GB2516930A - Organic Light-Emitting Device - Google Patents

Abstract

A method of forming a light-emitting device, the method comprising the steps of: forming a quantum dot light-emitting layer 105 comprising light-emitting quantum dots over an electrode 103 for injecting charge carriers of a first type; forming an adjacent layer 107 on the quantum dot light-emitting layer by depositing onto the quantum dot light-emitting layer a formulation comprising one or more solvents and one or more materials selected from charge-transporting and light emitting materials, and evaporating the one or more solvents; and forming an electrode 109 for injecting charge carriers of a second type over the adjacent layer. The first electrode 103 may comprise an anode and the second electrode 109 may comprise a cathode. A hole injection layer and/or a hole transport layer may be provided between the anode 103 and the quantum dot light emitting layer 105. The light emitting device may comprise an organic light-emitting material, wherein the material may be a phosphorescent organic light emitting material.

Description

Organic Light-Emitting Device

Background of the Invention

Electronic devices containing act.ive organic materials are attracting increasing attention for use in devices such as organic light. emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices containing active organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device niaiiulacture, foT example inkjet printing or spin-coating.

An ()LED may comprise a substrate carrying an anode, a cathode and one or more organic light-entitLing layers between the anode and cathode.

Holes are injected into the device through the anode and electrons are injected through the cathode during operation of the device. Holes in the highest occupied molecular orbital (IIOMO) and electrons in the lowest unoccupied molecular orbital (UJMO) of a light-emitting material combine to form an cxciton that releases its energy as light.

Suitable light-emitting materials include small molecule, polymeric and dcndrimeric materials. Suitable light-emitting polymers include poly(arylene vinylenes) such as polyp-phenylene vinylenes) and polyarylenes such as polyfluorenes.

A light emitting layer may comprise a seniiconducting host material and a light-emitting dopant wherein energy is transferred from the host material to the light-emitting dopant. For example, J. AppI. Phys. 65, 3610, 1989 discloses a host material doped with a fluorescent light-emitting dopant (that is, a light-emitting material in which light is emitted via decay of a singlet excit.on).

Phosphorescent dopants are also known (that is, a light-emitting dopant in which light is emitted via decay of a triplet exciton).

Layers containing quantum dots (QDs) may he provided. WO 2009/123763 discloses light-emitting devices containing a layer of QDs.

WC) 2008/105792 discloses deposition ol an ink containing a solvent and nanocrystalline semiconductors.

Summary of the Invention

The invention provides a method of forming a light-emitting device, the method comprising the steps of: forming a quantum dot light-emitting layer comprising light-emitting quantum dots over an electrode for injecting charge caniers of a first type; forming an adjacent layer on the quantum dot light-emitting layer by depositing onto the quantum dot light-emitting layer a formulation comprising one or more solvents and one or more materials selected from charge-transporting and light-emitting materials, and evaporating the one or more solvents; and forming an electrode for injecting charge carriers of a second type over the adjacent layer.

A layer formed "over" an underlying layer as used herein includes formation a layer directly on and in physical contact with the underlying layer, and formation of the layer over the underlying layer hut spaced apart therefrom by one or more intermediate layers.

A layer formed "on" an underlying layer as used herein means Formation oF a layer directly on and in physical conLact with die underlying layer.

Description of the Drawings

The invention will now be described in more detail with reference to the Figures, in which: Figur IA illustrates a light-emitting device according to a first embodiment of the invention; Figure lB illustrates a light-emitting device according to a second embodiment of the invention; Figure 1C illustrates a light-emitting device according to a third embodiment of the invention; Figure ID illustrates a light-emitting device according to a fourth embodiment of the invention; Figure 1E illustrates a light-emitting device according to a fifth embodiment of the invention; Figurc iF illustrates a light-emitting device according to a sixth embodiment of the invention; Figure 2 shows photolumineseence spectra of a layer formed from a composition of a QI) light-emitting material and a charge-transporling material before and after solvent treatment of Ihe layer; Figure 3 shows absorption spectra for the layer of Figure 2; Figure 4 shows photoluminescence spectra of a neat layer of a QD light-emitting material formed on glass, before and after solvent treatment of the layer; Figure 5 shows absorption spectra for the layer of Figure 4; Figurc 6 shows absorption spectra for a neat layer of a QD light-emitting material formed on a crosslinked polymer, before and after solvent. treatment. of the layer; and Figure 7 shows the electroluminescent spectra for light-emitting devices according t.o embodiments of the invention and a comparative device.

Detailed Description of the Invention

Figure IA illustrates a light-emitting device according to an embodiment of the invention.

The device 100 comprises a substrate 101, an anode 103, a cathode 109, and a quantum dot (QD) light emitting layer 105 and an organic light-emitting layer 107 between the anode 103 and the cathode 109.

The QD light-emit.I.ing layer 105 contains a QD light-emitting material. In operat.ion of the device, quantum dots of layer 105 emit light QD1. Organic light-emitting layer 107 contains an organic light-emitting material. h operation of the device, the organic light-emitting material of layer 107 emits light OL. The light emitted from the light-emitting layers may combine to produce white light.

The QI) light-emitting layer 105 may he formed by depositing a formulation containing QI)s and one or more solvents, followed by evaporation of the solvent or solvents, The organic light-emitting layer 107 may be formed by depositing a fonnulation containing the organic light-emitting material of the second layer and one or niore solvents onto the QI) light-emitting layer 105 followed by evaporation of the solvent or solvents.

Figure lB illustrates a light-emitting device 200 according to a further embodiment of the invention. The device is as described in Figure lA except that the organic light-emitting layer 107 is replaced with a second QI) light-emitting layer Ill containing quantum dots.

The second lighi-eniiiting layer ill may be formed by depositing a formulation containing the quantum dots of the second layer and one or more solvents onto the first QD light-emitting layer 105 followed by evaporation of the solvent or solvents.

Figure 1C illustrates a light-emitting device 300 according to a further embodiment. [he dcvicc 300 as dcscrihcd in Figure lÀ cxccpt that the dcvice has a single light-emitting layer containing QDs. and a non-enñssive electmn-transporting layer 113 in place of second light-emitting layer 105 of Figure IA. The electron-transporting layer is formed by depositing a formulation containing one or more elecb'on-transporting materials and one or more solvents onto the QD light-enntt.ing layer 105. and evaporating the solvent or solvents.

Figure 1D illustrates a light-emitting device 400 according to a further embodiment of the invention having a layer i07 containing an organic light-emitting material and a layer 105 containing a QD light-emitting material and an electron Uansporting layer 113.

Figure 1E illustrates a light-emitting device 500 according to a further embodiment of the invention. In this embodiment, a quantum dot light-emitting layer 105 is provided between an organic light-emitting layer 105 and ahole-transporting layer uS.

Figure iF illustrates a light-emitting device 600 according t.o a further embodiment. of the invention. In this embodiment, a quantum dot light-emitting layer 105 is sandwiched between two hole-transporting layers 115. An organic light-emitting layer 107 is provided over the hole-transporting layers.

The layer or layers cont.aining the QDs may consist essentially of QDs or may contain one or more further materials, for example one or more light-emitting materials and / or one or more charge-u-ansporting materials.

Devices of the invention contain at least one layer containing at least one QD light-emitting material, and may cont.ain one or more light-emitting materials selected from QD light.-emitting materials and organic light-emitting materials. Ihe one or more light-emitting materials may he provided in separate layers from the at least one QI) light-emitting material, for example as described in Figures lÀ. lB 1D, 1E and iF. or may be mixed with the at least one QD light-emitting material.

The QI)s of a QI) light-emitting byer may he dispersed in a matrix, for example a charge-transporting host material or inert material; for example, QDs of layer 105 of Figure IA may be dispersed within a hole-transporling matrix and QDs of layer 105 of Figure ID may be dispersed within an electron-transporting matrix. Preferred matrix materials are polymers.

In one arrangement, a QD light-emitting layer 105 may he formed by depositing a formulation containing the QI) and a crosslinlcable matrix material followed by evaporation of the solvent(s) of the formulation and cmsslinking of the matrix material to provide a light-emitting layer 105 in which the quanwm dot light-emitting material is dispersed in a crosslinked, insoluble matrix. Crosslinlcing of the matrix material may prevent QDs from being washed away upon deposition of the formulation used to form an overlying layer of (he device.

Tn another arrangement, the QT) light-emitting layer 105 is not crosslinked. In this case, the QD light-emitting layer 105 may be formed using a formulation containing QDs and optionally one or more further materials, for example a matrix material. If a matrix material is present, it is not erosslinked. In a preferred embodiment, the QD light-emitting layer 105 consists essentially of the QDs. The present inventors have surprisingly found that an uncrosslinked QI) light-emitting layer 105, for example a layer consisting essentially of QI)s, remains at least partially intact following deposition of a formulation used to fonn an overlying layer of the device; the QD light-entitLing layer 105 remains despite exposure of this layer to the solvent or solvents used to deposit the material or materials of the overlying layer.

If a QD light-emitting layer contains a charge-transporting material in addition to the Q[) material then the QD material: charge transport material weight ratio may be in the range of about 0.5: 1 -5: 1, optionally about 1:1-3:1.

In order to provide efficient charge transport, OLEDs of the invention may contain a hole- transporting layer containing an organic hole-transporting material and / or an electron- transporting layer containing an organic electron-transporting material, wherein the hole-transporting layer or layers are provided between the anode and the electron-transporting layer or layers. These layers may be non-emissive charge-transporting layers, in which ease QDs may he provided in separate light-emitting layers. These charge transporting layers may he emissive charge-transporting layers, wherein the layers contain QDs and I or other enlissive materials. QDs may he mixed with one or more of these charge-transporting layers to fonn one or more emissive charge-transporting layers. Accordingly, first light-enutting layer 105 may be a mixture of an organic hole-transporting material and QDs and second light-emitting layer 107 may he a mixture of an electron-transporting material and QDs.

The thickness of light-emitting layers containing QDs may be in the range of about 0.5-100 nm, optionally about 1-50 nm. Optionally, the thickness of the QI) layer is above 10 nm.

The thickness of a light-emitting layer may he controlled, at least in part, by the concentration of QDs in the formulation used to form the light-emitting layer. QD concentration is these formulations may be in the range of about 0.05 -20 weight %, optionally about 0.1 -10 weight. %.

Light.-enut.t.ing layers containing QDs may be fonned by deposition of a fonnulation of one or more solvents containing the QDs dispersed therein followed by evaporation of the one or more solvents. Ic formulation may contain one or more further components, for example one or more host. materials, dispersed or dissolved therein. Preferably, any further components arc dissolved in the formulation.

Other layers of an OLED, including light-emitting layers that do not contain QDs, charge-transporting layers, charge-blocking layers and charge-injecting layers, may he formed by any process including formation by evaporation and by deposition of a formulation containing the components of the layer dispersed or dissolved in one or more solvents.

Devices of the invention contain at least one QD layer having a layer formed thereon by deposition of a formulation.

Methods for depositing formulations include coating and printing methods. Exemplary coating methods include spin-coating. dip-coating, doct.or bla&e coating and roll coat.ing.

Exemplary printing methods include inkjet printing and Ilexographie printing.

Ouantum dots [ftc light-emitting quantum dots as described herein comprise light-emitting nanoparticles, in particular inorganic semiconductor nanocrystals.

Exemplary quantum dots include Ill-V and ti-VI semiconductors, for example gallium nitride, tin oxide, zinc oxide, zinc sulfide. cadmium sulfide, cadmium selenide, and lead oxide, and the like. TI-VT semiconductors are preferred.1 Quantum dots are nanometer sized particles thai can have optical properLies arising from quantum confinement. Quantum dots can emit light when subjected to a stimulating radiation.

The particular composition(s), sbucture, 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 specUum by changing their size. See C. B. Murray, CR. Kagan, and MG.

Bawendi, Annual Review qf Material Sri.. 2000, 30: 545-6T0 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). and preferably in a range from about 1 to about 100 nm. Quantum dots have an average particle sue in a range Ironi 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 20 nni); for example, quantum dots have an average particle size in a range from about 1 to about 10 nm. Quantum dots can have an average diamcter less than about 150 Angstroms (A). In certain embodiments, quantum dots having an average diameter in a range from about i2 to about ISO A can be particularly desirable. However, depending upon the composition, structure, and desired eniission wavelength of the quantum dot, the average diameter may be outside of thcsc ranges.

A quantum dot can comprise one or more senüconductor materials. Examples of inorganic semiconductor materials that can he included in a quantum dot. (including, e.g., semiconductor nanocrystal) include, but are not limited to, a Group IV element, a Group II-VT compound, a Group TI-V compound, a Group Ill-VT compound, a Group Ill-V compound, a Group IV-VI compound, a Group 1-111-VI compound. a Group 11-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, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe, GaSh, HgO. HgS. llgSe, HgTe, InAs. InN, TnP. TnSh, AlAs, A1N, AlP, A1Sh, FIN, liP, FlA,IiSh, PhO, PbS, PhSc, Phie, 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.

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 the outer surface of the core. A quantum dot including a core and shell is also referred to as a "core/shell" structure. Examples 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 seniiconductor materials on a surlace ol the core. Tn a core/shell quantum 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.

The overcoating has a thickness ftoni about one to about ten nionolayers. An overcoating can also have a thickness greater than ten monolayers. More than one overeoating can he included on a core.

A surrounding "shell" material can have a band gap greater than the band gap of the core material. In certain other embodiments. the surrounding shell material can have a hand gap less than the band gap of the core material.

A shell can further he chosen so as to have an atomic spacing close to that of the "core" substrate. In certain other embodiiuents. the shell and core materials can have the same crystal stracture.

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

QDs may he core-shell materials having an emissive core, for example (TdSe, coated with a higher handgap material, for example ZnS. Other examples include, without limitation: (CdSe)CdZnS, CdZnSe)CdZnS, and CdS)CdZnS (core)shell quantum dot.s.

Quantum dots can he commercially purchased or can he prepared by known methods. (I)ne example of a preferred method of making quantum dots is a colloidal growth process.

B

The inorganic surface of a QD may be substituted with one or more ligands. The inorganic surface may be a nanopartiele core or a shell coating of a nanopartiele core. Exemplary ligands include: -0-20 ailcyl wherein one or more non-adjacent. C atoms of the alkyl group may he replaced with 0, S, -NR-, SiR2, C(=O) or COO, wherein R in each occurrence is independently a Cl-20 hydroearhyl, preferably C1-20 alkyl; -aryl or heteroaryl, such as phenyl or fluorene, that may be unsubstituted or substituted with one or more substituents, for example one or more C 1-20 alkyl groups; and -a chain of aryl or hcteroaryl groups as described above, for example a C2-10 oligorneric chain of aryl or heteroaryl groups.

Ligands may be used to improve the solution processing characteristics of QDs. For example, use of alkyl substituents may improve dispersion of QDs in non-polar solvents such as tolucne or xylene.

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

A coordinating solvent is a compound having a donor lone pair that, for example, a lone electron pair available t.o coordinate to a surface of the growing quantum dot (including, e.g., a semiconductor nanoerystal).

A quantum dot surface that includes ligands derived from the growth process or otherwise can be modified by repeated exposure to an excess of a competing ligand group (including, e.g., but not limited to, coordinating group) to forni an overlayer. For example, a dispersion of the capped quantum clots can he treated with a coordinating organic compound. such as pyridine, to produce erystallites which disperse readily in pyridine, methanol, and aromaties hut no longer disperse in aliphatic solvents. Such a surface exchange process can be 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 terniinal.e in a moiety having an affinity for a suspension or dispersion medium. Such affinity improves the stability of the suspension and discourages flocculation 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 carboxylic acids, alkyl phosphines, alkyl phosphine oxides, alkyl phosphonic acids, or ailcyl phosphinic acids, pyridines, furans, and aniines. More specific examples include, hut are not limited to, pyridine, tri-n-octyl phosphine (IDP), tn-n-octyl phosphine oxide (TOFU) and tnis-hydmxylpropylphosphinc (tIIUP). More specific examples include, but are not. limited to. pyridine, tni-n-oct.yl phosphine (TOP), tri-n-octyl phosphine oxide (I'OPO), primary amines, e.g., CI-l3(Cl-l2)Nl-l2 wherein n = 4-19 (e.g., hutylaniinc, pcntylaminc, hexylaminc, hcptylamine, octylaminc, nonylamine, dccylaminc, undecylamine, dodecylamine, tnidecylamine, tetradecylarnine, pentadecylamine, hexadecylamine, heptadecylaniine, octadecylarnine, nonadecylaniine, eicosylaniine), secondary amines, c.g., (CH1(CH9)),Nll wherein n = 3-11 (e.g., dihutylamine, dipentylamine, dihexylamine, diheptylamine, dioctylanrine, dinonylamine, didecylamine, didundecylamine, didodecylamine), phenylbut.yl amine, 4-phenyihutyl amine, 3,3-diphenylpropylamine, (2,3-diphenvlpropyl)a.mine, oleie acid, hexylphosphonic acid, tciradceylpliosplioiuc acid, ocy1phosplionic acid, oc(adeeylphosphonic acid, propylencdiphosphonic acid, phenylphosphonic acid, , phenyiphosphonic acid, arninohexylphosphonie acid, henzylphosphonic acid, etc. Other ligands can he readily ascertained by the skilled artisan.

If a QD is used with an organic host material, substituents of the QD may be selected so as to enhance compatibility of the QD and the host material. For example, a QD having fluorene or oligofluorene suhstituents may he used in combination with a polyfluorene host material.

QDs preferably have an average site in the range of 1-20 mu.

A QI) may have a singlet excited energy level and triplet excited energy level, wherein fluorescence may arise from decay of a singlet cxciton from the singlet excited energy level and, according to quantum mechanical selection rules, triplet excitons do not undergo luminescenl decay from the triplet excion energy level. However, in reality, the triplet, state may emit (phosphoresce), hut this phosphorescence may occur with a eiy low probability due to triplet excitons being very long lived or metastable.

The singlet and triplet excited energy levels in a QI) may have a very small energy separation, e.g.. comparable with kT. Because of this small separation, and taken together with Ihe relalively long lifetime of triplet excitons, triplet excitons of the QD may be excited from the triplet excited state up to the singlet excited state from where they can eniit (fluorcsce).

Since the triplet may in reality emit (phosphoresce), the negligible energy diffcrencc between the singlet and triplet excited states may mean that the actual emission from QD is an indistinguishable combination of tluoreseence and phosphorescence. Thus, while the cmissivc statc of thc QIJ may hc an excited state having singlet character, in view of the above, it may further have sonic triplet character.

QDs may also emit from defect st.ates, surface st.ates and from shell states andlor core states.

A QD may emit red, green or blue light. The colour of light emitted by a quantum dot niay depend at least in part on the material it is formed from and the size and shape of the quantum dot, and so the colour of a QD may he obtained by selecting the material of the QD and controlling its size and shape during QD production.

A blue emitting material may have a photolumninescent. spectrum wit.h a peak in the range of 400-490 nm. optionally 420-490 mn.

A green emitting material may have a photoluminescent spectrum with a peak in the range of more than 490nm up t.o 580 nm, optionally more than 490 mu up t.o 540 nm A red emitting material may optionally have a peak in its photoluminescent spectum of more than 580 nm up to 630 mu, optionally 585 nm up to 625 mn.

Charge-Transporting Materials Charge transporting materials may he used in devices of the invention in combination with QDs in a QD emitting layer; as host materials used in combination with a fluorescent or phosphorescent organic light-emitting material in an organic light-emitting layer; or as a charge-transporting material of a non-eniissive charge-transporting layer.

Charge-transporting materials nuay be small molecule materials or may be polymeric materials. Polymeric charge uansporting materials may have a non-conjugated backbone having charge-transporting repeat units pendant from the charge-transporting backbone, or may have an at least partially conjugated backbone with charge-transporting repeat units provided as repeal units within the polymer backbone.

A polymeric charge-transporting material may contain individual repeal units or chains of conjugated repeat units thai provide charge-transporting functionality. hole-transporting functionality may be provided by amine repeat units, as described in more detail below.

Electron transporting functionality may be provided by individual electron transporting units or may he provided by a conjugated chain of arylene repeat units, for example a conjugated chain of phenylene and / or tluorene repeat units as described below.

A hole transporting material may have a low electron affinity (2eV or lower) and low ionisation potential (5.8 cv or lower, preferably 5.7 cv or lower, more preferred 5.6 cv or lower).

An electron-transporting unit may have a high electron affinity (1.8 cv or higher, preferably 2ev or higher, even more preferred 2.2ev or higher) and high ionisation potential (5.8ev or higher) Suitable electron transport groups include groups disclosed in, for example, Shirota and Kageyama, Chcm. Rev. 2007, 107, 953-1010, Electron affinities and ionisation potentials niay he measured by cyclic voltamnietry (CV) wherein the working electrode potential is ramped linearly versus time.

When cyclic voltanimetry reaches a set potential the working electrode's potential ramp is inverted. I'his inversion can happen multiple times during a single experiment. Ihe current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace.

Apparntus to measure 1-IDMO or I lIMO energy levels by CV may comprise a cell containing a tert-hutyl amnionium perchlorate/ or tertbu tyl ammoniu m hexafluorophosphate solution in acetonitrile. a glassy carbon working electrode where the sample is coated as a film, a platinium counter electrode (donor or acceptor of electrons) and a reference glass electrode no leak Ag/AgC1. Ferrocene is added in the cell at the end of the experiment for calculation purposes. (Measurement of the difference of potential between Ag/AgCl/fermcene and saniple/Fen'ocene).

Method and settings: 3mm diameter glassy carbon working electrode Ag/AgCl/no leak reference electrode Pt wire auxiliary electrode 0.1 M tetrabutylanunonium hexalluorophosphate in acetonitrile LIJMO = 4.5 -ferrocene (peak to peak maximuni average) + onset Sample: I drop of 5mg/mL in toluene spun @3000rpm LUMO (reduction) measurement: A good reversible reduction event is typically ohscrvcd for thick films measured at 200 mV/s and a switching potential of 2.5'sT. The reduction events should be nieasured and compared over 10 cycles, usually measurements are taken on the cycle. The onset is taken at the intemeetion of lines of best fit at the steepest part of the reduction event and the baseline.

During device fabrication, if a layer is to he formed by depositing a fonnulation of charge- transporting and I or light-emitting materials over an underlying layer containing a charge-transporting material then the charge-transporting material may be crosslinked to avoid dissolution of thc underlying layer upon deposition of the formulation used to form the overlying layer. In one arrangement, erosslinking may be achieved by providing a crosslinkable compound in combination with the other components of the underlying layer.

In another, preferred arrangement the charge-transporting material of an underlying layer is substituted with a crosslinkable group that undergoes crosslinking upon thernial trcatnmcnt or irradiation. Exemplary crosslinkable groups include groups containing a double bond such as a vinyl or aerylate unit, and groups containing a benzocyclobut.ane unit.

If a charge-Lransporting niaterial is used as a host material for an organic light-emitting material, or in combination with a QI) light-emitting material, then the charge transporting material preferably has an excited state energy level that is no more than 0.1 cv lower than, and preferably at least the same as or higher than, the corresponding excited state energy level of the light-emitting material that the charge-transporting material is used with, such that excitons may he transferred from the excited state energy level of the charge-transporting material to that of the light-enutting material and such thai there is little or no downconversion or quenching of excitons formed on the light-emitting material.

The relevant excited state energy level is the lowest singlet excited state energy level S1 in the case of a fluorescent. organic light-emitting material, and is the lowest triplet excited state energy level Ii in the ease of a phosphorescent organic light-emitting material. A charge-transporting material used with a QD light-emitting material preferably has both Si and Ti levels higher than that of the QD light-emitting material (in which Si and T1 levels are commonly close together, as described above).

Exemplary host polymers include polymers having a non-conjugated backbone with charge-transporting groups pendant from the non-conjugated backbone, for example poly(vinylcathazole), and polymers comprising conjugated repeal units in the backbone of the polymer.

Exemplary repeal units ol a conjugated polymer include optionally substituted iionocyclic and polycyclic arylene repeat units as disclosed in for example, Adv. Mater. 2000 12(23) 1737-1750 and include: 1,2-, 1,3-and 1,4-phenylene repeat units as disclosed in J. AppI.

Phys. 1996, 79, 934; 2,7-Iluorene repeat. units as disclosed in EP 0842208; indenofluorene repeat unit.s as disclosed in, for example, Macromolecules 2W0, 33(6), 2016-2020; and spirofluorene repeat units as disclosed in, for example I iP 0707020. I lach of these repeat units is optionally substituted. Examples of substitucnts include soluhilising groups such as C120 alkyl or alkoxy; electron withdrawing groups such as Iluorine, niEro or cyano; and substituents for increasing glass transition temperature çI'g) of the polymer.

One exemplary class of arylene repeat units is optionally substituted fluorene repeat units, such as repeat units of formula TV: R1 R1 (TV) wherein R' 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 R1 is preferably a substituent, and each R1 may independently he selected from the group consisting ol: -optionally substituted alkyl, optionally C120 alkyl, wherein one or more non-adjacent C atoms may he replaced with optionally substituted aryl or heteroaryl, (i) S, -optionally suhstiwted aryl or hetcroaryl; -a linear or branched chain of aryl or heteroaryl, each of which groups may independently be substituted, for example a group of fonnula -(Ar6), as described below with reference to formula (VU); and -a erosslinkable-group, for example a group comprising a double bond such and a vinyl or aerylate group, or a benzoeyclobutane group.

in the case where R' comprises aryl or heteroaryl ring system, or a linear or branched chain of aryl or heteroarvl ring systems. the or each aryl or heteroaryl ring system may be substituted with one or more suhstituents R3 selected from We group consisting of: alkyl, for example C120 alkyl. wherein one or more non-adjacent C atoms may he replaced with 0. 5, substituted N, C=() and -COO-and one or more II atonis of the alkyl group may be replaced with F or aryl or heteroaryl optionally substituted with one or more groups aryl or heteroaryl optionally substituted with one or more groups NR52, OR5, SR5. and fluorine, nitro and eyano; wherein each R4 is independently alkyl, for example C120 alkyl, in which one or nìore non-adjacent C atoms may he replaced with 0, S, substituted N, C=O and -COO-and one or more II atoms of the alkyl group may he replaced with F, and each R5 is independently selected from the group consisting of alkyl and aryl or het.emaryl optionally substitut.ed with one or more alkyl groups.

Optional substituent.s for the fluorene unit, other than subst.it.uent.s R1, are preferably selected from the group consisting of alkyl, for example C1-20 alkyl, wherein one or more non-adjacent C atoms may he replaced with 0, 5, Nil or substituted N, C=O and -COO-, optionally substituted aryl. optionally substituted heteroaryl, alkoxy. alkylthio. fluorine, cyano and arylalkyl. Particularly preferred substituents include C120 alkyl and substituted or unsubstituted aryl, for example phenyl. Optional suhstituents for the aryl include one or more C171 alkyl groups.

Where present, substituted N may independently in each occurrence be NR6 wherein R6 is alkyl, optionally Ciio alkyl, or optionally substituted aryl or heteroaryl. Optional substituents for aryl or heteroaryl R6 may he selected from R4 or R5.

Preferably, each R' is selected from the group consisting of C1.20 alkyl and optionally substituted phenyl. Optional substituents for phenyl include one or more C 1-20 alkyl groups.

If the compound of fonnula (I) is providcd as a side-chain of the polymer then at least. one R' may comprise a compound of formula (I) that is either bound directly to the 9-position of the fluorene unit or spaced apart from the 9-position by a spacer group.

The repeat unit of formula (IV) may he a 2,7-linked repeat unit of formula (IVa): R1 R1 (TVa) Optionally, the repeat unit of formula (Wa) is not substituted in a position adjacent to the 2-or 7-positions.

The extent of conjugation of repeat units of formulae (IV) may he limited by (a) linking the repeat unit through the 3-and / or 6-positions to limit, the extent of conjugation across the repeat unit, and! or (h) substituting the repeat unit with one or more further substituents It' in or more positions adjacent to the linking positions in order to create a twist with the adjacent repeat unit, or units, for example a 2,7-linked fluorene carrying a C -20 alkyl suhst.it.uent in one or both of the 3-and 6-positions.

Another exemplary class of arylene repeat units is phenylene repeat. units, such as phenylene repeat units of formula (V): (V) whercin p isO, 1, 2, 3 or4, optionally I or 2, and R2 indcpcndcntly in cach occurrcncc is a substituent, optionally a substituent R1 as described above, for example C,20 alkyl, and phenyl that is unsubstituted or substituted with one or more C,-20 alkyl groups.

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

If the repeat unit of fonnula (V) is 1,4-linked and if p is 0 then the extent. of conjugation of repeat unit of formula (V) to one or both adjacent repeat units may he relatively high.

If p is at least. 1, and I or the repeat. unit is 1,2-or 1,3 linked, then the extent of conjugation of repeat unit of formula (V) to one or both adjacent repeat units may he relatively low. In one preferred arrangement, the repeat unit of formula (V) is 1,3-linked and p isO, 1,2 or 3. In another preferred arrangement, the repeat unit of formula (V) has formula (Va): (Va) Charge-transporting units CT of a charge-transporting polymer may be hole-uansporting units or electron transporting units.

Exemplary hole-transporting units CT include optionally substituted (hetero)aiylamine repeat units, for example repeat. unit.s of formula (VI): ( (Ar (N_(Ar5))) (VI) wherein Ar4 and A? in each occurrence are independently selected from optionally suhslilutcd aryl or heteroaryl, n is greater than or equal to 1, preferably 1 or 2. R5 is II or a substituent, preferably a substituent. and x andy are each independently 1,2 or 3.

Ar4 and Ar5 may each independently be a monocyclic or fused ring system.

R8, which maybe the same or different in each occurrence when n> 1, is preferably selected from the group consisting of alkyl. for example C120 alkyl, Ar6, a branched or linear chain of Ar6 groups, or a crosslinkable unit that is bound directly to the N atom of formula (VI) or spaced apart therefrom by a spacer group, wherein Ar6 in each occurrence is independently ophonally substituted aryl or heteroaryl. Exemplary spacer groups are as described above, for example C1.20 alkyl, phenyl and phenyl-C 120 alkyl.

Ar° groups may be substituted with one or more substituents as described below. An exemplary branched or linear chain of Ar6 groups may have formula -(Ar6X, wherein Ar6 in each occurrence is independently selected from aryl or heteroaryl and r is at least I, optionally 1, 2 or 3. An exemplary branched chain of Ar6 groups is 3,5-diphenylhenzene.

Any of Ar4, Ar5 and Ar6 may independently be substituted with one or more substituents.

Preferred substitucnts arc sclccted from the group 1( consisting of: alkyl, for example C120 alkyl, wherein one or more non-adjacent. C atoms may be replaced with 0, 5, substituted N, C=O and -COO-and one or more I-I atoms of the alkyl group may he replaced with F or aryl or heteroaryl optionally substituted with one or more groups R4.

aryl or het.eroaryl optionally substituted with one or more groups R4.

NR52, OR5, SR5.

fluorine, nitro and eyano; wherein each R4 is independently alkyl, for example C120 ailcyl, 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 U atoms of the alkyl group may he replaced with I, and each R5 is independently selected from the group consisting of alkyl and aryl or hctcroaryl optionally substituted with onc or more alkyl groups.

Any of Ar4, A? and, if present, Ar6 in the repeat unit of 1"ormula (VI) may he linked by a dnect bond or a divalent. linking atom or group to another of A?, Ar5 and Ar6. Preferred divalent linking atoms and groups include 0, 5; substituted N; and substituted C. Where prcscnt, substituted N or substituted C of R3. R4 or of the divalcnt linking group may independently in each occurrence he NR6 or CR62 respectively wherein R6 is alkyl or optionally substituted aryl or heteroaryl. Optional substituents For aryl or heteroaryl R6 may he selected from R4 or R5.

Tn one preferred arrangement, Rs is Ar6 and each ol Ar4, Ar5 and Ar6 are independently and optionally substituted with one or more C120 alkyl groups.

Particularly preferred units satisfying Formula (VI) include units of Formulae 1-4: Ac /r5) ( Ac/Ar5) N Ar5-N / I Ar6 Ar6 Ar6 1 2 (J)Ar5) (Ac/AISt Ar6 Ar6 NR52 3 4 Where present, preferred substituents for Ar6 include substituents as described forAr4 and Ar5. in particular alkyl and alkoxy groups.

Ar1, A? and Ar6 are preferably phenyl, each of which may independently he substituted with one or more substituents as described above, In anothcr preferred arrangement, Ar4, Ar4 and Ar6 arc phcnyl, each of which may be substiluled with one or more C120 alkyl groups, and r 1.

In another preferred arrangement, Ar4 and Ar4 are phenyl, each of which may he substituted with one or more C12o alkyl groups, and R8 is 3.5-diphenylbcnzenc wherein each phenyl may be substituted with one or more Ci.20 alkyl groups.

In another preferred arrangement, n, x and y are each I and Ar4 and Ar are phenyl linked by an oxygen atom to form a phenoxazine ring.

Triazines form an exemplary class of electron-transporting units, for example optionally substituted di-or tri-(hetero)aryltriaiine attached as a side group through one or the (hetero)aryl groups. Other exemplary electron-transporting units are pyrimidines and pyridines; sulfoxides and phosphine oxides; benzophenones; and horanes, each of which may he unsuhstiiuted or subsdtuted with one or more substituents, for example one or more (120 alkyl groups.

Exemplary electrun-transporting units CT have formula (XIV): f-(VII) wherein Ar4, Ar5 and Ar6 are as described with reference to formula (VI) above, and may each independently he substituted with one or more suhstituents described with reference to Ar1, Ar5 and Ar6, and 7 in each occurrence is independently at least I, optionally 1, 2 or 3 and Y is N or CR7, wherein R' is H or a substituent, preferably H or C110 alkyl.. Preferably, Al.

Ar5 and Ar6 of formula (VII) are each phenyl, each phenyl being optionally and independently substituted with one or more C 120 alkyl groups.

In one preferred embodiment, all 3 groups Y are N. If all 3 groups Y are CR7 then at least one of Ar', Ar2 and Al is preferably a heteroaromatic group comprising N. Each of Ar4. Ar5 and Ar6 may independently he substituted with one or more substituents. In one arrangement, Ar4, A? and Ar6 are phenyl in each occurrence. Exemplary substituents include k3 as described above with reference to formula (VII), for example Ci2øalkyl or alkoxy.

Ar6 of formula (VII) is preferably phenyl, and is optionally substituted with one or more C1.20 alkyl groups or a erosslinkable unit. The crosslinkable unit may or may not he a unit. of formula (I) bound directly to Ar6 or spaced apart front Ar6 by a spacer group.

The charge-transporting units CT may be provided as distinct repeat units formed by polymerising a corresponding monomer. Alternatively, the one or more CT units may form part of a larger repeat unit, for example a repeat unit of formula (VIII): f (Ar3)q-Sp-CT-Sp-(Ar3)q) (VIII) wherein CF represents a conjugated charge-transporting group; each Ar' independently represents an unsubstituied or substituted aryl or heLeroaryl; q is aL least 1; and each Sp independently represents a spacer group fornnng a break in conjugation between A? and CT.

Sp is preferably a branched, linear or cyclic C120 alkyl group.

Exemplary CT groups may he units of formula (VI) or (VII) described above.

Ar' is preferably an unsubstituted or substituted aryl, optionally an unsubstituted or substituted phenyl or fluorene. Optional substituents for Ar3 may he selected from R' as deescribed above, and are preferably selected from one or more C,20 alkyl substit.uen.s.

q is preferably 1.

The polymer may comprise repeat units that. block or reduce conjugation along the polymer chain and thereby increase the polymer handgap. I"or example, the polymer may comprise units that arc twisted out of the planc of the polymcr hackhonc, rcducing conjugation along (he polymer backbone, or units that. do not provide any conjugation path along the polymer backbone. Exemplary repeat units thai reduce conjugation along the polymer backbone are substituted or unsuhstituted 1,3-substituted phenylene repeat units, and I,4-phenylene repeat substituted with a C120 alkyl group in the 2-and / or 5-position, as described above with reference to formula (V).

Preferrcd methods for preparation of conjugatcd polymers, such as polymers comprising one or more of repeat units of formulae (IV), (V), (VI), (VII) and (VIII) as described above, comprise a "metal insertion" wherein the metal atom of a metal complex catalyst is inserted between an aryl or het.eroaryl group and a leaving group of a monomer. Exemplary metal insertion methods are Suzuki polymerisation as described in, for example, WO @0/53656 and Yamamoto polymerisation as described in, for example, T. Yamamoto, "Electrically Conducling And Thermally Stable pi-Conjugated Poly(arylene)s Prepared by Organoinetallic Processes", Progress in Polymer Science 1993, 17, 1153-1205. In the case of Yamamoto polymerisation, a nickel complex catalyst is used; in the case of Suzuki polymerisation, a palladiutn complex catalyst is used.

For example, in the synthesis of a linear polymer by Yarnamoto polymnerisat.ion, a monomer having two reactive halogen groups is used. Similarly. according to the method of Suzuki polymerisation. at least one reactive group is a boron derivative group such as a horonic acid or horonic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine.

It will therefore he appreciated that repeat units illustrated throughout this application may be derived from a monomer canying suitable leaving groups. Likewise, an end group or side group may he hound to the polymer by reaction of a suitable leaving group.

Suzuki polymerisation may he used to prepare regioregular, block and random copolynicrs.

In particular, honiopolymers or random copolymers may he prepared when one reactive group is a halogen and the other reactive group is a boron derivative group. Alternatively, block or regioregular copolymers may he prepared when both reactive groups of a first monomer are boron and both reactive groups of a second monomer are halogen.

As alternatives to halides, other leaving groups capable of participating in metal insertion include sulfonic acids and sulfonic acid esters such as tosylate, mesylate and triflate.

Organic Light-Emitting Materials Organic light-emitting materials that may he used in combination with QD light-emitting materials in devices of the invention include small molecule, dendrimeric and polymeric light-emitting materials. Organic light-emitting materials may he fluoresecnt or phosphorescent materials.

Exemplary phosphorescent organic light-emitting materials are phosphorescent metal complexes, for example substituted or unsubstitut.ed complexes of fommula (IX): ML'qL2rL3s (DC) wherein hi is a metal; each of L1, L2 and L3 is a coordinating group; q is an integer; rand s arc each independently 0 or an integer; and the sum of(a. q) + (b. r) + (c.s) is equal to the number of coordination sites available on M. wherein a is the number of coordination sites on L'. b is the number of coordination sit.es on L2 and c is the number of coordination sites on I? Heavy elements M induce strong spin-orbit, coupling t.o allow rapid intersystem crossing and emission from triplet or higher states, Suitable heavy metals M include d-block metals, in particular those in rows 2 and 3 i.e. elements 39 to 42 and 72 to 20, in particular ruthcnium, rhodium, palladium, rhenium, osniiuni, iridium, platinum and gold. Iridium is particularly preferred.

Exemplary ligands L1, L2 and L3 include carbon or nitrogen donors such as porphyrin or bidentale ligands of fonnula (X): r6 (X) wherein Ar5 and Ar6 niay he the same or different and are independently selected from substituted or unsubstituted aryl or heteroaryl; X1 and Y1 may he the same or different and arc indcpcndently sclected from carbon or nthogcn; and A? and Ar6 may he fused together.

Ligands wherein X1 is carbon andY' is nitrogen are preferred, in particular ligands in which A? is a single ring or lused heteroaromatic ol N and C atoms only, br example pyridyl or isoquinolinc, and Ar6 is a single ring or fused aromatic, for example phenyl or naphthyl.

Examples of bidcntate ligands are illustrated below: & Each of Ar5 and Ar6 may carry one or more suhst.it.uents. Two or more of these suhstituents may be linked to form a ring, for example an aromatic ring.

Other ligands suitable for use with d-hlock elements include diketonates, in particular acetylacetonate (acac); triarylphosphines and pyridine, each of which may he substituted.

Exemplary substituents include groups as described above with reference to Formula (VI).

Particularly preferred substituents include fluorine or trifluoromethyl which may he used to blue-shift the emission of the complex, for example as disclosed in WO 02/45 466, WO 02/44189, TJS 2002-117662 and TJS 2002-1 82441; alkyl or alkoxy groups, for example C1,0 alkyl or alkoxy, which may he as disclosed in JP 2002-324679; carhazole which may he used to assist hole transport to the complex when used as an eniissive niaterial, for example as disclosed in W() 02/Si44; and dendrons which may be used to obtain or enhance solution processahility of the metal complex, for example as disclosed in WO 02/66552.

A light-emitting dendrimer typically comprises a light-emitting core bound to one or more dendrons, wherein cach dendron comprises a branching point and two or more dendritic branches. Preferably, the dendron is at least partially conjugated, and at least one of (he branching points and dendritic branches comprises an aryl or heteroaryl group, for example a phenyl group. In one arrangement, thc branching point group and thc branching groups arc all phenyl, and each phenyl may independently he substituted with one or niore suhstituenis, for example alkyl or alkoxy.

A dendron may have optionally substituted formula (XI) / BP\ (XI) wherein BP represents a branching point for attachment to a core and 61 represents first generation branching groups.

The dendron may be a first, second, third or higher generation dendron. 6 maybe substituted with two or more second generation branching groups 2, and so on, as in optionally substituted formula (XIa): p pJG3 j/2fG3 8/ G2c \ L\G<FG3 -u

V (XIa)

wherein u is 0 or 1; v is 0 if u is 0 or may be 0 or 1 if u is 1; BP represents a branching point for attachment to a core and G1. G2 and G3 represent first, second and third generation dendron branching groups. In one preferred embodiment, each of BP and Ui, U... G is phcnyh and each phenyl HP, 0, 02 (3, is a 3,5-linked phenyl.

A preferred dcndron is a substituted or unsubstituted dcndron of formula (XIh): (XIh) wherein * represents an attachment pint of the dendron to a core.

HP and I or any group U may he substituted with one or more substituents, for example one or more (71.20 alkyl or alkoxy groups.

Phosphorescent light-emitting materials may he provided in a light-emitting layer with a host polymer of the invention.

The phosphorescent light-emitting material may he physically mixed with the host polymer or may be covalenily bound thereto. The phosphorescent light-emitting material may be provided in a side-chain, niain chain or end-group of the polymer. Where the phosphorescent material is provided in a polymer side-chain, the phosphorescent material may be directly bound to thc backbone of thc polynicr or spaced apart thcrcfrom by a spaccr group, for example a C12o alkyl spacer group in which one or more non-adjacent C atoms may be replaced by 0 or S. White Lthht-Emitting Device Devices of the invention may contain multiple light-emitting materials (including at least one QD material) of different colours such that light emitted from the device is white. The device may contain a blue light-emitting QI) material and onc or more further light-emitting materials selected from QD or organic light-emitting materials, including red and green light-emitting materials.

The light emitted from a white OLED may have CIE x coordinate equivalent to that emitted by a black body at a tetnperaturc in the range of 2500-9000K and a CIE y coordinate within 0.05 or 0.025 of the CIE y co-ordinate of said light emitted by a black body, optionally a CIE x coordinate equivalent to that emitted by a black body at a temperature in the range of 2700-4500K.

Charge transporting and charge blocking layers A hole transporting layer may be provided between the anode and the light-emitting layer or layers of light-emitting devices of the invention, Likewise, an electron transporting layer may he provided between the cathode and the light-emitting layer or layers.

Similarly, an electron blocking layer may he provided between the anode and a light-emitting layer and a hole blocking layer may be provided between the cathode and a light-emitting layer. Transporting and blocking layers may be used in combination. Depending on its HOMO and LUMO levels, a single layer may both transport one of holes and electrons and block the other of holes and electrons.

A hole transporting layer may contain a hole-transporting (hetem)arylamine, such as a homopolynier or copolymer comprising hole transporting repeat units of formula (VI).

Exemplary copolymers comprise repeat units of formula (VI) and optionally substituted (hetero)aiylene co-repeat. units, such as phenyl, fluorene or indenofluorene repeat. units as described above, wherein each of said (hetero)arylene repeat units may optionally he substituted with one or more substituents such as alkyl or alkoxy groups. Specific co-repeat units include Iluorene repeat units of formula (IV) and optionally substituted phenylene repeat units of formula (V) as described above. A hole-transporting copolynier containing rcpcat units of formula (VI) may contain 25-95 mol % of repeat units of formula (VI).

An electron transporting layer may contain a polymer comprising a chain of optionally substituted aiylene repeat units, such as a chain of fluorenc repeat units.

hole iniection layers A conductive hole injection layer, which may be formed from a conductive organic or inorganic material, may he providcd on the anodc to assist hole injection from the anode into (he layer or layers of seniiconduct.ing polymer. A hole transporting layer may be used in combination with a hole injection layer.

Examples of doped organic hole injection materials include optionally substituted, doped poly(etliylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in liP 0901176 and liP 0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion ®; polyaniline as disclosed in US 5723873 and US 5798170; and optionally substi(uted polythiophene or poly(thienothiophene). Examples of conductive inorganic materials include transition metal oxides such as VOx MoOx and RuOx as disclosed in Journal of Physics D: Applied Physics (1996), 29(11), 2750-2753.

Cathode I'he cathode is selected fmm materials that have a workfunction allowing injection of electrons into the light-emitting layer or layers. (I)ther [actors influence the selection of the cathode such as the possibility of adverse interactions between the cathode and the light-emitting materials. The cathode may consist of a single material such as a layer of aluminium. Alternatively, it may comprise a plurality of metals, for example a hilayer of a low workfunction material and a high workfunction material such as calcium and aluminium as disclosed in WI) 98/1062 1. The cathode may contain a layer containing elemental barium, for example as disclosed in WI) 98/57381, Appl. Phys. Left. 2002, 8 1(4), 634 and WO 02/84759. l'he cathode may contain a thin (e.g. 1-5 nm thick) layer of metal compound between the light-emitting layer(s) of the OLED and one or more conductive layers of the cathode. such as one or more metal layers. Exemplary metal compounds include an oxide or fluoride of an alkali or alkali earth metal,to assist electron injection, for example lithium Iluoride as disclosed in WO 00/48258; barium Iluoride as disclosed in Appl. Phys. Lea. 2001, 79(5), 2001; and barium oxide. In order to provide efficient injection of elecirons mb the device, the cathode preferably has a workfunction of less than 3.5 cv, more preferably less than 3.2 eV, most preferably less than 3 cv. Work functions of metals can he found in, for example, Michaelson. J. AppI. Phys. 4S(11), 4729, 1977.

The cathode may he opaque or transparent. Transparent cathodes are particularly advantageous for active matrix devices because emission through a transparent anode in such devices is at leasi. partially blocked by drive circuitry located underneath (he enlissive pixels.

A transparent cathode comprises a layer of an electron injecting material that is sufficiently thin to he transparent. Typically, thc lateral conductivity of this layer will bc low as a result of its thinness. In this case, (he layer of electron injecting material is used in combination with a thicker layer of transparent conducting material such as indium tin oxide.

It will be appreciated that a transparent cathode device need not have a transparent anode (unless, of course, a fully transparent device is desired), and so the transparent anode used for bottom-emitting devices may be replaced or supplemented with a layer of reflective material such as a layer of aluminium. Examples of transparent cathode devices are disclosed in, for

example, NB 2348316.

The invention is described herein with reference to examples in which one or more QD layers are lormed over an anode electrode and a cathode is formed over (he QI) layer(s), however it will be appreciated that the QD layer may be fonned over a cathode electrode and thaL an anode electrode may he formed over the QIl) layer(s).

Encapsulation Organic optoeleetronie devices tend to he sensitive to moisture and oxygen. Accordingly, the substrate 1 preferably has good harrier properties for prevention of ingress of moisture and oxygen into the device. The suhstrat.e is commonly glass, however alternative substrates may be used, in particular where flexibiliby of the device is desirable. For example, the substrate may comprise a plastic as in US 6268695 which discloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in I iP 0949850.

The device may he encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having suitable barrier properties such as silicon dioxide, silicon monoxide, silicon nitride or alternating stacks of polymer and dielectric as disclosed in, for example, WC) 01/81649 or an airtight container as disclosed in, for example, WO 01/19142. In the case of a transparent cathode device, a transparent encapsulating layer such as silicon monoxide or silicon dioxide may be deposited to micron levels of thickness, although in one preferred embodiment the thickness of such a laycr is in the range of 20-300 nm. A gcttcr material for absorption of any atmospheric nioisture and / or oxygen that niay permeate through the substrate or encapsulant may he disposed between the substrate and the encapsulant.

Solution processing Solvents for forming solution processable formulations of QDs, chargc-transporting materials, organic light-emitting materials and compositions (hereof may he selected from common organic solvents, such as mono-or poly-allcylbenzenes such as toluene and xylene.

Exemplary solution deposition techniques for forming layers of devices of the invention include printing and coating techniques such spin-coating, dip-coating, roll-to-roll coating or roll-to-roll printing, doctor blade coating, slot die coating, gravure printing, screen printing and inlçjet printing.

Coating methods, such as those described above, are particularly suitable for devices wherein patterning of the light-emitting layer or layers is unnecessary -for example for lighting applications or simple monochrome segmented displays.

Printing is particularly suitable for high information content displays, in particular full colour displays. A device may he inkjet printed by providing a patterned layer over the first electrode and defining wells for printing of one colour (in the case of a monochrome device) or multiple colours (in the case of a multicolour, in particular full colour device). The patterned layer is typically a layer of photoresist that is patterned to define wells as described in, for example, EP 0880303.

As an alternative to wells, the ink may he printed into channels defined within a patterned layer. In particular, the photoresist may he patterned to form channels which, unlike wells, extend over a plurality of pixels and which may be closed or open al the channel ends.

The same coating and printing methods may be used to fonii other layers of an OLED including (where present) a hole injection layer, charge transporting layers and organic light-emitting layers.

Examples

01) layer stability exijerinient (I) -01) / ixlyrner layer A formulation of quantum dot.s and a erosslinkable hole-transporting polymer (3: 1 QD polymer weight ratio) in mixed xylenes was spin-coated onto quartL glass. The solvent, was evaporated and ihe polymer was crosslinked by thennal treatment. The QD polymer layer was rinsed with mixed xylenes. Surprisingly, the Qi) layer remains in place despite being rinsed in the solvent that was used to deposit it and despite the high ratio of QD material to polymer material.

Figure 2 shows thai ite photolumineseent intensity of the layer is very similar before and after QI) layer formation, indicating little loss of QDs from the QI) layer upon rinsing.

Likewise, Figure 3 shows substantially the same degree of absorhance by the QD layer upon rinsing.

The hole-transporting polymer used to form the composition producing the spectrum of Figure 2 was formed by Suzuki polymerisation as described in WO 00/53656 of the following monomers: Br-.%ç- C H _/LN)%) -BrBr mol % 10 mol % S mol % BrN-CB1

Y mo! %

The hole-transporting polymer used to form ite composition producing the spectrum or Figure 3 was fonned by Suzuki polymerisation as described in W() 00/53656 of the following monomers: Br&N\/5QBr SOniol% 42.Smol% \ / Br \ I \ / Br 7.5 mol % OD layer stability experiment (ii) -neat (3D layer on thass A QD layer was formed as described with reference to QD layer stability experiment (i), except that the formulation and resultant layer did not contain any polymer and the resultant layer was a neat QD layer. Surprisingly, the neat QD layer remains in place despite being rinsed in the solvent that was used to deposit it and despite the absence of a crosslinked matrix or other material to prevent the QDs from being washed away.

The photoluminescent spectra and absorbance spectra of the QD layer before and after rinsing of Figures 4 and 5 respectively show that the QD layer remains after rinsing.

01) layer stability (iii) -neat 01) layer on polymer layer A crosslinkable hole-transporting polymer was spin-coated onto quartz glass and cross-linked. A neat QD layer was formed on the crosslinked polymer layer as described in QD layer stability (iii). Surprisingly, the QD layer remained in place upon rinsing with tnixed xylenes despite being rinsed in the solvent that was used to deposit it and despite the high rat.io of Q[) material to polymer material.

The absorption spectra of Figure 6 show a proportion of absorhance attributable to the polymer and a proportion of ahsorhance attrihutahle to the QDs remaining after rinsing.

Device Example I

A device having the following structure was prepared: ITO 111Th / IITL/ PEL I QDEL I ETh I Cathode wherein ITO is an indium-tin oxide anode supported on a glass suhsttate; IIIL is a hole injection layer of a conductive polymer available from Plextronies, mc,; HTL is a hole-transporting layer of Hole Iransporting Polymer I; P11. is a green light-emitting layer of Green Polymer 1; QDEL is a quantum dot emission layer consisting of light-emitting quantum clots fonned to a thickness of greater than 10 nm; and ETL is an electron-transporting layer of Electron Transporting Material I. Each of the HIL, Hit, PEl QDE1. and Eli. layei were formed by spin-coating a formulation containing one or more solvents and the material or materials to form each said layer, followed by evaporation of the solvent or solvents. IITL and PEL were each formed from a polymer having cmsslinkahle groups that were crosslinked by Uermal trealmeni following spin-coating of the material forming that layer in order to prevent dissolution of the layer upon formation of the overlying layers, PEI. and QDEL. The cathode was formed from a first layer of sodium fluoride, a second layer of aluminium and a third layer of silver. The QI) layer remained intact despite spin-coating of the ETL onto the QI) layer.

Hole Transporting Polymer 1 was formed by Suzuki polymerisation as described in WO 00/53656 of the following monomers: Br&N\/QB1 C5 H13 5OmoI% 42.5moI% Br ____N_-I/Br 7.5 rno % Gnen Polymer 1 was fornicci by Suzuki polymerisation as described in WO 00153656 of the following monomers in the given inoar percentages:

C H

CH

% 24 % 5% *

I \1/' 5% Br

NN \ . V

I ( K Th

AN C6H13

5% 5% 1% Elecfton-Transporting MaLerial I was formed by Suzuki polymerisation as described in WO 00/53 656 of the following monomers: C6H13 V 35mo1% l5moI% B:r > C6H13 mol Ye 10 mol % It will be understood that a fluorescent or phosphorescent red light emitting material may he added to the device as descriftd in this example to provide a white light-emitting device.

The red emitter may he pnivided in one of the light-emitting layers of the device or in a separate light-emitting layer.

Device Example 2

A device was prepared as described in Device Example 1 except that the QD light-emitting layer was formed to a thicluiess in the range of 1-10 mm Comparative I)evice A device was prepared as descnbed in Device Example I except. that the QD light-emitting layer was omitted.

With rnference to Figure 7, emission from the Q[) layer of Device Examples 1 provides blue light having a peak wavelength at around 450 nm, and light from Green Polymer 1 provides light of a wavelength having a peak of around 520 nm. With reference to the Comparative Device, it is apparent that blue light emission from Device Examples I originates from the QD layer because this blue light emission is absent from the electroluminescent spectrum ol the Comparative Device, \Vhich does not contain QDs.

As shown in Figure 7, the thickness of the QD layer may he selected to control the amount of QI) emission relative to emission from any other layer.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.

Claims (16)

1. Claims 1. A method of forming a light-emitting device, the method comprising the steps of: forming a quantum dot light-emitting layer comprising light-emitting quantum dots over an electrode for injecting charge carriers of a first type; forming an adjacent layer on the quantum dot light-emitting layer by depositing onto the quantum clot light-emitting layer a formulation comprising one or more solvents and one or more materials selected from charge-transporting and light-emitting materials, and evaporating the one or more solvents; and forming an electrode for injecting charge carriers of a second type over the adjacent layer.
2. 2. A method according to claim I wherein the quantum dot light-emitting layer comprises the light-emitting quantum dots and a charge-transporting material.
3. 3. A method according to claim 2 wherein the charge-transporting material is a polymer.
4. 4. A method according to claim 2 or 3 wherein the charge-transporting material is a crosslinked material.
5. 5. A method according to any of claims 1-3 claim wherein the quantum dot light-emitting layer is not crosslinked.
6. 6. A method according to claim 5 wherein the quantum dot light-emitting layer consists essentially of quantum dot.s including one or niore ligands attached to an outer surface.
7. 7. A method according t.o any preceding claim wherein the quantum dot light emitting layer is formed hy depositing a quantum dot formulation comprising the light-emitting quantum dots and at least one solvent, and evaporating the at least one solvent.
8. S. A method according to claim 7 wherein one or more of the at least one solvent of the quantum dot fornmlation is a non-polar solvent, and or more of the at least one solvent of the formulation used to fonu the adjacent. layer is a non-polar solvent..
9. 9. A method according t.o claim 7 or 8 wherein one or more of the at least one solvent of the quantum dot formulation is the same as one or more of the at least one solvent of the formulation used to form the adjacent layer.
10. 10. A method according to any preceding claim wherein the first electrode is an anode and the second electrode is a cathode.
11. 11. A method according to claim 10 claim wherein a conductive hole injeedon layer is formed between the anode and the first light-emitting layer.
12. 12. A method according to claim 10 or II wherein a hole transporting layer is formed between the anode and the quantum dot light-emitting layer.
13. 13. A method according to any preceding claim wherein the device comprises an organic light-emitting material.
14. 14. A method according t.o claim 13 wherein the organic light-eniitt.ing material is a phosphorescent organic light-emitting material.
15. 15. A method according to claim 13 or 14 wherein the organic light-emitting material is provided in the quantum dot light-emitting layer.
16. 16. A method according to claim 13 or 14 wherein the organic light-emitting material is provided in a 1igh-emitt.ing layer separate from the quantum dot light-emitting layer.