US20180155622A1

US20180155622A1 - Lighting devices with prescribed colour emission

Info

Publication number: US20180155622A1
Application number: US15/831,953
Authority: US
Inventors: Hagai Arbell; Uri Banin
Original assignee: Yissum Research Development Co of Hebrew University of Jerusalem; Qlight Nanotech Ltd
Current assignee: Yissum Research Development Co of Hebrew University of Jerusalem; Qlight Nanotech Ltd
Priority date: 2010-01-28
Filing date: 2017-12-05
Publication date: 2018-06-07
Also published as: JP2013518166A; WO2011092646A3; EP2528989B1; EP2528990A2; JP2013518932A; EP2528989A2; US20130026506A1; US20160013370A1; KR20120131173A; US9109163B2; CN102844403B; JP5828154B2; KR20180025982A; CN103108940B; EP2528990B1; WO2011092647A3; KR101519509B1; US20130181234A2; US20160289556A1; WO2011092646A2

Abstract

Optical conversion layers based on semiconductor nanoparticles for use in lighting devices, and lighting devices including same. In various embodiments, spherical core/shell seeded nanoparticles (SNPs) or nanorod seeded nanoparticles (RSNPs) are used to form conversion layers with superior combinations of high optical density (OD), low re-absorbance and small FRET. In some embodiments, the SNPs or RSNPs form conversion layers without a host matrix. In some embodiments, the SNPs or RSNPs are embedded in a host matrix such as polymers or silicone. The conversion layers can be made extremely thin, while exhibiting the superior combinations of optical properties. Lighting devices including SNP or RSNP-based conversion layers exhibit energetically efficient superior prescribed colour emission

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/799,793 filed Jul. 15, 2015, which is a continuation of U.S. application Ser. No. 13/575,845 filed Oct. 16, 2012, which claims priority from U.S. Provisional Patent Applications Nos. 61/299,012 filed Jan. 28, 2010 and 61/299,018 filed Jan. 28, 2010, all of which are hereby incorporated herein by reference in their entirety.

FIELD AND BACKGROUND

Embodiments of the invention relate in general to optical devices which comprise semiconductor nanoparticles and in particular to lighting devices which include conversion layers having semiconductor quantum confined nanoparticles.
Light emitting diodes (LED) offer significant advantages over incandescent and fluorescent lamps with respect to their high energy efficiency and long lifetimes. LEDs are applicable in diverse applications including displays, automobile and signage lighting and domestic and street lighting.
A LED can emit monochromatic light in different regions of the spectrum, depending on the inorganic semiconductor compound used to fabricate it. However, “white” light, which is required for a very large portion of the lighting industry, cannot be generated using a conventional LED. Current solutions of producing white light include the use of three or more LEDs with various colours (e.g. Red, Green and Blue or “RGB”), or the use of a colour conversion layer of phosphor material (e.g. Cerium:YAG) to generate a broad white spectral emission from the ultraviolet (UV) or blue emission of a LED. However, such white light is almost always non-ideal and has in many cases undesired or unpleasant characteristics which may require improvement or correction.
Colloidal based semiconductor quantum dots (QD) offer the possibility of obtaining a colour gamut similar to and even better than the one obtained with the multi-LED solution, using the narrow-band emission of a QD tunable by size. Conversion layers incorporating ODs are known, see e.g. U.S. Pat. Nos. 7,264,527 and 7,645,397 and US patent applications 2008/0173886 and 2009/0162011. However, conversion layers based on QDs have challenges. These include for example losses due to re-absorption effects, whereby the QD emission is reabsorbed by other QDs in the layer. Generally this will occur for a red QD absorbing the emission emanating from QDs which emit more to the blue. This undesired process leads to reduced energy efficiency of a regular QD conversion layer and also to changes in the colour composition. The inherent size distribution of QD samples already provides different colours around a central colour. Therefore, re-absorption will take place inherently within such a layer. In devices where phosphor is used as part of a light conversion scheme to produce green light, the QD layers will absorb partially the light from the phosphor as well, leading to both re-absorption losses and colour changes.
In some cases, a close-packed conversion layer is desired. Close-packed QD conversion layers suffer from the phenomenon known as Fluorescence Resonant Energy Transfer (FRET), see e.g. Joseph R. Lakowicz, “Principles of Fluorescence Spectroscopy”, 2^ndedition, Kluwer Academic/Plenum Publishers, New York, 1999, pp. 367-443. FRET occurs between a donor QD which emits at a shorter (e.g. bluer) wavelength relative to an acceptor QD positioned in close proximity and which emits at longer wavelength. There is a dipole-dipole interaction between the donor emission transition dipole moment and the acceptor absorption transition dipole moment. The efficiency of the FRET process depends on the spectral overlap of the absorption of the donor with the emission of the acceptor. The FRET distance between quantum dots is typically 10 nm or smaller. The efficiency of the FRET process is very sensitive to distance. FRET leads to colour change (red shift) and losses in the efficiency of light conversion.
Core/shell nanoparticles (NPs) are known. These are discrete nanoparticles characterized by a heterostructure in which a “core” of one type of material is covered by a “shell” of another material. In some cases, the shell is grown over the core which serves as a “seed”, the core/shell NP known then as a “seeded” NP or SNP. The term “seed’ or “core” refers to the innermost semiconductor material contained in the heterostructure. FIG. 1A and FIG. 1B show schematic illustrations of known core/shell particles. FIG. 1A illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. FIG. 1B illustrates a rod shaped (“nanorod”) SNP (RSNP) which has a core located asymmetrically within an elongated shell. The term nanorod refers to a nanocrystal having a rod-like shape, i.e. a nanocrystal formed by extended growth along a first (“length”) axis of the crystal with very small dimensions maintained along the other two axes. A nanorod has a very small (typically less than 10 nm) diameter and a length which may range from about 6 nm to about 500 nm.
Typically the core has a nearly spherical shape. However, cores of various shapes such as pseudo-pyramid, cube-octahedron and others can be used. Typical core diameters range from about 1 nm to about 20 nm. FIG. 1C illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. The overall particle diameter is d₂, much larger than the core diameter d₁. The magnitude of d₂compared with d₁affects the optical absorbance of the core/shell NP.
As known, a SNP may include additional external shells which can provide better optical and chemical properties such as higher quantum yield (QY) and better durability. The combination may be tuned to provide emitting colours as required for the application. The length of the first shell can range in general between 10 nm and 200 nm and in particular between 15 nm and 160 nm. The thicknesses of the first shell in the other two dimensions (radial axis of the rod shape) may range between 1 nm and 10 nm. The thickness of additional shells may range in general between 0.3 nm to 20 nm and in particular between 0.5 nm to 10 nm.
In view of the numerous deficiencies of QD conversion layers mentioned above, there is a need for and it would be advantageous to have conversion layers which do not suffer from such deficiencies. In particular, there is a need for and it would be advantageous to have nanoparticle-based thin conversion layers with negligible re-absorption (of both same and different colour), negligible clustering and high-loading effects and negligible FRET.

DEFINITIONS

The term “core material” to the semiconductor material from which the core is made. The material may be II-VI, III-V, IV-VI, or I-III-VI₂semiconductors or combinations thereof. For example, the seed/core material may be selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂, Cu₂(ZnSn)S₄, Cu₂(InGa)S₄, alloys thereof, and mixtures thereof.
The term “shell material” refers to the semiconductor material from which each of the non-spherical elongated shells is made. The material may be a II-VI, III-V IV-VI, or I-III-VI₂semiconductor or combinations thereof. For example, the shell material may be selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, GaN, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂, Cu₂(ZnSn)S₄, Cu₂(InGa)S₄, alloys thereof, and mixtures thereof.
The term “host matrix” refers to a material which incorporates the SNPs or other suitable nanoparticles. The host matrix may be a polymer (formed from liquid or semisolid precursor material such as monomer), an epoxy, silicone, glass or a hybrid of silicone and epoxy. Specific (but not limiting) examples of polymers include fluorinated polymers, polymers of polyacrylamide, polymers of polyacrylic acids, polymers of polyacrylonitrile, polymers of polyaniline, polymers of polybenzophenon, polymers of poly(methyl methacrylate), silicone polymers, aluminium polymers, polymers of polybisphenol, polymers of polybutadiene, polymers of polydimethylsiloxane, polymers of polyethylene, polymers of polyisobutylene, polymers of polypropylene, polymers of polystyrene and polyvinyl polymers, polyvinyl-butyral polymers or perfluorocyclobutyl polymers.
The term “ligand” refers to an outer surface coating of the nanoparticles. The coating passivates the SNP to prevent agglomeration or aggregation to overcome the van der Waals binding force between the nanoparticles. Ligands in common use: phosphines and phosphine oxides such as trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), tributylphosphine (TBP), phosphonic acids such as dodecylphosphonic acid (DDPA), tridecylphosphonic acid (TDPA), octadecylphosphonic acid (ODPA) or hexylphosphonic acid (HPA), amines such as dodecyl amine (DDA), tetradecyl amine (TDA), hexadecyl amine (HDA) or octadecyl amine (ODA), thiols such as hexadecane thiol or hexane thiol, mercapto carboxylic acids such as mercapto propionic acid or mercapto undecanoic acid and other acids such as myristic or palmitic acid.

SUMMARY

Embodiments of the invention disclose optical conversion layers (or simply “layers”) incorporating at least one SNP species and/or other nanoparticles that have the needed characteristics rendering this layer with its unique optical properties. One such layer according to an embodiment of the invention is referred to henceforth as a “SNP conversion layer” or simply “SNP layer”. References will be made henceforth also to “SNP sub-layers” representing part of a SNP layer, and to SNP multilayers, representing a structure with a plurality of SNP layers. Similar terms will be used for RSNP based layers with “SNP” replaced by “RSNP”. To clarify, henceforth in this description, “layer” is equivalent to “conversion layer”. Embodiments of the invention further disclose the application of SNP conversion layers for transforming light, particularly for conversion of LED monochromatic emission of short wavelength (e.g. blue or UV) to longer wavelengths in the VIS/NIR range to produce light of different colours. In particular, SNP layers of the invention may be used with one or more LEDs to provide a white light device with high energy efficiency and good optical properties such as high CRI (Colour Rendering Index) and desired CCT (correlated colour temperature). In other lighting applications, a SNP layer can provide a necessary and beneficial spectral output such as large gamut coverage, or specific colour bands.
In an embodiment, a SNP layer may include one type (species) of SNP emitting essentially at a single colour. In another embodiment, a SNP layer may include a mixture of several types of SNPs emitting at different colours. In some embodiments, a SNP layer may include SNPs incorporated in a host matrix, with or without ligands. A SNP layered structure may comprise several sub-layers, each of which may include a mixture of SNPs, or may include a different type of SNP.
In some embodiments, the SNP layer thickness may be equal to or thinner than 200 μm. In some embodiments, the SNP layer thickness may be equal to or thinner than 50 μm. In some embodiments, the SNP layer thickness may be equal to or thinner than 2 μm. In other embodiments, the SNP layer may have a thickness ranging between ca. 50 and 1000 nm.
In some embodiments, a SNP layer may include SNPs having a high-loading ratio within a polymer matrix, epoxy or resin. In some embodiments, the high-loading ratio may be up to 40%. In some embodiments, the high-loading ratio may be up to 80%. In some embodiments, the high-loading ratio may be up to 100%.
SNP conversion layers according to embodiments disclosed herein provide functionalities and advantageous properties which are unknown in QD conversion layers and which were not previously discovered. These include:
1) Negligible re-absorption (both same colour and different colour). The re-absorption in a SNP conversion layer is reduced significantly (in comparison to that in a QD conversion layer) because of low red absorbance. In general, re-absorption leads to loss of energy. For example, assume a typical QY of 0.8. In a single re-absorption event, the OY is reduced to 0.8×0.8=0.64. In two such events, the QY is reduced further to 0.8³=0.51. This loss is avoided in a SNP conversion layer. Hence, the efficiency is improved. Re-absorption also leads to a red shift, which is also avoided in such a SNP conversion layer. The aspect of negligible re-absorption is present not only for one colour on itself (e.g. red to more red), but also in green emitting SNPs or phosphors. Namely, with a QD conversion layer, red QDs will reabsorb green emission, leading to reduced efficiency and colour shift. With a SNP conversion layer, this re-absorption is minimized Both same colour and different colour re-absorption avoidance functions are unique features of SNP layers, whether densely-packed or not.
2) Very efficient “funneling” of energy from blue excitation to red emission. An SNP conversion layer acts essentially as an “optical antenna” in the spectral sense. It performs this task much more efficiently that a regular QD conversion layer, since it has very high absorbance in the blue and strong red photoluminescence (PL) accompanied by minimal red re-absorption, see point (1) above.
3): FRET avoidance or minimalization. In a regular QD conversion layer, The typical length scale of FRET is ˜10 nm, with 1/R⁶dependence, where R is the distance between two QD particles. For example, if the initial emission QY is 0.8, the QY is reduced to 0.64 after a single FRET process. FRET also leads to red shift, which is avoided in a SNP layer. A densely-packed SNP conversion layer (with small distances between the SNPs, e.g. 0-50 nm) owing to their unique characteristics will avoid the losses and deficiencies related to FRET commonly encountered in densely-packed QD layers. Exemplarily, “densely-packed” as applied to SNPs in conversion layers of the invention may include ˜85% SNPs and ˜15% ligands dispersed in a host matrix.
In an embodiment, a SNP layer may be coated on an optical device such as a LED to improve its emission spectrum. In another embodiment, a separate SNP layer may be positioned in the optical path of light emitted by one or more LEDs for the same purpose. In yet another embodiment, a layered SNP structure comprising a plurality of different SNP conversion sub-layers may be coated on a LED. In yet another embodiment, the layered SNP structure may be positioned in the optical path of light emitted by LEDs. In some embodiments, a SNP layer may be spaced apart from a LED by a coupling layer which may be an air gap, an optical filter such as short wavelength (UV, blue) pass filter, a long wavelength (e.g. green or red) reflection filter, or an index matching layer which minimizes energy loss by reflection. The spacing between the LED and the SNP layer can for example be used to minimize heating due to heat flow from the LED to the SNP layer.
A combination of a LED and a SNP conversion layer or layered structure may be used in a lighting device (i.e. a domestic light, a signage light, a vehicle light, a portable light, a back light or any other light). In some embodiments, a lighting device may further comprise an optical element such as a lens, a waveguide, a scatterer, a reflective element, a refractive element or a diffractive element. The optical element may be placed between the SNP layer and the light source or before the SNP layer in the optical path, or on the sides of the layer (for example a reflective element for using scattered light). In some embodiments, the lighting device may include a combination of two or more optical elements from the list above in addition to a LED and one or more SNP layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIGS. 1A-1C are schematic illustrations of known core/shell particles: (A) core QD/shell QD; (B) RSNP; (C) SNP;

FIGS. 2A and 2B show experimental results of optical absorption and emission of a core/shell QD material vs. a RSNP material used in embodiments of the invention: (A) for green light; (B) for orange light;

FIG. 3 shows normalized absorption curves of three types of red emitting SNPs having different lengths;

FIGS. 4A and 4B illustrate schematically the FRET effect in densely-packed QDs, in which it is efficient, and in densely-packed RSNPs, in which it is blocked;

FIG. 5A shows schematically a light conversion device which includes a SNP layer according to one embodiment of the invention;

FIG. 5B shows schematically a light conversion device which includes a SNP layer according to another embodiment of the invention;

FIG. 6A shows schematically a light conversion device which includes a SNP layer according to yet another embodiment of the invention;

FIG. 6B shows schematically a light conversion device which includes a SNP layer according to yet another embodiment of the invention;

FIG. 7 shows schematically a lighting device which includes a SNP layer according to an embodiment of the invention;

FIG. 8 shows schematically a lighting device which includes a SNP layer according to another embodiment of the invention;

FIG. 9 shows schematically a lighting device which includes a SNP layer according to yet another embodiment of the invention.

FIG. 10A shows a LED coupled to a waveguide that has an SNP layer embedded therewithin;

FIG. 10B shows schematically a LED coupled to a waveguide that has an SNP layer positioned on a top surface thereof;

FIG. 11 shows the absorption (dotted line) and the PL (full line) of a conversion layer which comprises 33×7 nm CdSe/CdS RSNPs embedded in polymer PVB film;

FIG. 12A shows the light spectrum of a lighting device comprising a 455 nm blue LED with a conversion layer which comprises 33×7 nm CdSe/CdS RSNPs embedded in a PVB film with added BaSO₄particles;

FIG. 12B shows the light spectrum of a lighting device comprising a 455 nm blue LED with a conversion layer which comprises 27×5.5 nm CdSe/CdS RSNPs embedded in a Silicone RTV film;

FIG. 13A shows the absorption (dotted line) and the PL spectra (full line) of a dense spin coated red emitting RSNP layer on glass;

FIG. 13B shows the absorption (dotted line) and the PL spectra (full line) of a dense spin coated green emitting RSNP layer on glass;

FIG. 14A shows the normalized light spectrum of a lighting device comprising a broad band LED based element with a SNP conversion layer.

FIG. 14B shows the normalized light spectrum of a lighting device comprising a broad band LED based element with another SNP conversion layer.

FIG. 15 shows a CIE chart with the two outputs of FIGS. 14A, B marked as shown.

FIG. 16 shows the absorption (dotted line) and the PL spectra (full line) of the SNP film of Example 7.

DETAILED DESCRIPTION

Embodiments of SNP layers, SNP layers used to condition LED light and lighting devices including such layers are now described in more detail. In particular, advantageous properties and features of such layers are described next with reference to FIGS. 2A, 2B, 3, 4A, and 4B. The various SNP layers mentioned below may be prepared using procedures detailed in Examples below.
Reference is now made to FIGS. 2A and 2B, which show a comparison between the absorption and emission of a known conventional CdSe/ZnS core/shell QD layer and two types of RSNP layers according to embodiments of the invention: a green emitting RSNP layer (FIG. 2A) and an orange emitting RSNP layer (FIG. 2B). The comparison is between the absorption and normalized emission of the QD layer vs. the SNP layers having a matched absorption at the excitation wavelength of 450 nm. The Green RSNP layer included CdSe/CdS core/shell RSNPs with dimensions 4×27 nm (diameter×length), emitting at a center wavelength (CWL) or peak wavelength of 540 nm with a full width half maximum (FWHM) of 29 nm. The Orange RSNP layer included CdSe/CdS RSNPs with dimensions 5×40 nm, a CWL at 600 nm and FWHM of 28 nm. Both Orange and Green emitting layers were prepared in a similar way, and both were 190 μm-thick, with diameter of 42 mm.
The PL quantum yield (QY) of both QD and RSNP nanoparticles was similar and on the order of 50%. This is a typical value. In other prepared samples, the QY ranged from 5-100%, more often between 20-90% and even more often between 50-80%. The absorption is measured in relative optical density (OD) units, where the scale shown is normalized to the range [0 1] for convenience. Significantly, for the green light emitting layers in FIG. 2A, the OD of the QD layer in the emission wavelength range (e.g. 520-550 nm) is 10 times higher than that of the RSNP layer (0.64 vs. 0.065). The OD difference for the orange emitting layers in FIG. 2B is even higher (0.575 vs. 0.037, a factor of ˜15). In other examples (not shown), the OD in the emission range of a QD layer was found to be 3-30 times higher than that of a RSNP layer. Therefore, losses due to self-absorbance are significant for the QD layer case and negligible for the RSNP layer case. This property is used in various SNP layers of the invention (whether densely-packed or not) to provide far superior products over existing layers based on quantum dots.
The inventors have further determined that SNP layers of the invention have a feature of very efficient “funneling” of energy from blue excitation to red emission. An SNP layer acts essentially as an “optical antenna” in the spectral sense. It performs this task much more efficiently that a regular QD layer, since it has very high absorbance in the blue and strong red PL accompanied by minimal red re-absorption, see FIG. 3.
FIG. 3 shows normalized absorption curves of three types of RSNP layers prepared as described in Example 1 below and comprising in each case red emitting RSNPs (CdSe/CdS) having different overall dimensions and nearly similar emission spectra: a curve 300 for 5.8×16 nm RSNPs with 622 nm emission, a curve 302 for 4.5×45 nm RSNPs with 625 nm emission and a curve 304 for 4.5×95 nm RSNPs with 628 nm emission. These curves illustrate the funneling effect in different conversion layers. The absorption curves are normalized to OD 1 at 455 nm. The “absorption ratio” between absorption at 455 nm to that at the emission wavelengths is respectively 1:5, 1:12 and 1:23 for SNP layers with RSNPs of lengths 16, 45 and 95 nm. This shows that the funneling is more efficient for layers comprising longer RSNPs and that the absorption ratio is “tunable” by varying the RSNP length. Note that for SNPs which are not rod-shaped, a similar tuning can be achieved by increasing the shell to core diameter ratio. This tunability is very useful in SNP layers since it allows the SNP layers to act as the efficient spectral antenna to convert blue light to red light desired in a light source and application. An additional parameter resulting from this special characteristic of SNP layer is that it allows to efficiently balance the light between the different spectral regions of the visible spectrum (say green-yellow emitted by CE:YAG and the red emitted by SNPs) to obtain light with required characteristic (such as CCT and CRI).
The inventors have further determined that layers with densely-packed SNPs have significantly smaller FRET losses than layers having densely-packed QDs. FIGS. 4A and 4B illustrate schematically the FRET effect in densely-packed conversion layers of QDs and RSNPs. FIG. 4A, for the QD conversion layer case, shows some QDs acting as donors (D, 410) and some acting as acceptors (A, 420), with a typical distance between donor and acceptor denoted by the arrow 430. In such a typical QD conversion layer, the smaller QDs act as donors to larger QDs which act as acceptors. The typical center-to-center distance is on the order of the FRET distance of ˜10 nm, hence FRET is efficient in such a densely-packed QD conversion layer. FIG. 4B, for the SNP layer case, shows that the special geometry induces on average large distances between a RSNP 440 emitting a colour slightly bluer as compared to another RSNP 450. The typical core-to-core distance in this case (indicated by 460) is around half of the RSNP length, and is engineered to exceed the FRET distance, leading to a significantly reduced probability for FRET processes.
To reiterate, known QDs do not provide such a large distance and consequently, in a densely-packed layer arrangement, their FRET losses are inherent, leading to reduced conversion efficiency. In addition, in a densely-packed QD conversion layer, the FRET process leads to a red shift of the emission. In contrast, in a SNP layer as disclosed herein, the emission is maintained at the tailored and desired wavelength, providing the required colour and higher energy efficiency.
To reemphasize, the optical properties of the SNP layers of the invention provide significant advantages over existing QD conversion layers due to low re-absorption and small energy transfer losses and light colour changes. The capability to minimize re-absorption implies that higher absorption (by longer optical paths and/or higher concentration of the SNP) may be used. As a result, significant absorption of the blue or UV LED light may be achieved and higher efficiency devices are enabled, exemplified by the spectral antenna characteristic of the SNP conversion layers disclosed herein.
In known QD conversion layers, the formation of clusters of QD material may lead to energy losses via FRET, as described above. Clustering can occur even at low loading, while high loading can occur without dense packing (the latter correlated with extremely high loading). Since QDs are densely-packed in a cluster, the distance between neighbouring QDs is small and energy transfer processes may become significant. These will reduce the emission output and the efficiency of the devices and will also affect the light colour output. SNP clustering in a conversion layer does not lead to energy transfer losses, and therefore losses in efficiency or changes in the light colour output are avoided. Therefore, devices based on SNP will function even if clusters are formed. This enables the use of thinner layers.
Known QD materials for light conversion are embedded in a host material (matrix) in a low-loading ratio to avoid losses by mechanisms such as by FRET. As a result, a QD conversion layer must be thick (typically thicker than 100 μm in most cases), yet still contain sufficient amount of material to achieve effective absorption of blue light for conversion, thereby inherently leading to re-absorption losses. In addition, for thick layers, manufacturing methods become less accurate and more resource consuming In sharp contrast, high-loading ratio SNP layers may be made very thin. For example, thin SNP layers may be produced using spin-coating deposition techniques, see Examples 4 and 5 in which the layers are respectively 510 nm and 230 nm-thick. In general, for SNP/RSNP conversion layers of the invention, absorption and emission can be controlled to provide tailored colour and optical characteristics, power and efficiency. Densely-packed, high-loading thin SNP layers have an additional advantage in that they may be made with excellent uniformity over large length scales, from a few millimetres to centimetres and even more.
High-loading ratio SNP layers may be prepared using a polymer, epoxy or resin matrix, or simply by having a layer of close-packed SNPs. The polymer or additive may serve additional purposes such as for encapsulating the optically active nanoparticles to prevent oxidation or photo-degradation, serve as a medium easy for mechanical integration in the lighting device and as a medium which can also enhance the light extraction from the layer due to its refractive index and surface roughness. A host material (matrix) can also serve as a matrix for diffusive particles such as SiO₂, Al, BaSO₄or TiO₂, which can enhance the scattering within the layer. The loading ratio may be used to control the refractive index of the SNP layer. Layers of low-loading ratio may have a refractive index as low as 1.5 and even lower, while layers with a high-loading ratio may have a refractive index of 1.8 and even higher. Typically, for polymers with a refractive index of 1.3-1.5, the refractive index will not change up to ˜15% loading ratio. Typically, with ligands, the refractive index may be 1.8 and more.
Tables 1-3 summarize various exemplary embodiments of SNP/RSNP conversion layers made according to the invention. Other embodiments of conversion layers having advantageous physical parameters and optical performance are possible and can be made according to the teachings disclosed herein. Therefore, these exemplary embodiments should not be considered as limiting the invention in any way.

TABLE 1

Parameters for Red-emitting RSNP conversion layers

								PL
		SNP		Layer				Red
Conversion	Embedding	length	Emission	Thickness				Shift^d
layer/RSNP type	material	[nm]	[nm]	[μm]	AR^a _red	AR^b _green	OD^c	[nm]

CdSe\CdS	Ligands^e	8-	580-	0.1-2	For RSNP	For RSNP	0.07-	<5
ZnSe\CdS		150	680		length 8-	length 8-	2.0
CdSe\CdS\ZnS					100 nm,	110 nm,
CdSe\CdZnS					AR > 3.5:1	AR > 2.5:1
CdSe\CdZnS\ZnS					For RSNP	For RSNP
					length 60-	length 60-
					150 nm,	100 nm,
					AR > 7:1	AR > 6:1
CdSe\CdS	Polymer^f	8-	580-	1-	For SNP	For RSNP	0.07-	<5
ZnSe\CdS	or	150	680	5000	length 8-	length	2.0
CdSe\CdS\ZnS	Silicon	^g				100 nm,	8-110 nm,
CdSe\CdZnS					AR > 3.5:1	AR > 2.5:1
CdSe\CdZnS\ZnS					For RSNP	For RSNP
					length 60-	length
					150 nm,	60-100 nm.
					AR > 7:1	AR > 6:1

Markings in Table 1:
^aAR_redis the ratio between the absorbance at 455 nm to the maximal absorbance in a wavelength range between 580-700 nm, i.e. AR_red= (Absorbance_{455 nm}/max(Absorbance_{580-700 nm});
^bAR_greenis the ratio between the absorbance at 455 nm to the maximal absorbance in a wavelength range between 520-580 nm, i.e. AR_green= (Absorbance_{455 nm}/max(Absorbance_{520-580 nm});
^cOD is measured at 455 nm;
^dPL Red shift is the difference in nanometers between the CWL measured in Toluene at low OD (<0.1) and the CWL measured for the sample;
^eLigands can be selected from list given in definitions;
^fThe polymer can be selected from list given in definitions;
^gSilicone with suitable optical and mechanical properties can be selected from various commercial suppliers.

TABLE 2

Parameters for Green-emitting RSNP conversion layers

							PL
		SNP		Layer			Red
Conversion	Embedding	length	Emission	Thickness			Shift^c
layer/RSNP type	material	[nm]	[nm]	[μm]	AR^agreen	OD^b	[nm]

CdSe\CdS	Ligands^d	8-	520-580	0.1-5	For RSNP length 8-	0.07-	<5
ZnSe\CdS		150			100 nm,	2.0
CdSe\CdS\ZnS					AR > 3.5:1
CdSe\CdZnS					For RSNP length
CdSe\CdZnS\ZnS					45:150 AR > 7:1
CdSe\CdS	Polymer^e	8-	520-580	1-10	For RSNP length 8-	0.05-	<5
ZnSe\CdS	or	150			100 nm,	2.0
CdSe\CdS\ZnS	Silicone^f				AR > 3.5:1
CdSe\CdZnS					For RSNP length
CdSe\CdZnS\ZnS					45:150, AR > 7:1

Markings in Table 2:
^aAR_greenis the ratio between the absorbance at 455 nm to the maximal absorbance in a wavelength range between 520-580 nm, i.e. AR_green= (Absorbance_{405 nm}/max(Absorbance_{520-580 nm});
^bOD is measured at 405 nm;
^cPL Red shift is the difference in nanometers between the CWL measured in Toluene at low OD (<0.1) and the CWL measured for the sample;
^dLigands can be selected from list given in definitions;
^eThe polymer can be selected from list given in definitions;
^fSilicone with suitable optical and mechanical properties can be selected from various commercial suppliers.

TABLE 3

Parameters for Green- and Red-emitting SNP conversion layers

			Layer				PL Red
Conversion layer/SNP	Embedding	Emission	Thickness				Shift^d
type	material	[nm]	[μm]	AR^a _green	AR^b _red	OD^c	[nm]

CdSe\CdS	Ligands^e	580-	0.1-5		AR >	0.07-	<5
ZnSe\CdS		680			3:1	2.0
CdSe\CdS\ZnS
CdSe\CdZnS
CdSe\CdZnS\ZnS
CdSe\CdS	Polymer^f	580-	1-10		AR >	0.05-	<5
ZnSe\CdS	or	680			3:1	2.0
CdSe\CdS\ZnS	Silicone^g
CdSe\CdZnS
CdSe\CdZnS\ZnS
CdSe\CdS	Ligands^e	520-	0.1-5	AR >		0.07-	<5
ZnSe\CdS		580		3:1		2.0
CdSe\CdS\ZnS
CdSe\CdZnS
CdSe\CdZnS\ZnS
CdSe\CdS	Polymer^f	520-	1-10	AR >		0.05-	<5
ZnSe\CdS	or	580		3:1		2.0
CdSe\CdS\ZnS	Silicone^g
CdSe\CdZnS
CdSe\CdZnS\ZnS

Markings in Table 3:
a) AR_greenis the ratio between the absorbance at 405 nm to the maximal absorbance in a wavelength range between 520-580 nm, i.e. AR_green= (Absorbance_{405 nm}/max(Absorbance_{520-580 nm});
b) AR_redis the ratio between the absorbance at 455 nm to the maximal absorbance in a wavelength range between 580-680 nm, i.e. AR_red= (Absorbance_{455 nm}/max(Absorbance_{580-680 nm});
c) OD is measured at 455 nm for nanoparticles emitting at 580-680 nm and at 405 nm for nanoparticles emitting at 520-580 nm;
d) PL Red shift is the difference in nanometers between the CWL measured in Toluene at low OD (<0.1) and the CWL measured for the sample;
e) Ligands can be selected from list given in definitions;
f) The polymer can be selected from list given in definitions;
g) Silicone with suitable optical and mechanical properties can be selected from various commercial suppliers.

FIG. 5A shows schematically a lighting device 500 a which includes a SNP layer 502 a according to one embodiment of the invention. Light produced by a suitable source 504 a (exemplarily a LED emitting UV light) is directed at SNP layer 502 a. Layer 502 a comprises SNPs that convert the light from blue and/or UV to longer wavelengths. Different populations (types) of SNPs (having different cores or shell sizes or materials) will emit different colours. The colours emitted by the SNP layer may be combined with the light produced by source 504 a or used independently to form different light combinations. In order to improve and tune the spectral properties of the emission, more than one type of SNPs can be used, e.g. mixtures of blue, green and red emitting SNPs (their light exemplarily marked RGB). The various colours may be chosen so as to provide white light. Other colour combinations, as desired for a specific lighting application, can be generated by tailoring the SNP conversion layer.
FIG. 5B shows schematically a lighting device 500 b which includes a SNP layer 502 a according to another embodiment of the invention. In this embodiment, light produced by a suitable source 504 b (exemplarily a LED emitting blue light) remains partially un-converted (i.e. passes through unaffected) by a SNP layer 502 b. Layer 502 b incorporates SNPs that convert the light from blue and shorter wavelengths to green and red. Layer 502 b further incorporates diffusive structures or particles that spread and mix the unabsorbed light in a tailored pattern, to conform with the spatial and optical characteristics of the photoluminescence of the SNPs incorporated therein. That is, these structures scatter both the incoming blue light and the SNP-emitted light such that the combined light has the same angular diversion when it exits the SNP layer as a “white” light. In addition, the white has high quality green and red light added to the LED blue light to provide a large colour gamut for a display backlight.
FIG. 6A shows schematically a lighting device 600 a which includes a SNP layer 602 a according to yet another embodiment of the invention. In this embodiment, a “colour mixture light” source 604 a is improved or corrected by SNP layer 602 a. Layer 602 a includes a plurality of SNP species which may have different cores or shell sizes, different materials and/or different spectral properties. The SNPs act to convert the colour mixture light source into an improved colour mixture light. In an embodiment, the improved colour mixture light output from the lighting device can be “white light” with a CCT in the range of 2500-6000K with high CRI. In another embodiment, the improved colour mixture light can be white light with a CCT in the range of 2700-4500K range with high CRI. The source light may be white light with high CCT (for example 5000-10000K). Alternatively, it may be a light combination which cannot be defined as white light but which includes light in the range of the visible spectrum. The improvement includes for example addition of red light to the emission, thereby providing a lower CCT and better CRI.
FIG. 6B shows schematically a light conversion device 600 b which includes a SNP layer 602 b according to yet another embodiment of the invention. In this embodiment, layer 602 b includes, in addition to a plurality of SNP species such as in layer 602 a, diffusive structures or particles that spread and mix the un-absorbed light from source 604 b in a tailored pattern to produce a further improved colour mixture light.
In alternative embodiments, the lighting device can include several SNP layers, each providing a separate function, may be used instead of a single SNP layer. Scattering and controlling the transmission characteristics (e.g. homogenization) of transmitted and emitted light may be achieved by incorporating in one or more of the SNP conversion layers either refractive particles such as small SiO₂beads or reflective particles such as metal particles or light diffusing particles such as BaSO₄and TiO₂by adding a patterning (e.g. diffusive) layer, or by patterning the surface of at least one of the layers.
FIG. 7 shows schematically a lighting device 700 which includes a SNP conversion layer according to an embodiment of the invention. Device 700 includes a blue or UV LED light source, an optional spacer layer (or air as spacer) 704, a SNP conversion layer 706, an optional encapsulation layer 708, an optional transmissive optical element 710 for light extraction to desired directionality, an optional refractive element such as a lens 712 to collimate or focus the light, and an optional reflective element 714 placed behind and around the LED element to collect and direct emission from large angles to the correct output direction. In some embodiments, the high refractive index of a SNP layer with a high-loading ratio is preferred in order to increase the light extraction from the LED chip.
FIG. 8 shows schematically a lighting device 800 which includes a SNP layer according to another embodiment of the invention. In device 800, an optical filter 806 is between a SNP layer 802 and a LED emitting chip 804. Optical filter 806 is a filter which transmits short wavelength 820 (e.g. blue or UV) light and reflects longer wavelength (e.g. green or red) light 822, thereby enabling light recycling and a more efficient device. While the light recycling increases the optical path of the emitted light in the SNP layer, due to the low self-absorbance, any extra loss would be minimized In contrast, with a QD layer, the extra loss will be significant. Optical elements between the light source and the SNP layer may also be used to shape or otherwise control the light source characteristics. Like device 700, device 800 further comprises an optional transmissive optical element 810 for light extraction to desired directionality, an optional refractive element such as a lens 812 to collimate or focus the light, and an optional reflective element 814 placed behind and around the LED element to collect and direct emission from large angles to the correct output direction. Placing the SNP layer at a distance from the LED element can diminish the light intensity at the SNP layer and the temperature level thereby increasing its durability.
FIG. 9 shows schematically a lighting device 900 which includes a SNP layer according to yet another embodiment of the invention. Device 900 includes a SNP layer shaped to fit in a curved optical element 902, serving to colour convert and also to diffuse the light, a LED 904 and additional layers 906 used for example for spatial patterning or optical filtering (e.g. additional UV filtering). SNP layers can be thin yet efficient, which represents a significant advantage over the performances of thick QD conversion layers.
FIG. 10A shows a LED 1002 a coupled to a waveguide assembly 1004 a that has an SNP layer 1006 a embedded therewithin. The waveguide includes a reflecting layer at a bottom 1008 a (which may be diffusive or reflective, patterned or homogenous) and an optional light extraction layer 1010 a. FIG. 10B shows schematically a LED 1002 b coupled to a waveguide assembly 1004 b that has an SNP layer 1006 b positioned on a top surface 1008 thereof and an optional light extraction layer 1010 b. In both embodiments, the SNP layer is shown excited by light coming from emitted by the LED through an edge 1012 a or 1012 b of the waveguide. As light propagates in the waveguide, it passes through the SNP layer again and again. Light converted in the SNP layer is then transmitted across the waveguide over a relatively great distance, which can be in the millimetres to centimetres to tens of centimetres range. In this application, the low self-absorbance of the SNP layer may be critical, since the light travels over a long optical path. The reflective and/or diffusive optical elements (1008 a, 1008 b 1010 a, 1010 b and 1012 b) may be placed at all areas of the device where the light can be emitted not in the needed direction. These elements will return the light into the waveguide and increase its efficiency

EXAMPLES

Example 1

Lighting Device with RSNP Conversion Layer Within Polymer Host Providing Red Light

RSNPs were synthesized following similar procedures to those described in L. Carbone et al. “Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach” Nano Letters, 2007, 7 (10), pp 2942-2950. In a first step, CdSe cores with diameter of 3.8 nm were synthesized. In a second step, red emitting CdSe/CdS RSNPs were synthesized using the CdSe cores as seeds. The resulting RSNPs had dimensions of 33×7 nm with an emission maximum is at 635 nm with FWHM of 30 nm when measured in a Toluene solution.
A RSNP conversion layer was prepared as follows: 0.5 g of Poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVB), a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility and commercially available from Sigma-Aldrich (3 Plaut St., Rabin Park, Rehovot 76100, Israel) were dissolved in 4 ml Toluene. 12 mg of RSNPs were dissolved in 1 ml Toluene to form a RSNP solution. The RSNP solution was added to the polymer mixture while stirring. The mixture was transferred to a pattern vessel which was inserted into a dessicator and vacuumed for 15 hours, after which the mixture became solid. The resulting film thickness was 190 μm. The optical characteristics of the conversion layer are presented in FIG. 11, which shows the absorption (dotted line) and the PL (full line) spectra. The emission maximum is at 635 nm, with a FWHM of 30 nm. The absorption OD is 1.18 at 455 nm, 0.07 at 540 nm and <0.046 at 600-750 nm, i.e. 25 times smaller than the OD at 455 nm. This RSNP layer therefore funnels light from blue to red emission.
The RSNP layer was incorporated in a lighting device similar to that of FIG. 5A. In the lighting device, a UV LED at 360 nm was used to illuminate the RSNP layer, providing light output in the red, at 635 nm. Negligible UV output was detected, as the UV light was absorbed and converted very effectively by the RSNP layer.

Example 2

Lighting Device with Diffusive RSNP Conversion Layer Within Polymer Host Providing Combination of Blue and Red Light

A diffusive RSNP layer was prepared using the procedure in Example 1, with a modification that 1.3 mg of RSNP was dissolved in 1 ml Toluene and that after the 10 minutes stirring of RSNPs in polymer, 5 mg of BaSO₄particles were added to the solution and stirred for another 15 minutes. The resulting film had diffusive properties that enhanced the optical emission and increased the extraction of the light in a required direction.
The RSNP layer was incorporated in a lighting device as shown in FIG. 5B. A blue LED emitting at 455 nm was used to illuminate the RSNP layer. The lighting output was measured and the light spectrum is presented in FIG. 12A, which shows a combination of a blue remnant from the blue LED and a red component from the RSNP conversion layer.

Example 3

Lighting Device with RSNP Conversion Layer Within Silicone RTV Providing Combination of Blue and Red Light

A RSNP layer in Silicone RTV was prepared as follows: 1 g of RTV615A (Momentive, 22 Corporate Woods Boulevard, Albany, N.Y. 12211 USA) was stirred with 0.1 g of RTV615B for 10 min. 1 5 mg of CdSe/CdS RSNPs with overall dimensions of 27×5.5 nm emitting at 635 nm was dissolved in 250 μl Toluene. The RSNP solution was added to the silicone mixture while stirring, then vacuumed until no bubbles remained. The solution was then deposited on a glass substrate and sandwiched using another glass substrate. 600 μm-thick spacers were positioned between the two glass substrates to obtain the desired film thickness. The sandwiched structure was then placed on a hot plate at 150° C. for 15 minutes, after which the solution became solid. The measured film thickness was 600 μm.
The RSNP layer was incorporated in a lighting device as shown in FIG. 5B. A blue LED emitting at 455 nm was used to illuminate the RSNP layer. The lighting output was measured and the light spectrum is presented in FIG. 12B, which shows a combination of a blue remnant from the blue LED at 455 nm and a red component from the RSNP layer at 635 nm with a FWHM of 30 nm.

Example 4

Lighting Device with Thin Dense Spin-Coated RSNP Conversion Layer Providing Red Light

A dense RSNP layer was prepared as follows: first prepared was a solution of 35×5.4 nm CdSe/CdS RSNPs, emitting at 635 nm, in Toluene with 1:4 weight/volume (mg/μL) ratio. 20 μL of the solution was drop cast on a soda lime glass substrate and spread by spin coating at 2000 rpm. The deposited film was measured to have an absorbance OD of 0.51 at 455 nm, and OD of 0.9 at 360 nm. The thickness was 0.510 um as measured by a profilometer. FIG. 13A shows the absorption (dotted line) and the PL (full line) spectra of this RSNP layer. The emission maximum is at 633 nm, with a FWHM of 33 nm. The absorption OD is 0.96 at 360 nm, 0.5 at 455 nm, 0.035 at 540 nm and 0.025 at 600-750 nm, the latter 20 times smaller than the OD at 455 nm.
The RSNP layer was incorporated in a lighting device as shown in FIG. 5A A UV LED at 360 nm was used to illuminate the RSNP layer, and provided light output in the red at 633 nm (not shown).

Example 5

Lighting Device with Thin Dense Spin-Coated RSNP Conversion Layer Providing Green Light

A dense RSNP layer was prepared as follows: A solution of green emitting 20×3.5 nm CdSe/CdS RSNPs in Toluene with 1:5 weight/volume (mg/μL) ratio was prepared. 20 μL of solution containing the RSNPs was drop cast on a soda lime glass substrate and spread by spin coating at 2000 rpm. The deposited film was measured to have an absorbance OD of 0.07 at 455 nm and a thickness of 230 nm as measured by a profilometer. FIG. 13B shows the absorption (dotted line) and the PL (full line) spectra of this RSNP layer. The emission maximum is at 540, with a FWHM of 33 nm. The absorption OD is 0.165 at 360 nm, and 0.008 at 540 nm, the latter 20 times smaller than the OD at 360 nm.
The RSNP layer was incorporated in a lighting device as shown in FIG. 5A. A UV LED at 360 nm was used to illuminate this RSNP layer, and provided light output in the red, at 540 nm (not shown).

Example 6

Lighting Devices with RSNP Conversion Layers Providing White Light

Two RSNP layer samples were prepared using the methods described above for PVB with scatterers (example 2) RSNP layer CL14A had 10 mg of red emitting RSNPs and 25 mg of BaSO₄, inserted into 0.5 g of PVB. RSNP layer CL14B had 20 mg of red emitting RSNP and 25 mg of BaSO₄, inserted into 0.5 g of PVB. Each of the two samples was 190 μm-thick and had a diameter of 42 mm.
The RSNP layers were incorporated in two lighting devices as shown in FIG. 6B. In both lighting devices, the RSNP layers were placed at the aperture of a LED module composed of a blue LED source and a specially prepared Ce:YAG based phosphor layer in a silicone matrix. The light output was measured and is shown in FIGS. 14A and B, for a lighting device with layers CL14A and CL14B, respectively. The light seen is composed of contributions of blue light from the 455 nm LED, a broad peak around 580 nm for the Ce-YAG based phosphor and red light from the RSNP layer. The CIE 1931 coordinates were calculated and the location of the two lighting devices on the CIE Chromaticity Diagram is shown in FIG. 15. The CCT for CL14A is 3420 K and for CL14B is 2730 K while the CRI for CL14A is 95 and for CL14B is 92.

Example 7

Lighting Devices with SNP Conversion Layers Providing White Light

The SNP used was a non-rod shaped CdSe\CdZnS SNP with CdSe core diameter of 3.9 nm and overall diameter of 8.9 nm. 0.5 gr of PVB were dissolved in 4 ml Toluene. 2 mgr of SNP was dissolved in 1 ml Toluene. The SNP solution was added to the polymer mixture while stirring. After 10 min stirring, the mixture had a shining glow. The mixture was then transferred to a pattern vessel which was inserted in a dessiccator and vacuumed for 15 hours, after which the mixture became solid. The final film thickness was 190 um. FIG. 16 shows the absorption (dotted line) and the PL spectra of this film. The CWL is at 626 nm and the FWHM is 33 nm. The absorption ratio between absorption at 455 to maximum absorption in 600-700 nm range is 1:6 (0.156 to 0.026). The SNP layer was incorporated in a lighting device as shown in FIG. 5A. A UV LED at 360 nm was used to illuminate this RSNP layer, and provided light output in the red, at 626 nm (not shown).
In conclusion, various embodiments of the invention provide devices incorporating novel conversion layers based on SNPs. Conversion layers disclosed herein are characterized by low re-absorption in the emission region compared with the absorbance in the exciting wavelength. SNP/RSNP conversion layers disclosed herein are suitable for enhancing the properties of LED devices to provide white emission with a CCT<4000K with a high CRI>80 and even >85, in particular a CCT<3500 and even a CCT˜2700K with CRI>89. Polymer embedded SNP conversion layers of the invention can be further prepared to provide a white colour for display applications composed of three or more primary colours with a narrow FWHM<60 nm and even a FWHM<40 nm.
The invention has been described with reference to embodiments thereof that are provided by way of example and are not intended to limit its scope. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the described invention and embodiments of the invention comprising different combinations of features than those noted in the described embodiments will occur to persons of ordinary skill in the art. The scope of the invention is limited only by the following claims.
All patents, patent applications and publications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent, patent application or publication was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

1. A light conversion layer configured for converting light of a first spectral range to light of at least one second wavelength longer than wavelengths of said first wavelength range; the conversion layer comprising a plurality of semiconductor seeded nanoparticles (SNPs) embedded in a matrix, said SNPs having material composition, geometry and shape selected to convert light of said first spectral range into light of said at least one second wavelength, said conversion layer comprising predetermined refractive index and surface roughness selected to enhance extraction of light of said second wavelength emitted.

2. The conversion layer of claim 1, wherein said SNP comprise rod shapes SNPs (RSNPs) having diameter below 10 nm and a length between 6 nm and 500 nm.

3. The conversion layer of claim 1, wherein said matrix comprises polymer host matrix.

4. The conversion layer of claim 1, wherein said conversion layer further comprises diffusive particles enhancing light scattering within the conversion layer.

5. The conversion layer of claim 1, being configured and operable as a spectral antenna to convert said exciting light of the first spectral range into light of the second wavelengths having balance between different spectral regions such that said light of the second wavelengths is characterized by desired correlated colour temperature (CCT) and color rendering index (CRI).

6. The conversion layer of claim 1, wherein said SNPs comprising SNPs emitting in three or more primary colours with emission bandwidth having full width half max (FWHM) of below 60 nm.

7. The conversion layer of claim 1, wherein said conversion layer comprises one or more sub-layers comprising respectively SNPs different in at least one of material composition, geometry and shape property.

8. The conversion layer of claim 1, wherein said SNPs comprise a core material selected from at least one of the following: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂, Cu₂(ZnSn)S₄and Cu₂(InGa)S₄.

9. The conversion layer of claim 1, wherein said SNPs comprise a shell material selected from at least one of the following: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, GaN, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, Cu₂S, Cu₂Se, CuInS₂, CuInSe₂, Cu₂(ZnSn)S₄and Cu₂(InGa)S₄.

10. The conversion layer of claim 1, wherein said conversion layer is associated with at least one optical filter located in optical path between the light source and the at least one conversion layer; said at least one optical filter being configured for transmitting exciting light of the first spectral range and reflecting emitted light of the at least one second wavelength range.

11. The conversion layer of claim 1, wherein said conversion layer is associated with at least one transmissive optical element configured for extracting emitted light from said conversion layer towards a desired direction.

12. The conversion layer of claim 1, wherein said first wavelength range comprises at least one of UV and blue wavelength range.

13. The conversion layer of claim 1, configured for use in backlight unit for display device.

14. The conversion layer of claim 1, wherein loading ratio of SNPs embedded in said matrix is greater than 40%.

15. The conversion layer of claim 1, configured for converting input colour mixture light having correlated colour temperature (CCT) in the range of 5000-10000K to provide emitted light having CCT in the range of 2500-6000K.