An electroluminescent device (EL device) is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules, and aluminum complexes as materials for forming a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
An organic EL device (OLED) is a device changing electrical energy to light by applying electricity to an organic electroluminescent material, and generally has a structure comprising an anode, a cathode, and an organic layer between the anode and the cathode. The organic layer of an organic EL device may be comprised of a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer (which comprises host and dopant materials), an electron buffer layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc., and the materials used for the organic layer are categorized by their functions in hole injection material, hole transport material, electron blocking material, light-emitting material, electron buffer material, hole blocking material, electron transport material, electron injection material, etc. In the organic EL device, due to an application of a voltage, holes are injected from the anode to the light-emitting layer, electrons are injected from the cathode to the light-emitting layer, and excitons of high energies are formed by a recombination of the holes and the electrons. By this energy, luminescent organic compounds reach an excited state, and light emission occurs by emitting light from energy due to the excited state of the luminescent organic compounds returning to a ground state.
The most important factor determining luminous efficiency in an organic EL device is light-emitting materials. A light-emitting material must have high quantum efficiency, high electron and hole mobility, and the formed light-emitting material layer must be uniform and stable. Light-emitting materials are categorized into blue, green, and red light-emitting materials dependent on the color of the light emission, and additionally yellow or orange light-emitting materials. In addition, light-emitting materials can also be categorized into host and dopant materials according to their functions. Recently, the development of an organic EL device providing high efficiency and long lifespan is an urgent issue. In particular, considering EL characteristic requirements for a middle or large-sized panel of OLED, materials showing better characteristics than conventional ones must be urgently developed. The host material, which acts as a solvent in a solid state and transfers energy, needs to have high purity and a molecular weight appropriate for vacuum deposition. Furthermore, the host material needs to have high glass transition temperature and high thermal degradation temperature to achieve thermal stability, high electro-chemical stability to achieve a long lifespan, ease of forming an amorphous thin film, good adhesion to materials of adjacent layers, and non-migration to other layers.
Iridium(III) complexes have been widely known as phosphorescent materials, including (acac)Ir(btp)2 (bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate)), Ir(ppy)3 (tris(2-phenylpyridine)iridium) and Firpic (bis(4,6-difluorophenylpyridinato-N,C2)picolinato iridium) as red-, green- and blue-emitting materials, respectively.
A light-emitting material can be used as a combination of a host and a dopant to improve color purity, luminous efficiency, and stability. Since host materials greatly influence the efficiency and lifespan of the EL device when using a dopant/host material system as a light-emitting material, their selection is important. At present, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known as phosphorescent host materials. Recently, Pioneer (Japan) et al., developed a high performance organic EL device using bathocuproine (BCP) and aluminum(III) bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq), etc., as host materials, which were known as hole blocking materials.
Although these materials provide good luminous characteristics, they have the following disadvantages: (1) Due to their low glass transition temperature and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum. (2) The power efficiency of the organic EL device is given by [(Tr/voltage)×luminous efficiency], and the power efficiency is inversely proportional to the voltage. Although the organic EL device comprising phosphorescent host materials provides higher luminous efficiency (cd/A) than one comprising fluorescent materials, a significantly high driving voltage is necessary. Thus, there is no merit in terms of power efficiency (Im/W). (3) Furthermore, the operational lifespan of the organic EL device is short, and luminous efficiency is still necessary to improve.
Thus, in order to embody excellent properties of the organic EL device, materials constituting the organic layers in the device, in particular host or dopant materials constituting a light-emitting material, should be suitably selected. In this regard, WO 2013/146942 A1 discloses the compounds linked with two carbazoles via arylene group, as a host material. However, the organic EL devices comprising the compounds recited in the above publication still does not satisfy efficiency, lifespan, etc.
In this regard, the present inventors have tried to find host compounds that can provide superior efficiency and long lifespan compared to the conventional host materials, and have found that the compounds of the present disclosure provide a device with high luminous efficiency and long lifespan.