Star-shaped dendritic hosts based on carbazole moieties for highly efficient blue phosphorescent OLEDs

An efficient carbazole-based dendritic host material Cz-TCB for solution processed blue phosphorescent organic light-emitting devices (PhOLEDs) was designed and synthesized. The nonplanar star-shaped configuration of Cz-TCB limited the effective extension of their p-conjugation, leading to the same triplet energy of 2.86 eV. No noticeable signals of Cz-TCB related to glass transition is observed in the DSC curves. Atomic force microscopy ( AFM) measurements indicate the excellent thermal and morphological stability of the dendrimer. The performance of the novel host material based devices is far superior to those of the corresponding 1,3,5-tri(N-carbazolyl) benzene ( TCB) based devices, which is outstanding for a solution-processed blue PhOLED. The best performance of iridium(III) bis(4,6-( di-fluorophenyl)-pyridinato-N,C' (FIrpic) based devices show a turn-on voltage of 4.6 V, a maximum external quantum efficiency of 13.0%, and a maximum luminance efficiency of 25.7 cd A(-1). Moreover, the best performance of iridium( III) tris( 3,5-difluoro-4-cyanophenyl) pyridinato-N,C' (FCNIrpic) based deep-blue-emitting devices exhibits a turn-on voltage of 5.7 V, a maximum efficiency of 10.9 cd A(-1), and a maximum external quantum efficiency 6.4%. Our results demonstrate a promising approach to well-designed dendritic host materials for solution-processed blue PhOLEDs.