A Research Of High-performance Organic Electroluminescent Materials And Devices

Organic light-emitting diodes (OLED) have attracted much attention because of their potential applications for multicolor emission with many advantages such as active emission, fast response, wide view-angle and simple fabrication process. 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 OLED. However, exploiting organic light-emitting materials with high efficiency and stable physic characteristics, choosing appropriate electrode materials, searching for new film fabrication process, 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. The investigation of essential mechanism and characterization in organic electroluminescence (EL) field is imperative for fulfilling commercialization requirements. Consequently, this work is dedicated to the systematic study of following issues.By controlling the reaction conditions and optimizing the process parameters, we get the high purity crude products of electron-transporting material Alq and hole-transporting material NPB, whose purity reached 98.2% and 95.5% respectively. Through the method of elevating the temperature of the three zones, we carry out further purification sublimation and get the higher purity material of Alq and NPB, the purity of which is higher than 99.9%.This method not only increased the purity of the materials, but also greatly enhanced the efficiency of the purification and reduced the product losses during the purification process. The data of XRD showed that with the increase of the purification times, the crystal size of the Alq and NPB material increased and the performance of the material was improved. The OLED devices were made with the various purity of Alq and NPB as electron-transporting and hole-transporting layer respectively. The experiment results showed that with the increase of the material purity, the performance of the devices was improved. With the improvement of the material purity, the concentration of the impurities or defects in the materials was reduced, which would reduce the luminescence quenching effect caused by impurities or defects. And the concentration decrease of impurities or defects will help to the movement of the exciton and increasing the possibility of the recombination of holes and electrons in the devices, resulting in the performance improvement of the devices.Through the co-doping method, we obtained blue, green and red light-emitting devices based on a wide band gap host material ADN. The structure of the blue device was ITO (80nm)/NPB (30nm)/ ADN: DPAVB: TBPE (30nm)/ Alq (30nm)/ LiF (1 nm)/Al(100nm), the maximum luminance brightness of which reached 5626 cd/m2 and the maximum current luminance efficiency of which got to 6.2 cd/A with CIEx,y=0.15,0.19.The structure of the green device was ITO (80 nm)/NPB (40nm)/ ADN: C545T: DMQA (30nm)/ Alq (30nm)/LiF (1nm)/Al(100nm), the maximum luminance brightness of which reached 15153 cd/m2 and the maximum current luminance efficiency of which got to 10.8 cd/A with CIEx,y=0.30,0.62.The structure of the blue devise was ITO (80 nm)/NPB (40 nm)/ ADN: C6: DCJTB (30 nm)/ Alq (30nm)/LiF (1nm)/Al(100 nm), the maximum luminance brightness of which reached 12847cd/m2 and the maximum current luminance efficiency of which got to 4.9 cd/A with CIEx,y=0.61,0.38.The bipolar character of ADN serves to trap the excess holes generated at high current drive condition thus preventing the imbalance of the carriers injected in the device and improving the performance of the device. In addition, co-dopants of the heterogeneous light-emitting molecules may decrease the possibility of self-quenching from the interaction of the homogenous molecules at the same doping concentration, which will also result in the improvement of the color purity and emission efficiency.Based on the design of the molecule, we have synthesized a new non-doped blue light-emitting material TOBP and a new red-doped material DADIN. TOBP is one kind of phenanthroline derivatives with oxadiazole group and it has good thermal stability with glass transition temperature Tg = 142℃and the thermal decomposition temperature Td = 325℃. The structure of the blue device with TOBP as the emitting layer is ITO (80nm)/NPB (30nm)/ TOBP (30nm)/ Alq (30nm)/LiF (1 nm)/Al(100nm), the maximum luminance brightness of which reached 4078 cd/m2 and the maximum current luminance efficiency of which got to 2.7 cd/A with CIEx,y=0.15,0.10. DADIN is an electroluminescent material with symmetrical structure. Compared with Kodak classic red dopant material DCJTB, the synthesis and purification process of DADIN is simpler and it is easier to achieve large-scale preparation. The structure of the red device with DADIN as the dopant is ITO (80 nm)/NPB (40nm)/ Alq:DADIN (30 nm)/ Alq (30nm)/LiF (1nm)/Al(100nm), the EL wavelength of which was 650 nm and the maximum current luminance efficiency of which got to 2.3 cd/A with CIEx,y=0.64,0.36 that was very close to the standard red color. Compared to the device with DCJTB as the dopant material, the device with DADIN as the dopant material had showed higher color purity of and higher current luminance efficiency.