FIELD OF THE INVENTION

The invention relates to the field of electroluminescent devices, such as OLED devices, with two transparent electrodes, wherein at least one optical element is applied to the second electrode opposing the substrate and wherein the optical element at least partially covers the surface area of the second transparent electrode and reflects and/or refracts light into the direction of the first transparent electrode.

BACKGROUND OF THE INVENTION

Conventional electroluminescent (EL) devices are usually produced by deposition of the electrodes and the required electroluminescent layer(s) on a transparent substrate such as glass or a polymer foil through which the light is emitted. In such EL devices the electrode opposing the substrate, usually the cathode, can be deposited as thin metal layer, e.g. an aluminum layer. Due to the properties of such a thin metal layer that is typically 100 nm thick the EL device - in the off- state - has a mirror- like appearance if looked upon from the substrate side.

Another type of conventional EL devices comprises two transparent electrodes. Such EL devices emit light through both the front, i.e. the substrate, and the back, i.e. the second transparent electrode.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an EL device with two transparent electrodes wherein the optical appearance in the off- and/or on-state can be con- trolled. A further object of the invention is the provision of a method for the production of such an EL device.

This object is achieved by an EL device according to claim 1 and the method according to claim 12. Particularly, an EL device is disclosed comprising a substrate and stacked thereon in the order of mention a first transparent electrode, an elec- tro luminescent stack, a second transparent electrode and at least one optical element, wherein the optical element is in optical contact with the second transparent electrode and partially or completely covers the surface area of the second transparent electrode and is adapted for reflecting and/or refracting light into the direction of the first transparent electrode.

The present invention is based on the unexpected finding that the applica- tion of an optical outcoupling element to the back electrode, i.e. the electrode opposing the electrode deposited onto the substrate, can be utilized to reflect and/or refract light travelling in the direction of the second transparent electrode back into the EL device, i.e. into the direction of the first transparent electrode. In other words, light generated by the EL device and/or incident light entering the EL device through the first transparent electrode can be redirected by the optical element to leave the EL device through the substrate electrode, i.e. the first transparent electrode. If the optical element only partially covers the second transparent electrode, for example in a given pattern, light will be redirected back through the first transparent electrode only by such areas where the optical element is present. Consequently, light will leave the EL device through the substrate in the pattern of the optical element. Therefore, the appearance of the EL device can be designed, adapted and/or altered in the on- and/or off- state by means of the optical element applied to the second transparent electrode.

The EL device can be any EL device known to the skilled person and/or any device for the generation of light based on electroluminescent diodes. Preferably the EL device is an organic EL device, i.e. an OLED device. In further embodiments the EL device of the present invention is used as or comprised by a light source, a lamp, or is comprised by a monitor, switch or display. Thus, also a light source, a lamp, a monitor, a switch and a display comprising the inventive EL device are encompassed by the present invention. In the following the basic structure of an organic EL device is described comprising a substrate and stacked thereon a first transparent electrode, an organic electroluminescent stack, and a second transparent electrode. However, various other basic structures of EL devices, and particularly organic EL devices, are known to the skilled person, all of which are meant to be encompassed by the present invention. An exemplary basic EL device comprises two transparent electrodes, i.e. an anode and a cathode, wherein the anode is usually disposed on a substrate such as glass or flexible polyethylene terephtalate (PET) foil. On top of the substrate electrode, i.e. - without loss of generality - the anode, the EL stack is disposed comprising at least one emitter layer comprising at least one type of EL molecules. A second transparent electrode, i.e. the cathode acting as the counter electrode, is disposed on top of said electroluminescent stack. The skilled person will be aware of the fact that various other layers may be incorporated for the production of such an EL device, for example, a hole transport layer that may contact the anode, an electron transport layer that may contact the cathode, a hole injection layer - preferably made from poly(3,4- ethylendioxythiophene)/polystyrolsulfonate (PEDOT/PSS) - disposed between the anode and the hole transport layer and/or a electron injection layer - preferably a very thin layer made from lithium fluoride, or cesium fluoride - disposed between the electron transport layer and the cathode. Furthermore, it is known to the skilled person that EL devices may comprise an EL stack wherein more than one emitter layer is present.

In one embodiment the EL device is an OLED device, i.e. the electroluminescent emission layer(s) comprise organic molecules. In further preferred embodi- ments the organic molecules comprise polymers (PLEDs) or small molecules (SMO- LEDs). In another preferred embodiment, the EL device is a phosphorescent organic light-emitting diode (PHOLED) device. The present invention is not restricted to specific organic molecules provided such are suitable for the use as electroluminescent molecules in EL devices. Various electroluminescent and/or organic electroluminescent molecules are known to the skilled person, all of which are meant to be encompassed by the present invention. As used in the present invention "electroluminescent molecules" preferably mean "organic electroluminescent molecules". In preferred embodiments the polymers of a PLED are conjugated polymers such as derivates of poly(p-phenylen- vinyls) (PPV) and the small molecules of an SMOLED are organo -metallic chelates, such as for example Alq3, and/or conjugated dendrimers.

The substrate is transparent and can comprise any suitable material known to the skilled person. In the present invention the term "transparent" refers to the transmission of in the visible range of > 50 % light in the given material. The remaining light is thus either reflected and/or absorbed. "Transparent" includes "semi-transparent" refer- ring to a material that exhibits a transmission of light in the visible range of between > 10 % and < 50 %. Thus, whenever reference is made to a "transparent" material this also explicitly discloses a "semi-transparent" material if not stated otherwise. Preferably light in the visible range has a wavelength of between > 450 nm and < 650 nm. Thus, for example, a transparent substrate or electrode absorbs and/or reflects less than 50 % of the incident light.

In preferred embodiments of the invention the substrate is made from glass, plastics, ceramics, and/or comprises at least one of gold and silver. Further preferred materials for the substrate comprise polymer sheets or foils, more preferably with a suitable moisture and oxygen barrier to essentially prevent moisture and/or oxygen entering the EL device. The substrate may further comprise additional layers, e.g. for optical purposes such as light out-coupling enhancement and the like. The substrate can have any suitable geometry, shape or form but is preferably flat and may, if a flexible material is utilized, be shaped or bent into any three- dimensional shape that is required.

The transparent electrodes can be made from any suitable material known to the skilled person. In preferred embodiments the electrodes are made from a metal, diamond- like carbon, or comprise at least one of the following materials: indium tin oxide (ITO), aluminum, silver, ZnO, doped ZnO or an oxide layer. More preferably, the electrodes are made from a transparent conductive oxide (TCO) such as ITO, or ZnO. Optionally the substrate electrode is undercoated with SiO ₂ and/or SiO to suppress diffusion of mobile atoms or ions from the substrate into the electrode. In a further embodiment thin Ag or Au layers are used as electrodes. Preferably, such electrodes have a thickness of > 3 nm and < 20 nm, more preferably of > 5 nm and < 15 nm and most preferably of about 8 or 10 nm. Most preferably, at least the second transparent electrode is an Ag or Au layer.

Electrodes comprising a thin metal layer, such as an Ag or Au layer, pre- ferably have a transparency of > 50 % and < 100 %, more preferably of > 60 % and < 80 % and most preferably of about 66 %. Electrodes comprising a TCO preferably have a transparency of > 60 % and < 100 %, more preferably of > 70 % and < 90 % and most preferably of about 80 %.

In a further preferred embodiment, the first transparent electrode disposed onto the substrate, i.e. the front or substrate electrode, is the anode and the second transparent electrode disposed onto the EL stack, i.e. the counter or back electrode, is the cathode. Preferably, the electrodes are connected to a voltage/current source through electrical conductors.

The EL stack can be any EL stack known to the skilled person and/or suitable for an EL device. As described above an EL stack comprises at least one EL emitter layer comprising EL molecules. A single EL emitter layer preferably has a thickness of about 10 nm.

Preferred EL stacks comprise more than one EL layer, each comprising at least one type of EL molecule. Preferably, the EL layers emit light of different colors. This is especially advantageous if color tuneable EL devices are required. In a further embodiment of the invention the EL stack comprises at least two EL emission layers having different emission colors. This means that if the EL device of the present invention is induced to emit light by application of electric voltage/current each of the at least two emission layers will emit light at a different wavelength. If n > 2 emission layers are present between 2 and n-1 emission layers preferably have a different emission color than the other emission lay er(s).

Different emission colors are usually achieved by use of different EL molecules that are comprised by the EL emission layers. Each EL emission layer can comprise a single or, more than one type of EL molecules. In more preferred embodiments, the EL stack comprises three EL emission layers emitting red, green and blue light, re- spectively.

The optical element can be any means that allows the reflection and/or refraction of light traveling in the direction towards the second transparent electrode. As explained above, the optical element is disposed in optical contact with the second transparent electrode and positioned on the side of the second transparent electrode facing away from the substrate and/or the EL stack. Thus, light traveling in the direction towards the second transparent electrode passes this electrode, is reflected and/or refracted by means of the optical element and again traverses the second transparent electrode, the EL stack and the first transparent electrode to finally exit the EL device through the substrate. The optical element may have any suitable form and/or shape that allows for a reflection and/or refraction of incident light according to the invention.

In a preferred embodiment of the invention the optical element comprises a reflecting back surface. The term "back surface" means that this surface is on the side of the optical element opposing the second transparent electrode. This has the advantage that the reflection of light is further enhanced. More preferably the reflecting back surface comprises Al, Ag, and/or a dielectric mirror. The optical element may cover the complete surface area of the second transparent electrode lying underneath or it may, in a preferred embodiment, only cover part of said surface area. This is especially advantageous to display information and/or a design since only light traversing the second transparent electrode that impinges upon the optical element will be reflected and/or refracted back to leave the EL device through the substrate. Various embodiments of an optical element covering only part of the surface area of the second electrode will be disclosed below.

In preferred embodiments the at least one optical element is or has the form and/or shape of at least one layer, at least one foil, at least one spherical shaped element, at least one ellipsoid shaped element, at least one triangular shaped element, at least one prismatic element, at least one irregular shaped element, at least one pattern, at least one geometrical pattern, at least one artistic pattern, at least one symbol, at least one character, at least one image, and/or least one sign. In further preferred embodiments the optical element is cut with a shape used in gemology, preferably it has a diamond cut. As will be apparent to the skilled person from the above also more than one optical element can be arranged to form a certain pattern, shape, image, geometry, character, symbol, sign and/or form. Thus, the optical element can be arranged as a single optical element or in a group and/or pattern of more than one optical element. For example, a character may be formed by arranging a number of optical elements in a dot matrix. As another example, the character or number "8" may be formed by seven optical elements arranged in the way used with seven-segment displays known to the skilled person. In a preferred embodiment the characters and symbols are formed using the dot matrices of segment displays known to the skilled person.

The skilled person will furthermore readily understand that the optical element can have nearly any desired geometry and that thus the optical appearance of the EL device in the off- and/or on-state may be changed as desired. In other words, the appearance of the EL device in the on- and/or off- state can be changed, designed and/or modified to display this desired geometry, i.e. any desired geometry is visible in the on- and/or off-state of the EL device. Advantageously this can be used to provide EL devices that, preferably in the off-state, display any type of information and/or design. For example, if the optical element has the form of the three characters "O", "F" and "F" the word "OFF" will be visible to a user even in the off-state of the EL-device.

Preferably, if the second electrode has a flat surface, the optical element will at least have one flat surface as well that will oppose the flat surface of the electrode.

The at least one optical element may comprise any material that allows for a reflection and/or refraction of incident light according to the invention.

In preferred embodiments the at least one optical element comprises a material selected from the group consisting of glass, plastics, PMMA, crystals, ZrO ₂ , diamond, an adhesive, a pigmented adhesive, a metal and/or ceramic.

In other preferred embodiments the at least one optical element comprises a natural material, more preferably a thin slice of a natural material, such as paper, wood, stone, a woven textile, a fabric. This has the advantage that EL devices displaying natural textures and/or structures may be provided.

In more preferred embodiments the at least one optical element is colored and/or clear, i.e. not colored. Colored optical elements have the advantageous effect that the optical appearance of the EL device in the on- and/or off- state can be further changed by coloring. In further preferred embodiments the optical element comprises different colors and/or different optical elements comprise a different color.

A coloring of the optical element(s) can be achieved by various means known to the skilled person. In preferred embodiments pigments are used that are com- prised by and/or deposited onto the optical element(s). Such pigments can be applied over the whole optical element(s) and/or applied partially, for example in a specific pattern as explained above, i.e. without limitation as characters, signs, images, sentences, logos and so on. The application can, inter alia, be carried out by printing, transfer- printing and/or screen-printing methods or various other printing methods known to the skilled person. In a preferred embodiment the application is carried out by non-contact printing methods to advantageously protect the sensitive EL device.

In another embodiment the pigments are incorporated into the material of the optical element(s) during production of such, for example during production of a colored plastic optical element.

All suitable pigments known to the skilled person can be used. In preferred embodiments the pigments are selected from the group consisting of metal effect pigments, coated and/or uncoated metal flakes, aluminum flakes, mica particles, Tiθ2 coated particles, Irodion pigments obtainable from Merck, Darmstadt, Germany, interference effect pigments such as pigments based on Tiθ2 and/or Fe2θ3 coated particles such as mica particles, all colored and/or uncolored pigments such as TiO ₂ , BaSO ₄ , ZnO, SnO, Fe ₂ Os, phtalocyanines, pigments based on carbon, such as graphite, black soot pigments, glassy carbon, carbon nanotubes etc, and all other pigments used in coatings and/or lacquers and combinations thereof.

In another preferred embodiment the optical element(s) is/are at least partially colored dark, more preferably black. This has the advantage that an enhanced contrast is achieved. In a particularly preferred embodiment of the invention the at least one optical element is comprised by a layer and/or a foil that is even more preferably laminated to the second transparent electrode. In the following, when reference is made to a foil, a layer is also referred to.

In such an embodiment the foil is used to attach the at least one optical element to the EL device, i.e. the second transparent electrode. In other words, while the foil itself may cover the whole surface area of the second transparent electrode the optical elements it carries may only cover part of said surface area and is used to reflect and/or refract the impinging light. The optical element may have any form, particularly those described above, i.e. at least one spherical shaped element, at least one ellipsoid shaped element, at least one triangular shaped element, at least one prismatic element, at least one irregular shaped element, at least one pattern, at least one geometrical pattern, at least one artistic pattern, at least one symbol, at least one character, at least one image, and/or least one sign. The use of at least one foil as the optical element entails various advantages as explained below. In one embodiment the foil is used to seal and/or encapsulate the EL device in an inexpensive way, i.e. the second transparent electrode is capped by the foil. As the EL components of a typical EL device, for example an OLED device, are extremely sensitive to moisture and degradation by oxygen, EL devices need to be protected from such environmental influences. Therefore, the optical element in form of a foil can advantageously be used to encapsulate the EL device. Preferably, the foil is impermeable to water and/or oxygen and/or it comprises a metal foil and/or a glass sheet. In further pre- ferred embodiments the foil as an encapsulation means is used as the only encapsulation means or together with various different encapsulation means known to the skilled person, for example a cavity lid.

In another embodiment the optical element comprised by the foil is a pattern, preferably printed and/or painted. The pattern is preferably present on the surface of the foil and/or embedded in the material itself. Advantageously such a pattern can provide a patterned optical appearance to the EL device in the on- and/or off-state of the device as explained above.

In a further embodiment the foil comprises surface corrugations and/or patterns as optical element(s) that reflect and/or refract the light in the on- state of the device.

In another embodiment the at least one optical element comprised by the foil is highly reflective and/or scattering and thus enhances the outcoupling of light.

In a further embodiment of the invention the foil is electrically conductive, for example a metal foil, and thus advantageously provides the electrical contact to the second transparent electrode of the EL device.

In a further embodiment the at least one optical element comprised by the foil is a colored part and/or region of the foil. This has the advantage that EL devices displaying a colored optical appearance in the on-and/or off-state can be provided.

In another preferred embodiment the foil itself is the optical element and covers part of or the complete surface area of the second transparent electrode. In such embodiments the foil may be attached to the second transparent electrode in any desirable shape, form or pattern, preferably those described above for the optical element, for example, may have any form, particularly those described above, i.e. at least one spherical shaped element, at least one ellipsoid shaped shape, at least one triangular shape, at least one irregular shape, at least one pattern, at least one geometrical pattern, at least one artistic pattern, at least one symbol, at least one character, at least one image, and/or least one sign. If more than one foil is used patterns may be formed by arranging different foils side by side and/or on top of each other.

In another particularly preferred embodiment of the invention the at least one optical element is comprised by a layer of an organic binder and/or an adhesive. In the following, the term "adhesive" is meant to comprise the term "binder". This has the advantage that an inexpensive, easy and reliably capping of the EL device and thus a protection from environmental influences may be achieved.

In such an embodiment the at least one optical element is embedded in the adhesive layer. In other words, while the adhesive layer itself may cover the whole sur- face area of the second transparent electrode the optical elements it carries may only cover part of said surface area and is used to reflect and/or refract the impinging light. The optical element may have any form, particularly those described above, i.e. at least one spherical shaped element, at least one ellipsoid shaped element, at least one triangular shaped element, at least one prismatic element, at least one irregular shaped element, at least one pattern, at least one geometrical pattern, at least one artistic pattern, at least one symbol, at least one character, at least one image, and/or least one sign.

The adhesive may be any suitable adhesive known to the skilled person. It can be of natural or synthetic origin. In a preferred embodiment the adhesive is transparent. In further preferred embodiments the adhesive is a drying adhesive, a contact adhe- sive, a hot adhesive, a UV and/or light curing adhesive and/or a pressure sensitive adhesive. Drying adhesives, such as rubber cements, are a mixture of compounds, typically polymers, dissolved in a solvent. As the solvent evaporates, the adhesive hardens. Contact adhesives are applied to both surfaces of the items to be glued together and allowed some time to dry before the two surfaces are pushed together. However, once the sur- faces are pushed together, the bond forms very quickly. Thus, this offers the advantage that it is not necessary to apply pressure for a long time or to use clamps. Hot adhesives are thermoplastics which are applied hot and harden as they cool. Such adhesives can advantageously be applied by means of a glue gun. Ultraviolet and/or light curing adhesives are advantageous due to their rapid curing time and strong bond strength. Such adhesives can cure in as little as a second and many known formulations exist which can bond dissimilar substrates and withstand harsh temperatures. Furthermore, they can be used to seal and coat products. Thus, ultraviolet and/or light curing adhesives are espe- cially suited for the EL device of the present invention. Pressure sensitive adhesives form a bond by the application of light pressure to marry the adhesive with the adherend.

In more preferred embodiments the adhesive is an epoxy adhesive, a polymeric binder, a fast curing adhesive, a fast curing epoxy adhesive, a silicon gel, a po- lyurethane adhesive and/or a two component adhesive such as "UHU 5 minutes curing glue".

In a further preferred embodiment the adhesive is the optical element itself. Preferably the adhesive as optical element only partially covers the second transparent electrode. In such embodiments the adhesive may be attached to the second transpa- rent electrode in any desirable shape, form or pattern, preferably those described above for the optical element, for example, may have any form, particularly those described above, i.e. at least one spherical shaped element, at least one ellipsoid shaped shape, at least one triangular shape, at least one irregular shape, at least one pattern, at least one geometrical pattern, at least one artistic pattern, at least one symbol, at least one charac- ter, at least one image, and/or least one sign. In addition, shapes, forms, patterns etc. may also be formed by using more than one type of adhesive on the OLED in the described way.

In a further preferred embodiment the adhesive is colored, more preferably by means of the pigments and/ or a combination of the pigments described above. The adhesive may be applied to the second transparent electrode by any suitable means known to the skilled person. Preferably, the adhesive is applied by printing methods as described above. As will be readily apparent to the skilled person, the adhesive may be applied to the complete surface area of the second transparent electrode or it may preferably only partially cover the surface area of said electrode. Thus, the above describe different textures, shapes, forms, patterns, characters etc., preferably using different colors, may be readily applied to the second transparent electrode to change the optical appearance of the EL device in the on- and/or off- state as desired.

If an adhesive is used for the production of an EL device according to the present invention preferably a bake-out procedure, more preferably with a temperature of below 80° C, may be employed to improve the curing and/or the stability of the adhesive. However, such procedures highly depend on the type of adhesive employed and thus the skilled person will readily apply the curing method best suited for the respective adhesive utilized.

In another embodiment of the invention the optical element is not an adhesive and not comprised by an adhesive and the EL device further comprises an adhesive layer disposed between the second transparent electrode and the at least one optical element. This has the advantage that the optical element(s) can easily and reliably be affixed to the second transparent electrode. The adhesive layer is thus used to optically couple and/or optically contact the second transparent electrode and the optical element. In other words, it must be possible for the light to traverse the adhesive layer to reach the optical element and be reflected and/or refracted back into the second transparent electrode. In a preferred embodiment the refractive index of the adhesive layer is adapted to the refractive index of the second transparent electrode, the refractive index of the EL stack and/or the refractive index of the optical element(s).

The adhesive can be any suitable adhesive known to the skilled person and preferably is an adhesive as detailed above. More preferably, the adhesive is transpa- rent.

In a further preferred embodiment the adhesive' s refractive index is matched to the refractive index of the EL stack. This has the advantages that also light modes that are trapped in the organic modes of the EL stack may reach the optical elements). More preferably, the adhesive has a refractive index of n > 1.4 and < 1.9, more preferably of n > 1.56 and n < 1.8. The adaptation of the refractive index of the adhesive can be achieved by mixing of the adhesive with small, preferably nano-sized, i.e. > 1 nm and < 100 nm, particles. Preferably such particles have a high refractive index, such as TiC>2. Utilization of such particles advantageously increases the average refractive index of the adhesive without adding too much to the scattering properties. Furthermore, at least one optical element with a high refractive index is preferably used in combination with said adhesive, for example ZrC>2, diamond and/or glass with a high refractive index. In a further embodiment of the invention the EL device comprises at least one additional transparent layer that is disposed between the second transparent electrode and the adhesive layer. This has the advantage that the transparency may be en- hanced and/or that the transparent electrode is protected from the adhesive. The at least one additional layer may be made from any suitable material such as the materials described above for the substrate, from any inorganic material such as e.g. ITO, ZnO, other oxides etc or organic material, such as e.g. AIq ₃ etc and preferably is a non-conductive layer. This has the further advantage that an additional electrical insulation is achieved.

In a further embodiment the EL device further comprises a power and/or voltage source. Preferably said power and/or voltage source is connected to the elec- trades, i.e. anode and cathode, of the EL device. More preferably, the power and/or voltage source provides DC voltage between anode and cathode. In another preferred embodiment the power and/or voltage source provides a voltage of > 1 V and < 20 V, more preferably of > 2 V and < 10 V.

In even another aspect the invention is directed to a method of producing an electroluminescent device according to the invention comprising the steps of: providing a substrate; depositing onto the substrate in the order of mention: a first transparent electrode, an electroluminescent stack, a second transparent electrode and at least one optical element, wherein the optical element is in optical contact with the second trans- parent electrode, at least partially covers the surface area of the second transparent electrode and is adapted for reflecting and/or refracting light into the direction of the first transparent electrode.

Preferred embodiments of the method according to the invention will be apparent to the skilled person when reading the description regarding the EL device above. However, in the following some of the preferred embodiments will explicitly be disclosed.

In a preferred embodiment, the method further comprises the step of applying a transparent adhesive to the second transparent electrode. Even more preferably the method further comprises the additional step of applying at least one transparent layer for the protection of the second electrode and/or for an enhanced transparency before the application of the transparent adhesive.

The electrodes can be deposited by any suitable means. Preferably, the electrodes are deposited using a vacuum processing system for vapor deposition.

Various methods for the deposition of the EL layers are known to the skilled person all of which are meant to be encompassed by the present invention. Preferably, electroluminescent layers based on small molecules are disposed by vacuum evaporation or by organic vapor phase deposition. In further preferred embodiments electro- luminescent layers based on polymers, i.e. molecules of greater length, are first solubi- lized in suitable solvents and subsequently deposited by printing or spin-coating methods.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 shows a schematic view of an EL device according to the present invention.

Fig. 2 shows a schematic top view of the EL device of Fig. 1.

Fig. 3 shows a schematic view of a different EL device according to the present invention.

Fig. 4 shows a schematic view of a different EL device according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a schematic view of an EL device according to the present invention. The EL device comprises a flat glass substrate 1 onto which a transparent ITO anode 2 has been deposited by CVD. On top of the electrode an OLED stack 3 comprising a red, a green, and a blue emission layer separated by conductive layers (not shown) and the transparent cathode 4, i.e. the back electrode, have been deposited. The cathode is an Ag metal electrode and has a thickness of about 10 nm.

A transparent adhesive layer 5 has been applied to the cathode by a non- contact printing method. The transparent adhesive is the two component epoxy adhesive "UHU 5 minutes curing glue". Furthermore, the refractive index of the adhesive was matched to the refractive index of the OLED stack (n = 1.8) by addition of nano-sized Tiθ2 particles to the adhesive prior to printing. The printing of the adhesive was carried out in a glove box filled with N ₂ . Subsequently 14 optical elements 6 (not all of them are shown in Fig. 1) were affixed to the cathode via the adhesive. This procedure was likewise carried out in the glove box. The optical elements are made from Zrθ2. After application of the optical elements a bake-out procedure was carried out at a temperature of T = 60° C in order to improve the curing and the stability of the adhesive.

Finally, the EL device was encapsulated by capping with a cavity lid made from glass (not shown).

Fig. 2 shows a schematic top view of the OLED device of Fig. 1 and the arrangement of the 14 protruding optical elements made from ZrC>2 glued to the cathode. The rhomboid shaped optical elements thus partially cover the surface area of the cathode and are arranged in the manner of a typical 7-segment display. Fig. 3 shows a schematic view of a different EL device according to the present invention. The OLED device comprises a flat glass substrate 2 onto which a transparent ITO anode 2 has been deposited by sputtering. On top of the electrode an OLED stack 3 and a transparent ZnO cathode 4, have been deposited.

The optical element 6 comprising the two component epoxy adhesive "UHU 5 minutes curing glue" has been applied to the cathode by a non-contact printing and covers the complete surface area of the cathode. By this means the OLED device is efficiently encapsulated and protected against environmental influences.

Furthermore, a pattern has been incorporated into the optical element by printing of areas 7 consisting of the adhesive comprising a colored pigment. The printing of the adhesive was carried out in a glove box filled with N2 and a bake-out procedure was carried out at a temperature of T = 65° C in order to improve the curing and the stability of the adhesive.

An encapsulation of the OLED device by capping was no longer necessary. Fig. 4 shows a schematic view of a different EL device according to the present invention.

The OLED device comprises a flat glass substrate 2 onto which a transparent ITO anode 2 has been deposited by CVD. On top of the electrode an OLED stack 3 and a transparent ITO cathode 4, have been deposited. A transparent adhesive layer 5 has been applied to the cathode by a screen-printing method. The transparent adhesive is a polyurethane adhesive. The printing of the adhesive was carried out in a glove box filled with Ar. The optical element 6 in the form of a colored polymer foil was applied to the electrode via the adhesive layer by means of lamination. By this means the OLED device is efficiently encapsulated and protected against environmental influences.

Furthermore, a pattern of different colored areas 7 has been painted on the surface of the foil.

An encapsulation by capping with a glass cavity lid (not shown) was subsequently carried out to protect the colored areas 7.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be consi- dered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.