Abstract:
An electrode comprises an inorganic composite layer of a mixture of at least one insulating inorganic material and at least one at least partially conducting inorganic material. In an application of such an electrode, an organic electroluminescent device comprises a first and second conductor layers. An organic layer is disposed between the first and second conductor layers. The aforementioned composite layer is disposed between the organic layer and the first conductor layer. Methods of fabricating such an electrode and such a device are also described.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to inorganic electrodes for organic electroluminescent devices. 
     BACKGROUND ART 
     There is continuing interest in developing electroluminescent devices, such as light emitting diodes, based on organic materials. The primary motivation for this continuing interest is that many organic materials have high fluorescence quantum efficiencies in the visible spectrum, an thus have significant potential for colour display applications. U.S. Pat. No. 5,608,287 describes an example of a typical organic light emitting diode (OLED). Such a device usually comprises a first electrode layer disposed on a substrate, at least one organic layer disposed on the first electrode layer, and a second electrode layer disposed on the organic layer. In operation, a voltage is applied across the organic layer via the electrodes. One of the electrodes (the cathode) injects electrons into the organic layer. The other electrode (the anode) injects holes into the organic layer. Radiative recombination of the oppositely charged carriers produces photon emissions from the device. 
     The anode is preferably fabricated from a material with a relatively high work function in the interests of providing effective hole injection. Because light has to be transferred out of the device efficiently, it is desirable for at least one of the electrodes to be transparent. In most conventional applications, light is transferred through a transparent anode. Such anodes typically consist of Indium Tin Oxide (ITO) having a work function in the range of 4.2 to 4.8 eV, Zinc Oxide, or Aluminium doped Zinc Oxide. To optimise device performance, the cathode is preferably formed from a material having a relatively low work function preferably aligned to the electron affinity of the organic layer. U.S. Pat. No. 5,677,572 and U.S. Pat. No. 5,776,623 describe multi-layer electrode structures in which different functional layers are stacked to provide a collective effect. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is now provided an electrode comprising an inorganic composite layer of a mixture of at least one insulating inorganic material and at least one at least partially conducting inorganic material. 
     This differs from conventional multi-layer electrode structures in which such materials are segregated into discrete layers. Instead, in embodiments of the present invention, at least one insulating inorganic material is mixed with at least one at least partially conducting inorganic material in a modifiable combination. The or each insulating material has energy levels for facilitating efficient injection of charge carriers. In an anode configuration, such charge carriers comprise holes. However, in a cathode configuration, such charge carriers comprise electrons. The at least partially conducting material effectively renders the composite layer at least partially conducting. The composite layer may be regarded as composed of at least one insulating or semiconducting inorganic material and at least one semiconducting or metallic inorganic material. 
     The insulating material and at the at least partially conducting material may comprise oxide, fluoride, chloride, bromide, iodides, or sulphide, hydroxide, sulphite, sulphate, or carbonate, for example. In an anode configuration, the composite layer preferably has an average work function greater than 4 eV. In a cathode configuration, the composite layer preferably has an average work function less than 4 eV. The composite layer preferably has a resistivity lower than 1000 Gigaohm-meters. Preferably, the grain length of the materials of the composite layer is greater than 1 nanometer. Preferred examples of electrodes embodying the present invention also comprise a conductive layer. The conductive layer may comprise a metal such as Nickel, Tungsten or Cobalt or the like. Equally, alloys of such metals could be employed. Similarly, semimetals such as graphite, intrinsic or doped semiconductor materials or conductive organic materials could be used. 
     The present invention extends to a device comprising a substrate and an electrode as herein before described. 
     For example, in a particularly preferred embodiment of the present invention, there is provided an organic electroluminescent device comprising: first and second conductor layers; an organic layer disposed between the first and second conductor layers; and a composite layer disposed between the organic layer and the first conductor layer, the composite layer comprising a mixture of at least one insulating inorganic material and at least one at least partially conducting inorganic material. The device may comprise a substrate. The first conductor layer may be disposed between the substrate and the organic layer. Alternatively, the second conductor layer may be disposed between the substrate and the organic layer. In another preferred embodiment of the present invention, there is provided an organic electroluminescent device comprising: a first and second conductor layers; an organic layer disposed between the first and second conductor layers; a first composite layer disposed between the organic layer and the first conductor layer, and a second composite layer disposed between the organic layer and the second conductor layer, the first and second composite layers each comprising a mixture of at least one insulating inorganic material and at least one at least partially conducting inorganic material. A performance enhancing layer may be disposed between the first conductor layer and the first organic layer. The performance enhancing layer may be light reflective. Alternatively, the performance enhancing layer may be light absorbent. The first conductor layer may form part of an anode. Alternatively, the first conductor layer may forms part of a cathode. In the interests of optimising performance of the device, the or each composite layer preferably comprises a region adjacent the organic layer in which region the at least partially conducting material is depleted. 
     Viewing the present invention from another aspect, there is now provided a method of fabricating an electrode comprising the step of forming an inorganic composite layer of a mixture of an insulating inorganic material and an at least partially conducting inorganic material. The forming step may comprise forming the composite layer on a conductive layer. The forming step may also comprise exposing the conductive layer to a precursor. The precursor may comprise Hydrogen or water, for example. The forming step may comprise exposing the conductive layer to a surface treatment, such as oxygen plasma treatment, Ultra Violet (UV)-Ozone treatment, ion bombardment, and ion implantation. The method may also comprise the step of adjusting the ratio of the insulating material to the at least partially conducting material in the mixture. The adjusting step may comprise a quantitative measurement of precursor amount. The adjusting step may comprise exposing the composite layer to a surface treatment such as oxygen plasma treatment UV-ozone treatment, ion bombardment, ion implantation, irradiating the composite layer with X rays, and, etching the composite layer with an etchant for depleting one of the materials in the mixture. The present invention also extends to a method for fabricating an organic electroluminescent device comprising fabricating an electrode as herein before described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of an example of an OLED embodying the present invention; 
         FIG. 2  is a cross-sectional view of part of another example of an OLED embodying the present invention; 
         FIG. 3  is a cross-sectional view of a modification of the OLED illustrated in  FIG. 2 . 
         FIG. 4  is a cross sectional view of another example of an OLED embodying the present invention; 
         FIG. 5  is a cross sectional view of yet another example of an OLED embodying the present invention; 
         FIG. 6  is a cross sectional view of a further example of an OLED embodying the present invention; 
         FIG. 7  is a cross sectional view of another example of an OLED embodying the present invention; 
         FIG. 8  is a cross sectional view of yet another example of an OLED embodying the present invention; and, 
         FIG. 9  is a cross sectional view of another example of an OLED embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1 , an OLED embodying the present invention comprises an anode  40  disposed on a substrate  10 . The substrate  10  comprises a single crystal semiconductor. In other embodiments of the present invention, the substrate may comprise glass, plastic foils, ceramic, and the like. Examples of suitable semiconductor materials for implementing the substrate include Silicon, Germanium, and Gallium Arsenide. An organic layer  50  is disposed on the anode  40 . In other embodiments of the present invention, there may be multiple organic layers disposed on the anode  40 . A cathode  60  is disposed on the organic layer  50 . The cathode  60  may be transparent and may comprise a conductive layer  65 . The conductive layer  65  may be formed from low work function metals, alloys of such metals, or from Indium Tin Oxide, Zinc Oxide, or the like in combination with organic and inorganic injection layers. In operation, a voltage is applied across the organic layer  50  via the anode  40  and the cathode  60 . The cathode  60  injects electrons into the organic layer. Similarly, holes are injected into the organic layer  50  by the anode  40 . 
     The anode  40  comprises a conductive layer  20  disposed on the substrate  10  and a composite inorganic layer  30  disposed between the conductive layer  20  and the organic layer  50 . Examples of materials suitable for implementing the conductive layer  20  include metals, semimetals such as graphite, conductive polymers, chemically doped organic layers and doped semiconductors. Particularly suitable materials for implementing the conductive layer  20  include, without limitation, Nickel, Tungsten, Silicon, Molybdenum, Cobalt, Silver Aluminium, and Graphite. 
     Referring to  FIG. 2 , the composite layer  30  comprises at least one first, inorganic insulating component  80  and one or more second, at least semiconducting, inorganic components  70 . The first component  80  provides a sufficiently high work function to permit effective injection of holes into the organic layer  50 . The second component  70  makes the composite layer  30  at least partially conducting. 
     An example of an OLED embodying the present invention was constructed in the following manner. First, a conductive layer  20  of Nickel was deposited on a silicon substrate  10 . The Nickel layer  20  was exposed to a precursor and then exposed to an oxygen plasma. Examples of suitable precursors include Hydrogen and water. The exposure of the Nickel to the oxygen plasma created a relatively thick layer  30  composed of a mix of Nickel Hydroxide and Nickel Oxide on the surface of the Nickel. In this example, the Nickel Oxide provides the first component  80  of the composite layer  30  and the Nickel Hydroxide provides the second component  70  of the composite layer  30 . 
     Nickel is not an efficient hole-injecting material. However, Nickel Oxide is suitable for hole injection into organic materials. This is because Nickel Oxide is an insulating oxide having a relatively high ionisation potential. Nickel Hydroxide is a semi-insulating material having a resistivity of approximately 10 Gigaohm centimeters. Tests indicate that Nickel Hydroxide is not suitable for hole injection into organic materials. This may be due to the relatively low ionisation potential of Nickel Hydroxide. Deposition of a relatively thick layer of insulating material such as Nickel Oxide on the conductive layer  20  reduces device efficiency, because the insulating material presents a barrier to charge flow. Tests indicate that deposition of relatively thin (about 1 nm) insulating layers of Lithium Fluoride (LiF) or Vanadium oxide films on the conductive layer  30  provide more efficient devices. However, it is difficult to make such layers at the desired thickness in a high volume manufacturing environment. The conductivity of the composite layer is increased by chemical interaction between the Nickel Oxide and Nickel Hydroxide. This permits efficient hole injection without introducing an insulating barrier. Referring to  FIG. 2 , the path of hole current when an electric field is applied across the OLED is illustrated by the arrow. The holes follow the path of least resistance to current flow. Initially, this path passes through conducting paths provided by the Nickel Hydroxide and/or conducting paths provided by the interaction between the Nickel Hydroxide and the Nickel Oxide. However, at the interface between the organic layer and the composite layer  30 , the path passes instead through the Nickel Oxide. This is because Nickel Oxide has a relatively high ionisation potential and therefore presents no significant barrier to holes entering the organic layer. Charge flows from the composite layer  30  into the organic layer  50  via those domains offering the lowest barrier. Preferred performance in terms of hole injection is obtained if the domain sizes of the first and second components of the composite layer  30  are relatively small (in the range of nanometers) and there is sufficient material having a high ionisation potential at the interface between the composite layer  30  and the organic layer  50 . 
     Referring now to  FIG. 3 , in a modification of the example of the present invention herein before described, a region  90  of the composite layer  30  adjacent to the organic layer  50  is made rich in Nickel Oxide by Hydroxide depletion. Such Hydroxide depletion may be brought about by exposing the surface of the composite layer  30  to surface treatments such as oxygen plasma, Ultraviolet (UV) Ozone and the like. 
     The current carrying characteristics of the composite layer  30  allow the composite layer  30  to cover a range of thickness from the nanometer scale to relatively thick layers (e.g.: in the region  90  of several micro meters) without degrading device performance. 
     As mentioned earlier, in the examples of the present invention herein before described, the Nickel Oxide provides the first component  80  of the composite layer  30  and the Nickel Hydroxide provides the second component  70  of the composite layer  30 . However, in other embodiments of the present invention, the first component  80  and the second component ( 70 ) may be provided by other oxides, or alternatively by fluorides, chlorides, iodides, sulphides or hydroxides, or alternatively by sulphites, sulphates, and carbonates. Other suitable compositions will be apparent to those skilled in the art. The present invention also extends to arrangement in which the second component  70  comprises vacancies in the first component  80 . 
     In the example of the present invention herein before described with reference to  FIG. 3 , Hydroxide depletion was achieved by exposing the surface of the composite layer  30  to an oxygen plasma. However, in other embodiments of the present invention, an analogous effect may be achieved by eliminating the component having the lower ionisation potential from the region  90  through the use of a selective etchant. For example, in a device comprising a composite layer  30  of Nickel Oxide and Nickel Carbonate or Nickel Phosphate, such an etchant may be employed to deplete the Nickel Carbonate or Nickel Phosphate from the region  90  adjacent to the organic layer  50 . Additionally, fluoridation, iodination, or oxygenation of a more insulating material of higher ionisation potential can be optionally performed to enhance the conductivity of the material. 
     In some embodiments of the present invention, the composite layer  30  may be deposited as a film on the conductive layer  20 . However, in other embodiments of the present invention ,the composite layer  30  may be fabricated by modification of the conductive layer  20  via techniques such as exposure to oxygen plasma, X-ray radiation, ozone exposure, dry chemical etching, and the like. The ratio of the first component  80  to the second component  70  in the composite layer  30  can be set by techniques such as oxygen plasma, UV-ozone, X ray irradiation, ion implantation, wet chemical processing and the like. 
     Embodiments of the present invention have been described in which the performance of an electrode is enhanced by forming the electrode  40  from a conductive layer  20  and a composite layer  30 . The composite layer  30  comprising an insulating material having a relatively high energy level intermixed or doped with an at least partially conducting inorganic material. In some embodiment of the present invention, the composite layer  30  may include silicon dioxide, silicon nitrite or the like doped with an at least partially conducting material. 
     In the preferred embodiments of the present invention herein before described, electrodes according to the present invention are employed as anodes for a organic light emitting diode. When employed as an anode, electrodes according to the present invention achieved especially desirable results when the composite layer  30  has a work function greater than 4 eV, a resistivity lower than 1000 gigaohm-meters, and a grain length greater than 1 nanometer. However, in other embodiment of the present invention, electrodes according to the present invention may also be employed as cathodes. For example, referring now to  FIG. 4 , in another embodiment of the present invention, the cathode  60  is provided by the composite layer  30  and the conductive layer  65  and the anode  40  is provided by the conductive layer  20 . The cathode  60  is disposed between the organic layer  50  and the substrate  10  and the anode  20  is disposed on the side of the organic layer  50  remote from the substrate. It will be appreciated that many other arrangements embodying the present invention are possible. For example, with reference to  FIG. 5 , in another embodiment of the present invention, the cathode  60  is provided by the conductive layer  65  and the anode  40  is provided by the conductive layer  20  and the composite layer  30 . The cathode  60  is disposed between the substrate  10  and the organic layer  50  and the anode  40  is disposed on the side of the organic layer  50  remote from the substrate  10 . Referring now to  FIG. 6 , in yet another arrangement embodying the present invention, the cathode  60  comprises the composite layer  30  and the conductive layer  65  and the anode  40  comprises the conductive layer  20 . The anode  40  is disposed between the organic layer and the substrate  10 ; and, the organic layer  50  is disposed between the cathode  60  and the anode  20 . 
     Introduction of the composite layer allows the conductive layers to be optimised for device performance. For example, in the  FIG. 6  arrangement, in the interests of enhancing performance and/or contrast, the anode  40  may be made highly reflective through use of high reflectivity materials such as Silver and Aluminium for the anode or highly absorbent through the use of high absorption material such as graphite for the anode. 
     Referring now to  FIG. 7 , in a modification to the  FIG. 6  arrangement, a performance enhancing layer  25  is disposed between the organic layer  50  and the anode  40 . The performance enhancing layer may be highly reflective or highly absorbent as herein before described. Referring now to  FIG. 8 , in yet another arrangement embodying the present invention, the cathode  60  comprises a composite layer  31  and the anode  40  comprises a composite layer  32 . In the cathode  60 , the composite layer  31  is disposed between the conductor  65  and the organic layer  50 . Similarly, in the anode  40 , the composite layer  32  is disposed between the organic layer  50  and the conductor  20 . With reference to  FIG. 9 , in a modification of the  FIG. 8  arrangement, the positions of the anode  40  and cathode  60  relatively to the substrate  10  are interchanged. 
     Embodiments of the present invention have been herein before described with reference to an OLED. However, it will be appreciated that the present invention is not limited to OLED applications and organic layers sandwiched between two electrodes, but instead extends to other devices both within and outside the display field.