GB2516929A - Light Emitting Device - Google Patents

Abstract

A composition comprising: a host material; a quantum dot material; and a first phosphorescent material, wherein the lowest triplet energy state of said host material is at a higher energy than a lowest triplet excited state of said phosphorescent material wherein the emission energy level of said quantum dot is at a higher energy that the lowest triplet excited state of said phosphorescent material. Also disclosed is a light emitting device comprising the above composition. The composition may comprise one or more further quantum dot materials and/or one or more further phosphorescent materials. The composition may comprise a second phosphorescent material which has a lower emissive energy state than that of the first phosphorescent material, wherein the first material may be a green light emitting material and the second material may be a red light emitting material.

Description

LIGHT EMFI'IING DEVICE HELD OF TIlE INVENTION [his invention generally relates to compositions, a light emitting device and a method of generating white light, and more particularly to an organic light emitting device (OLED).

BACKGROUND TO THE INVENTION

Displays fabricated using organic light emithng diodes (OLEDs) provide a number of advantages over other fiat panel technologies. I'hey are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates.

Organic light. emitting devices (OLEDs) provide lighE emission by electroluminescence. Two types of 01.1 l) are polymer (P-UI El)) and smafl molecule (SM-OLED). Both types emit light by recombination in a light-emission layer of eleelrons injected from a cathode and holes injected from an anode. Further layers may however be incorporated into an OLED, for example to enhance or block electron or hole injection into the light cmission region or other regions of the device as appropriate.

P-OLEDs arc, in general, solution-processable. Although some SM-OLEDs are fabricated by solution processing most are fabricated by vacuum evaporation methods. In contrast, the eniissive layer ol a solution-processable P-0l i-il) may he deposited without a vacuum, e.g., by printing or spin-coating.

OLEDs are emissive devices (rather than transmissive devices such as liquid crystal devices), and thus may not require additional elements such as hacklights and filters, and may he particularly suited to thin andlor flexible displays, possibly at low operating voltages. They may also he particularly suited to lighting applications. In addition to the red, green and blue OLEDs available for use in display and lighting applications, white light-emitting devices are now desirable for general lighting applications. However, it can he relatively difficult to produce a white light-emitting device, for example one having optimum characteristics (spectrum, efficiency CR1 dc). Thus, there is a need for an improved system or solution for white light emission.

To help in understanding the invention, we refer to the following documents: -Organic Light-Emitting Materials and Devices, , edited by Zhigang Li and hong Meng, CRC Press, Taylor and Francis, ISBN I -57444-574-X (2007); -I'riplet Harvesting in Hybrid White Organic Light-emitting Diodes, (3. Schwartz et al, Advanced Functional Materials 2009, 19, pp. 1 -15; -IJS 6,777,706 BI, Tcsslcr ct al., August 17, 2004; -Electroluminescence from CdSe quanturn-dotlpolymer composites, Dabbousi B.O. et aL, Appl. Phys. Lett. 66, March 13, 1995. p.1316; -High quality CdSeS nanocrystals synthesiLed by facile single injection process and their electrolumi nescence, Jang IL ct al., Chcni. Commun., 2003, pp.2964 -2965; -Contmlling the Optical Properties of Inorganic Nanoparticles, Scholes (i., Advanced Functional Materials 2008, 18, pp. 1157-1172; and -Korean patent KR2009064079, application no. KR2007-131643, 14.12.2007, Lee, Seong Hun, Seoul National University, Industry-Academy Cooperation Foundation, S. Korea; Samsung Electronics Co., Ltd..

SUMMARY

Accoiing to a first aspect of the present invention, there is provided a composition comprising: a host material; a quantum dot material; and a first phosphorescent material, wherein the emission energy level of said quantum dot is at a higher energy that the lowest tnpleL exciLed slate of said phosphorescent material.

In operation in a light-emitting device containing the composition. light may he eIiiitted from the QD's emission energy level and from the lowest triplet excited state(s) of the phosphorescent material(s). [he QD's emission energy level is its lowest singlet excited state. The QD's lowest singlet and triplet excited states may he degenerate at room temperature, and triplets formed on (he QD' s lowest excited triplet state may access the emission energy level.

Advantageously, in such a composition, excitons generated in the host material may transfer to both the QD(s) and the phosphorescent material(s), whereas exciton transfer generally may not occur from either the QD or (he phosphorescent material to the host Because the QD emissive state is at a higher energy than the phosphorescent material(s)' lowest triplet (1') excited states, triplet exciton transfer from QD(s) to phosphorescent. material(s) may occur. Coniplete energy transfer may however be less desirable as some dot ennssion is advantageous. Moreover, and particularly where an optimum concentration of QDs is provided in the composition, which may he a blend of the host, QI) and phosphorescent material(s), light emission from each of the QD and phosphorescent niaterial(s) present in the composition may be achieved. Thus. luminescence of the or each QD and the or each phosphorescent material, advantageously in sonic embodiments without direct emission from the host, niay occur.

The host material may be a material that is inherently capable of electroluminescing in the absence of QDs or phosphorescent materials. In one preferred enibodinient all light emission is from the QD(s) and phosphorescent material(s). This may be of advantage or example for generation of white light where the QD and two phosphorescent. materials provide red, green and blue luminescence, as further described below. In general, while sonic host emission may be advantageous depending on the application, generally it is preferred to havc substantially no host emission. In one embodiment singlet and triplet excitons are transferred from the host to the QD(s) and the phosphorescent niatcrial(s). In another embodiment, the QDs may he directly excited, i.e. without energy transfer from the host. Energy niay also he transferred from the QI)(s) to the phosphorescent material(s).

Guest-host systems are advantageous to provide a matrix containing the emitting units, so enabling easy control over incorporalion level of each guest, e.g. to provide Ihe charge transport, charge balance etc for the OLED, and to keep the guests far enough apart to avoid concentration quenching'.

Compositions of the invention may he deposited from a formulation of the composition in one or more solvents, followed by evaporation of the one or more solvents lo fonn a layer of die composition. One oL sonic of or all of the host material and the phosphorescent material(s) may be dissolved in the formulation. One of, some of or all of the host material, the QD(s) and the phosphorescent material(s) may form a suspension in (he one or more solvents, for example a eolloid. Preferably, the host and phosphorescent material(s) are dissolved in the formulation, and the one or more QI)s form a suspension in the one or more solvents. The formulations may be depositcd using solution processing methods including printing and coaling methods.

To illustrate, Fig. 1 shows an embodiment having a QD blue eniittcr and phosphorescent red and green emitters. Specifically, and as in any embodiment described herein, Fig. I shows that the host may have a singlet state SI, which is higher than the lowest triplet energy slate Ti thereof (and the higher lying triplet stales In can lie between TI and Si and above SI. or can lie wholly above Si).

Furthermore, such a host may have a singlet energy higher than a singlet energy of the QD, so that the host may act as a singlet source for the QD. Thus, the host may act as a source of singlets for the QD such that singlets may transfer to the QD and the QD may then emit a photon.

In an embodiment, host triplet energy is the same as or higher than QD singlet energy, and therefore higher than QD triplet energy. Therefore host triplets also transfer. QD lowest singlet and triplet, excited stale energy levels are closely spaced so triplet.s can access the singlet level thennally, and emit. Regarding the host energy relative to all guests (QD(s) and phosphorescent material(s)), levels are advantageously as shown in Pig. I. SI and II of the host may then he of higher energy than all guest emissive states. Si of the host may then transfer to all guests. Tl of the host may then transfer to all the guests. Singlets and triplets from the QD may then transfer to the phosphorescent materials. hxcitons on the green emitter may then transfer to the red emitter.

In another embodiment.. the lowest triplet. excited st.ate level of the host. may have a lower energy than the QD lowest triplet excited slate. The present inventors have surprisingly found that emission from a QD may occur even if the QD emission energy level is higher than that of the host materiaL Without wishing to he bound by any theory, it is believed that there is hunted or no transfer of triplet exeitons from Ihe QD emission energy level to such a host material.

An excited state energy level of a material may he determined from its luminescence spectrum. For example, S1 and T1 levels S and T1 levels of a material may be measured from the material's fluorescence and phosphorescence spectra respectively.

Considering particular constituents of the coniposition defined above, the host material may he a polymer or non-polymeric material, e.g, a small molecule organic material such as a vacuum-or solution-depositahle material. For example, where the QD5 arc provided in solution-processible (e.g., colloidal) form, any solution-processible material may he used as the host, this being particularly advantageous for fabrication of P-OLFDs. For example, for the combination of a blue QD and green and red phosphorescent materials, a polymer, may he used as a host. Host polymers may he hole-and I or electron-transporting polymers. Host polymers niay have a non-conjugated backbone with charge transporting groups pendant from the backbone, for example PVK, or may have a conjugated backbone in which adjacent repeat units of the polymer are conjugated to each other in the polymer backbone, such thai sonic or all repeal units of the polynier are a conjugating repeat unit dat provides a conjugation path between repeat units adjacent to the conjugating repeat unit. Polymeric hosts include solution-processihle polymers, preferably soluble polymers, that would electroluminescence in a light-emitting layer of an OLE[) in Ihe absence of any guest materials. Siniilarly, any evaporable material niay be used where the QDs are evaporable, this being applicable to SM-Ol NDs.

The at least one QD may be a semiconductor or conductor whose exeitons are confined in all three spatial dimensions. [he or each QI) may have dimensions below mn and preferably about mm to about S0nni, more preferably about 2nm t.o about I Onm. Particularly advantageously, the QI)s form a colloid in a formulation of the composition and one or more solvents. Exemplary solvents include organic solvent and waler. One or more of the QDs may be provided with one or more surface modification substituents at a surface of the QDs to improve dispersion of the QD(s) in thc onc or morc solvents or to improve dispersion of thc QI)s within the host. A specific example of suitable QDs may be colloidal core-shell QDs, wherein the eniissive core, e.g., CdSe, is coated with a higher bandgap material such as ZnS, and to this shell there are attached one or more surface modification groups.

In an embodiment, excitons form on the phosphorescent material by one or both of two mechanisms: exciton transfer from a higher energy host/guest (e.g., excitons on the green guest from either host or blue QD), or excitons form directly on the phosphorescent material if the phosphorescent material acts as either a hole transporter/trap or an electron transporter/trap. Once a hole or electron is on the phosphorescent material, it may then capture an opposite charge and an exeiton may form. The at least one phosphorescent material of an embodiment may comprise two or more phosphorescent materials enutting light of different colours. Phosphorescent dopants used in an embodiment may include metal complexes of organic materials of Ir, Pt, for example. Thus, red and green light. emitting phosphorescent materials may be used. Light-emitting spectra for a given metal, for example iridium complexes, may he different depending on ligands of the organic metal complexes. Suitable examples are b(ppy)3 (tris (2-phenylpynidine) iridium), which is a phosphorescent dopant for green light emission, and Ir(piq)3 (ths( I -phenylisoquinoline)iridium(III)), which is a phosphorescent metal complex for red light emission. Since a blue phosphorescent miaterial miay he more difficult to achieve, an embodiment advantageously uses blue QD(s).

The aforesaid lowest triplet of the host is an excited energy state Ti. Moreover, any reference to an energy "state" or to a state' in this application may be to an energy "level" which may in turn be a reference to an average, e.g. median energy level of a distribution of energy levels.

Furthennore. and as described in the detailed description below, references to triplet and singlet energy states throughout this specification may not he to exclusively triplet or singlet states. For example, any fluorescent singlet state may have some triplet character so that the emissive state may he of mixed singletItriplet character, bul be predominanLly or mainly singlet in characier.

In view of (he above! there may further he provided the composition wherein the emissive energy slates of any one or more of the phosphorescent material(s) is an excited state having triplet character, or at least the enlissive energy states are not pure singlets, i.e., of pure singlet character. The emissive states of phosphorescent materials are complex as they may have niixed singlet and triplet character. The eniissive state may have some charge transfer character. Thus, the phosphorescent material emissive energy state may be exclusively a triplet state or, alternatively, may he a mixed singleiltriplet slat.e having, for example, predoniinanily triplet character.

(To explain this further, a material will generally have a ground state that is a singlet state S0. Above the ground state lie excited states, which may he singlets and triplets.

Singlcts emit (fluoresce). In a phosphorescent material for example, singlel and Iriplet states are generally mixed, and spin is not a good quantum number. Consequently, the triplet state, which lies below the singlet state, may gain some singlet character and become emissive.) [he QI) may have only one enlissive state, and may further have lower lying non-enhissive slates. Singlel and triplet energy levels in the QD may have a very small energy separation, e.g., comparable with kT. Because of this small separation, and taken together with the long triplet lifetime, the triplet exeitons of the QI) may he excited from the triplet level up to the singlet level from where they can emit (fluoresce). Further, since Ihe triplet may in realily also enul (phosphoresce), the negligible energy difference between the singlet and triplet may mean that the actual emission from QI) is an indistinguishable combination of fluorescence and phosphorescence. Thus, while the enhissive slate of the QD may be an excited stale having singlet character, in view of the above, it may further have some triplet character. In principle, dots may also emit from defect states, surface states and from Shell slates andlor Core states.

[he composition comprises at least one phosphorescent material and may comprise a firsi phosphorescent material and a second phosphorescent maicrial, wherein said first phosphorescent material has a higher emissive energy state than an emissive energy state of said second phosphorescent material. Moreover, an embodiment may comprise one or more additional phosphorescent materials each having a successively lower enussive energy state than the one before, e.g., a third phosphoisceni inalerial having lower enussive energy stale Ihan the second phosphorescent material. Where the first phosphorescent material is a green light-emitting phosphorescent material and the second is a red light-emitting phosphorescent material, the emissive state of the green phosphorescenl malerial may then lie above the ernissive slate of the red phosphorescent material. This is of advantage where the QD(s) emits blue light and, for example, white light emission may thus he obtained by combining blue froni the QD(s) and red and green from phosphorescent materials.

Consistent with (he above, there may he provided the above coniposiuon. wherein the host material is a polymer. The choice of host material may depend on the colour of QI) and phosphorescent material guests which it hosts, and may further depend on the desired overall colour of enussion. This may be advantageous for white light generation and may be provided by using, e.g., PVK as the host polymer. More generally, a polymer or a solution-processable small molecule material may each be suitable to host solution processable QDs.

Exemplary polymeric hosts include homopolymei and co-polymers containing two or more dilTerent repeal unit.s wherein the polymers contain arylene and I or heteroarylene repeat units, each of which may be unsubstituted or substituted with one or more substituents. I ixemplary arylene repeat units include phenylene, fluorene, indenofluorene and phenanthrene repeat units.

One exemplary class of arylene co-repeat units is optionally substituted fluorene group comprising repeat units, such as repeat units of formula IX: R9 R9 (TX) wherein R9 in each occurrence is the same or different and is H or a substituent, and wherein the two groups may be linked to form a ring.

Each R9 is preferably a substituent, and each R9 may independently he selected from the group consisting of: -optionally substituted alkyl. optionally C120 alkyl, wherein one or more non-adjacenL C atoms may be replaced with optionally substituted aryl or heteroaryl. 0, S. substituted N, C=O or -COO-; -optionally substituted aryl or heteroaryl; -a linear or branched chain of aryl or heteroaryl, each of which groups may independently be substhut.ed, for example a group of formula -(A?). as descrihed ahove with relerence to krrnula (VI); and -a eross-linkable group, for example a group comprising a double bond such as a vinyl or acrylatc group, or a benzocyclobutane group.

In the case where R9 comprises aryl or hcteroaryl ring system, or a linear or branched chain of aryl or heteroaryl ring systems, the or each aryl or het.eroaryl ring system niay be substituted with one or more suhstituents R3 sdected from the group consisting ol: -alkyl, for example C 1-20 alkyl, wherein one or morc non-adjacent C atoms may he replaced with 0, 5, substituted N, C=O and -COO-and one or more H atoms of the alkyl group may he replaced with F or aryl or heteroaryl optionally substituted with one or more groups R4, -aryl or heteroaryl optionally substituted with one or niore groups -NR'2, OR', 5k', and -fluorine, nitro and cyano; wherein each R4 is independently alkyl, for example C120 alkyl, in which one or more non-adjacent C atoms may he replaced with 0, 5, substituted N, C=O and -COO-and one or more II atoms of (he alkyl group may he replaced with F, and each R5 is independently selected from the group consisting of alkyl and aryl or heteroary optionally substituted with one or more tilkyl groups.

Optional substituents for one or more of the aromatic carbon atoms of the fluorene unit are preferably selected from the group consisting of alkyl, for example C -20 alkyl, wherein one or more non-adjacent C atoms may be replaced with 0, S. NIT or substituted N, C=Q and -COO-, optionally substituted aryl, optionally substituted heteroaryl, alkoxy, alkylthio. fluorine, cyano and aiylalkyl. Particularly preferred substituents include C120 alkyl and substituted or unsubstituted aryl, for example phenyl. Optional substituents for the aryl include one or more Ci20alkyl groups.

Where present, substituted N may independently in each occurrence be NR6 wherein R6 is alkyl, optionally C1-20 alkyl, or optionally substituted aryl or heteroaryl.

Optional substituents for aryl or heteroaryl R6 may he Selected from R5.

Preferably, each R9 is selected from the group consisting of C120 alkyl and optionally substituted phenyl. Optional substituents for phenyl include one or more C1-20 alkyl groups.

The rcpcat unit of formula (IX) may he a 2,7-linked repeat unit of formula (IXa): R9 R9 (IXa) In one embodiment, the repeat unit oF Formula (IXa) is not substituted in a position adjacent to the 2-or 7-positions.

The extent of conjugation of repeat units of formulae (IX) may he limited by (a) linking the repeat unit through the 3-and I or 6-positions to limit the extent of conjugation across the repeat unit, and I or (h) substituting the repeat unit with one or more further suhstitucnts R9 in or more positions adjacent to the linking positions in order to create a twist with the adjacent repcat unit or units, for example a 2,7-linked fluorene canving a C120 alkyl subst.ituent in one or both of the 3-and 6-çxisitions.

Another exemplary class of arylene repeat units is a phenylene repeat unit that may he unsubsliluied or substituted with one or more substituents. such as a repeal unit comprising a phenylene group of fonnula (X): (X) wherein v is 0, 1, 2, 3 or 4, optionally 1 or 2, and R1° independently in each occurrence is a substiluent, optionally a substilueni R9 as described above with reference to formula (IX), for example C1-20 alkyl, and phenyl that is unsubstituted or substituted with one or more C1 -2'] alkyl groups.

The repeat unit of formula (X) may he 1,4-linked, 1,2-linked or 1,3-linked.

If the repeat unit of formula (X) is 1,4-linked and if v is 0 then the extent of conjugation of repeat unit of fonnula (X) t.o one or both adjacent repeat units may be relatively high.

If v is at least 1, and / or the repeat unit is 1,2-or 1,3 linked, thcn the extent of conjugation of repeat unit of formula (X) to one or both adjacent repeat units may be relatively low. In one preferred arrangement, the repeat unit of Ibrinula (X) is 1,3-linked and v is 0, 1, 2 or 3. In another preferred arrangement, the repeat. unit of formula (X) has formula (Xa): (Xa) Further exemplary co-repeat units include repeat units comprising triazine.

Triazine-containing materials are described in more detail in WO 2008/0259 97 and include, for example, optionally substituted Iriphenyltriazine.

An exemplary triazine-compristrig co-repeat unit of the polymer according to the present invention has fonnula (XI): + N Nt (XI) wherein Ar8 in each occurrence is independenl.ly selected from aryl or heteroaryl groups, each of which may he unsuhstituted or substituted with one or more substitucnts, and z in cach occurrdncc is indcpcndcntly at least 1, optionally 1, 2 or 3.

Preferably, Ar8 in each occurrence is aryl, optionally phenyl.

Preferred substituents are selecl.ed from Ihe group R3 consisting of: alkyl, for example C120 alkyl, wherein one or more non-adjacent C atoms may he replaced with 0, S, substituted N, C=O and -COO-and one or more I-I atoms of thc alkyl group may he replaced with F or aryl or heteroaryl optionally substituted with one or more groups R4.

aryl or heteroaryl optionally substituted with one or more groups R4, NR52. OR5, SR5.

fluorine. nitro and cyano; wherein each R4 is independently alkyl, for example C120 alkyl, in which one or more non-adjacent C atoms may he replaced with 0, S, substituted N, C=O and -COO-and one or more H atoms of thc alkyl group may he replaced with I', and each R5 is independently selected from the group consisting of alkyl and aryl or heteroaryl optionally substituted with one or more alkyl groups.

Preferably, each Ar8 of formula (XI) is phenyl, each phenyl being optionally and independently substituted with one or more C120 alkyl groups.

[he extent of conjugation of the polymer backbone may he limited to avoid quenching of emission from the phosphorescent material(s). The extent of conjugation of the polymer may be controlled by selection of the linking position of polymeric repeat units.

Additionally or alternatively, conjugation of the polymer may he limited by repeat units that form a twist in the polymer backbone and I or conjugation-breaking repeat units.

An example of a repeat unit that may cause a twist in the polymer backbone (for example by steric hindrance) is I,4-phenylene substituted with one or more groups such as one or more alkyl or alkoxy groups, e.g. C1-20 alkyl or alkoxy groups, in particular 2,5-disubst.ituted-1,4-phenylene repeat. unit.s.

A conjugation breaking repeat unit that breaks conjugation bet.ween repeat units either side of the conjugation breaking repeat unit include repeat units having formula -Ar9-Sp'-Ar9-wherein each A? is an optionally substituted aryl or heteroaryl group and S'p1 is a spacer atom or chain comprising at least one non-conjugating atom between the two Ar groups. Exemplary Ar groups include optionally substituted phenyl.

Optional suhstituents may be one or more suhstituents R3 as described above, in particular one or more alkyl or alkoxy groups, e.g. C120 alkyl or alkoxy groups.

Exemplaiy groups Sp' include groups of formula -(CH2)11-wherein iii is at least 1, for example an integer between 1-10, and wherein each II may independently be replaced with a suhstituent, for example an alkyl group, and wherein one or more carbon atoms may he replaced with a hetcroatom, for example 0 or S. Optionally, host. polymers contain one or more arylene repeal units, for example fluorene or phenylene repeat units, and one or more charge-transporting repeat units, for example triazine-containing repeat units or amine-containing repeat units.

Exemplary monomers and repeat units that may he used to form a host polymer are illustrated in Figures lOa-lOe. Conjugated polymers may he fonned by Suzuki polymerisation as described in WO 00/53656.

There may further be provided a formulation comprising the host material. QD and phosphorescent material(s) in one or more solvents. The choice ol solvent(s) and surface modification groups for the Ql)s may provide for uniform distribution of the QDs in Leh formulation, and unifonn distribution of QDs in a layer formed by solution processing of the fonnulation. Optionally, the composition may further comprise other constituents such as an inert material such as an insulating polymer, e.g., PM M A (po]y(mcthyl niethacrylate)).

Whcn dcpositing QDs from a formulation, this may he done using various methods such as spinning, ink-jet printing, dmpcasting, etc. Where in alternative embodiments the QDs are not in a formulation, evaporation, chemical deposition or vapour deposition may he more suitable.

Quantum dots and phosphorcsccnt materials may each make up least about 0.5 mol% of the composition of quantum dot.s, host material and phosphoiescent material(s).

However, the concentration of the various ingredients may depend on their activity in the composition, particularly if thc blend is used as a film in a light-emitting device.

For example, if the phosphorescent. materials and/or QDs are only optically active and electrically inert (i.e., do not contribute to thc elcetrical characteristics of the device), then they each may be provided in a concentration of a few mol %, e.g., about 5 mol or less for each phosphorescent material and/or QI) colloid. I'he remainder of the composition (e.g. about 85mo1% or more) may then be the host niaLerial. The proportion of each guest required to generate white light may be specific to, inter alia, the device geometry (e.g., layer materials and thicknesses) used. For example, in an 0111], a change of emission layer material or thickness may result in a different proportion of each emitting guest being required.

On the other hand, if the phosphorescent materials and/or QD have electrical activity as well as optical activity, i.e., if they take part in electrical transport in the device, then achieving the correct ratio may require that optical and electrical requirements for the device he simultaneously satisfied. In this case, a niaxinium amount for each guest may be about 15 mol %. Again, the remainder of the composition may be the host material, e.g. a host polymer.

There may further be provided the composition, wherein a concentration of the or each phosphorescent material in said composition is equal to or greater than 1 mol%.

This may mean that each different phosphorescent. material (as described above, there may he one or more phosphorescent materials) is present in a concentration of 1 niol Particularly advantageously for white lighi generation. at least one phosphorescent material may comprise green light emission phosphorescent material (i.e., a phosphorescent material having peak emission within a wavelength range of about 495mn to 570nm). An example may he iac*-tris(2-phenyipyriciine)iridium.

Furthennore. at least one phosphorescent material may comprise a red light emission phosphorescent material (i.e., a phosphorescent material having peak emission in the wavelength of about O2Onm to about 750nm). Similarly, the at least one QD may, particularly advantageously when combined with the above red and green phosphorescent materials for white light generation, comprise blue light emission QD(s) (i.e., QD(s) having peak emission in a wavelength range from about 400nm up to and not including green and longer wavelengths from about 495nm). Thus, the above first phosphorescent material may be a green light emission phosphorescent material and the above second phosphorescent material may be a red light emission phosphorescent material, this being advantageous when combined wiLh the above blue light emission QD for generating white light. (Where thc peak emission wavelength of an emission spectrumi is within a particular range as described above, spectral tails of the emission spectrum may nevertheless extend to higher and/or lower wavelengths).

There may further he provided the composition, wherein the or each said quantum dot has one or more, preferably a plurality, of surface modification groups distributed over the surface of a said quantum dot. Such surface modification groups may comprise groups that are suitable for increasing QI) compatibility with the host material (e.g., polymer), for example such that the QD is unifornily distributed in the host material in a layer formed by deposition of a formulation of the composition and one or more solvents. The use of a surface modification group is of particular advantage when the QDs are in dispersion. For example, the surface modification groups miay be optimiscd so that the formulation comiprising the polymer or small molecule host substantially prevents aggregation of the QDs. This may mean matching the surface modification group to the host material, for example using a surface modification group containing a stmctural unit that is substantially the same as a structural unit of the host. Furthermore, the lengths of the surface modification groups niay be optimised to allow/block exciton transfer and charge transfer as desired. For example, to reduce or prevent charge transfer, long and/or bulky surface modification groups or transport blocking (e.g. shallow LUMO, deep HOMO) surface modification groups may he advantagcous. Conversely, to cnhance transfer, short surface niodification groups or charge-transporting surface modification groups may be advantageous. As an example, a surface modification group formed from a chain of fluorene groups may he suitable for transporting electrons. I'hus, the average (i.e. median) length of each surface modification grous may be of significance to charge transfer, e.g., exciton transfer. The surface modification groups may have an average length of about lOnm or less.

At least one said surface modification group may comprise, for example, an alkyl chain, an alkoxy chain or a polycthylene glycol chain andlor at least one chain of fluorene groups. A polytluorene chain may be particularly advantageous for matching to the host, and matching may minimise aggregation of the QDs.. Moreover, a surface modification group may have a hydrophilic tail end, which may enhance hydrogen bonding with a polar solventlenvironment, e.g.. water and may improve uniformity of dispersion in the solvent.. Surface modification groups miay comprise elemenl.s to make the QD(s) incompatible with the host where it may he advantageous to phase separate the layer formed by depositing a formulation containing the host and QI)(s).

Thus, a QD colloid-polymer blend may he achieved wherein the QDs remain uniformly dispersed in the host (e.g., polymer). In other words, the QDs remain uniformly suspended (rather than dissolved as in a solution) throughout the blend. A suitable tail end may comprise an amuno group for interacting with the polymer.

Nevertheless, the above is not intended to limit the different materials that may be used for the surface modification gmup. The surface modification group may be any chemical structure which helps blend the host and guest advantageously together in such a way that the guest advantageously does not aggregate, and the optimum choice of surface modification group may he specific to the particular host used.

According to a second aspect of the present invention, there is provided a light emitting device comprising the composition of the first aspect, the composition advantageously having any combination of one or more of the above optional features and advantages of the first aspect. the device may he a white light emitting device, such as a while P-OLED wherein the host is a polymer, or a while SM-OLED wherein the host is a small molecule malerial.

According to a third aspect of the presenL invention, there is provided a polymer organic light emitting diode (P-OLED) comprising: an emission layer comprising a polymer; an anode for hole injection towards said emission layer; a cathode for electren injection towards said emission layer, wherein: said emission layer comprises: at least one blue emission quantum dot material; a green light emission phosphorescent material; and a red light emission phosphorescent material, and wherein: a lowest triplet energy stale of said polymer is at a higher energy than an eniissive energy state of said at least one quantum dot; said quantum dot emissive energy state is higher than an emissive energy state of said green light emission phosphorescent material; and said cniissive energy state of said green emission phosphorescent material is higher than an emissive energy state of said red light emission phosphorescent material.

Said injection towards an emission layer may comprise injection directly into the emission layer or indirectly via a further layer, e.g., a HII. or a hole transport layer (HTL) otherwise known as an intcrlayer (HTL).

An embodiment. of such a P-OLE[) is shown in Fig. 4. which includes the emission layer (labelled as "EU'), which is generally a thin layer, e.g. less than lOOnm thick, of semiconducting polymer containing the QIDs and phosphorescent materials. It will he understood that the light-emitting layer may he the only layer between the anode and cathode, and that one or more other layers are optionally present in any combination.

Regarding specific layers of the P-OLED, the polymer of an OLED embodiment may have one or more of the repeat units of a host polymer as described above in relation t.o the composition of the first, aspect.

The anode provides a conducting electrode with a higher work function than the cathode. One of the anode and cathode is formed on a supporting substrate, for example glass, and further layers of the device are formed over the said anode or cathode. The cathode may be formed of, e.g., Ba or Ca /Al. The anode or cathode may he transparent to aflow emission from the device. An exemplary material for fonning a transparent anode is indium tin oxide (ITO).

The QD(s) may he made from CdSe or other materials inducing Cd. They are preferably made from Ill-V or TI-VT compounds, but can be made from any organic or inorganic materials or combination thereof, provided the emission energy of the dot is lower than the triplet energy of the host such that energy transfer will occur from host to clot.

Optionally, and as lurther shown in Fig. 4, the P-OTFD may have a hole injection layer (H1I4, which may he considered conceptually as part of the anode. ftc HIL may aid charge injection from the anode by providing an additional, and smaller, potential energy step for the holes and/or by blocking electrons from reaching the anode where they would he wasted. The 1(11. may comprise a conjugated polymer for carrying positive charges, e.g., may comprise a PEDOI' material (PEDJI' is a material based on poly(3,4-ethyleneclioxythiophene)). For example, such a material may be PEDOT-PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) or PEDOT-TMA (poly(3,4-ethylenedioxythiophene)-tetramethacmylate)). the HIL may additionally or alternatively comprise molybdenum trioxide MoO3. Ihe I-IlL may transport holes as well as assist injection of holes and may therefore have higher hole mobility than electron mobility. For example, the 11Th mnay comprise a conducting polymer layer that transports and injects holes into the active layers. The HIL may he provided between the emission layer and the anode. Similarly, an electron transport layer (ETL), which may have higher electron mobility than hole mobility, may be provided between the emission layer and the cathode for improving electron transport to the emission layer from the cathode.

There may further he an optional layer between the 11IL and emission layer called the ITole Transport Layer (11Th) comprising a hole-transporting material. The IIIL may comprise polymer. Moreover, the TIlL may be used for transporting holes to the exeiton formation region in the emission layer. The interlayer may enable substantial improvements in efficiency and/or stability, e.g., device lifetime, to he achieved. the TIlL may be a thin solution-proecssed layer and may be incorporated to assist with balancing hole and electron injection and transport, thereby improving the efficiency

S

of radiative recombination that leads to light emission and controlling where the light is elniLted in the device.

Thus, a P-OLED may he provided wherein the emission layer has a composition comprising a host polymer corresponding to the host IllaLerlal of the coniposition of the first aspect, green and light emission phosphorescent materials as the at least one phosphorescent material of the first aspect and at least one QD as in the first aspect.

Since the QD is a blue enussion QD, (he ennssion layer may then comprise a composition suitable for white light generation as described in relation the first aspect above. Moreover, the cxeitons from the host may transfer to the blue, green and red guests without direct ennssion from the host and may achieve a high efficiency white light generating P-OLED.

Aceoding to a fourth aspect of the present invention, there is provided a small molecule organic light emitting diode (SM-OLED) comprising: an emission layer (EML); an anode for hole injection towards said EML; and a cathode for electron injection towards said EML, whcrein said EML comprises: a host material; at least one blue emission quantum dot; green light emission phosphorescent material; and red light emission phosphorescent material, and wherein: a lowest triplet energy state of said host material is at a higher energy than an cniissivc state of said at least one quantum dot; said quantum dot emissive energy state is higher than an emissive energy state of said green light emission phosphorescent material; and said emissive energy state of said green light emission phosphorescent material is higher than an emissive energy state of red light emission phosphorescent material.

Furthennore, the SM-OLED may optionally additionally comprise an electron transport layer (EEL) between said EML and said cathode for transporting electrons derived from said cathode toward said EML, and/or a hole transport layer (HTL) between said EML and said anode for transporting holes from said anode towards said EML. The ETL may have higher electron Illobility than hole mobility. The IITL may have higher hole mobility than electron mobility.

Said injection towards the EML may comprise injection directly into the EML or indirectly via a further layer, e.g., 1-EEL or ETL.

Advantageously, the emission layer of the SM-OI A-]) may he provided in the form of a composition as described in the first aspect including any optional features thereof in any combination. Particularly advantageously, the SM-OLED may be a white light emitting device where the composition is suitable for white light emission as described above.

An example structure of such a small molecule OLED as in the above fourth aspect is shown in (lie Fig. 5. however, this is a relatively detailed embodiment since, at a more basic level, the SM-OLED may generally be considered as an anode-EML-cathode structure, i.c., may comprise only the anodc, cathodc and EML, othcr layers being opuonally present in any combinahon. As menuoned above, at least the light emission layer of such a SM-OLED may be deposited by vacuum deposition such as evaporation, or by solution processing. Specifically, the anode may he provided as ITO coaled on a glass substrate and the IITL (hole transport layer) deposited thereon.

For enhancing hole transport in a light-emitting device such as an SM-OLED, NPB (N,NS-di(naphthalen-1 -yfl-N,NS-diphenyl-henzidine) may be used. An electroluminescent complex for the EML may be for example Alq3. The electron transport layer (ETL) and cathode (e.g., aluminium) may then be vapour deposited on top of the EML. A suitable cathode material may be LiF/Al. The ETL may he for improving electron injection from the cathode into the EML, and may additionally or alternatively he for blocking holes from reaching the cathode. The Hi], and/or liii.

may provide a more gradual change of work function from the adjacent electrode to the EML. The FIlL may comprise NPB as indicated above, and may enhance hole injection into the I Ml, andlor assist in blocking electrons from reaching the anode.

lor the EM!. of the SM-OIEI), a blue emitter small molecule material is preferably used. This is of particular advantage for white light generation similarly as described for the above composition and polymer OLED.

According to a fifth aspect of the present invention, there is provided a method of generating white light, comprising electrically stimulating exciton generation in a composition, wherein said coniposition comprises: a host material; at least one blue light emission quantum dot; a green light emission phosphorescent material; and a red light emission phosphorescent material, wherein: a lowest triplet energy state of said host material is at a higher energy than an enlissive energy state of said at least one quantum dot; said quantum dot einissive energy state is higher than an emissive energy state of said green light emission phosphorescent material; and said emissive energy state of said green light emission phosphorescent material is higher than an eniissive energy state of said red light emission phosphorescent material.

The host material may he a polymer or a small molecule material, e.g., a small molecule organic matrix.

In such an embodiment, as in the above first to fourth aspects, the excitons may be generated and may then recomhine to emil photcns. In other words, electrons and holes are injected and excitons created which recomhine via photo-emission. This principle is applicable to any instance of light generation in the present specification.

The composition used in the fifth aspect may be a composition as described in the first aspect and may have in any combination one or more of the advantages and optional features of the first aspect. Thus, the host polymer used in the method may he comprised in the active layer of a white light generating device.

According to further aspects, there are provided a white light emitting device corresponding to the above method, a method for making each of the above compositions, and methods corresponding to the above devices.

Preferred embodiments are defined in [lie appended dependent claims.

BRIEF DESCRIPI'ION OEM-IF DRAWINGS For a better understanding of the invention and to show how it may he carried into effect, reference will now he made, by way of example only, to the accompanying drawings, in which: Fig. I shows an energy diagram of an embodiment comprising a host.. QD, and green and red phosphorescent materials. Downward arrows are shown to indicate radiative exciton transitions to ground states in the QDs and phosphors. An upward arrow indicates that, when stimulated, a transition in the host may occur from a ground state to a higher state, e.g., electric stimulation may generate an exciton in the host); Fig. 2a is a photoluminescence spectrum for a film formed from a composition of PMMA and 0.1 weight % of a blue QD; Fig. 2h is a phot.oluunnescence spectrum for a film fonued from a composition of IIostP, which is a polymeric host having repeal units illustrated in Figure 6, 0.1 weight % of a blue QD and 1 weight % of the green phosphorescent material illustrated in Figure 7; Fig. 2c is a photoluminescence spectrum for a film formed from a composition of HostP, illustrated in Figure 6, 0. I weight % of a blue QD and I weight % of the green phosphorescent material illustrated in Figure 7; Figure 2d is a photoluminescence spectrum for a film formed from a composition of IIostP and 0.1 weight % of a blue QD; Figure 2e shows the spectra of Figures 2a-2d together; Fig. 3a is a photoluniinescence spectrum for a film fonned from a composition of PMMA and 0.1 weight % of a blue QI); Fig. 3b is a phot.oluniinescence spectrum for a film fonned from a composition of HostP and 0.5 weight % of a blue QD; Fig. 3c is a photoluminescence spectrum for a film formed from a composition of HostP, 0.5 weight % of a blue QD and I weight % of the green phosphorescent material illustrated in Figure 7; Figure 3d is a photoluniinescence spectrum for a film fonned from a composition of I-lostP and I weight % of the green phosphorescent material illustrated in Figure 7; Figure 3e shows the spectra of I igures 3a-3d together; Fig. 4 shows an example structure of layers in a P-OIl ii); 11g. 5 shows an example structure of layers in an SM-OiiI); 11g. 6 shows two polymeric repeat unit which, when combined in a 1:1 ratio as an alternating AB eopolymer, form a host polymer, IIostP Fig. 7 shows a chemical representation of PCi, which is shown as comprising 3 nitrogen atoms situated amund an iridium atoni (1676.31 being Ge molecular weight); Fig. 8 shows a chemical representation of NPB usable in a SM-OLED embodiment; Fig. 9 shows an example composition (C) embodiment. having phosphorescent materials (PH) and a QD in polymer (P), the QD having surfactants in the form of ligands (L); Figs. lOa-c show various monomers and repeat units, with names merely for ease of reference, any combination of one or niore of which may used in an embodiment.; Fig. 11 illustrates an advantage of an embodiment by means of energy diagrams of phosphorescent material green host, phosphorescent materia' green emitter, small molecule fluorescent blue emitter and QD blue emitter; Fig. 12 shows light emission intensity against wavelength for a blend of blue QD, green phosphorescent. material and red phosphorescent. material, wherein the QDs are present in various concentrations (50, 100, 200 and 300%); and Fig. 13 shows nonnalised ennssion intensity against wavelength for two different blends, each comprising blue quantum dots, green phosphorescent material and red phosphorescent material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments described below provide compositions and, for example, a light-emitting device having improved characteristics such as, inter alia, lifetime and/or efficiency. An embodiment in the form of a composition (C), which may he incorporated into a light-emitting device embodiment such as a P-OLED, is shown in Fig. 9.

The iniproved characteristic(s) niay be achieved by consideration of singlet and triplet states of different constituents of a light-emitting composition. In this regard, a ground state of a luminescent material is generally a singlet state. Above that (i.e., at energies greater than that of the ground state), there may he singlet and triplet excited states or excitons (electron-hole pairs). Excited singlets may emit (fluoresce). In contrast, triplets generally do not emit, or eniit only rarely.

In more detail, excited states in which exeitons (hole-electron pairs) have a total spin of magnitude 1 (in units of 7) are triplet energy states. Conversely, a spin slate corresponding to a total spin of zero is a singlet state. Since a particle, e.g., proton or electron, having spin quantum number 1⁄2 may he spin-up or spin-down, there are three possible combinations of spin basis states for a triplet, and only one such combination for a singlet. Consequently, when excited electrically, there may be expected to be present in a material about 75% of excitons in triplet states and about 25% of excitons in singlet states.

Luminescent. (e.g. fluorescent or phosphorescent.) material in an embodiment may absorb energy of a specific wavelength and generally re-emits energy at a different, longer, wavelength. In other words, the absorption of a first photon may trigger photon emission at a longer wavelength, albeit sometimes accompanied by heat and/or vibrations. A fluorescent molecule generally comprises a fluorophor, which is a group of atoms the excitation and dc-excitation of which gives rise to fluorescence.

Moreover, fluorescence may occur due a radiative reconihinalion event that is a transition froni an excited singlet state to a singlet ground state.

A phosphorescent material in the embodiment may emit light after either photo-or electro-exeitation to its emissive excited state. Phosphorescence may occur due to a radiative recombination event that is a transition from a me.tastahle electronic state (e.g., triplet state) to a lower energy state (e.g., singlet ground state). Such transitions are relatively infrequent, since a transition requiring a hip in the spin state is classically forbidden. ftc frequency of transition from triplet to ground state in a phosphorescent material is increased by mixing S character into triplet state e.g. by including heavy metal such as fr in the phosphorescent material. This results in the mixing of S and F states as discussed below.

It is however noted that singlet and triplet states in the phosphorescent material may be mixed (spin is not a good quantum number), so that the triplet state, which lies below' the singlet state, gains some singlet character and becomes emissive, i.e. the triplet state becomes emissive. Such a mixed state, which may have predominantly triplet character, may thus be described niore bmadly as an enussive' stale.

In an embodinienL (he QI) enussive level lies below' that of the lowest. Iriplet exciled state for the host, so that both singlets and triplets from Ge host may transfer 10 the dot, in addition to transferring to phosphorescent materials. Ihus, efficiency approaching 100% of exciton use may he achievable in an embodiment in which emission occurs from dot. and phosphorescent material guests. e.g., by using host material with phosphorescent materials and QDs as described below. In this regard, the emissive state of a QD may be considered primarily as a fluorescent singlet statc, even though II may take on sonic triplet character so that the QD enussive state may be of mixed singletltriplet character, e.g. predominantly singlet. In other embodiments, the QI) emissive level lies above that of the lowest triplet excited state for the host.

The white light. may he generated in such an emhodinient froni two phosphorescent materials and a QD material, e.g., red and green phosphorescent materials and a blue QD material, for which the narrow singlet-triplet splitting of the QD material is advantageous.

Triplet trapping, in a general ease, may occur when an exciton becomes localised by a low' triplet energy st.at.e of a first. constituent (e.g., a non-QD fluorescent. material) such that it cannot transfer as desired to a higher triplet energy state of a further constituent (e.g., phosphorescent material). In this case, desired light emission from the further constituent may he reduced. Thus, triplet trapping in a composition may reduce efficiency of exciton use and consequently reduce overall elTiciency (input (e.g., optical power or intensity, or electrical power): output (output light emission power or intensity)).

The following describes triplet trapping specifically in relation t.o a system comprising a fluorophor (non-QD material), phosphorescent material and exciton-gencrating material.

Where the fluorophor has a lower triplet state than a triplet state ol the phosphorescent material, the lower triplet state may energetically trap triplet exeitons obtained from the exciton-generating material. (Such material may he blended in the composition, or located to allow diffusion of the generated excitons to the composition).

Consequently, the exdilon may become localised by the Iluorophor and unable to transfer to phosphorescent material. Such trapping may thus reduce instances of triplet-singlet (e.g., triplet to ground state) transitions in the phosphorescent material.

Triplet trapping on the fluorophor guest may occur if the singlet-triplet energy level splitting on that guest is large enough to prevent a thermal excitation from Ij to S1, or from T1 on the Iluorophor to the emissive state on a phosphorescent material for the case in which the fluorophor T1 energy level is lower than the phosphorescent material Ii energy level. For example, a blue fluorescent material may have a triplet excited state energy level that is lower than that. of a green phosphorescent. material, resulting in emission from the singlet excited state of the fluorescent material but at least some quenching of phosphorescence from the green phosphorcscent material.

Embodiments described hcrein may advantageously have substantially no triplet trapping, this being particularly achievable by use of QDs and phosphorescent niatcrial(s).

Generally speaking, the QD(s) of the embodiment may he semiconductors that provide three-dimensional confinement of exeitons (electron-hole pairs). The confinement is generally of size smaller than the wavelength of electrons or holes.

Consequently, a QD has quantized energy levels, i.e., the electron and hole energy levels are separate, discrete levels so that they are generally not considered as a continuous hand, and similarly the exciton (emission) energies are quantized.

Furthermore, the energy levels of a Q[), and thus the wavelength of light emission from the QD. may be tuned by adjusting the QD size..Specillcally, when a QD energy state is excited, e.g., by photon absorption, an electron-hole pair (exciton) may he generated and, upon recombination, light may he emitted. In other words, the QD may fluoresce. The QD may he excited optically or electrically. The handgap tunability and discrete energy levels thus advantageously allow a precise wavelength of emission to he obtained.

In this regard, a QD used in an embodiment may have any diameter less than about nm. More advantageously, the QD has a diameter of about 2nm to about lOnm.

Furtheniiore, a hatch of QDs may have a size distnhut.ion, i.e., dispersion, e.g., about 5% to about 10%. This can also he designed to he larger to give a broader spectrum if required. The dispersion deterniines at least in part the Full Width half Maximum (FWIIM). i.e., bandwidth, of the emission spectrum of the QD batch. (Narrower size distributions yield smaller FWHM).

Colloidal semiconductor QDs may be synthesized based on precursor compounds and organic surfactants for dissolving in solutions. Colloidal QDs may include cadmium selenide, cadmium sulfide, indium arsenide. and indium phosphide. Advantageously, an embodiment may use colloidal CdSe QDs. Further advantageously, the QDs are corc-shcll QDs, which comprise a small core of onc material surrounded by a shell of another material with larger handgap e.g., a CdSe core and ZnS shell.

A specific embodiment. of the invention as described above uses blue eniission QDs combined with red and green phosphorescent materials to generate white light. The combination of QDs and phosphorescent materials may be provided in a host, e.g., exciton generating material, such as a polymer. Moreover, the QDs may he provided in thc cniissivc layer or layers of a P-OLED, and suitable QDs may be as detailed below. Suitable host polymers are shown in Figs. 9 and 10.

More specifically in relation to light emission from embodiments described above, photoluminescence experiments using blue QDs show that a blend of polymer, blue QDs and green phosphorescent. material provides emission from both t.he blue QDs and the green phosphorescent material.

Moreover, a blend of polymer, blue QDs and green and red phosphorescent materials provides the white light generation embodiments descTihed above, advantageously with very low', or substantially no, loss in internal QE (Quantum Efficiency). Such low' loss light emission may arise if the energy structure of the blend substantially prevents triplet trapping as described above.

Fig. 1 show's energy levels associated with a host polymer, a QD and green and red phosphorescent materials, in order of decreasing energy, respectively. Fig. 1 does not necessarily indicate spatial positions of these constituents, which may he blended together in a single composition, e.g., device layer. In this regard, it is noted that the materials may he blended in an embodiment such that Ql)s and phosphors are mixed into a host polymer matrix, each with approximately uniform density, advantageously with substantially no aggregation of the QDs.

Specifically, Fig. 1 show's that. the QD has a singlet slate and a triplet st.ate. Moreover, as described above, the emissive state of the QD may be the singlet stale. While the triplet slate in reality may emit, this phosphorescence generally occurs with a yen' low probability.

In more detail, Fig. I shows a transition to the ground state from an cmissivc stale of a blue emission QD, and from enussive states of red and green phosphorescent iaterialc..'I'he positions of the blue QI) enhissive state and of the emissive states of the red and green phosphorescent materials relative to one another may be partly related to the desired wavelengths, i.e., colours, of these emitters. The phosphorescent material emissive states may have mixed singlet-triplet character; generally speaking, they may he predominantly of triplet character so that the states can then reasonably accurately be referred t.o as triplet. states. Since the host triplet state is higher than both the QD emissive state and the cmissive slates of the phosphorescent materials, triplets from the host may transfer to the blue, green and red light emission constituents, advantageously with substantially no direct emission from the host. Conversely, energy transfer may advantageously not occur from the QD or phosphorescent materials to the host. Thus, the energy diagram of Fig. 1 advantageously corresponds to a blend of host material, blue QDs and red and green phosphorescent materials that may have high exciton use efficiency or even be lossless.

Fig. 1 further shows a QD with singlet-triplet energy difference AE. This may he sufficiently small that the triplet energy level of the QD is higher than the emissive (generally, triplet) level of both of the phosphorescent nmterials. The difference in energy AE depends on temperature and may be substantially zero. For example, at room temperature (25° C) the singlet and thplet st.ates of the QI) may he substantially degenerate. In this case, there niay be substantially no distinction between fluorescence and phosphorescence in the QI).

Further as described above, the energy separation between singlet and triplet energy levels in a QI) may be comparable with kr. (In a polymer, the equivalcnt energy separation may he 30 kT). Because of this small separation, combined with the long lifetime of a triplet exciton, the actual emission from a QI) may he a combination of fluorescence and phosphorescence. Thus, a QD in an embodiment, particularly at room temperature, need not be regarded as either exclusively a fluorophor or exclusively a phosphorescent material, since the emission may not he exclusively from eithcr a singlet or triplet state.

(All descriptions of embodiments provided herein are applicable at least whcre the QDs are at or near room temperature, e.g., about 25° C, or at higher temperatures which may occur for example due to heat generation in or near an embodiment -for example, there may he some heat emission from a host/QD/phosphorescent material blend of an enthodiment., andlor an embodiment in the fomi of a light-emitting device such as an OLED may be near heat-generating electronic/optical components).

Regarding the role of the host, for example where guest emitters (e.g., R, (3 and/or B) are blended with the host, the host is generally not required to emit, except in some of eases where a deep blue contribution to the total spectrum is desired.

Moreover, a function of the host may he to transport charges (electrons and holes) from electrodes (anode, cathode) to a reconihination region where excitons are formed. Some exeitons may he formed on the host, and some excitons may he formed directly on the guest emitters. Excitons formed on the host may then transfer to the guest emitters.

In an OLEI), exciton generation in the host occurs as a result. of electrical excitation, i.e. by injecting electrons and holes either sequentially or simultaneously into the host from a cathode and an anode, respectively. (In some cases the host exeiton generation may by photo-excitation or photocxcitat.ion combined with electrical excitation; however, it may be less advantageous to design an enihodimenL to work using photoexcitation. In other words, preferred embodiments work substantially only by electrical excitation).

Specifically regarding a polymer host electrically excited in a suitable device structure, the exeiton generation may occur by excitons forming on the polymer chains. Consistent with the above, 10 -25% ol excitons may be singlets (and emit light through electroluminescence) while 75 -90 % of the exeitons may he triplets.

(Iriplets are generally not harvested in fluorophors, hut may he harvested by phosphorescent materials).

Regarding a specific energy transfer mechanism associated with the energy stales shown in Fig. 1, exciLon transfers occur from the host singlet and triplet levels to each of blue QID. green and red phosphorescent materials. Furthermore, exeiton transfers may occur from the QD to the green and red phosphorescent materials, and from the green phosphorescent material to the red phosphorescent material. (Regarding the host singlet and triplet states of Fig. 1, the above comments concerning low probability of triplet emission apply similarly to emission from the triplet state in the host, e.g., a polymer host).

With reference to Figs. 2a -e and Figs. 3a -e, compositions containing 0.1 wt % of blue QD show relatively weak QD emission (Figure 2) , however this emission is stronger at 0.5 wt % QI) (Figure 3). Accordingly, QUs are preferably provided in an amount of more than 0.1 wi %, optionally at least 0.5 wE. % of the light-emitting composition.

A composition containing the host polymer, blue QD and green phosphorescent emitter shows both blue and green emission (see Figure 3c for example), indicating that the blue QD does not quench emission from the green phosphorescent emitter, or vice versa.

Quantum dots are nanometer sized particles that can have optical properties arising from quantum confinement. Quantum dots can emit light when subjected to a stimulating radiation.

The particular composition(s), structure, and/or size of a quantum dot can he selected to achieve the desired wavelength of light to he emitted from the quantum dot upon stimulation with a particular excitation source. In essence, quantum dots may he tuned to emit light across the spectrum by changing their size. See C.B. Murray, CR.

Kagan, and M.G. Bawendi, Annual Review ot Material Sri.. 2000, 30.545-610 hereby incorporated by reference in its entirety.

Quantum dots can have an average particle size in a range from about 1 to about 1000 nanometers (nm), anti preferably in a range from about. 1 to about. 100 nil!. In certain enthodiments, quantum dots have an average particle size in a range from about I to about 20 nm (e.g., such as about 5. 6, 7, 8, 9, 10, 11. 12, 13, 14, 15, 16, 17, 18, 19, or nm). In certain embodiments, quantum dots have an average particle size in a range from about I to about 10 nm. Quantum dots can have an average diameter less than about 150 Angstroms (A). In certain embodiments, quantum dots having an average diameter in a range from about 12 to about 150 A can be particularly desirable. However, depending upon the composition, structure, and desired emission wavelength of the quantum dot, the average diameter may he outside of these ranges.

A quantum dot can comprise one or more semiconductor materials. Examples of inorganic semiconductor materials that can he included in a quantum dot (including, e.g., semiconductor nanocrystal) include, hut are not limited to, a Group iv element, a Group Il-VI compound, a Group Il-V compound, a Group Ill-VT compound, a Group Ill-V compound, a Group IV-VI compound, a Group I-Ill-VT compound, a Group II-tv-Vt compound, a Group Il-tV-V compound, an alloy including any of the foregoing, and/or a mixture including any of the foregoing, including ternary and quaternary mixtures or alloys. A non-limiting list of examples include ZnO. ZnS.

ZnSe. ZnTe, CdO. CdS. CdSe, (idle, MgS, MgSe. GaAs, GaN, GaP. GaSe. GaSh, ilgO, FIgS, llgSe, HgTe, InAs, InN. InP, InSh, AlAs, A1N. AlP, AlSh. TIN, T1P, TIAs, T1Sh, PhD, PbS, PbSe, PhTe, Ge, Si, an alloy including any of the foregoing, and/or a mixture including any of the foregoing, including ternary and quaternary mixtures or alloys.

In certain embodiments, quantum dots can comprise a core comprising one or more semiconductor materials and a shell comprising one or more semiconductor materials, wherein the shell is disposed over at least a portion, and preferably all, of thc outer surface of the core. A quantum dot. including a core and shell is also referred I.o as a "core/shell" structure. Hxamples of semiconductor materials that can he included in a core and/or a shell include, hut are not limited to, those listed above.

A shell can he a semiconductor material having a composition that is the same as or different from the composition of the core. The shell can comprise an overcoat including one or more semiconductor materials on a surface of the core. In a core/shell quantuni dot, the shell or overcoating may comprise one or more layers.

The overcoating can comprise at least. one semiconductor material which is the same as or different from the composition of the core. l'he overcoating has a thickness from about one 10 about ten monolayers. An overcoating can also have a thickness greater than len nionolayers. In certain embodiments, more than one overcoating can be included on a core.

In certain embodiments, the surrounding shell' material can have a band gap greater than the hand gap of the core material. In certain other embodiments, the surroundhig shell material can have a band gap less (han the band gap of the core material.

Tn certain embodiments, the shell can he chosen so as to have an atomic spacing close to that of the core substrate. in certain other embodiments, the shell and core materials can have the same crystal structure.

Examples of quantum dot (e.g., semiconductor nanocrystal) (core)shell materials include, without limitation: red (e.g., (CdSe)CdZnS (core)shell). green (e.g., (CdZnSc)CdZnS (eorc)shell, etc.), and blue (e.g., (CdS)CdZnS (corc)shell.

Quantum dots can have various shapes, including, but not limited to, sphere, rod, disk, other shapes, and mixtures of various shaped particles.

Quantum dots can be commercially purchased or can be prepared by known methods.

One example of a preferred method of making quantum dots is a colloidal growth process.

The narrow size distribution of the quantum (lots (including, e.g., semiconductor nanocrystals) allows tl'e possibility ol light emission in narrow spectral widUs.

Monodisperse semiconductor nanociystals have been described in detail in Murray et al. (J. Ant Chem. Soc., ii5:S706 (1993)). An example of an overcoating process is described, for example, in U.S. Patent 6,322,90i. By adjusting the temperature of the reaction mixture during overeoating and monitoring the absorption spectrum of the core, overcoated materials having high emission quantum efficiencies and narrow size distributions can he obtained. The foregoing are hereby incorporated herein by reference in their entireties.

[he process of controlled growth and annealing of the quantum dots in the coordinating solvent that follow's nucleation can also result in uniform surface derivatization and regular core structures. A c(x)rdinating solvent can help control the growth of the quantum dot. A coordinating solvent is a compound having a donor lone pair thai, for example, a lone electron pair available to coordinate to a surface of the growing quantum dot (including, e.g., a semiconductor nanocrystal). Solvent coordination can stabilize the growing quantum dot. I ixamples of coordinating solvents include alkyl phosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkyl phosphinic acids, however, other coordinating solvents, such as pyridines, thrans, and amines may also bc suitable for the quantum dot (e.g., semiconductor nanocrystal) production. Quantum dots can alternatively be prepared with use of non-coordinating solvent(s).

The emission spectra of quantum dots can he tuned continuously over the wavelength range of 300 nm to 5 microns, or from 400 nni to 800 nm.

Quantum dots preferably include one or more ligands attached to an outer surface.

Ligands can be derived from a coordinating solvent that may he included in the reaction mixt.um during the growth process. Ligands can also he added to the reaction mixture and/or derived from a reagent or precursor included in the reaction mixture for synthesizing the quantum dots.

A quantum dot. surface that includes ligands derived from the growth process or otherwise can he modified by repeated exposure to an excess of a competing ligand group (including, e.g.. but not limited to, coordinating group) to form an overlayer.

For example, a dispersion of the capped quantum dots can he treated with a coordinating organic compound, such as pyridine, to produce crystallites which disperse readily in pyridine, methanol, and aromaties but no longer disperse in aliphaUc solvents. Such a surface exchange process can he carried out with any compound capable of coordinating to or bonding with the outer surface of the nanoparticle. including, for example, hut. not limited to, phosphines, thiols, amines and phosphates.

For example, a quantum dot can he exposed to short chain polymers which exhibit an affinity for the surface and which terminate in a moiety having an affinity for a suspension or dispersion medium. Such affinity improves the stability of the suspension and discourages liocculadon of the quantum dot.

Examples of ligands that can he attached to a quantum dot include, hut are not limited to, alkyl carboxylic acids, aromatic carhoxylic acids, alkyl phosphines, allcyl phosphinc oxides, ailcyl phosphonic acids, or alkyl phosphinic acids, pyridines, flirans, and amines. More specific examples include, hut are not limited to, pyridine, tri-n-octyl phosphine (TOP), tri-n-octyl phosphine oxide (TOPO) and Ens-hydroxylpropylphosphine (tHPP). More specific examples incthde, hut are not limited to, pyridinc, tri-n-octyl phosphinc (TOP), tri-n-octyl phosphinc oxide (TOPO), primary amines, e.g., CII3(CII2)NII2 wherein n 4-19 (e.g., hut.ylamine, pentylamine, hexylarnine, heptylamine, octybmi ne, nonylamine, decylaini ne, undecylamine, dodecylaminc, tridccylaminc, tctradccylaminc, pcntadecylamine, hexadecylamine, heptadecylamine, octadecylamine, nonadecylamine, eicosylamine), secondary amines, e.g., (CH3(CI-l))2NI-l wherein n = 3-I I (e.g., dihutylamine, dipentylamine, dihexylainine, dihcpty!aminc, dioctylaminc, dinonylaminc, didecylamine, didundecylamine, didodecylanilne), phenylbutyl amine, 4-phenylbutyl amine, 3,3-diphenylpropylanune, (2,3-diphenylpropyl)amine, oleic acid, hexy!p!iosphomc acid, tetradecvlphosphonic acid, octyiphosphonic acid. octadecyiphosphonic acid, propylenediphosphonic acid, phenyiphosphonic acid, , phenyiphosphonic acid, aininohcxylphosphoinc acid, benzylphosphonic acid, etc. Other ligands can bc readily ascertained by the skilled artisan.

The emission from a quantum dot. capable of emitting light can he a narrow Gaussian emission hand that can he tuned through the complete wavelength range ol the ultraviolct. visible, or infra-red rcgions of the spectrum by varying the size of the quantum dot, the composition of the quantum dot, or both.

The nanow size distribution of a population of quantum dots capable of eimtting light can result in emission of light in a narrow spectral range. The population can he monodisperse prcferably exhibits lcss than a 15% rms (root-mean-square) dcviation in dianiet.er of such quantum dots, more preferably less than 10%, most preferably less than 5%. Spectral emissions in a narrow range of no greater than about 75 nrn, preferably no cater than about 60 nm, more preferably no greater than about 40 nm, and most preferably no greater than about. 30 mn full width at half max (FWHM) for such quantum dots that emit in the visible can he observed. The breadth of the emission decreases as the dispersity of the light-emitting quantum dot diameters decreases.

The narrow FWHM of quantum dot.s can result in saturated color emission. The broadly tunable, saturated color emission over the entire visible spectrum of a single material system is unmatched by any class of organic chromophores (see, for example, Dabbousi et cii., J. Phys. Cheni. 101. 9463 (1997), which is incorporated by reference in its entirety). A monodisperse population of quantum dots will emit light spanning a narrow range of wavelengths.

Quantum dots can have cniission quantum efficiencies such as greater than 10%, 20%, 30%,40%,50%,60%,70%,80%,or90%.

In an embodiment, the host and Ql)(s) and/or phosphorescent material(s) may he combined in a blend, for example where the host is polymer. Such a blend may form a device layer such as an emission layer of an ()LED, e.g., a emission layer of a solution-processahie, P-OLED. Advantageously, such a layer may comprise a polymer such as a polyphcnylcncvinylcnc (PPV) or polyfluorcnc (P1) as the host.

Alternatively, such a layer may be an EML of a small molecule OLED (SM-OLED).

An optimum QD concentration for any embodiment may he influenced by the soluhilizing groups applied to the QD, (advantageously over the majority of the surface). The optimum concentration needed may depend on the relative transfer rate of excitons from the host to the QD(s) and to each phosphorescent material(s) and in each direction between the QD(s) and phosphorescent material(s). Thus, the optimum concentration may depend on lengths of the QD solubiliting grnups, e.g., QD ligands which may have lengths of about 5 Angstroms to about 10 Angstrorns.

Pig. 4 shows a structure of a solution-proccssahlc P-OPEl) having a composition of an embodiment as the emission layer, which is between the cathode and the anode provided for electron and hole injection to the emission layer, respectively. As further shown in Fig. 4, there may optionally he any combination of one or more of the following layers: Hole Injection Layer (HIL; e.g., comprising poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) for hole transport and/or work function matching to the anode to enhance hole injection; interlayer (IITL) (otherwise known as JIole Transport Layer (IITL)) for improving hole/electron injection balance; and/or electron transport layer (Eli) for electron transport form the cathode to the emission laycr. Other laycrs known in thc ficld of P-OIiiDs may additionally or alternatively be incorporated.

Regarding the basic structure of organic (which here includes organoinettillic) LEE) (OLED) embodiments described herein, they may be fabricated using niaterials including polymers, small molecules and dcndrimcrs, in a range of colours which depend upon the materials employed. Examples of P-OLEDs are described in WO 90/13148 WO 95/06400 and WO 99/48160, and examples of dendrimer-based materials are descrihcd in WO 99/21935 and WO 02/067343. QDs may he incorporated in such cxamplc devices in cmbodinicnts as described herein.

Such an OLED enihodinient. may comprise a substrate, typically 0.7 111111 or 1.1 111111 glass but optionally clear plastic or somc other substantially transparcnt material. An anode layer is deposited on the substrate, typically comprising around 150 nm thickness of tIC) (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 mn of aluminium, or a layer of aluminium sandwiched between layers of chronic, and this is sometimes referred to as anode metal. Glass substrates coated with FF0 and contact metal are available from Coming, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, for example for external contacts to the device. The contact metal is removed from the 110 where it is not wanted, for example where it would otherwise ohseure the display, by a standard process of photolithography followed by etching.

A substantially transparent hole transport layer, e.g., JIlL, may deposited over the anode layer, followed by an electroluminescent layer, and a cathode. . The IJIL, which may help match the holc energy levels of the anode layer and electroluminescent layer, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonatc-doped polycthylcne-dioxythiophene) from II C Starck of Germany. In a typical polymer-based device the JilL may comprise around 200 nni of PI-])O'F; a light emitting polymer layer is typicafly around 70 mu in thickness.

These organic layers may he deposited by spin coating, dip coaling, doctor blade coaling (afterwards removing nialerial from unwanted areas by plasma etching or laser ablation). Alternatively selective deposition techniques wherein the organic material is only deposited in desired areas, such as inlcjet printing or laser induced (hernial imaging (LITI), may he employed.

The cathode layer typically comprises a low work Function metal (typically less than 3.5 cv, more preferably less than 3.0 cv) such as calcium or barium (for example deposited by physical vapour deposition or sputtering) covered with a thicker, capping layer of aluminium. Optionally a (Mn (1-5 nm) layer of a metal compound may be provided between the light-emitting layer or layers and the cathode. Exemplary metal compounds include metal, e.g. alkali or alkali earth, halides and oxides. Exemplary halides are fluorides.

Fig. 5 shows a structure of a SM-OLEI) having a composition of an embodiment as the emission layer (EMI4, which is similarly between an anode and a cathode provided for electron and hole injection to the EML. As further shown in Fig. 5, there may optionally he any combination of one or more of the following layers: Hole Transport. Layer (HTL) to transport holes frnm anode to EML; and/or electron transport layer (ETL) for electron transport from the cathode to the EML. Other layers may additionally or alternatively be incorporated. Furthermore, the comments as provided regarding the basic structure for OLEDs above may also be employed for small molecule embodiments described herein, wherein the light emitting material is however typically deposited by vacuum evaporation. Examples of small molecule based devices in which QDs may be incorporated in an embodiment are described in TJS 4,539,507.

Thus, in comparison to the above embodiments comprising polymer and QDs, a small molecule embodiment composition or light-emitting device may he formed, for example by substituting the polymer and QDs with solubilised QDs and solution-processed small molecule material and depositing this by physical or chemical vapour deposition between other sequentially deposited layers of the device. Ilenee, in any of the above non-P-OI.EI) devices, the polymer of a similar P-OUt-]) device may he substituted for a small molecule material to provide an SM-OLED. When depositing QDs from solution to form any embodiment described herein, eg., P-OLED or SM-OLED, this can he done using any of the typical methods, e.g., spinning/ink-jet printing, dropcasting, etc.. In comparison, chemical or vapour deposition may he less preferable for solutions. similarly to evaporation.

Fig. 11 illustraLes an advantage of an embodiment by means of energy diagrams. In detail, this figure shows singlet and triplet energy levels SI, TI above ground state for, from left to right, a green phosphorescent material host (HostP), a grcen phosphorescent. material emitter, and for a small molecule fluorescent blue emitter such as perylene. Furthermore, the ligure shows an emission energy level of the quantum dot blue emitter is shown as I l. As indicated, a low triplet energy level of a blue emitter generally quenches green phosphorescent material emission. In contrast, by alternatively using a quantum dot, such quenching may be avoided as a result of the SI and TI levels of the quantum dot of this embodiment being degenerate. Thus, a blue QD:green phosphorescent mnaterial:red phosphorescent inaLcrial blend embodiment allows high efficiency white light emission.

Fig. 12 shows light eniission intensity against wavelength for blends containing blue QD, green phosphorescent material and red phosphorescent material, wherein the QDs are present in various concentrations (50, 100, 200 and 300%). White emission with 70% photoluminescent quantum efficiency (PTQE) was achieved using this

example blend.

Fig. 13 shows nonnalised eniission intensity against wavelengih for two different blends, each comprising blue quantum dots, green phosphorescent material and red phosphoresccnt material. Both compositions show 13% External Quantum Efficiency (EQE). Thus, this figure shows that a high EQE device and quantum (tot emission may be achieved with QDs present in a blend.

Detailed information on device stmctures and methods of making both polymer and small molecule OLED devices are described in the book "Organic Light-Emitting Materials and Devices", edited by Zhigang Li and Hong Meng, published by CRC Press (Taylor and Francis) in 2007 (ISBN 1-57444-574-X), especially Chapters 2 and 8 for polymer materials and devices, and Chapters 3 and 7 for sinai] molecule materials and devices.

No doubt many other effective alternatives will occur to the skilled person. It. will he understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.