WO2013178975A1

WO2013178975A1 - Organic light emitting device with metallic anode and polymeric hole injection layer

Info

Publication number: WO2013178975A1
Application number: PCT/GB2013/000240
Authority: WO
Inventors: William Young; Ilaria Grizzi
Original assignee: Cambridge Display Technology Limited
Priority date: 2012-05-28
Filing date: 2013-05-23
Publication date: 2013-12-05
Also published as: GB201209431D0

Abstract

An organic light-emitting device comprising an anode (102), a cathode (105), a light- emitting layer (104) between the anode and the cathode and a hole injection layer (103) between the anode and the light-emitting layer, wherein the anode comprises a layer comprising a metal or metal alloy; the hole injection layer is in contact with the anode layer comprising an aluminium alloy; and the hole injection layer comprises a conductive polymer comprising substituted or unsubstituted thiophene repeat units and a non-conductive polymer. The weight ratio of the conductive polymer: the non- conductive polymer is 1:n wherein n is less than 5.

Description

ORGANIC LIGHT EMITTING DEVICE WITH METALLIC ANODE AND

POLYMERIC HOLE INJECTION LAYER

This invention relates to organic light-emitting devices and methods of making the same. Background of the Invention

Electronic devices comprising active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes, organic photovoltaic devices, organic photosensors, organic transistors and memory array devices. Devices comprising 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 manufacture, for example inkjet printing or spin-coating.

A typical organic light-emissive device ("OLED") is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide ("ITO"). A layer of a thin film of at least one electroluminescent organic material is provided over the first electrode. Finally, a cathode is provided over the layer of electroluminescent organic material.

In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic light- emitting layer to form excitons which then undergo radiative decay to give light.

In WO90/13148 the organic light- emissive material is a conjugated polymer such as poly(phenylenevinylene). In US 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as tris-(8-hydroxyquinoline) aluminium ("Alq₃").

Charge transporting, charge injecting or charge blocking layers may be provided between the anode and the light-emitting layer and / or between the cathode and the light-emitting layer.

Light-emission may occur through the anode or the cathode. In the case of emission through a transparent cathode, the anode may comprise a reflective material in order that at least some of the light emitted from the light-emitting layer towards the anode is reflected for emission through the cathode. WO 98/04610 discloses an OLED wherein a hole-injecting layer of

poly(ethylenedioxythiophene) polystyrene sulfonate is provided between the anode and the light-emitting layer.

US 2008/0248313 and WO 2009/111675 disclose a conductive composition comprising a sulfonated polythiophene and poly(4-vinylphenol).

Summary of the Invention

In a first aspect the invention provides an organic light-emitting device comprising an anode, a cathode, a light-emitting layer between the anode and the cathode and a hole injection layer between the anode and the light-emitting layer, wherein the anode comprises a layer comprising a metal or metal alloy; the hole injection layer is in contact with the anode layer comprising a metal or metal alloy; and the hole injection layer comprises a conductive polymer comprising substituted or unsubstituted thiophene repeat units and a non-conductive material.

Optionally, the thiophene repeat units include sulfonated thiophene repeat units. Optionally, the conductive polymer is a copolymer.

Optionally, about 25% - 90 % of the thiophene repeat units are sulfonated thiophene repeat units.

Optionally, the thiophene repeat units include thiophene repeat units substituted with a polyether group.

Optionally, the non-conductive polymer is optionally substituted polystyrene.

Optionally, the non-conductive polymer is optionally substituted poly(vinylphenol).

Optionally, a weight ratio of the conductive polymer : the non-conductive polymer is 1 : n wherein n is less than 14.

Optionally, n is no more than 5, optionally no more than 3. Optionally, the layer comprising a metal or metal alloy is reflective and the cathode is transparent, in which case the layer comprising a metal or metal alloy optionally has a thickness of at least 50 nm.

Optionally, the layer comprising a metal or metal alloy is transparent, in which case the layer comprising a metal or metal alloy optionally has a thickness of no more than 20 nm.

In a second 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 anode comprising a layer comprising a metal or metal alloy;

forming the hole injection layer on the layer comprising a metal or metal alloy;

forming the light-emitting layer over the hole injection layer; and

forming the cathode over the light-emitting layer.

Optionally, the layer comprising a metal or metal alloy is formed by evaporation or sputtering.

Optionally, the hole injection layer is formed by depositing a formulation comprising the conductive material, the non-conductive material and at least one solvent onto the layer comprising a metal or metal alloy, and evaporating the at least one solvent.

Description of the Drawings

The invention will now be described in more detail with reference to the drawings, wherein:

Figure 1 illustrates an OLED according to an embodiment of the invention;

Figure 2 illustrates an inket-printed pixel of a device according to an embodiment of the invention; and

Figure 3 illustrates an OLED according to another embodiment of the invention. Detailed Description of the Invention

Figure 1 illustrates an OLED comprising an anode comprising anode layer 102, supported on an opaque or transparent substrate 101, a hole injection layer 103, a light-emitting layer 104 and a transparent cathode 105 through which light is emitted.

One or more further layers (not shown) may be provided between the anode and cathode, for example one or more layers selected from a hole transporting layer and / or an electron blocking layer between the hole injection layer 103 and the light-emitting layer 104; an electro-transporting layer and / or a hole-blocking layer between the cathode 105 and the light-emitting layer 104; and one or more further light-emitting layers. In one preferred embodiment, a hole-transporting layer is provided between the hole injection layer 103 and the light-emitting layer 104.

Anode

In the embodiment of Figure 1, anode layer 102 is a conductive metal or metal alloy layer. Anode layer 102 is preferably reflective for at least some of the wavelengths of light emitted by the light-emitting layer 104. It will be appreciated that the thickness of layer 102 required for reflectivity will depend on the materials of anode layer 102, however a preferred thickness range is at least 50 nm, optionally 50-500 nm or 50-200 nm.

Exemplary materials for forming anode layer 102 include aluminium and alloys thereof, and silver and alloys thereof.

Anode layer 102 may be formed by any process, for example evaporation or sputtering. Sputtering is preferred. If the anode layer requires patterning, for example patterning in stripes for a passive matrix OLED having cathode stripes substantially perpendicular to the anode stripes, or patterned to form individual pixel electrodes for an active matrix OLED, then the anode layer 102 may be deposited in a pattern or may be deposited as an unpatterned film followed by patterning using methods known to the skilled person, for example photolithography.

The anode may consist of anode layer 102 alone or of two or more conductive layers. For example, anode layer 102 may be formed from a layer of silver. However, to reduce the cost associated with a single layer of silver of sufficient thickness to provide reflectivity, a silver anode layer 102 may be used in combination with a layer of a reflective metal such as aluminium.

Hole injection layer

The hole injection layer 103 comprises a conducting polymer comprising thiophene repeat units and a non-conducting material. The hole injection layer 103 optionally has a thickness in the range of 5-500 nm, optionally 10-200 nm or 10-100 nm. The thickness of the hole injection layer may be selected so that the cavity defined by the anode and cathode is of a size to maximise light emission of a wavelength of light-emitted by the light-emitting layer.

Exemplary conducting polymers are conjugated polymers comprising thiophene repeat units of formula (I):

wherein R⁷ independently in each occurrence represents H or a substituent.

The conductive polymer preferably comprises repeat units of formula (I) wherein at least one R⁷ is an acid group, preferably a sulfonic acid group.

The conductive polymer preferably comprises repeat units wherein at least one group R⁷ is a polar, aprotic substituent, for example a polyether group.

The polymer may comprise repeat units carrying one of an acid group and a polar aprotic group and / or repeat units carrying both of an acid group and a polar aprotic group.

An exemplary conductive polymer comprises repeat units of formulae (la) and (lb):

(la) (lb) wherein p is at least 1 and is preferably 2. A sulfonated polymer of this type may be formed by sulfonation of a regioregular polymer comprising repeat units of formula (I) substituted with a polyether group, for example a repeat units of formula (lb). A polymer of this type is available from Plextronics, Inc. under the trade name Plexcore ®.

In one embodiment, the conductive polymer is a polythiophene wherein substantially all repeat units of the polymer are repeat units of formula (I), for example a copolymer consisting essentially of repeat unit of formula (la) and (lb). The polymer may be a random, block or alternating copolymer.

The non-conducting polymer preferably has a substantially non-conjugated polymer backbone.

An exemplary non-conducting polymer is polystyrene substituted with one or more polar, protic groups, for example a polymer comprising repeat units of formula (III):

wherein Y is a polar group, for example OH. An exemplary non-conducting polymer is poly-4-vinylphenol (PVP).

The hole-injection layer is formed by depositing a composition comprising the conducting polymer, the non-conducting polymer and one or more solvents, and evaporating the solvents. Solvents may be evaporated using heat and / or vacuum.

The composition may comprise water and at least one organic solvent that is miscible with water.

Suitable deposition methods include coating and printing methods.

Exemplary coating methods include spin-coating, dip-coating, roll coating or roll-to-roll printing, doctor blade coating, slot die coating. Exemplary printing methods include roll-printing, gravure printing, screen printing and inkjet printing.

Coating methods, such as those described above, are particularly suitable for devices wherein patterning 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. .

The light-emitting layer 104, and any charge-tranporting, charge blocking or additional light- emitting layers, may be deposited using a method that is the same as or different to the method used to deposit the hole injection layer 103. In one preferred arrangement, the hole injection layer 103 and the light-emitting layer 104 are deposited from solution.

A device may be inkjet printed by providing a patterned layer over the anode and defining wells into which the hole injection material and a light-emitting material is printed. A light- emitting material having one colour of emission may be printed into each well the case of a monochrome device, or multiple light-emitting materials of different colours may be printed 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, for example as described in EP 0880303.

With reference to Figure 2, a pixel of a display having a structure shown in Figure 1 comprises anode layer 202 comprising a plurality of anodes of which one anode 202 is shown, and a patterned layer of photoresist 210 defining a well over each anode. The hole- injection layer 203 and light-emitting layer 204 are formed by inkjet printing into each well. Further layers may be printed into each well, for example a hole-transporting layer between the anode and the light-emitting layer and / or one or more further light-emitting layers.

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 form channels which, unlike wells, extend over a plurality of pixels and which may be closed or open at the channel ends.

Cathode

Cathode 105 is transparent to light emitted from the light-emitting layer. An exemplary cathode is a metal that is sufficiently thin to allow light to pass through it. The thickness of a metal layer required for transparency of that layer will depend on the metal, however it is preferred that the a metal layer of a transparent cathode has a thickness of less than 20 nm. A preferred transparent metal is silver.

The layer of transparent metal may be overlaid with another layer to form a bilayer transparent cathode, for example metal / ITO and metal / SiO.

Another transparent cathode structure comprises a layer of an n-doped organic

semiconductor, for example an electron-transporting layer doped with an organic or inorganic donor capped with a layer of transparent, conductive material, for example a layer of ITO.

A device having a transparent cathode is particularly advantageous for active matrix OLED devices comprising drive circuitry on substrate 101 because emission through a transparent anode in such devices is at least partially blocked by the drive circuitry.

Charge transporting layers

A hole transporting layer may be provided between the hole-injection layer 103 and the light- emitting layer 104 (or light-emitting layers). Likewise, an electron transporting layer may be provided between the cathode and the light-emitting layers.

Similarly, an electron blocking layer may be provided between the hole-injection layer and the light-emitting layer and a hole blocking layer 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.

If present, a hole transporting layer located between the hole-injection layer and the light- emitting layers preferably has a HOMO level of less than or equal to 5.5 eV, more preferably around 4.8-5.5 eV. The HOMO level of the hole transport layer may be selected 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 hole transport between these layers.

If present, an electron transporting layer located between the light-emitting layers and cathode preferably has a LUMO level of around 3-3.5 eV. HOMO and LUMO levels may be measured by cyclic voltammetry. A hole transporting layer may contain a hole-transporting (hetero)arylamine, such as a homopolymer or copolymer comprising hole transporting repeat (hetero)arylamine repeat units.

Exemplary (hetero)arylamine repeat units have formula (IV):

(IV) wherein Ar¹ and Ar² in each occurrence are independently selected from optionally substituted aryl or heteroaryl groups, z is greater than or equal to 1, preferably 1 or 2, R is H or a substituent, preferably a substituent, and x and y are each independently 1, 2 or 3.

R is preferably alkyl, for example Ci-20 alkyl, Ar³, or a branched or linear chain of Ar³ groups, for example -(Ar³)_r, wherein Ar³ in each occurrence is independently selected from aryl or heteroaryl and r is at least 1, optionally 1, 2 or 3.

Any of Ar , Ar and Ar may independently be substituted with one or more substituents. Preferred substituents are selected from the group R³ consisting of: alkyl, for example Ci-20 alkyl, wherein one or more non-adjacent C atoms may be replaced with O, S, substituted N, C=0 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,

NR ₂, OR⁵, SR⁵, fluorine, nitro and cyano, and crosslinkable groups; wherein each R⁴ is independently alkyl, for example Ci_2o alkyl, in which one or more non- adjacent C atoms may be replaced with O, S, substituted N, C=0 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 alkyl and aryl or heteroaryl optionally substituted with one or more alkyl groups.

R may comprise a crosslinkable-group, for example a group comprising a polymerisable double bond such and a vinyl or acrylate group, or a benzocyclobutane group such that the hole-transporting layer may be crosslinked following its deposition, in particular if the light- emitting layer is deposited from solution.

Any of the aryl or heteroaryl groups in the repeat unit of Formula (IV) may be linked by a direct bond or a divalent linking atom or group. Preferred divalent linking atoms and groups include O, S; substituted N; and substituted C.

Where present, substituted N or substituted C of R³, R⁴ or of the divalent linking group may independently in each occurrence be NR⁶ or CR⁶ ₂ respectively wherein R⁶ is alkyl or optionally substituted aryl or heteroaryl. Optional substituents for aryl or heteroaryl groups R⁶ may be selected from R⁴ or R⁵.

In one preferred arrangement, R is Ar³ and each of Ar¹, Ar² and Ar³ are independently and optionally substituted with one or more Ci-20 alkyl groups.

Ar¹, Ar² and Ar³ are preferably phenyl, each of which may independently be substituted with one or more substituents as described above, preferably one or more Ci_-2o alkyl groups.

In another preferred arrangement, Ar , Ar and Ar are phenyl, each of which may be substituted with one or more Ci_-2o alkyl groups, and r = 1.

In another preferred arrangement, Ar¹ and Ar² are phenyl, each of which may be substituted with one or more Ci.₂o alkyl groups, and R is 3,5-diphenylbenzene wherein each phenyl may be substituted with one or more alkyl groups.

Arylamine repeat units may be provided in a copolymer in an amount of at least 1 mol %, optionally at least 5 mol %.

Exemplary copolymers comprise repeat units of formula (IV) and optionally substituted (hetero)arylene co-repeat units. Exemplary arylene co-repeat units are disclosed in for example, Adv. Mater. 2000 12(23) 1737-1750 and include: phenylene repeat units, for example 1,4-Iinked phenylene repeat units; fluorene repeat units, for example 2,7-linked fluorene repeat units, indenofluorene repeat units and spirobifluorene repeat units.

Phenylene repeat units are disclosed in, for example, J. Appl. Phys. 1996, 79, 934; 2,7- fluorene repeat units are disclosed in, for example, EP 0842208; indenofluorene repeat units are disclosed in, for example, Macromolecules 2000, 33(6), 2016-2020; and spirobifluorene repeat units are disclosed in, for example EP 0707020.

Each of these repeat units is optionally substituted. Examples of substituents include solubilising groups such as Ci.₂o alkyl or alkoxy; electron withdrawing groups such as fluorine, nitro or cyano; crosslinkable-groups, for example groups comprising a polymerisable double bond such and a vinyl or acrylate group, or a benzocyciobutane group; and substituents for increasing glass transition temperature (Tg) of the polymer.

Light-emitting layer

Suitable organic light-emitting materials for use in light-emitting layer 104 include small molecule, polymeric and dendrimeric light-emitting materials.

Exemplary light-emitting polymers include polymers having a non-conjugated backbone with light-emitting groups in polymer side-groups, and polymers having a conjugated backbone with light-emitting groups in the backbone of the polymer and / or in polymer end-groups or side-groups. Exemplary conjugated light-emitting polymers include polyarylenevinylenes, for example polyphenylenevinylenes, and polymers comprising arylene and / or arylamine repeat units as described above with reference to charge-transporting layers.

The light-emitting layer may consist only of a light-emitting material or it may comprise one or more further materials, for example one or more charge-transporting materials.

The light-emitting layer may comprise a semiconducting host material and a fluorescent or phosphorescent light-emitting dopant, for example a light-emitting transition metal complex dopant.

The light-emitting layer may be formed by any process, including solution deposition methods as described above, or evaporation of the material or materials forming the light- emitting layer. The device may comprise more than one light-emitting layer. For example, a white light- emitting OLED may comprise a plurality of light-emitting layers that, in combination, provide white light.

Encapsulation

Organic light-emitting devices tend to be sensitive to moisture and oxygen. Accordingly, the substrate 101 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 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 EP 0949850. In the embodiment of Figure 1, the substrate may be formed from an opaque material because emission is through the cathode.

The device may be 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, WO 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 layer is in the range of 20-300 nm. A getter material for absorption of any atmospheric moisture and / or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.

Figure 3 illustrates another exemplary OLED comprising transparent substrate 301, transparent anode layer 302, hole injection layer 303, light-emitting layer 304 and cathode 305. This OLED is substantially as described with respect to Figure 1 except that light- emission is through the anode layer 302 and substrate 301, and the metal or metal alloy used to form the anode is sufficiently thin to be transparent. It will be appreciated that the thickness of the anode layer 302 required for transparency will depend on the material or materials used to form it. For example, in the case where anode layer 302 is formed from silver, the thickness of this is preferably less than 20 nm. The cathode 305 may be as described above with reference to Figure 1, in which case light may be emitted through both the anode 302 and cathode 305. In another embodiment, the cathode 305 is reflective so that at least some of the light emitted from light-emitting layer 304 towards cathode 305 is reflected for emission through the anode 302.

The cathode 305 may consist of a single layer of a conductive material, such as a layer of metal (e.g. aluminium) or a metal alloy. Alternatively; it may comprise a plurality of layers.

Exemplary cathodes 305 comprising multiple layers include:

- one or more layers of a high workfunction material (e.g. greater than 3.5 eV) and a layer of a lower workfunction material (e.g. less than 3.5 eV or less than 3 eV) between the light- emitting layer 304 and the one or more layers of high workfunction material, for example as disclosed in WO 98/10621, WO 98/57381, Appl. Phys. Lett. 2002, 81(4), 634 and WO 02/84759. Exemplary cathodes include bilayer cathodes, for example Ba / Al or Ca / Al, or trilayer cathodes, for example Ca / Al / Ag.

- one or more conductive layers, such as one or more metal layers, and a thin layer of metal compound between the light-emitting layer and the one or more conductive layers.

Exemplary metal compounds include an oxide or fluoride of an alkali or alkali earth metal, for example lithium fluoride as disclosed in WO 00/48258; barium fluoride as disclosed in Appl. Phys. Lett. 2001, 79(5), 2001; and barium oxide. The metal compound layer may have a thickness of no more than 5 nm. The one or more conductive layers preferably includes at least one layer of a high workfunction material, for example a high workfunction metal (e.g. greater than 3.5 eV). Exemplary cathodes include bilayer cathodes, for example LiF / Al, or trilayer cathodes, for example LiF / Al / Ag or LiF / Ca / Al.

Exemplary low workfunction materials include low workfunction metals, for example calcium or barium. Exemplary high workfunction materials include high workfunction metals, for example aluminium or silver. Work functions of metals can be found in, for example, Michaelson, J. Appl. Phys. 48(11), 4729, 1977.

Examples

Organic light-emitting devices having the following structure were formed on a glass substrate: Anode (100 nm) / HIL (20 nm) / HTL (22 nm) / EL (60 nm) / Cathode wherein Anode is preferably an aluminium alloy deposited by sputtering (specifically an Al:Ni:La alloy as described in Liu et al. Electrochem. Solid-State Lett. 2011, vol. 14, issue 2, H57-H59); HIL is a layer of hole-injection material comprising a conductive polymer having repeat units of formulae (la) and (lb) and the non-conductive polymer poly(4-vinylphenol); HTL is a hole transport layer comprising a crosslinked hole transporting polymer; EL is a light-emitting layer; and Cathode is a transparent cathode comprising a layer of metal fluoride (1-5 nm), a layer of magnesium (2nm) and a layer of silver (15-20 nm). The cathode is capped with a layer of transparent silicon dioxide.

(I)

Each of HIL, HTL and EL were formed by spin-coating.

The hole transporting polymer was crosslinked following deposition to avoid dissolution of this layer during deposition of the light-emitting 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

1. An organic light-emitting device comprising an anode, a cathode, a light-emitting layer between the anode and the cathode and a hole injection layer between the anode and the light-emitting layer, wherein the anode comprises a layer comprising a metal or metal alloy; the hole injection layer is in contact with the anode layer comprising a metal or metal alloy; and the hole injection layer comprises a conductive polymer comprising substituted or unsubstituted thiophene repeat units and a non-conductive material.

2. A composition according to claim 1 wherein the thiophene repeat units include

sulfonated thiophene repeat units.

3. A composition according to claim 1 or 2 wherein the conductive polymer is a

copolymer.

4. A composition according to claim 3 wherein about 25 % - 90 % of the thiophene repeat units are sulfonated thiophene repeat units.

5. A composition according to any of claims 2-4 wherein the thiophene repeat units include thiophene repeat units substituted with a polyether group.

6. An organic light-emitting device according to any preceding claim wherein the non- conductive material is a non-conductive polymer.

7. An organic light-emitting device according to claim 6 wherein the non-conductive polymer is an optionally substituted polystyrene.

8. An organic light-emitting device according to claim 7 wherein the non-conductive polymer is optionally substituted poly(vinylphenol).

9. An organic light-emitting device according to any preceding claim wherein a weight ratio of the conductive polymer : the non-conductive material is 1 : n wherein n is less than 14.

10. An organic light-emitting device according to claim 9 wherein n is no more than 5, optionally no more than 3.

11. An organic light-emitting device according to any preceding claim wherein the layer comprising a metal or metal alloy is reflective and the cathode is transparent.

12. An organic light-emitting device according to claim 11 wherein the layer comprising a metal or metal alloy has a thickness of at least 50 nm.

13. An organic light-emitting device according to any of claims 1-10 wherein the layer comprising a metal or metal alloy is transparent.

14. An organic light-emitting device according to claim 13 wherein the layer comprising a metal or metal alloy has a thickness of no more than 20 nm.

15. A method of forming an organic light-emitting device according to any preceding claim comprising the steps of:

forming the anode comprising a layer comprising a metal or metal alloy;

forming the light-emitting layer over the hole injection layer; and

forming the cathode over the light-emitting layer.

16. A method according to claim 15 wherein the layer comprising a metal or metal alloy is formed by evaporation or sputtering.

17. A method according to claim 15 or 16 wherein the hole injection layer is formed by depositing a formulation comprising the conductive material, the non-conductive material and at least one solvent onto the layer comprising a metal or metal alloy, and evaporating the at least one solvent.