An organic electroluminescence device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of 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 a laminate structure using tris(8-hydroxyquinolinol)aluminum for the light emitting layer and a triphenyldiamine derivative for 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 among 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.
Conventionally, trials for raising efficiency of light emission of the organic EL device by providing an electron injecting/transporting layer have been repeated. In this case, there were drawbacks that formation of exciplex was found, and that lifetime of light emission was short in spite of obtaining highly illuminant light emission. Further, a long time electric drive caused separation between a metal-electrode and an organic compound or crystallization of the organic compound layer and the electrode resultantly whitening, further decreasing luminance of light emission, and it was necessary to prevent such a phenomena.
As the examples of heterocyclic compound having nitrogen atom such as pyrazine compound, quinoline compound, quinoxaline compound and so on employed as a constituting component of the organic EL device, there are 2,3,5,6-tetraphenyl pyrazine, 2,3,4-triphenyl quinoline and 2,3-diphenyl quinoxaline that are all disclosed in U.S. Pat. No. 5,077,142. However, because melting points of these compounds is low, there were disadvantages of such as inviting unfavorable fact such that even employing these compounds as amorphous thin-film layer of the organic EL device soon induces crystallization and disturbs light emission almost at all. Also, there were disadvantages that an electric drive caused the above separation, which made the lifetime short.
Furthermore, a blue-light-emitting device with the use of a heterocyclic compound having nitrogen atom is disclosed in Japanese Unexamined Patent Application Laid-Open No. 2001-6877, and an organic EL device with the use of a heterocyclic compound having nitrogen atom for a light emitting material or a hole injection transport material is disclosed in Japanese Unexamined Patent Application Laid-Open No. 2001-35664. The invention of Japanese Unexamined Patent Application Laid-Open No. 2001-6877 provides blue-light emission having peak wave-length in 430 to 480 nm, and the invention of Japanese Unexamined Patent Application Laid-Open No. 2001-35664 provides a luminance of about 500 cd/m2 under the application of an electric voltage of 6 V when using the heterocyclic compound having nitrogen atom as hole injection material, and also provides a luminance of about 2300 cd/m2 under the application of electric voltage of 12 V when using the compound as a light emitting material. However, the voltage was too high to keep practical performance and an organic EL device having excellent efficiency of light emission under the application of lower electric voltage was required.