A display for displaying images is one of light emitting devices indispensable in the modern life and takes various forms such as a so-called TV monitor, a liquid crystal display developed rapidly in recent years, an organic EL display expected to be developed, and the like depending on the usage. Among the above, the organic EL display is best noted as a next generation flat panel display device.
A light emission mechanism of an electroluminescent device constituting the organic EL display is such that a luminescent layer made from a luminescent composition is provided between electrodes so that electrons injected from a cathode are recombined with holes injected from an anode at the recombination center of the luminescent layer to form molecular excitons when a current is supplied and photons discharged when the molecular excitons return to the ground state are used for the light emission. Accordingly, one of preconditions for manufacturing a light emitting device of good efficiency is to inject the holes and the electrons efficiently into the luminescent layer made of an organic thin film or the like.
Under typical electroluminescent device operation conditions, a current of about 100 mA/cm2 is injected into the organic thin film inherently having a high electrical resistance. In order to realize such high density current injection, it is necessary to keep the sizes of a barrier against the holes injected from the anode and a barrier against the electrons injected from the cathode as small as possible. That is to say, it is necessary to use a metal having a small work function for the cathode and to select an electrode having a large work function for the anode. By selecting various metals and alloys for the cathode, it is practically possible to control the work function at will. In contrast, since transparency is required of the anode in general electroluminescent devices, the material to be used for the anode is limited to transparent conductive oxides under the current situation, and there is no alternative but to select some oxide conductive films such as an indium-tin oxide (hereinafter abbreviated to ITO) film in view of stability, transparency, resistivity, and the like at present. A work function of the ITO film can be changed to a certain degree by a history of film formation and a surface treatment, but such process has its limit. This inhibits the reduction in the hole injection barrier.
As one of methods to reduce the barrier against hole injection from the ITO cathode, an insertion of a buffer layer on the ITO film is known. By optimizing an ionization potential of the buffer layer, it is possible to reduce the hole injection barrier. The above-described buffer layer is called a hole injection layer. Materials which can function as the hole injection layer are generally classified into metal oxides, low molecular organic compounds, and high molecular compounds. Examples of the metal oxides are vanadium oxide, molybdenum oxide, ruthenium oxide, aluminum oxide, and the like (Non-Patent Documents 1 and 2). Examples of the low molecular organic compounds are starburst amines (Non-Patent Document 3) such as m-MTDATA, metal phthalocyanine (Patent Document 1, Non-Patent Document 4), and the like. As the high molecular compounds materials, conjugated polymers such as polyaniline (Non-Patent Document 5) and a polythiophene derivative (Non-Patent Document 6) are known. By using the above-described materials for the hole injection layer, the hole injection barrier is reduced and the holes are efficiently injected, thereby improving the efficiency and life of the electroluminescent device and reducing a driving voltage.
[Non-Patent Document 1]
    S. Tokito, Et al., J. Phys. D 1996, 29, 2750–2753[Non-Patent Document 2]    Tsunataka Kurosaka, et al., Shingakugiho, 1998, 98, 63–68[Non-Patent Document 3]    Y Shirota, et al., Appl. Phys. Lett. 1994, 65, 807–809[Patent Document 1]    S. A. Vanslyke, et al., U.S. Pat. No. 4,720,432[Non-Patent Document 4]    S. A. Vanslyke, et al., Appl. Phys. Lett. 1996, 69, 2160–2162[Non-Patent Document 5]    Y. Yang, et al., Appl. Phys. Lett. 1994, 64, 1245–1247[Non-Patent Document 6]    S. A. Carter, et al., Appl. Phys. Lett. 1997, 70, 2067–2069
Among the above-described materials, the low molecular compounds such as the starburst amines and the metal phthalocyanine, and the high molecular materials such as polyaniline are frequently used (see, for example, Patent Documents 5 and 6). Particularly, copper phthalocyanine is one of the most-frequently-used hole injection materials. This material can be obtained easily and is chemically and thermally stable. However, metal phthalocyanines have a remarkably low solubility and it is difficult to chemically modify the compounds. Further, though the materials have excellent characteristics as the hole injection material, they are colored in many cases to disadvantageously color a light emission surface itself of the device. Therefore, there is a demand for an electroluminescent device using a hole injection material having no absorption or small absorption intensity in the visible region.