1. Field of the Invention
The present invention relates to an electroluminescent (EL) device having an emitting layer made of an emitting substance, which utilizes an electroluminescence phenomenon that the emitting substance emits light by applying an electric current to the emitting layer. More particularly, it is concerned with an organic EL device in which the emitting layer is made of an organic emitting substance.
2. Description of the Prior Art
As such type of EL devices, there have been known a device having two layers each made of an organic compound, as shown in FIG. 1, in which an organic fluorescent film or emitting layer 3 and an organic positive-hole transport layer 4 are arranged between a metal electrode or cathode 1 and a transparent electrode or anode 2, the two layers being layered on each other. There have been also known an EL device of three-layer structure, as shown in FIG. 2, in which an organic electron transport layer 5, an organic fluorescent film 3 and an organic positive-hole transport layer 4, are arranged between a metal electrode 1 and a transparent electrode 2. The organic positive-hole transport layer 4 herein has a function to facilitate the injection of positive holes from an anode as well as another function to block electrons, while the organic electron transport layer 5 has a function to facilitate the injection of electrons from a cathode.
In these EL devices where a glass substrate 6 is arranged outside the transparent electrode 2, excitons are generated from the recombination of the electrons injected from the metal electrode 1 with the positive holes injected to the organic fluorescent film 3 from the transparent electrode 2. At the stage where the excitons are inactivated through radiation, they emit light radiating toward outside through the transparent electrode 2 and the glass substrate 6 (the Japanese Patent Laid-open No. 59-194393).
As is disclosed in the Japanese Patent Laid-open No. 264692/1988, there have been also known stably emitting el devices comprising a fluorescent film formed from an organic host material and a fluorescent guest material.
It is however expected to develop an EL device capable of emission at a further high luminance, although the conventional EL devices with such aforementioned composition comprising organic compounds can emit light at a lower voltage.