Nonconjugated Carbazoles: A Series of Novel Host Materials for Highly Efficient Blue Electrophosphorescent OLEDs

Abstract

A series of carbazole derivatives was synthesized and their electrical and photophysical properties were investigated. It is shown that the triplet energy levels of these hosts are higher than that of the most popular blue phosphorescent material iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2′] picolinate (FIrpic) and the most extensively used phosphorescent host material 4,4′-N,N′-dicarbazole-biphenyl (CBP). These new host materials also showed good thermal stability and high glass transition temperatures (Tg) ranging from 78 to 115 °C as the linkage group between the carbazoles was altered. Photophysical measurements indicate that the energy transfer between these new hosts and FIrpic is more efficient than that between CBP and FIrpic. Devices incorporating these novel carbazole derivatives as the host material doped with FIrpic were fabricated with the configurations of ITO/NPB (40 nm)/host:FIrpic (30 nm)/BCP (15 nm)/AlQ (30 nm)/LiF (1 nm)/Al (150 nm). High efficiencies (up to 13.4 cd/A) have been obtained when 1,4-bis (4-(9H-carbazol-9-yl)phenyl)cyclohexane (CBPCH) and bis(4-(9H-carbazol-9-yl)phenyl) ether (CBPE) were used as the host, respectively. Efficiencies obtained from these devices are significantly higher than the efficiency obtained with CBP as the host. At a bias voltage of ∼20 V the maximum luminances of 20 342 and 17 766 cd/m2 were achieved for CBPCH- and CBPE-based devices, respectively, which demonstrates that CBPCH and CBPE can be excellent host materials for blue electrophosphorescent OLEDs.