We investigate the influence of the hole-transporting layer (HTL) on the performance of bilayer vapor-deposited organic light-emitting diodes. Three different HTL materials were used: m-MTDATA, triphenyl-diamine, and naphthyl-phenyl-diamine. In all cases, Alq3 was the electron-transporting layer (ETL). We measure and compare the current density-voltage (J-V) and luminance-voltage (L-V) characteristics of these devices and we conclude that the operating voltage is controlled by the type of HTL used and the nature of the hole-injecting indium tin oxide/HTL interface. We found that the device quantum efficiency depends not only on the electron transport characteristics of the ETL but also on the energetics of the HTL/ETL interface. Analysis of the J-V characteristics suggests that current flow in bilayer devices cannot be described sufficiently by a single carrier theory; both hole and electron currents should be considered. © 1999 American Institute of Physics.