The application of a cathode system comprising of an ultrathin CsF layer and an overlaid ytterbium (Yb) metal in organic electroluminescent devices is reported. This cathode shows dramatically reduced operating voltage and improved current and power efficiencies, both in small molecule or polymer based devices. The improvement is attributed to the dissociation of CsF upon Yb deposition to liberate low-work function Cs which reduces the electron injection barrier at the cathode/organic interface. Thermodynamically, the CsF dissociation is a substrate-independent process, and should be applicable for most organic based devices. Devices with this bilayer cathode system also show good operational stability. (C) 2003 Elsevier Science B.V. All rights reserved.