Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultrathin hole-injection layer of molybdenum oxide

We show that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing the thickness of a hole-injection layer (HIL) of molybdenum oxide (MoO3) inserted between indium tin oxide and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). From results of the electroluminescence (EL) characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO/MoO3/α-NPD.