1. Field of the Invention
The present invention relates to a light emitting device, in particular, an organic electroluminescence (EL) device.
2. Description of the Related Art
Applied researches have been vigorously conducted on an organic EL device because of the potential of the device to serve as a light emitting device having high-speed responsiveness and high luminous efficiency.
The light emitting layer of an organic EL device is known to be formed only of a light emitting material or to be formed of a light emitting dopant and a host material. The reason why the light emitting layer is formed of two components, that is, the light emitting dopant and the host material is that the concentration quenching (self-quenching) of the light emitting dopant should be suppressed. In general, when the concentration of the light emitting dopant in a medium is high, energy transfer between molecules of the light emitting dopant is apt to occur, and the molecules form an association body to produce a heat inactivation path, with the result that the emission quantum efficiency of the device reduces. Therefore, the light emitting layer of a conventional organic EL device has been generally formed of a light emitting dopant at a content of about 0.1% to 20% and a host material, and, in an ideal case, the light emitting dopant must be dispersed in the host material at a molecular level.
An organic EL device using a phosphorescent dopant as a light emitting dopant has been attracting attention in recent years because the device can stably emit light with high efficiency. A metal coordination compound using any one of various metals as a center metal is used as the phosphorescent dopant.
In addition, organic EL devices using nanoparticles as light emitting centers have been announced. Those devices are classified into a device using nanoparticles each made of an inorganic light emitting material such as CdSe or CdSeS (S. Coe et al., “Nature” 2002, 420, p. 800, and E. Jang et al., “Chemical Communications” 2003, p. 2964-2965) and a device using metal oxide nanoparticles or metal nanoparticles none of which is a light emitting material (Japanese Patent Application Laid-Open No. 2004-335438 and Japanese Patent Application Laid-Open No. 2003-257671).
The idea on which each of the devices using nanoparticles each made of an inorganic light emitting material described in S. Coe et al., “Nature” 2002, 420, p. 800, and E. Jang et al., “Chemical Communications” 2003, p. 2964-2965 is based is interesting, but the external quantum efficiency of each of the devices is remarkably as low as 1% or less.