Manipulation of electron deficiency of delta -carboline derivatives as bipolar hosts for blue phosphorescent organic light-emitting diodes with high efficiency at 1000 cd m super(-2)

The effect of different linkages on thermal, electrochemical, photophysical, and optoelectronic properties was discussed in detail to illuminate the structure-property relationship of four delta -carboline derivatives, 5-(3'-(9H-carbazol-9-yl)-[1,1'-biphe nyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb), 3,3'-bis(5H-pyrido[3,2-b]i ndol-5-yl)-1,1'-biphenyl (BDCbBP), 5-(5-(3-(9H-carbazol-9-yl)phenyl)pyridin-3-yl)-5H-pyrido[3, 2-b]indole (Cz35PyDCb), and 5-(6-(3-(9H-carbazol-9-yl)phenyl)pyridin-2-yl)-5H-pyrido[3, 2-b]indole (Cz26PyDCb). Their triplet energies (E sub(T) = 2.78-2.96 eV) and the lowest unoccupied molecular orbital (LUMO) energy could be manipulated by the linkage and the number of delta -carboline units. These materials exhibited bipolar features with good hole- and electron-transporting properties. These host materials when doped with bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium([sm a ll caps]iii)) (FIrpic) demonstrated efficient blue phosphorescent organic light emitting diode (OLED) with reduced driving voltage, high efficiency, and negligible roll-off. Among these four materials, Cz35PyDCb performed as a host material for blue OLED with ideal current and power efficiencies (CE sub(max) at 44.7 cd A super(-1), and PE sub(max) at 40.2 lm W super(-1)), as well as good external quantum efficiency (EQE sub(max) at 22.3%). Most importantly, at the normal 1000 cd m super(-2) operation brightness, the efficiencies remained at 42.6 cd A super(-1) for CE (95% of CE sub(max)), 31.7 lm W super(-1) for PE (79% of PE sub(max)), and 20.1% for EQE (90% of EQE sub(max)), which indicated well-balanced carriers transporting and good confinement of the triplet excitons in the emitting layer, even at high current density. Moreover, introduction of the delta -carboline moiety could improve the thermal and morphological stabilities.