WO2010131156A1

WO2010131156A1 - Cooling of electroluminescent devices

Info

Publication number: WO2010131156A1
Application number: PCT/IB2010/051942
Authority: WO
Inventors: Soeren Hartmann; Herbert F. Boerner
Original assignee: Koninklijke Philips Electronics N. V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2009-05-14
Filing date: 2010-05-04
Publication date: 2010-11-18
Also published as: TW201106511A

Abstract

The present invention provides an electroluminescent device (1) comprising a substrate (2) and stacked thereon a first electrode, an electroluminescent stack (3), a second electrode, a heat conducting layer (4) and a heat sink (5), wherein the electro- luminescent device is sealed by the heat conducting layer that is in contact with said heat sink. Furthermore, methods of producing such an electroluminescent device are provided.

Description

COOLING OF ELECTROLUMINESCENT DEVICES

FIELD OF THE INVENTION

The invention relates to the field of electroluminescent devices, such as organic light emitting devices (OLED), wherein a heat conducting layer is utilized to seal the device and furthermore contact a heat sink to the device.

BACKGROUND OF THE INVENTION

Despite the high level of efficiency - when compared to ordinary light sources - organic electroluminescent (OLED) devices, especially those delivering light of a high brightness, still generate considerable amounts of heat which makes it necessary to cool the OLED device in order to ensure a reliable and continuous operation.

Conventional 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. Furthermore, as the EL components of a typical EL device, for example an OLED device, are extremely sensitive to moisture and corrosive gasses, EL devices are usually sealed by means of a cavity lid opposing the substrate in order to protect the device from such detrimental environmental influences. The substrate and cavity lid either hamper the transfer of heat away from the EL device or they do not offer an area large enough to cope with the amount of heat generated by the EL device. In particular, as the light needs to exit the EL device through the substrate, the substrate has to be kept transparent and thus only a marginal fraction of its area at the edges of the EL device can be used for the dissipation of heat. On the other hand, the cavity of the cavity lid on the backside of the EL device is usually filled with Nitrogen or Argon that have a small thermal conductivity and consequently block the heat transfer.

SUMMARY OF THE INVENTION It is an object of the invention to provide an EL device with improved heat dissipation properties that can be operated continuously and reliably, particularly at a high brightness. 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 13. Particularly, an EL device is disclosed comprising a sub- strate and stacked thereon a first electrode, an electroluminescent stack, a second electrode, a heat conducting layer and a heat sink, wherein the electroluminescent device is sealed by the heat conducting layer that is in contact with said heat sink.

The present invention is based on the unexpected finding that the replacement of the cavity lid of a conventional EL device by a heat conducting layer seal- ing the EL device and contacting said layer with a heat sink significantly improves the dissipation of heat from the EL device, thus leading to a more reliable EL device that can be continuously operated even at a high brightness. It has additionally been found that advantageously an adhesive can be used to form the heat conducting layer allowing for a direct, quick and easy mounting of the heat sink to the EL device. 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, a display or television. Thus, also a light source, a lamp, a mon- itor and a television 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 electrode, an organic electroluminescent stack, and a second electrode. However, various other basic structures of EL devic- es, 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 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 anode, i.e. the substrate elec- trade, the organic EL stack is disposed comprising at least one emitter layer comprising at least one type of EL molecules. A second electrode, i.e. the cathode acting as the counter electrode, is disposed on top of said electroluminescent stack, followed by the sealing heat conductive layer and the heat sink. 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 that may be p-doped - disposed between the anode and the hole transport layer and/or a electron injection layer - preferably a very thin layer made from lithium fluoride, cesium fluoride or alternatively a n-doped electron transport layer - disposed between the electron transport layer and the cathode. The introduction of additional hole and electron blocking layers may be advantageous. Furthermore, it is known to the skilled person that EL devices may com- prise an EL stack wherein more than one - pure or doped - light emitting layer is present, e.g. in white OLEDs. Furthermore in so-called stacked EL devices adjacent OLED device like structures can be separated by transparent or semi-transparent doped connecting layers

Further basic exemplary structures of EL devices comprise bottom emis- sion/top emission devices, wherein a transparent or semi-transparent bottom and top electrode, respectively is utilized; transparent LED devices, such as TOLEDS that use a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent); inverted EL devices such as inverted OLEDs, in which the anode is placed on the substrate, and fur- ther more.

In a further embodiment the EL device is an OLED device, i.e. the electroluminescent emission layer(s) comprise organic molecules. In further preferred embodiments 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. In a further preferred embodiment, the EL device comprises a combination of phosphorescent and fluorescent emitters (Hybrid OLED). In another preferred embodiment the OLED is a fluorescent light emitting diode. The present invention is not restricted to specific organic molecules provided such are suitable for the use as electroluminescent molecules in EL devices. Various electro- luminescent 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 electrolumi- nescent molecules". In preferred embodiments the polymers of a PLED are conjugated polymers such as derivates of poly(p-phenylen-vinylene) (PPV), or polyfluorenes and the small molecules of an SMOLED are organo -metallic chelates, such as for example Alq3, Alq2p (BAIq), Ir(ppy)3, Ir(MDQ)2acac, and/or conjugated dendrimers such as a-NPD, TCTA, TBPI, n-MTDATA.

In a further embodiment the substrate is transparent or semi-transparent. The term "transparent" is meant to refer to a substrate and/or cover layer that shows a transmission of light in the visible range of > 50 %. The remaining light is thus either reflected and/or absorbed. The term "semi-transparent" refers to a substrate that exhibits a transmission of light in the visible range of between > 10 % and < 50 %. Preferably light in the visible range has a wavelength of between > 450 nm and < 650 nm.

In further preferred embodiments the substrate is made from glass, 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 pre- ferably flat and may, if a flexible material is utilized, be shaped or bent into any three- dimensional shape that is required.

In the context of the present invention an EL device comprising a combination of a transparent substrate, a transparent substrate electrode and a non-transparent counter electrode, i.e. the electrode opposing the substrate which is usually a reflective electrode, emitting the light through the substrate is called "bottom-emitting". In case of an EL device comprising a combination of a non-transparent substrate and/or non- transparent substrate electrode and a transparent counter electrode, emitting light through the counter electrode is called "top-emitting". In preferred embodiments the EL device is bottom-emitting or top-emitting, or comprises two transparent electrodes and emits into both directions.

In one embodiment of the invention, at least one of the electrodes, i.e. the anode and/or the cathode and/or a connecting layer, if present, is transparent or semi- transparent. The term "transparent" is meant to refer to an electrode and/or connecting layer that shows a transmission of light in the visible range of > 50 %. The remaining light is thus either reflected and/or absorbed. The term "semi-transparent" refers to an electrode and or connecting layer that exhibits a transmission of light in the visible range of between > 10 % and < 50 %. Preferably light in the visible range has a wavelength of between > 450 nm and < 650 nm. The electrode may also be reflective, preferably in case of the counter electrode.

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 such as ITO, or ZnO. Optionally the electrodes are undercoated with Siθ2 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, about 8-15 nm thick - may be used alone or in combination with the electrode layers. If a metal and/or metal foil is used as the electrode it preferably may take the role of the substrate and/or substrate electrode, i.e. either anode and/or 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.

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 layer(s). Different emission colors are usually achieved by use of different electroluminescent molecules that are comprised by the electroluminescent emission layers. In a preferred embodiment the electroluminescent molecule is an electrophosphorescent met- al complex, such as Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate). In another preferred embodiment the electroluminescent molecule is a singulett-emitter, such as 4-(dicyano-methylene)-2-methyl-6-(p-dimethyl aminostyryl)]-4H-pyran, 4- Dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-lH,5H-benzo[ij]quinolizin-8- yl)vinyl]-4H-pyran, or Rubrene (5,6,11,12-tetraphenylnaphthacene). Further preferred embodiments for such electroluminescent molecules comprise Ir(MDQ)2acac (Bis(2- Methyldibenzol[F,H]quinoxaline)(Acetylacetonate)-IR-(III)), emitting red; Ir(ppy)3 (Tris-(Phenyl-Pyridil)-IR), emitting green; and SpiroDPVBi (4,4-bis-2, 2-diphenylvinyl- 1, 1-spirobiphenyl), emitting blue light. Each EL layer can comprise one or more EL molecules.

In more preferred embodiments, the EL stack comprises three EL emission layers emitting red, green and blue light, respectively.

In a further preferred embodiment the EL emission layers emit complementary or nearly complementary colors, for example yellow and blue, red and green, or red and cyan. More preferably, the emitted colors of the at least two emission layers cover most of the spectrum of the visible light. Even more preferably, three EL emission layers are present that cover most of the spectrum of the visible light, for example red, green and blue. This has the advantage that a wide range of colors and white light emission can be achieved. The heat conductive layer can comprise any suitable material, preferably a material that acts as a barrier for moisture and/or gas, such as oxygen. Preferably, the material has a DIN 4108-4 s_<j value of > 0.5 m, more preferably of > 1500 m. Preferably the material has a thermal conductivity of > 0.024 W/(m * K), > about 0.033 W/(m * K),

> about 0.1 W/(m * K), > about 0.13 W/(m * K), > 0.3 about W/(m * K), > about 0.59 W/(m * K), > about 0.6 W/(m * K), > about 0.7 W/(m * K), > about 0.8 W/(m * K), > about 1 W/(m * K), > about 2 W/(m * K), > about 3 W/(m * K), > about 5 W/(m * K),

> about 7 W/(m * K), > about 30 W/(m * K), or > about 100 W/(m * K).

The heat conductive layer can be of variable thickness. Preferably, the heat conductive layer is as thin as possible, more preferably it has a thickness of > 1 μm and < 10 mm, > 50 μm and < 10 mm, > 100 μm and < 10 mm, > 500 μm and < 8 mm, or

> 1 mm and < 5 mm. If the heat sink comprises a cavity lid then the thickness of the heat conductive layer is preferably about the height of the cavity of the cavity lid. More pre- ferably, the heat conductive layer completely fills out the cavity of the cavity lid and thus thermally bridges the electrode with all inner surfaces of the cavity lid. This has the advantage that an optimal thermal coupling is achieved. The heat conductive layer is preferably applied to the electrode in liquid or semi-liquid form, which may harden later on. This has the advantage that the cavity of the cavity lid can easily be filled completely.

In preferred embodiments the heat conductive layer comprises an adhesive. This has the advantage that the heat sink can directly, quickly and reliably by connected to the EL device. In other words, the heat sink is directly glued to the electrode. As used herein, the term "adhesive" is meant to comprise any compound in a liquid or semi- liquid state that adheres or bonds items together. The adhesive can be of natural or synthetic origin. In preferred embodiments the adhesive is a drying adhesive, a contact adhesive, 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 which preferably does not harm the OLED. 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 surfaces 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 use clamps. Hot adhesives are thermoplastics which are applied hot and harden as they cool. The temperature of the glue and the pressure applied may be limited due to the physical properties of one or more of the compounds used in the EL device. 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 formulations, 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 especially 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 a thermally conductive adhesive. Such adhesives are known to the skilled person, for example Dow Corning X3-6211.

In another especially preferred embodiment the heat conductive layer fur- ther comprises a getter material.

In a further preferred embodiment of the invention the heat conductive layer is additionally surrounded by a rim comprising a standard adhesive known to the skilled person and used for the encapsulation of EL devices. This has the advantageous effect that the penetration and/or diffusion of moisture into the heat conductive layer is prevented. In other words, an additional glue rim is applied to the heat conducting layer that preferably completely surrounds said heat conductive layer and seals and/or protects it from environmental influences. In an especially preferred embodiment a getter material is incorporated into the adhesive surrounding the heat conductive layer. In another embodiment of the invention the heat sink is in direct thermal contact with the heat conducting layer. Preferably, the heat conducting layer is an adhesive and the heat sink is adhering to the adhesive. In other words, the heat sink is preferably glued to the electrode by means of the heat conducting layer, i.e. the adhesive. This has the advantage that a quick and reliable assembly of the EL device can be carried out and that an extremely good heat dissipation is achieved. Thus, the EL device of the present invention can be operated reliably and continuously even at a high brightness.

The heat sink can be any means suitable for the dissipation of heat. A heat sink is an object that absorbs and dissipates heat from another object using thermal contact, either direct or radiant. A heat sink efficiently transfers thermal energy, i.e. heat from an object at a relatively high temperature to a second object at a lower temperature with a much greater heat capacity. This rapid transfer of thermal energy quickly brings the first object into thermal equilibrium with the second, lowering the temperature of the first object, fulfilling the heat sink's role as a cooling device. Efficient function of a heat sink relies on rapid transfer of thermal energy from the first object to the heat sink, and the heat sink to the second object. Various designs of heat sinks are known to the skilled person, including without limitation, radial heat sinks, liquid cooled heat sinks or pin fin heat sinks.

In a further embodiment of the invention the heat sink is made of at least one material with a high thermal conductivity and/or a thermal conductivity that is pre- ferably equal or larger than the thermal conductivity of the heat conducting layer, preferably > 10 W/(m * K), > 20 W/(m * K), > 50 W/(m * K), > 70 W/(m * K), > 100 W/(m * K), > 150 W/(m * K), > 200 W/(m * K), > 300 W/(m * K), or > 400 W/(m * K). The heat sink preferably comprises metals, more preferably aluminum, silver, copper and/or gold, or liquids such as water or oil.

In another embodiment of the invention the heat sink has a geometry resulting in a large surface area, e.g. fins. The high thermal conductivity of the heat sink combined with its large surface area results in the rapid transfer of thermal energy to the surrounding, cooler material, e.g. air. This cools the heat sink the heat conducting layer and thus the EL emission layer in thermal contact with it.

In a preferred embodiment the heat sink further comprises means to mix the surrounding material, e.g. the air. Preferably such a means is a fan or a pump. Preferably, the heat sink has a surface area that is larger than the contact surface area between heat conducting layer and heat sink. More preferably, the heat sink has a surface area that is > 1.5, > 2, > 3, > 5, > 10, > 20, > 30, > 50, or > 100 times larger than the area of the contact surface between heat conducting layer and heat sink.

In another embodiment the heat sink comprises a conventional cavity lid as known to the skilled person. In such an embodiment the cavity of the cavity lid is filled with the thermally conductive material of the thermally conductive layer. More preferably, the cavity is filled with an adhesive, either partially or most preferably completely. This has the advantage that the cavity of the cavity lid is no longer filled with Nitrogen or another inert gas - having a low thermal conductivity and thus blocking the heat dissipation - but with the thermally conductive material. If the thermally conductive material is an adhesive the assembly of the EL device is furthermore sped up and simplified and, in addition, results in a more reliable and more robust EL device. It will be apparent to the skilled person that this heat sink comprising a cavity lid may additionally comprise any of the elements described above, e.g. fin like structures, different materials and/or fans.

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 electrodes, 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 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 electrode, an electroluminescent stack, a second electrode; and applying a heat conducting layer and a heat sink to the second 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 deposited EL stack comprises three EL emission layers. Even more preferably, said three EL emission layers emit red, green and blue light, respectively

The electrodes can be deposited by any suitable means. Preferably, the electrodes are deposited using a vacuum processing system for vapor deposition. In further embodiments the vapor deposition is selected from the group consisting of physical vapor deposition (PVC), chemical vapor deposition (CVD), low pressure CVD (LPCVD) or plasma enhanced CVD (PECVD). Preferably, the vapor deposition is

LPCVD or PECVD. In further preferred embodiments the vapor deposition is a deposition of metal oxide, more preferably a deposition of ZnO or indium tin oxide (ITO). ITO or ZnO layers show premium performance. Most preferably the vapor deposition is a ZnO or ITO LPCVD, or a ZnO or ITO PECVD. 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 thermal evaporation in vacuum. In further preferred embodiments electroluminescent layers based on polymers, i.e. molecules of greater length, are first solubilized in suitable solvents and a subsequently deposited by printing or spin-coating methods.

In another preferred embodiment an adhesive is applied as the heat conductive layer. The adhesive can be applied by any possible means, preferably depending on the state of the adhesive. Preferably, the adhesive is sprayed, poured or printed.

The heat sink can be applied to the heat conductive layer by any suitable means. Preferably, the heat conductive layer comprises an adhesive and the heat sink is applied to the adhesive by exertion of pressure, i.e. the heat sink is pressed onto the adhesive. The adhesive may have to harden or may have to be cured before and/or after the application of the heat sink. Such a hardening or curing may preferably result from contacting the heat sink and the adhesive and/or applying pressure and/or by radiation utilizing a light source, preferably UV light and/or by heating.

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 a conventional EL device. Fig. 2 shows a schematic view of an EL device according to the present invention.

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

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a schematic view of a conventional EL device comprising a substrate A, an EL stack B and a cavity lid C. The application of the cavity lid leads to a cavity D usually filled with Nitrogen. Since Nitrogen gas has a low thermal conductivity of about 0.024 W/(m * K), this hampers the heat dissipation. Consequently, the EL de- vice may be damaged, function incorrectly and/or be subject to an increased wear.

Fig. 2 is a schematic view of an EL device 1 according to the present invention. The EL device comprises a flat glass substrate 2 onto which a transparent ITO anode (not shown) 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 has been deposited, followed by the cathode (not shown).

A thermally conductive adhesive 4 was applied to the cathode in liquid form, covering and thus sealing the EL stack. Thus, the adhesive acts as an encapsulation for the EL stack, protecting the stack from environmental influences such as moisture and/or oxygen. The heat sink 5 was applied to the liquid adhesive by application of minor pressure. The heat sink comprises aluminum and comprises multiple fins in order to increase the surface area. After the positioning of the heat sink the adhesive was hardened by drying.

Fig. 3 is a schematic view of another OLED device 1 according to the present invention. The EL device comprises a flat glass substrate 2 onto which a TCO anode (not shown) has been deposited by PECVD. On top of the electrode an OLED stack 3 comprising two types of polymer EL molecules has been deposited, followed by the cathode (not shown).

A thermally conductive adhesive 4 was applied to the cathode in semi- liquid form, covering and thus sealing the EL stack, followed by the application of a heat sink 5 comprising a cavity lid 6 combined with a copper block 7 having fin-like struc- tures. The adhesive completely fills the cavity of the cavity lid and thermally couples the cathode to the heat sink, i.e. the cavity lid combined with the copper block.

The heat sink 5 was applied to the liquid adhesive by application of minor pressure. After the positioning of the heat sink the adhesive was hardened by UV radiation. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered 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.

Claims

CLAIMS:

1. An electroluminescent device (1) comprising a substrate (2) and stacked thereon a first electrode, an electroluminescent stack (3), a second electrode, a heat conducting layer (4) and a heat sink (5), wherein the electroluminescent device is sealed by the heat conducting layer that is in contact with said heat sink.

2. The electroluminescent device (1) according to claim 1, wherein the electroluminescent device (1) is an OLED

3. The electroluminescent device (1) according to claim 1, wherein the substrate (2) is selected from the group consisting of a transparent or semi- transparent substrate, a glass substrate, a flexible substrate and/or a polymer substrate.

4. The electroluminescent device (1) according to claim 1, wherein at least one of the electrodes is transparent or semi-transparent.

5. The electroluminescent device (1) according to claim 1, wherein the electroluminescent stack (3) comprises at least one electroluminescent layer com- prising at least one type of electroluminescent molecules.

6. The electroluminescent device (1) according to claim 5, wherein the electroluminescent stack (3) comprises at least two electroluminescent layers comprising different types of electroluminescent molecules and emitting different colors, wherein adjacent electroluminescent layers are separated by a connector layer.

7. The electroluminescent device (1) according to claim 1, wherein the heat conductive layer (4) comprises an adhesive.

8. The electroluminescent device (1) according to claim 7, wherein the adhesive is has a thermal conductivity of > 0.024 W/(m * K), > about 5 W/(m * K), > about 30 W/(m * K), or > about 100 W/(m * K).

9. The electroluminescent device (1) according to claim 7, wherein the adhesive is applied in liquid or semi-liquid form and/or hardens by drying and /or after application of light, UV light, pressure and/or heat.

10. The electroluminescent device (1) according to claim 1, wherein the heat sink (5) has a thermal conductivity that is equal to or larger than the thermal conductivity of the heat conducting layer (4) and/or wherein the heat sink (5) has a surface area that is > 1.5, or > 20 times larger than the area of the contact surface between heat conducting layer (4) and heat sink (5).

11. The electroluminescent device (1) according to claim 1 , wherein the heat sink (5) comprises a cavity lid.

12. A light source, lamp, day-light lamp, monitor or television comprising the electroluminescent device (1) according to claim 1.

13. Method of producing an electroluminescent device (1) according to claims 1-11 comprising the steps of: a) providing a substrate (2); b) depositing onto the substrate (2) in the order of mention: a first electrode, an electroluminescent stack (3), a second electrode; and c) applying a heat conducting layer (4) and a heat sink (5) to the second electrode.

14. The method according to claim 13, wherein the heat conductive layer (4) comprises an adhesive that is applied in liquid or semi-liquid form, further comprising the step of hardening and/or curing the adhesive by means of pressure, light, UV light and/or heat.