An electroluminescent (hereinafter referred to as “EL”) device has the advantages of a wide viewing angle, superior contrast, and a fast response time as an emissive display device.
EL devices are categorized as inorganic EL devices and organic EL devices according to materials for forming a light-emitting layer, wherein the organic EL has merits of superior luminescence, driving voltage and response speed characteristics as well as multi-coloration.
Existing EL devices generally consist of an anode electrode for injecting a hole, a hole injecting layer for injecting and transporting the hole, a hole transport layer, a light-emitting layer, an electron transport layer and a cathode electrode.
In an organic EL device illustrated in FIG. 1, a hole extends from an anode electrode 12 formed on a substrate 11 to a light-emitting layer 15 through a hole injection layer 13 and a hole transport layer 14, and electrons are moved from a cathode electrode 18 to the light-emitting layer 15 through an electron transport layer 16 so that a light-emitting substance in the light-emitting layer is emitted to generate light by excitation of electrons due to the difference in energy levels in the light-emitting layer. Alternatively, an organic EL device can be fabricated by further comprising an electron transport layer 17 to increase electron formation efficiency. The organic EL device is formed of organic thin films consisting of organic compounds, such as the hole injection layer 13, hole transport layer 14, light-emitting layer 15, electron transport layer 16, and electron transport layer 17.
The driving principle of the organic EL device having the foregoing structure is as follows:
A hole injected from the anode extends to the light-emitting layer from the hole transport layer when a voltage is applied between the anode and cathode. On the other hand, electrons are injected from the cathode into the light-emitting layer via the electron transport layer so that carriers are bonded again in the light-emitting layer region to form an exciton. The exciton is changed from the excited state into the ground state. Accordingly, fluorescent molecules of the light-emitting layer are emitted to form an image.
On the other hand, Eastman Kodak developed an organic EL device using a low molecular aromatic diamine and aluminum complex as a light-emitting layer forming material for the first time in 1987 (Appl. Phys. Lett. 51, 913, 1987). Although compounds such as diphenylanthracene, tetraphenylbutadiene and distyrylbenzene derivatives have been developed as blue light-emitting substances, it is known that they tend to be easily crystallized because of their low thin film stability. Idemitsu Corporation developed a diphenyidistyryl based blue light-emitting substance in which branched phenyl groups hinder crystallization so that thin film stability is improved [H. Tikailin, H. Higashi, C. Hosogawa, EP 388,768 (1990)], and Kyushu University developed distyrylanthracene derivatives that improve thin film stability by having electron donor and acceptor [PRO. SPIE, 1910, 180 (1993)].
Furthermore, it is disclosed in Japanese Patent Laid-open Publication No. Heisei10-261488 that life cycle of the compound is extended by using distyrylanilin derivatives between 2.6 eV and 3.2 eV of electron affinity as blue light-emitting compounds, thereby improving thin film stability.
However, development of a new blue light-emitting compound must be immediately settled without delay to develop a blue light-emitting device or a full light-emitting device since these compounds have also low light-emitting efficiencies and are required to further improve thin film stability compared with other color light-emitting compounds.