GB2518879A

GB2518879A - Electroluminescent devices

Info

Publication number: GB2518879A
Application number: GB1317604.5A
Authority: GB
Inventors: Richard Wilson; Martin Humphries; Oscar Fernandez
Original assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Current assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Priority date: 2013-10-03
Filing date: 2013-10-03
Publication date: 2015-04-08
Also published as: GB201317604D0

Abstract

A white light emitting electroluminescent device comprises, in sequence, (i) an anode, (ii) a hole injection layer, (iii) a first electroluminescent layer contacting the hole injecting layer, (iv) an optional spacer layer, (v) a second electroluminescent layer and (vi) a cathode. The first electroluminescent layer comprises a phosphorescent green light emitting material and the second electroluminescent layer comprises a fluorescent blue light emitting material. Either or both of the first and second electroluminescent layers and/or the spacer layer comprise a phosphorescent red light emitting material. The first electroluminescent layer has a hole mobility that is greater than its electron mobility, and hence there is no requirement for an interlayer between the first electroluminescent layer and the anode. The phosphorescent green light emitting material may be a small molecule or dendrimer, for example with an iridium centre, which is optionally chemically bound to a polymer.

Description

ELECTROLUMINESCENT DEVICES

The present invention relates to electroluminescent devices and their manufacture. It relates particularly to devices which emit white light.

Organic light emitting diodes (OLEDs) are usually formed from functional layers sandwiched between an anode layer, which is often made of indium tin oxide (ITO) on a glass or light transmitting polymer substrate, and a cathode, which often comprises a low work-function cathode layer, for example aluminium/lithium fluoride.

These functional layers usually include one or more electroluminescent layers and one or more hole transport layers, wherein the ho!e transport layers may also interchangeably be termed interlayers or primer layers, and/or one or more electron transport layers.

Charge/carriers (electrons or holes) are transported between the anode and cathode via the 1 5 electroluminescent layers and the hole transport layers and/or electron transport layers.

Generally charge carriers injected from an anode or cathode layer into an electroluminescent layer will travel to or from the interface between that layer and the hole and/or electron transport layers, from or towards the other of the anode or cathode layers at the other surface of the relevant charge transport layer.

An interlayer is normally positioned between the anode (or, if present, a hole injection layer overlying the anode) and an electroluminescent layer. Such an interlayer is known to enhance device lifetimes and/or quantum efficiencies, and may serve to facilitate charge transport and to protect the electroluminescent layer from adverse effects of being adjacent a hole transport layer.

However, providing an interlayer entails additional cost and complexity for manufacturing whether by solvent deposition or vapour deposition. It would be beneficial to be able to prepare white light emitting OLEDs that did not require the presence of an interlayer but * :. 30 retained the advantages of such a layer in terms of lifetime and/or efficiency.

The present invention provides a white light emitting OLED comprising in sequence (i) an anode, (ii) a hole injection layer, (iii) a first electroluminescent layer contacting the hole ""s injecting layer, (iv) an optional spacer layer, (v) a second electroluminescent layer and (vi) a " 35 cathode; wherein the first electroluminescent layer comprises a phosphorescent green light * ., emitting material, the second electroluminescent layer comprises a fluorescent blue -light emitting material, and one or more of the first and second electroluminescent layers and the *5**** * S optional spacer layer comprise a phosphorescent red light emitting material, and wherein the first electroluminescent layer has a hole mobility that is greater than its electron mobility.

By "red light emitting material" or "red light emitting polymer" or equivalents thereof is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 580-680 nm, preferably 590-660 nm, more preferably 590-640 nm and most preferably having an emission peak around 590-620 nm.

By "green light emitting material" or "green light emitting polymer" or equivalents thereof is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 500-580 nm, preferably 51 0-550 nm.

By "blue light emithng material" or "blue light emitting polymer" or equivalents thereof is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 380-500 nm, more preferably 430-500 nm. Aptly the emitters will be covalently bound into a polymer or mixed with a polymer.

"White light' as described herein 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 CIE x coordinate equivalent to that emitted by a black body at a temperature in the range of 2700-4500K.

In one form, the electroluminescent layers are formed from materials such as non-polymeric or "small molecule" materials. Such materials are capable of forming a vapour without significant decomposition) so that at least these layers of the devices may be fabricated by vacuum deposition.

In a favoured form, the electroluminescent layers and preferably the hole injection layer and spacer layer if present are formed from polymers such that the layers of the devices may be fabricated by deposition from solution. * .

The green light emitting material present in the first layer may be a material capable of vapour deposition such as for example a small molecule material. However, it can be 35 advantageous for the green light emitting material to be incorporated into a polymer. Aptly, * *. the green light emitting material forms part of the backbone of the polymer, or may be pendant from groups that form a part of the backbone of the polymer, or may be attached at *..*.* * one or both termini of the polymer if desired. Suitable green light emitting materials can include iridium containing dendrimers, for example those described in US Patent Application No 2010/003386, the contents of which is incorporated herein by cross reference.

As used herein, the term dendrimer" encompasses a core such as a metal complex, e.g. an iridium core, to which is attached at least one ligand having at least one branchpoint, such a ligand being termed herein a "dendron".

Such dendrimers may have an iridium core and three ligands of which at least one is a dendron. The ligands and dendrons may include phenylpyridine containing moieties or derivatives thereof bound to iridium or another metal, for example a dendrimer derived from an lr(ppy)3 core.

In such exemplary dendrimers the pyridyl, or more aptly the phenyl, residue will be substituted by a phenyl or biphenyl group and these groups in turn may be substituted by one or two phenyl groups each of which is optionally substituted by a group (sometimes referred to as a surface group) which may be C1-C20 alkyl or alkoxy, more usually C1-C10 alkyl or alkoxy, for example CrC6 alkyl or alkoxy such as a t-butyl group.

Suitable specific dendrimers include those of the formulas:

R :.". * * * .

S

**5**S * . * ** * * * *S*S * S S.*. * .

P

x j1_1 x -/ (Bu /_\ N' 2 CM9 \Jl a

-I S* S

OOC4H.

S S

S * *

45*555 C41-lg **.* -* * *.** * *****. * S * S. * S *.** S * (CH2) tBu\ tBu

I C4H9

C6 H13 I_CH

NX N

-- : --

* . I I C4H9 CH9 *.* * * *** *...$ In which R independently represent surface groups, and X independently represent Cl, Br, or covalent attachment points, for example to a polymer. A wavy line interrupting a bond :.:::: 10 similarly represents a covalent attachment point, for example to a polymer. * S

A suitable monomer for use as such or for incorporation into light emitting polymers is that of formula (I): Br LN_IKkYBr t.BU7f1öLt1tBU (I) The use of such dendrimers aid in ensuring that the first electroluminescent layer is more effective at hole transportation than electron transportation. Without thereby being limited by theory, it is believed that such dendrimers in general may possess good hole conduction properties.

The phosphorescent red light emitting material may be in either or both of the first and second electroluminescent layers. It is presently thought particularly apt to include it in the first electroluminescent layer only. Conventional phosphorescent red lighting materials may be employed. These may be small molecules or dendrimers, or more aptly comprised within a polymer. The skilled person is aware of a plethora of such emitters, for example a red emitter is derived from: t-Bu01A-Su [H

N N

* * 1Th flY N fl ** * . S...

S S

The devices of this invention are unusual in that they can achieve enhanced lifetimes and efficiencies without the need for an interlayer between the hole injection layer and the first electroluminescent layer. It is possible to have the first electroluminescent layer immediately adjacent the hole injection layer by selecting its components to provide better hole transport properties than electron transport properties.

The skilled person is aware of which monomers to employ, for example for inclusion into an electroluminescent polymer, to achieve the desirable hole transporting properties. One apt way is to include amine containing residues, such as for example triarylamines, in such polymers. Such components aptly comprise 1 to 30 mole percent of the components of the polymer, for example 5 to 20 mole percent of the components of the polymer.

The optional spacer layer may be composed of materials, for example one or more polymers, which impede triplet excitons passing between the second and first electroluminescent layers for example by exhibiting a higher first triplet energy level (Ti) relative to the materials in adjacent layers. Preferably, the materials of the spacer layer also exhibit high hole and electron conductivities, e.g. with a LUMO level of about -1.8 to -3.0 eV.

Such layers are well known and the skilled person is aware of a plethora of suitable materials.

The optional spacer layer between the first and second electroluminescent layers is preferably present. This semi conducting layer may be comprised of materials known to the skilled person for providing semi conducting layers, for example, semi conducting polymers.

The layers of the devices of the invention may be formed by vacuum deposition of their *. PS components. The skilled person is well acquainted with methods of vacuum deposition as these are employed in the manufacture of experimental and commercial devices.

It is particularly apt in making the device of the invention to employ polymers dissolved in * 35 organic solvents to form the layers of the device. The choice of solvents and conditions are well known to the skilled person as conventional methods may be employed. * S * I...

S I

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.

Spin-coating is particularly suitable for devices. lnkjet printing is suitable. lnkjet printing of OLEDs is described in, for example, EP 0 880 303.

Other solution deposition techniques include dip-coating, roil printing, nozzle printing, slot-dye coating, and screen 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.

The conductive hole injection layer may be formed of a doped organic material located between the anode and the first electroluminescent layer assists hole injection from the anode into the layer or layers of semiconducting polymer. Examples of doped organic hole injection materials include poly(ethylene dioxythiophene) (PEDT), polyaniline as disclosed in US 5723873 and US 5798170, and poly(thienothiophene). Exemplary acids include PEDT doped with polystyrene sulfonate (PSS) as disclosed in EP 0901176 and EP 0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nation ®.

The electroluminescent layers may consist of the electroluminescent material (e.g. polymer) 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 99/481 60, or may comprise a luminescent dopant in a semiconducting host matrix. Alternatively, the ** ** * electroluminescent material may be covalently bound to a charge transporting material and / or host material.. ** S

*° The cathode may be selected from materials that have a workfunction allowing injection of electrons into the electroluminescent layer. Other factors influence the selection of the cathode such as the possibility of adverse interactions between the cathode and the SOS electroluminescent material. The cathode may consist of a single material such as a layer of * S.SS * 0 aluminium. Alternatively, it may comprise a plurality of metals, for example a bilayer of a low workfunction material and a high workfunction material such as calcium and aluminium as disclosed in WO 98/10621; elemental barium as disclosed in WO 98/57381, App!. Phys. Let 2002, 81(4), 634 and WO 02/84759; or a thin layer of metal compound, in particular an oxide or fluoride of an alkali or alkali earth metal, to assist electron injection, for example lithium fluoride as disclosed in WO 00/48258 or barium fluoride as disclosed in AppI. Phys. Left.

2001, 79(5), 2001. In order to provide efficient 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.

As stated previously, the cathode may be transparent or, if the anode is transparent, it may be opaque. 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 will comprises a layer of an electron injecting material that is sufficiently thin to be transparent.

Typically, the lateral conductMty 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 ITO.

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.

Suitable electroluminescent and/or charge transporting polymers include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes.

Polymers preferably comprise a first repeat unit selected from arylene repeat units as disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.

Exemplary first repeat units include: 1,4-phenylene repeat units as disclosed in J. AppI.

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 * :" 35 units is optionally substituted. Examples of substituents include solubilising groups such as * ** C120 alkyl or alkoxy; electron withdrawing groups such as fluorene, nitro or cyano; and substituents for increasing glass transition temperature (Tg) of the polymer.

*..e.. * S

Particularly preferred polymers comprise optionally substituted, 2,7-linked fluorenes, most preferably repeat units of formula I: (1) 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 of Formula 2: Ar1_J_Ar2 n (2) wherein Ar1 and Ar2 are optionally substituted aryl or heteroaryl groups, n is greater than or equal to 1, preferably 1 or 2, and Fl is H or a substituent, preferably a substituent. Fl is :. preferably alkyl or aryl or heteroaryl, most preferably aryl or heteroaryl. Any of the aryl or heteroaryl groups in the unit of formula 1 may be substituted. Preferred substituents include alkyl and alkoxy groups. Any of the aryl or heteroaryl groups in the repeat unit of Formula 1 may be linked by a direct bond or a divalent linking atom or group. Preferred divalent linking 30 atoms and groups include 0, S; substituted N; and substituted C. * ** Particularly preferred units satisfying Formula 2 include units of Formulae 3-5: Ar1 Ar1 Ar1 Ar1 Ar1 Ar'- __ / NNV NNV Ar3 Ar2_N\ Ir ir Ar'Ar (3) (4) (5) wherein Ar1 and Ar2 are as defined above; and Ar3 is optionally substituted aryl or heteroaryl.

Where present, preferred substituents for Ar3 include alkyl and alkoxy groups.

-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 6-20: * N N87 N (6) wherein FR6 and FR7 are the same or different and are each independently hydrogen or a substituent group, preferably alkyl, aryl, pertluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl or arylalkyl. For ease of manufacture, FR6 and FR7 are preferably the same. More preferably, they are the same and are each, say, a phenyl group.

Ph Ph Ph Ph

H 20ji

(7) (8) *"": Ph Ph Ph Ph

H H

N N N N

: (9) (10)

S

*5*SS* * S PhPh C8H17C8H17 (11) C8H17 C5H17 C8H17 C8H17 0 NN (13) NN (14) 0 N,N S (16) (15) (18) N,N (17) S N,N (19) (20) 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.

S

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 wa 01/62869. *5*s

55*555 Phosphorescent emitters; in addition to those referred to hereinbefore, commonly utilise metal complexes, such as those comprising optionally substituted complexes of formula (V): ML1qL2rL3s wherein M is a metal; each of L1, L2 and L3 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 L3.

Heavy elements M induce strong spin-orbit coupling to allow rapid intersystem crossing and emission from triplet or higher 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, pallaidum, 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 d-block metals are particularly suitable for emission from triplet excited states. These metals form organometallic complexes with carbon or nitrogen donors such as porphyrin or bidentate ligands of formula (VI): * * **** S..... * . (VI)

wherein Ar4' and Ar5' 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.

Ugands wherein X1 is carbon and Y1 is nitrogen are particularly preferred.

Examples of bidentate ligands are illustrated below: ç cW C& 0/ Each of Ar4' and Ar5' may carry one or more substituents. Two or more of these substituents may be linked to form a ring, for example an aromatic ring. Particularly preferred 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 2002-1 82441; 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, The first electroluminescent layer, for example of polymer, may comprise a first fluorene based electroluminescent polymer to act as a phosphorescent host for a green and optionally red emitting iridium complex.

With advantage, the first electroluminescent layer will include an amine compound in an electroluminescent polymer. This will aptly comprise 1 to 30 mole % of the polymers, for example 10 to 20 mole % of the polymer. This contributes to the hole transportation properties of the layer.

Generally each of the electroluminescent layers of polymer in the device of this invention, is generally 10 nm to 200 nm thick, more favourably from 10 nm to 100 nm thick, for example about 25 nm to 50 nm thick, such as 30 nm thick.

Preferred methods for preparation of the polymers according to the invention are Suzuki polymerisation as described in, for example, WO 00/53656 and Vamamoto polymerisation as described in, for example, T. Yamamoto, "Electrically Conducting And Thermally Stable -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 r" 35 leaving group. * 4* 1 4 * *.

* 4 15 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 such as tosylate, mesylate and trif late.

In the following Examples (including comparative examples) the devices were produced by conventional disposition of polymer layers from solution in xylene. The thickness of the layers were conventional and were sufficiently similar in examples of the invention and in comparative examples not to affect test results. The hole injection layer was as described, for example, in W006036755 or W006008648.

Relative charge carrier mobilities ban be determined by techniques that are well known in the literature, such as the carrier time-of-flight (TOF) technique, the dark injection space-chargelimited (DISCL) current transient method, and transient EL as described by Poplavskyy et al. (J. AppI. Phys., 98, 014501 (2005), and Khan et al in J. Physical Review B, 75(3), 035215 (2007). Clearly if differences are to be measured the same technique should be used for both materials.

Example 1: Two Comparative Devices This example demonstrates why interlayers have been employed in former devices: Device 1: ITO/H IL/IL/Single Component Blue layer/Cathode Device 2: ITO/H IL/Single Component Blue layer/Cathode * * * * C In these devices the ITO layer was about 50 nm thick, the hole injection layer (HIL) of was about 35 nm thick, the interlayer (IL) of Device 1 was about 23 nm thick, the single **.* component blue (3GB) light emitting layer was about 70 nm thick and the cathode was :: 35 NaF/Al/Ag. * ** * . * S...

The IL comprised a polymer derived from of 50% monomer A, 42.5% monomer B, 7.5% monomer C. The material of the SCS layer was the same polymer in both devices and was derived from 36% monomer D, 14% monomer E, 45% monomer F, 4% monomer 0, and 1% monomer H. The T (from 1000 cd/A) of Device 1 was greater than 80 hrs but was only 12 hr in Device 2.

This difference in lifetime illustrates why interlayers have been used in conventional devices.

Examnle 2: Two White Light Emitting Devices This demonstrates that the interlayer may be omitted from devices in which the first electroluminescent layer is more effective at conducting holes than electrons.

Device 3: ITO/HI L/interlayer/EL1 Ispacer/EL2/Cathode Device 4: ITO/HI L/EL1 /spacer /EL2/Cathode In these devices the ITO layer was 145 nm thick, the hole injection layer (HIL) was about 48 nm thick, the interlayer of Device 3 was 21 nm thick, the first electroluminescent layer (ELi) was 28 nm thick, the spacer layer was about 19 nm thick, the second electroluminescent layer (EL2) was 49 nm thick and the cathode was NaF/Al/Ag. The first electroluminescent layer included green and red emitters and the second electroluminescent layer included a blue emitter.

The IL was as described for Device 1. The ELI comprised a polymer derived from 50% monomer A, 34.9% monomer B, 5% monomer C, 5% monomer I, 5% monomer J, and 0.1% monomer K. The spacer layer comprised a polymer derived from 50% monomer A, 20% monomer B, 10% monomer L, 5% monomer M, 5% monomer N and 5% monomer I and 10% monomer 0. The EL2 was as described for SCB of Device 1.

The performances obtained for the two devices were substantially similar with Device 3 having a T50 (from 1000 cd/A) of 88 hrs and Device 4 having a T50 of 84 hours.

* .t..

Example 3 36

The following Device 5 was prepared: ITO/HIL:50 nm/ELi:29 nm/Spacer:29 nm/EL2:50 nm/Cathode The cathode was as described for Device 1. The ELi comprised a polymer derived from 40% compound A, 10% compound P, 32.445% compound D, 1% compound L, 6.5% compound Q, 5% compound N, 5% compound R, 0.11% compound S. The Spacer comprised a polymer derived from 50% compound A, 19.92% compound B, 12% compound L, 8% compound 0, 10% compound N and 0.15% compound S. The EL2 comprised a polymer derived from 50% compound D, 30% compound T, 14% compound E and 6% compound C. An analogous device was also prepared which contained in addition an interlayer 23 nm thick between the NIL and ELi. This interlayer had the composition of that of the interlayer of Device 3. When tested it was found that the T5 of the two devices were not significantly affected by the presence or absence of the interlayer with both T50 values being in excess of 200 hrs. The efficiency of Device 5 was not quite as good as that of Device 4, indicating that the presence of the amino component in Device 4 produced an added benefit.

Example CIEx CIEy EQE Device 5 0.470 0.449 13.2 Device 4 0.461 0.438 13.54 Key to Monomers Compound A: C6H13 Nj_os)=\ 1° a * * * * S *:... 25 Compound B. S... * .. * . * *.*

**SSSS

S

Compound C: Compound D: cm Compound E: C8H17 C8H17 Compound F; Br Compound G: Br *:* 20 Compound H: *0. N * S S... * I *.*. * . C4H * ** * . * *.*. **.*.

* . 19 Compound I: Compound J: 1 0 Compound K: Compound L:

N -N

-I * .. *

*.**** * * * 0 0*e * 0 * S. * 0 * **** * *** SO.

Compound M: Br Compound N: Br Compound 0: Br -j'--N -c5--Br Compound P: * * S * * Compound 0: * *5*S* * * *..* * S ****

S

5S5S5. * * ** * * * *S** *

****** * 21

-I Br

D

Compound ft / Compound S:

N N

N N * * * **.**

* Compound T: 0** **** * Br Br * 0 S. S..

S S

Claims

CLAIMS1. A white light emitting electroluminescent device, comprising in sequence (i) an anode, (ii) a hole injection layer, (iii) a first electroluminescent layer contacting the hole injecting layer, (iv) an optional spacer layer, (v) a second electroluminescent layer and (vi) a cathode; wherein the first electroluminescent layer comprises a phosphorescent green light emitting material, the second electroluminescent layer comprises a fluorescent blue light emitting material, and one or more of the first and second electroluminescent layers and the optional spacer layer comprise a phosphorescent red light emitting material, and wherein the first electroluminescent layer has a hole mobility that is greater than its electron mobility.
2. A device according to claim 1, wherein at least one of said electroluminescent layers is formed from materials capable of vapour deposition.
3. A device according to claim 1, wherein at least one of said electroluminescent layers comprises a polymer.
4. A device according to claim 1, wherein the hole injection layer comprises a polymer.
5. A deviceaccording to claim 1 comprising a spacer layer comprising a polymer.
6. A device according to any one of claims 1 to 5, wherein the phosphorescent green light emitting material is a small molecule or dendrirner.
7. A device according to any one of claims 1 to 6, wherein the phosphorescent green light emitting material forms a part of a polymer.
8. A device according to claim 7, wherein the phosphorescent green light emitting material forms part of the backbone of said polymer.*
9. A device according to claim 7, wherein the phosphorescent green light emitting 0*e** * material is pendant to the backbone of said polymer. S...

S ens
10. A device according to any one of claims 6 to 9, wherein the phosphorescent green light emitting material is an iridium containing dendrimer. S * S *.*.S

SS* SI 23
11. A device according to claim 7, wherein the phosphorescent green light emifting material comprises a compound of the formula, or comprises a compound derived from a compound of the formula: wherein each X independently represents Br, Cl or an attachment point to said polymer, and each B independently represents a surface group.
12. A device according to claim 11, wherein each surface group B is independently selected from C1-C10 alkyl or alkoxy.* 15
13. A device according to claim 12, wherein each surface group B is independently selected from C2-C6 alkyl or alkoxy. S....*
14. A device according to claim 13, wherein each surface group B is t-butyl. S...

S

* 20
15. A device according to claim 7, wherein the phosphorescent green light emitting material is formed by incorporation into said polymer a compound of the formula:

S *...*
16. A device according to any of claims ito 15, wherein the phosphorescent red light emitting material is present in the first electroluminescent layer-
17. A device according to claim 16, wherein the phosphorescent red light emitting material forms a part of a polymer.
18. A device according to claim 17, wherein the phosphorescent red light emitting material and the phosphorescent green light emitting material form parts of the same polymer.
19. A device according to any of claims 17 or 18. wherein the red light emitting material is formed by incorporation into said polymer a compound of the formula: IOu
20. A device according to any of claims 1 to 19, wherein the first electroluminescent layer comprises a polymer comprising one or more repeat units capable of hole conduction. * .
21. A device according to claim 20, wherein the one or more repeat units comprise amine containing repeat units. * * * 0SS.. **S
22. A device according to claim 21, wherein the polymer comprises from 1 to 30 mole percent amine containing repeat units.
23. A device according to claim 22, wherein the polymer comprises from 5 to 20 mole percent amine containing repeat units. *. .. t. S 0 0*0OS * . fls% *fl*S**..*fr * S S. *0 * *4*$SI.. .*.