We report on high-efficiency polymer light-emitting diodes (PLEDs) based on poly [2-methoxy-5-(3',7'-dimethyloctyloxyl)]-1,4-phenylene vinylene (OClC10) with LiF-modified cathodes. Devices with different cathodes are made and characterized by the electroabsorption technique to measure their built-in voltage. Devices with a LiF/Al bilayer cathode or a LiF:Al composite cathode, all show significantly improved performance as compared to those with bare Al cathodes. The improvement is correlated with enhanced electron injection due to a decrease of the electron injection barrier, which is also indicated by the electroabsorption measurements. The same effect is also observed with thin film between Ag and OClC10 does not improve LiF(0.6 nm)/Mg cathodes. However, inserting the same LiF the device performance. Cathodes composed of ultra-thin films of LiF(0.6 nm)/Al(1 nm) or LiF:Al(2 nm) covered by Ag (100 nm) show the same performance as LiF(0.6 nm)/Al bilayer cathode or a LiF:Al composite cathode, indicating that the enhancement is specific to LiF and Al. Our experiments can be explained by assuming that Li-ions can dissociate from LiF and diffuse into the OClC10 layer, leading to an n-type zone close to the polymer/cathode interface. This n-doped layer at the interface facilitates electron injection at the cathode/polymer interface and eventually leads to the formation of an Ohmic contact. (C) 2004 Elsevier B.V. All rights reserved.