Synthesis and characterization of carbazole-based dendrimers as bipolar host materials for green phosphorescent organic light emitting diodes

A group of novel, carbazole-based dendrimers comprised of the electron-accepting dibenzothiophene core and the electron-donating oligo-carbazole dendrons, namely G1 SF and G2 SF, are synthesized utilizing the Ullmann C–N coupling reaction. The dendrimers are designed in such a way to show good solubility in common organic solvents, excellent thermochemical stability with decomposition temperatures(Td) up to430 8C, and high HOMO levels in a range from 5.45 e V to 5.37 e V. Results of density functional theory calculations(DFT) indicate G2 SF has an almost complete separation of HOMO and LUMO levels at the holeand electron-transporting moieties; while G1 SF exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Green phosphorescent OLEDs were fabricated by the spin coating method with the dendrimers as hosts and traditional green iridium phosphor as doped emitter. Under ambient conditions, a maximum luminance efficiency(hL) of 19.83 cd A~-(1)and a maximum external quantum efficiency of 5.85% are achieved for G1 SF, and 15.50 cd A (-1)and 4.57% for G2 SF.