Electroluminescence (EL) describes the direct conversion of electricity to light. It is the result of radiative recombination of electrons and holes in a material (usually a semiconductor), whereby excited electrons release their energy as photons (i.e., light). Prior to recombination, electrons and holes are separated either as a result of doping of the material to form a p-n junction (e.g., in semiconductor electroluminescent devices such as LEDs) or through excitation by impact of high-energy electrons accelerated by a strong electric field (e.g., as with the phosphors in electroluminescent lamps).
Devices based on the latter, e.g., phosphor-based, electroluminescent mechanism include, but are not limited to, backlights for liquid crystal displays, membrane keypads and a variety of other applications. In such devices, the EL material generally must be enclosed between at least two electrodes, at least one of which should be at least semi-transparent to allow produced light to escape. As used herein, a layer of material or a sequence of several layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some but less than 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be “semi-transparent.”
Currently, the most commonly used transparent electrodes are transparent conducting oxides (TCOs), specifically indium-tin-oxide (ITO) on glass. However, ITO, with its relatively brittle structure, can be an inadequate solution for many emerging applications (e.g., flexible optoelectronic devices), and the indium component of ITO is rapidly becoming a scarce commodity. Additionally, ITO deposition usually requires expensive, high-temperature sputtering, which can be incompatible with many device process flows. Hence, a more robust, abundant and compatible transparent conductor material is required.