WO2009050497A1

WO2009050497A1 - Organic light emitting electronic device

Info

Publication number: WO2009050497A1
Application number: PCT/GB2008/003559
Authority: WO
Inventors: Stephen Clemmet; Deborah Stokes; Mark Peace
Original assignee: Polymertronics Limited
Priority date: 2007-10-20
Filing date: 2008-10-20
Publication date: 2009-04-23
Also published as: GB2458096A; GB0720618D0

Abstract

A multi-layer organic electronic device comprising an active portion sandwiched between first and second contact layers. The first and second contact layers are arranged to allow application of a potential difference across the active portion and the active portion comprises a mixture of a light curable resinous material and a solvent in which is dissolved a crystalline material. Said crystalline material is selectively luminescent upon application of a potential difference via the first and second contact layers. A method of manufacturing the multi-layer organic electronic device is also disclosed.

Description

Organic Light Emitting Electronic Device

The present invention relates to light curable organic electronic devices and, more particularly, to devices arranged to emit electromagnetic radiation during use, which may be of a wavelength which is visible to the human eye.

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 layer, 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 6V to 15V) 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 electroluminescence.

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 practical solution for general use display applications.

The Applicant owns co-pending patent applications as follows: PCT/GB2007/002328, entitled " M ulti- Layered Ultra-Violet Cured Organic Electronic Device"; GB0813663.2 entitled "UV-LED Controller" and GB0813662.4 entitled "OLED Controller". The details of those applications are incorporated herein by reference. Those applications describe how OLED's can be manufactured using inkjet printing techniques and electronically controlled, with the significant benefit that print customisation 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 a material that may be inkjet printed, for example using the process disclosed within PCT/GB2007/002328. When printing the device, the process disclosed in GB0813663.2 may be used for pinning purposes. Once the device is deposited, it may be electronically controlled using the process disclosed within GB0813662.4

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 electroluminescent crystalline material.

The provision of the light emitting crystalline material in solvent solution can result in significantly improved uniformity of light dispersion from the organic electronic device, when compared to crystals in a resin not in a solvent solution. Furthermore the luminosity of the electronic device may be 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 of a light emitting material in solute form can greatly improve 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 may be 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 blend of polymers. Up to 80% by weight of selectively electroluminescent material may 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 comprise a monomer resin. The resinous material may comprise a monomer, or a polymer resin.

The resinous material may be an oligomer, or a polymer. The curing reaction of said material 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 within the reactant in curing of the material. Both are typically initiated by exposure to incident UV light of a wavelength typically in the range 200-400nm light. It will be appreciated that the wavelengths for curing may be matched with that of the material's curing requirements.

Light curing and, in particular UV light curing, may help to address the issue of cost effective printing for small and medium print-runs. For printing, it may increase product throughput and products may generally have better print registration than with solvent based printing. Additionally, because UV curable inks only cure when exposed to UV radiation, inks do not tend to cure in an unwanted manner, and thereby block, a printer's print-head nozzles. Thus UV curable devices may result in reduced 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 solvent may comprise a polar solvent.

An application of the present invention is in the activation of photosensitive drugs, as used in photodynamic therapy. Photodynamic therapy (PDT) is also called photoradiation therapy, phototherapy, or photochemotherapy. Photodynamic therapy is treatment that uses drugs, called photosensitising agents, along with light to destroy cancerous cells. The drugs only work after they have been activated or by light of the appropriate wavelength.

Traditional PDT light sources use lasers, or halogen lamps. Where halogen lamps are used, an appropriate filter with an adjustable iris of 30 - 55mm is used. The lamp delivers a stable and uniform distribution of red light in the range 560 - 670nm, with a peak intensity at 636nm. The light penetrates into the skin's target area, where it is absorbed by the photosensitive drug. Lasers can be mechanically scanned across the area to be treated.

A photosensitising agent is either injected into the bloodstream or put on the skin. After the drug is absorbed by the cells, light is irradiated to the area to be treated. The light causes the drug to react with oxygen, which forms a chemical that destroys cells. Examples of photosensitive drugs are Verteporfin, a benzoporphyrin derivative and Methyl ester. Verteporfin is an intravenous photosensitive drug with an absorption peak wavelength of 690 nm. Methyl ester of aminolevulinic acid (ALA) is activated with a wavelength 570 - 670nm.

A device according to the present invention may be applied to a person's skin in order to activate photosensitive drugs by emission of light of a suitable wavelength. The device may be applied to the skin using an adhesive or else using a positioning aid such as a bandage or sleeve. One of said contact layers may be positioned against the wearer's skin.

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 an electroluminescent material, wherein the electroluminescent material comprises a ruthenium component.

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 the chemical composition for the formulation of a selectively electroluminescent active portion of an OLED according to any previous aspect, wherein the selectively electroluminescent component is dispersed within a light curable resinous material.

According to another aspect of the invention, there is provided a process to aid the surface whetting of the layer upon which the light emissive layer is deposited. The purpose of which is to improve inkjet printed OLED dot registration. 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.

Practicable 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 a more 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 an electron scanned microscopy (SEM) image of one embodiment of an OLED according to the present invention having been cast in a solvent solution;

Figure 5 shows an inkjet printed OLED device having a composition according to one embodiment of the present invention;

Figure 6 shows the spectral response of a mercury metal halide UV light source suitable for use in the curing process;

Figure 7 shows the spectral response of a mercury metal halide-free UV light source suitable for use in the curing process;

Figure 8 shows the spectral response of a UV emissive LED light source suitable for use in the dot-fixing process; and, Figure 9 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 effect UV curable device which may be scaled up in size without the drawbacks of resultant low product yield associated with vapour deposition and thermal deposition processes. 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.

It will be appreciated that where, the term 'polymer' is used below in reference to an electronic device, said device has a composite organic light emissive material within it.

In Figure 1 a first embodiment of a multi-layered organic electronic device is shown generally at 18. The device 20 comprises an organic light emitting diode (OLED) having an active portion 14, and two contact layers 12, 16, 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 high work-function transparent material such as indium tin oxide (ITO), poly(3,4-ethylene dioxy-2,4-thiophene)-polystyrene sulfonate (PEDOT-PSS), derivatives thereof, 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.

ITO also offers excellent performance as an anode layer due to its low surface resistance per unit area. Whilst ITO is preferred for contact layer 12, a number of other 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 material PEDOT- PSS can be formulated to be suitable for printing purposes.

The active portion 14, is fabricated onto the first contact layer 12 and comprises a composite light emissive material and resin. The polymer layer 14 is deposited on the anode contact layer 12. The active layers 14, is typically < 100nm thick and may comprise any suitable polymer, for example, acrylate polymers (polyacrylate), acrylated polyurethane, and a suitable light emissive material, or a plurality of light emissive materials, are described in further detail hereinafter.

Each layer thickness is determined by the substrate's surface properties, drop size, viscosity and temperature.

A second of the contact layers 16 is fabricated directly onto the light-emissive layer 14 and comprises a conducting film with electronic properties suitable for the film 16 to function as a cathode. Typically, for example, the cathode comprises an appropriate conducting low work-function material. The contact layer 16 may comprise a relatively low work function material such as calcium, aluminium, magnesium, a magnesium/silver alloy or the like. It will be appreciated that the contact layer 16 may comprise a single layer, or may comprise a plurality of different material layers.

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

The active portion 14 is configured such that the application of an appropriate potential difference across the contact layers 12, 16, causes electrons to be injected into the emitter layer 14. The polymer's characteristics are selected, and film thicknesses engineered such that charge carriers combine to form tightly bound electron-hole pairs (excitons) in the emissive layer. The chemical structure of the emissive layer may be configured to result in a specific peak wavelength emission.

The substrate 10 comprises a flexible thin insulating sheet material, for example, a clear plastics material, vinyl, or the like. Whilst 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.

Layer 14 may comprise a UV curable binding polymer poly(tert-butyl methacrylate-co- glycidylmethacrylate). The photoinitiator for which may (4-phenoxyphenyl) diphenylsulfonium triflate, or the like of Table 4. Bis[4-(vinyloxy)butyl]isophthalate is highly light transmissive, so will not significantly absorb (degrade) the device's irradiance.

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 is fabricated onto the substrate, the active portion 14 onto the cathode layer and the anode layer onto the active portion 14. The transparency of the substrate and/or anode layers may be selected according to the application.

In Figure 2, a further embodiment of a multi-layered organic electronic device is shown generally at 32. The device layers 36 comprises an organic light emitting diode (OLED) having an active portion 34, and two contact layers 24, 30, fabricated on an appropriate substrate 22. It will be appreciated that the OLED 36 may also be referred to as a polymer light emitting diode (PLED).

The device of Figure 2 may have components which are identical or similar to the corresponding components in figure 1 and may be cross-referenced with respect to Figure 1 as follows:

Device shown at 36 with active portion 34 in Figure 2, has layer 22 which may be equivalent to layer 10 in figure 1. Layer 24 may be equivalent to layer 12. Layer 28 may be equivalent to layer 14 and layer 30 may be equivalent to layer 16. It will be appreciated that the explanations and attributes associated with Figure Vs layers and substrate, may be applied to device 32.

Layer 26 comprises PEDOT-PSS, or a derivative thereof. The purpose of which is to increase surface whetting of layer 24, for layer 26. For inkjet printing, PEDOT-PSS increases reduces dot-spread pre-pinning and / or pre-curing. Layer 28 may comprise a

UV curable binding polymer poly(tert-butyl methacrylate-co-glycidylmethacrylate). The photoinitiator for which may (4-phenoxyphenyl) diphenylsulfonium triflate, or the like of

Table 4, bis[4-(vinyloxy)butyl]isophthalate is highly light transmissive, so will not significantly absorb (degrade) the device's irradiance.

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, a pulse width modulated (PWM) combined with pulse amplitude modulated (PAM), referenced PWM-PAM voltage signal is applied across the OLED device. PWM-PAM prevents a thermal increase in said OLED device, that is present when a constant fixed voltage is applied. The use of PWM-PAM has the advantage that it does not result a thermal runaway of the OLED device. Thermal runaway results in degradation of said device, which results in a further temperature increase of the device. By altering the PWM-PAM signal as the device ages under operational conditions, said devices irradiance can be held constant. Said PWM-PAM is disclosed in application GB0813662.4. Further device degradation 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. Said encapsulation may be inkjet printed.

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

Turning now to Figure 3, shown generally at 42 is 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 38 of size generally in the region of 15μm across or smaller. In this embodiment, the particulate material forms roughly 30-40% by weight of the mix, and typically around 35%. It has been found that crystals not dissolved in a solvent are more suitable to spin-coating processes, rather than inkjet printing.

The particulate light emitting material 38 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⁺. There is a range of negatively charged counter-ions available, including Cl , PF_{6 1} CIO₄ and BF₄ . 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 1 :

Table 1

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 ruthernium based compound can be dispersed in a solvent along with an acid photoinitiator material, which can then be blended with the resinous material. In order to achieve a material which can be inkjet printed, it has been found that a relatively low molar mass solvent is required. Low molar mass of the solvent results in a better dispersion of the RuX and photoinitiator when printed on the substrate.

Volatile polar solvents have been used in order to establish the impact of the solvent on the operation of the resulting device. The solvents are identified in Table 2. It will be appreciated that derivatives of the solvents in Table 2 may be suitable, or blend of solvents may be used:

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 device, 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 will 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 in the Application 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 glycidylmethacrylate materials:

Cross-linkable Polymers poly(tert-butyl methacrylate-co-glycidylmethacrylate) poly(methyl methacrylate-co-glycidylmethacrylate) poly(styrene-co-glycidylmethacrylate)

Table 3

Acid photoinitiators which are sensitive at wavelengths most suitable types for rapid curing by mercury halide UV lamps are detailed in Table 4:

Cationic Photoinitiator

(4-phenoxyphenyl)diphenylsulfonium triflate

Bis(tert-butylphenyl)iodonium perluoro-1 -butanesulfonate

Diphenyliodonium

2-(4-Methoxystyryl)-4,6-bis(trichloromethyl)-1 ,3,5-triazine

5,7-diiodo-3-butoxy-6-fluorone

Table 4 Acid photoinitiators which are sensitive at wavelengths most suitable types for rapid curing by mercury-iron halide UV lamps have additional photoinitiators Table 5:

Cationic Photoinitator

(4-Methylthiophenyl)methyl phenyl sulfonium trifluoromethanesulfonate

(4-Phenylthiophenyl)diphenylsulfonium trifluoromethanesulfonate,

1-Naphthyl diphenylsulfonium triflate

Table 5

The acid photoinitiators, Tables 4 and 5, allow curing at wavelengths that are at peak emissions from the respective UV source. It will be appreciated that shorter exposure times will reduce degradation of the Ruthenium complexes.

5,7-diiodo-3-butoxy-6-fluorone has peak sensitivity at 470nm, but there is a significant absorbance (approximately 50% λ_max) around 406nm, 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 proporation, 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/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-glycidylmathacrylate). The photoinitiator for this polymer is (4- phenoxyphenyl) diphenylsulfonium triflate (C₂₅H₁₉F₃O₄S₂).

The UV curable monomer resin which yields a high perfomance device is Bis[4- (vinyloxy)butyl]isophthalate. The photoinitiator for which is (4-phenoxyphenyl) diphenylsulfonium triflate (C₂SH₁₉F₃O₄S₂).

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 3 as follows:

Ru dissolved in methanol (CH₃OH), or dichloromethane-methanol mixture (CH₂CI₂- CH₃OH);

[Ru(bpy)₃](BF₄)₂ dissolved in methanol (CH₃OH), or acetone (CH₃COCH₃), or dichlorometane-methanol mixture (CH₂CI₂-CH₃OH);

[Ru(bpy)₃](CIO₄)₂ dissolved in methanol (CH₃OH), or acetonitrile (CH₃CN);

[Ru(bpy)₃](PF₆)₂ dissolved in acetone (CH₃COCH₃), acetonitrile (CH₃CN), dimethyl sulfoxide ((CH₃)₂SO), or dimethylformamide ((CH₃)₂NC(O)H). [Ru(bpy)₃](PF₆)₂ does not dissolve as readily in methanol as it does in acetone, acetonitrile, dimethyl sulfoxide, or dimethylformamide. However with enough methanol it does dissolve. Methanol is 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 eleven or more ruthenium-solvent derivatives that are suitable for blending with either the UV curable monomer, or UV curable polymer. It will be further appreciated that some solvents are less suitable for inkjet printing due to their volatility, despite having suitable a viscosity, some solvents evaporate too quickly. All solvents have been found to be suitable for spin-coating processes. 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 charge transport between the opposing layers of the film can occur through the light emitting layer. Furthermore the viscosity of the blended materials prior to curing must be such that the material can be printed. It is an important consideration for the present invention that 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 electroluminescence 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 OLED polymer may have the formula 27-37% polymer poly(tert-butyl methacrylate- co-glycidylmathacrylate), 3% photoinitiator (4-phenoxyphenyl) diphenylsulfonium triflate (C₂₅H₁₉F₃O₄S₂), blended with 60-70% [Ru(bpy)₃](PF₆)₂. The ruthenium compound and photoinitator being dissolved in acetonitrile (CH₃CN).

An OLED polymer may have the formula 27-37% monomer Bis[4- (vinyloxy)butyl]isophthalate. The photoinitiator for which is (4-phenoxyphenyl) diphenylsulfonium triflate (C₂₅H₁₉F₃O₄S₂), blended with 60-70% [Ru(bpy)₃](PF₆)₂. The ruthenium compound and photoinitator being dissolved in acetonitrile (CH₃CN). 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. A reduction in the UV curable polymer may affect the cohesion properties to the substrate. 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 print-head 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. However the irradiance will be greater when a solvent is used to dissolve said crystals.

An inkjet printer is used to place small droplets of ink onto a substrate. The dots are positioned very precisely by digital registration. InkJet 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 / translucent substrate may be used for the anode. The cathode could be indium-gallium (InGa). InGa is a liquid at approximately 16degC and has a sufficiently low viscosity to be inkjet printed, if an inert gas environment is maitained. To maintain device integrity, a final protective UV curable lacquer would encapsulate said device. This prevents water ingress and /or contamination of said device.

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

Table 7

Other printing processes are vacuum thermal evaporation (VTE) and organic vapour phase deposition (OVPD). OVPD generally employs a nitrogen, or other inert carrier gas to transport the OLED vapour to a substrate, where it condenses upon said substrate. It will be appreciated that any ink, 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 electroluminescent 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 depends on the binder polymer molecular weight. At the same weighin of polymer into 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, adjust the viscosity for the print-head being used to spray the device.

Improvement of the quality of the solution RuX with photoiniator and polymer poly(tert- butyl methacrylate-co-glycidylmethacrylate) solutions by employing an extra sub-micron filtration step prior to deposition. This removes any micro-particles of solution that hasn't 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.

In Figure 4, an example of an electron-scan microscopy image is shown at 46. An RuX layer 44 is deposited onto a substrate 48.

The chemical composition and deposition process must result in a flat surface morphology of the device. This is essential to result an even light emission of the emissive layer and to ensure that subsequent layers deposited onto the emissive layer 44 do not result in an electronic short between the anode and the cathode layers, or that weak areas result in the device. Weak areas may be thinner, so result in areas of brighter light. Thinner areas are liable to fail.

It will be appreciated that the emissive layer will only have a flat surface morphology if the layer it is deposited on is flat. In Figure 5, an example of an inkjet printed RuX OLED emissive layer is shown generally at 54. InkJet drops of 1 pi, shown typically at 52 have been deposited on to a substrate 50.

If the droplets are deposited close enough together, then they will blend to form a single amorphous light emissive layer. For the droplets to result in a film for a light emissive product, the substrate should either consist of 'wells' to contain droplets, or if flat, then a pre-deposited film of PEDOT-PSS is required.

An improved emissive layer will result if the printer platform, with the substrate upon it is pre-heated to aid driving off the solvent. This has the advantage of driving the solvent off, so reducing dot-spread and increasing the printing spread.

The application GB0813662.4 discloses how to effectively light an OLED where the deposition process results in gaps in the emissive layer.

Figure 6 shows a typical spectrum of a mercury-halide arc lamp, whilst Figure 7 is that for a mercury-iron arc lamp. Both are suitable for free-air, or inert environment curing of printed films. The 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 wavelength peaks, and UV-C region peaks are beneficial for the controlled polymerisation of thin films and for ensuring complete curing.

In Figure 6, the potentially usable wavelength regions can be seen to be approximately 238-265nm, 278-292nm, 310-316nm and 355-375nm. In Figure 7, the UV emissive band is much wider. Greater care must be taken against skin exposure when using mercury-iron lamps.

Figure 8 shows a typical spectrum of 365nm and 375nm UV emissive LEDs. Both are suitable for 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 - 315nm) 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 overall the energy is the 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 cathode to the anode 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, electrons combine with electron holes. Transfer of charge can by ionic migration through the emissive layer At said event, the electron gives up energy in the form of a photon emission. The wavelength / waveband being a function of the organic molecules.

The colour of the light depends on the type of organic molecule in the emissive layer. Manufacturers 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 9 shows a typical spectral response for the base OLED material [Ru(bpy)₃](CIO₄)_2l which is taken shortly after the device has been activated. The peak wavelength is approximately 600nm.

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 blend of a light curable resinous material and a solvent in which is dissolved a crystalline material, said crystalline material being selectively electroluminescent by application of said potential difference thereto.

2. A multi-layer organic device according to claim 1 , wherein a cross-linkable matrix is used to support Ru(bpy)₃ ²⁺.

3. A multi-layer organic electronic device according to a previous claim, wherein said crystalline material emits light of a visible wavelength.

4. A multi-layer organic electronic device according to a previous 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 any one of claims 3 to 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'bipyridyl) ruthenium(ll).

8. A multi-layer organic electronic device according to any one of claims 4 to 6, wherein the negatively charged counter-ion comprises any of Cl , PF₆ , CIO₄ or BF₄ .

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.

12. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises bis(tert-butylphenyl)iodonium perluoro-1-butanesulfonate.

13. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises bis(tert-butylphenyl)iodonium perluoro-1-butanesulfonate.

14. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises diphenyliodonium.

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.

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 claim 10, wherein the photoinitiator comprises (4-Methylthiophenyl)methyl phenyl sulfonium trifluoromethanesulfonate.

18. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises (4-Phenylthiophenyl) diphenylsulfonium trifluoromethanesulfonate.

19. A multi-layer organic electronic device according to claim 10, wherein the photoinitiator comprises 1-Naphthyl diphenylsulfonium triflate.

20. A multi-layer organic electronic device according to any one of claims 1 to 9, wherein the resinous material comprises a free-radical photoinitiator.

21. A multi-layer organic electronic device according to any one of the preceding claims wherein the resinous material comprises a glycidyl group polymer.

22. A multi-layer organic electronic device according to claim 21 , wherein the polymer comprises poly(tert-butyl methacrylate-co-glycidylmethacrylate).

23. A multi-layer organic electronic device according to claim 21 , wherein the polymer comprises poly(methyl methacrylate-co-glycidylmethacrylate).

24. A multi-layer organic electronic device according to claim 21 , wherein the polymer comprises poly(styrene-co-glycidylmethacrylate).

25. A multi-layer organic electronic device according to any one of claims 1 to 20, wherein the resinous material is a vinyl.

26. A multi-layer organic electronic device according to claim 25, wherein the resinous material is a monomer material comprising bis[4-(vinyloxy)butyl]isophthalate.

27. A multi-layer organic electronic device according to any one of the preceding claims wherein the resinous material comprises an oligomer.

28. A multi-layer organic electronic device according to any one of the preceding claims, wherein the active portion comprises between 55% and 80% by weight of said crystalline material.

29. A multi-layer organic electronic device according to any one of the preceding claims, comprising a polar solvent.

30. A multi-layer organic electronic device according to any one of the preceding claims comprising a low molar mass solvent, wherein the molar mass of the solvent is 85 g/mol or less.

31. A multi-layer organic electronic device according to claim 30, wherein the solvent comprises any of acetone, acetonitrile, methanol, dichloromethane, dimethyl sulfoxide, or dimethylformamide.

32. A multi-later organic electronics device according to claim 6, further comprising the material poly(3,4-ethylene dioxy-2,4-thiophene)-polystyrene sulfonate, or a derivative thereof, between the active portion and either of said contact layers and for the purpose of whetting for said ruthenium complex.

33. A method of manufacturing a multi-layer organic electronic device according to any one of claims 1 to 32 comprising: forming an active material by blending a light curable resinous material with a solvent in which is dissolved an electroluminescent crystalline material; applying a first contact layer to a substrate; applying said active material to said first contact layer so as to form an active portion of the device; applying said second contact layer such that the active portion is sandwiched between said 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 curing at least said active portion.

34. The method of claim 33, wherein the light curable resinous material comprises a cationic photoinitiator material which is substantially evenly distributed in the solvent by the blending.

35. The method of claim 33, wherein the light curable resinous material comprises a free-radical photoinitiator material which is substantially evenly distributed in the solvent by the blending.

36. The method of claim 33, further comprising filtering the active material and/or solvent having the crystalline material dissolved therein.

37. The method of claim 36, wherein the filtering removes particles of size greater than approximately 1 μm.

38. The method of any one of claims 33 to 37, comprising applying said first layer to the substrate, applying said active portion to said first layer, applying said second layer to said active portion and curing said device.

39. The method of any one of claims 33 to 37, wherein the active portion, the first and second contact layers are each individually cured once they have been applied.

40. The method of any one of claims 33 to 39, wherein the organic electronic device is inkjet printed.

41. The method of any one of claims 33 to 40, wherein the substrate is heated to aid evaporation of the solvent.

42. The method of any one of claims 33 to 40, wherein the curing is carried out using a mercury-iron UV lamp.

43. An active portion for use in a multi-layer organic electronic device according to any one of claims 1 to 32.

44. An active portion according to claim 43, wherein the active portion has a viscosity of lOOmPA.s or less at ambient.

45. An active portion according to claim 43, wherein the active portion has a viscosity of 20mPA.s or less at 55°C.