Light-emitting characteristics of organic light-emitting diodes with the MoO_x-doped NPB and C₆₀/LiF layer

The hole ohmic properties of the MoO_x-doped NPB layer have been investigated by analyzing the current density-voltage properties of hole-only devices and by assigning the energy levels of ultraviolet photoemission spectra. The result showed that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing both the thickness and the doping concentration of a hole-injecting layer (HIL) of N, N′-diphenyl-N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) doped with molybdenum oxide (MoO_x) which was inserted between indium tin oxide (ITO) and NPB. For the doping concentration of above 25%, the device composed of a glass/ITO/MoO_x-doped NPB (100 nm)/Al structure showed the excellent hole ohmic property. The investigation of the valence band structure revealed that the p-type doping effects in the HTL layer and the hole concentration increased at the anode interfaces cause the hole-injecting barrier lowering. With both MoO_x-doped NPB as a hole ohmic contact and C₆₀/LiF as an electron ohmic contact, the device, which is composed of glass/ITO/MoO_x-doped NPB (25%, 5 nm)/NPB (63 nm)/Alq₃ (37 nm)/C₆₀ (5 nm)/LiF (1 nm)/Al (100 nm), showed the luminance of about 58,300 cd/m² at the low bias voltage of 7.2 V.