WO2003080758A2

WO2003080758A2 - Electroluminescent device

Info

Publication number: WO2003080758A2
Application number: PCT/GB2003/001252
Authority: WO
Inventors: Poopathy Kathirgamanathan; Subramaniam Ganeshamurugan; Gnanamoly Paramaswara
Original assignee: Elam-T Limited
Priority date: 2002-03-22
Filing date: 2003-03-24
Publication date: 2003-10-02
Also published as: AU2003219292A1; GB0206736D0; WO2003080758A3; TW200305352A; AU2003219292A8

Abstract

An electroluminescent device with a deeper blue emitted colour has an electroluminescent layer comprising a mixture of lithium quinolate and α-NPB.

Description

Electroluminescent Device

The present invention relates to electroluminescent devices.

Materials which emit light when an electric current is passed through them are well known and used in a wide range of display applications. Liquid crystal devices and devices which are based on inorganic semiconductor systems are widely used, however these suffer from the disadvantages of high energy consumption, high cost of manufacture, low quantum efficiency and the inability to make flat panel displays

Typical electroluminescent devices comprise an anode, normally of an electrically light transmitting material, a layer of a hole transmitting material, a layer of the electroluminescent material, optionally a layer of an electron transmitting material and a metal cathode. There can be other layers, such as buffer layers and the layers can be combined using mixtures of one or more of the hole transmitting material, electroluminescent material and the electron transmitting material.

Patent application WO98/58037 describes a range of lanthanide complexes which can be used in electroluminescent devices which have improved properties and give better results. Patent Applications PCT/GB98/01773, PCT/GB99/03619, PCT/GB99/04030, PCT/GB99/04028. PCT/GB00/00268 describe electroluminescent complexes, structures and devices using rare earth chelates.

Patent Application WO 00/32717 discloses the use of a lithium quinolate as an electroluminescent material in electroluminescent devices. The lithium quinolate of this application emits light in the blue spectrum.

As described in this patent application the lithium quinolate is preferably synthesised by the reaction of a metal alkyl or alkoxide with 8-hydroxy quinoline in the liquid phase. The metal compound can be dissolved in an inert solvent added to the 8- hydroxyquinoline. The metal quinolate can be separated by evaporation or when a film of the metal quinolate is required, by deposition onto a suitable substrate.

The preferred alkyls are ethyl, propyl and butyl with n-butyl being particularly preferred. With metal alkoxides he preferred alkoxides are ethoxide, propoxides and butoxides. The method is particularly suitable for the preparation of group I, II and III metals such as lithium, sodium potassium, zinc, cadmium and aluminium alkoxides. the reaction, in an inert solvent, e.g. acetonitrile, of 8-hydroxyquinoline with a lithium alkyl or alkoxide preferably n-butyl lithium. The lithium quinolate is an off white or white solid at room temperature.

An article by C. Schmitz, H Scmidt and M. Thekalakat entitled Lithium Quinolate Complexes as Emitter and Interface Materials in Organic Light-Emitting Diodes in Chem. Mater, 2000, 12, 3012-3019 discloses the use of a layer of lithium quinolate together with hole transporting materials in electroluminescent devices.

As well as the lithium salt of 8-hydroxyquinoline, the term quinolate in this specification includes salts of substituted 8-hydroxyquinoline for example

where the substituents are the same or different in the 2, 3, 4, 5, 6 and 7 positions and are selected from alky, alkoxy, aryl, aryloxy, sulphonic acids, esters, carboxylic acids, amino and amido groups or are aromatic, polycyclic or heterocyclic groups.

Alkali metal alkyls are difficult compounds to handle practically as they are highly reactive and can catch fire spontaneously in air. For this reason they would not normally be chosen as reactants. We have found that the blue colour emitted by lithium quinolate can be made a deeper blue if there is a layer of lithium quinolate mixed with the hole transmitting compound α-NPB.

According to the invention there is provided an electroluminescent device which comprises sequentially an anode, a layer of a mixture of α-NPB and lithium quinolate and a cathode.

The formula of α-NPB is given in fig. 1 of the accompanying drawings.

The ratio of the mixture of α-NPB and lithium quinolate can vary widely for example the mixture can comprise from 95to 1% weight per cent of the α-NPB and from 5 to 99% weight per cent of the lithium quinolate.

Preferably the mixture of α-NPB and lithium quinolate comprises from 80 to 95 weight per cent of the α-NPB and from 20 to 5 weight per cent of the lithium quinolate.

The mixture of α-NPB and lithium quinolate can be deposited on the substrate directly by evaporation from a solution of the mixture in an organic solvent. The solvent which is used will depend on the material but chlorinated hydrocarbons such as dichloromethane are suitable in many cases.

Alternatively the material can be deposited by spin coating or by vacuum deposition from the solid state e.g. by sputtering or any other conventional method can be used. The lithium quinolate is preferably made by reacting 8-hydroxyquinoline with a lithium alkyl or alkoxide in an inert solvent which consists of or contains acetonitrile. The preferred alkyls are ethyl, propyl and butyl with n-butyl being particularly preferred. With lithium alkoxides the preferred alkoxides are ethoxide, propoxides and butoxides. The preferred method of synthesis is by the reaction, in an inert solvent, e.g. acetonitrile, of 8-hydroxyquinoline with n-butyl lithium. The lithium quinolate is an off white or white solid at room temperature. The lithium quinolate/ α-NPB mixture can be prepared by any conventional method e.g. by mixing and grinding together powders of the two compounds, by co-vacuum evaporation, by co-precipitation from solution etc.

There can be a layer of a hole transmitting material between the anode and the lithium quinolate/α-NPB layer and a preferred hole transmitting layer is α-NPB, other suitable hole transmitting materials include amine complexes such as poly (vinylcarbazole), N, N'-diphenyl-N, N'-bis (3-methylρhenyι) -1,1' -biphenyl -4,4'-diamine (TPD), an unsubstituted or substituted polymer of an amino substituted aromatic compound, a polyaniline, substituted polyanilines, polythiophenes, substituted polythiophenes, polysilanes etc. Examples of polyanilines are polymers of

(I) where R is in the ortho - or meta-position and is hydrogen, Cl-18 alkyl, Cl-6 alkoxy, amino, chloro, bromo, hydroxy or the group

where R is alky or aryl and R' is hydrogen, Cl-6 alkyl or aryl with at least one other monomer of formula I above.

Or the hole transporting material can be a polyaniline, polyanilines which can be used in the present invention have the general formula

(II) where p is from 1 to 10 and n is from 1 to 20, R is as defined above and X is an anion, preferably selected from Cl, Br, SO₄, BF , PF₆, H₂PO₃, H₂PO₄, arylsulphonate, arenedicarboxylate, polystyrenesulphonate, polyacrylate alkysulphonate, vinylsulphonate, vinylbenzene sulphonate, cellulose sulphonate, camphor sulphonates, cellulose sulphate or a perfluorinated polyanion.

Examples of arylsulphonates are p-toluenesulphonate, benzenesulphonate, 9,10- anthraquinone-sulphonate and anthracenesulphonate, an example of an arenedicarboxylate is phthalate and an example of arenecarboxylate is benzoate.

We have found that protonated polymers of the unsubstituted or substituted polymer of an amino substituted aromatic compound such as a polyaniline are difficult to evaporate or cannot be evaporated, however we have surprisingly found that if the unsubstituted or substituted polymer of an amino substituted aromatic compound is deprotonated the it can be easily evaporated i.e. the polymer is evaporable.

Preferably evaporable deprotonated polymers of unsubstituted or substituted polymer of an amino substituted aromatic compound are used. The de-protonated unsubstituted or substituted polymer of an amino substituted aromatic compound can be formed by deprotonating the polymer by treatment with an alkali such as ammonium hydroxide or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. The degree of protonation can be controlled by forming a protonated polyaniline and de-protonating. Methods of preparing polyanilines are described in the article by A. G. MacDiarmid and A. F. Epstein, Faraday Discussions, Chem Soc.88 P319 1989.

The conductivity of the polyaniline is dependant on the degree of protonation with the maximum conductivity being when the degree of protonation is between 40 and 60% e.g. about 50% for example.

Preferably the polymer is substantially fully deprotonated

A polyaniline can be formed of octamer units i.e. p is four e.g.

The polyanilines can have conductivities of the order of 1 x 10^"1 Siemen cm^"1 or higher.

The aromatic rings can be unsubstituted or substituted e.g. by a Cl to 20 alkyl group such as ethyl.

The polyaniline can be a copolymer of aniline and preferred copolymers are the copoiymers of aniline with o-anisidine. m-sulphanilic acid or o-aminophenol, or o- toluidine with o-aminophenol, o-ethylaniline, o-phenylene diamine or with amino anthracenes.

Other polymers of an amino substituted aromatic compound which can be used include substituted or unsubstituted polyaminonapthalenes, polyaminoanthracenes, polyaminophenanthrenes, etc. and polymers of any other condensed polyaromatic compound. Polyaminoanthracenes and methods of making them are disclosed in US Patent 6,153,726. The aromatic rings can be unsubstituted or substituted e.g. by a group R as defined above.

Other hole transporting materials are conjugated polymer and the conjugated polymers which can be used can be any of the conjugated polymers disclosed or referred to in US 5807627, PCT/WO90/13148 and PCT/WO92/03490.

The preferred conjugated polymers are poly (p-phenylenevinylene)-PPN and copolymers including PPN. Other preferred polymers are poly(2,5 dialkoxyphenylene vinylene) such as poly (2-methoxy-5-(2-methoxypentyloxy-l,4-phenylene vinylene), poly(2-methoxypentyloxy)- 1 ,4-phenylenevinylene), poly(2-methoxy-5-(2- dodecyloxy-l,4-phenylenevinylene) and other poly(2,5 dialkoxyphenylenevinylenes) with at least one of the alkoxy groups being a long chain solubilising alkoxy group, poly fluorenes and oligofluorenes, polyphenylenes and oligophenylenes, polyanthracenes and oligo anthracenes, ploythiophenes and oligothiophenes.

In PPN the phenylene ring may optionally carry one or more substituents e.g. each independently selected from alkyl, preferably methyl, alkoxy, preferably methoxy or ethoxy.

Any poly(arylenevinylene) including substituted derivatives thereof can be used and the phenylene ring in poly(p-phenylenevinylene) may be replaced by a fused ring system such as anthracene or naphthlyene ring and the number of vinylene groups in each polyphenylenevinylene moiety can be increased e.g. up to 7 or higher.

The conjugated polymers can be made by the methods disclosed in US 5807627, PCT/WO90/13148 and PCT/WO92/03490.

The thickness of the hole transporting layer is preferably 20nm to 200nm. The polymers of an amino substituted aromatic compound such as polyanilines referred to above can also be used as buffer layers with or in conjunction with other hole transporting materials.

The structural formulae of some other hole transporting materials are shown in Figures 1, 2, 3, 4 and 5 of the drawings, where R₁₌ R₂ and R₃ can be the same or different and are selected from hydrogen, and substituted and unsubstituted hydrocarbyl groups such as substituted and unsubstituted aliphatic groups, substituted and unsubstituted aromatic, heterocyclic and polycyclic ring structures, fluorocarbons such as trifluoryl methyl groups, halogens such as fluorine or thiophenyl groups; Kχ_ R and R₃ can also form substituted and unsubstituted fused aromatic, heterocyclic and polycyclic ring structures and can be copolymerisable with a monomer e.g. styrene. X is Se, S or O, Y can be hydrogen, substituted or unsubstituted hydrocarbyl groups, such as substituted and unsubstituted aromatic, heterocyclic and polycyclic ring structures, fluorine, fluorocarbons such as trifluoryl methyl groups, halogens such as fluorine or thiophenyl groups or nitrile.

Examples of Ri and/or R₂ and/or R₃ include aliphatic, aromatic and heterocyclic alkoxy, aryloxy and carboxy groups, substituted and substituted phenyl, fluorophenyl, biphenyl, phenanthrene, anthracene, naphthyl and fluorene groups alkyl groups such as t-butyl, heterocyclic groups such as carbazole.

There can be a hole transporting material mixed with the mixture of α-NPB and lithium quinolate, for example m-MTDATA as in Fig. 1.

Optionally there is a layer of an electron injecting material between the cathode and the electroluminescent material layer, the electron injecting material is a material which will transport electrons when an electric current is passed through electron injecting materials include a metal complex such as a metal quinolate e.g. an aluminium quinolate, lithium quinolate, a cyano anthracene such as 9,10 dicyano anthracene, cyano substituted aromatic compounds, tetracyanoquinidodimethane a polystyrene sulphonate or a compound with the structural formulae shown in figures 6 and 7 of the drawings in which the phenyl rings can be substituted with substituents R as defined above, instead of being a separate layer the electron injecting material can be mixed with the electroluminescent material and co-deposited with it.

The cathode can be any low work function metal e.g. aluminium, calcium, lithium, silver/magnesium alloys, rare earth metal alloys etc., aluminium is a preferred metal. A metal fluoride such as an alkali metal, rare earth metal or their alloys can be used as the second electrode for example by having a metal fluoride layer formed on a metal.

The anode is preferably a transparent substrate such as is a conductive glass or plastic material which acts as the anode, preferred substrates are conductive glasses such as indium tin oxide coated glass, but any glass which is conductive or has a conductive layer such as a metal or conductive polymer can be used. Conductive polymers and conductive polymer coated glass or plastics materials can also be used as the substrate.

Either or both electrodes can be formed of silicon and the electroluminescent material and intervening layers of a hole transporting and electron transporting materials can be formed as pixels on the silicon substrate. Preferably each pixel comprises at least one layer of the electroluminescent mixture and an (at least semi-) transparent electrode in contact with the organic layer on a side thereof remote from the substrate.

Preferably, the substrate is of crystalline silicon and the surface of the substrate may be polished or smoothed to produce a flat surface prior to the deposition of electrode, or electroluminescent mixture. Alternatively a non-planarised silicon substrate can be coated with a layer of conducting polymer to provide a smooth, flat surface prior to deposition of further materials. In one embodiment, each pixel comprises a metal electrode in contact with the substrate. Depending on the relative work functions of the metal and transparent electrodes, either may serve as the anode with the other constituting the cathode.

When the silicon substrate is the cathode an indium tin oxide coated glass can act as the anode and light is emitted through the anode. When the silicon substrate acts as the anode the cathode can be formed of a transparent electrode which has a suitable work function, for example by a indium zinc oxide coated glass in which the indium zinc oxide has a low work function. The anode can have a transparent coating of a metal formed on it to give a suitable work function. These devices are sometimes referred to as top emitting devices or back emitting devices.

The metal electrode may consist of a plurality of metal layers, for example a higher work function metal such as aluminium deposited on the substrate and a lower work function metal such as calcium deposited on the higher work function metal. In another example, a further layer of conducting polymer lies on top of a stable metal such as aluminium.

Preferably, the electrode also acts as a mirror behind each pixel and is either deposited on, or sunk into, the planarised surface of the substrate. However, there may alternatively be a light absorbing black layer adjacent to the substrate.

In still another embodiment, selective regions of a bottom conducting polymer layer are made non-conducting by exposure to a suitable aqueous solution allowing formation of arrays of conducting pixel pads which serve as the bottom contacts of the pixel electrodes.

As described in WO00/60669 the brightness of light emitted from each pixel is preferably controllable in an analogue manner by adjusting the voltage or current applied by the matrix circuitry or by inputting a digital signal which is converted to an analogue signal in each pixel circuit. The substrate preferably also provides data drivers, data converters and scan drivers for processing information to address the array of pixels so as to create images.

In one embodiment, each pixel is controlled by a switch comprising a voltage controlled element and a variable resistance element, both of which are conveniently formed by metal-oxide-semiconductor field effect transistors (MOSFETs) or by an active matrix transistor.

The invention is illustrated in the following examples.

Example 1 Lithium 8-hvdroxyquinolate LJ(CQH₆ON)

2.32g (0.016 mole) of 8-hydroxyquinoline was dissolved in acetonitrile and 10ml of 1.6M n-butyl lithium (0.016 mole ) was added. The solution was stirred at room temperature for one hour and an off white precipitate filtered off . The precipitate was washed with water followed by acetonitrile and dried in vacuo. The solid was shown to be lithium quinolate. As shown in WO 00/32717 the lithium quinolate had CIE colour coordinates x,y:0.19,0.28.

Example 2 Mixture Of Lithium Quinolate and α-NPB

The lithium quinolate powder prepared as in example 1 was mixed with α-NPB by mixing and grinding powders of the two materials together to form a mixed lithium quinolate/ α-NPB powder comprising 15% weight of lithium quinolate and 85% weight α-NPB. Example 3 Electroluminescent Device

An electroluminescent device was made by depositing sequentially from solution onto a indium tin oxide glass anode layers of copper phthalocyanine (buffer layer) 8nm; α-NPB (30nm); a mixture of lithium quinolate and α-NPB prepared as in Example 2 (30nm); lithium quinolate (20nm) and an aluminium cathode. The device is shown schematically in fig. 9 where (1) is the ITO anode; (2) is the copper phthalocyanine buffer layer; (3) is an α-NPB layer; (4) is the mixed lithium quinolate/α-NPB layer; (5) is a lithium quinolate layer and (6) is an aluminium cathode.

An electric current was passed between the aluminium cathode and ITO anode and the device emitted a blue light. The spectral radiance was measured and the results shown in fig. 8.

The properties of the device were measured and the results shown in Table 1. The colour is shown by the x and y coordinates which are the co-ordinates in the colour chart CIE 1931.

Table 1

Example 3

Example 4

An electroluminescent device was made as in Example 3 by depositing sequentially from solution onto a indium tin oxide glass anode layers of copper phthalocyanine (buffer layer) lOnm; p-PMTDATA (45nm); a mixture of lithium quinolate and α- NPB prepared as in Example 2 (20nm); lithium quinolate (lOnm) and an aluminium cathode. The p-PMTDATA is shown in fig. la.

The properties of the device were measured and the results shown in Table 2. The colour is shown by the x and y coordinates which are the co-ordinates in the colour chart CIE 1931

Table 2

Example 5

An electroluminescent device was made as in Example 3 by depositing sequentially from solution onto a indium tin oxide glass anode layers of copper phthalocyanine (buffer layer) lOnm; p-PMTDATA (45nm); a mixture of lithium quinolate and α- NPB prepared as in Example 2 (20nm) except that composition of the mixture was 90% α-NPB and 10% lithium quinolate; lithium quinolate (lOnm) and an aluminium cathode. The p-PMTDATA is shown in fig. la.

The properties of the device were measured and the results shown in Table 3. The colour is shown by the x and y coordinates which are the co-ordinates in the colour chart CIE 1931

Table 3

Example 6

A device was made as in Example 3 and the lithium quinolate/ α-NPB layer had p- MTDATA mixed with it.

The device had the structure - an indium tin oxide glass anode layers of copper phthalocyanine (buffer layer) 25nm; m-MTDATA (55nm); a mixture of lithium quinolate, α-NPB and m-MTDATA (40.41nm); lithium quinolate (5nm); lithium fluoride (0.4nm) and an aluminium cathode.

The a mixture of lithium quinolate, α-NPB and m-MTDATA comprised 99% lithium quinolate, 0.975% α-NPB and 0.025% m-MTDATA.

The properties were measured and the results shown in Fig. 10. Example 7

The device had the structure - an indium tin oxide glass anode layers of copper phthalocyanine (buffer layer) 25nm; m-MTDATA (72nm); a mixture of lithium quinolate, α-NPB and m-MTDATA (40.41nm); lithium quinolate (5nm); copper phthalocyanine (3nm), lithium quinolate (3nm) and an aluminium cathode. The a mixture of lithium quinolate, α-NPB and m-MTDATA comprised 99% lithium quinolate, 0.975% α-NPB and 0.025% m-MTDATA.

The properties were measured and the results shown in Fig. 11.

Claims

1. An electroluminescent device which comprises sequentially an anode, a layer of a mixture of α-NPB and lithium quinolate and a cathode.

2. An electroluminescent device as claimed in claim 1 in which the mixture of α-NPB and lithium quinolate comprises from 80 to 95 weight per cent of the α-NPB and from 20 to 5 weight per cent of the lithium quinolate.

3. An electroluminescent device as claimed in claim 1 or 2 in which the lithium quinolate is made by the reaction of 8-hydroxyquinoline with a lithium alkyl or alkoxide in the liquid phase.

4. An electroluminescent device as claimed in claim 1 or 2 in which the lithium quinolate is made by the reaction of 8-hydroxyquinoline with a lithium alkyl or alkoxide in a solvent consisting of or comprising acetonitrile.

5. An electroluminescent device as claimed in claim 4 in which the lithium alkyl is n- butyl lithium.

6. An electroluminescent device as claimed in any one of claims 1 to 5 in which there is a layer of a hole transmitting material between the anode and the lithium quinolate/α-NPB layer.

7. An electroluminescent device as claimed in claim 6 in which the hole transmitting material is α-NPB.

8. An electroluminescent device as claimed in claim 6 in which the hole transmitting material is selected from amine complexes such as poly (vinylcarbazole), N, N'- diphenyl-N, N'-bis (3-methylphenyl) -1,1' -biphenyl -4,4'-diamine (TPD), an unsubstituted or substituted polymer of an amino substituted aromatic compound, a polyaniline, substituted polyanilines, polythiophenes, substituted polythiophenes, polysilanes.

9. An electroluminescent device as claimed in claim 5 in which the hole transmitting material is selected from compounds of structural formulae shown in Figures 1, 2, 3, 4 and 5 of the drawings

10. An electroluminescent device as claimed in any one of claims 1 to 9 in which there is a hole transmitting material mixed with the mixture of α-NPB and lithium quinolate.

11. An electroluminescent device as claimed in any one of claims 1 to 10 in which there is a layer of an electron injecting material between the cathode and the electroluminescent material layer.

12. An electroluminescent device as claimed in claim 11 in which the electron injecting material is a metal quinolate, a cyano anthracene such as 9,10 dicyano anthracene, a cyano substituted aromatic compound, tetracyanoqumidodimethane a polystyrene sulphonate or a compound with the structural formulae shown in figure 6 or 7 of the drawings.

13. An electroluminescent device as claimed in any one of claims 1 to 12 in which the cathode is selected from aluminium, calcium, lithium, silver/magnesium alloys and rare earth metal alloys.

14. An electroluminescent device as claimed in any one of claims 1 to 13 in which the anode is a conductive glass anode.

15. An electroluminescent device as claimed in any one of claims 1 to 12 in which either the anode and/or the cathode is formed of silicon and the electroluminescent material and intervening layers of a hole transporting and electron transporting materials are formed as pixels on the silicon substrate.

16. An elecfroluminescent device as claimed in any one of claims 1 to 6 comprising sequentially an indium tin oxide glass anode; a layer of copper phthalocyanine ; a layer of α-NPB; a layer of a mixture of lithium quinolate and α-NPB; a layer of lithium quinolate and an aluminium cathode.