Design And Synthesis Of Host Materials For Blue Phospho Rescent Organic Light-emitting Diodes

Organic light-emitting diodes (OLEDs) have attracted broad attentions from both academia and industry over the last three decades because of their potential applications in flat panel display and solid state lighting. After development of thirty years, the efficiency of OLED improved significantly, especially after the proposal of phosphorescent Organic light-emitting diodes (PHOLEDs). It is known that the PHOLEDs have great potential in achieving high internal quantum efficiency-100% by harvesting both singlet and triplet excitons. The phosphor emitters should be doped uniformly in the host materials to suppress the multi-particle quenching effects. For now, the efficiency and lifetime of red or green PHOLEDs are high enough for industrialization, but the efficiency of blue PHOLEDs is still too low and the blue PHOLEDs are not stable, so it is important to do research to improve the performance of blue PHOLEDs. Compared with red and green PHOLEDs, blue PHOLEDs have more strict requirements for the host materials.In order to improve the performance of blue PHOLEDs, it is essential to develop better host materials for blue PHOLEDs. In this thesis, based on 1,2,4-oxadiazole, we designed and synthesized new host materials for vacuum evaporated blue PHOLEDs and electron-transport host materials for solution processed deep blue PHOLEDs, and hole-tranpsort host materials for solution processed blue PHOLEDs based on triphenylamine annulated fluorene.1. Symmetric host materials for blue PHOLEDs based on 1,2,4-oxadiazole:in the four different isomers of oxadiazole,1,2,4-oxadiazole is not so widely used compared with 1,2,5-oxadiazole and 1,3,4-oxadiazole, and 1,2,4-oxadiazole has the highest triplet energy which could be used in blue PHOLEDs. Thus, in this part of work, we firstly utilized 1,2,4-oxadiazole as the electron-withdrawing group in the design and synthesis of two bipolar host materials for blue PHOLEDs. Triplet energies of the two materials are 2.71 eV and 2.81 eV, respectively. So they are suitable host materials for FIrpic in PHOLEDs.2. Asymmetric host materials for blue PHOLEDs based on 1,2,4-oxadiazole:in the last part of work we got two materials with high triplet energy based on 1,2,4-oxadiazole. In this part we used the asymmetry property of 1,2,4-oxadiazole, designed and synthesized two asymmetric host materials for blue PHOLEDs. The small changes in the molecule structures greatly enhanced the performance of blue PHOLEDs.3. Electron-conducting host materials for solution processed deep blue PHOLEDs based on 1,2,4-oxadiazole:although vacuum thermal evaporation has been generally used as a fabrication process of OLEDs, the vacuum deposition method has several problems such as inefficient use of material, poor scalability, high equipment cost, high vacuum pressure, and complicated color patterning processes. These problems can be solved by introducing solution coating processes, but solution processed OLEDs have higher requirements for their host materials such as good solubility, good film-forming property and high glass-transition temperature. Although PVK and OXD-7 system is widely used in solution processed OLEDs, the triplet energy of OXD-7 is only 2.69 eV, not high enough for deep blue PHOLEDs. So we replaced 1,3,4-oxadiazole with 1,2,4-oxadiazole to obtain three new electron-conducting host materials which could be used in deep blue PHOLEDs.4. Hole-conducting host materials for solution processed blue PHOLEDs based on triphenylamine annulated fluorene:in the last part of work, we designed and synthesized three electron-conducting host materials working with PVK, the efficiencies of the devices were not very high but the work voltages of the devices are very high. Because PVK is a polymer, work voltages of the devices are high. Therefore we designed and synthesized four hole-conducting host materials working with OXD-7, the work voltages of the devices were much lower than that of the PVK hosted device.