The effect of carrier injection and transport of hosts on organic electrophosphorescent device

Recently the organic electrophosphorescent host materials have attracted increasing attention. The selection principle of suited host materials is important to obtain high efficient electrophosphorescence. In this paper, we report the effect of carrier injection and transport properties of hosts to organic electrophosphorescent devices. A system, consisting of a red phosphorescent [Ru(4,7-Ph2-phen)3]2+ dopant and two blue-emitting hosts THPH and TCPC introducing carbazole substituent groups, was studied. Both the hosts showed similar electronic properties, forming film ability and energy transfer degree to dopant, however, EL efficiency with TCPC host significantly exceeds that with THPH host. The Cyclic Voltammogram curves and single carrier devices of hosts TCPC and THPH show that TCPC with carbazole groups possesses stronger hole injection and transport ability, and this induced TCPC is more suited host than THPH. This result demonstrates our previous study that the electrochemical properties of dopant [Ru(4,7-Ph2-phen)3]2+ molecule, which reflects the carrier trapping ability of guest, may be a basic design criterion for the selection of host.