WO2008029103A1

WO2008029103A1 - Organic opto-electrical devices

Info

Publication number: WO2008029103A1
Application number: PCT/GB2007/003312
Authority: WO
Inventors: William Young
Original assignee: Cambridge Display Technology Limited
Priority date: 2006-09-04
Filing date: 2007-09-03
Publication date: 2008-03-13
Also published as: GB0903094D0; GB2455241A; GB0617381D0

Abstract

An organic opto-electrical device comprising: a substrate having a first electrode disposed thereon for injecting charge carriers of a first polarity; an organic semiconductive region disposed over the first electrode; and a transparent second electrode disposed over the organic semiconductive region for injecting charge carriers of a second polarity, wherein the second electrode comprises a charge injecting layer, a buffer layer disposed over the charge injecting layer, and a transparent conductive layer disposed over the buffer layer, the buffer layer comprising an inorganic metal oxide material.

Description

ORGANIC OPTO-ELECTRICAL DEVICES

Field of Invention

The present invention relates to organic opto-electrical devices, to methods of making such devices and the use of cathodes therein.

Background of Invention

One class of organic opto-electrical devices is that using an organic material for light emission in which electrical energy is transformed into light. While the present specification describes opto-electrical devices in terms of these light-emissive devices, it will be understood that the same structures can be used for converting light into electrical energy, so called photovoltaic devices.

Organic light emissive devices (OLEDs) generally comprise a cathode, an anode and an organic light emissive region between the cathode and the anode. Light emissive organic materials may comprise small molecular materials such as described in US4539507 or polymeric materials such as those described in PCT/WO90/13148. The cathode injects electrons into the light emissive region and the anode injects holes. The electrons and holes combine to generate photons.

Figure 1 shows a typical cross-sectional structure of an OLED. The OLED is typically fabricated on a glass or plastics substrate 1 coated with a transparent anode 2 such as an indium-tin-oxide (ITO) layer. The ITO coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and cathode material 4 of low workfunction metal such as calcium is applied, optionally with a capping layer of aluminium (not shown). Other layers may be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.

There has been a growing interest in the use of OLEDs in display applications because of their potential advantages over conventional displays. OLEDs have relatively low operating voltage and power consumption and can be easily processed to produce large

COMFiRMATiOM COPY area displays. On a practical level, there is a need to produce OLEDs which are bright and operate efficiently but which are also reliable to produce and stable in use.

The structure of the cathode in OLEDs is one aspect under consideration in this art.

In certain device applications it is necessary for the cathode to be transparent. This is particularly the case where drive circuitry or other structures are situated adjacent to the anode thereby obstructing light emission through the anode. These devices are frequently termed "top emitting devices". Figure 2 shows in diagrammatic form a typical cross-sectional structure of a top emitting OLED. An anode material 22 such as ITO may be situated on a metal mirror 25 which is positioned over an active matrix back plane 21. An optional hole injecting material 26 is situated between the anode 22 and an emissive layer 23. Optionally, a hole transporting layer 27 may also be applied between the hole-injection layer and the emissive layer 23.

Electron injecting layer 24 is situated over the light emitting layer 23 and is generally a layer of metal such as barium or metal compound such as barium oxide, which is able to inject electrons into the emissive layer. A buffer layer 28 is deposited over the electron injecting layer 24 and an indium tin oxide (ITO) layer 29 is deposited over the buffer layer to provide a relatively transparent layer of lateral conductivity to compensate for the relatively low conductivity of the barium cathode. Finally, a transparent encapsulation layer (not shown) is applied over the ITO layer so as to protect the device from ingress of oxygen and moisture.

A problem arises with the fabrication of the cathode of this arrangement. ITO is deposited by high energy processes such as sputtering or ion beam deposition. However, sputtering is a highly damaging process and if the ITO were sputtered directly onto an unprotected electron injecting layer, the electron injecting layer and light emitting layer would be damaged. It is for this reason that the buffer layer is used. The buffer layer must not interfere with electron-injection but may make some contribution to the optical properties of the device, if required. Buffer materials may be metals. However, in order to achieve the buffering protection required, a relatively high thickness of metal would need to be used and this may have adverse effects on the transparency of the cathode. Common buffers therefore tend to be highly transparent compounds such as barium fluoride or zinc sulphide. These buffer materials, however, suffer from a disadvantage that they are poor conductors.

An article in the Journal of Applied Physics 96(1), 709-714 (2004) acknowledges that sputter damage is a problem in the fabrication of top emitting organic light emitting devices. The authors suggest that Al doped SiO thin films, which can be deposited by thermal evaporation, provide an attractive alternative method of laying down a transparent cathode for a top emitting OLED. The authors conclude that a multilayer stack of LiF/Al/Al:SiO may be applicable as a cathode in which the fraction of Al in the films is a key parameter to optimise the electrical conductivity, electron-injection and optical transparency of the cathode. The cathode structures described all therefore require a layer of LiF/Al to provide adequate electron-injection. A relatively high level of aluminium is required to provide adequate conductivity which has the undesirable effect of reducing the transparency of the cathode.

An article in Chemical Physics Letters 366 (2002) 128-133 discloses that Ytterbium can be used to prepare a transparent cathode by co-evaporating Yb and Ag to form a Yb: Ag alloy electrode. This article also refers to a paper in Applied Physics Letters 68 (1996) 2606 which discloses a transparent OLED which uses a very thin Mg:Ag alloy layer capped by a sputter-deposited ITO film. It is stated that since the work function of Yb (2.6eV) is lower than those of Li dissociated from LiF (2.9eV), Mg:Ag (3.68eV) and Ag (4.2eV), the energy barrier for electron injection between the organic light emitting layer and the cathode is lower using Yb. However, Yb is very expensive. Furthermore, lower current density and luminance were obtained when the cathode is thin. This is because as the Yb: Ag cathode layer becomes thinner, the current density decreases as a result of the lower conductance of the thinner cathode layer. However, the luminance only rises slightly compared with the more significant increase in current density on increasing the thickness of the layer and this is attributed to more reflection/absorption caused by the thicker Yb: Ag layer. A large proportion of light is lost by total internal reflection as Ag has low transparency. The present applicant's earlier application published as WO 2006/016153 discusses the aforementioned prior art and discusses alternative arrangements for an organic light emissive device with improved properties, including a cathode which does not suffer from the drawbacks of cathode structures of the prior art discussed above. In particular, WO 2006/016153 discusses the possibility of using transparent composites comprising a low work function metal and a transparent metal compound which is codepositable with the low work function metal. In this arrangement, the separate buffer layer and electron injecting layer are replaced by a single layer comprising a composite which may act both as a buffer layer and an electron injecting layer. In this way, the need for a separate buffer layer is avoided. This simplifies the construction of OLEDs giving the potential to reduce manufacturing costs.

However, codeposition of materials is more difficult to control than laying down separate layers of material. Accordingly, while WO 2006/016153 teaches a good solution to the problems associated with prior art transparent cathode arrangements, alternative solutions are still sought.

US 5,739,545 discloses a transparent cathode structure comprising an electron injecting layer, a buffer layer of ZnSe, ZnS, or alloys thereof, and a transparent conductive layer comprising ITO or a thin metal layer.

US 2006/0035015 discloses a non-transparent cathode structure comprising a 0.5nm thick layer of LiF as the electron injecting layer, a buffer comprising a bilayer including a 30nm thick layer of Al and a 5 or lOnm thick layer of WO_x (where 0<x<3) thereover, and a 90nm thick layer of Mg as a conductive capping layer.

It is an aim of the present invention to provide alternative arrangements for an organic opto-electrical device with improved properties, including a transparent electrode structure which does not suffer from the drawbacks of the cathode structures discussed above. Summary of Invention

According to a first aspect of the present invention there is provided an organic opto- electrical device comprising: a substrate having a first electrode disposed thereon for injecting charge carriers of a first polarity; an organic semiconductive region disposed over the first electrode; and a transparent second electrode disposed over the organic semiconductive region for injecting charge carriers of a second polarity, wherein the second electrode comprises a charge injecting layer, a buffer layer disposed over the charge injecting layer, and a transparent conductive layer disposed over the buffer layer, the buffer layer comprising an inorganic metal oxide material.

It has been found that the use of an inorganic metal oxide buffer layer in a transparent electrode is advantageous as metal oxides have been found to provide good transparency and good conductivity (in a direction perpendicular to the plane of the buffer layer) and good protection for the underlying charge injecting layer and organic semiconductive region.

Preferably, the inorganic metal oxide material in the buffer layer is a transition metal oxide, more preferably, a metal oxide of W, Mo, Pd, V or Ru, most preferably W. It has been found that these metal oxides, and particularly WO_x (where 0<x<3), provide the best functional properties being sufficiently conductive and transparent while providing good protection for the underlying charge injecting layer and organic semiconductive region.

If the metal oxide is non-stoichiometric then conductivity is increased but transparency is reduced. Accordingly, it is preferred that the metal oxide is substantially stoichiometric, for example x > 2 for WO_x.

Preferably, the buffer layer has a thickness in the range 20nm to 200nm, more preferably in the range 30nm to lOOnm, more preferably 30nm to 75nm. If the buffer layer is too thin then inadequate protection is provided for the underlying layers. If the buffer layer is too thick then its transparency becomes too low. The buffer layer should be deposited by a relatively low energy process in order to avoid damaging the underlying electron injection and organic layers. Suitable low energy deposition techniques include evaporation techniques, such as electron beam evaporation or thermal evaporation.

It is advantageous for the electrode layers to have good charge transport properties to reduce voltage drops - this criterion is particularly important for the buffer layer. An appropriate figure for the allowable vertical resistivity is to assume that the requirement for current density is 1 A/cm² and an allowable voltage drop is 0.1V. This implies a resistivity-thickness product of 0.1 ohm cm . If the films we are considering are less than 50nm thick, putting a resistivity limit of < 20000 ohm cm, i.e. 0.1/(5OxIO^"7).

An additional buffer layer may be provided in combination with the inorganic metal oxide buffer layer. An example of such an additional buffer layer is a thin metal layer, such as a thin layer of Al, having a thickness in the range 1 to IOnm, more preferably 1 to 5nm.

Preferably, the transparency of the cathode in the device is at least 60%, more preferably at least 70%, still more preferably at least 80%, and most preferably at least 90%. The buffer layer typically has a transparency of at least 70%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95% (when measured at 50nm thickness).

It is particularly preferred that the transparency ranges set out above are met across all of the visible wavelengths, typically 400 to 700nm.

Preferably, the first electrode is an anode and the second electrode is a cathode, i.e. the first charge carriers are positive holes and the second charge carriers are electrons. However, it is possible that the anode may be provided as the overlying electrode including the buffer layer. Preferably, the charge injecting layer has a thickness in the range 1 to IOnm, more preferably in the range 1 to 6nm, and most preferably in the range 3 to 6nm. It has been found that thin layers provide adequate charge injection, can be formed quickly, and are highly transparent and conductive in a direction perpendicular to the plane of the layer.

The cathode further comprises a transparent conductive layer in electrical contact with the side of the buffer layer furthest from the emissive region. The purpose of the transparent conductive layer is to provide lateral conductivity across the cathode. The transparent conductive layer typically has a thickness of from IOnm to lOOnm and a transparency of at least 80%, preferably at least 90%. A preferred transparent conductive layer is a layer of transparent conductive oxide, particularly ITO. Where a transparent conductive layer such as a transparent conductive oxide like ITO is applied to the device by a high energy deposition process, the buffer acts to protect the underlying layers from damage.

As an alternative, the transparent conductive layer may be a thin metal layer. If a metal layer is provided, it preferably has a thickness in the range 5 to 40nm. If a thicker layer is provided then the layer is not transparent enough whereas if a thinner layer is provided then it does not provide sufficient conduction in a lateral direction in the plane of the layer. Examples of suitable metals include Ag and Al.

The device is preferably encapsulated with an encapsulant to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having suitable barrier properties such as 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. 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.

Where the organic light emissive device comprises a top-emitting device, little or no light emission would be expected or desired through the anode. In one arrangement, the anode may be a transparent material such as ITO that is provided on a substrate comprising a metal mirror typically configured to reflect light emitted from the emissive layer out of the device through the cathode. In another arrangement, the metal mirror can also serve as the anode, thus eliminating the need for a separate anode layer such as ITO. An active matrix back plane may be provided at the other side of the substrate.

As an alternative to the provision of multiple discrete layers, in another arrangement the buffer layer and the charge injecting layer may be combined in a composite as described in WO 2006/016153. For example, inorganic metal oxides of W, Mo, Pd, V, Ru or Ir, have been found to have good functional properties for transparent electrodes, may be provided in a composite with a charge injecting material such as, for example, Ba, BaO or LiF. In this case, a separate buffer layer is not required as the composite layer can be codeposited to a sufficient thickness to act both as a buffer and a charge injecting layer for the underlying organic semiconductive region.

The co-deposition process may be controlled so as to have a higher concentration of charge injection material at a side of the composite nearest the organic semiconductive region and a higher concentration of the metal oxide buffer material at a side of the composite farthest away from the light-emitting layer.

Summary of the Drawings

The present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings in which:

FIGURE 1 shows in diagrammatic form a typical cross-sectional structure of an OLED;

FIGURE 2 shows in diagrammatic form a typical cross-sectional structure of a top emitting OLED which is also common to embodiments of the present invention;

FIGURE 3 shows a cross-sectional structure of a cathode stack according to an embodiment of the present invention; and

FIGURE 4 shows in diagrammatic form a cross-sectional structure of an OLED according to another aspect of the present invention. Detailed Description of Preferred Embodiments

Figure 3 shows a cross-sectional structure of a transparent cathode stack according to an embodiment of the present invention. The anode side of the device is not shown for clarity but may comprise the anode side structure illustrated in Figure 2.

The cathode stack comprises, in sequence on an electroluminescent (EL) layer, a low work function injection layer, a buffer layer, a lateral conduction layer, and a primary encapsulant. Further layers, such as an electron transport layer and / or hole blocking layer, may be provided between the electroluminescent layer and low work function injection layer.

Figure 4 shows in diagrammatic form a cross-sectional structure of a top emitting OLED according to another aspect of the present invention. An anode material 32 such as ITO may be situated on a metal mirror 35 which is positioned over an active matrix back plane 31. Hole injecting material 36 is PEDT/PSS and is situated between anode 32 (ITO) and emissive layer 33. Optionally, a hole transporting layer 37 may be applied between the hole injecting layer 36 and the light emissive layer 33.

Composite combined cathode/buffer layer 34 is deposited over the light emitting layer 33 by electron beam coevaporation or thermal coevaporation. An example of such a composite layer is a composite of WOx and Ba. An indium tin oxide layer 39 is deposited by sputtering over the composite layer. Finally, a transparent encapsulation layer (not shown) is applied over the ITO layer so as to protect the device from ingress of oxygen and moisture. The encapsulation layer is generally a dielectric or polymer- dielectric composition.

The electroluminescent layer may consist of the electroluminescent material alone or may comprise the electroluminescent material in combination with one or more further materials. In particular, the electroluminescent material may be blended with hole and / or electron transporting materials as disclosed in, for example, WO 99/48160. Alternatively, the electroluminescent material may be covalently bound to a charge transporting material.

Suitable electroluminescent polymers include poly(arylene vinylenes) such as poly(p- phenylene vinylenes) and polyarylenes such as: polyfluorenes, particularly 2,7-linked 9,9 dialkyl polyfluorenes or 2,7-linked 9,9 diaryl polyfluorenes; polyspirofluorenes, particularly 2,7-linked poly-9,9-spirofluorene; polyindenofluorenes, particularly 2,7- linked polyindenofluorenes; polyphenylenes, particularly alkyl or alkoxy substituted poly-l,4-phenylene. Such polymers as disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.

Polymers preferably comprise a first repeat unit selected from arylene repeat units, in particular: 1,4-phenylene repeat units as disclosed in J. Appl. Phys. 1996, 79, 934; 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 C_1-20 alkyl or alkoxy; electron withdrawing groups such as fluorine, nitro or cyano; and substituents for increasing glass transition temperature (Tg) of the polymer.

Particularly preferred polymers comprise a first repeat unit of optionally substituted, 2,7- linked fluorenes, most preferably repeat units of formula:

wherein R¹ and R² are independently selected from hydrogen or optionally substituted alkyl, alkoxy, aryl, arylalkyl, heteroaryl and heteroarylalkyl. More preferably, at least one of R¹ and R² comprises an optionally substituted C₄-C₂₀ alkyl or aryl group. A polymer comprising the first repeat unit may provide one or more of the functions of hole transport, electron transport and emission depending on which layer of the device it is used in and the nature of co-repeat units. In particular:

- a homopolymer of the first repeat unit, such as a homopolymer of 9,9-dialkylfluoren- 2,7-diyl, may be utilised to provide electron transport.

- a copolymer comprising a first repeat unit and a triarylamine repeat unit, in particular a repeat unit selected from formulae 1-6, may be utilised to provide hole transport and / or emission:

wherein X, Y, A, B, C and D are independently selected from H or a substituent group. More preferably, one or more of X, Y, A, B, C and D is independently selected from the group consisting of optionally substituted, branched or linear alkyl, aryl, perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl and arylalkyl groups. Most preferably, X, Y, A and B are C_1-10 alkyl. The aromatic rings in the backbone of the polymer may be linked by a direct bond or a bridging group or bridging atom, in particular a bridging heteroatom such as oxygen.

Particularly preferred hole transporting polymers of this type are AB copolymers of the first repeat unit and a triarylamine repeat unit.

A copolymer comprising a first repeat unit and heteroarylene repeat unit may be utilised for charge transport or emission. Preferred heteroarylene repeat units are selected from formulae 7-21:

wherein R₆ and R₇ are the same or different and are each independently hydrogen or a substituent group, preferably alkyl, aryl, perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl or arylalkyl. For ease of manufacture, R₆ and R₇ are preferably the same. More preferably, they are the same and are each a phenyl group.

10 11

12

13

4 15

16 17

18 19

20

21 Electroluminescent copolymers may comprise an electroluminescent region and at least one of a hole transporting region and an electron transporting region as disclosed in, for example, WO 00/55927 and US 6353083. If only one of a hole transporting region and electron transporting region is provided then the electroluminescent region may also provide the other of hole transport and electron transport functionality.

The different regions within such a polymer may be provided along the polymer backbone, as per US 6353083, or as groups pendant from the polymer backbone as per WO 01/62869.

Preferred methods for preparation of these polymers are Suzuki polymerisation as described in, for example, WO 00/53656 and Yamamoto polymerisation as described in, for example, T. Yamamoto, "Electrically Conducting And Thermally Stable π - Conjugated Poly(arylene)s Prepared by Organometallic Processes", Progress in Polymer Science 1993, 17, 1153-1205. These polymerisation techniques both operate via a "metal insertion" wherein the metal atom of a metal complex catalyst is inserted between an aryl group and a leaving group of a monomer. In the case of Yamamoto polymerisation, a nickel 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 polymerisation, 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 will therefore be appreciated that repeat units and end groups comprising aryl groups as illustrated throughout this application may be derived from a monomer carrying 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, in particular AB, 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 groups include tosylate, mesylate and triflate.

A single polymer or a plurality of polymers may be deposited from solution to form a layer. Suitable solvents for polyarylenes, in particular polyfluorenes, include mono- or poly-alkylbenzenes such as toluene and xylene. Particularly preferred solution deposition techniques are spin-coating and inkjet printing.

If multiple layers of the device are formed by solution processing then the skilled person will be aware of techniques to prevent intermixing of adjacent layers, for example by crosslinking of one layer before deposition of a subsequent layer or selection of materials for adjacent layers such that the material from which the first of these layers is formed is not soluble in the solvent used to deposit the second layer.

Spin-coating is particularly suitable for devices wherein patterning of the electroluminescent material is unnecessary - for example for lighting applications or simple monochrome segmented displays. InkJet printing is particularly suitable for high information content displays, in particular full colour displays comprising red, green and blue electroluminescent material. InkJet printing of OLEDs is described in, for example, EP 0880303.

By "red electroluminescent material" is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 600-750 nm, preferably 600-700 nm, more preferably 610-650 run and most preferably having an emission peak around 650-660 nm.

By "green electroluminescent material" is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 510-580 nm, preferably 510-570 nm.

By "blue electroluminescent material" is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 400-500 nm, more preferably 430-500 nm.

The emitter: may be a fluorescent or a phosphorescent emitter. Preferred phosphorescent materials comprise transition metal complexes.

Numerous hosts are described in the prior art for phosphorescent emitters including "small molecule" hosts such as 4,4'-bis(carbazol-9-yl)biphenyl), known as CBP, and (4,4',4"-tris(carbazol-9-yl)triphenylamine), known as TCTA, disclosed in Ikai et al. (Appl. Phys. Lett., 79 no. 2, 2001, 156); and triarylamines such as tris-4-(N-3- methylphenyl-N-phenyl)phenylamine, known as MTDATA. Polymers are also known as hosts, in particular homopolymers such as polyvinyl carbazole) disclosed in, for example, Appl. Phys. Lett. 2000, 77(15), 2280; polyfluorenes in Synth. Met. 2001, 116, 379, Phys. Rev. B 2001, 63, 235206 and Appl. Phys. Lett. 2003, 82(7), 1006; poly[4-(N- 4-vinylbenzyloxyethyl, N-methylamino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv. Mater. 1999, 11(4), 285; and poly(para-ρhenylenes) in J. Mater. Chem. 2003, 13, 50-55. Copolymers are also known as hosts.

The emitting species may be a metal complex. Preferred metal complexes comprise optionally substituted complexes of formula:

wherein M is a metal; each of L , L and L is a coordinating group; q is an integer; r and s are 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 L¹, b is the number of coordination sites on L² and c is the number of coordination sites on L .

Heavy elements M induce strong spin-orbit coupling to allow rapid intersystem crossing and emission from triplet states (phosphorescence). Suitable heavy metals M include: lanthanide metals such as cerium, samarium, europium, terbium, dysprosium, thulium, erbium and neodymium; and - 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.

Suitable coordinating groups for the f-block metals include oxygen or nitrogen donor systems such as carboxylic acids, 1,3-diketonates, hydroxy carboxylic acids, Schiff bases including acyl phenols and iminoacyl groups. As is known, luminescent lanthanide metal complexes require sensitizing group(s) which have the triplet excited energy level higher than the first excited state of the metal ion. Emission is from an f-f transition of the metal and so the emission colour is determined by the choice of the metal. The sharp emission is generally narrow, resulting in a pure colour emission useful for display applications. The d-block metals form organometallic complexes with carbon or nitrogen donors such as porphyrin or bidentate ligands of formula:

(VI) wherein Ar⁴ and Ar³ may be the same or different and are independently selected from optionally substituted aryl or heteroaryl; X¹ and Y¹ may be the same or different and are independently selected from carbon or nitrogen; and Ar⁴ and Ar⁵ may be fused together. Ligands wherein X¹ is carbon and Y¹ is nitrogen are particularly preferred.

Examples of bidentate ligands are illustrated below:

Each

ed substituents include fluorine or trifluoromethyl which may be used to blue-shift the emission of the complex as disclosed in WO 02/45466, WO 02/44189, US 2002-117662 and US 2002-182441; alkyl or alkoxy groups as disclosed in JP 2002-324679; carbazole which may be used to assist hole transport to the complex when used as an emissive material as disclosed in WO 02/81448; bromine, chlorine or iodine which can serve to functionalise the ligand for attachment of further groups as disclosed in WO 02/68435 and EP 1245659; and dendrons which may be used to obtain or enhance solution processability of the metal complex as disclosed in WO 02/66552.

Other ligands suitable for use with d-block elements include diketonates, in particular acetylacetonate (acac); triarylphosphines and pyridine, each of which may be substituted. Main group metal complexes show ligand based, or charge transfer emission. For these complexes, the emission colour is determined by the choice of ligand as well as the metal.

The host material and metal complex may be combined in the form of a physical blend. Alternatively, the metal complex may be chemically bound to the host material. In the case of a polymeric host, the metal complex may be chemically bound as a substituent attached to the polymer backbone, incorporated as a repeat unit in the polymer backbone or provided as an end-group of the polymer as disclosed in, for example, EP 1245659, WO 02/31896, WO 03/18653 and WO 03/22908.

A wide range of fluorescent low molecular weight metal complexes are known and have been demonstrated in organic light emitting devices [see, e. g., Macromol. Sym. 125 (1997) 1-48, US-A 5,150,006, US-A 6,083,634 and US-A 5,432,014], in particular tris- (8-hydroxyquinoline)aluminium. Suitable ligands for di or trivalent metals include: oxinoids, e. g. with oxygen-nitrogen or oxygen-oxygen donating atoms, generally a ring nitrogen atom with a substituent oxygen atom, or a substituent nitrogen atom or oxygen atom with a substituent oxygen atom such as 8 -hydroxy quinolate and hydroxyquinoxalinol-10-hydroxybenzo (h) quinolinato (II), benzazoles (III), schiff bases, azoindoles, chromone derivatives, 3-hydroxyflavone, and carboxylic acids such as salicylate amino carboxylates and ester carboxylates. Optional substituents include halogen, alkyl, alkoxy, haloalkyl, cyano, amino, amido, sulfonyl, carbonyl, aryl or heteroaryl on the (hetero) aromatic rings which may modify the emission colour.

While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. An organic opto-electrical device comprising: a substrate having a first electrode disposed thereon for injecting charge carriers of a first polarity; an organic semiconductive region disposed over the first electrode; and a transparent second electrode disposed over the organic semiconductive region for injecting charge carriers of a second polarity, wherein the second electrode comprises a charge injecting layer, a buffer layer disposed over the charge injecting layer, and a transparent conductive layer disposed over the buffer layer, the buffer layer comprising an inorganic metal oxide material.

2. An organic opto-electrical device according to claim 1, wherein the inorganic metal oxide material is a transition metal oxide.

3. An organic opto-electrical device according to claim 2, wherein the transition metal oxide is an oxide of W, Mo, Pd, V, Ru or Ir.

4. An organic opto-electrical device according to claim 3, wherein the transition metal oxide is WO_x, where 0<x<3.

5. An organic opto-electrical device according to claim 4, wherein x > 2.

6. An organic opto-electrical device according to any preceding claim, wherein the buffer layer has a thickness in the range 20nm to 200nm, more preferably in the range 50nm to lOOnm.

7. An organic opto-electrical device according to any preceding claim, wherein the buffer layer has a resistivity of < 20000 ohm cm.

8. An organic opto-electrical device according to any preceding claim, wherein the second electrode further comprises a second buffer layer disposed above or below the buffer layer.

9. An organic opto-electrical device according to claim 8, wherein the second buffer layer buffer layer is a metal layer.

10. An organic opto-electrical device according to claim 9, wherein the metal layer has a thickness in the range 1 to lOnm, more preferably 1 to 5nm.

11. An organic opto-electrical device according to claim 9 or 10, wherein the metal layer comprises Al.

12. An organic opto-electrical device according to any preceding claim, wherein the second electrode has a transparency of at least 60%, more preferably at least 70%, still more preferably at least 80%, and most preferably at least 90%.

13. An organic opto-electrical device according to any preceding claim, wherein the buffer layer typically has a transparency of at least 70%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%.

14. An organic opto-electrical device according to claim 12 or 13, wherein the transparency is met across all of the visible wavelengths from 400 to 700nm.

15. An organic opto-electrical device according to any preceding claim, wherein the first electrode is an anode and the second electrode is a cathode.

16. An organic opto-electrical device according to any preceding claim, wherein the charge injecting layer has a thickness in the range 1 to lOnm, more preferably in the range 1 to 6nm, and most preferably in the range 3 to 6nm.

17. An organic opto-electrical device according to any preceding claim, wherein the transparent conductive layer has a thickness of from lOnm to lOOnm.

18. An organic opto-electrical device according to any preceding claim, wherein the transparent conductive layer has a transparency of at least 80%, preferably at least 90%.

19. An organic opto-electrical device according to any preceding claim, wherein the transparent conductive layer is a layer of transparent conductive oxide.

20. An organic opto-electrical device according to any preceding claim, wherein the transparent conductive oxide is ITO.

21. An organic opto-electrical device according to any one of claims 1 to 18, wherein the transparent conductive layer is a transparent metal layer.

22. An organic opto-electrical device according to claim 21, wherein the transparent metal layer has a thickness in the range 5 to 40nm.

23. An organic opto-electrical device according to claim 21 or 22, wherein the transparent metal layer is Ag or Al.

24. An organic opto-electrical device comprising: a substrate having a first electrode disposed thereon for injecting charge carriers of a first polarity; an organic semiconductive region disposed over the first electrode; and a transparent second electrode disposed over the organic semiconductive region for injecting charge carriers of a second polarity, wherein the second electrode comprises a composite layer comprising a charge injecting material and an inorganic metal oxide being an oxide of W, Mo, Pd, V, Ru, or Ir, and a transparent conductive layer disposed over the composite layer.

25. An organic opto-electrical device according to claim 24, wherein the inorganic metal oxide is WOx, where 0<x<3.

26. A method of manufacturing the organic opto-electrical device of any preceding claim, wherein the transparent conductive layer is deposited by a high energy deposition process.

27. A method according to claim 26, wherein the high energy deposition process is selected from sputtering and ion beam deposition.