WO2002090465A1

WO2002090465A1 - Electroluminescent devices

Info

Publication number: WO2002090465A1
Application number: PCT/GB2002/002092
Authority: WO
Inventors: Poopathy Kathirgamanathan
Original assignee: Elam-T Limited
Priority date: 2001-05-04
Filing date: 2002-05-07
Publication date: 2002-11-14
Also published as: WO2002090466A1

Abstract

An electroluminescent device has an anode, a layer of a rare earth electroluminescent compound and a cathode in which the anode is silicon, preferably there is a layer of a hole transmitting material between the anode and the electroluminescent compound and a layer of an electron transmitting material between the electroluminescent compound and the silicon cathode.

Description

Electroluminescent Devices

The present invention relates to a method of forming 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.

Organic polymers have been proposed as useful in electroluminescent devices, but it is not possible to obtain pure colours, they are expensive to make and have a relatively low efficiency.

Another compound which has been proposed is aluminium quinolate, but this requires dopants to be used to obtain a range of colours and has a relatively low efficiency.

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/GB99/04024, PCT/GB99/04028, PCT/GB00/00268 describe electroluminescent complexes, structures and devices using rare earth chelates.

US Patent 5128587 discloses an electroluminescent device which consists of an organometallic complex of rare earth elements of the lanthanide series sandwiched between a transparent electrode of high work function and a second electrode of low work function with a hole conducting layer interposed between the electroluminescent layer and the transparent high work function electrode and an electron conducting layer interposed between the electroluminescent layer and the electron injecting low work function anode. The hole conducting layer and the electron conducting layer are required to improve the working and the efficiency of the device. The hole transporting layer serves to transport holes and to block the electrons, thus preventing electrons from moving into the electrode without recombining with holes. The recombination of carriers therefore mainly takes place in the emitter layer.

We have now devised improved electroluminescent devices and structures using a silicon cathode.

The invention provides an electroluminescent device comprising (i) a first electrode, (ii) a layer of a rare earth chelate electroluminescent compound and (iii) a second electrode in which one of the electrodes comprises silicon.

The silicon electrode can function as an anode or as a cathode in use or both the anode and cathode can comprise silicon. The layer of the electroluminescent compound can be formed directly on the silicon layer or optionally there can be intermediate layers as described below.

When the silicon electrode functions as a cathode optionally there is a layer of an electron injecting material between the silicon cathode and the electroluminescent material layer, as described below.

Optionally there can be a layer of a metal fluoride such as an alkali metal, rare earth metal or their alloys and preferably lithium fluoride on the silicon cathode for example by having a metal fluoride layer deposited on the silicon.

Electrical connections can be formed on the silicon electrode using conventional techniques e.g. by means of indium/gallium contact. When the cathode is silicon preferably the first electrode is a transparent substrate such as a conductive glass e.g. glass coated with indium tin oxide, antimony oxide, indium antimony oxide, cadmium oxide cadmium tin oxide.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.

For example 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 electrode comprises the cathode the silicon anode can be a thin layer of silicon which is transparent and can be in contact with the electroluminescent layer directly or through a layer such as a layer of a hole transporting material as described below. The silicon anode can be formed on a transparent substrate such as a conductive glass or plastic material, 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.

When the cathode is silicon an n-type silicon can be used and when the anode is silicon a p-type silicon can be used.

If the silicon layer is opaque i.e. it is too thick to be transparent, the light is emitted from the device around the edges of the silicon and interdigitated structures are preferred e.g. the electrode is formed of strips of the silicon.

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 cathode can be a transparent thin layer of a metal such as gold, platinum etc. with a suitable work function or a work function adjusted to a suitable value.

The preferred electroluminescent materials are preferably rare earth chelates. Rare earth chelates are known which fluoresce in ultra violet radiation and A. P. Sinha (Spectroscopy of Inorganic Chemistry Vol. 2 Academic Press 1971) describes several classes of rare earth chelates with various monodentate and bidentate ligands.

Group III A metals and lanthanides and actinides with aromatic complexing agents have been described by G. Kallistratos (Chimica Chronika, New Series, 11, 249-266 (1982)). This reference specifically discloses the Eu (III), Tb (III), U (III) and U (IV) complexes of diphenyl-phosponamidotriphenyl-phosphoran.

EP 0744451A1 also discloses fluorescent chelates of transition or lanthanide or actinide metals and the known chelates which can be used are those disclosed in the above references including those based on diketone and triketone moieties.

Examples of electroluminescent compounds which can be used as the electroluminescent materials in the present invention are of general formula (Lα)_nM where M is a rare earth, lanthanide or an actinide, Lα is an organic complex and n is the valence state of M.

Preferred electroluminescent compounds which can be used in the present invention are of formula

where Lα and Lp are organic ligands, M is a rare earth, transition metal, lanthanide or an actinide and n is the valence state of the metal M. The ligands Lα can be the same or different and there can be a plurality of ligands Lp which can be the same or different.

For example (Li)(L₂)(L₃)(L..)M (Lp) where M is a rare earth, transition metal, lanthanide or an actinide and (Lι)(L )(L₃)(L...) are the same or different organic complexes and (Lp) is a neutral ligand. The total charge of the ligands (Lι)(L₂)(L₃)(L..) is equal to the valence state of the metal M. Where there are 3 groups Lα which corresponds to the III valence state of M the complex has the formula (Lι)(L₂)(L₃)M (Lp) and the different groups (L (L₂)(L₃) may be the same or different

Lp can be monodentate, bidentate or polydentate and there can be one or more ligands Lp.

Preferably M is metal ion having an unfilled inner shell and the preferred metals are selected from Sm(III), Eu(II), Eu(III), Tb(III), Dy(III), Yb(III), Lu(III), Gd (III), U(III), U(VI)O₂, Tm(iπ), Th(IV), Ce (III), Ce(IV), Pr(III), Nd(III), Pm(III), Dy(III), Ho(III), Er(III) and more preferably Eu(IH), Tb(III), Dy(III), Gd (III).

Further electroluminescent compounds which can be used in the present invention are of general formula (Lα)_nMιM₂ where Mi is the same as M above, M₂ is a non rare earth metal, Lα is a as above and n is the combined valence state of Mi and M . The complex can also comprise one or more neutral ligands Lp so the complex has the general formula (Lα)_n Mi M₂ (Lp), where Lp is as above. The metal M₂ can be any metal which is not a rare earth, transition metal, lanthanide or an actinide examples of metals which can be used include lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, copper (I), copper (II), silver, gold, zinc, cadmium, boron, aluminium, gallium, indium, germanium, tin (II), tin (IV), antimony (II), antimony (TV), lead (II), lead (IV) and metals of the first, second and third groups of transition metals in different valence states e.g. manganese, iron, ruthenium, osmium, cobalt, nickel, palladium(II), palladiurn(IV), platinum(II), platinum(IV), cadmium, chromium, titanium, vanadium, zirconium, tantulum, molybdenum, rhodium, iridium, titanium, niobium, scandium, yttrium.

For example (Lι)(L₂)(L₃)(L..)M (Lp) where M is a rare earth, transition metal, lanthanide or an actinide and (Lι)(L₂)(L₃)(L...) and (Lp) are the same or different organic complexes.

Further organometallic complexes which can be used in the present invention are binuclear, trinuclear and polynuclear organometallic complexes e.g. of formula

where L is a bridging ligand and where M_\ is a rare earth metal and M₂ is M] or a non rare earth metal, Lm and Ln are the same or different organic ligands Lα as defined above, x is the valence state of M] and y is the valence state of M₂.

In these complexes there can be a metal to metal bond or there can be one or more bridging ligands between Mj and M₂ and the groups Lm and Ln can be the same or different.

By trinuclear is meant there are three rare earth metals joined by a metal to metal bond i.e. of formula (Lm)_xM M, ( n)_y— M₂(Lp)₂

or

where Mi, M₂ and M₃ are the same or different rare earth metals and Lm, Ln and Lp are organic ligands Lα and x is the valence state of Mi, y is the valence state of M₂ and z is the valence state of M₃. Lp can be the same as Lm and Ln or different.

The rare earth metals and the non rare earth metals can be joined together by a metal to metal bond and/or via an intermediate bridging atom, ligand or molecular group. For example the metals can be linked by bridging ligands e.g.

(Lr Λ i M₃(|_n)_y M₂(_Lp)₂

or

where L is a bridging ligand By polynuclear is meant there are more than three metals joined by metal to metal bonds and/or via intermediate ligands

M₁ M₂ M₃ M₄ or

M₁ M₂ M₄ M₃ or

or

M M₃

where Mi, M₂, M₃ and M are rare earth metals and L is a bridging ligand.

The metal M₂ can be any metal which is not a rare earth, transition metal, lanthanide or an actinide examples of metals which can be used include lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, copper, silver, gold, zinc, cadmium, boron, aluminium, gallium, indium, germanium, tin, antimony, lead, and metals of the first, second and third groups of transition metals e.g. manganese, iron, ruthenium, osmium, cobalt, nickel, palladium, platinum, cadmium, chromium, titanium, vanadium, zirconium, tantulum, molybdenum, rhodium, iridium, titanium, niobium, scandium, yttrium etc.

Preferably Lα is selected from β diketones such as those of formulae

where R_1; 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; Rι_; R₂ 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.

Some of the different groups Lα may also be the same or different charged groups such as carboxylate groups so that the group L] can be as defined above and the groups L₂, L _... can be charged groups such as

(IV) where R is Ri as defined above or the groups L_ls L₂ can be as defined above and L_3. etc. are other charged groups.

R_1; R₂ and R₃ can also be

where X is O, S, Se or NH.

(V)

A preferred moiety Ri is trifluoromethyl CF₃ and examples of such diketones are, banzoyltrifluoroacetone, p-chlorobenzoyltrifluoroacetone, p-bromotrifluoroacetone, p-phenyltrifluoroacetone, 1 -naphthoyltrifluoroacetone, 2-naphthoyltrifluoroacetone, 2-phenathoyltrifluoroacetone, 3-phenanthoyltrifluoroacetone, 9- anthroyltrifiuoroacetonetrifluoroacetone, cinnamoyltrifluoroacetone, and 2- thenoyltrifluoroacetone.

The different groups Lα may be the same or different ligands of formulae

(VI) where X is O, S, or Se and R] R₂ and R₃ are as above

The different groups Lα may be the same or different quinolate derivatives such as

(VII) (VIII) where R is hydrocarbyl, aliphatic, aromatic or heterocyclic carboxy, aryloxy, hydroxy or alkoxy e.g. the 8 hydroxy quinolate derivatives or

(IX) (X) where R, Ri, and R₂ are as above or are H or F e.g. Ri and R₂ are alkyl or alkoxy groups

(XI) (XII) As stated above the different groups Lα may also be the same or different carboxylate groups e.g.

(XIII) where R₅ is a substituted or unsubstituted aromatic, polycyclic or heterocyclic ring a polypyridyl group, R₅ can also be a 2-ethyl hexyl group so L_n is 2-ethylhexanoate or R₅ can be a chair structure so that L„ is 2-acetyl cyclohexanoate or Lα can be

R

(XIV)

where R is as above e.g. alkyl, allenyl, amino or a fused ring such as a cyclic or polycyclic ring.

The different groups Lα may also be

(XV) (XVI)

M (XVII) Where R, R\ and R₂ are as above.

The groups Lp can be selected from

Ph Ph

O N Ph

Ph Ph

(XVIII) Where each Ph which can be the same or different and can be a phenyl (OPNP) or a substituted phenyl group, other substituted or unsubstituted aromatic group, a substituted or unsubstituted heterocyclic or polycyclic group, a substituted or unsubstituted fused aromatic group such as a naphthyl, anthracene, phenanthrene or pyrene group. The substituents can be for example an alkyl, aralkyl, alkoxy, aromatic, heterocyclic, polycyclic group, halogen such as fluorine, cyano, amino. Substituted amino etc. Examples are given in figs. 1 and 2 of the drawings where R, Ri, R_2> R₃ and R₄ can be the same or different and are selected from hydrogen, hydrocarbyl groups, substituted and unsubstituted aromatic, heterocyclic and polycyclic ring structures, fluorocarbons such as trifluoryl methyl groups, halogens such as fluorine or thiophenyl groups; R, R_1; R _; R₃ and Rt can also form substituted and unsubstituted fused aromatic, heterocyclic and polycyclic ring structures and can be copolymerisable with a monomer e.g. styrene. R, R_1; R_2; R₃ and ^ can also be unsaturated alkylene groups such as vinyl groups or groups

-CH, ^:CH, R where R is as above.

L_p can also be compounds of formulae

(XVIV) (XX) (XXI) where R_ls R and R₃ are as referred to above, for example bathophen shown in fig. 3 of the drawings in which R is as above or

(XXII) (XXIII)

where R_l5 R₂ and R₃ are as referred to above.

L_p can also be

Ph Ph Ph

N- 0^: :P N P= 0

Ph Ph or Ph Ph

(XXIV) (XXV) where Ph is as above.

Other examples of L_p chelates are as shown in figs. 4 and fluorene and fluorene derivatives e.g. a shown in figs. 5 and compounds of formulae as shown as shown in figs. 6 to 8.

Specific examples of Lα and Lp are tripyridyl and TMHD, and TMHD complexes, α, α', α" tripyridyl, crown ethers, cyclans, cryptans phthalocyanans, porphoryins ethylene diamine tetramine (EDTA), DCTA, DTPA and TTHA. Where TMHD is 2,2,6,6-tetramethyl-3,5-heptanedionato and OPNP is diphenylphosphonimide triphenyl phosphorane. The formulae of the polyamines are shown in fig. 9.

Other electroluminescent materials which can be used include metal quinolates such as lithium quinolate, and non rare earth metal complexes such as aluminium, magnesium, zinc and scandium complexes such as complexes of β-diketones e.g. Tris -(l,3-diphenyl-l-3-propanedione) (DBM) and suitable metal complexes are A1(DBM)₃, Zn(DBM)₂ and Mg(DBM)_2., Sc(DBM)₃ etc.

Preferably there is a layer of a hole transporting material between the anode and the layer of electroluminescent material. The hole transporting layer serves to transport holes and to block the electrons, thus preventing electrons from moving into the electrode without recombining with holes. The recombination of carriers therefore mainly takes place in the emitter layer.

The hole transporting material can be an amine complex 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 etc. Examples of polyanilines are polymers of

(XXVI) 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

(XXVII) 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, BF₄, PF₆, H₂PO , 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 copolymers 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)-PPV and copolymers including PPV. Other preferred polymers are poly(2,5 dialkoxyphenylene vinylene) such as poly (2-methoxy-5-(2-methoxypentyloxy-l,4-phenylene vinylene), poly(2-methoxypentyloxy)-l ,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 PPV 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 11, 12, 13, 14, 15 and 16 of the drawings, where Rj_, 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; Ri, R₂ 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.

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 figure 10 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.

When the silicon electrode is the anode 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 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.

Optionally the hole transporting material can be mixed with the electroluminescent material and co-deposited with it.

The hole transporting materials, the electroluminescent material and the electron injecting materials can be mixed together to form one layer, which simplifies the construction.

The display of the invention may be monochromatic or polychromatic. Electroluminescent rare earth chelate compounds are known which will emit a range of colours e.g. red, green, and blue light and white light and examples are disclosed in Patent Applications WO98/58037 PCT/GB98/01773, PCT/GB99/03619, PCT/GB99/04030, PCT/GB99/04024, PCT/GB99/04028, PCT/GB00/00268 and can be used to form OLEDs emitting those colours. 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 on the silicon substrate. Alternative arrangements can comprise sequentially a silicon cathode : layer electron transmitting material : layer of electroluminescent material : layer of hole transporting material : anode, silicon : layer electron transmitting material : layer of electroluminescent material : layer of hole transporting material : silicon anode. silicon cathode : layer electron transmitting material : layer of electroluminescent material : layer of hole transporting material : silicon anode.

The silicon can be a porous silicon or a crystalline silicon and the surface of the silicon substrate may be polished or smoothed to produce a flat surface prior to the deposition of the adjacent layers. 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.

Example 1

Figure 17 shows a device with a silicon anode in which (1) is a transparent cathode, (2) is a layer of an electron transmitting material (3) is a layer of the electroluminescent material (4) is a layer of the hole transporting material and (5) is the silicon anode. When an electric current is passed through the device light is transmitted through the transparent cathode.

An electroluminescent device was fabricated to this structure, omitting the hole transporting layer (4), by depositing sequentially on an anode comprising crystalline p-type silicon, layers of a terbium (III) green electroluminescent organo metal complex (70nm) based on Tb(III)(TMHD)₃OPNP, and a layer of an aluminium quinolate as an electron transmitting layer (45nm). There was transparent indium tin oxide coated glass as the cathode. An indium gallium contact was formed on the silicon cathode. The silicon anode was formed of a p-type silicon of orientation (100) of conductivity 0.1 S/cm and thickness 0.5mm. At a voltage above 9 volts green light was emitted through the ITO coated glass cathode. A plot of current density against bias voltage is shown in fig. 18.

Claims

1. An electroluminescent device comprising (i) a silicon anode, (ii) a layer of an electroluminescent compound comprising a metal chelate and (iii) a second electrode.

2. An electroluminescent device as claimed in claim 1 in which the anode is a transparent layer of silicon.

3. An electroluminescent device as claimed in claim 1 or 2 in which the silicon anode is formed of strips of silicon on a substrate to form an interdigitated structure.

4. An electroluminescent device as claimed in any one of claims 1 to 3 in which the silicon anode is formed on a transparent substrate.

5. An electroluminescent device as claimed in any one of claims 1 to 4 in which the second electrode (the cathode) is a low work function metal selected from aluminium, calcium, lithium, silver/magnesium alloys and rare earth metal alloys.

6. An electroluminescent device as claimed in any one of claims 1 to 5 in which there is a layer of an electron transmitting material between the second electrode and the electroluminescent compound.

7. An electroluminescent device as claimed in claim 6 in which the electron transmitting material is a metal quinolate, a cyano anthracene, 9,10 dicyano anthracene, a polystyrene sulphonate or a compound of formulae as shown in fig. 9 or 10.

8. An electroluminescent device as claimed in claim 7 in which the metal quinolate is lithium, sodium, potassium, zinc, magnesium or aluminium quinolate.

9. An electroluminescent device as claimed in any one of claims 1 to 8 in which the silicon second electrode (the cathode) is formed of a transparent layer of silicon on a transparent substrate.

10. An electroluminescent device as claimed in claim 9 in which the substrate is a conductive glass or plastic or a transparent layer of metal.

11. An electroluminescent device as claimed in any one of claims 1 to 10 in which there is a layer of a hole transmitting material between the first electrode and the electroluminescent compound.

12. An electroluminescent device as claimed in claim 11 in which the hole transmitting layer is an aromatic amine complex.

13. An electroluminescent device as claimed in claim 1 in which the hole transmitting layer is formed from a poly(vinylcarbazole), N,N'-diphenyl-N,N'-bis (3- methylphenyl) -1,1' -biphenyl -4,4'-diamine (TPD), polyaniline, or a substituted polyaniline.

14. An electroluminescent device as claimed in claim 11 in which the hole transmitting material has the formula of any of figures 11 to 16.

15. An electroluminescent device as claimed in any one of claims 11 to 14 in which there is a layer of a metal fluoride on the second electrode.

16. An electroluminescent device as claimed in any one of claims 1 to 15 in which the second electrode (the cathode) is formed of a transparent material such as a conductive glass or plastic or a transparent layer of a metal.

17. A structure or device as claimed in any one of the preceding claims in which the

electroluminescent compound is of the formula V^Lcvfi ^{,vι L}-P where Lα, n,

M, Lp are as defined herein.

18. A structure or device as claimed in claim 17 in which the electroluminescent compound is (Lα)_nMιM₂ where Mi, is as defined herein, M₂ is a non rare earth metal, Lα is as defined herein and n is the combined valence state of Mi and M₂.

19. A structure or device as claimed in claim 17 in which the electroluminescent compound is of the general formula (Lα)_n Mi M₂ (Lp), where Mi, M₂, Lα and Lp are as defined herein and n is the combined valence state of Mi and M₂.

20. A structure or device as claimed in claim 17 or 18 in which M and Mi are selected from Sm(III), Eu(II), Eu(III), Tb(III), Dy(IH), Yb(III), Lu(III), Gd (III), U(III), U(VI)O₂, Tm(III), Th(IV), Ce (IH), Ce(TV), Pr(Ifl), Nd(III), Pm(III), Dyfffl), Ho(iπ), Er(III).

21. A structure or device as claimed in any one of claims 18 to 20 in which M is lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, copper (I), copper (II), silver, gold, zinc, cadmium, boron, aluminium, gallium, indium, germanium, tin (II), tin (IV), antimony (II), antimony (IV), lead (II), lead (IV) and metals of the first, second and third groups of transition metals in different valence states e.g. manganese, iron, ruthenium, osmium, cobalt, nickel, palladium(II), palladium(IV), platinum(II), platinum(IV), cadmium, chromium, titanium, vanadium, zirconium, tantulum, molybdenum, rhodium, iridium, titanium, niobium, scandium, or yttrium.

22. A structure or device as claimed in any one of claims 1 to 21 in which the electroluminescent compound is a binuclear, trinuclear and polynuclear organometallic complexes of formula (Lm )_xM,^^L ₂(Ln)_y or

(Lm)_xM _! M₃(_Ln)_y— M₂(Lp)_z or

or

^L- ,

(Lm)_xM M, (Ln)_y ^M ₂(Lp)_z v, x . / or

or

M₁ M₂ M₃ M₄ or

M₁ M₂ M₄ M₃ or M- - - Mo

' * ^≠ 1

M₃- - " M₄ or N ^ ^-L .-

M ₁ M₂ M„ „

V, v, ^~L where Mi, M₂, M₃ and M₄ are rare earth metals where Mi , M₂ and M₃ are the same or different rare earth metals and Lm, Ln and Lp are organic ligands Lα as defined herein, and x is the valence state of M , y is the valence state of M₂ and z is the valence state of M_3ι Lp can be the same as Lm and Ln or different, and L is a bridging ligand.

23. A structure or device as claimed in any one of claims 1 to 22 in which the electroluminescent compound is an electroluminescent non rare earth metal complex of formula (Mq)_n(Lα)_n where Mq is the metal, n is the valency state of the metal and Lα is as specified herein.

24. A structure or device as claimed in claim 23 in which the metal Mq is selected from lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, copper, silver, gold, zinc, cadmium, boron, aluminium, gallium, indium, germanium, tin, antimony, lead, and metals of the first, second and third groups of transition metals e.g. manganese, iron, ruthenium, osmium, cobalt, nickel, palladium, platinum, cadmium, chromium, titanium, vanadium, zirconium, tantulum, molybdenum, rhodium, iridium, titanium, niobium, scandium, yttrium etc. which emit light when an electric current is passed through it.

25. A structure or device as claimed in any one of claims 1 to 24 in which the electroluminescent compound is an electroluminescent metal quinolate.

26. A structure or device as claimed in claim 25 in which the metal quinolate is selected from lithium, aluminium, zinc, barium, cadmium, sodium, potassium and magnesium quinolate.