Materials and structures of organic light emitting devices have been improved rapidly since C. W. Tang, et al. of Eastman Kodak Company demonstrated a high-luminance in 1987, and the devices have recently been put into practical use in displays of car audio systems and cellular phones, etc. To further widen the application of these organic EL (electroluminescent) devices, materials for increasing the light emitting efficiency or the durability, full-color display systems, etc. are now being actively developed. Particularly in view of applying the devices to middle- or large-sized panels and illuminators, the light emitting efficiency needs to be increased to achieve a higher luminance. However, conventional organic EL devices utilize light emission from an excited singlet state, that is, fluorescence, and since the formation ratio of singlet excitons to triplet excitons is 1/3 in electroexcitation, the upper limit of the internal quantum efficiency in organic light emitting device is 25% (equivalent to the external quantum efficiency of 5% when the external extraction efficiency is 20%).
Under the circumstances, M. A. Baldo, et al. disclosed that an iridium complex, etc. capable of emitting phosphorescence in the excited triplet state at the room temperature can achieve the external quantum efficiency of 7.5% (equivalent to the internal quantum efficiency of 37.5% when the external extraction efficiency is assumed to be 20%), which exceeds the conventional external quantum efficiency upper limit of 5%. Further, a higher efficiency of almost 20% was achieved by modifying a host material or structure of the device (Appl. Phys. Lett., Vol. 90, P. 5048, 2001), and this has been attracting attention as a method for achieving an extra-high efficiency.
However, this phosphorescent iridium complex is a low molecular weight compound and is formed into a film by a vacuum deposition method. Though the vacuum deposition method has been widely used for forming films of low molecular weight light emitting materials, the method is disadvantageous in that a vacuum apparatus is required and that the larger the area of the organic film to be formed is, the more difficult it is to form the film with a uniform thickness or a highly defined pattern. Thus, the method is not necessarily suitable for mass production of a large area panel.
In the circumstances, in relation to method suitable for producing device having a larger light-emission area and mass production method therefor, methods of forming light emitting polymer materials into films by spin coating methods, ink-jet methods, printing methods, etc. have been developed. These technologies have been widely used for fluorescent polymer materials and also, application of such a method in phosphorescent polymer materials is being developed. It has been reported that an external quantum efficiency of more than 5% can be obtained by using a phosphorescent polymer material with a side chain containing a phosphorescent moiety and a carrier transporting moiety (Proceedings of The 11th International Workshop on Inorganic and Organic Electroluminescence (EL2002), p. 283-286, 2002).
However, the above phosphorescent polymer material shows an external quantum efficiency of about 6%, which is only slightly more than the external quantum efficiency limit 5% of the fluorescent devices. Thus, this material cannot achieve the expected high external quantum efficiency of the phosphorescent devices.
Meanwhile, many research institutes have been making various attempts to apply pi-electron-based organic compounds to optical materials or electronic function materials. Particularly, as to boron compound having a boron atom in its molecule, the presence of an empty p-orbital in the boron atom is expected to contribute to exhibiting highly specific optical and electronic properties. However, generally, a boron compound, which has a disadvantage of being unstable in air and water, is not suitable for use as a material. In relation to this problem, there have been recent reports that a boron compound can be made stable in air and water by allowing the compound to have a bulky structure (see, for example, Journal of American Chemical Society, Vol. 120 p. 10776, 1998), and thus the potentiality of boron compound to be used in non-linear optical material, organic EL material or the like has been explored. However, such study reports only describe about the fluorescent property of the compound in a solution state, and it is still the case that study results so far obtained are too immature to put a boron compound having a bulky structure into practical use. Particularly, there are strong demands for application of boron compound to organic EL material, and although studies on technologies of this kind have been vigorously made, none of the studies has developed a technology where sufficient properties are exhibited. An organic EL device basically comprises a structure where a charge-transporting layer and/or an organic compound serving as light-emitting material are sandwiched between two electrodes. An organic EL device is desired to have a high efficiency at low power consumption, and for this purpose, it is necessary to use an organic compound serving as a material which exhibits high luminous efficiency.
There is an article on a research where a boron compound containing a heterocyclic ring is used as a charge-transporting agent (Journal of American Chemical Society, Vol. 120 p. 9714, 1998), however, the article does not refer to light emitting property or suitability as a light-emitting material of the boron compound at all. The article only includes a report implying that a device containing the boron compound, owing to its lower current density, exhibited an improved luminous efficiency at the same luminance level as compared with a device not containing a boron compound. Moreover, the boron compound used therein, being a low-molecular weight compound, requires a film-forming method by vacuum deposition or the like as described above which is not always suitable for mass-production process of large-area panels. The international publication WO00/40586 discloses an example using a boron compound in an organic EL device, however, the boron compound used therein is also a low-molecular weight compound and only exhibits a low efficiency, leaving unsolved the problem involved in the above-mentioned research article.