We report low operating voltage and high efficiency organic electroluminescence devices by using an ultrathin CsF layer with the conventional Mg:Ag cathode. At 5 V, devices with CsF/Mg:Ag cathode show a luminance of 800 cd/m(2) and a current efficiency of 4 cd/A compared to those (130 cd/m(2) and 2.6 cd/A) in standard Mg:Ag devices. In addition, the insertion of the CsF layer increases the maximum power efficiency from 1.6 to 3.3 lm/W. Mechanism responsible for this performance improvement is attributed to the substantial reduction of electron injection barrier by the free Cs metal atoms released at the organic/metal interface. This shallow contact also acts as a diffusion barrier for Mg, suppressing the formation of MgO and the degradation of Alq(3) molecules, and thus improves the long-term operational stability. (C) 2003 Elsevier B.V. All rights reserved.