New carbazole-based bipolar hosts for efficient blue phosphorescent organic light-emitting diodes

Four novel bipolar host materials, namely 9,9'-(4-(pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2CzPy), 9,9'-( 4-(pyridin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (2TCzPy), 5,5'-(4-(pyridin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzPy) and 9',9 '''-(4-(pyridin-2-yl)-1,3-phenylene)bis(9-phenyl-9H,9'H-3,3'-bicarbazole) (2PBCzPy) were designed and synthesized. Within each host material, one pyridine group serving as electron-transporting unit and two identical electron-donating groups serving as hole-transporting motifs were attached to the central benzene ring. These two donor units are located ortho and para to pyridine group, respectively. The four compounds showed excellent thermal stability with the glass-transition temperature up to 192 degrees C and decomposed temperature up to 571 degrees C. These hosts possessed triplet energies over 2.80 eV, indicating that they could be used as suitable hosts for blue phosphorescent organic light-emitting diodes (PHOLEDs). The bis[2-(4,6-difluorophenyl)-pyridinato-C2,N](picolinato) iridium(III) (FIrpic) based PHOLEDs hosted by 2CzPy and 2BFCzPy exhibited high device performance with maximum external quantum efficiencies (EQEs) of 17.8% and 17.4%, respectively. More importantly, 2CzPy exhibited a low efficiency roll-off at a brightness of 1000 cd m(-2) with an external quantum efficiency of 17.1% (3.9% roll-off). The results demonstrate that 2CzPy could be a promising host for blue PHOLEDs.