METHOD FOR FORMING FILMS OR LAYERS

Abstract: An improved method for forming films of organo-metallic complexes in which an organic complex and a labile metal salt are vaporised together or sequentially and the vapour condensed on to a substrate to form a film or layer of the organo-metallic complex on the substrate. The method produces electroluminescent films with improved performance.

Description: Method for Forming Films or Layers The present invention relates to a method of forming films or layers of organo- metallic complexes and more particularly it relates to a method of forming organo- metallic complexes and depositing them on a substrate. When forming a complex of an organic ligand and a metal, a salt of the metal and the organic ligand are reacted together under the appropriate conditions in solution in a suitable solvent. A very large number of reactions are well known and a very large number of organo-metallic complexes are known and are formed by this process. A particular class of organo-metallic complexes are formed from transition metals. lanthanides and actinides and these complexes have a very large range of applications in catalysis, electrical and electronic devices etc. Some organo-metallic complexes have electroluminescent or photoluminescent properties and have been described in an article in Chemistry Letters pp 657-660, 1990 Kido et al and in an article in Applied Physics Letters 65 (17) 24 October 1994 Kido et al but these were unstable in atmospheric conditions and difficult to produce as films. Photoluminescent complexes which are rare earth chelates which fluoresce in ultra violet radiation are known 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, 1 1. 249-266 (1982)). This reference specifically discloses the Eu(III), Tb(III) and U(III) complexes of diphenyl-phosponamidotriphenyl-phosphoran. EP 0744451A1 also discloses fluorescent chelates of transition or lanthanide or actinide metals. When forming an electroluminescent or photoluminescent device which incorporates an organo-metallic complex as the electroluminescent or photoluminescent active material, a film of the electroluminescent or photoluminescent compound has to be formed on a substrate. This is normally done by deposition of the compound from solution onto the substrate so as to obtain a film or layer of the right thickness etc. The organo-metallic complex can be formed in solution and deposited from this solution or it can be separated and dissolved in another solvent and deposited from this solution etc. or it can be formed by vacuum evaporation of the solid material. We have invented an improved method of forming a film or layer of an organo- metallic complex on a substrate which does not require a solution to be formed nor does it require multiple syntheses. According to the invention there is provided a method for forming a film or layer of an organo-metallic complex on a substrate which method comprises vaporising a metal compound and vaporising an organic complex and condensing the vapour on to a substrate to form a film or layer of the organo-metallic complex on the substrate. In one embodiment the invention comprises mixing a powder comprising a metal compound with a powder comprising an organic complex, heating the mixture formed under a vacuum so that the mixture is vaporised and condensing the vapour on to a substrate to form a film or layer of the organo-metallic complex on the substrate. In another embodiment of the invention the metal compound and the organic complex are vaporised sequentially. The term vaporised includes all forms of going from the solid state to the vapour or gaseous state such sublimation etc. The invention is particularly useful with transition metals, lanthanides and actinides which can form stable and useful complexes with organic ligands by this method. The metal preferably is in the form of a salt such as a halide e.g. chloride or bromide which is labile so that it can be vaporised or sublimed, or an organo-metallic compound e.g. diketo complexes, acetyl acetonates, although any metal compound which will vaporise under the conditions can be used. The invention is particularly useful for the preparation of films or layers of photoluminescent and electroluminescent compounds including those incorporating lanthanides or actinides such as samarium, dysprosium, lutetium, thorium, yttrium. gadolinium, europium, terbium, uranium and cerium in the appropriate valence states. Any metal ion having an unfilled inner shell can be used as the metal and the preferred metals are Sm(III), Eu(III). Tb(III). Dy(III), Yb(III), Lu(III), Gd (III), Eu(II). CE(III), Gd(III) U(III),UO (VI), Th(IlI). as well as metals with an incomplete inner shell e.g. Th(IV), Y(III), La(III), Ce(IV). Mixtures of more than one metal compound and more than one organic complex can be used to obtain a range of mixed organo-metallic complexes. This can be achieved by mixing the compounds before vaporisation or by vaporising the compounds sequentially so as to obtain a multilayered or a mixed layer comprising organo- metallic complexes. In another embodiment of the invention the metal compound can be deposited on the substrate and then a mixture of the metal compound and the organic compound deposited by vaporisation of a mixture of these to obtain various types of layered structures. The organic ligands in the organo-metallic complex formed can be any organic complex which can react with the metal compound to form an organo-metallic complex or which, when co-deposited with the organic complex forms an organo- metallic complex. It may be, particularly with sequential deposition that the organo- metal complex formed has a variable composition through the layer. The organic ligands which can be used include or where R' is the same or different at different parts of the molecule and each R' ' and R' is a hydrocarbyl group e.g. alkyl, a substituted or unsubstituted aromatic or heterocyclic ring structure a fluorocarbon or R" is -CF or hydrogen or R ' ' is copolymerised with a monomer e.g. or R' is t-butyl and R' ' hydrogen. Preferably each of R', R". and R' is an alkyl group preferably a -C(CH3) group, or X X I I Φ Φ 0= P — N = P — φ — X Φ Φ I I x x where X can be the same or different in each case and can be H, F, Me, Et. OMe. or OEt. Preferred organic complexes are 2,2,6, 6-tetramethyl-3,5-heptanedionato(TMHD); « ' α"-| α"' tripyridyl, dibenzoyl methane(DBM), diphenylphosphonimide triphenyl phosphorane(OPNP), bathophen (4,7-diphenyl-l,l-phenanthroline), 1 ,10 phenanthroline (Phen) and crown ethers, cryptands, substituted calix(4)arene based ligands and other organic complexes Particularly preferred organo-metallic complexes are the Th, Y, Eu, Dy, and SM complexes of tripyridyl and TMDH and complexes such as thorium bathophen, yttrium tripyridyl and TMHD, and europium (III) (TMHD) 3 complexes, terbium (TMHD) 3 OPNP. europium (II) (TMHD) 2 and europium (II) (TMHD) 2 OPNP. The organic complex used to form the organo-metallic complex should be in a form which is stable in powder form under the conditions of the vaporisation, for organic complexes which are unstable in oxygen an inert atmosphere can be used. When mixed powders of the metal compound and the organic complex are vaporised they should be of a size such that intimate mixing can take place, compounds can be bought commercially in powder form and these are often suitable without further treatment, if smaller size particles are required, the starting material can be ground. If the organic complex is formed by precipitation from a solution this can be formed as a powder which, after drying, can be mixed with the metal compound. In one embodiment of the invention an organo-metallic complex such as a complex of a transition metal, lanthanide or actinide and any of the above specified organic ligands is mixed with an organic complex e.g. as referred to above and the resultant mixture heated in vacuum to vaporise the mixture and to deposit the organo metal complex formed onto a substrate. In another embodiment of the invention an organo-metallic complex such as a complex of a transition metal, lanthanide or actinide and any of the above specified organic ligands is heated in vacuum to vaporise the mixture and to deposit the organo- metallic complex on the substrate and then an organic complex e.g. as referred to above and is heated in vacuum to vaporise the complex and to deposit it on top of the orano-metallic compound to form the organo-metal complex layer on the substrate. The starting materials should be dry and. if necessary they can be dried prior to heating to remove any water or other liquid compounds. The vaporisation takes place under a vacuum which is preferably below 10 torr and -6 more preferably below 10 torr. The metal compound and the organic complex or a mixture of these can be placed in a suitable coater, the temperature is not critical but should be sufficient to vaporise or o υ sublime the mixture. Temperatures above 100 C and up to 250 C and higher can be used. The vapour is then condensed or deposited onto a substrate which is held at a lower temperature as in conventional vapour deposition processes e.g. chemical vapour deposition, vacuum coating, electron beam coating etc. The substrate which can be used will depend for the purposes for which the device is to be used, as the substrate need not be subjected to heating conventional substrates can be used. The method is particularly useful in forming electroluminescent devices in which a layer of an electroluminescent compound is deposited on to a conductive substrate optionally with other layers such as layers of hole transporting compounds and electron transporting compounds to arrive a multilayered structure in which an anode and cathode are attached to form an electroluminescent device. In a typical electroluminescent device there is a transparent substrate which is a 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 can be used. The electroluminescent material can be deposited on the substrate by the method of the invention or preferably there can be a hole transporting layer deposited on the transparent substrate and the electroluminescent material is deposited on the hole transporting layer. 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. Hole transporting layers are used in polymer electroluminescent devices and any of the known hole transporting materials in film form can be used. The hole transporting layer can be made of a film of an aromatic amine complex such as poly(vinylcarbazole), N,N"-diphenyl-N,N'-bis (3-methylphenyl) -1 ,1 ' -biphenyl - 4.4'-diamine (TPD) or polyaniline. There is preferably an electron transporting layer between the cathode and the electroluminescent layer. This electron transporting layer can be a metal complex such as a metal quinolate e.g. an aluminium quinolate which will transport electrons when an electric current is passed through it or an organic compound such as PBD or a polymer e.g. containing cyano groups. The cathode can be any metal. In a preferred structure there is a substrate formed of a transparent conductive material which is the anode on which is successively deposited a hole transportation layer, the electroluminescent layer and an electron transporting layer which is connected to the cathode. Optionally dyes such as fluorescent laser dyes, luminescent laser dyes can be included to modify the colour spectrum of the emitted light and also enhance the photoluminescent and electroluminescent efficiencies. The thickness of the layer deposited on the substrate by the method of the invention can be varied according to the conditions of the vaporisation and deposition and in general layers of from one molecule thick up to any desired thickness can be formed. It has been surprisingly found that for electroluminescent and photoluminescent organo-metallic complexes formed by the method of the invention have superior properties to those complexes where the formed organo-metallic complex is deposited on a substrate from a solution e.g. by evaporation or spin deposition. The reason for this is not known and was unexpected. Types of electroluminescent devices are shown in figs la, l b, l c and I d of the drawings in which (1 ) is layer deposited in accordance with the invention. (2) is ITO, (3) is an electron transporting layer (4) is a hole transporting layer and (5) is an elctron transporting layer. The invention enables a emissive layer to be deposited on ITO or on to a layer previously applied to the ITO. Examples. Thin films were made according to the invention and compared with films made by- chemical synthesis. The Photoluminescence (PL) Efficiency and colour were measured and the results shown in Table 1. The colour was measured using colour chart CIE 1931. Example 1 Thin film Tb(TMHD), OPNP (i) 50mg of Tb(T HD) (7xl0°moles) and 33.68mg of OPNP(7.05xl θ ": ' moles) were ground together in a mortar and pestle and a 3mg portion was placed on a molybdenum boat in an Edwards(E306) vacuum coater. Spectrosil slides (UVgrade) -7 were secured on a sample holder. The vacuum coater was evacuated to 10 torr and the sample was heated using an electrical heater at 10 to 90 A and 10V for up to twenty seconds to give a film of Tb(TMHD), OPNP. Comparative experiments were carried out on (ii) a film of Tb(TMHD) OPNP (3 mg) pre-synthesised from a solution and (iii) and a film of Tb(TMHD) (3mg) purchased from Strem chemicals, Cambridge. The spectra are shown in fig. 3. Example 2 Thin Film Eu(DBM) 3 OPNP (i)50mg of Eu(DBM) 3 (6 x 10° moles) was mixed with 57.43mg of OPNP (1.2 xl0 ~ moles) and ground in a mortar and pestle. The mixture was evaporated as in Example 1 to form a film of Eu(DBM) 3 (OPNP) n . (ii) A Comparative experiment was carried out on a chemically synthesised film of Eu(DBM) 3 (OPNP) . The spectra are shown in fig. 4. Table 1 Example System Relative PL CIE(x) CIE(y) Efficiency 1(0 Tb(TMHD) OPNP(50nm) 2 0.35 0.58 l (ii) Tb(TMHD) 3 OPNP (40nm) 1 0.35 0.69 l(iii) Tb(TMHD) 3 (50nm) lxio "3 0.29 0.62 2(i) Eu(DBM) 3 OPNP n (50nm) 1.4 0.62 0.32 2(ii) Eu(DBM) 3 OPNPn (50nm) 1 0.58 0.30 Example 3 Fabrication of an Electroluminescent Devices Based on Tb(TMHD) OPNP (i) A 3mg mixture of Tb(TMHD) OPNP prepared as in Example 1 was evaporated onto a patterned ITO electrode to give a film of 50 nm thickness. The patterned ITO had been previously coated with 20 nm of a hole transporting layer formed of TPD. Aluminium quinolate (Alq ) was then evaporated on top the layer give a film of 20nm thickness to act as an electron-transporting layer. An aluminium top contact (900 nm) was made to form the structure of fig. 2 of the drawings. (ii) A comparative experiment was carried out using the same structure and configuration but using a chemically synthesised Tb(TMHD), OPNP. The luminous efficiency was measured and the results shown in Table 3. Table 3 System Normalised Absolute Efficienc\ (// /ltn w ) (i) Tb(TMHD) 3 OPNP 6.0 (ii) TbfTMHD OPNP 1.0 As can be seen the device based on a film formed by the method of the invention has a much higher luminous efficiency. Example 4 Fabrication of an Electroluminescent Device Based on Eu(DBM) Phen The structure of Example 4 was fabricated using an equimolar mixture of Eu(DBM) and 1,10 Phenanthroline in place of the Tb(TMHD) OPNP so as to form a film of Eu(DBM) Phen. The experimental conditions and the structural configuration were as in example 4 The luminous efficiency was measured and the results shown in Table 4. Table 4 System Normalised Absolute Efficiency (i) Eu(DBM) Phen 1.1 (ii) Eu(DBM) 3 Phen .0 As can be seen the device based on a film formed by the method of the invention has a 10% higher luminous efficiency.

Claims: Claims 1. A method for forming a film or layer of an organo-metallic complex on a substrate which method comprises vaporising a metal compound and vaporising an organic complex and condensing the vapour on to a substrate to form a film or layer of the organo-metallic complex on the substrate. 2. A method as claimed in claim 1 which comprises mixing a powder comprising a metal compound with a powder comprising an organic complex, heating the mixture formed so that the mixture is vaporised and condensing the vapour on to a substrate to form a film or layer of the organo-metallic complex on the substrate. 3. An electroluminescent device as claimed in claim 1 in which the metal compound and the organic complex are vaporised sequentially. 4. A method as claimed in claim any one of claims 1 to 3 in which the metal compound is a labile salt of a metal selected from transition metals, lanthanides and actinides or selected from labile organo-metallic compound of transition metals, lanthanides and actinides. 5. A method as claimed in claim 4 in which the metal is Sm(III). Eu(III), Tb(III), GGdd Eu(II). CE(III), Gd(III) U(III),UO 2 (VI), Th(III), Th(IV), Y(III), La(III) or Ce(IV). 6. A method as claimed in any one of claims 1 to 5 in which mixtures of more than one metal compound and more than one organic complex are vaporised to obtain a mixed organo-metallic complex. 7. A method as claimed in claim 6 in which the compounds are mixed before vaporisation. 8. A method as claimed in claim 6 in which the compounds vaporised sequentially so as to obtain a multilayered or a mixed layer comprising organo-metallic complexes. 9. A method as claimed in claim 6 in which the metal compound is deposited on the substrate and then a mixture of the metal compound and the organic compound deposited by vaporisation of a mixture of these to obtain a layered structures. 10. A method as claimed in any one of the preceding claims in which the organic complex incorporates a moiety of formula or where R' is the same or different at different parts of the molecule and each R' ' and R' is a hydrocarbyl group e.g. alkyl. a substituted or unsubstituted aromatic or heterocyclic ring structure a fluorocarbon or R' ' is -CF or hydrogen or R' ' is copolymerised with a monomer e.g. or R' is t-butyl and R' ' hydrogen. or X X φ φ I I 0= P — N =P — φ — X Φ Φ X X where X can be the same or different in each case and can be H, F, Me, Et, OMe, or OEt. 1 1. A method as claimed in claim 10 in which each of R', R", and R' is an alkyl group. 12. A method as claimed in claim 10 in which the organic complex is selected from 2,2,6,6-tetramethyl-3,5-heptanedionato(TMHD); α' α" α"' tripyridyl, dibenzoyl methane(DBM). diphenylphosphonimide triphenyl phosphorane(OPNP), bathophen (4, 7-diphenyl- 1 ,1 -phenanthroline), 1,10 phenanthroline (Phen) and crown ethers, cryptands. substituted calix(4)arene based ligands and other organic complexes 13. A method as claimed in claim 10. 1 1 or 12 in which the organo-metallic complexes is selected from the Th. Y, Eu. Dy, and SM complexes of tripyridyl and TMDH and thorium bathophen, yttrium tripyridyl and TMHD, and europium (III) (TMHD) 3 complexes, terbium (TMHD) 3 OPNP, europium (II) (TMHD) 2 and europium (II) (TMHD) 2 OPNP. 14. A method for forming electroluminescent devices in which a layer of an electroluminescent compound is deposited on to a conductive substrate by the method of any one of the preceding claims.

Inventors: KATHIRGAMANATHAN POOPATHY, KATHIRGAMANATHAN, POOPATHY
Applicants: KATHIRGAMANATHAN POOPATHY, SOUTH BANK UNIV ENTPR LTD, KATHIRGAMANATHAN, POOPATHY, SOUTH BANK UNIVERSITY ENTERPRISES LTD.
Application year: 1999
Main classification: C09K11
IPC codes: B05D7, C09K11, C23C14, H01L51, H05B33