Patent Publication Number: US-2004052931-A1

Title: Method of fabricating an organic electroluminescent device

Description:
RELATED APPLICATION  
     [0001] This Application is a division of application Ser. No.: 09/361137, filed Jul. 27, 1999. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally electroluminescent devices and more specifically relates to an organic electroluminescent device having a thing film optical interference layer to reduce reflectance from ambient light.  
       BACKGROUND OF THE INVENTION  
       [0003] Electroluminescent devices (ELDs) are well known and are generally constructed of several layers of different materials. These layers typically-consist of a transparent front-electrode layer, an electroluminescent layer and a back-electrode layer. When a voltage is applied across the electrodes, the electroluminescent layer becomes active, converting some portion of the electrical energy passing therethrough into light. This light is then emitted out through the front-electrode, which is transparent to the emitted light, where it is visible to a user of the device.  
       [0004] Electroluminescent devices can be particularly useful as computer displays and are generally recognized as high-quality displays for computers and other electronic devices used in demanding applications such as military, avionics and aerospace where features such as high reliability, low weight, and low power consumption are important. Electroluminescent displays are also gaining recognition for their qualities in automotive, personal computer and other consumer industries, as they can offer certain benefits over other displays such as cathode-ray tubes (“CRT”) and liquid crystal displays (“LCD”).  
       [0005] One feature of electroluminescent displays is the ability to add think films between the layers to vary the characteristics of the display. It is known to use thin film layers in electroluminescent displays to improve selected display characteristics, such as signal-to-reflected-ambient light ration (“SRA”) and contrast ratio “CR”). For greater clarity, signal-to-reflected ambient light ratio can be defined as:  
       SRA   =       L   em       R   ×     L   amb                       
 
       [0006] where:  
       [0007] SRA=Signal-to-reflected ambient light ratio  
       [0008] L em =Emitted luminance of the device  
       [0009] R=Reflectance of the device  
       [0010] L amb =The ambient illuminance, or the ambient light incident on the display  
       [0011] and, in a pixilated device, contrast ratio can be defined as:  
       CR   =         L     o                 n       +     R   ×     L   amb             L   off     +     R   ×     L   amb                         
 
       [0012] where:  
       [0013] CR=Contrast Ratio  
       [0014] L on =Emitted luminance of active or “on” pixels  
       [0015] L off =Emitted luminance of inactive or “off” pixels  
       [0016] R=Reflectance of the device  
       [0017] L amb =The ambient illuminance, or the ambient light incident on the display  
       [0018] One particular type of thin-film layer that can be used to improve contrast ratio in electroluminescent devices is a substantially transparent optical interference layer placed between one or more of the layers of the electroluminescent device, as taught in U.S. Pat. No. 5,049,780 to Dobrowolski. As will be apparent to those of skill in the art, improvements to the contrast ratio of an electroluminescent device is generally desirable and particularly important in avionics and military applications where poor contrast and glare can have serious consequences. Using the principle of destructive interference, the optical interference layer can result in the reduction of the amplitude of ambient light by superimposing of two or more, out-of-phase, electromagnetic waves, which can be generated by reflection and/or transmission at the interfaces of thin-film layer(s). By selecting appropriate thicknesses of the layers, optical destructive interference at the electromagnetic wavelengths of interest (typically visible ambient light waves reflected off of the display) can result in an exceptional contrast ratio and/or signal-to-reflected ambient light ratio.  
       [0019] Dobrowolski is generally directed to voltage-driven inorganic electroluminescent devices, where the electroluminescent layer is formed of an inorganic material, and which typically require one or more additional transparent dielectric layers to reduce electrical-breakdown of the inorganic electroluminescent layer. Such inorganic electroluminescent devices are typically voltage-driven, powered with alternating current (“ac”) in order to reduce charge build-up within the device. While Dobrowolski does generally contemplate the use of direct current (“dc”) electroluminescent devices without transparent dielectric layers, such inorganic devices are still voltage-driven, and are generally prone to electrical breakdown of the electroluminescent layer.  
       [0020] With the advent of modern current-driven organic electroluminescent devices which offer certain advantages (such as colour improvements and a reduced barrier to current flow to reduce the necessary drive voltage) compared to voltage-driven inorganic electroluminescent devices, there is now a need to improve the contrast ratio and/or signal to ambient light ratio of these organic devices, and it can be seen that the prior art does not teach a suitable optical interference electroluminescent device to address this need.  
       SUMMARY OF THE INVENTION  
       [0021] It is therefore an object of the present invention to provide a novel optical interference organic electroluminescent device which obviates or mitigates at least one of the disadvantages of the prior art.  
       [0022] In an embodiment of the invention there is provided an optical interference electroluminescent device for displaying an image to a viewer in front of the device, comprising: an anode layer, a cathode layer, at least one of the anode layer and the cathode layer being substantially transparent to at least a portion of emitted electroluminescent light; at least one organic electroluminescent layer disposed between the anode layer and the cathode layer, the electroluminescent layer having a first energy characteristic being the amount of energy required to extract an electron from a highest occupied molecular orbital of the electroluminescent layer, and a second energy characteristic being the amount of energy required to extract an electron from a lowest unoccupied molecular orbital of the electroluminescent layer; and at least one optical interference member disposed between two of the layers, and having a work function substantially equal to the first energy characteristic when the optical interference member is between the anode and the electroluminescent layer, and having a work function substantially equal to the second energy characteristic when the optical interference member is between the cathode and the electroluminescent layer, the optical interference member being of a thickness and material such that the spectral reflectance of the electroluminescent device is so modified that the reflectance of ambient light by the electroluminescent device towards the viewer is reduced.  
       [0023] In another embodiment of the invention, there is provided an electroluminescent device to emit light in a selected spectrum, comprising: an anode layer, a cathode layer, wherien one of the anode layer and the cathode layer are substantially transparent to at least a portion of the selected spectrum emitted by the electroluminescent device. The electroluminescent device further comprises an organic electroluminescent layer between the anode layer and the cathode layer, the electroluminescent layer having a highest occupied molecular orbital respective to the anode layer and having a lowest unoccupied molecular orbital respective to the cathode layer. The device further includes an optical interference member having a selected work function and operable to reduce ambient light reflected through the transparent layer, the optical interference member being between the electroluminescent layer and one of the anode layer and the cathode layer, wherein the difference between the selected work function and an energy level required to extract an electron from a respective molecular orbital approaches zero.  
       [0024] In another embodiment of the invention, there is provided a method of fabricating an electroluminescent device for displaying an image to a viewer in front of the device, comprising the steps of:  
       [0025] depositing an anode layer onto a substrate;  
       [0026] depositing an organic electroluminescent layer onto the anode layer, the electroluminescent layer having a first energy characteristic associated with an anode side, and a second energy characteristic associated with a cathode side;  
       [0027] depositing an optical interference member onto the electroluminescent layer, the optical interference member for reducing the reflectance of ambient light towards the viewer, the optical interference member having a work function substantially equal to the second energy characteristic;  
       [0028] depositing a cathode layer onto the optical interference member, and sealing the device.  
       [0029] In another embodiment of the invention there is provided a method of assembling an electroluminescent device for displaying an image to a viewer in front of the device, comprising the steps of:  
       [0030] depositing an anode layer onto a substrate;  
       [0031] depositing, an optical interference member. onto the anode layer; the optical interference member for reducing the reflectance of ambient light towards the viewer, the optical interference member having a work function;  
       [0032] depositing an organic electroluminescent layer onto the anode layer, the electroluminescent layer having an energy characteristic being the amount of energy required to extract an electron from the electroluminescent layer, the energy characteristic being substantially equal to the work function;  
       [0033] depositing a cathode layer onto the electroluminescent layer, and sealing the device.  
       [0034] In another embodiment of the invention, there is provided a method of displaying an image to a viewer comprising the steps of:  
       [0035] emitting light from an organic electroluminescent layer between an anode and a cathode, said electroluminescent layer having a first energy characteristic respective to said anode and a second energy characteristic respective to said cathode; and  
       [0036] receiving ambient light incident towards said electroluminescent layer; and  
       [0037] forming destructive interference from said ambient light at the incident surface of an optical interference member, said optical interference member having a selected work function and disposed between said electroluminescent layer and one of said anode and said cathode, the difference between said work function and a respective energy characteristic approaching zero.  
       [0038] The appropriate selection of material of the at least one optical interference member ensures proper current flow through the device, thus reducing the likelihood of electrical breakdown of the organic electroluminescent layer, and improving the overall energy efficiency of the device, while still reducing reflectance towards a viewer. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0039] The present invention will now be described, by way of, example only, with reference to certain embodiments shown in the attached Figures in which:  
     [0040]FIG. 1 is a schematic diagram of a cross-section of through a portion of an optical interference organic electroluminescent device in accordance with a first embodiment of the invention; and  
     [0041]FIG. 2 is a schematic diagram of a cross-section of through a portion of an optical interference organic electroluminescent display in accordance with a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0042] Referring now to FIG. 1, an electroluminescent device in accordance with a first embodiment of the invention is indicated generally at  10 . Device  10  comprises an electroluminescent transmitting anode  12 , an electroluminescent layer  14  disposed behind anode  12 , an optical interference member  16  disposed behind the electroluminescent layer  14  and a cathode  18  disposed behind the interference member  16 . Device  10  is connected to a current source  20  via anode  12  and cathode  18  in order to drive a constant current through device  10 .  
     [0043] Electroluminescent transmitting anode  12  is any conducting material which is transparent to at least a portion of emitted electroluminescent light, such as indium tin oxide (ITO) or zinc oxide (ZnO). In a present embodiment, anode  12  is a layer of indium tin oxide preferably having a thickness of about fifteen-hundred angstroms (1500Å).  
     [0044] It is to be understood that electroluminescent transmitting anode  12  can have different thicknesses, and can be in the range of, for example, from about one-thousand angstroms (1000Å) to about three-thousand angstroms (3000Å), or from about twelve-hundred angstroms (1200Å) to about two-thousand angstroms (2000Å).  
     [0045] Electroluminescent layer 14 is an organic electroluminescent material such as tris(8-quinolinolato aluminum) (Alq3) or poly(n-vinyl carbozale) (PVC z ) wherein photons of light are emitted when electrons drop from a lowest unoccupied molecular orbital (“lumo”) of layer  14 , where they combine with holes in the highest occupied molecular orbital (“homo”) of layer  14 . Accordingly, a current flow through electroluminescent layer  14  can produce an emission of light. In a present embodiment, layer  14  is preferably made from tris(8-quinolinolato aluminum), preferably having a thickness of about one-thousand angstroms (1000Å), although those of skill in the art will be able to select other appropriate thicknesses of this layer. As is known to those familiar with the electrical properties of tris(8-quinolinolato aluminum), the energy required to extract an electron from the highest occupied molecular orbital is about 5.4 electron-volts, which can also be described as the energy required to extract an electron from the surface of layer  14 , or, the work function E HOMO  of layer  14 . Further, the energy required to extract an electron from the lowest unoccupied molecular orbital of tris(8-quinolinolato aluminum) is about 3.0 electron-volts, which can be referred to as E LUMO . Thus the difference between E HOMO  and E LUMO  of tris(8-quinolinolato aluminum) is about 2.4 electron-volts. In other embodiments of the invention, other organic materials can be so chosen (i.e. by selection of material or modification of material with dopants) so that the difference between E HOMO  and E LUMO  is in the range from about 1.5 electron-volts to about 3.0 electron-volts, which covers the spectrum of visible light.  
     [0046] In a present embodiment, optical interference member  16  comprises a semi-absorbent layer  16   a  and a transparent layer  16   b . Semi-absorbent layer  16   a  is partially reflective, partially absorbing and partially transmissive of light in the visible spectrum, and in a present embodiment, is made from magnesium silver (Mg:Ag) having a thickness of about one-hundred-and-eighty-five angstroms (185Å). Other suitable materials can include Inconel™, Nickel (Ni), Titanium, or a suitable organic material and appropriate thicknesses of such layers can be determined by those of skill in the art. The extinction coefficient of the material and its thickness, should be selected so that the reflection from layer  16   a  at a preselected wavelength, neglecting optical interference, should preferably be at least about thirty-five percent, with the remainder of light energy being absorbed and dissipated in the form of heat. Similarly, transmission through layer  16   a  at a preselected wavelength, neglecting optical interference, will preferably be at least about thirty-five percent.  
     [0047] It is to be understood that the extinction coefficient of layer  16   a  and its thickness can be selected so that the transmission through layer  16   a  at a preselected wavelength, neglecting optical interference, can be from about thirty percent to about forty percent. Overall, the amount of light transmitted through layer  16   a , after two passes, should be substantially equal to the amount of light that is originally reflected from layer  16   a , in order to achieve the appropriate destructive interference at the reflective surface of layer  16   a , as will be understood by those of skill in the art.  
     [0048] Substantially transparent conducting layer  16   b  is made from indium tin oxide (ITO) and has a thickness of about eight-hundred-and-forty angstroms (840Å). Other suitable materials and layer thicknesses can be used as will occur to those of skill in the art, such as zinc oxide (ZnO). The extinction coefficient of the material of layer  16   b  and its thickness is selected so that the transmission through layer  16   b  at a preselected wavelength, neglecting optical interference, is greater than about eighty percent, but is preferably at least about ninety percent. As known to those of skill in the art, it is generally preferred that the preselected wavelength(s) for layer  16   b  should be substantially equal to the preselected wavelengths used to choose layer  16   a.    
     [0049] A wavelength of about five-hundred-and-fifty nanometers (550 nm), the centre of the spectrum of visible light, is a presently preferred preselected wavelength used for the purpose of determining appropriate thicknesses and materials of layers  12  and  14  and member  16 , as the resulting device  10  can have the desired optical interference characteristics across the visible light spectrum. As will be understood by those of skill in the art, an incidental benefit to the selection of this wavelength can result in a device which reflects electromagnetic energy outside the visible spectrum, including infra-red, thus reducing the heating of the display. However, it will occur to those of skill in the art that other wavelengths can be selected, as desired.  
     [0050] Optical interference member  16  is also quantifiable in terms of its work function Φ OIM , or the amount of energy required to extract an electron from the surface of member  16 . In a present embodiment, positive-charges or holes flow from current source  20  through anode  12  and into electroluminescent layer  14 , where they combine with electrons which flow from current source  20  through cathode  18 , optical interference member  16  and into electroluminescent layer  14 . Thus, in order to facilitate the injection of electrons into the lowest unoccupied molecular orbital of layer  14 , the materials chosen for member  16  are such that the difference X between E LUMO  and Φ OIM , expressed as an absolute value, approaches zero. This expression can be expressed mathematically as:  
       X   =            Φ   OIM     -     E   LUMO                    Where            lim             X   →   0                         
 
     [0051] In order for the difference X to approach zero, it is believed that the X can be in the range of from about 0.0 to about 1.5 electron-volts (eV). It is believed that the difference X can be in the range of from about 0.1 to about 1.3 electron-volts. More preferably, it is believed that the difference X should be in the range of from about 0.6 to about 1.0 electron-volts. In the present embodiment, where layer  16   a  is made from magnesium silver and layer  16   b  is made from indium tin oxide, the overall work function Φ OIM  of member  16  is about 3.6 electron-volts. As E LUMO  is about 3.0 electron-volts, the difference X is about 0.6 electron-volts.  
     [0052] Cathode  18  is magnesium silver having a thickness of about five-hundred angstroms (500Å), and in the present embodiment is reflective. In other embodiments, it is believed that cathode  18  can have a thickness between about two-hundred-and-fifty angstroms (250Å) to about two-thousand-angstroms (2000Å). However, it will occur to those of skill in the art that other suitable conducting materials and thicknesses can be used.  
     [0053] It now will be apparent that the thickness and material(s) of member  16  and its components can be determined with an operation that considers, at least in part, the thicknesses of and/or materials of anode  12 , electroluminescent layer  14 , and cathode  18 .  
     [0054] Device  10  can be fabricated using techniques known in the art. In the foregoing embodiment, anode  12  is vacuum-deposited onto a glass substrate, and the subsequent layers are formed thereon also using vacuum deposition. The entire device  10  is then sealed using techniques known in the art. Other suitable substrates and means of fabricating device  10  will occur to those of skill in the art. For example, the substrate can be plastic. Further, where electroluminescent layer  14  is poly(n-vinyl carbozale), then spin-coating can be an appropriate fabrication technique for layer  14 .  
     [0055] The operation of device  10  will now be discussed. It will be appreciated by those of skill in the art that the following is a simplified model for purposes of explanation, and that other physical phenomena occurring during the operation of device  10  are assumed, for the purposes of this discussion, to have a negligible influence on the operation. Current source  20  is ‘on’, so that holes are driven into electroluminescent layer  14  via anode  12 . These holes then combine with electrons delivered into layer  14  from source  20 , via cathode  18  and optical interference member  16 . The fact that the difference between E LUMO  and Φ OIM , expressed as an absolute value, approaches zero, reduces barriers to current flow through device  10 , preventing or inhibiting breakdown of layer  14 . The recombination of holes and electrons in layer  14  causes light to be emitted out through the front or exterior face of anode  12  and towards a viewer, as indicated by arrow L em .  
     [0056] At the same time, ambient light is incident upon device  10 , as indicated by arrow L amb  and passes through anode  12  and electroluminescent layer  14 . Ambient light L amb  incident upon semi-absorbing layer  16   a  is partially reflected, partially absorbed and partially transmitted. The light transmitted through semi-absorbing layer  16   a  passes through transparent layer  16   b , where it reflects off cathode  18  and back through transparent layer  16   b , at which point this reflected light is inverted one-hundred-and-eighty degrees out of phase with the partially reflected light from layer  16   a , and thus these two reflections destructively interfere and substantially cancel each other out The energy otherwise found in these two reflections is absorbed by semi-absorbing layer  16   a  and cathode  18 , where it is dissipated as a relatively small amount of heat. The result is that reflected light (L ref ) back towards the viewer from device  10  is reduced. In a present embodiment, reflected light (L ref ) is reduced by about ninety percent, compared to an electroluminescent device assembled without optical interference member  16 .  
     [0057] It is believed that in other embodiments of the invention, reflected light (L ref ) can be reduced by as much as about 99.5 percent by choosing different materials, thicknesses and extinction coefficients for optical interference member  16  and by selected appropriate thicknesses and materials for the other layers in device  10 , although still within the aforementioned acceptable parameters and ranges such that the difference between E LUMO  and Φ OIM , expressed as an absolute value, approaches zero. However, while a higher difference in energy levels between E LUMO  and Φ OIM , still within the acceptable ranges, can achieve reduced reflection, it can also result in reduced electrical efficiency in the current flow through device  10 .  
     [0058] In other embodiments of the invention, a suitably modified optical interference member  16  can be disposed in series with other layers of device  10 . Referring now to FIG. 2, an electroluminescent device in accordance with a second embodiment of the invention is indicated generally at  10   a . Like components to those shown in FIG. 1 are indicated with like reference numbers. Device  10   a  comprises a electroluminescent transmitting anode  12 , an optical interference member  16 ′ disposed behind anode  12 , an electroluminescent layer  14  disposed behind interference member  16 ′, and a cathode  18  disposed behind electroluminescent layer  14 . Device  10   a  is connected to a current source  20  via anode  12  and cathode  18  in order to drive a constant current through device  10  and layer  14 .  
     [0059] Electroluminescent transmitting anode  12  is any conducting material which is transparent to at least a portion of emitted electroluminescent light, such as indium tin oxide (ITO) or zinc oxide (ZnO). In a present embodiment, anode  12  is a layer of indium tin oxide preferably having a thickness of about fifteen-hundred angstroms (1500Å).  
     [0060] It is to be understood that electroluminescent transmitting anode  12  can have different thicknesses, and can be in the range of, for example, from about one-thousand angstroms (1000Å) to about three-thousand angstroms (3000Å), or from about twelve-hundred angstroms (1200Å) to about two-thousand angstroms (2000Å).  
     [0061] Electroluminescent layer  14  is an organic electroluminescent material such as tris(8-quinolinolato aluminum) (Alq3) or poly(n-vinyl carbozale) (PVC z ) wherein photons of light are emitted when electrons drop from a lowest unoccupied molecular orbital (“lumo”) of layer  14 , where they combine with holes in the highest occupied molecular orbital (“homo”) of layer  14 . Accordingly, a current flow through electroluminescent layer  14  can produce an emission of light In a present embodiment, layer  14  is preferably made from tris(8-quinolinolato aluminum), preferably having a thickness of about one-thousand angstroms (1000Å), although those of skill in the art will be able to select other appropriate thicknesses of this layer. As is known to those familiar with the electrical properties of tris(8-quinolinolato aluminum), the energy required to extract an electron from the highest occupied molecular orbital is about 5.4 electron-volts, which can also be described as the energy required to extract an electron from the surface of layer  14 , or, the work function E HOMO  of layer  14 . Further, the energy required to extract an electron from the lowest unoccupied molecular orbital of tris(8-quinolinolato aluminum) is about 3.0 electron-volts, which can be referred to as E LUMO . Thus the difference between E HOMO  and E LUMO  of tris(8-quinolinolato aluminum) is about 2.4 electron-volts. In other embodiments of the invention, other organic materials can be so chosen (i.e. by selection of material or modification of material with dopants) so that the difference between E HOMO  and E LUMO  is in the range from about 1.5 electron-volts to about 3.0 electron-volts, which covers the spectrum of visible light.  
     [0062] In a present embodiment optical interference member  16 ′ comprises a substantially transparent conducting layer  16   c  made from indium tin oxide (ITO) and having a thickness of about seven-hundred-and-forty-five-angstroms (745Å). Other suitable materials can be used as will occur to those of skill in the art, such as zinc oxide (ZnO) or a suitable organic material. The indium tin oxide of layer  16   c  is modified (by controlling its stoichiometry) so that it behaves as a quarter-wave-stack at a light wavelength of five-hundred-and-fifty nanometers (550 nm), and chosen so that the transmission through layer  16   c  is greater than about eighty percent, and preferably at least about ninety percent. However, it will occur to those of skill in the art that other wavelengths can be selected, as desired.  
     [0063] As discussed previously, optical interference member  16 ′ is also measurable in terms of work function Φ OIM . In a present embodiment, holes flow from current source  20  through anode  12  and optical interference member  16 ′, and into electroluminescent layer  14 , where they combine with electrons which flow from current source  20  through cathode  18  and into electroluminescent layer  14 . Thus, in order to facilitate the extraction of electrons from the highest occupied molecular orbital of electroluminescent layer  14 , the materials chosen for member  16 ′ are such that the difference Y between E HOMO  and Φ OIM , expressed as an absolute value, approaches zero. This expression can be expressed mathematically as:  
       Y   =            Φ   OIM     -     E   HOMO                    Where            lim             Y   →   0                         
 
     [0064] In order for the difference between E HOMO  and Φ OIM  to approach zero, it is believed that the difference Y can be in the range of from about 0.0 to about 1.5 electron-volts (eV). It is believed that the difference Y can be in the range of from about 0.1 to about 1.3 electron-volts. More preferably, it is believed that the difference Y should be in the range of from about 0.4 to about 1.0 electron-volts. In the present embodiment, where layer  16   a  is made from magnesium silver and layer  16   b  is made from indium tin oxide, the overall work function Φ OIM  of member  16  is about 5.0 electron-volts. Since E HOMO  is about 5.4 electron-volts, the difference Y is about 0.4 electron-volts (eV).  
     [0065] Cathode  18  is magnesium silver having a thickness of about five-hundred angstroms (500Å), and in the present embodiment is reflective. In other embodiments, it is believed that cathode  18  can have a thickness between about two-hundred-and-fifty angstroms (250Å) to about two-thousand-angstroms (2000Å). However, it will occur to those of skill in the art that other suitable conducting materials and thicknesses can be used.  
     [0066] It now will be apparent that the thickness and material(s) of member  16 ′ and its components can be determined with an operation that considers, at least in part, the thicknesses of and/or materials of anode  12 , electroluminescent layer  14 , and cathode  18 .  
     [0067] Device  10   a  can be fabricated using techniques known in the art. In the foregoing embodiment, anode  12  is vacuum-deposited onto a glass substrate, and the subsequent layers are formed thereon also using vacuum deposition. The entire device  10   a  is then sealed using techniques known in the art. Other suitable substrates and means of fabricating device  10   a  will occur to those of skill in the art. For example, the substrate can be plastic. Further, where electroluminescent layer  14  is poly(n-vinyl carbozale), then spin-coating can be an appropriate fabrication technique for layer  14 .  
     [0068] The operation of device  10   a  will now be discussed. It will be appreciated by those of skill in the art that the following is a simplified model for purposes of explanation, and that other physical phenomena occurring during the operation of device  10   a  are assumed to have a negligible influence on the operation. Current source  20  is ‘on’, so that holes or positive-charges are driven into electroluminescent layer  14  via anode  12  and optical interference member  16 . These positive-charges then combine with electrons delivered into layer  14  from source  20  via cathode  18 . Because the difference between E HOMO  and Φ OIM  approaches zero, barriers to current flow through device  10   a  are reduced, preventing or inhibiting breakdown of layer  14 . The recombination of electrons and holes in layer  14  causes light to be emitted out through the front face of anode  12  and towards a viewer, as indicated by arrow L em .  
     [0069] At the same time, ambient light is incident upon device  10   a  as indicated by arrow L amb  and passes through anode  12  and is incident upon transparent layer  16   c . About half of ambient light L amb  incident upon layer  16   c  is reflected, while the remainder is transmitted. The light transmitted through layer  16   c  reflects off the surface of electroluminescent layer  14  and back through layer  16   c . Due to the quarter-wavelength thickness of layer  16   c , this reflected light from layer  14  is inverted one-hundred-and-eighty degrees out of phase with the reflected light from layer  16   c , and thus these two reflections destructively interfere and substantially cancel each other out. The energy otherwise, found in these two reflections is transmitted through layer  16   c . The result is that reflected light (L ref ) back towards the viewer from device  10  is reduced. In the present embodiment, reflected light (L ref ) is believed to be reduced by a range of from about 0.5 percent to about two percent, compared to electroluminescent devices without layer  16 ′.  
     [0070] While only specific combinations of the various features and components of the present invention have been discussed herein, it will be apparent to those of skill in the art that desired sub-sets of the disclosed features and components and/or alternative combinations of these features and components can be utilized, as desired. For example, the embodiments discussed herein can be combined to provide multiple optical interference members disposed between different layers of the electroluminescent device, and therefore disjoined from each other, in order to further reduce reflectance from the device. For example, layer  16 ′ and layer  16  can be included on respective sides of layer  14 .  
     [0071] Furthermore, each optical interference member can be a transparent layer or a combination of a transparent layer and a semi-absorbing layer in order to achieve different results, and it will be apparent that these different types of optical interference members can also be placed at different locations throughout the device. For example, it is contemplated that a transparent layer, without a semi-absorbing layer can be used between the organic electroluminescent layer and the cathode, and similarly, a combination of a transparent layer and a semi-absorbing layer can be used between the anode and the electroluminescent layer, by having the difference between work function of the optical interference member and the energy required to extract an electron from either highest occupied molecular orbital, and/or the lowest unoccupied molecular orbital, approach zero, as appropriate.  
     [0072] The present invention can be suitable for a computer display. For example, a pixilated organic electroluminescent computer display can be formed where the anode comprises a plurality of generally parallel and spaced anodes to compose the front layer of an organic electroluminescent computer display, and the cathode comprises a number of spaced cathodes which are generally perpendicular to the anodes. It will be further understood that the anode and the cathode can be patterned in a variety of ways, other than pixilated, to create different recognizable patterns to a user of device  10 . When such a display is pixilated or patterned, it will be appreciated that individual pixels or patterns can be fired using known techniques such as pulsed-DC, and/or adding a periodic reverse-polarity ‘refresh’pulse to reduce built-up charge. The device can also be hybrid-display having an active matrix, as can be found in notebook computers.  
     [0073] In addition, the present invention can be suitably modified for use in colour organic electroluminescent devices. As known to those of skill in the art, such multi-colour and full-colour devices can be formed from stacked transparent organic electroluminescent layers. Where the optical interference member is between two of these stacked layers, it will be apparent that the difference between its work function and the energy level required to extract an electron from the lowest unoccupied molecular orbital of the electroluminescent layer nearest the anode should approach zero, and the difference between the work function and the energy level required to extract an electron from the highest occupied molecular orbital of the electroluminescent layer nearest the cathode should also approach zero. More preferably, the work function of the optical interference member should be the average of the lowest unoccupied molecular orbitals of the surrounding electroluminescent layers.  
     [0074] Multi-coloured and full-coloured devices can also be provided through a patterned red-green-blue organic layer (i.e. by selecting materials having inherent colour properties, or by appropriately doping the patterns on the layer). Other colourizing techniques can including the use of a white-emitter and appropriate filters. It will be apparent that the teachings of the present invention can be modified to accommodate these and other colour devices.  
     [0075] The present invention can be suitable for use as a backlight for a liquid crystal display.  
     [0076] It will also be understood that the present invention can be suitably modified for organic electroluminescent devices which have an electron transport layer between the cathode and the electroluminescent layer, and/or a hole transport layer between the anode and the electroluminescent layer. In this embodiment, the optical interference member can thus be placed between the transport layer and the electroluminescent layer, or the transport layer can be placed between the electroluminescent layer and the optical interference member. In this embodiment of the invention, it will be apparent that the optical interference member is still selected to have a work function such that the difference between the work function and the energy levels associated with the respective anode side and/or cathode side of the electroluminescent layer(s) approach zero, in order to facilitate current flow through the entire device. It is also contemplated that an appropriate transport layer can be incorporated into an optical interference member.  
     [0077] Furthermore, while the embodiments herein refer to a front anode and a rear cathode, it will be apparent that the present invention can be suitable to a device having a front cathode and a rear anode, as long at least the front layer is transparent to at least a portion of emitted electroluminescent light.  
     [0078] The present invention provides a novel organic electroluminescent device having an optical interference member which reduces the overall reflectance from the device. The optical interference member is selected to have a thickness which causes at least some destructive optical interference of ambient light incident on the display. In addition, the material(s) of the optical interference member are chosen to have a work function such that the difference between the work function and the energy level required to extract an electron from the highest occupied molecular orbital, or the lowest unoccupied molecular orbital of the electroluminescent layer approaches zero, depending on the direction of current flow and location of the optical interference member in relation to the electroluminescent layer. The appropriate selection of material for the optical interference member improves proper current flow through the device, thus reducing the likelihood of electrical breakdown of the organic electroluminescent layer and improving the overall energy efficiency of the device. Finally, in embodiments where a semi-absorbent layer and transparent layer are combined to form the optical interference member, then placement of such an optical interference member in contact with the electrode at the back of the device can actually increase the reflectance of infra-red ambient signatures, compared to absorbing films, thus reducing the heating of the display and reducing the likelihood of damage to the electroluminescent layer.