An organic electroluminescence device (hereinafter, referred to as an organic EL device) is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied.
Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al used tris(8-hydroxyquinolinol)aluminum as the light emitting layer and a triphenyldiamine derivative as the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased and that excited particles formed within the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials and development of a device exhibiting color images is expected (For example, Japanese Patent Application Laid-Open Nos. Heisei 8(1996)-239655, Heisei 7(1995)-138561 and Heisei 3(1991)-200289).
A device using a phenylanthracene derivative as the light emitting material is disclosed in Japanese Patent Application Laid-Open No. Heisei 8(1996)-12600. The phenylanthracene derivative is used as the light emitting material emitting blue light. In general, this compound is used as the layer emitting blue light as a laminate in combination with a layer of tris(8-quinolinolato)aluminum (Alq). However, the efficiency of light emission and the life are insufficient for practical application.
In Japanese Patent Application Laid-Open No. Heisei 11(1999)-312588, a laminate having a layer of a phenylanthracene derivative and a layer of tris(8-quinolinolato)aluminum is used. However, this laminate has a problem in that the efficiency of light emission is as low as 1 to 2 cd/A. One of the causes of the problem is that the amounts of electrons and holes are not in a good balance since the injection of electrons into the layer of phenylanthracene is harder than the injection into the layer of Alq and a portion of the electric current is not used for the recombination. Another cause of the problem is that Alq is degraded by electric current when a hole current as a portion of the electric current not used for the recombination flows into the layer of Alq.
Organic EL devices emitting light of a high luminance at a high efficiency and having a long life have been reported recently. However, the performance of these devices are not always satisfactory.
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