A typical OLED consists of two electrodes with one or more polymer layers disposed between the electrodes.  As shown in FIG.  1, these devices commonly include the following layers, arranged in the following order:                (i) a substrate 2 often made from plastic or glass;        (ii) an anode 3 disposed on the substrate and commonly made from a transparent conductive material, which may be an organic material or an inorganic oxide such as Indium Tin Oxide (ITO);        (iii) a hole transport layer 5 disposed on the anode;        (iv) an electroluminescent (EL) layer 6 disposed on the hole transport layer; and        (v) a cathode 8 disposed on the EL layer, the cathode being made from a first layer 9 of Calcium or Magnesium and a second layer 10 of Silver or Aluminum.         
The anode 3 is biased positively with respect to the cathode 8.  In use, the electrons and holes are injected into the EL layer 6, where they migrate in the electric field until they recombine to produce a photon.  The total organic layer thickness is usually around 50-150 nm, and the bias is about 2-20 V. The anode 3 is typically transparent and the work functions of the cathode materials are chosen to match the energy levels of the polymer HOMO for the anode and LUMO for the cathode.  Hence, the materials most widely used for OLED cathodes are calcium, magnesium and aluminum and lithium alloys. 
In recent times considerable improvements have been made in the materials and device architecture of OLEDs, which has resulted in devices with luminance values exceeding 100,000 cd/m2 and external quantum efficiencies in excess of 4%.  These new generation devices have been found to be useful in large area illuminators and flat panel displays. 
Although remarkably stable electroluminescent (EL) devices have been demonstrated operating in inert environments, the lifetimes of such devices have been limited by the formation and growth of non-emissive areas known as “dark spots.” 
A number of factors contribute to the formation and growth of dark spots.  One such factor is cathode defects such as pinholes.  These pinholes are often formed during resistive-heating evaporation.  The oxidized cathode reduces the electron injection efficiency of the device and a higher driving bias is therefore needed.  This significantly increases the chance of local shorting.  Although sputtering is one of the methods commonly used in the semiconductor industry for the deposition of cathodes, this technique has, until now, not been suitable for OLEDs.  This is because OLEDs are extremely sensitive and are damaged by the radiation, charging, heating and ion bombardment involved in the sputtering process.  Although sputtering produces cathodes with few or no pinholes, it has not been used on OLEDs because of the damage caused by the technique. 
Another factor in dark spot formation and growth is the evolution of oxygen and moisture in the OLED materials and its permeation through the device during operation.  The diffusion of moisture and oxygen through the layers of the OLED is a significant cause of the growth of dark spots on the cathode.  Magnesium and Calcium are often used as cathode materials in OLED devices because of their low work function.  However, Calcium and Magnesium are very sensitive to oxygen and water vapor.  The dark spot and defect growths are due to the oxygen and moisture produced within the OLED materials which then permeate through the various layers and substrates and reacts with the electrodes. 
An additional factor in dark spot growth is the migration of the electrode materials during electrical stress.  This metal migration causes peaks of electrical shorts and strong electrical fields in various parts of the OLED. 
A further factor which contributes to the formation of dark spots is local heating.  This heating is often caused by high currents.  It expedites material failure, increases the possibility of inter-diffusion of the organic layers and leads to dark spot growth. 
A further problem faced by prior art OLEDs is that they do not have very good polymer stability.  One of the more significant degradation mechanisms in conjugated polymers has been found to be photo-oxidation.  Such photo-oxidation induces the chain scission of the vinyl double bond on the polymer backbone thereby reducing the conjugation length by the formation of carbonyl groups, resulting in lower photoluminescence efficiencies. 
In recent times it has been found that the performance of OLEDs is enhanced not only by the proper selection of organic materials but also by the efficient carrier injection of electrodes and the controlled electron-hole recombination within a well-defined zone.  Research that has focused on increasing the charge injection and carrier confinement has led to significant improvements in device performance. 
FIG.  2 shows an advanced form of such prior art OLEDs which consists of a multi-layer, high efficiency, structure.  In addition to the layers labeled with common numerals to those of FIG.  1, the advanced prior art OLED also includes an electron injecting layer 15 and a dielectric layer 14. 
The electron injecting layer 15 is disposed between the electroluminescent (EL) layer 6 and the cathode 8 and serves to efficiently control hole and electron injection. 
The dielectric layer 14 is disposed between the anode 3 and the hole transport layer 5.  If the thickness of the dielectric layer 14 is within tunneling thickness, the effective barrier to electron injection is lowered and the injection of holes to the emitting layer is reduced.  As a result, this lowering of the effective barrier to carrier injection leads to more balanced injection of electrons and holes and the quantum efficiency of the OLED increases. 
This more advanced prior art OLED device therefore exhibits superior luminescence and efficiency.  However, because these advanced OLEDs also exhibit dark spot formation and growth they therefore have a relatively short lifespan. 
There is therefore a need for an improved OLED which is adapted to inhibit the formation and growth of dark spots.