Studies On Novel High Performance Organic Light Emitting Devices

Organic light-emitting diodes (OLEDs) have received increasing attention in the development of advanced flat panel display technology because of its low weight, broad visual angle, low cost, fast response speed, active emitting, high brightness, high efficiency and the capability to support full color display. Thanks to the efforts of several world famous companies, the performance of OLEDs has been improved significantly with the adoption of novel materials, suitable structures and process technologies. However, although the production of OLEDs has been commercialized, much work remains to enhance the product performance and achieve market advantage. In this dissertation, we have made some meaningful works relating to the improving efficiency and high brightness of OLEDs. It includes the following items:(1) The improvement of carrier injection and transport. Firstly, we investigate the influence of the hole injection layer on the performance of the OLED. Evidence shows that m-MTDATA exhibits a dense film structure and fine surface morphology, leading to easy hole migration. Furthermore, a low-voltage driving of OLEDs has been achieved by doping 4F-TCNQ into m-MTDATA layer. Secondly, we obtain a highly efficient OLED by using a new oxidiazoles, an efficient electron-transporting molecular materials. Finally, we have realized the efficient electron injection and transport in organic light-emitting diodes using an electron-transport layer composed of BPhen with Liq co-deposited layer. With this cohost strategy, we improve the power efficiency by～21 % while reduce the driving voltage by～13%.(2) Enhanced efficiency in white OLED due to the improvement of charge-injection balance. Firstly, we have developed highly efficient white organic electroluminescent devices based on a blue fluorescent styrylamine dopant EBDP. For the white device, the maximum luminance of 32291cd/m², maximum efficiency of 8.31cd/A and the luminance of 1413cd/m² at a current density of 20mA/cm² are obtained. The slow decrease in efficiency with the increase in current density indicates weak exciton-exciton annihilation, which is resulted from the large steric hindrance caused by the non-planar structure of the fluorescence dye EBDP. These performance are further improved by inserting m-MTDATA:4F-TCNQ between the ITO and NPB layers and Bphen: Liq between the cathode and EML. We also study the white device efficiency where the relative electron - hole balance is adjusted by means of molecular doping with different concentration of 4F-TCNQ.(3) Tandem organic light emitting diode. The charge generation layer of WO₃ with high transmittance can generate electrons on one side and holes on the other side efficiently, and it is an effective intermediate layer for highly efficient tandem organic light-emitting devices. We produce a pure-blue tandem organic light-emitting device (OLEDs) employing the blue fluorescent emitter, ter (9,9-phenylfluorene) fluoride (TPFF). The charge generation region consists of a Li-doped electron transport layer and a highly transparent WO₃ thin film. We obtain highly efficient pure-blue emission from the device. The maximum luminous efficiency of 2.08 cd/A with 1931 Commission Internationale De L'Eclairage coordinates of (0.156, 0.078) are attained. The efficiency of the white OLED has been improved by using tandem structure with two low-voltage white units connected by charge generation layer of Bphen:Li/WO₃.(4) Enhancement of red organic light-emitting diodes via cascade energy transfer. We have demonstrated that efficient red electroluminescence can be realized via cascade energy transfer from Alq₃ to fluorescent dye C545, then from C545 to DCJTB. An additional dopant, C545, is used to assist the energy transfer from Alq₃ to the red dopant. Compared with devices where the emitting layer is composed of Alq₃ and DCJTB only, the emission efficiency and color purity are improved. We attribute these improvements to the assistant dopant C545, which leads to the increased efficiency in energy transfer from Alq₃ to DCJTB.