Heterolayer light-emitting diodes based on poly-phenylene vinylene

In order to enhance the quantum efficiency of poly-p-phenylene vinylene (PPV) light-emitting diodes (LEDs), we have fabricated metal/insulator/polymer (MIP) LEDs and heterolayer LEDs based on PPV and oxadiazole polymers. The current-voltage (I-V) characteristics and electroluminescence (EL) intensity of the MIP structures display a pronounced dependence of the insulator thickness and we detect an increase in the quantum efficiency of more than a factor of 30 at an AlOx, layer thickness of 3-6 nm. The device characteristics are qualitatively understood within inorganic metal insulator semi-conductor (MIS) theory and can be explained by a voltage-dependent barrier for minority carrier injection in connection with a hole-blocking barrier at the PPV/insulator interface. Our oxadiazole polymers used in the heterolayer polymeric devices are characterized by a high thermal stability and excellent film-forming properties. These materials act as efficient hole-blocking, electron transport and injection layers in PPV-based LEDs and we measure a significantly improved device performance with external quantum efficiencies of more than 0.5%. Temperature-dependent investigations point to a relatively balanced charge carrier injection and reveal the influence of space charge limited currents on the device performance at low temperature. (C) 1997 by John Wiley & Sons, Ltd.