Role of ultrathin Alq3 and LiF layers in conjugated polymer light-emitting diodes

We report that an ultrathin layer of tris-(8-hydroxyquinoline) aluminum (Alq(3)) interposed between a bilayer cathode of LiF/Al and an emissive layer can significantly improve electroluminescence efficiency in a polymer light-emitting device (PLED). At a current density of 35 mA/cm(2), a poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenyl-enevinylene] (MEH-PPV)-based PLED with a cathode of Alq(3)/LiF/Al has a quantum efficiency of 2.5%, considerably higher than that of a MEH-PPV-based PLED with a bare Al or a LiF/Al cathode. Interfacial characteristics of MEH-PPV/LiF/Al and MEH-PPV/Alq(3)/LiF/Al were investigated by photoelectron spectroscopy, and the results were used to explain the difference in device performance. (C) 2003 American Institute of Physics.