A transparent electrode is generally used for: a liquid crystal display, an electroluminescent display, a plasma display, an electrochromic display, a solar battery, a touch panel, and an electronic paper.
An organic EL element (it may be called as an organic-field light-emitting element), which utilizes electroluminescence (hereinafter, it is abbreviated as “EL”) of an organic material, has a configuration of interposing a light emitting layer containing an organic compound between a pair of opposed electrodes. Emission light generated in the light emitting layer passes through the electrode and it is extracted to the outside. Therefore, at least one of the two electrodes is composed of a transparent electrode.
As for a transparent electrode, oxide semiconductor materials, such as indium tin oxide (SnO2—In2O3: or abbreviation name ITO), are generally used. The transparent electrode made of ITO is usually produced with a sputtering method. The transparent electrode produced with a sputtering method only will have a large sheet resistance, and it exhibits remarkable voltage decrease from the power supplying point.
In order to decrease a sheet resistance, it was investigated a method of laminating an ITO layer and a silver layer for reducing resistance (for example, refer to Patent documents 1 and 2).
Patent document 1 discloses a transparent electrode having a structure of laminating an ITO film and a silver film. Patent document 2 discloses a transparent electrode having a structure of interposing a silver film with ITO films. However, when the used silver film was made thin to an extent of not inducing loss of light transmittance, the resistance did not sufficiently decrease. Therefore, it was required to combine with a metal oxide such as ITO. Since ITO employs a rare earth metal In, the cost of material is high. In addition, it is required to perform an annealing treatment at a temperature condition of around 300° C. for decreasing the resistance. It was difficult to use a resin substrate at such temperature condition. When silver was employed, high electric conductivity is obtained. On the other hand, it has a problem of trade-off between resistance and light transmittance.
Patent document 3 proposes the structures for replacing a metal oxide such as ITO. One structure contains a thin film metal material such as silver having high electric conductivity. The other structure contains a mixture of silver with aluminum. This mixture enabled to achieve higher electric conductivity with a thinner film than silver. However, a transparent electrode composed of silver and aluminum having high electric conductivity had slightly insufficient sheet resistance. A metal thin film is usually is required to have a large thickness to increase sheet resistance. Consequently, light transmittance will be deteriorated. As described above, the resistance property and the light transmittance are in a trade-off relationship. It was difficult to achieve sufficient conductivity and light transmittance at the same time. Further, in a silver-aluminum alloy, aluminum has a property of easily oxidized. It has a problem of increasing the resistance by oxidization. Therefore, it has been required a transparent electrode enabling to achieve both resistance (conductivity) and light transmittance.