1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device including an organic layer interposed between a pair of electrodes, wherein the organic layer includes a titanium oxide derivative layer or a layer doped with a titanium oxide derivative so as to obtain higher luminescent efficiency and longer lifetime than a conventional organic electroluminescent device.
2. Description of the Related Art
Organic electroluminescent devices are self-emissive devices in which when a current is provided to a fluorescent or phosphorescent organic layer, electrons and holes are combined together in the organic layer, thereby emitting light. Organic electroluminescent devices are lightweight, and can be relatively easily manufactured using a small amount of components. In addition, organic electroluminescent devices can realize high-quality images, have wide viewing angles, and can realize moving pictures. Furthermore, organic electroluminescent devices can realize high color purity at low power consumption and a low operating voltage, and are thus suitable for portable electronic devices and large-scale screens.
In 1987, Tang and VanSlyke of Eastman Kodak Co. in the USA developed a stack-type thin film device that showed a luminosity of 1000 cd/m2 or more at a low operating voltage of 10 V or less. This development became thereafter a basis for organic EL research. In this patent, high luminosity can be realized by decreasing a thickness of all of the organic layers to about 130 nm through deposition of an emissive layer having an electron transporting capability and a hole transport layer, and by improving electron injection efficiency by including an Mg.Ag alloy having a low work function in a cathode.
Generally, an organic layer of an organic electroluminescent device is not formed of an emissive layer alone. Instead, the organic layer of an organic electroluminescent device has a multi-layer structure including, for example, an electron injection layer, an emissive layer, and a hole transport layer etc, to obtain higher efficiency and a lower operating voltage. For example, Japanese Patent Laid-open Publication No. 2002-252089 discloses an organic electroluminescent device including a hole transport layer.
In order to improve luminescent efficiency of an organic electroluminescent device, a charge balance application method, a charge generation layer, and TiO2 nanoparticles etc. were introduced. For example, U.S. Pat. No. 5,869,199 discloses an organic electroluminescent device including a triazole layer, and U.S. Pat. No. 6,447,934 discloses an organic electroluminescent device including two emissive layers interposed between a pair of electrodes. In addition, it was reported that use of TiO2 nanoparticles increase luminescent efficiency of a device, and a multi-layer device in which TiO2 nanoparticles are dispersed showed higher luminosity and luminescent efficiency than a mono-layer polymer device (Synthetic Metals 111-112, 2000. 207-211). However, TiO2 nanoparticles must be uniformly dispersed to obtain a uniform layer.