As an emission type electronic displaying device, there is an electroluminescent device (ELD). As elements constituting the ELD, there are an inorganic electroluminescent device and an organic electroluminescent device. The inorganic electroluminescent device has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the device.
An organic EL device has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The device emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the device can emit light by applying a relatively low voltage of from several volts to several decade volts. The device has a wide viewing angle and a high visuality since the device is of self light emission type. Further, the device is a thin, complete solid device, and therefore, the device is noted from the viewpoint of space saving and portability.
However, an organic EL device for practical use is required which efficiently emits light with high luminance at a lower power.
A conventional organic EL device employs emission (i.e., fluorescence) through an excited singlet state. When light emitted through such an excited singlet state is used, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (for example, see Non-Patent Document 1 below), study on materials emitting phosphorescence at room temperature has been actively. The similar device as above was disclosed in Non-Patent Document 2 or Patent Document 1 below.
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the excited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet can obtain the same performance as a cold cathode tube, and can be applied to illumination, which is noticed in various fields.
For example, many kinds of heavy metal complexes such as iridium complexes has been synthesized and studied (see Non-Patent Document 3 below).
An example employing tris(2-phenylpyridine)iridium as a dopant has been studied (see Non-Patent Document 1 below).
Further, M. E. Tompson et al. studies an example employing as a dopant L2Ir (acac) (in which L represents a bidentate ligand, and “acac represents acetyl acetone) such as (ppy)2Ir (acac) in “The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)”. Moon-Jae Youn. Og, Tetsuo Tsutsui et al. studies an example employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3}, tris(benzo-[h]-quinoline)iridium {Ir(bzq)3}, or Ir(bzq)2ClP (Bu)3 in “The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu).
In addition to attempts employing various iridium complexes as described above (see the Non-Patent Document 3 above-described), Ikai et al. employ a hole transporting material as a host of a phosphorescent compound in order to increase emission efficiency in “The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)”.
M. E. Tompson et al. employ various kinds of electron transporting materials doped with a new iridium complex as a host of a phosphorescent compound. Tsutsui et al. obtain high emission efficiency by incorporation of a hole blocking layer.
An external qauntum efficiency of around 20%, which is a theoretical threshold, is attained in green light emission, but the light emission efficiency extremely lowers in a high luminance region. Further, a sufficient emission efficiency is not attained in another color emission, particularly in blue light emission, where there is room to be improved.
In order to obtain high efficiency, a light emission layer containing plural materials has been sought. There is, for example, an organic electroluminescent device comprising a light emission layer containing a dopant and a hole transporting material and/or an electron transporting material. In such a device employing the plural materials and emitting a green light, high efficiency is obtained employing a phosphorescent dopant (see Patent Documents 2 to 4). However, a blue light-emitting device employing a fluorescent dopant as a light emission material is inferior in efficiency to a device employing a phosphorescent dopant, and is insufficient in efficiency and lifetime (see Patent Document 5).    Patent Document 1: U.S. Pat. No. 6,097,147    Patent Document 2: Japanese Patent O.P.I. Publication No. 2002-184581    Patent Document 3: Japanese Patent O.P.I. Publication No. 2003-68465    Patent Document 4: Japanese Patent O.P.I. Publication No. 2003-68466    Patent Document 5: Japanese Patent O.P.I. Publication No. 2004-311231    Non-Patent Document 1: M. A. Baldo et al., Nature, 395, p. 151-154 (1998)    Non-Patent Document 2: M. A. Baldo et al., Nature, 40, 17, p. 750-753 (2000)    Non-Patent Document 3: S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001).