Application Of Quinolate Chelates In Organic Light-Emitting Diode Devices

Organic light-emitting diode (OLED) is a type of flat panel displays with the following characteristics: The materials adopting organic/polymer have wide range of options, through the design, assembly and tailoring of the structure of organic molecules, the different need of displaying any color from red to blue light can be realized; with low driving voltage characteristics, only 3¹2V direct current voltage; high luminance and luminous efficiency; wide lighting vision and fast response; ultra-thin, light weight and full curing self-luminous; the device can be made on flexible substrates and bent; wide operating temperature range; the relatively simple molding process and without size restrictions.The OLED devices mainly consist of anode indium tin oxide (ITO) glass, the organic layers and cathode low work function metal materials. In order to improve the interface performance of metal Al and the organic layer, a thin interface layer of LiF or other small molecule material is usually asserted between the Al and organic layer, so as to improve the electron injection and device stability. Although the small molecule compounds have favorable film forming, high carrier mobility and nice thermal stability, they are prone to crystallization which results in decreased device stability. Therefore the metal chelates with stable structure are worth concerning. In this thesis, we designed and synthesized a series of chelates with 8-hydroxyquinoline and its derivatives as the ligands. These compounds are well thermal stabilized and not easily crystallized, they can be used as the electron injection layer materials in the OLED devices.Firstly, we synthesized compounds 2-formyl-8-hydroxyquinoline, 2-diphenyl methyl-quinoline, 2,2'-(1,4-phenyl)-bi-(2,1-divinyl)-quinolin-8-ol and 2,2-vinyl-bi-quinolin-8-ol. Secondly, we prepared sodium-quinolate chelates by dehydration reaction between the ligands 8-hydroxyquinoline, 2-methyl-8- hydroxyquinoline, 2-diphenylmethyl-quinoline, 2,2'-(1,4-phenyl)-bi-(2,1-divinyl)- quinolin-8-ol, 2,2-vinyl-bi-quinolin-8-ol and sodium hydroxide in anhydrous solvent. Finally, we selected tris(8-hydroxyquinoline)aluminum (Alq₃) and 2-methyl-9, 10-bis(naphthalene-2-yl)anthracene (MADN) as the electron transport material separately, then prepared two group multi-layer OLED devices. The devices performance were investigated and the conclusions are as follows:(1) Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) results show that all the sodium-quinolate chelates have high glass transition temperature (higher than 130℃), and will not crystalline. The thermal decomposition temperature is above 250℃, which show good thermal stability of compounds, it can guarantee the stability of OLED devices.(2) Cyclic voltammetry (CV) test and density functional theory (DFT) calculation results show that the energy level values of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of sodium-quinolate chelates mainly depend on the the ligand 8 - hydroxyquinoline and its derivatives, the contribution of metal ions is almost negligible. Furthermore the energy level values of LUMO are lower (nearly -1.0eV), which can meet the requirements of electron injection materials.(3) The results of device characterization show that: when Alq₃ was the electron transport layer, the sodium-quinolate chelates can replace the commonly used LiF as electron injection layer material and also get good device performance similarly. The light-emitting efficiency is about 10.5cd/A. But when the MADN was used as the electron transport layer, the small conjugated quinoline metal complexes Naq and NaMeq show better performance than LiF. the light-emitting efficiency is about 14.0cd/A, twice of the LiF device. The device with Ph₂Naq as the electron injection layer shows the lowest light emitting efficient. Despite the introduction of two benzene groups on 8-hydroxyquinoline increases the conjugation degree, it results in lower relative density of metal.