Highly efficient multilayer organic pure blue light emitting diodes with substituted carbazoles compounds in the emitting layer

Bright blue organic light-emitting diodes (OLEDs) based on 1, 4, 5, 8, N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole (N, N'-diethyl-3, 3'-bicarbazyl) (DEC) as emitting layers or as dopants in a 4, 4'-bis(2, 2'-diphenylvinyl)-1, 1'-biphenyl (DPVBi) matrix are described. Pure blue light with the CIE coordinates (x = 0.153, y = 0.100), electroluminescence efficiency ?EL of 0.4?cd?A?1, external quantum efficiency ?ext of 0.6% and luminance L of 236?cd?m?2 (at 60?mA?cm?2) were obtained with PMC as an emitter and the 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenantroline (BCP) as a hole-blocking material in five-layer emitting devices. The highest efficiencies ?EL of 4.7?cd?A?1 and ?ext = 3.3% were obtained with a four-layer structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158, y = 0.169, ?peak = 456?nm). The ?ext value is one the highest reported at this wavelength for blue OLEDs and is related to an internal quantum efficiency up to 20%.