Research Of Organic Electroluminescent Materials And Organic Light-emitting Devices

Organic light-emitting devices(OLEDs) is not only a great point of new flat panel displays, but also one of the first choices of green lighting source of the 21 century, during to their merits such as low drive voltage, high brightness, wide visual angle, quick response, and simple fabrication technics. As the present time, low power-conversion efficiency, short useful life and bad long-term stability are the critical problems to block the utility and marketization of OLEDs. However, exploiting organic light-emitting materials with high efficiency and stable physic characteristics, choosing appropriate electrode materials, searching for new film fabrication technics, and optimizing device configuration, improving the efficiency and useful life of device, and questing for the best scheme to realize full color are still the primary aims of study work. This paper summarizes and introduces the research in my doctor period, accompanied by the national Natural Science Foundation of China(NO.60276026) and provincial Natural Science Foundation of Gansu(ZS031-A25-012-G). The key points are listed blow: study of fabrication technics and characteristics of blue light-emitting devices;research of the surface and interface characters of emitting layer and the action of buffer layer in device based on the study of light-emitting efficiency, useful life and stability;white OLED is triumphantly fabricated based on the study of the infection to device by adulteration, and the luminescence principle and the characteristics of optics and electrics are researched;improvement mechanism is analyzed on the base of the synthesis of electrode study. The analyses are listed blow:1. The characters of blue organic light-emitting material, the fabrication technics of blue OLED and current illumination characteristic of device are studied, and infections of buffer material CuPc in BOLED is analyzed systematically. The characters and synthesis technics of blue organic light-emitting material LiBq₄ are described systematically, and the structure token of LiBq₄ is carried out by infrared absorption spectrum, nuclear magnetic resonance spectrum, absorption spectrum and fluorescence spectrum. So we can conclude: fluorescence of LiBq₄ takes place on 8-hydroxyquinoline ligand, and LiBq₄ is an organic ligand (quinoline ring)luminescent material which is disturbed by central atom;the blue shift of the peak in fluorescence spectrum is a result of infection of LiBq4 structure and B-0 bonding intensity and the unbond between B atom and N atom.The fabrication technics of BOLEDs using LiBq4 as emitting layer are described systematically, and the current illumination characteristic of device with buffer layer is analyzed compared to the device with no buffer layer. The drive voltage is found to have increased and the brightness becomes lower under the same electric voltage after CuPc is added as a buffer layer. The main reason may be: CuPc has depressed the potential barrier of hole injection, and increased the number of holes on the interface of PVK:TPD/LiBq4. Inverted drift electric field comes into being in HTL, and the hole injection is blocked, which induces the current decrease under the same electric voltage;Meanwhile, the number of holes injected into emitting layer LiBq4 is also decreased, which causes the decrease of the probability of exciton formation. The build-in electric field in LiBq4 is strengthened, which increases the probability of exciton dissociation, and the brightness and efficiency of devices using CuPc is decreased.2. The surface and interface of organic layer and buffer layer in OLEDs are analyzed by AFM and XPS. The research of interface is carried out based on the study of the luminescence character of TPD/Alq3, and we find: there is an interaction on the interface between TPD molecules and Alq3 molecules, and this caused a red shift of the peak in luminescence spectrum compared to the fluorescence spectrum of Alq3. So this action is the reason of exciplex formation on the interface of TPD/Alq3 in devices.Through the analysis of surface and interface of LiBq4/ITO using AFM and XPS, LiBq4 is found to has decomposed by thermal reason when it is evaporated and deposited onto the ITO film, which causes the dissociation of B atoms on the interface and the number of B atoms is decreased;The interface of LiBq4/ITO is not smooth and there are many cracks and interspaces, which will absorb a great deal of gas molecules.The analysis of buffer layer CuPc in OLEDs using AFM and XPS validates that CuPc and PTCDA can cover the defects on ITO surface basically;In organic light-emitting devices with multi layer structure, the addition of CuPc or PTCDA can restrain the diffusion of chemical constituent to HTL by ITO, which reduces the luminescence quenching centers, and weakens the retrograde effect of ITO to HTL.This improves the injection efficiency and stability of devices, which is propitious to improve the characteristics and useful life.3. White OLED is triumphantly fabricated based on the study of the infection to device by adulteration, and the luminescence principle and the characteristics of optics and electrics are researched. During the manufacture of WOLED, Rubrene is added into 8-hydroxyquinoline Lituium (Liq) film. When the ratio is 1.1%, white light is got (color coordinate is x=0.286,y=0.357). The drive voltage is 8v, and the brightness is 1400cd/m2 when the voltage increases to 25v. The recombination and emission mechanism of carriers is found to be a result of the conjunct action among the capture of carriers, Forster energy transfer and charge transfer process.4. Improvement mechanism of electrode is also studied based on the synthesis of research process of electrode. To the anode material ITO, Carbon pollution is the main infection to the work function of ITO. Moderate oxygen plasma disposal is a ideal method relatively, which is not only can increase ITO work function but also reduce the resistivity, and the square resistance is decreased. To cathode material, alloy electrode, Al/LiF double layer electrode and compound electrode are thought to be well. A thin insulator layer is added in metal/insulator electrode to improve electron injection efficiency, and this paper has analyzed the three kinds of explanations to the improvement of equilibrium condition of electron and hole.