WO2006027736A1

WO2006027736A1 - Electroluminescence arrangement

Info

Publication number: WO2006027736A1
Application number: PCT/IB2005/052889
Authority: WO
Inventors: Herbert Friedrich Boerner; Wolfgang Busselt; Edward Willem Albert Young
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2004-09-10
Filing date: 2005-09-05
Publication date: 2006-03-16
Also published as: TW200623476A

Abstract

An electroluminescence arrangement having a substrate (1), having at least one layered structure having an organic luminescence layer (2) between an anode (3) and a cathode (4), for the emission of light (10), having a dielectric liquid (15) provided for complete wetting of the layered structures to prevent electrical flashovers or to reduce leakage currents between the anode (3) and the cathode (4) due to layer defects caused for example by particles (13). The dielectric liquid (15) is largely chemically inert in this case to the layers situated between the anode (3) and the cathode (4), and has an oxygen concentration of more than 2 ppm. There is also an encapsulating device arranged around the layered structures to create an enclosed volume of space, the enclosed volume of space being intended for filling with the dielectric liquid (15).

Description

Electroluminescence arrangement

The invention relates to an encapsulated electroluminescence arrangement having an organic luminescence layer and having a dielectric liquid for the electrical passivation of the arrangement.

Organic electroluminescence arrangements (organic EL arrangements) comprise a layered structure (an EL structure) having a luminescing organic layer (the OLED layer), a layer of p-type conductivity, an anode and a cathode, all mounted on a substrate. The typical layer thicknesses are of the order of 100 nm. The typical voltages applied to the EL structure are between 3 V and 10V. Between the OLED layer and the cathode there is, in addition, an electron injection layer made of a material having a low work function, such as barium for example. Over time, there are two possible effects that have an adverse effect on the emission properties of the organic EL arrangement, these being on the one hand the growth of non-emitting dark spots (temporal degradation of the EL arrangement) and on the other hand the sudden failure of the entire arrangement due to leakage currents or short- circuits between the cathode and anode. In the prior art, the degradation of an organic EL arrangement due to the growth of dark spots could be attributed to a reaction of the layered structure with water/moisture, a reaction that increases as temperature rises. The EL structures are therefore provided with physical encapsulation and the intervening space is filled with dry gases that are totally inert chemically to the entire layered structure and that, at the same time, dissipate heat from the EL structure.

As described in document EP 0781075, the growth of dark spots can be further reduced with a dry, chemically inert dielectric liquid that is, in addition, free of oxygen (oxygen concentration less than 1 ppm). A major advantage of organic EL arrangements is however the possibility of being able to produce thin light-sources of large area. It is precisely in the case of OLED layers of large area of a few square centimeters or more that the presence of particles, of dust for example, cannot be prevented during the production process. When the layers are being produced, particles situated on the substrate cause defects in the form of holes, the nature of whose edges is undefined. Within holes of this kind, only a part of the layered structure is present or no layered structure at all. These defects give rise to leakage currents and short-circuits between the cathode and anode for which no tolerances can be set and with EL arrangements of large area they are by far the leading cause of EL arrangements that fail as a whole. Generally speaking, the short-circuits only occur in the course of operation of the OLEDs if, due to the diminishing light yield, the operating voltage has to be increased to enable the same amount of light to be generated.

It is therefore an object of this invention to provide an electrical passivation in organic EL arrangements of large area, which electrical passivation results in a clear reduction in the failure rate due to leakage current and short-circuits without the use of costly clean-room technology. This object is achieved by an electroluminescence arrangement having a substrate, having at least one layered structure having an organic luminescence layer between an anode and a cathode, for the emission of light, having a dielectric liquid provided for the complete wetting of the layered structures in order to prevent short-circuits and to reduce leakage currents, due to layer defects, between the anode and the cathode, the dielectric liquid being largely chemically inert to the organic luminescence layer and having an oxygen concentration of more than 2 ppm, and having an encapsulating device to create an enclosed volume of space around the layered structures that is intended for filling with the dielectric liquid. An advantageous minimum quantity of dissolved oxygen in the dielectric liquid causes oxidation of thin conductive bridges and rough surfaces at the edges of defects and thus, by reducing the electrical conductivity at the edges of defects, results in the prevention, or at least in a clear reduction, of leakage currents. By virtue of the electrical passivation ^• obtained by wetting the layered structure with a dielectric liquid, additional coating processes for passivating the layered structure can be dispensed with.

Advantageous dielectric liquids that are chemically inert to the organic luminescence layer are liquids made from vast fluorinated oils, perfluorinated oils, and/or fluorinated dielectric liquids. Silicone oil, mineral oil, paraffin oil, mineral diffusion-pump oil and castor oil on the other hand have been found not to be chemically inert to the organic luminescence layer. Something that is advantageous for the prevention of flashovers between the anode and cathode in the region of layer defects is wetting of the layered structure, at least in the region of the layer defects, with a dielectric liquid that has a dielectric constant that, at 4.5 > ε > 1.5, is very similar to the dielectric constant of the organic luminescence layer of approximately 3 and is thus appreciably higher that the dielectric constant of air (ε = 1).

For complete wetting of the EL structure precisely in the region of the layer defects, it is advantageous if the dielectric liquid has a surface tension of less than 25*10^"3 N/m at 25°C.

For complete wetting of the layered structure that exists during the entire time in operation, under operating conditions in which the electroluminescence arrangement heats up, it is advantageous if the dielectric liquid has a boiling point of more than 125°C.

Because of the complete wetting in the region of the layer defects, the dielectric liquid is in contact with all the layers whose durability and mutual adhesion show a sensitive reaction to water. It is therefore advantageous for the dielectric liquid to contain a proportion of water of less than 1 ppm.

In another embodiment, the dielectric liquid contains, in addition, a water- absorbing material to reduce the proportion of water in the dielectric liquid.

In a further embodiment, the encapsulating device contains a getter material for the chemical binding of water or moisture within the getter material, and for the reduction that this involves in the proportion of water or moisture in the volume of space enclosed by the encapsulating device.

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

In the drawings:

Fig. 1 is a side view of an encapsulated electroluminescence arrangement.

Fig. 2 is a side view of a hole defect due to a dust particle.

Fig. 3 is a side view of a hole defect due to a dust particle and of the dielectric liquid according to the invention that wets the layered structure.

Fig. 1 is a side view of an encapsulated electroluminescence arrangement. The layered structure of the electroluminescence arrangement includes a thin organic luminescence layer 2 (such as, for example, doped tris-(8-hydroxyquinolinato) aluminum) of a typical thickness in the 100 nm range, which layer is arranged between two electrodes (such as, for example, an anode 3 and a cathode 4 as shown in Fig. 1) at least one of which is transparent. What is usually used as a transparent conductive material is indium tin oxide (ITO). What is used as a non-transparent electrode is conductive material, usually a layer of metal, of a thickness of the order of 100 nm. There are however also arrangements in which both the electrodes are transparent. The layered structure is mounted on a substrate 1. A distinction is made in this case between what are termed top and bottom emitters. Bottom emitters emit the light 10 from the luminescence through the substrate 1, as shown in Fig. 1. In this case the anode 3 comprises an ITO layer and the cathode 4 a layer of aluminum. The layered structure may also be applied to the substrate in the reverse order. A top emitter of this kind then emits the light not through the substrate in the way shown in Fig. 1 but in the opposite direction. Between the organic luminescence layer 2 and the anode 4, there is generally arranged a layer of p-type conductivity, typically alpha-NPD (N,N'-Di(napthalen- 2-yl)-N,N'-diphenyl-benzidine), having a thickness of approximately 50 nm. Between the cathode 4 and the organic luminescence layer 2 is usually situated a thin electron injection layer 9 made of a material having a low work function, such as, for example, lithium, cesium or barium, which layer is important for a good injection of electrons into the luminescence layer. This electron injection layer shows a very sensitive reaction to moisture. Therefore, to provide protection against ambient moisture, electroluminescence arrangements are provided with an encapsulating device. This encapsulating device comprises a cover 5 that, by means of adhesive-bonded joints 7, encloses the layered structure having the organic luminescence layer 2 and is firmly connected thereto. An opening 12 that can be closed off is used to pump out the volume of space 6 situated between the layered structure and the encapsulation and/or for its possible refilling with dry gases or dry liquids. In addition, a getter material 11 may be arranged inside the encapsulation to reduce the proportion of moisture/water within the volume of space 6. In so-called top emitters, the encapsulation, or at least the cover 5, has to be transparent. The forms and positions shown here for the opening 12 able to be closed off, the getter material 11 and the cover 5 merely represent possible embodiments. In other embodiments, the positions and forms may also be of some other kind. To enable the layered structure situated inside the encapsulation to be driven electrically, conductive tracks 8 and 3 are run out of the encapsulation. In addition to the layered structure shown in Fig. 1, additional layers for improving the coupling-out of light may be added between the anode and the organic luminescence layer, such as micro-cavity layers, layers for changing or improving colors, scattering layers and/or hole injection layers. These possible additional layers do not change anything in the way in which, as described, the basic object is achieved in accordance with the invention.

The layered structure of an EL arrangement comprises individual thin layers, a large number of which are produced by dry, directed coating processes such as, for example, vacuum vapor deposition and/or sputtering. In such directed coating processes, the presence of particles 13, such as dust particles for example, leads to the substrate that is being coated or part of the layered structure being shaded off, and hence to layer defects of the kind shown in Fig. 2. The dimensions of such particles are usually appreciably larger that the thicknesses of the individual layers. Due to the shading-off during the coating process, none, or only some, of the layers that will subsequently be present outside the layer defect are present inside it. The size and shape of the layer defects depend on the position and geometry of the particle and on the point in time from which the particle was present on the growing layered structured during the production of the thin layers. If, due to a particle 13, the high-resistance organic luminescence layer 2 is no longer present in the region of a layer defect, flashovers 14 may take place between the two electrodes 3 and 4. With a typical operating voltage of 3 to 10V between the electrodes and a typical electrode spacing of 100 nm, a field of 30- 100 kV/mm is applied to the EL structure. Locally, the edges of a layer defect even result in substantially higher field strengths due to the very small radius of curvature of the edges. If the volume of space 6 created by the encapsulation is filled with air, the difference in the dielectric constants of the organic luminescence layer (ε ~ 3) and air (ε = 1) results in a further increase in field strength in the critical region formed by the edges of the layer defect. In addition, the dielectric strength of air is substantially lower than that of the organic luminescence layer, which increases the risk of an electrical flashover to a further degree. What is more, a flashover 14 between the cathode 4 and anode 3 leads not only to an uncontrolled flow of current but also to local heating of the layered structured, which may result in a localized release of carbon in the organic luminescence layer 2. This carbon settles on the edges of a layer defect and increases the electrical conductivity at the edge of the layer defect, which is even more conducive to the occurrence of further flashovers or leakage currents. This self-accentuating process results in destruction of the EL arrangement. The occurrence of this process does not depend on the number of organic layers between anode and cathode.

The probability of layer defects increases with the area of the organic EL arrangement. However, one advantage of organic luminescence layers is precisely the possibility of their being of a form that is large in area. However, large-area organic EL arrangements can only be produced to have a low failure rate when flashovers can be avoided between the electrodes. The electrical passivation according to the invention of such layer defects within the EL arrangement (see Fig. 3) represents an effective and inexpensive solution. The complete EL structure is wetted with a dielectric liquid 15 that is largely chemically inert to the organic luminescence layer 2 and that has an oxygen concentration of more than 2 ppm. Because, due to complete wetting of the EL structure in the region of the layer defects, the dielectric liquid is in contact with the organic luminescence layer 2, the liquids that may be used are only ones that do not have an adverse effect on the luminescent properties of the organic layer 2 or on the strength of the layered structure. There is a guarantee that this will be the case when dry dielectric liquids made from perfluorinated oils, and/or fluorinated dielectric liquids, are used. To protect the electron injection layer 9, the water content of the dielectric liquid should be appreciably less than the proportion that is typically soluble in such liquids of 7-13 ppm. The water content of a dielectric fluid can be reduced significantly by temperature enhanced degassing in vacuum. What is particularly advantageous in this case is a proportion of water of less than 1 ppm, which can be even further reduced by adding water-absorbing additives to the dielectric liquid. In another embodiment, a water-absorbing and/or moisture-absorbing getter material 11 may be arranged in the enclosed volume of space 6 in the encapsulating device, which reduces the proportion of water/moisture in the volume of space 6 to an additional degree.

The use of the dielectric liquid 15 having an oxygen content of more than 2 ppm where the solubility-determined content between 30 ppm and 100 ppm makes possible a desired oxidation of the surfaces of the cathode 4, particularly at the rough edges of a layer defect. This is immaterial to the operation of the El structure in the undamaged regions. In the region of layer defects, unintended conductive bridges between the cathode 4 and anode 3 are oxidized and hence their conductivity is at least greatly reduced. Rough edges are oxidized too and in this way the risk of flashovers is reduced. Possible oxidation of the organic luminescence layer 2 does not affect the operation of the EL arrangement in this case. Life tests (duration: 5 months) using dielectric liquids that were saturated with oxygen showed a clear reduction in leakage currents by a factor of more than 10 without the properties of the luminescence layer 2 being adversely affected thereby. The saturation of the dielectric fluid with oxygen was achieved with flowing dry oxygen through the fluid. This technique can also be used to adjust any oxygen level below the saturation level. To reduce the risk of flashovers between the cathode 4 and anode 3, the dielectric liquid should have an appreciably higher dielectric constant than air. Dielectric liquids where 4.5 > ε > 1.5 are therefore advantageous. The liquids should also have a dielectric strength that is appreciably higher than that of air (~ 4-5 kV/rnm). To enable successful electrical passivation to be performed by the dielectric liquid in the regions of the layer defects, the dielectric liquid has to completely displace the remaining gases in the regions of the layer defects, between the layer edges and the particles that may possibly still be clinging on there. To be able to penetrate into cavities that may possibly be small, the dielectric liquid needs to have a suitably low surface tension. What are particularly advantageous in this case are dielectric liquids having a surface tension of less than 25*10^"3 N/m. For long-lasting wetting of the layered structure, it is also necessary for the dielectric liquid to have a boiling point above the local temperatures that occur in the course of the operation of the EL arrangement. This condition is met by a boiling point above 125°C. In addition to its electrically passivating effect, the dielectric liquid may also be used to dissipate the heat from the operation of the EL arrangement by means of thermal contact with the encapsulating device. By means of the electrical passivation according to the invention of the EL structure, it has been possible for the failure rate due to leakage currents and flashovers between the cathode and anode to be brought down by a factor of 20 in comparison with encapsulated EL arrangements not having dielectric liquids in the volume of space 6.

Dielectric liquids such as, for example, the Fomblin Y series from Solvay Solexis, Tyreno Fluid 12/25 V from Klϋber or FC-43 from ³M have boiling points above 180⁰C, dielectric constants ε of between 1.9 and 2.1, surface tensions of 16-22 mN/m, appreciably higher dielectric strengths than air, a maximum relative solubility for water of 7- 13 ppm, are largely chemically inert to and compatible with organic materials, and thus meet the conditions given in the description.

In high-voltage engineering, gases such as SF6 are also known to be electrical insulators. It is not advantageous for insulating gases to be used in organic EL arrangements due to the small distances between the anode and cathode. Insulating materials such as dielectric liquids are considerably better suited to this application because of their higher density.

The filling of the volume of space 6 between the EL structure and the encapsulation 5 and 7 can be performed by means of vacuum impregnation. In this, the volume of space 6 is evacuated, by which means the gases situated in the particles are also largely removed. Then, the dielectric liquid is introduced into the volume of space 6, while excluding the outside air, after which the opening 12 is closed off with an airtight seal. The volume of space 6 may be wholly or partly filled with the dielectric liquid in this case. In case of partly filled volume of space 6 an encapsulation 5 can be used without any opening 12. The encapsulation is placed with adhesive-bonded joints 7 up ride in order to be filled with dielectric fluid. After filling, the EL arrangement will be glued to the adhesive-bonded joints 7. In the case of top emitters, the dielectric liquid and also the cover 5 have to be transparent. The complete wetting of the EL structure with dielectric fluid will be maintained also with partly filled volume of space 6 if the EL arrangement is positioned vertically. Another approach to the achieving of the object on which this invention is based, namely reducing the number of layer defects by means of very complicated and costly clean-room technology, would mean a sharp rise in the costs of production and, precisely with EL arrangements of large area, would not be capable of totally preventing any layer defects from occurring. The embodiments that have been elucidated by reference to the drawings and in the description are only examples of an EL arrangement and are not to be construed as limiting the invention to these examples.

Claims

1. An electroluminescence arrangement having

- a substrate (1),

- at least one layered structure having an organic luminescence layer (2) between an anode (3) and a cathode (4), for the emission of light (10), - a dielectric liquid (15) provided for complete wetting of the layered structures to prevent electrical flashovers (14) and to reduce leakage currents (14), due to layer defects, between the anode (3) and the cathode (4), the dielectric liquid (15) being largely chemically inert to the layers situated between the anode (3) and the cathode (4), and having an oxygen concentration of more than 2 ppm, and - an encapsulating device (5, 7, 12) to create an enclosed volume of space (6) around the layered structures that is intended for filling with the dielectric liquid (15).

2. An electroluminescence arrangement as claimed in claim 1 , characterized in that the dielectric liquid (15) contains at least vast fluorinated oils, perfluorinated oils and/or fluorinated liquids.

3. An electroluminescence arrangement as claimed in claim 2, characterized in that the dielectric liquid (15) has a dielectric constant of 1.5 < ε < 4.5 to prevent flashovers between the anode (3) and cathode (4).

4. An electroluminescence arrangement as claimed in claim 2, characterized in that the dielectric liquid (15) has a surface tension of less than 25*10^"3 N/m at 25°C.

5. An electroluminescence arrangement as claimed in claim 2, characterized in that the dielectric liquid (15) has a boiling point above 125⁰C.

6. An electroluminescence arrangement as claimed in claim 1 , characterized in that the dielectric liquid (15) has a proportion of water < 1 ppm.

7. An electroluminescence arrangement as claimed in claim 6, characterized in that the dielectric liquid (15) contains an additive composed of a water-absorbing material.

8. An electroluminescence arrangement as claimed in claim 6, characterized in that the encapsulating device (5, 7, 12) is provided with a getter material (11) for absorbing water/moisture.