1. Field of the Invention
The present invention relates to an organic electroluminescent element (hereinafter referred to as an organic EL element) capable of inducing luminescence by the recombination of an electron and a hole when they are injected into an organic compound layer of the EL element.
2. Discussion of Background
In recent years, in line with the trend toward diversification of information processing equipment and space-saving for location of such equipment, there is an increasing demand for a plans display element which can be operated with a low electric power and occupies a small space as compared with the cathode ray tube (CRT) display. A liquid crystal has been already proposed as such a plane display element, but much attention has been paid to an organic EL element of a self-emitting type because the EL element can indicate the information clearly and can be driven with a direct-current low voltage system.
A luminescent layer of the organic EL element is of a single-layered type, or a laminated type which comprises a carrier transporting layer and a luminescent layer. The luminance of the laminated luminescent layer is higher than that of the single-layered one. The above-mentioned carrier transporting layer for use in the laminated luminescent layer serves to transport a hole or an electron.
A variety of triphenylamine compounds, which have been developed for an organic electrophotographic photoconductor, are proved to be usable as materials for transporting the hole (hereinafter referred to as hole transporting materials) Some of them are found to have relatively high thermal stability in the amorphous condition.
On the other hand, some of oxadiazole compounds, triazole compounds and peryleneimide compounds are know as electron-transporting materials, but there are few compounds that have no absorption in the visible region. Further, the electron-transporting materials with high thermal stability are very rare among the above-mentioned compounds.
From the viewpoint of the layer structure of the EL element, the organic EL element comprising an electron-transporting layer has the advantages that various hole transporting materials can be selected for a luminescent layer, an exciton can be trapped in the small luminescent layer, and the deactivation of the exciton caused by the mutual action between the exciton and a cathode can be prevented. Thus, the organic EL element with a high luminance can be obtained using a variety of luminescent materials with different fluorescent colors. However, because the conventional electron-transporting materials do not have high stability, the durability of the conventional EL element comprising the electron-transporting layer is regarded as poor.
For instance, there is known an organic EL element using an oxadiazole compound as a luminescent material and electron-transporting material, as stated in "Nippon Kagaku Kaishi 1991, (11), pages 1540-1548". However, the problem of the film stability of the above-mentioned oxadiazole compound for use in the EL element remains unsolved, so that there is no organic EL element that shows high luminance and high reliability at the present stage.
It is considered that the poor film stability of the oxadiazole compound results from the crystallization thereof Some of the conventional oxadiazole compounds cannot be made into an amorphous film, and others can be formed into an amorphous film, but the amorphous film thus formed induces crystallization during a long period of storage. Therefore, there is an increasing demand for an organic EL element employing an electron-transporting material which has good film-forming properties and does not easily induce crystallization.