GB2458096A

GB2458096A - Organic electroluminescent devices

Info

Publication number: GB2458096A
Application number: GB0720618A
Authority: GB
Inventors: Stephen Clemmet; Deborah Stokes; Mark Peace
Original assignee: POLYMERTRONICS Ltd
Current assignee: POLYMERTRONICS Ltd
Priority date: 2007-10-20
Filing date: 2007-10-20
Publication date: 2009-09-09
Also published as: GB0720618D0; WO2009050497A1

Abstract

An electroluminescent ink comprises a blend of a light curable resin and a crystalline organic semiconductor material dissolved in a solvent. The light emitting material is formed by crushing an ionic transition metal complex based on ruthenium (II) and dissolving it in a solvent such as acetone, acetonitrile, methanol or dicholoromethane. The light curable resin may be a cross-linkable polymer. The ink can be deposited using inkjet printing and it does not block the inkjets. The use of an ink with dissolved electroluminescent material improves the uniformity of light emission from the deposited films.

Description

Light Emitting Electronic Device The present invention relates to light curable electronic devices and, more particularly, to devices arranged to emit electromagnetic radiation during use.

The use of semi-conducting polymers in the fabrication of electronic devices and in particular light emitting diodes (LEDS) is known. One simple form of Organic LED (OLED) typically comprises a multi-layered structure fabricated on a substrate, so as to provide an anode, an active polymer portion and a cathode.

The substrate is typically glass, on which the anode material has been deposited.

Typically, the anode comprises a transparent, hole-injecting material such as indium tin oxide (ITO) and the cathode comprises an electron injector material such as aluminium or calcium.

In its simplest form the active polymer portion comprises a single emissive kiyr, although the active polymer portion would typically comprise multiple layers, such as two or three layers, of different types. The cathode has a metallic layer, which acts as the negative contact for the device. In operation a voltage (typically of magnitude 3V to 9V) is applied between the contacts to induce light emission from the emissive layer as a result of electroluminescence.

OLEDs emit light in a similar manner to conventional LEDs, through a process called electrophosphorescence.

The most widely perceived application of organic LED technology is in the fabrication of thin film displays. In the case of monochromatic displays each organic LED generally represents a single pixel, or in the case of colour displays, one of three pixel components (red, green, blue). Organic LED displays have the significant advantage over conventional amorphous / poly-silicon thin film transistor liquid crystal displays (TFT-LCD) that OLEDs do not require backlighting. OLED displays can therefore be manufactured without the need for complex and expensive backlighting circuitry.

In display applications device quality and longevity are vital in order for the resulting product to be commercially viable. Organic materials, however, tend to degrade rapidly over time when compared to silicon based alternatives.

Furthermore, the thin films of emissive polymer used are extremely vulnerable to pinholes and the like, which result from dust, or other contaminant particles settling on top of, or beneath the active layer. The perception, therefore, is that the development of OLEDs should focus on research into manufacturing technologies and/or device architectures, which minimise the risk of contamination and maximise the life of the device. Such concerns have favoured precision production technologies which involve the use of controlled environments. The setup and running costs of such production processes is inhibitive to the realisation of OLED technologies as a practicai soiutiii for general use display applications.

The Applicant owns co-pending patent application PCT/2007/002328, entitled "Multi-Layered Ultra-Violet Cured Organic Electronic Device", the details of which are incorporated herein by reference. That document describes how OLED's can be manufactured using inkjet printing techniques, with the significant benefit that print customization and reproduction can be achieved at relatively low cost. The use of such printing techniques broadens the viable scope of OLED technology and allows the practical application of OLEDs to printed displays such as screens, signage or marketing materials in the form of banners, posters, plaques or the like.

The present invention represents developments over and above the process disclosed within PCTI2007/002328, which have been found to improve the functional characteristics of UV curable organic electronic devices.

According to one aspect of the present invention, there is provided an active portion for use in a multi-layer organic electronic device, wherein the active portion is arranged to be sandwiched within the organic electronic device between first and second contact layers and comprises a mixture of a light curable resinous material and a solvent in which is dissolved a selectively luminescent crystalline material.

The provision of the light emitting crystalline material in solution results in significantly improved uniformity of light dispersion from the organic electronic device. Furthermore the luminosity of the electronic device is improved during operation. Accordingly, the luminosity of OLEDs can be made comparable to that of other forms of conventional LED, which significantly improves the scope of uses for OLED technology.

Aside from the operational characteristics, the provision oi a iiyiit emuttng material in solute form greatly improves the properties of the active material for manufacture of the multi-layer organic electronic device. The viscosity of the resin and solvent mixture can be better controlled and adapted for passage through a nozzle or else can be tailored to suit other printing processes such as, for example, screen printing. Problems associated with the handling and dispersion of a thin layer using a non-homogenous, sludge-like mixture are effectively obviated. In particular the possibility of suspended particulate material clogging inkjet nozzles or the like is removed.

The light emitting solution may be blended with a UV curable resin. The photo-emissive solution may be blended with the light curable resin in variable ratios to achieve a desired viscosity for a particular printing process. Said resin is typically a polymer and may form part of a complex polymer. Up to 60 or 70% by weight of selectively luminescent material can thus be incorporated into the mixture.

It will be appreciated that the term light curable is intended to cover materials which cure upon exposure to electromagnetic radiation and is not limited only to visible light. In one embodiment, the light curable material is UV curable.

The resinous material may be an oligomer and may be cationic or a free radical.

Exposure of both of these types of resin to incident light within predetermined wavelength boundaries causes a chemical interaction with the electrovalent bonds in monomers to result in curing of the material. Both are typically initiated by exposure to incident UV light of typically a wavelength of 200-4lOnm light.

Light curing and, in particular UV light curing, addresses the issue of cost effective printing for small and medium print-runs. For printing, it increases product throughput and products generally have better print registration than solvent based printing. Additionally, because UV curable inks only cure when exposed to UV radiation, inks do not cure, and thereby biui.k, a pnntcr' printhead nozzles. Thus UV curable devices generally result in less down time and less maintenance of printers than non-UV curable OLED devices. Near-instant curing also provides a means to increase a prints area size, whilst retaining the product yield.

The resinous material may comprise a monomer resin. The resinous material may comprise a polymer resin.

According to a second aspect of the present invention there is provided an active portion for use in a multi-layer organic electronic device, the active portion being arranged to be sandwiched within the organic electronic device between first and second contact layers and comprising a mixture having a light curable resinous material and a selectively luminescent crystalline material, wherein the light curable resinous material comprises ruthenium.

Said active portion may comprise an electron promotion layer.

According to a further aspect of the present invention there is provided a multi-layered organic electronic device comprising: a substrate, first and second contact layers, and an active portion; said first contact layer being formed on said substrate, and the active portion being formed between said first and second contact layers; wherein said active portion comprises an active polymer layer; and wherein said polymer layer comprises a polymer network polymerised using a UV light source.

According to another aspect of the present invention there is provided a chemical composition for the formulation of a selectively luminescent active portion of an OLED according to any previous aspect, wherein the selectively luminescent component is dispersed within a light curable resinous material.

According to a further aspect of the present invention, there is provided an active portion for use in a multi-layer organic electronic device, comprising a cross-linkable matrix material used to support a selectively luminescent material, said s&ectively luminescent material comprising ruthenium.

According to another aspect of the present invention there is provided a method of manufacturing an organic electronic device according to any one or more of the above aspects.

Further preferable features of the invention are recited in the dependent claims.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the attached figures, of which: Figure 1 shows an embodiment of a basic organic light emitting diode according to the invention; Figure 2 shows an embodiment of an advanced organic light emitting diode according to the invention; Figure 3 shows a magnified view of a particulate photo-emissive material suspended in a resinous material; Figure 4 shows the spectral response of a mercury metal halide UV light source suitable for use in the curing process, depending on the choice of photo-initator; Figure 5 shows the spectral response of a mercury-iron UV light source suitable for use in the curing process, depending on the choice of photo-initator;; Figure 6 shows the spectral response of a UV emissive LED light source suitable for use in the dot-fixing process; and, Figure 7 shows the spectral response of an OLED material which changes its spectral response as it ages.

The present invention addresses the problems of OLED manufacture by providing a cost effective UV curable device which can be scaled up in size without the drawbacks of resultant low product yield. Furthermore the resultant product displays improved processability, irradiance levels and extended operational life. This is at least in part achieved by way of a novel blending formulation for the active portion of the OLED.

In figure 1 a first embodiment of a multi-layered organic electronic device is shown generally at 20. The device 20 comprises an organic light emitting diode (OLED) having an active portion 22, and two contact layers 12, 18, fabricated on an appropriate substrate 10. It will be appreciated that the OLED 20 may also be referred to as a polymer light emitting diode (PLED).

A first of the contact layers 12 is fabricated directly onto the substrate 10 and comprises a conducting film of electronic properties suitable for the film 12 to function as an anode. By way of example, the contact layer 12 may comprise an appropriate conducting polymer film. Alternatively, or additionally, the contact layer 12 comprises a relatively low work function transparent material such as indium tin oxide (ITO), or the like, suitable for acting as a hole-transporter. The use of a conducting polymer film, however, is particularly advantageous because it can be printed onto the substrate relatively cheaply, whilst a completely transparent polymer film is desirable, especially for pixelated displays, many applications only require a semi-transparent / translucent film. It will be appreciated that the contact layer 12 may comprise a single layer, or may comprise a plurality of different material layers.

ITO also offers excellent performance as an anode layer due to its low surface resistance. Whilst ITO is preferred for contact layer 1, a number ol uLhi materials are viable such as nano or micro-silver particle suspensions, carbon nano-tube suspensions, both of which may be inkjet printable. In addition, the conductive polymer PDOT-PSS (poly(3,4-ethylene-dioxythiophene): polystyrene sulphonic acid) may also produce an anode suitable for printing purposes.

The active portion 22, is fabricated onto the first contact layer 12 and comprises first and second active semi-conducting polymer layers. The first polymer layer 14 comprises a p-type' emissive layer deposited on the anode contact layer 12.

The second polymer layer 16 comprises an n-type' electron-transporter layer 16 fabricated on the hole-transport layer 14. The active layers 14, 16 are typically <500nm, and preferably <lOOnm, thick and may comprise any suitable polymer, for example, acrylate polymers (polyacrylate), acrylated polyurethane, and suitable polymer formulations are discussed in further detail below.

Each layer thickness is determined by the substrate's surface, drop size and viscosity. For lOpi drop size, layers for UV curable inks are approximately 8pm and 1pm for Ipi drop sizes.

A second of the contact layers 18 is fabricated directly onto the second polymer layer 16 and comprises a conducting film of electronic properties suitable for the film 18 to function as a cathode. Typically, for example, the cathode comprises an appropriate conducting polymer film. Alternatively, or additionally, the contact layer 18 may comprise a relatively low work function material such as calcium, aluminium, magnesium, a magnesium/silver alloy or the like, suitable for acting as acting as an electron-injector. The use of a conducting polymer film is particularly advantageous because it can be printed onto the active layer relatively cheaply. It will be appreciated that the contact layer 18 may comprise a s!.çI'? Jyer, or may comprise a plurality of different material layers.

One particular embodiment of the cathode layer is based on a low work function metal gallium, such as, Gallium-Indium (Ga:ln).

The active portion 22 is configured such that the application of an appropriate potential difference across the contact layers 12, 18, causes electrons to be injected into the emitter 16 from the cathode, and holes to be injected into the hole-transport layerl4 from the anode. The polymer characteristics are selected, and film thicknesses engineered such that charge carriers combine to form tightly bound electron-hole pairs (excitons), near the interface between the polymer layers, thereby to decay with the emission of tight through the transparent anode.

The band gaps of the active polymer layers are configured to give desired emission characteristics, for example, to give visible light of a required wavelength and hence colour (according to ?.=hIE).

The substrate 10 comprises a flexible thin insulating sheet material, for example, a clear plastics material, vinyl, or the like. Whilst a flexible plastics, or other material is preferable for many applications the substrate could comprise glass, or a rigid plastic.

The device 20 (and all the devices detailed) may be further provided with a scratch resistant encapsulation polymer layer (not shown) to protect the device once fabricated. Alternatively if the uppermost layer of the device is a polymer layer it could be fabricated to act as the scratch resistant encapsulation layer It will be appreciated that the device could alternatively be fabricated onto a non-transparent substrate. In such devices the anode contact layer could comprise a reflective conducting polymer layer and the cathode a transparent, semi-transparent, and/or translucent layer such that the photons generated in the active layer are visible through the cathode layer.

In a further embodiment of the multi-layered organic electronic device the cathode conducting layer 18 is fabricated onto the substrate, the active portion 22 onto the cathode layer 18 and the anode layer 12 onto the active portion 22. The active polymer layers 14, 16 in such a device are reversed, and the transparency of the substrate, cathode, and/or anode layers selected according to the application.

Another embodiment of the multi-layered organic electronic device the cathode conducting layer 18 and the anode layer 12 are the same polymer, ITO type, or similar, both being transparent, semi-transparent, and/or translucent.

In figure 2, a further embodiment of a multi-layered organic electronic device is shown generally at 36. The device 36 comprises an organic light emitting diode (OLED) having an active portion 38, and two contact layers 26, 34, fabricated on an appropriate substrate 24. It will be appreciated that the OLED 36 may also be referred to as a polymer light emitting diode (PLED).

Figure 2 with respect to figure 1 can be cross-referenced. The device generally shown at 36 with active portion 22, has layer 24 which is equivalent to layer 10, layer 26 is equivalent to layer 12, layer 28 is equivalent to layer 14, layer 30 is equivalent to layer 16 and layer 34 is equivalent to layer 18. It is appreciated that the explanations and attributes associated with figure l's layers and substrate, are applied to device 36.

The hole-transport layer 32, enhances the movement of holes from the emissive layer 32. The effect being an increased light output of device 36, relative to device 20. Layer 34 may be gallium-indium, which is an eutectic alloy and can result in unstable devices. Layer 32 also acts as barrier to prevent atom migration between the layers that sandwich said layer.

It wifl he appreciated that layer 32 can be between layers 28 and 26. Further, as well as layer 32 being in situ as illustrated in figure 2, a similar layer-type may exist between layers 26 and 28.

Layer 32 may comprise a blend of 2-(4 biphenyl)-5-(4-tert-butylphenyl)-1,3,4- oxidazole, UV curable binding polymer poly(tert-butyl methacrylate-co-glycidylmethacrylate). The photoinitiator for which may (4-phenoxyphenyl) diphenylsulfonium triflate, or the like of table... Bis[4-(vinyloxy)butyl]isophthalate is highly light transmissive, so will not significantly absorb (degrade) the device's i rradiance.

In accordance with any of the above embodiments of the present invention there may be provided an electronic means to mask the effect of the polymer matrix degrading with the advance of time. Typically, the applied voltage is increased as the device ages. During use, conducting paths which do not emit light form in the polymer matrix, so the current does not noticeably drop in relation to the change in irradiance.

Further device degredation can be mitigated by suitable encapsulation of the device. Encapsulation prevents the ingress of oxygen and water. Numerous encapsulation processes are available for the prevention of degradation. Many do absorb / reflect environmental UV light.

An optically transmissive layer can be deposited on top of the device, post all other layers, to encapsulate the device and prolong the device lifetime in air. One such encapsulation solution is a UV photpolymerisable blend consisting of pentaerythritol triacrylate 90-97% and a photoinitiator 3-10%.

Turning now to figure 3, one embodiment of the light emitting polymer layer is based upon a light emitting material suspended in a UV curable resin 40. The light emitting material is provided by way of a crystalline material which has been crushed to form particles 42 of size generally in the region of roughly 1-l5pm. In this embodiment, the particulate material forms roughly 30-40% by weight of the mix, and typically around 35%.

The particulate light emitting material 42 is an ionic transition metal complex based on Ruthenium (II), where tris(2,2'bipyridyl) ruthenium (II) is the cation with the formula [Ru(bpy)3]2. Other transitional metal cations can also be used including those based on osmium, for example [Os(bpy)2L]2. There is a range of negatively charged counter-ions available, including Cl, PF6, CLO4 and BL4.

These transition metal complexes will be collectively referred to as RuX and the individual names of the four Ruthenium (II) complexes which have been used in the production of a suitable light emitting layer are given in Table I below.

Assigned name Chemical Formula Ru Ru(bpy)3CI2 RuB [Ru(bpy)3](BF4)2 RuC [Ru(bpy)3](CI04)2 RuP [Ru(bpy)3](PF6)2

Table I

However it has been found that the particulate nature of the light emitting material can lead to non-uniform light emission from the assembled device. A non-uniform dispersion of RuX has been found to jeopardise the operation of the device. It has been determined that the ruthenium based compound can be dcperced n snlvent along with a photo initiator material, which can then be blended with the resinous material. The photoinitiator may be a cationic acid. In order to achieve a material which can be inkjet printed, it has been found that a relatively low molar mass solvent is preferable. Low molar mass of the solvent results in a better dispersion of the RuX and photoinitiator when printed on the substrate.

The RuX is dissolved in the solvent and, upon curing, the particles are reformed as the solvent is driven off.

A number of solvents have been used in order to establish the impact of the solvent on the operation of the resulting device. The volatile solvents used are identified in Table 2 below: Name Chemical Formula Molar Mass (g/mol) Acetonitrile CH3CN 41.05 Methanol CH3OH 32.04 Dichloromethane CH2CI2 84.93 Acetone CH3COCH3 58.09

Table 2

Turning now to the issue of the selection of suitable resinous materials, polymers have been identified which are suitable for curing by way of bulbs which can be found on conventional printing apparatus. Both free-radical and cationic polymers are proposed. However, for light emitting devices, it has been generally found that cationic crosslinking is superior to free-radical processes. The resin material is important since it contributes to both the mechanical and electrical properties of the OLED when formed. Free-radicals are small molecules with unpaired electrons. Cationic initiators are positively charged ions. They can be cured by exposure to UV light of typically a wavelength of between 200-41 Onm.

It has been found that conventional lamps on commercial printers have a peak wavelength of approximately 365nm. However it has been found that there are also a number of side bands which can also be used for curing, such as, for example, those in the region of 238-265 nm. Accordingly a photoinitiator can be selected which matches an appropriate UV wavelength range.

In cationic vinyl polymerization, the initiator is a cation. The carbon-carbon double bond will be attracted to the cation, and wiJi leave the carbon-carbon double bond to form a single bond with the initiator. This leaves one of the former double bond carbons carrying a positive charge. This new cation will react with a second monomer molecule in the same manner as the initiator reacted with the first monomer molecule.

A carbocation is a cation where the positive charge is on a carbon atom.

Carbocations are very unstable. A carbocation interacts with the electrons in the double bond of a monomer molecule. The carbocation forms a single bond with the monomer molecule and generates another carbocation. This can react with another monomer, and then another to create a long polymer chain.

The mechanism for cross-linking initiation in the polymer matrix by the acid photoinitiator is directed at glycidyl substituents of the polymer chains, which ring-open the epoxy functional groups of the glycidyl moities to form ether bonds with different polymer chains in the film leading to cross-linking. Table 3 details suitable glycidyl group materials.

Cross-Linkable Polymers poly(tert-butyl methacrylate-co-glycidylmethacrylate) poly(methyl methacrylate-co-glycidylmethacrylate) poly(styrene-co-glycidylmethacrylate) 1l,

Table 3

Examples of acid photoinitiators which are sensitive at wavelengths suitable for rapid curing by mercury halide UV lamps are shown in table 4.

Cationic Photoinitiator for Mercury + Halide Lamps (4-phenoxyphenyl)diphenylsulfonium triflate Bis(tert-butylphenyl)iodonjum perluoro-1 -butanesulfonate Diphenyiiodonium 2-(4-Methoxystyryl)-4,6-bis(trichloromethyl)1,3,5-triazine 5, 7-diiodo-3-butoxy-6-fluorone

Table 4

Additional examples of acid photoinitiators which are sensitive at wavelengths suited for rapid curing by mercury-iron halide UV lamps are shown in table 5.

Cationic Photoinitator for Mercury + Iron Lamps (4-Methylthiophenyl)methyl phenyl sulfonium trifluoromethanesulfonate (4-Phenylthiophenyl)diphenylsulfonium trifluoromethanesulfonate, 1 -Naphthyl diphenylsulfonium triflate

Table5

The acid photoinitiators of tables 4 and 5 allow curing at wavelengths that are at peak emissions from the respective UV source, It will be appreciated that shorter cxpcure trnes wil! red1!r' degradation of the Ruthenium complexes.

5,7-diiodo-3-butoxy-6-fluorone has peak sensitivity at 470 nm, but there is a significant absorbance (approximately 50% Amax) around 406 nm, where mercury halide curing lamps have a high spectral irradiance. Care must be taken to prepare the solutions in the dark to avoid premature curing. This has the added benefit that post-curing will occur upon ejection of the solution from the print head onto the substrate.

This process of propagation, by which monomer after monomer is added to form a polymer, is terminated when no new chains are started. When termination happens, the polymerization is complete.

The resins which have been found to produce good quality transparent films are

listed in Table 6.

Monomer I polymer poly(tert-butyl methacrylate-co-glycidylmethacrylate) Bis[4-(vinyloxy)butyl] isophthalate

Table 6

The UV curable polymer resin which yields a high perfomance device is poly(tert-butyl methacrylate-co-glycidylmethacrylate). The photoinitiator for this polymer is (4-phenoxyphenyl) diphenylsulfonium triflate (C25H 19F304S2).

The UV curable monomer resin which yields a high perfomance device is Bis[4- (vinyloxy)butyl]isophthalate. The photoinitiator for which is (4-phenoxyphenyl) diphenyisuiioritutii tiuflate (C2H 1F3Q4S).

The ruthenium-based light emitting organic crystals listed in Table 1 have been dissolved in different ways for blending with the cationic monomer, or polymer listed in Table 6 as follows: Ru dissolved in methanol (CH3OH), or dichlorometane-methanol mixture (CH2CI2-CH3OH); [Ru(bpy)3J(BF4)2 dissolved in methanol (CH3OH), or acetone (CH3COCH3), or dichlorometane-methanol mixture (CH2CI2-CH3QH); [Ru(bpy)31(C104)2 dissolved in methanol (CH3OH), or acetonitrile (CH3CN); [Ru(bpy)3J(PF6)2 dissolved in acetone (CH3OH), or acetonitrile (CH3COCH3).

[Ru(bpy)3](PF6)2 does not dissolve as readily in methonal as it does in acetone, or acetonitrile. However with enough methanol it does dissolve. Methanol has been found to be a better volatile solvent (faster evaporation) than acetone, or acetonitrile, so will result in better dispersion of the film on the substrate.

It will be appreciated that there are nine or more ruthenium -solvent derivatives that are suitable for blending with either the UV curable monomer, or UV curable polymer.

When blending the light-emitting metal complex with the polymer, a number of factors must be considered. The concentration of the light emitting material must be great enough that there is sufficient connectivity that electron transport between the opposing layers of the film can occur throughout the light emitting layer. Furthermore the viscosity of the blended materials prior to curing must be such that the material can be printed. Preferably the material can pass through an inkjet nozzle without clogging.

Prior to curing the OLED device by UV exposure, it is preferable to evaporate off the solvent with latent heat as required by a given solvent, It will be appreciated that a UV curable resin may thermally cure as a consequence of applied thermal energy. It is therefore desirable to maintain the temperature at a level that does not result in thermal curing.

Using a cationic resins has been found to allow the same luminescence output as for a free-radical resin to be achieved with a lower mass quantity of OLED crystals. Accordingly an increased level of light can be achieved using a cationic resin by adding further RuX without significantly jeopardising the viscosity of the mixture.

An example OLED polymer may have the composition, 27-37% polymer poly(tert-butyl methacrylate-co-glycidylmethacrylate), 3% photoinitiator (4-phenoxyphenyt) diphenylsulfonium triflate (C25H19F304S2), blended with 60-70% [Ru(bpy)3}(PF6)2.

The ruthenium compound and photoinitator are dissolved in acetonitrile (CH3COCH3).

Another OLED polymer may have the composition, 27-37% monomer Bis[4- (vinyloxy)butyl]isophthalate. The photoinitiator for which is (4-phenoxyphenyl) diphenylsulfonium triflate (C25H19F304S2), blended with 60-70% [Ru(bpy)3](PF6)2.

The ruthenium compound and photoinitiator are dissolved in acetonitrile (CH3COCH3). A substantial portion of the solvent is driven off before or during curing. However a small amount of solvent will typically be locked into the cured layer.

It will be appreciated that the more RuX in the solution, the greater the irradiance.

However a reduction in the UV curable polymer may affect the cohesion properties to the substrate and so a suitable balance needs to be achieved.

lrradiance of said OLEDS can be achieved without a solvent, by grinding a given ruthenium compound crystals to a size that will not block the printhead nozzles.

The crystals may simply be suspended in the resin, reducing the amount of resin needed. It will be appreciated that an even dispersion of light may not be achieved. lrradiance will typically be greater when a solvent is used to disperse said crystals.

An inkjet printer is used to place small droplets of ink onto paper to create an image. The dots are positioned very precisely by digital registration. lnkjet technology greatly reduces the cost of OLED manufacturing and allows OLEDs to be printed onto very large flexible and rigid films for large displays like TV screens or electronic signage.

When inkjet printing, a pre-coated ITO transparent I translucent substrate may be used for the anode. The cathode could be indium-gallium (InGa). lnGa is a liquid at approximately l6degC and has a sufficiently low viscosity to be inkjet printed.

To maintain device integrity, a final protective UV curable lacquer would encapsulate said device. This prevents water ingress and br contamination of said device.

Preferred material parameters for the resulting active mixture are given in Table 7.

Preferred Parameter Parameter Descnption Value Surface energy (48mN/m) @ ambient 50-IOOmPA.s @ ambient Ink viscosity 10-2OmPA.s @ 55°C Average Particle size if not < 1 5pm, dissolved Preferably < 5p.m Ink droplet volume 1-lOpi Thermal curing 70 100°C temperature -Cure time 0.4 -15s ________________________ Preferably < 4s

Table 7

Other printing processes are vacuum thermal evaporation (VTE) and organic vapour phase deposition (OVPD). OVPD generally employs a nitrogen carrier gas to transport the OLED vapour to a substrate, where it condenses upon said substrate.

It will be appreciated that any ink, whether UV curable or otherwise, should ideally be matched to the printing system being employed to spray an OLED device. OLED device polymer blends should be such that the resultant parameters are suitable for printing. The viscosity of the polymer blend is greatly affected by the ratio of the luminescent material present in the mixture.

Accordingly a polymer with lower viscosity allows an increased amount of RuX to be added, thus improving the properties of the cured active layer.

Viscosity of solutions strongly depend on the binder polymer molecular weight.

At the same ratio of polymer to solvent, viscosity can be controlled over a wide range by choice of molecular weight. The same polymer in different molecular weights can be used. Blending with high/low molecular weights of polymers can be used to adjust the viscosity for the print-head being used to spray the device.

The quality of the solution RuX with photoiniator and polymer poly(tert-butyl methacrylate-co-glycidylmethacrylate) solutions can be improved by employing an extra sub-micron filtration step prior to deposition. This removes any micro-particles of solution that are not dissolved and any pre-cured particles. It will be appreciated that the better the filtration process, the more particles removed, so the better the final film properties.

Figure 4 shows a typical spectrum of a mercury metal halide lamp, whilst figure 5 is that for a mercury metal halide-free lamp. Both are suitable for free-air curing of the printed fi!ms. ThA spectrum includes peaks in the UV-A (315nm -380nm), the UV-B (280nm -315nm) and the UV-C (200nm -280nm) regions. UV-A region peaks are particularly beneficial in the curing of thick polymer layers, UV-B region peaks support and maintain the triggered cross-linking reaction and promotes improved curing over the shorter wave-length peaks, and UV-C region peaks are beneficial for the controlled polymerisation of thin films and for ensuring complete curing.

In figure 4, the potentially usable wavelength regions for chemical bonding of the organic molecules are approximately 238-265nm, 278-292nm, 297-302nm, 310- 316nm, 355-375nm and 400-412nm.

In figure 5, the spectral response is broad, providing a wider scope of photoinitiators to be used. If the OLEDs are being printed with UV curable colour inks, then mercury-iron lamps have been found to be less suitable. In said instance the UV-C band can cure the surface of the colour ink before through-cure, thus trapping any solvents within the solution, which could be detrimental to the finished graphics.

Figure 6 shows a typical spectrum of 365nm and 375nm UV emissive LEDs. Both are suitable for free-air dot-fixing of the printed films. 365nm is the preferred device, though a 375nm device may have sufficient energy in its sidebands to dot-fix. UV emissive LEDs will partially cure an organic device. The spectrum includes peaks in the UV-A (315nm -380nm). There is no response in the UV-B (280nm -31 5nm) and the UV-C (200nm -280nm) regions.

A plurality of parameters of the UV light are controlled to ensure optimum curing of the layer, or layers being cured. The photon flux at a given depth within the polymer is a function of the polymers' absorbance and the incident flux at a given wavelength according to the Bouger-Lambert law. A low irradiance for a relatively long period is not equivalent to a higher irradiance for a short period, even if the ovcrc!! the energy s thA same. Typically peak intensity and energy are used as control parameters. Controlled UV peak intensity and dosage, for example, may be used to accurately control the curing process whilst maximising longevity of the organic device being fabricated.

Furthermore it has been found that prolonged exposure to UV radiation can degrade the operation of the device once formed. This may also be a problem due to individual layers being laid down and cured separately, since lower layers will be repeatedly exposed to the UV light source.

If necessary the UV parameters are controlled to minimise the effect of curing newly deposited layers, on existing layers cured earlier during fabrication, by avoiding over exposure of the existing cured layers to UV radiation. Such over exposure can lead to significant reductions in longevity.

During use, the device is connected to a power supply which applies a voltage across the OLED's anode and cathode. An electrical current flows from the anode to the cathode through the organic layers. The cathode gives electrons to the emissive layer of organic molecules. The anode removes electrons from the conductive layer of organic molecules. At the boundary between the emissive and the conductive layers, e'ectrons combine with electron holes. At said event, the electron gives up energy in the form of a photon emission. The wavelength / waveband is a function of the organic molecules.

The colour of the light depends on the type of organic molecule in the emissive layer. Manufacturers may place several types of organic films on the same OLED to make colour displays. The intensity or brightness of the light depends on the applied voltage.

Figure 7 shows a typical spectral response for the base OLED materials Ru(bpy)3Cl2 Ru(bpy)3](BF4)2, [Ru(bpy)3](C104)2 and [Ru(bpy)3](PF6)2, which is taken shcrt!y after the tIvice has been activated. The peak wavelength is approximately 600-650nm.

Claims

Claims: 1. A multi-layer organic electronic device comprising an active portion sandwiched between first and second contact layers, wherein the first and second contact layers are arranged to allow application of a potential difference across said active portion and wherein said active portion comprises a mixture of a light curable resinous material and a solvent in which is dissolved a crystalline material, said crystalline material being selectively luminescent by application of said potential difference thereto.
2. A multi-layer organic device according to claim 1, wherein said light curable material comprises a cross-linkable matrix used to support said selectively luminescent material, which comprises Ru(bpy)32.
3. A multi-layer organic electronic device according to claim I or claim 2, wherein said crystalline material emits light of a visible wavelength.
4. A multi-layer organic electronic device according to any preceding claim, wherein said crystalline material is an organic semiconductor.
5. A multi-layer organic electronic device according to any preceding claim, wherein said crystalline material is an ionic transition metal complex.
6. A multi-layer organic electronic device according to claim 5, wherein said ionic transition metal complex comprises ruthenium.
7. A multi-layer organic electronic device according to claim 6, wherein the cation in said ionic transition metal complex is tris(2,2'bipyndyl) ruthenium(Jl).
8. A multi-layer organic electronic device according to any one of claims 5 to 7, wherein the negatively charged counter-ion comprises any of Ci, PF6, CLO4 or BL4.
9. A multi-layer organic electronic device according to any one of the preceding claims, wherein the light curable resinous material is UV curable.
10. A multi-layer organic electronic device according to any one of the preceding claims wherein the resinous material comprises a cationic photoinitiator.
11. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises (4-phenoxyphenyl) diphenylsulfonium triflate for curing with mercury halide UV lamp source.
12. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises bis(tert-butylphenyi)iodonium perluoro-1 -butanesulfonate for curing with a mercury halide UV lamp source.
13. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises bis(tert-butylphenyi)iodonium perluoro-1 -butanesulfonate, for curing with a mercury halide UV lamp source.
14. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises diphenyliodonium, for curing with a mercury halide UV lamp source.
15. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises 2-(4-Methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, for curing with a mercury halide UV lamp source.
16. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises 5,7-diiodo-3-butoxy-6-fluorone.
17. A multi-layer organic electronic device according to any one of claims 10 to 16, wherein the photoinitiator is suitable for curing with a mercury halide UV lamp source.
18. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises (4-Methylthiophenyl)methyl phenyl sulfonium trifluoromethanesulfonate.
19. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises (4-Phenyithiophenyl)diphenylsulfonium trifluoromethanesulfonate.
20. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises 1-Naphthyl diphenylsulfonium triflate.
21. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator is suitable for curing with a mercury-iron UV lamp source
22. A multi-layer organic electronic device according to any one of claims 1 to 9, wherein the resinous material comprises a free-radical photoinitiator.
23. A multi-layer organic electronic device according to any one of the preceding claims wherein the resinous material comprises a glycidyl group polymer.
24. A multi-layer organic electronic device according to any preceding claim, wherein the resinous material comprises poly(tert-butyl methacrylate-co-glycidylmethacrylate).
25. A multi-layer organic electronic device according to any preceding claim, where in the resinous material comprises poly(methyl methacrylate-co-glycidylmethacrylate).
26. A multi-layer organic electronic device according to any preceding claim, where in the polymer comprises poly(styrene-co-glycidylmethacrylate)
27. A multi-layer organic electronic device according to any preceding claim, wherein the resinous material is a vinyl.
28. A multi-layer organic electronic device according to any preceding claim, wherein the resinous material is a monomer material comprising bis[4- (vinyloxy)butyl]isophthalate.
29. A multi-layer organic electronic device according to any one of the preceding claims wherein the resinous material comprises an oligomer.
30. A multi-layer organic electronic device according to any one of the preceding claims, wherein the active portion comprises between 55% and 75% by weight of said crystalline material.
31. A multi-layer organic electronic device according to any one of the preceding claims, comprising a low molar mass solvent.
32. A multi-layer organic electronic device according to any one of the preceding claims wherein the solvent comprises a volatile solvent having a molar mass of 85 g/mol or less.
33. A multi-layer organic electronic device according to any one of the preceding claims, wherein the solvent comprises any of Acetone, Acetonitrile, Methanol, Dichloromethane, or Dichiorometane-Methanol.
34. An active portion for use in a multi-layer organic electronic device according to any one of claims I to 33.
35. An active portion according to claim 36, wherein the active portion has a viscosity of IOOmPA.s or less at ambient.
36. An active portion according to claim 34 or claim 35, wherein the active portion has a viscosity of 2OmPA.s or less at 55°C.
37. A method of manufacturing a multi-layer organic electronic device according to any one of claims Ito 31, comprising applying said first layer to a substrate, applying said active portion to said first layer, applying said second layer to said active portion and curing said device.
38. A method according to claim 40, wherein the active portion, the first and second layers are individually cured once they have been applied.
39. A method according to claim 40 or 41, wherein the organic electronic device is inkjet printed.
40. A method according to any one of claims 37 to 39 further comprising filtering of said active portion.
41. A method according to claim 40, wherein the filtering removes particles from said active portion having a particle size greater than 1 Jm.
42. A method according to any one of claims 37 to 41, wherein the selectively luminescent material and/or a photoinitiator material are substantially evenly distributed in said solvent.