Toward high efficiency green phosphorescent organic light-emitting diodes by fine tuning the charge transporting properties of 1,2,4-thiadiazole based hosts

Abstract

Two novel host materials, from bipolar-type BzCzTHZ (5-(2-(9H-carbazol-9-yl)phenyl)-3-(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-1,2,4-thiadiazole) to electron-type DBzTHZ (3,5-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-1,2,4-thiadiazole), were designed and synthesized utilizing the unsymmetric intermediate of 3-bromo-5-chloro-1,2,4-thiadiazole as the center. Their thermal, photoelectronic, energy levels, charge transport mobility properties can be easily tuned by changing the radio of benzimidazole/carbazole moieties. Using a common device structure ITO/MoO3 (10nm)/TAPC (50nm)/TCTA (5nm)/host: Ir(ppy)3 (3wt%) (20nm)/TmPyPB (45nm)/LiF (1nm)/Al (100nm), highly efficient green electrophosphorescent devices were successfully achieved. Devices hosted by DBzTHZ and BzCzTHZ achieved maximum power efficiencies (ηp, max) of 95.2 and 86.2lm/W, corresponding to maximum quantum efficiency (ηEQE, max) of 23.4 and 22.2%, respectively. Moreover, the electron-type host DBzTHZ based device exhibits an ideal turn-on voltage (2.5V at 1.0cd/m2). These results demonstrate that electron-type hosts are competitive with bipolar hosts.