GB2508191A

GB2508191A - Organic light emissive device

Info

Publication number: GB2508191A
Application number: GB1221085.2A
Authority: GB
Inventors: Thomas Kugler
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2012-11-23
Filing date: 2012-11-23
Publication date: 2014-05-28
Anticipated expiration: 2032-11-23
Also published as: CN103840083A; TW201425374A; JP2014112674A; KR20140066643A; GB201221085D0; CN103840083B; GB2508191B; TWI605071B

Abstract

An organic light-emitting device comprising an anode, a cathode and a light-emitting layer between the anode and the cathode, wherein the light-emitting layer comprises a phosphorescent light-emitting material and a non-emissive transition metal complex, wherein the non-emissive transition metal complex preferably has a HOMO level no more than 0.2 eV further from vacuum level than a HOMO level of the phosphorescent light-emitting material. The non-emissive transition metal complex may have formula (IV): M1L11q1L21r1L31s1 (IV) wherein M1 is a metal selected from elements 39 to 48 and 72 to 80, and each of L11, L21 and L31 is a coordinating group; q1 is a positive integer; r1 and s1 are each independently 0 or a positive integer; and the sum of (a1. q1) + (b1. r1) + (c1.s1) is equal to the number of coordination sites available on M, wherein a1 is the number of coordination sites on L11, b1 is the number of coordination sites on L21 and c1 is the number of coordination sites on L31. Also shown is a method of forming an organic light-emitting device.

Description

ORGANIC LIGHT EMISSIVE DEVICE

Background of the Invention

Electronic devices containing active 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 matenals offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic matenals allows use of solution processing in device manufacture, for example inkjet printing, flexographic or gravure printing, or spin-coating.

An OLED may comprise a substrate carrying an anode, a cathode and one or more organic light-emitting 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 (HOMO) and electrons in the lowest unoccupied molecular orbital LUMO) of a light-emitting material combine to form an exciton that releases its energy as light.

Suitable light-emitting materials include small molecule, polymeric and dendrimeric materials. Suitable light-emitting polymers include poly(aiylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as polyfluorenes.

A light emitting layer may comprise a semiconducting 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 singlet excitons).

Phosphorescent dopants are also known (that is, a light-emitting dopant in which light is emitted via decay of triplet excitons). Known phosphorescent dopants include complexes of heavy transition metals.

US 2004/0 155238 discloses a device of a hole-transporting phosphorescent dopant and an electron-transporting oxadiazole in an inert host material. It is reported that light is emitted solely from the dopant.

Summary of the Invention

In a first aspect the invention provides an organic light-emitting device comprising an anode, a cathode and a light-emitting layer between the anode and the cathode wherein the light-emitting layer comprises a phosphorescent light-emitting material and a non-emissive transition meta' complex.

Tn a second aspect the invention provides a composition comprising a phosphorescent light-emitting material, a host material and non-emissive transition metal complex.

In a third aspect the invention provides a formulation comprising a composition according to the second aspect and at least one solvent.

In a fourth aspect the invention provides a method of forming an organic light-emitting device according to the first aspect, the method comprising the steps of forming the light-emitting layer over one of the anode and cathode, and forming the other of the anode and cathode over the light-emitting layer.

The non-emissive transition metal complex may be a material that is inherently capable of phosphorescing, but that does not phosphoresce when in use in a device of the present invention.

Descriptioll of the Drawillgs

The invention will now be described in more detail with reference to the Figures, in which: Figure iflustrates an OLED according to an embodiment of the invention; Figure 2 illustrates lowest triplet excited state energy levels of materials of the light-emitting layer of the device of Figure 1; Figure 3A illustrates HOMO and LUMO levels of the device of Figure 1; Figure 3B illustrates HOMO and LUMU levels of a device according to a further embodiment of the invention; Figure 4 is a graph ofluminance vs. voltage for a device according to an embodiment of the invention and comparative devices; Figure 5 is a graph of current density vs. voltage for a device according to an embodiment of the invention and comparative devices; and Figure 6 shows electroluminescence spectra of a device according to an embodiment of the invention and comparative devices.

Detailed Description of the Invention

Figure 1, which is not drawn to any scale, illustrates schematically an OLED according to an embodiment of the invention. The OLED is carried on substrate 1 and comprises an anode 2, a cathode 4 and a light-emitting layer 3 between the anode and the cathode.

Further layers (not shown) may be provided between the anode and the cathode including, without limitation, charge-transporting layers, charge-blocking layers and charge injection layers. The device may contain more than one light-emitting layer.

Exemplary OLED structures including one or more further layers include the following: Anode / Hole-injection layer / Light-emitting layer / Cathode Anode I Hole transporting layer I Light-emitting layer / Cathode Anode / Hole-injection layer / Hole-transporting layer / Light-emitting layer / Cathode Anode / Hole-injection layer / Hole-transporting layer / Light-emitting layer / Electron-transporting ayer / Cathode.

Tn one preferred embodiment, the OLED comprises at least one, optionally both, of a hole injection layer and a hole transporting layer.

Light-emitting layer 3 contains a phosphorescent light-emitting matenal and a hole-transporting transition metal complex. Light-emitting layer 3 further preferably contains a host material, preferably a charge transporting host material, most preferably an electron-transporting host material.

Figure 2 illustrates triplet energy levels of the materials in light-emitting layer 3 of the device of Figure i.

The excited triplet energy level T1 of the hole-transporting metal complex (Ti HTMC) is higher than that of the host material (Ii Host), and L Host is higher than that of the phosphorescent light-emitting material (Ti Emitter). In operation, triplet excitons formed on T1 HTMC or T1 Host may be transferred to the phosphorescent light-emitting material, and substantially all phosphorescent light emission hv from light-emitting layer 3 is from the phosphorescent light-emitting material. It will be appreciated that the hole-transporting transition metal complex may be a material that is inherently capable of phosphorescing. However, the host material and a phosphorescent emitter both have T1 energy levels lower than the T1 energy level of the hole-transporting transition metal complex and so triplets formed on the hole-transporting transition metal complex do not undergo radiative decay but are instead transferred to the phosphorescent emitter, either directly or via the host material.

The hole-transporting transition metal complex may itself be a material that is capable of phosphorescence if not in contact with a phosphorescent light-emitting material having a similar or lower T1 energy level, By use of such a hole-transporting material, loss of trip'et excitons to non-radiative decay pathways maybe minimized.

The triplet excited state energy levels of the materials illustrated in Figure 2 meet the following inequality to provide for efficient transfer of triplet excitons to the phosphorescent light-emitting material: T1 HTMC > Ti Host> Ti Emitter However, it will be appreciated that T1 Host may be the same as or no more than 2kT higher than T1 HTMC. and that T1 Emitter may be the same as or no more than 2kT higher than Ti Host and! or T1 HTMC.

The Tl energy levels for each of the host, emitter, and HTU may be determined in "gated" low temperature photolurninescence measurements of the energy of the very weak T1 to So transition (i.e. phosphorescence)s. Light-sampling is done by delaying sensing following excitation by a light pulse, thus allowing differentiation of phosphorescence from fluorescence.

Figure 3A illustrates HOMO levels (Fl) and LUMO levels (L) of the materials of in the light-emitting layer 3 of the device of Figure 1.

The HOMO level of the host material is too deep to provide efficient hole transport for the phosphorescent light-emitting material. The HOMO of the host material may be at least 0.4 eV or at least 0.5 cv deeper (further from vacuum) than the HOMU level of the phosphorescent light-emitting material.

The HOMO levels of the hole-transporting transition metal complex and the phosphorescent light-emitting material illustrated in the embodiment of Figure 3A are similar. The HOMO level of the hole-transporting transition metal complex is preferably no more than 0.2 eV deeper than the HOMO level of the phosphorescent Ught-emitting material and may be up to 0.2 eV shallower than the HOMO level of the phosphorescent light-emitting material. Optionally, the HOMO level of the hole-transporting transition metal complex is in the range 5.0-5.4 eV, optionally 5.1 -5.3 eV.

Figure 3B illustrates I-IOMO and LUMO levels of a light-emitting device according to a further embodiment of the invention. In this embodiment, a hole-transporting layerS of a hole-transporting material HT is provided between the anode 2 and the light-emitting layer 3. The energy levels of the materials of the light-emitting layer 3 are as described in Figure 3A. The hole-transporting material MT has a HOMO level that is preferably no more than 0.3 eV deeper than the HOMO of the hole-transporting material HTMC, and is preferably no more than 0.2 eV shallower than the HOMO of the hole-transporting material HTMC.

In operation, holes injected from the anode are transported through the hole-transporting layer 5 (if present) into light-emitting layer 3 and electrons are injected from cathode 4 into the LUMO of the host material. Hole transport in light-emitting layer 3 is provided by either the phosphorescent light-emitting material or by the hole-transporting transition metal complex. Although hole transport may be provided by the phosphorescent light-emitting material alone, the present inventors have surprisingly found that the presence of a hole-transporting transition metal complex in addition to the phosphorescent light-emitting material may improve device performance. Without wishing to be bound by any theory. it is believed that holes are located on the d-orbitals of the metal of the hole-transporting transition metal complex, and these d-orbitals are sterically protected by ligands of the hole-transporting transition metal complex that obstruct potentially deleterous interactions with other components of the light-emitting layer.

Furthermore, by providing a ho'e-transporting transition metal comp'ex that is separate from the phosphorescent emitter, a wider range of efficient light-emitting compositions may be accessible. For example. if the concentration of a phosphorescent emitter in a solution-processed light-emitting layer is limited by a relatively low solubility of the phosphorescent emitter in a given solvent or solvent mixture then the phosphorescent emitter may be used in combination with a separate, relatively high solubility hole- transporting transition metal complex such that efficiency of hole transport in the light-emitting layer is not limited by solubility of the phosphorescent emitter.

HOMO and LUMO levels as described anywhere herein may be HOMO and LUMO levels as measured by square wave cyclic voltammetry (SQ CV).

Plotting applied potential for forward and backward scans against resulting current gives a typical Cyclic Voltammogram. The cyclic voltammogram may be used to establish HOMO and/or LUMO levels of a material.

The excitation signal in SQWV consists of a symmetrical square-wave pulse (for example. of amplitude 25mV) superimposed on a staircase waveform of step height (for example, 4mV) where the forward pulse of the square wave coincides with the staircase step. The resulting current is obtained by taking the difference between the forward and reverse currents. The peak height is directly proportional to the concentration of the electroactive species. An exemplary frequency used for HOMO and LUMO measurements is 15Hz. Oxidation/reduction events (HOMO/LUMO) in Squarewave voltammographs take the shape of peaks with a peak maximum describing occurrence of the event (redox potential).

A solution of the material is spun on a glassy carbon electrode with approximate thickness of 7Onm. The electrode is transferred into an electrochemical cell having a reference electrode (commonly Ag/AgC1), and a Pt counter electrode, both immersed in MeCN with 0. iM supporting electrolyte (typically TBAPF6). Measurements of the material spun on the glassy carbon electrode in a negative potential region (vs Ag/AgC1) give rise to reductive currents (LUMO level), and a positive region is resulting from oxidative currents (HOMO level). All measurements are re-referenced vs HOMO level of a standard molecule -Fenocene equal to -4.8 eV.

Solution CV of a material may be performed in a similar fashion but instead of spun film -dissolved material is used in the dectrochemical cell.

Cyclic voltammetry is described in A.J. Bard, L.R. Faulkner, Electrochemical Methods.

Fundamentals and Appflcations, seconded., Wiley, New York, 2001. Voltammetric Techniques, Samuel P. Kounaves, Tufts University, page 720, and Anal.

Chem., 1969, 41 (ll).pp 1362-1365.

Phosphorescent light-emitting material The phosphorescent light-emitting material is preferably a light-emitting transition metal complex, and may be a red, yellow, green or blue light-emitting material, with the proviso that the T1 energy level of the phosphorescent light-emitting material is no more than 2kT higher than, preferably the same as or lower than, the T1 energy level of the hole-transporting transition metal complex. Optionally, the T1 level of the hole-transporting transition metal complex is at least 0.1 eV or at least 0.2 eV higher than the T1 level of the phosphorescent emitter, in order to avoid quenching of the emitter.

A blue light-emitting material may have a photoluminescent spectrum with a peak in the range of 400-490 nm A green emitting phosphorescent material may have a photoluminescent spectrum with a peak in the range of more than 490-560 nm.

A yellow emitting phosphorescent material may have a photoluminescent spectrum with a peak in the range of more than 560-590 nm.

A red emitting phosphorescent material may have a peak in its photoluminescent emission spectrum at around more than 590-750 nm.

Exemplary phosphorescent light-emitting materials include metal complexes comprising substituted or unsubstituted complexes of formula (I): ML1q L2rL3s () wherein M is a metal; each of L', L2 and L3 is a coordinating group; q is a positive integer; r and s are each independently 0 or a positive integer; and the sum of (a. q) -4-(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 sites on L2 and c is the number of coordination sites on [2.

Heavy elements M induce strong spin-orbit coupling to 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 48 and 72 to 80, in particular ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum and gold.

Iridium is particularly prefered.

Exemplaiy ligands L', L2 and L3 include carbon or nitrogen donors such as porphyrin or bidentate ligands of formula (II): r6 (1:1 wherein A? and Ar6 may be the same or different and are independently selected from substituted or unsubstituted aryl or heteroaryl; X1 and Y1 may be the same or different and are independently selected from carbon or nitrogen; and Ar5 and Ar6 may be fused together. Ligands wherein X' is carbon and Y' is nitrogen are preferred, in particular ligands in which Ar5 is a single ring or fused heteroaromatic of N and C atoms only, for example pyridyl or isoquinoline, and Ar6 is a single ring or fused aromatic, for example phenyl or naphthyl.

Examples of bidentate ligands are illustrated below: Q_<N QN1RQ) Qc'QWCdD OCHO 0%3 wherein R1 and R2 are each independently a substituent.

Each of Ar5 and Ar6 may independently carry one or more substituents. Two or more of these substituents may be linked to form a ring, for example an aromatic ring.

Further ligands L'. L2. L3 suitable for use with d-block elements include diketonates, in particular acetylacetonate (acac); triarylphosphines and pyridine, each of which may be Metal complexes of formula (I) maybe homoleptic or heteroleptic. Homoleptic metal complexes may contain only ligands of formula (11), wherein all ligands are the same.

Heteroleptic ligands may contain only ligands of formula (II) wherein two or more ligands of formula (X) are different, or may contain one or more ligands of formula (TI) and one or more further ligands.

Each of L', L2 and L3 may independently be unsubstituted or substituted with one or more substituents. Exemplary substituents include C140 hydrocarbyl, for example C120 alkyl or aryl (for example phenyl) substituted with one or more C120 ailcyl groups; fluorine or trifluoromethyl: Ci20 alkoxy: carbazole which may be used to assist hole transport to the complex when used as an emissive material; and a dendron.

Fluorine or trifluoromethyl substituents may blue-shift emission of the metal complex.

Dendrons. such as hydrocarbyl dendrons. may be used to obtain or enhance solution processability of the metal complex, for example as disclosed in WO 02/66552.

A light-emitting dendrimer comprises a light-emitting core, such as a complex of formula (I). substituted with one or more dendrons, wherein each dendron comprises a branching point and two or more dendritic branches. Preferably, the dendron is at least partially conjugated, and at least one of the branching points and dendritic branches comprises an aryl or heteroaryl group, for example a phenyl group. hi one arrangement, the branching point group and the branching groups are all phenyl, and each phenyl may independently be substituted with one or more substituents. for example C12oalky oralkoxy.

A dendron may have optionally substituted fomiula (III) !0 /1

B (TIT)

wherein BP represents a branching point for attachment to a core and Gi represents first generation branching groups.

The dendron may be a first, second, third or higher generation dendron. Gi may be substituted with two or more second generation branching groups G2, and so on, as in optionally substituted formula (lila): Jc3 k2 G3 1G3 H G3 \

L

V ffla)

wherein u isO or 1; v isO ifu isO or may be 0 or 1 ifu is 1; BP represents a branching point for attachment to a core and 01, G and @3 represent first, second and third generation dendron branching groups. In one preferred embodiment, each of BP and G1, G, is phenyl, and each phenyl BP, G. G2... G is a 3.5-linked phenyl.

A prefelTed dendron is a substituted or unsubstituted dendron of formula (11Th): Ii (llIb) wherein represents an attachment point of the dendron to a core.

BP and I or any group G may be substituted with one or more substituents, for example one or more C120 alkyl or alkoxy groups.

The phosphorescent emitter may be provided in an amount of at least 0.5 weight % of the light-emitting layer. optionally in the range of 1 -50 weight %, optionally 1-40 weight Hole-transportinR transition metal complex Exemplary hole-transporting transition metal complexes include metal complexes of formula (IV): M1L1 q L2' ri L3' (IV) wherein M' is a metal selected from elements 39 to 48 and 72 to 80, iii particular ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum and gold. Iridium is preferred.

Each of L", L21 and L3' is a coordinating group; qi is a positive integer; ri and si are each independently U or a positive integer; and the sum of (al. ql) + (bl. ri) + (cl.sl) is equal to the number of coordination sites avaflable on M, wherein al is the number of coordination sites on L11, b 1 is the number of coordination sites on ii and ci is the number of coordination sites on L31.

Preferably, each of L11, L2' and L3' is selected from L1, L2 and L3 respectively, and each of qi, ri and si is as described with reference to q, rand s of formula (I). Accordingly, the hole-transporting transition metal complex and the phosphorescent material may both be transition metal complexes, provided that the hole-transporting transition metal complex has a higher T' energy level than the phosphorescent material. The hole-transporting transition metal complex may be a be a material that is inherently capable of emitting blue phosphorescence (but which is non-emissive in devices of the invention) and the phosphorescent material may be one or more of a green, red and yellow phosphorescent material.

The hole-transporting transition metal complex may be provided in an amount of equal to or greater than I weight % in the light-emitting ayer, optionally in the range of I -40 mol %.

Host Material The host material may be a polymer or a non-polymeric compound.

The phosphorescent material and the hole-transporting transition metal complex may each be mixed with the host material, or one or both of the phosphorescent material and the hole-transporting transition metal complex may be bound to the host material. In the case where the host material is a polymer, the phosphorescent material and / or the hole-transporting transition metal complex may be covalently bound as a side chain group of the polymer; a backbone repeat unit of the polymer; or an end-capping group of the polymer.

In the case where the phosphorescent material and/or the hole-transporting transition metal complex is provided as a side group. it may be directly bound to a main chain of the polymer or spaced apart from the main chain by a spacer group. Exemplary spacer groups include C120 ailcyl groups, aryl-C120 ailcyl groups and C120 ailcoxy groups.

If the phosphorescent material and! or the hole-transporting transition metal complex is bound to a host polymer comprising conjugated repeat units then it may be bound to the polymer such that there is no conjugation between the conjugated repeat units and the phosphorescent material and I or the hole-transporting transition metal complex, or such that the extent of conjugation between the conjugated repeat units and the phosphorescent material and I or the hole-transporting transition metal complex is limited.

The host material preferably has au energy level that is at least 0.05eV or at least 0.1 eV higher than that of the phosphorescent emitter. The host material preferably has a T1 energy level that is at least 005 eV or at least 0.1 eV lower than that of the hole-transporting transition metal complex.

Exemplary host polymers include polymers having a non-conjugated backbone with charge-transporting groups pendant from the non-conjugated backbone, for example poly(9-vinylcarbazole), and polymers comprising conjugated repeat units in the backbone of the polymer. If the backbone of the polymer comprises conjugated repeat units then the extent of conjugation between repeat units in the polymer backbone may be limited in order to maintain a triplet energy level high enough to avoid significant quenching of phosphorescent emission.

Exemplaiy repeat units of a conjugated polymer include optionally substituted monocyclic and polycyclic arylene repeat units as disclosed in for example, Adv. Mater.

2000 12(23) 1737-1750 and include: 1,2-. 1.3-and l,4-phenylene repeat units as disclosed in J. AppI. Phys. 1996, 79, 934; 2,7-fluorene repeat units as disclosed in EP 0842208; indenofluorene repeat units as disclosed in, for example, Macromolecules 2000, 33(6), 2016-2020; and spirofluorene repeat units as disclosed in, for example EP 0707020. Each of these repeat units is optionally substituted. Examples of substituents include solubilising groups such as C120 alkyl or alkoxy; electron withdrawing groups such as fluorine, nitro or cyano; and substituents for increasing glass transition temperature (Tg) of the polymer.

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

Each R9 is preferably a substituent, and each R9 may independently be selected from the group consisting of: -optionaHy substituted alkyl, optionally C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with optionally substituted aryl or heteroaryl, -optionally substituted aryl or heteroaiyl; -a linear or branched chain of aryl or heteroaryl. each of which groups may independently be substituted, for example a group of formula -(1r6)r wherein each Ar6 is hdependently selected from aryl or heteroaryl, r is at least 2 and the group -(Ar6)r forms a linear or branched chain of aromatic or heteroaromatic groups, for example 3,5-diphenylbenzene wherein each phenyl may be substituted with one or more C120 alkyl groups.

-a crosslinkable-group, for example a group comprising a double bond such and a vinyl or acrylate group. or a benzocyclobutane group.

In the case where R9 comprises aryl or heteroaryl nng system. or a linear or branched chain of aryl or heteroaryl ring systems, the or each aryl or heteroaryl ring system may be substituted with one or more substituents R3 selected from the group consisting of: ailcyl, for example C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with 0. S, substituted N, C=O and -COO-and one or more H atoms of the alkyl group may be replaced with F or aryl or heteroaryl optionally substituted with one or more groups NR52, OR5. SR5. and fluorine, nitro and cyano; wherein each R4 is independently alkyl, for example C1-20 alkyl, in which one or more non-adjacent C atoms may be replaced with 0, S, substituted N, C=O and -COO-and one or more H atoms of the ailcyl group may be replaced with F, and each R5 is independently selected from the group consisting of C1-20 alkyl and aryl or heteroaryl optionally substituted with one or more alkyl 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 C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with 0, S. NH or substituted N, C=O and -COO-, optionally substituted aryl, optionally substituted heteroaryl. alicoxy, allcylthio, fluorine, cyano and arylallcyl. Particularly preferred substituents include C1-20 alkyl and substituted or unsubstituted aryl, for example phenyl. Optional substituents for the aryl include one or more C1-20 alkyl groups.

Where present, substituted N may independently in each occurrence be NR6 wherein R6 is ailcyl, optionally C1-20 ailcyl, or optionally substituted aryl or heteroaryl. Optional substituents for aryl or heteroaryl R6 may C120 ailcyl.

Preferably, each R9 is selected from the group consisting of C140 hydrocarbyl, including C1-20 alkyl, unsubstituted phenyl, and phenyl substituted with one or more C20 alkyl groups.

If the phosphorescent emitter is provided as a side-chain of the polymer then at least one may comprise the phosphorescent emitter that is either bound directly to the 9-position of a fluorene unit of formula (V) or spaced apar from the 9-position by a spacer group.

The repeat unit of formula (V) may be a 2,7-linked repeat unit of formula (Va): P9 P9 (Va) The extent of conjugation of repeat units of formulae (V) may be 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 / or (b) substituting the aromatic carbon atoms of the repeat unit with one or more further substituents R9 in or more positions adjacent to its linking positions in order to create a twist with the adjacent repeat unit or units, for example a 2,7-linked fluorene carrying a C120 alkyl substituent 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 (VT): (VI) wherein w is 0, 1, 2, 3 or 4, optionally 1 or 2, and R'° independently in each occurrence is a substituent, optionally a substituent R9 as described above with reference to formula (V), for example C120 alkyl, and phenyl that is unsubstituted or substituted with one or more C120 alkyl groups.

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

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

If w is at east I. and/or the repeat unit is 1,2-or 1,3 linked, then the extent of conjugation of repeat unit of formula (VI) to one or both adjacent repeat units may be relatively low. In one preferred arrangement, the repeat unit of formula (VI) is 1,3-linked and w isO, 1,2 or 3. Tn another preferred arrangement, the repeat unit of formula (VI) has formula (VIa): (Via) The host polymer may be an electron-transporting host having a high electron affinity (1.8 eV or higher, preferably 2 eV or higher, even more prefened 2.2 eV or higher) and high ionisation potential (5.8 eV or higher) Suitable electron transport groups include groups disclosed in. for example, Shirota and Kageyama, hern. Rev. 2007, 107, 953-1010.

Triazines form an exemplary class of electron-transporting units, for example optionally substituted di-or th-(hetero)aryltriazine attached as a side group through one of the (hetero)aryl groups. Other exemplary electron-transporting units are pyrimidines and pyddines; sulfoxides and phosphine oxides; benzophenones; and boranes, each of which may be unsubstituted or substituted with one or more substituents, for example one or more C120 alkyl groups.

Exemplary electron-transporting units have formula (VII): 7-(Ar4)2 (VII) wherein Ar4, ArS and Ar6 in each occurrence are independently selected from aryl or heteroary, each of which may independently be unsubstituted or substituted with one or more substituents; z is greater than or equal to 1, optionally 1. 2 or 3; and X in each occurrence is N or CR7, wherein R7 is H or a substituent, preferably H or C110 alkyl.

In one preferred embodiment, all 3 groups X are N and Ar4, Ar5 and Ar6 are each unsubstituted or substituted phenyl.

If all 3 groups X are CR7 then at least one of Ar4, Ar5 and Ar6 is preferably a heteroaromatic group comprising N. Any of Ar4, Ar5 and Ar6 may independently be substituted with one or more substituents.

Preferred substituents are selected from the group R1' consisting of: alkyl, for example C1-20 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 H atoms of the alkyl group may be 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, nhro and cyano; wherein each R4 is independently alkyl, for example C1-20 alkyl. in which one or more non-adjacent C atoms may be replaced with 0, S. substituted N. C=O and -COO-and one or more H atoms of the alkyl group may be replaced with F, and each R5 is independently selected from the group consisting of C1-20 alkyl and aryl or heteroaryl optionally substituted with one or more ailcyl groups.

Where present, substituted N of R' or R4 may independently in each occurrence be NR6 or CR62 respectively wherein R6 is C1-20 alkyl or optionally substituted aryl or heteroaryl.

Optional substituents for aryl or heteroaryl R6 are C120 alkyl.

Ar4, Ar5 and Ar6 are preferably phenyl, each of which may independently be unsubstituted or substituted with one or more C120 alkyl groups.

Electron-transporting units may be provided as distinct repeat units formed by polymerising a corresponding monomer. Alternatively, electron-transporting repeat units ET units may form part of a larger repeat unit, for example a repeat unit of formula (VIII): -(-(Ar3)q__Sp-ET__Sp-(Ar3)q)_ (VIII) wherein CT represents a conjugated charge-transporting group; each Ar3 independently represents an unsubstituted or substituted aryl or heteroaryl; q is at least 1, optionally 1, 2 or 3; and each 5p independently represents a spacer group forming a break in conjugation between Ar3 and CT.

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

Exemplary CT groups may be units of formula (VII) described above.

Ar1 is preferably an unsubstituted or substituted aryl. optionally an unsubstituted or substituted phenyl or Iluorene, Optional substituents for Ar3 may be selected from R3 as described above, and are preferaNy selected from one or more Ci2oakyl substituents.

qis preferably 1.

Charge transporting and charge blocking layers A hole transporting layer may be provided between the anode and the light-emitting layer or layers. Likewise, an &ectron transporting ayer may be provided between the cathode and the light-emitting layer or layers.

Similarly, an electron blocking layer may be provided between the anode and the light-emitting layer and a hole blocking ayer may be provided between the cathode and the 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 charge-transporting layer or charge-blocking layer may be crosslinked, particularly if a layer overlying that charge-transporting or charge-blocking layer is deposited from a solution, The crosslinkable group used for this crosslinking may be a crosslinkable group comprising a reactive double bond such and a vinyl or acrylate group, or a benzocydobutane group.

The FIOMO level of the hole transport layer maybe s&ected so as to be within 0.2 eV, optionally within 0.1 eV, of an adjacent layer (such as a light-emitting layer) in order to provide a small barrier to hose transport between these layers.

Exemplaiy hole transporting materials may be materials having a electron affinity of 2.9 eV or lower and an ionisation potential of 5.8 eV or lower, preferably 5.7 eV or lower.

Optionally, devices of the invention have a hole-transporting layer comprising a hole-transporting polymer, the hole-transporting polymer comprising repeat units of formula (DC): ((Ar8)( tN9)d \\R13 \ g (TX) wherein Ar8 and Ar9 in each occurrence are independently selected from substituted or unsubstituted aryl or heteroaryl, g is greater than or equal to 1, preferably 1 or 2, R is H or a substituent. preferably a substituent, and c and dare each independently I, 2 or 3.

R'3, which may be the same or different in each occurrence when g > 1, is preferably selected from the group consisting of alkyl, for example C120 alkyl, Ar'°, a branched or linear chain of Ar'° groups, or a crosslinkable unit that is bound directly to the N atom of formula (VIII) or spaced apart therefrom by a spacer group. wherein Ar'° in each occurrence is independently optionally substituted aryl or heteroaryl. Exemplary spacer groups are C120 ailcyl, pheny and phenyl-C129 alkyl.

Any of Ar8. Ar9 and, if present, Ar'° in the repeat unit of Formula (IX) may be linked by a direct bond or a divalent linking atom or group to another of Ar8, Ar9 and Ar'°.

Preferred divalent linking atoms and groups include 0, S; substituted N; and substituted C. Any of Ar8, Ar9 and, if present, Ar1° may be substituted with one or more substituents.

Exemplary substituents are substituents R'°, wherein each R1° may independently be selected from the group consisting of: -substituted or unsubstituted alkyl. optionally C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with optionally substituted aryl or heteroaryl, 0, S, substituted N, C=O or -COO-and one or more H atoms may be replaced with F; and -a crosslinkable group attached directly to Ar8, Ar9 or Ar1° or spaced apart therefrom by a spacer group, for example a group comprising a double bond such and a vinyl or acrylate group, or a benzocyclobutane group Preferred repeat units of formula (IX) have formulae 1-3: ( Ac Ar) ( Ac/i9) ( Ac/I9) N-Ar9-N / \ Ar10 Aria Ar10 Ar1° 1 2 3 In one preferred arrangement. R'3 is Ar'° and each of Ar8, Ar9 and Ar'° are independently and optionally substituted with one or more C120 alkyl groups. Ar8, Ar9 and Ar1° are preferably phenyl.

In another preferred arrangement, the central Ar9 group of formula 1 linked to two N atoms is a polycyclic aromatic that may be unsubstituted or substituted with one or more substituents R'°. Exemplary polycyclic aromatic groups are naphthalene, perylene, anthracene and fluorene.

In another preferred arrangement, Ar8 and Ar9 are phenyl, each of which may be substituted with one or more CL20 aWyl groups, and R'3 is -(Ar15r wherein r is at least 2 and wherein the group -(Ar'°), forms a linear or branched chain of aromatic or heteroaromatic groups, for examp'e 3,5-diphenylbenzene wherein each phenyl may be substituted with one or more C120 alkyl groups.Tn another preferred alTangement. c, d and g are each 1 and Ar8 and Ar9 are phenyl linked by an oxygen atom to form a phenoxazine ring.

A polymer comprising repeat units of formula (IX) maybe a homopolymer or a copolymer comprising one or more repeat units of formula (TX) and one or more further co-repeat units. Exemplary co-repeat units are aryene co-repeat units, for example repeat units of formulae (V) and (VI) as described above. A copolymer comprising one or more repeat units of formula (lx) may contain 10-80 mol %, optionally 20-50 mol %, of repeat units of formula (IX).

If present, an electron transporting ayer located between the light-emitting ayers and cathode preferably has a LUMO level of around 2.5-15 eV as measured by square wave cyclic voltammetry. For example, a layer of a silicon monoxide or silicon dioxide or other thin dielectric layer having thickness in the range of 0.2-2nm may be provided between the light-emitting layer nearest the cathode and the cathode. HOMO and LUMO levels may be measured using cyclic voltammetry.

An electron transporting layer may contain a polymer comprising a chain of optionaUy substituted arylene repeat units, such as a chain of fluorene repeat units.

If a charge-transporting layer is provided adjacent a phosphorescent light-emitting ayer then the trplet energy evel of the material or materials of the charge transporting layer are preferably the same as or higher than the phosphorescent light-emitting material.

White OLEDs OLEDs of the invention may, for example. emit white light.

The emitted white light may have CIE x coordinate equivalent to that emitted by a black body at a temperature 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 Cifi x coordinate equivalent to that emitted by a black body at a temperature in the range of 2700-4500K.

White light may be provided by emission from the phosphorescent emitter, and one or more fluorescent or phosphorescent materials that, together with emission of the phosphorescent emitter provide white light.

A white-emitting OLED may have a single light-emitting layer emitting white light, or may contain two or more Hght-eniitting layers wherein the light emitted from the two or more layers combine to provide white light.

Polymer synthesis Preferred methods for preparation of conjugated polymers, such as polymers comprising one or more of repeat units of formulae (V) -(lx) as described above, comprise a "meta' insertion" wherein the metal atom of a metal complex catalyst is inserted between an aryl or heteroary group and a leaving group of a monomer. Exemplary metal insertion methods are Suzuki polymerisation as described in. for example. WO 00/53656 and Yamamoto polymerisation as descr bed in, for example, T. Yamamoto, Electrically Conducting And Thermafly Stable pi-Conjugated P61y(arylene)s Prepared by Organometallic Processes', Progress in Polymer Science i993, U, 1153-1205. In the case of Yamamoto polymerisation, a nicke' complex catalyst is used; in the case of Suzuki polymerisation, a palladium complex catalyst is used.

For example, in the synthesis of a linear polymer by Yamamoto polymerisation, a monomer having two reactive halogen groups is used. Similarly, according to the method of Suzuki polyrnerisation, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen.

Preferred halogens are chlorine, bromine and iodine, most preferably bromine.

It \vifl therefore be appreciated that repeat units illustrated throughout this appfication may be derived from a monomer carrying suitable leaving groups. Likewise, an end group or side group may be bound to the polymer by reaction of a suitable leaving group.

Suzuki polymerisation may be used to prepare regioregular, block and random copolymers. In particular, homopolymers or random copolymers may be prepared when one reactive group is a halogen and the other reactive group is a boron derivative group.

Alternatively, block or regioregular copolymers may be 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. mesyhte and triflate.

Hole injection layers A conductive hole injection layer. which may be formed from a conductive organic or inorganic material, may be provided between the anode and the light-emitting layer or layers of an OLED to improve hole injection from the anode into the layer or layers of semiconducting polymer. Examples of doped organic hole injection materials include optionally substituted, doped poly(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in EP 0901176 and EP 0947123, polyacrylic acid or a tluorinated sulfonic acid, for example Nation ®; poyaniline as disclosed in US 5723873 and US 5798170; and optionally substituted 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 The cathode is selected from materials that have a workfunction allowing injection of electrons into the light-emitting layer. Othei factors influence the selection of the cathode such as the possibility of adverse interactions between the cathode and the light-emitting material. The cathode may consist of a single material such as a layer of aluminium. Alternatively, it may comprise a plurality of metals, for examp'e a bilayer of a low workfunction material and a high workfunction material such as calcium and aluminium as disclosed in WO 98/10621. The cathode may contain a layer of elemental barium as disclosed in WO 98/57381, AppI. Phys. Lett. 2002, 81(4), 634 and WO 02/84759. The cathode may contain a thin (up to 5 nm) layer of metal compound between the one or more light-emitting layers of an OLED and one or more cathode layers of conductive material, for example one or more meta' layers. Exemplary metal compounds include an oxide or fluoride of an alkali or alkali earth metal, for example lithium fluoride as disclosed in WO 00/4825 8; barium fluoride as disclosed in Appl.

Phys. Left. 2001, 79(5), 2001; and barium oxide. In order to provide efticient injection of electrons into the device, the cathode preferably has a workfunction of less than 3.5 eV, more preferably less than 3.2 eV, most preferably less than 3 eV. Work functions of metals can be found in, for example, Michaelson, J. Appl. Phys. 48(11), 4729, 1977.

The cathode may be opaque or transparent. Transparent cathodes are particularly advantageous for active matrix devices because emission through a transparent anode in such devices is at least partially blocked by drive circuitry located underneath the emissive pixels. A transparent cathode comprises a layer of an electron injecting material that is sufficiently thin to be transparent. Typically, the lateral conductivity of this layer will be low as a result of its thinness. In this case, the 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, GB 2348316.

Encapsulation Organic optoelectronic devices tend to be sensitive to moisture and oxygen.

Accordingly, the substrate preferably has good barrier properties for prevention of ingress of moisture and oxygen into the device, The substrate is commonly glass, however alternative substrates may be used, in particular where flexibility of the device is desirable. For example, the substrate may cornpnse one or more plastic layers, for example a substrate of alternating plastic and dielectric barrier layers or a laminate of thin glass and plastic.

The device may be encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsubnts include a sheet of glass,fi1s having suitable barrier properties such as silicon dioxide, silicon monoxide, silicon nitride or alternating stacks of polymer and dielectric or an airtight container. 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 layer is in the range of 20-300 nm. A getter material for absorption of any atmospheric moisture and I or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.

Formulation processing With reference to Figure I, light-emitting layer 3 may be formed from a formulation of the phosphorescent emitter, the hole-transporting transition metal complex and the host material dispersed or dissolved in a solvent or mixture of two or more solvents. Light-emitting layer 3 may be formed by depositing the formulation and evaporating the solvent or solvents. All of the components of the composition may be dissolved in the solvent or solvent mixture, in which case the formulation is a solution, or one or more components may be dispersed in the solvent or solvent mixture. Suitable solvents for use alone or in a solvent mixture include aromatic compounds, preferably benzene, that may be unsubstituted or substituted. PreferaNy, substituents are selected from halogen (preferably chlorine). C1-10 alkyl and C110 alkoxy. Exemplary solvents are toluene.

xylene, chlorobenze, and anisole.

Techniques for forming layers from a formulation indude printing and coating techniques such spin-coating, dip-coating, roll printing, screen printing, flexographic printing, gravure printing, and inkjet printing.

Multiple organic layers of an OLED, for example charge-transporting layers and light-emitting layers, may be formed by deposition of formulations containing the active materials for each layer.

During OLED formation, a layer of the device may be crosslinked to prevent it from partially or completely dissolving in the solvent or solvents used to deposit an overlying layer. Layers that may be crosslinked include a hole-transporting layer prior to formation by solution processing of an overlying light-emitting layer, or crosslinking of one light- emitting layer prior to formation by solution processing of another, overlying light-emitting layer.

Suitable crosslinkable groups include groups comprising a reactive double bond such and a vinyl or acrylate group, or a benzocyclobutane group. Where a layer to be crosslinked contains a polymer, the crosslinkable groups may be provided as substituents of repeat units of the polymer.

Coating methods such as spin-coating are particularly suitable for devices wherein patterning of the light-emitting layer is unnecessary -for example for lighting applications or simple monochrome segmented disp'ays.

Printing methods such as inkjet printing are particulaidy suitable for high information content displays, in particular full colour displays. A device may be 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 be printed into channels defined within a patterned layer. In particular. the photoresist may be patterned to fomi channels which, unlike wells, extend over a plurality of pixels and which are closed or open at the channel ends.

Examples

Device Example I

A device having the following structure was prepared: ITO! HIL!HTL!LEL/Cathode wherein ITO is a 45 nm indium tin oxide anode layer supported on a glass substrate, HTL is a 35 nm hole-injection layer, HTL is a 22 nm hole-transporting layer and LEL is a 100 nm thick light-emitting layer.

ITO was cleaned using UV I ozone. The HIL was formed by spin-coating a hole-injection material availahie from Hextronics, Inc. The HTL was formed by spin-coating a 0.6 weight % solution of crosslinkable Hole-Transporting Polymer i in o-xylene followed by thermal crosslinking. The LEL was formed by spin-coating a 2.5 weight % o-xylene solution of a 71: 29 weight composition of Phosphorescent Polymer I: Hole-Transporting Transition Metal Complex 1. The cathode was fomied by depositing a first layer of sodium fluorde to a thickness of about 2 nm. then a layer of aluminium to a thickness of about 100 nrn, and finally a layer of silver to a thickness of about 100 nm.

Hole Transporting Transition Metal Complex 1, illustrated below, is as described in US 7659010: N1 Hole Transporting Polymer 1 was formed by Suzuki polymerization as described in WO 00/53656 of the following monomers: Br&N\/B1 n-C5H13 n-C5H13 SOmol% 3Omol% Br 12.5 mol% 7.Smol% Phosphorescent Polymer 1 is a block copolymer formed by Suzuki polymerization as described in WO 00153656. A first block is formed by polymerization of monomer group 1, and a second block is formed by adding monomer group 2 Monomer Group 1: B Br -d CH3 lmol% Smol% lOmo]% Monomer Group 2: 22mo1% 22mo1% 38mo1% Br7IO) 30 2mol% Phosphorescent Polymer 1 possesses a phosphorescent emitter tethered to a side-chain of the polymer. The backbone repeat units of the polymer form an electron-transporting host material. The backbone units of the polymer fonn a host having au energy level of about 2.5 eV. The T1 energy level of the tethered phosphorescent emitter is about 2.4 eV.

Hole-Transporting Transition Metal Complex 1 has aT1 energy level of about 2.8 eV.

Comparative Device I A device was prepared according to Device Example 1 except that Comparative Transition Metal Complex 1 was used in place of Hole-Transporting Transition Metal Complex I:

V

Comparative Transition Metal Complex 1 Comparative Transition Meta' Complex I has the same core light-emitting metal complex as Phosphorescent Emitter 1.

Comparative Device 2 A device was prepared according to Device Example 1 except that the light-emitting layer was formed by spin-coating Phosphorescent Polymer 1 only.

With reference to Figure 4, the drive voltage to achieve a given luminance is lower for Device Example 1 than for both Comparative Polymer 2, in which there is no additive to the Phosphorescent Polymer 1, and Comparative Polymer 1. in which the additive is the same as the phosphorescent light-emitting group of Phosphorescent Polymer I. With reference to Figure 5. Device Example I also shows higher current density than either Comparative Device 1 or 2.

Without wishing to be bound by any theory, it is believed that this improvement is attributable to improved hole transport in devices containing Hole-Transporting Transition Metal Complex 1. Furthermore, the relatively shallow LUMO of Hole-Transporting Transition Metal Complex 1 (1.87 eV, as compared to 2.2 eV for the phosphorescent metal complex emitter of Phosphorescent Polymer I) is believed to avoid any significant electron trapping Blue phosphorescence from Hole-Transporting Transition Metal Complex 1 is described in WO 2004/10 1707. However, with reference to Figure 6 the electroluminescent spectra of Device Example 1 and of Comparative Devices 1 and 2 are all very similar with a peak at about 520 nm. CIE (x, y) co-ordinates are also similar for all 3 devices, as shown in Table I. This indicates that essentially all light is emitted from Phosphorescent Polymer 1 (and / or Comparative Metal Complex 1 in the case of Device Example 1). The absence of any fight in the blue region of the spectra indicates that light indicates that little or no light is emitted from Hole Transporting Transition Metal Complex I.

Table 1

Device CIE-x CIE-y Comparative Device 2 0.30 0.63 Comparative Device 1 0.31 0.64 Device Example 1 0.29 0.65 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

CLAIMSAn organic light-emitting device comprising an anode, a cathode and a light-emitting layer between the anode and the cathode, wherein the light-emitting layer comprises a phosphorescent light-emitting material and a non-emissive transition metal complex.
2. An organic light-emitting device according to claim 1 wherein the non-ernissive transition metal comp'ex has a HOMO level no more than 0.2eV thither from vacuum level than a HOMO level of the phosphorescent light-emitting material.
3. An organic light-emitting device according to claim 1 or 2 wherein the non-emissive transition metal complex has a lowest triplet excited state energy level that is no more than 2kT lower than, preferably the same as or higher than, a lowest triplet excited state energy level of the phosphorescent light-emitting material.
4. An organic light-emitting device according to any preceding claim wherein the phosphorescent light-emitting material is a transition metal complex.
5. An organic light-emitting device according to claim 4 wherein the phosphorescent light-emitting transition metal complex has foniiula (I): ML1q L2L3 (I) wherein M is a metaL each of L1. L2 and V is a coordinating group; q is a positive integer; r and s are each independently 0 or a positive integer; and the sum of (a.q) + (b. r) + (cs) is equal to the number of coordination sites available on M, wherein a is the number of coordination sites on L1, b is the number of coordination sites on L2 and c is the number of coordination sites on L3.
6. An organic light-emitting device according to any preceding claim wherein the non-emissive transition metal complex has formula (IV): M1L1 qiL21riL31si (IV) wherein M' is a metal selected from elements 39 to 48 and 72 to 80, and each of L", L2' and L1 is a coordinating group; qi is a positive integer; ri and si are each independently 0 or a positive integer; and the sum of (al. qi) + (hi. ri) + (ci. s 1) is equal to the number of coordination sites available on M, wherein al is the number of coordination sites on L", hi is the number of coordination sites on L2' and ci is the number of coordination sites on L31.
7. An organic light-emitting device according to any preceding claim wherein the light-emitting layer comprises a host material.
8, An organic light-emitting device according to claim? wherein the host material has a HOMO level at least 0.4 cv further from vacuum than the HOMO level of the phosphorescent light-emitting material.
9. An organic light-emitting device according to claim? or 8 wherein the host is a polymer.
10. An organic light-emitting device according to claim 9 wherein the host has an at least partially conjugated backbone.
ii. An organic light-emitting device according to claim 9 or 10 wherein the phosphorescent light-emitting material is covalently hound to the host polymer.
12. An organic light-emitting device according to any of claims 7-11 wherein the host material has a lowest triplet excited state energy level that lies between the lowest triplet excited state energy levels of the phosphorescent light-emitting material and the non-ernissive transition metal complex.
13. An organic light-emitting device according to any preceding claim wherein the phosphorescent light-emitting material forms 0.5 -10 weight % of the light-emitting layer.
14. An organic light-emitting device according to any preceding claim wherein the non-emissive transition metal complex forms 1-40 weight % of the light-emitting layer.13. An organic light-emitting device according to any preceding claim wherein a hole-transporting layer is provided between the anode and the cathode.16. A composition comprising a phosphorescent light-emitting material, a host material and non-emissive transition metal complex.17. A formulation comprising a composition according to claim 16 and at least one solvent.18. A method of forming an organic light-emitting device according to any one of claims i-15, the method comprising the steps of forming the light-emitting layer over one of the anode and cathode, and forming the other of the anode and cathode over the light-emitting layer.19. A method according to claim 18 wherein the light-emitting layer is formed by depositing a formulation comprising the phosphorescent light-emitting material and non-emissive transition metal complex in at least one solvent, and evaporating the at least one solvent.20. A method according to claim 19 wherein the formulation is a formulation according to claim 17.