The host materials containing carbazole and oxadiazole fragment for red triplet emitter in organic light-emitting diodes

Three new oxadiazole substituted carbazole derivatives 9-{4-[5-(4-tert-butylphenyl)-[1,3,4] oxadiazol-2-yl]-benzyl}-9H-carbazole (t-CmOxa), 9-[4-5-phenyl-[1,3,4]oxadiazol-2-yl-benzyl]-9H-carbazole (p-CmOxa) and 9-[4-5-biphenyl-[1,3,4]oxadiazol-2-yl-benzyl]-9H-carbazole (d-CmOxa) were successfully synthesized and characterized by spectroscopy (NMR, UV–vis, mass spectrum and photoluminescence) and cyclic voltammetry measurements. Employing t-CmOxa as a host and Ir(DBQ)2(acac) (DBQ = dibenzo[f,h]quinoxaline, acac = acetylacetonate) as the dopant emitter, OLEDs with structures of ITO/NPB(30 nm)/Ir(DBQ)2(acac): t-CmOxa (30 nm, x%)/Alq3(30 nm)/Mg0.9:Ag0.1 were fabricated without using BCP as the hole blocking layer. Red emission was obtained with CIE coordinates (x = 0.66, and y = 0.34) at 5 V and a very high external electroluminescent (EL) quantum efficiency of 9.5 ± 0.1%, and an energy conversion efficiency of 9.9 ± 0.1 lm/W were achieved for the device when the doping concentration x is equal to 4%.