The Matching Of Host-guest Materials And Its Influence To The Organic Electrophosphorescent Devices

In passed decades, major breakthroughs have led to significant improvements inthe performance of organic light-emitting devices due to the use of phosphorescentheavy metal complexes. The triplet-triplet annihilation and triplet-polaronannihilation is occurred at high concentration of triplet state exciton, which is due tolong triplet exciton life of phosphorescent materials. Incorporatingelectrophosphorescent metal complexes into the host organic/polymer materials is aneffective way to resolve this problem. Generally, the compounds with aryl groups,for example phenyl and carbazole, were selected as host materials. The host materialshave to have good carrier transfer ability, wide energy bandgap, good film capability,high glass transformation temperature, longer exciton life, etc. While guest-hostrelationship also affects performance of electrophosphorescent devices. In this thesis,high efficient phosphorescent guests and organic/polymer hosts of different typeswere selected, and we studied factors affecting matching of guest and host andresolve these problems. In Chapter 2, The Ru(II) complex Ru(dpp) with good stability, highphosphorescent efficiency and reversible electrochemistry behavior was selected asguest, three blue light-emitting materials with different carrier transfer ability wereselected as hosts. The Cyclic voltammogram curve of Ru(dpp) shows that Ru(dpp)molecules have four reversible redox processes, i.e. three electroreduction processesand one electrooxidation process. The electrochemical properties of Ru(dpp)molecule, which reflects the carrier trapping ability of host, may be a basic designcriterion for the selection of polymer matrix and device configuration. When ann-type polymer PBD was used as host, the excess electrons were trapped soon bythree reduction states of Ru complex site, radiationless deactivation of high energyexcited states is bad, so the device performance was poor. Whereas the hole transfermaterial was used as host and a hole block layer was inserted, the probability ofmulti-electron trapping at the Ru(dpp) site is reduced, thus the efficiency of deviceswere effectively increased. The devices with Ru(dpp)-PVK blend as light-emittinglayer could achieve higher luminous efficiency up to 8.6 cd/A. The deviceperformance with PF of more-balanced charge transporting ability as host was bad,due to phase separation of host and guest.In Chapter 3, the blend of electron transporting material and hole transportingmaterial was selected as host to easily adjust charge carrier transporting ability ofhost. The blend of PVK and PBD was selected as host for Ru(dpp) guest. Theadjusting content of PBD in the host, the device performance is best at PBDconcentration of 40 %, the luminous efficiency reached 5.71 cd/A with simple devicestructure, which is remarkably higher than efficiency of devices with only PVK orPBD as host. Further, for Ru(dbeb)2+ guest the blend of PVK-PBD (40 %) was stillselected as host to fabricate devices. The adjusting concentration of guest, theexternal quantum efficiency reached 3.0 cd/A with simple device structure, and purered colour emitted. While the blend of small molecular hole transporting materialCBP and PBD was selected as host for Ru(dpp) to fabricate devices, and the deviceperformance is remarkably higher than devices with only CBP or PBD as host.In Chapter 4, to avoid phase separation of components in blend and defects ofself-polymer, the oligomer TCPC and THPH were selected as hosts for Ru(dpp) withspiting-coating method to fabricate devices, and device performance of Ru(dpp):TCPC is better. The Electrochemistry data and single charge carrier transportingdevices proved that hole inject and transfer of TCPC is stronger which of THPH, soin light-emitting layer of devices with TCPC as host the hole carrier is more, whichis favorable for efficient recombination of charge carriers. The luminous efficiency ofRu(dpp): TCPC devices reached 2.13 cd/A. when the TCPC is host for Ir(ppq) theluminous efficiency is 1.30 cd/A with simple device structure. When TCPC is hostand blue material doped green and red phosphorescent dyes the white light devicesreached maximum brightness of 2367 cd/m2 and efficiency of 1.53 cd/A. Theseresults proved that TCPC is good host material.