Abstract:
The invention relates to OLEDs ( 1 ) having a substrate ( 2 ), a first electrode layer ( 3 ), a layer of organic electroluminescence material ( 4 ), a second electrode layer ( 5 ), a cover layer ( 6 ), moisture-absorbing means ( 9 ) and a separating foil ( 10 ) of resilient material. According to the invention, said foil ( 10 ) is positioned between the second electrode layer ( 5 ) and the moisture-absorbing means ( 9 ). This feature results in a longer mean life-time of the OLED. Advantageously spacer structures ( 12 ) (preferably formed as dots) are applied on the separating foil ( 10 ) between the foil ( 10 ) and the cover ( 6 ). This prevents discoloring effects around the rim of the OLED material.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application claims the benefit or priority of and describes relationships between the following applications: wherein this application is a continuation of U.S. patent application Ser. No. 13/805,172, filed Dec. 18, 2012, which is the National Stage of International Application No. PCT/IB52591, filed Jun. 15, 2011, which claims the priority of foreign applications EP10179138.2 filed Sep. 24, 2010 and EP 10166885.3 filed Jun. 22, 2010, all of which are incorporated herein in whole by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of organic electroluminescence devices, comprising a substrate, a first electrode layer, a layer of organic electroluminescence material, a second electrode layer, moisture-absorbing means, a separating foil and a cover layer. Such devices are commenly referred to as OLEDs. 
     BACKGROUND OF THE INVENTION 
     Such a device is known from patent publication US 2008/0265753-A1. This document describes typical OLEDs having a substrate, a first electrode layer being deposited directly on the substrate, an electroluminescence layer (EL) and a second electrode layer being deposited on the EL. Said EL is composed of different sublayers, including an emission layer and a conductive layer. Upon applying a voltage in the range of 2-10 V between both electrode layers, the EL emits light. Depending on the type of the electroluminescence materials used in the EL, different light spectrums (colors) can be generated with OLEDs. At least one of the two electrode layers is transparant for light generated by the EL. The described layer package is contained in a cavity formed by a bowl-like cover layer. Said cover is secured at its rim to the glass substrate by means of a sealing material. 
     The known OLED also comprises moisture-absorbing means positioned at different locations in the cavity, like inside the sealing material, on the second electrode layer and on the inner surface of the cover layer. These moisture-absorbing means function as a getter for chemically or physically removing moisture from the inside of the OLED. Said moisture-absorbing means may comprise water-absorbing materials like metaloxide particles (f.e. CaO) being bound in a matrix of an organic polymer. In OLEDs, the anode/EL/cathode package is known to be extremely sensitive to moisture. Therefore, the presence of moisture-absorbing means has been shown to be essential in enhancing the life-span of OLEDs. If possible, the level of moisture should remain below 1000 ppm. 
     The described OLED further comprises a separating foil, which is positioned between the cover layer and the moisture-absorbing means which is deposited on the second electrode layer. Said separating foil acts as a compliant protection layer which should prevent direct contact between the cover and the EL. Such contact can cause mechanical damage to one the electrodes or shorting. In the described OLED, mechanical stresses due to flexing are absorbed by the separating foil of resilient material. 
     The here-described OLED is of the so-called ‘cavity-lid’ type, in which the EL is contained in a bowl-like cover. In principle, light generated by the EL exits the OLEDs of the cavity-lid type only via the substrate. Therfore, both the substrate and the first electrode layer deposited on it are transparant to the light generated by the EL. Often the substrate is made of glass, the first electrode layer of ITO (Indium Tin Oxide, or InSnO) and the second electrode of a thin layer of Al. The cover can be made of glass or metal. Another type of OLEDs is commonly referred to as the ‘flat-lid’ type. In this type, the mentioned layer package of the EL is sandwiched between the first and second electrode layers. Latter layers are covered on both main surfaces by flat substrates. In this OLED type, both substrates and both electrode layers can be more or less transparant for the light generated by the EL. In OLEDs of the flat-lid type, the substrate and the cover are often made of glass whereas the first electrode is made of ITO and the second electrode can be formed as a thin layer of Al. 
     It has been experienced that the known OLED shows certain drawbacks. It has appeared that, also in the absence of any mechanical pressures on their substrates, the known OLEDs show too many failures. This is especially observed during so-called ‘drop tests’, in which the OLEDs are dropped from a limited height (f.e. 1 meter) on the floor. Under these circumstances, the EL stops emitting light under appropriate voltages. As a consequence, the current mean life-span of the known OLEDs is considered to be still insufficient. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to overcome or at least mitigate these or other drawbacks of the known OLEDs. More particularly, the present invention aims at providing an OLED, in which the mean life-time is enhanced. 
     This and/or other objects are achieved by an organic electroluminescence device, comprising 1) a substrate on which a first electrode layer is provided, 2) a layer of organic electroluminescence material being provided on the first electrode layer, 3) a second electrode layer being provided on the layer of electroluminescence material, 4) a cover layer being positioned on the side of the second electrode layer that faces away from the layer of electroluminescence material, 5) moisture-absorbing means and 6) a separating foil of resilient material being positioned between the second electrode layer and the moisture-absorbing means. 
     The invention is amongst others based on the insight acquired by the inventors, that direct contact between the moisture-absorbing means and the second electrode layer causes chemical damage of said electrode, which on the long term results in destruction of the OLED. Said chemical damage is caused by reactive chemicals which are mainly responsible for capturing moisture in the moisture-absorbing means. If the electrode layer does not fully cover the EL, even reaction between the moisture-absorving material and the EL can occur. This also can result in failure of the OLED. Therefor, the separating foil preferably covers the whole surface of the second electrode. If the second electrode does not fully cover the surface of the EL, it is also preferred that the separating foil covers also the surface parts of the EL which are not covered by the electrode layer. 
     It has also been realized that within the gas-filled volume of the OLED, particles can be present. These particles can have various sources. They can be getter and/or binder particles which are released from the moisture-absorbing means, glass particles released from the glass substrate, debris already present on the unfinished OLED product or dirt originating from equipment for manufacturing the OLED. In principle, these particles are capable to damage the layer stack of the EL and the electrode layers. 
     In the invented OLED, no moisture-absorbing means are present between the separating foil and the second electrode layer. So, no chemical reactions of moisture-absorbing material with the electrode layer material can occur. Such reaction can result in corrosion of the electrode layer, especially if the electrode layer is of Al. In view of this is goes without saying that the separating foil is free of reactive chemicals, such as inorganic metal oxides (CaO, MgO, BaO, etc). The foil also prevents mechanical damage caused by particles of any source available in the OLED. In order to allow the foil to absorp mechanical stresses, for example caused by flexing, the material of the foil should be resilient. Materials having a Young&#39;s modulus (Ym) of 1 GPa or less can be used in these foils with great satisfaction. Foils having a Ym above 1 GPa show hardly any mechanical protection effect when present in OLEDs. Best results are obtained with foil materials having a Ym between 0.2 GPa and 0.8 GPa. 
     In a preferred embodiment of the OLED according to the invention, the separating foil is made of an organic resin material. These materials can easily and at low cost be manufactured with the required properties, like electrical resistance, impermeability to water, etc. A large scale of (modified) natural and synthetic organic resin materials can be used. Regarding synthetic resin materials, both thermoplastic and thermosetting polymers can be used. It is furter stressed that not only homopolymers but also copolymers and blockcopolymers can be applied as material for the separating foil. It is stressed that the foil contains only organic resin material without any inorganic additives. 
     Another preferred embodiment of the OLED according to the present invention is characterized in that the organic resin material is a polyolefin material. It has been found that this class of organic materials show very attractive properties, like the elasticity, electrical resistance and impermeability for moisture, which makes them particularly suitable for use as separating foil in the invented OLED. Typical examples of this class of materials are polypropene and polyisobutene. The best results are however obtained with (modified) polyethene (0.2 GPa&lt;Ym&lt;0.8 GPa), such as pure polyethene or polyvinilydenefluoride (Ym appr. 0.5 GPa). 
     A further interesting embodiment of the invented OLED has the feature that the organic resin material is a Si-containing polymer. Separating foils of this material are very resilient and can therefor absorp rather large mechanical stresses without failure of the OLED. These foils are also extremely inert towards chemically reactive materials. 
     Interesting is also the embodiment of the invented OLED in which the resin material of the separating foil is a foam material. The material of such foil has a porous structure. This has the advantage that moisture can diffuse through these foils. So, undesired moisture present between the electrode layer and the foil can now diffuse through the foil in the direction of the moisture-absorving means, by which it is captured. 
     Another embodiment of the invented OLED has the feature that the thickness of the foil is between 2 and 500 micrometer. Experiments and calculations with different types of foils have shown that the thickness of the foil should not be less than 2 micrometer. In that case, hardly any stress-absorbing effect of the foil is observed anymore. The thickness of the foil should however also not be more than 500 micrometer. In that case, the foil becomes too thick to be incorporated in the usual cavity-lid OLEDs. An optimal compromize between both negative effects is obtained in case that the invented OLED comprises a separating foil having a thickness between 10 and 50 micrometer. 
     Great interest is also shown for an embodiment of the invented OLED which shows the feature that the moisture-absorbing means are provided on the surface of the foil, which faces away from the EL. OLEDs with this feature can be simpler manufactured, as separating foil and moisture-absorbing means can be introduced in the cavity in a single handling. 
     Especially attractive is the embodiment of the OLED according to the present invention wherein the separating foil is provided on the second electrode layer and wherein the separating foil has spacer structures on the surface of the separating foil which faces away from the second electrode layer. In larger area OLEDs, the separating foil acts as a spacer layer, which, together with the layer of moisture-absorbing means, significantly occupies the space between the cover layer and the second electrode layer. In these circumstances, moisture entering the OLED cell via the sealing material cannot distribute over the whole free surface of the moisture-absorbing layer, but will react in first instance only at the outer rims of said layer. While completely saturating the rims area, the moisture will be able to degrade the second electrode layer and the EL, thus causing discolored area at the rims of the OLED lighting area. 
     In the OLED according to the present embodiment, the spacer structures arrange for a free space between the separating foil and the cover layer. Therefore incoming moisture can penetrate in this free space and react with the layer of moisture-absorbing means which is present between the separating foil and the cover layer. So, saturation of the absorbing means by the moisture will take place over the whole layer, not only at rim portions of the layer. As a result, the first visible features of cell degradation will be significantly delayed. This increases the mean life-span of such large area OLEDs. The spacer structures should be on the surface of the separating foil which faces away from the second electrode layer (i.e. in the direction of the cover layer), as the moisture-absorbing means are present between the separating foil and the cover layer. 
     Another embodiment of the OLED according to the invention shows the feature that the spacer structures comprise a series of protruding dots. These dots define a space between the cover layer and the separating foil. Although the dots in practise will be applied in regular pattern on the separating foil, irregular patterns will also function. The shape of the dots is also less important. Seen along the normal to the plane of the separating foil, the dots can have for instance a rectangular or a square form. In view of ease of application, dots having a circular or elliptic form are preferred. 
     Interesting is also the embodiment of the invented OLED in which the spacer structures comprise a cured adhesive material. When using such material for the spacer structures, the separating foil and the cover layer can be firmly fixed to each other in an easy way. Such fixation improves the rigidity of the OLED. Use of such materials has also a clear adventage during the manufacture of the OLEDs in mass production. This material can be easily applied in dots on the separating foil. In a subsequent step, the separating foil and the cover layer can be attached by means of the spacer structures, which are formed by the dots of the adhesive material. Subsequently the adhesive material is cured. 
     The total surface area by which the spacer structures contact the cover layer is less than the total surface of the separating foil on which these structures are provided. Practise has shown that the ratio between these surfaces can be chosen in a wide range without going outside the scope of the present invention. The contact ratio is defined as the ratio of the total contact surface area of the spacer structures divided by the total surface area of the separating foil. Practise has shown that a contact ratio less than 1% does not function, because possible stresses built up between the cover layer and the separating foil cannot be absorbed by the spacer structures anymore. On the other hand, a ratio more than 50% causes that the amount of readily accessible moisture-absorbing means becomes to less. An optimal compromize between both extremes is found when the contact ratio is in the range between 5% and 20%. 
     A further interesting embodiment of the invented OLED has the characteristic that the moisture-absorbing means are provided in a layer on the surface of the cover layer which faces towards the separating foil. The moisture-absorbing means can be provided in a continuous layer on the cover layer, so that in the finished OLED cell the spacer structures are in contact with the cover layer via the layer with moisture-absorbing means. In view of production techniques, the use of continuous layers is preferred over the use of discontinuous layers. 
     An interesting alternative embodiment of the invented OLED has the characteristic that the moisture-absorbing means are provided on the surface of the cover layer where the spacer structures do not contact the cover layer. In this embodiment, a non-continuous layer with moisture-absorbing means is applied on the cover layer. In finished OLED cells according to this embodiment, the attachment of the separation layer to the cover layer is independent of the change in physical properties of the moisture-absorbing means. 
     Most interesting is the embodiment of the invented OLED in which the moisture-absorbing means are incorporated in the spacer structures. In this embodiment the functions of both the spacer structures and the moisture-absorbing means are incorporated in a single structural element. From the view of production, such combination is interesting, as both functions can be applied in the OLED cell in a single handling step. 
    
    
     
       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 the cross-section of a first embodiment of an OLED according to the present invention. 
         FIG. 2  shows the cross-section of a second embodiment of an OLED according to the present invention, 
         FIG. 3  shows the cross-section of a third embodiment of an OLED according to the present invention, 
         FIG. 4  shows the cross-section of a fourth embodiment of an OLED according to the present invention, and 
         FIG. 5  shows a schematic top-view of the second embodiment of an OLED according to the present invention. 
     
    
    
     It is stressed that the drawing is schematic and not to scale. In the different Figures, same elements are denoted with the same reference numbers. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In  FIG. 1 , the cross-section of an OLED  1  (of the ‘cavity-lid’ type) according to the present invention is shown. It comprises a substrate  2  of glass on which a thin, first electrode layer  3  of ITO is sputtered (thickness appr 100 nm). On layer  3 , an electroluminence layer  4  of well-known materials is applied by known techniques. The properties of the materials and the transparancy window of the substrate  2  and the first electrode layer  3  are mutually tuned. EL  4  is composed of several sublayers, including a conduction layer and an emission layer. Additional layers like a hole blocking layer, an electron blocking layer may also be present in EL  4 . On top of EL  4 , a second electrode layer  5  of Al is provided by means of metal evaporation. Layer  5  has a thickness of appr 100 nm and fully covers the surface of EL  4 . 
     The OLED also comprises a cover layer  6  of metal (here Al), which is attached along its rim to substrate  2  with a sealing material  7 . Good results are obtained if a UV-setting glue is used. Electrical leads  8  being connected with the first electrode layer  3  and the second electrode layer  5  are routed through the sealing material  7  to the outside world. Applying a voltage of 2-10 Volts on these leads  8  will activate the EL  4 , thereby giving rise to illumination. Moisture-absorption means  9  formed as layer of CaO particles fixed in a polymer matrix is attached to the inside of the bowl-formed cover layer  6 . During the life of the OLED, moisture may most likely enter the cavity formed by the bowl-like cover layer  6  via the sealing material  7  by means of which it is attached to the substrate  2 . 
     The OLED further comprises a separating foil  10  of resilient material. According to the invention, this foil  10  is positioned between the second electrode layer  5  and the moisture-absorbing layer  9  and lies loose in the cavity. This foil  10  fully covers the surface of the second electrode layer  5 , and preferably is somewhat larger in its surface dimensions. In the present example, a sheet of Tyvak (polethylene spun bonded sheet with thickness of 20 nm, DuPont) was used. The Youngs modulus of this material is appr 0.3 GPa. Good results have also obtained with other foils made of other polyolifin materials. Especially good results were obtained with a foil of a Si-containing polymer material. The Young modulus of these materials was below 1 GPa. 
     In a comparative experiment, OLEDs according to the invention of the above-described design were compared with the same OLEDs in which moisture-absorption means formed als CaO containing layers  9  were present between the separating foil  10  and the second electrode layer  5  (so not according to the invention). In appeared that in the OLEDs of the second type (not according to the invention) showed more failures than the ones of the first type (according to the invention). 
       FIG. 1  also shows a second layer  11  of moisture-absorbing means. This layer  11  is provided on the surface of the foil  10 , which faces away from the EL  4 . In practise, having the moisture-absorption means formed as a layer  11  attached to the separating foil  10  presents a clear advantage during the manufacture of the invented OLED. An additional advantage is obtained in case that the organic resin material of the foil  10  is a foam material. Such material comprises pores through which moisture can be transported. In case that moisture is present between the foil  10  and the EL  4 , it can be rather easily transported through the foil ( 10 ) to the moisure absorbing means, which are attached as layer  11  to the surface of the foil that faces away from the EL  4 . 
     In  FIG. 2 , the cross-section of a second embodiment of an OLED  1  (of the ‘flat-lid’ type) according to the present invention is shown. It comprises a substrate  2  of glass (thickness appr 200 micrometer) on which a thin, first electrode layer  3  of ITO is sputtered (thickness appr 100 nm). On electrode layer  3 , a layer package comprising well-known electroluminence materials (EL)  4  is applied by known techniques. The properties of the materials and the transparancy window of the substrate  2  and the first electrode layer  3  are mutually tuned. EL  4  is composed of several sublayers, including a conduction layer and an emission layer. Additional layers like a hole blocking layer, an electron blocking layer may also be present in EL  4  as well. On top of EL  4 , a second electrode layer  5  of Al is provided by means of metal evaporation. Electrode layer  5  has a thickness of appr 100 nm and it fully covers the surface of EL  4 . 
     The OLED also comprises a thin cover layer  6  of metal (here steel), having a thickness of 100 micrometer. Most generally thicknesses of steel cover layers can range between 20 and 400 micrometers within the scope of the present invention. Said cover layer  6  is fixed along its rim to substrate  2  with a sealing material  7 , like a (UV-curable) glue is used. Electrical leads  8  being connected with the first electrode layer  2  and the second electrode layer  4  are routed through the sealing material  7  to the outside world. Applying a electrical voltage in the range of 2-10 V between these leads  8  will activate the EL  4 , thereby giving rise to illumination. Moisture-absorption means  9  formed as continuous layer comprising CaO particles fixed in a polymer matrix are attached to the inside surface of the flat cover layer  6 . During the life of the OLED, moisture may most likely enter the OLED cell via the sealing material  7  by means of which the cover layer  6  it is attached to the substrate  2 . 
     The OLED further comprises a separating foil  10  of polymer material. In this embodiment, separating foil  10  fully covers second electrode layer  5 . In the present situation a UV-cured polymer layer comprising fluoride side-groups was used. The UV-cured polymer may be prepared from a mixture comprising (meth)acrylate and/or epoxy monomers. The thickness of the separation layer  9  was 200 micrometer. According to an aspect of the invention, the separating foil  10  comprises spacer structures  12  on its surface  13  that faces away from the second electrode layer  5 . These spacer structures  12  are prepared of a cured adhesive material and have a thickness chosen in the range between 50 and 400 micrometers. The presence of spacer structures  12  arrange for space between cover layer  6  and separating foil  10 . As will be shown in more detail in  FIG. 5 , spacer structures  12  are formed as protruding dots. In this second embodiment of the invented OLED, the spacer structures  12  are not fixed directly to cover layer  6 , but are connected via a continuous layer in which moisture-absorbing means  9  are incorporated. 
       FIG. 3  shows a third embodiment of the invented OLED. This embodiment differs from the second embodiment in regard to moisture-absorbing means  9 . In this third embodiment, these means are not applied in a continuous layer on the inside surface of the cover layer  6 , but are only applied on cover layer  5  where the spacer structures  12  do not contact cover layer  6 . This has the advantage that the distance determined by spacer structures  12  can be more precisely achieved, as these spacer structures are in direct contact with cover layer  6 . 
       FIG. 4  shows a fourth embodiment of the invented OLED. In this embodiment, moisture-absorbing means  9  are incorporated into spacer structures  12 . This embodiment shows production technical advantages, as moisture absorbing-means  9  and spacer structures  12  can be applied in the OLED cell in a single handling. 
       FIG. 5  shows a schematic first top-view of the fourth embodiment as depicted in  FIG. 4 . For reasons of clarity, the schematic top-view was made in the absence of cover layer  6 . The OLED  1  comprises a substrate  2  on which a closed line of sealing material  7  is applied in square form around the anode/EL/cathode layer package. Opposite positioned electrical leads  8 , being attached to the first en second electrode layers  3  and  5  (not shown) are indicated as well. On separating foil  10  a series of protruding spacer structures  10 , formed as dots are applied. The contact ratio amount to approximately 8%. Moisture-absorbing means  9  are incorporated in spacer structures  12 . 
     Several experiments have been performed in which the OLEDs according to the present invention been compared with prior art OLEDs, in particular with OLEDs comprising a separating foil without spacer structures. These prior art OLEDs showed in many cases discolored area at the rim of the lighting area after having worked for a certain test period. OLEDs according to the present invention did not show such discolored area after having worked the same test period. This confirms the expectations of the inventors. 
     The OLEDs according to the invention can be manufactured as follows. On a suitable substrate  2 , preferably of glass, respectively a first electrode layer  3 , a package of layers comprising organic electrolyminescence material  4  and a second electrode layer  5  are deposited with well known deposition techniques. In a next step, a separating foil  10  with spacer structures  12  is positioned on and fixed to second electrode layer  5 . This can be done either in a single step or in a series of steps. In the single step method, the separating foil  10  is already provided with spacer structures  12  before it is positioned on and fixed to second electrode layer  5 . In the other method, separating foil  10  is applied on electrode layer  5  and spacer structures  12  are applied on separating foil  10  afterwards. In latter method, separating foil  10  can be produced ‘in situ’. Thus, a curable polymer solution can be applied on electrode layer  5 , which is subsequently cured into a separating foil  10 . In this method, UV-curable solutions comprising (meth)acrylate and/or epoxy monomers can be used with great advantabe in the ‘in situ’ preparation of separating foil  10 . It is preferrred to use fluor-containing components in the solution, which will arrange that separating foil  10  is impermeable for moisture. In a subsequent step, spacer structures  12  can be applied on the cured separating foil  10 . It is preferred to use a curable adhesive for this purpose in order to fix separating foil  10  and cover layer  6  via curable spacer structures  12  in a permanent way. 
     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.