Solution processable small molecular host materials for blue and white phosphorescence OLEDs

Abstract

Three new solution processable small molecular host materials based on bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescence (FIrpic dopant) organic light-emitting diodes. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (ET) of 2.73 eV, tetrakis-[3,3',5,5'-(9H-carbazol-9-yl)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-amine (CzPAPm) are two host materials potentially being bipolar for charge transport due to the electron deficient unit of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient ET (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum-thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpassing CzPAMe devices reach current efficiency as high as 9.32 cd/A (or power efficiency of 4.97 lm/W), which is one of the highest of the kind. Corresponding single-layer white phosphorescence OLEDs are also fabricated with the small molecular host material demonstrated herein.