Study Of The Carrier Transporting Characteristics And Luminescence Properties Of Phosphorescent Materials

Phosphorescent materials are crucial to improve the luminescence and efficiency of organic light emitting diodes(OLED), because its internal quantum efficiency can reach 100%. So the study of optical and electrical properties of phosphorescent materials is propitious for the further development of phosphorescent OLED. Phosphorescent materials were generally doped into different host materials as emitting components, not only played an important role in emitting light but also had a profound influence on carrier transporting properties.This thesis focuses on the studies of hole trapping ability, hole directly injecting and transporting abilities and emitting characteristic of material(t-bt)2Ir(acac). Firstly, based on the basic blue fluorescence device ITO/NPB/DPVBi/BPhen/Mg: Ag, inserted(t-bt)2Ir(acac) into different positions of device and discussed the influence on devices performance. The comparisons showed that(t-bt)2Ir(acac) has remarkable hole-trapping ability. Then I designed hole-only and electron-only devices to further prove the conclusion. The results showed(t-bt)2Ir(acac) nearly had influence on the injecting and transporting of electrons, but reduced the current density of holes. So(t-bt)2Ir(acac) surely had the hole-trapping ability. It indicated that it was possible to use a few phosphorescent materials to balance carriers and optimize the performance of florescent devices and even phosphorescent devices.Secondly, based on the excitons distribution of first experiment, inserted high triplet energy host material CBP between DPVBi and(t-bt)2Ir(acac) to enhance yellow light from(t-bt)2Ir(acac). Combined with blue light from DPVBi, the device got very stable white color light emitting with performance of 22405 cd/m2 and 3.84 lm/W. But because of the undoped inefficient fluorescent material DPVBi, efficiencies of the white devices were not very high. So doped DPVBi with host materials and added spacer layer to optimize the device. The host and spacer materials were CBP and CBP, TCTA and CBP, TCTA and BPhen, respectively. Devices used TCTA and BPhen got 25010 cd/m2 and 12.98 lm/W white light emitting.Finally, I studied about the hole directly injecting ability of(t-bt)2Ir(acac). Hole injecting and transporting layers were NPB, CBP, CBP:(t-bt)2Ir(acac) and TPBi:(t-bt)2Ir(acac), respectively. By comparing the performance of the four devices, conclusion that holes could directly inject into material(t-bt)2Ir(acac) and transport to emitting area was made. Then I optimized the hole directly injecting devices to reach the level of devices with NPB. Firstly investigated the influence of doping concentration changes, then selected suitable but not complicated doping ratio. Then changed the thickness of emitting layer and doped(t-bt)2Ir(acac) into electron transporting layer TPBi to balance carriers and broaden the area of the light-emitting region, finally got 45891 cd/m2 and 27.51 lm/W yellow phosphorescent devices, both higher than NPB devices. After that added blue phosphorescent material FIrpic to get white light-emitting devices. Doped FIrpic into CBP, TPBi and used TCTA as spacer layer, got 26020 cd/cm2 and 18.99 lm/W near white light emitting devices, eventually.