nanoparticles in the encapsulant precursor mixture (which, in a preferred embodiment, is basic). Additinally or alternatively the formulation may include additives to enhance the mechanical and/or tensile properties or the finally cured material, and/or the weathering of the final device, e.g. an LED (e.g. Ti0₂ nanopowders, silicone hydride containing siloxanes and siloxane polymers, fumed silica). The semiconductor nanoparticles in the first aspect which contain ions from groups 13 and 15 of the Periodic Table preferably contain one or more semiconductor material selected from the group consisting of InP, InAs, InSb, AIP, AIS, AIAs, AlSb, GaN, GaP, GaAs, GaSb and combinations thereof. In the second and third aspects of the present invention the semiconductor nanoparticles may contain ions selected from group 1 1 , 12, 13, 14, 15 and/or 16 of the periodic table, or said semiconductor nanoparticles may contain one or more types of transition metal ion or d-block metal ion. By way of example, said semiconductor nanoparticles may contain one or more semiconductor material selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AIP, AIS, AIAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, MgTe and combinations thereof.

The semiconductor nanoparticles may be dispersed directly into the encapsulation medium, incorporated into a plurality of discrete microbeads which are then dispersed or embedded within said encapsulation medium, or a combination may be used.

The term "beads" is used for convenience and is not intended to impose any particular size or shape limitation. Thus, for example, the beads may be spherical but other configurations are possible, such as disc- or rod-like. Where reference is made herein to "microbeads" this is intended to refer to "beads" as defined above having a dimension on the micron scale. The nanoparticle-containing optically transparent medium is provided in the form of a plurality of discrete, i.e. separate or distinct, microbeads. For the avoidance of doubt, reference to microbeads as being "discrete" is not intended to exclude composite materials formed by aggregations of microbeads since even in such materials each microbead retains its original beadlike structure despite being in contact with one or more other microbeads. By preloading small microbeads, which can range in size from 50 nm to 500 μηι or more preferably 25 nm to 0.1 mm or more preferably still 20 nm to 0.5 mm in diameter, with QDs, then incorporating one or more of these QD-containing beads into an LED encapsulation material on a UV or blue LED, it becomes a simple process to change, in a controllable and reproducible manner, the colour of the light emitted by the LED device. Moreover, it has been shown that this approach may be much simpler than attempting to directly incorporate the QDs directly into an LED encapsulate in terms of ease of colour rendering, processing, and reproducibility and offers greater QD stability to photo-oxidation. This approach can lead to better processing; the QD- containing beads can be made to the same size as the currently employed YAG phosphor material which range from 10 to ^"Ι ΟΟμηι and can thus be supplied to commercial manufacturers in a similar form to that of the current commercially used phosphor material. Moreover, the QD-containing beads are in a form that is compatible with the existing LED fabrication infrastructure.

The material from which the beads or microbeads are made is preferably optically transparent medium and may be made in the form of a resin, polymer, monolith, glass, sol gel, epoxy, silicone, (meth)acrylate or the like using any appropriate method. It is preferred that the resulting nanoparticle-containing bead is suitably compatible with the optically transparent encapsulating medium to enable the nanoparticle-containing beads to be embedded within the encapsulant such that the chemical and physical structure of the resulting composite material (i.e. the encapsulant with nanoparticle-containing beads embedded therein) remains substantially unchanged during further processing to incorporate the composite into a light emitting device and during operation of the resulting device over a reasonable lifetime for the device. Suitable bead materials include: poly(methyl (meth)acrylate) (PMMA); polyethylene glycol dimethacrylate) (PEGMA); polyvinyl acetate) (PVA); poly(divinyl benzene) (PDVB); poly(thioether); silane monomers; epoxy polymers; and combinations thereof. A particularly preferred bead material which has been shown to exhibit excellent processibility and light emitting device performance comprises a copolymer of PMMA, PEGMA and PVA. Other preferred bead materials employ polystyrene microspheres with divinyl benzene and a thiol co-monomer; silane monomers (e.g. 3-(trimethoxysilyl)propylmethacrylate (TMOPMA) and tetramethoxy silane (TEOS)); and an epoxy polymer (e.g. Optocast™ 3553 from Electronic Materials, Inc., USA). By incorporating at least some of the QDs into an optically transparent, preferably clear, stable bead material it is possible to protect the otherwise reactive QDs from the potentially damaging surrounding chemical environment. Moreover, by placing a number of QDs into a single bead, for example in the size range from 20 nm to 500 μηι in diameter, the subsequent QD-bead may be more stable, than free or "naked" QDs, to the types of chemical, mechanical, thermal and photo-processing steps which are required to incorporate QDs in most commercial applications, such as when employing quantum dots as down converters in a "QD-solid-state-LED" light emitting device. The formulation according to the present invention may contain a population of semiconductor nanoparticles distributed across a plurality of beads embedded within the optically transparent encapsulating medium. Any desirable number of beads may be embedded, for example, the encapsulating medium may contain 1 to 10,000 beads, more preferably 1 to 5000 beads, and most preferably 5 to 1000 beads.

Some or all of the nanoparticle-containing microbeads may include a core comprising a first optically transparent material and one or more outer layers or shells of the same or one or more different optically transparent materials deposited on said core. Nanoparticles may be confined to the core region of the microbeads or may be dispersed throughout the core and/or one or more of the shell layers of the microbeads.

Advantages of QD-containing beads over free QDs may include greater stability to air and moisture, greater stability to photo-oxidation and greater stability to mechanical processing. Moreover, by pre-loading small microbeads, which can range in size from a few 50 nm to 500 μηι, with QDs then incorporating one or more of these QD-containing beads into the encapsulating medium on a UV or blue LED, it may be a relatively simple process to change, in a controllable and reproducible manner, the colour of the light emitted by the LED-based light emitting device.

While incorporating semiconductor nanoparticles into beads may afford some or all of the aforementioned advantages, it is preferred that the formulation according to the first aspect of the present invention comprises at least some semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table which are dispersed directly into the encapsulation medium. That is, the formulation according to the first aspect preferably contains semiconductor nanoparticles containing group 13 and 15 ions which are not incorporated into beads or microbeads, and optionally contains further semiconductor nanoparticles, which may or may not contain group 13 and/or 15 ions, that are incorporated into beads or microbeads. In this way, the optical properties of the final device may be optimised and/or maximised for a particular application. For example, by using a mixture of bead-encapsulated QDs with none-bead-encapsulated QDs (i.e. "bare" or "naked" QDs), the final device may benefit from both the advantages of the bead- encapsulated QDs in terms of robustness, processibility etc, while also benefiting from advantages associated with using QDs dispersed directly within the encapsulating medium, such as increased brightness, optical clarity and/or quantum yield due to the ommision of the additional layer of encapsulating bead material and the associated additional processing steps required to combine the QDs with the beads. It is further preferred that the formulations according to the second and/or third aspects of the present invention comprise at least some semiconductor nanoparticles which are dispersed directly into the encapsulation medium. In a particularly preferred embodiment of the first, second and/or third aspect of the present invention all of the semiconductor nanoparticles are dispersed directly into the encapsulation medium, i.e. none of the semiconductor nanoparticles are incorporated into beads or microbeads dispersed within the encapsulation medium. In this way, the advantages of providing the QDs directly into the encapsulation medium, rather than first incorporating them into beads, can be obtained.

A fourth aspect of the present invention provides a method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table into an optically transparent poly(meth)acrylate encapsulation medium. The poly(meth)acrylate encapsulation medium is preferably derived from a (meth)acrylate monomer and a trivalent crosslinking compound. It is preferred that the fourth aspect of the present invention is applied to produce a formulation according to the first aspect of the present invention defined above or a preferred embodiment thereof.

A fifth aspect of the present invention provides a method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound. The monomer and crosslinking compound are preferably reacted in the presence of a photoinitiator. It is preferred that the fifth aspect of the present invention is applied to produce a formulation according to the second aspect of the present invention defined above or a preferred embodiment thereof.

A sixth aspect of the present invention provides a method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles com into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator.

It is preferred that the sixth aspect of the present invention is applied to produce a formulation according to the third aspect of the present invention defined above or a preferred embodiment thereof.

In the fourth, fifth and sixth aspects of the present invention it is preferred that said monomer, crosslinking compound and photoinitiator are combined to provide an encapsulant precursor mixture to which is then added the semiconductor nanoparticles prior to polymerisation of said monomer to provide said polymeric optically transparent encapsulation medium. In this way it is possible to avoid the need to include a nanoparticle capping agent, such as TOP or TOPO, in the mixture containing the nanoparticles and the polymerisable monomer.

In the fourth, fifth and sixth aspects of the present invention it is preferred that said semiconductor nanoparticles are produced by converting a nanoparticle precursor composition to the material of the nanoparticles in the presence of a molecular cluster compound under conditions permitting seeding and growth of the nanoparticles on the cluster compound. The nanoparticles may incorporate first and second ions, in which case the nanoparticle precursor composition may comprise separate first and second nanoparticle precursor species containing said first and second ions respectively for incorporation into the growing nanoparticles or the first and second ions may be combined into a single molecular species containing both types of ions for incorporation into the growing nanoparticles. A seventh aspect of the present invention provides a light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table, said nanoparticles being incorporated into an optically transparent (meth)acrylate encapsulation medium.

A further aspect of the present invention provides a method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table in an optically transparent poly(meth)acrylate encapsulation medium to produce a nanoparticle- containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles.

An eighth aspect of the present invention provides a light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

Another aspect of the present invention provides a method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles in an optically transparent (meth)acrylate encapsulation derived from a (meth)acrylate monomer and a trivalent crosslinking compound medium to produce a nanoparticle- containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles. A ninth aspect of the present invention provides a light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator. A further aspect of the present invention provides a method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles in an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator to produce a nanoparticle-containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles. The primary light source in any of the above-defined devices of methods may be selected from the group consisting of a light emitting diode, a laser, an arc lamp and a black-body light source.

In each of the devices according to the seventh, eighth and ninth aspects of the present invention it is preferred that said formulation is in accordance with the first, second or third aspects of the present invention respectively or preferred embodiments thereof.

The current invention provides a semiconductor nanoparticle formulation for use in the fabrication of light emitting devices, preferably with the devices incorporating an LED as a primary light source and the semiconductor nanoparticles as a secondary light source. In a preferred embodiment the formulation contains one or more types of semiconductor nanoparticles, such as QDs, incorporated into a plurality of polymeric beads which are embedded or entrapped within the optically transparent poly(meth)acrylate encapsulation medium.

In the Comparative Example below an LED-based light emitting device incorporated a formulation according to the present invention is tested against a light emitting device incorporating similar QDs embedded directly in a commercially available silicone LED encapsulant analogous to prior art devices. The device incorporating the formulation according to the present invention was observed to perform significantly better than the prior art device in that the device according to the present invention exhibited a significalty enhanced LED lifetime as compared to device incorporating QDs dispersed in the silicone LED encapsulant. The optically transparent medium may contain any desirable number and/or type of semiconductor nanoparticles. The medium may contain a single type of semiconductor nanoparticle, e.g. InP or CdSe, of a specific size range, such that the composite material incorporating the nanoparticles incorporated within the medium emits monochromatic light of a pre-defined wavelength, i.e. colour. The colour of the emitted light may be adjusted by varying the type of semiconductor nanoparticle material used, e.g. changing the size of the nanoparticle, the nanoparticle core semiconductor material and/or adding one or more outer shells of different semiconductor materials. Moreover, colour control can also be achieved by incorporating different types of semiconductor nanoparticles, for examples nanoparticles of different size and/or chemical composition within the optically transparent medium. Furthermore, the colour and colour intensity can be controlled by selecting an appropriate number of semiconductor nanoparticles within the optically transparent medium. Preferably the medium contains at least around 1000 semiconductor nanoparticles of one or more different types, more preferably at least around 10,000, more preferably at least around 50,000, and most preferably at least around 100,000 semiconductor nanoparticles of one or more different types.

Colour Indexing

The colour of the light output from the QD-bead-LED (the "secondary light") can be measured using a spectrometer. The spectral output (mW/nm) can then be processed mathematically so that the particular colour of the light emitting device can be expressed as colour coordinates on a chromaticity diagram, for example the 2 ° CIE 1931 chromaticity diagram.

The 2 ° CIE 1931 chromaticity coordinates for a particular spectrum can be calculated from the spectral power distribution and the CIE 1931 colour matching functions x, y, z. The corresponding tristimulus values can be calculated thus

Where p is the spectral power, and x, y and z are the colour matching functions. From X, Y, and Z the chromaticity coordinates x, y can be calculated according to X Y

x = and y =

X + Y + Z X + Y + Z

Using x, y as the coordinates, a two-dimensional chromaticity diagram (the CIE 1931 colour space diagram) can be plotted.

Colour Rendering

Colour rendering describes the ability of a light source to illuminate objects such that they appear the correct colour when compared to how they appear when illuminated by a reference light source. Usually the reference light source is a tungsten filament bulb which is assigned a colour rendering index (CRI) of 100. To be acceptable for general lighting, a white light emitting device source is required to have a CRI > 80. An example of poor colour rendering is the sodium street lamp which has very poor colour rendering capability, i.e. it is difficult to distinguish a red car from a yellow car illuminated by a sodium lamp, in the dark under a sodium lamp they both appear grey.

The present invention provides a light-emitting device comprising a population of QDs incorporated into an optically transparent medium. The QDs within the optically transparent medium are in optical communication with a primary solid-state photon/light source (e.g. an LED, laser, arc lamp or black-body light source) such that, upon excitation by primary light from the primary light source the QDs within the optically transparent medium emit secondary light of a desired colour. The required intensities and emission wavelengths of the light emitted from the device itself can be selected according to appropriate mixing of the colour of the primary light with that of the secondary light(s) produced from the down conversion of the primary light by the QDs. Moreover, the size (and thus emission) and number of each type of QD within the optically transparent medium can be controlled, as can the size, morphology and constituency of the optically transparent medium, such that subsequent mixing of the QD-containing media allows light of any particular colour and intensity to be produced.

It will be appreciated that the overall light emitted from the device may consist of effectively just the light emitted from the QDs, i.e. just the secondary light, or a mixture of light emitted from the QDs and light emitted from the solid-state/primary light source, i.e. a mixture of the primary and secondary light. Colour mixing of the QDs can be achieved either within the QD-containing media or a mixture of differently coloured optically transparent media with all the QDs within a specific medium being the same size/colour (e.g. some containing all green QDs and others containing all red QDs).

The present invention is illustrated with reference to the following non-limiting examples and figures in which: Figure 1 is a schematic representation of a QD-based light emitting device according to an aspect of the present invention;

Figure 2 is an ambient light photo of an LED filled with a cadmium-free QD acrylate resin according to as aspect of the present invention; and

Figure 3 is a plot of QD-photoluminescence intensity expressed as a percentage of the initial value versus time for the a device according to an aspect of the present invention and a device prepared using a common silicone QD encapsulant resin. EXAMPLES

The Example below describes the preparation of QD-containing formulations for use in the fabrication of new, improved QD-based light emitting devices in accordance with the present invention. In the Comparative Example a device in accordance with the present invention is tested against a device based on prior art principles using the same type of QDs to compare the performance of the two devices. Two methods for producing QDs suitable for incorporation into said formulations are first set out in the Synthetic Methods section below.

SYNTHETIC METHODS

Method 1

Preparation of CdSe/HDA Capped Nanoparticles

HDA (500g) was placed in a three-neck round bottomed flask and dried and degassed by heating to 120^QC under a dynamic vacuum for > 1 hour. The solution was then cooled to 60^QC. To this was added 0.718g of [HNEt₃]4[Cd₁₀Se₄(SPh)₁₆] (0.20mmols). In total 42mmols, 22.0ml of TOPSe and 42mmols, (19.5ml, 2.15M) of Me₂Cd^»TOP was used. Initially 4mmol of TOPSe and 4mmols of Me₂Cd^»TOP were added to the reaction at room temperature and the temperature increased to 1 10^QC and allowed to stir for 2 hours. The reaction mixture was a deep yellow colour. The temperature was progressively increased at a rate of ~1 ^QC/5min with equimolar amounts of TOPSe and Me₂Cd^»TOP being added dropwise. The reaction was stopped when the photoluminescence (PL) emission maximum had reached around 600nm, by cooling to 60^QC followed by addition of 300ml of dry ethanol or acetone. This produced a precipitation of deep red particles, which were further isolated by filtration. The resulting CdSe particles were recrystallized by re-dissolving in toluene followed by filtering through Celite followed by re-precipitation from warm ethanol to remove any excess HDA, selenium or cadmium present. This produced 10.1 Og of HDA capped CdSe nanoparticles. Elemental analysis C = 20.88%, H = 3.58%, N = 1 .29%, Cd = 46.43%. Max PL = 585nm, FWHM = 35nm. 38.98 mmols, 93% of Me₂Cd consumed in forming the QDs.

Preparation of CdSe/ZnS-HDA Capped Nanoparticles

HDA (800g) was placed in a three neck round-bottom flask, dried and degassed by heating to 120^QC under a dynamic vacuum for > 1 hour. The solution was then cooled to 60^QC. To this was added 9.23g of CdSe nanoparticles that have a PL maximum emission of 585nm. The HDA was then heated to 220^QC. To this was added by alternate dropwise addition a total of 20ml of 0.5M Me₂Zn^»TOP and 0.5M, 20ml of sulfur dissolved in octylamine. Three alternate additions of 3.5, 5.5 and 1 1 .0ml of each were made, whereby initially 3.5ml of sulphur was added dropwise until the intensity of the PL maximum was near zero. Then 3.5ml of Me₂Zn^»TOP was added dropwise until the intensity of the PL maximum had reached a maximum. This cycle was repeated with the PL maximum reaching a higher intensity with each cycle. On the last cycle, additional precursor was added once the PL maximum intensity been reached until it was between 5 - 10% below the maximum intensity, and the reaction was allowed to anneal at 150^QC for 1 hour. The reaction mixture was then allowed to cool to 60 ^QC whereupon 300ml of dry "warm" ethanol was added which resulted in the precipitation of particles. The resulting CdSe-ZnS particles were dried before re-dissolving in toluene and filtering through Celite followed by re-precipitation from warm ethanol to remove any excess HDA. This produced 12.08g of HDA capped CdSe-ZnS core-shell nanoparticles. Elemental analysis C = 20.27%, H =3.37%, N = 1 .25%, Cd = 40.1 1 %, Zn = 4.43%; Max PL 590nm, FWHM 36nm. Method 2

Preparation of InP Core Quantum Dots (500 - 700 nm emission)

Di-butyl ester (100ml) and myristic acid (10.0627g) were placed in a three-neck flask and degassed at 70^ under vacuum for one hour. After this period, nitrogen was introduced and the temperature increased to 90 °C. ZnS molecular cluster [Et₃NH₄][Zn_{1 0}S4(SPh)₁₆] (4.7076g) was added and the mixture allowed to stir for 45 minutes. The temperature was then increased to 100°C followed by the dropwise addition of ln(MA)₃ (1 M, 15ml) followed by (TMS)₃P (1 M, 15ml). The reaction mixture was allowed to stir while increasing the temperature to 140°C. At 140 ^, further dropwise additions of ln(MA)₃ (1 M, 35ml) (left to stir for 5 minutes) and (TMS)₃P (1 M, 35ml) were made. The temperature was then slowly increased to 180 ^<€ and further dropwise additions of ln(MA)₃ (1 M,55ml) followed by (TMS)₃P (1 M, 40ml) were made. By addition of the precursor in the manner above nanoparticles of InP could be grown with the emission maximum gradually increasing from 520nm up to 700nm, whereby the reaction can be stopped when the desired emission maximum has been obtained and left to stir at this temperature for half an hour. After this period, the temperature was decreased to 160°C and the reaction mixture was left to anneal for up to 4 days (at a temperature between 20 - 40°Cbelow that of the reaction). A UV lamp was also used at this stage to aid in annealing.

The nanoparticles were isolated by the addition of dried degassed methanol (approx. 200 ml) via cannula techniques. The precipitate was allowed to settle and then methanol was removed via cannula with the aid of a filter stick. Dried degassed chloroform (approx. 10 ml) was added to wash the solid. The solid was left to dry under vacuum for 1 day. This produced 5.60 g of InP core nanoparticles. Max PL = 630 nm, FWHM = 70 nm.

Post-Operative Treatments

The quantum yields of the InP QDs prepared above were increased by washing with dilute HF acid. The dots were dissolved in anhydrous degassed chloroform (~270ml). A 50ml portion was removed and placed in a plastic flask, flushed with nitrogen. Using a plastic syringe, the HF solution was made up by adding 3ml of 60% w/w HF in water and adding to degassed THF (17ml). The HF was added dropwise over 5hrs to the InP dots. After addition was complete the solution was left to stir overnight. Excess HF was removed by extracting through calcium chloride solution in water and drying the etched InP dots. The dried dots were re-dispersed in 50ml chloroform for future use. PLmax 567 nm, FWHM 60 nm. The quantum efficiencies of the core materials at this stage range from 25-90 %

Growth of a ZnS shell to provide InP/ZnS Core/Shell Quantum Dots

A 20ml portion of the HF-etched InP core particles was dried down in a 3-neck flask. 1 .3g myristic acid and 20ml di-n-butyl sebacate ester was added and degassed for 30 minutes. The solution was heated to 200 ^ then 1 .2g anhydrous zinc acetate was added and 2ml 1 M (TMS)₂S was added dropwise (at a rate of 7.93ml/hr) after addition was complete the solution was left to stir. The solution was kept at 200 ^ for 1 hr then cooled to room temperature. The particles were isolated by adding 40 ml of anhydrous degassed methanol and centrifuged. The supernatant liquid was disposed of and to the remaining solid 30ml of anhydrous degassed hexane was added. The solution was allowed to settle for 5hrs and then re-centrifuged. The supernatant liquid was collected and the remaining solid was discarded. PL emission peak Max. = 535 nm, FWHM = 65 nm. The quantum efficiencies of the nanoparticle core/shell materials at this stage ranged from 35-90%.

EXAMPLE QD-containing LED preparation

A solution of cadmium-free quantum dots (CFQDs) in toluene (e.g. 20mg of InP/ZnS core/shell QDs produced as described above) is dried under vacuum to leave a QD residue. To the residue an acrylate monomeric mixture was added and the dots incubated until a clear solution was formed. The acrylate mixture was composed of laurylmethacrylate (1 .85ml, 6.6mmol), a photoinitiator (Irgacure 819^® (bis(2,4,6- trimethylbenzoyl)-phenylphosphineoxide), 9mg) dissolved in a crosslinker (trimethylolpropanetrimethacrylate (1 .06ml, 3.3mmol)).

An aliquot of the monomer mixture (e.g. 1 .5 to 3μΙ) containing the QDs was used to fill a cup of an LED. The filled LED was then irradiated (Hamamatsu uv-LED lamp LC-L2, 365nm, 7500 mW/cm², 10min, 1 LED above & 1 LED below 8cm distance) to provide a CFQD-polymethacrylate filled LED having the structure shown in Figures 1 and 2. With reference to Figure 1 there is shown a light emitting device 1 comprising a conventional LED package 2 with a standard LED chip 3. Directly on top of the LED chip 3 within the LED well 4 is provided a sufficient volume of a commercially available silicone resin 5 so as to cover and submerge the LED chip 3. A sufficient volume of the QD-monomer mixture 6 is provided on top of the silicone layer 5 so as to substantially fill the LED well 4. Since the silicone resin 5 submerges the LED chip 3 there is a space between the chip 3 and the QD-monomer mixture 6 that is filled with the silicone resin 5. In this way, the QD-containing mixture 6 is insulated from the potentially harmful high temperatures generated by the chip 3 during operation. A UV curing epoxy resin 7 is provided around the periphery of the opening to the LED well 4, upon which is provided a thin layer of an encapsulating material 8, such as glass. The epoxy resin 7 and QD-monomer mixture 6 are then exposed to UV radiation as described above to cure the resin 7 and seal the device 1 , and to polymerise and crosslink the QD-monomer mixture 6.

COMPARATIVE EXAMPLE LEDs were fabricated using either a standard commercially available silicon resin (SCR101 1®, ShinEtsu) or an acrylate resin according to on the current invention. The LEDs were operated at 20mAmp and generated a 450nm blue light with 22mW intensity. Figure 3 demonstrates the difference in the stability and performance of the LEDs. The acrylate based resin according to the present invention provided a remarkable increase in stabilization of the encapsulated CFQDs on the LED as compared to the LED incorporating CFQDs dispersed in the commerically available silicon resin.

Claims

A formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table, said nanoparticles being incorporated into an optically transparent poly(meth)acrylate encapsulation medium.

A formulation according to claim 1 , wherein said poly(meth)acrylate encapsulation medium is derived from a (meth)acrylate monomer and a multivalent crosslinking compound.

A formulation according to claim 2, wherein said multivalent crosslinking compound is a trivalent crosslinking compound, such as trimethylolpropanetrimethacrylate.

A formulation according to claim 2 or 3, wherein said (meth)acrylate monomer is laurylmethacrylate.

A formulation according to any one of claims 2 to 4, wherein said monomer and crosslinking compound are reacted in the presence of a photoinitiator.

A formulation according to claim 1 , wherein said poly(meth)acrylate is selected from the group consisting of polylauryl(meth)acrylate, polystearyl(meth)acrylate, polytrimethylsilyl (meth)acrylate, polytrimethylsilyloxyalkyl (meth)acrylate, polyglycidyl(meth)acrylate, methyl(meth)acrylate and combinations thereof.

A formulation according to claim 1 , wherein said poly(meth)acrylate is polylaurylmethacrylate.

A formulation according to any preceding claim, wherein said semiconductor nanoparticles contain one or more semiconductor material selected from the group consisting of InP, InAs, InSb, AIP, AIS, AIAs, AlSb, GaN, GaP, GaAs, GaSb and combinations thereof.

9. A formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

10. A formulation according to claim 9, wherein said poly(meth)acrylate is selected from the group consisting of polylauryl(meth)acrylate, polystearyl(meth)acrylate, polytrimethylsilyl (meth)acrylate, polytrimethylsilyloxyalkyl (meth)acrylate, polyglycidyl(meth)acrylate, methyl(meth)acrylate and combinations thereof.

1 1 . A formulation according to claim 9, wherein said (meth)acrylate monomer is laurylmethacrylate.

12. A formulation according to any one of claims 9 to 1 1 , wherein said trivalent crosslinking compound is trimethylolpropanetrimethacrylate.

13. A formulation according to any one of claims 9 to 12, wherein said (meth)acrylate monomer and and said trivalent crosslinking compound are reacted in the presence of a photoinitiator.

14. A formulation according to any one of claims 9 to 12, wherein said semiconductor nanoparticles contain ions selected from group 1 1 , 12, 13, 14,

15 and/or 16 of the periodic table, or said semiconductor nanoparticles contain one or more types of transition metal ion or d-block metal ion.

15. A formulation according to any one of claims 9 to 13, wherein said semiconductor nanoparticles contain one or more semiconductor material selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AIP, AIS, AIAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, MgTe and combinations thereof.

16. A formulation for use in the fabrication of a light emitting device said formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a multivalent crosslinking compound reacted in the presence of a photoinitiator.

17. A formulation according to claim 16, wherein said multivalent crosslinking compound is a trivalent crosslinking compound, such as trimethylolpropanetrimethacrylate.

18. A formulation according to claim 16 or 17, wherein said semiconductor nanoparticles contain ions selected from group 1 1 , 12, 13, 14, 15 and/or 16 of the periodic table, or said semiconductor nanoparticles contain one or more types of transition metal ion or d-block metal ion.

19. A formulation according to claim 16 or 17, wherein said semiconductor nanoparticles contain one or more semiconductor material selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AIP, AIS, AIAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, MgTe and combinations thereof.

20. A formulation according to claim any preceding claim, wherein said population of semiconductor nanoparticles is incorporated into a plurality of discrete microbeads, said nanoparticle-containing microbeads being embedded within said encapsulation medium.

21 . A method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table into an optically transparent poly(meth)acrylate encapsulation medium.

22. A method according to claim 21 , wherein said poly(meth)acrylate encapsulation medium is derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

23. A method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

24. A method according to claim 22 or 23, wherein said monomer and crosslinking compound are reacted in the presence of a photoinitiator.

25. A method of preparing a formulation for use in the fabrication of a light emitting device said method comprising incorporating a population of semiconductor nanoparticles com into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator.

26. A method according to any one of claims 22 to 25, wherein said monomer, crosslinking compound and photoinitiator are combined to provide an encapsulant precursor mixture to which is then added the semiconductor nanoparticles prior to polymerisation of said monomer to provide said polymeric optically transparent encapsulation medium.

27. A method according to any one of claims 22 to 26, wherein said semiconductor nanoparticles are produced by converting a nanoparticle precursor composition to the material of the nanoparticles in the presence of a molecular cluster compound under conditions permitting seeding and growth of the nanoparticles on the cluster compound.

28. A method according to claim 27, wherein the nanoparticles incorporate first and second ions and the nanoparticle precursor composition comprises separate first and second nanoparticle precursor species containing said first and second ions respectively for incorporation into the growing nanoparticles.

29. A method according to claim 27, wherein the nanoparticles incorporate first and second ions and the nanoparticle precursor composition comprises a single molecular species containing said first and second ions for incorporation into the growing nanoparticles.

30. A light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table, said nanoparticles being incorporated into an optically transparent (meth)acrylate encapsulation medium.

31 . A light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent poly(meth)acrylate encapsulation medium derived from a (meth)acrylate monomer and a trivalent crosslinking compound.

32. A light emitting device including a primary light source in optical communication with a formulation comprising a population of semiconductor nanoparticles incorporated into an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator.

33. A device according to claim 30, 31 or 32, wherein said primary light source is selected from the group consisting of a light emitting diode, a laser, an arc lamp and a black-body light source.

34. A device according to any one of claims 30 to 33, wherein said formulation is in accordance with any one of claims 1 to 20.

35. A method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles comprising ions from groups 13 and 15 of the Periodic Table in an optically transparent poly(meth)acrylate encapsulation medium to produce a nanoparticle-containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles.

36. A method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles in an optically transparent (meth)acrylate encapsulation derived from a (meth)acrylate monomer and a trivalent crosslinking compound medium to produce a nanoparticle- containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles.

37. A method of fabricating a light emitting device comprising providing a population of semiconductor nanoparticles in an optically transparent polymeric encapsulation medium derived from a laurylmethacrylate monomer and a trivalent crosslinking compound reacted in the presence of a photoinitiator to produce a nanoparticle-containing formulation, and depositing said formulation onto a primary light source such that said primary light source is in optical communication with said population of semiconductor nanoparticles.

38. A method according to any one of claims 35 to 37, wherein said formulation is in accordance with any one of claims 1 to 20.