In general, the term “organic light-emitting phenomenon” refers to a phenomenon in which electric energy is converted to light energy by means of an organic material. The organic light-emitting diode using the organic light-emitting phenomenon has a structure usually comprising an anode, a cathode and an organic material layer interposed therebetween. Herein, the organic material layer may be mostly formed in a multilayer structure comprising layers of different materials, for example, the hole-injecting layer, the hole-transporting layer, the light-emitting layer, the electron-transporting layer, the electron-injecting layer and the like, in order to improve efficiency and stability of the organic light-emitting diode. In the organic light-emitting diode having such a structure, when a voltage is applied between two electrodes, holes from the anode and electrons from a cathode are injected into the organic material layer, the holes and the electrons injected are combined together to form excitons. Further, when the excitons drop to a ground state, lights are emitted. Such an organic light-emitting diode is known to have characteristics such as self-luminescence, high brightness, high efficiency, low drive voltage, wide viewing angle, high contrast and high-speed response.
The materials used for the organic material layer of the organic light-emitting diode can be classified into a light-emitting material and a charge-transporting material, for example, a hole-injecting material, a hole-transporting material, an electron-transporting material and an electron-injecting material, according to their functions. The light-emitting material can be classified into a high molecular weight-type and a low molecular weight-type, according to their molecular weight, or fluorescent materials, emitting light from singlet excited state of electrons, and phosphorescent materials, emitting light from, triplet excited state of electrons, according to their emitting mechanism. Further, the light-emitting material can be divided into a blue, green or red light-emitting material and a yellow or orange light-emitting material required for giving more natural color, according to a light-emitting color.
On the other hand, an efficiency of a device is lowered owing to maximum luminescence wavelength moved to a longer wavelength due to the interaction between molecules, deterioration of color purity and reduction in light-emitting efficiency when only one material is used for the light-emitting material, and therefore a host/dopant system can be used as the light-emitting material for the purpose of enhancing color purity and light-emitting efficiency through energy transfer. When dopants, having smaller energy band gap than that of hosts, are mixed into light-emitting layer in a small amount, excitons generated from the light-emitting layer are transported to the dopants and then emit light efficiently. In this case, since a wavelength of emitted light shifts from a wavelength of the hosts to a wavelength of the dopants, light having a desired wavelength can be obtained depending on the type of the dopant.
In order to allow the organic light-emitting diode to fully exhibit the above-mentioned excellent characteristics, a material constituting the organic material layer in the device, for example, a hole-injecting material, a hole-transporting material, a light-emitting material, an electron-transporting material and an electron-injecting material should be essentially composed of a stable and efficient material. However, the development of a stable and efficient organic material layer material for the organic light-emitting diode has not yet been fully realized. Accordingly, the development of new materials is continuously desired.