Conductivity mechanisms in poly(p-phenylene vinylene) light-emitting diodes at high and low bias

Polymer light-emitting diodes (LEDs) based on the structure ITO conducting glass /poly(p-phenylene vinylene)/metal (Al, In, Mg) have been investigated with particular emphasis on some anomalous and poorly understood features of the conduction mechanisms. At large forward bias (above 3 V, where electroluminescence is seen) the DC current is dominated by hole injection at the bottom ITO electrode and is not very sensitive to the top electrode metal or the fabrication conditions. It always increases exponentially with voltage at 20 degrees C, but studies on operating voltages and apparent ideality factors as a function of thickness indicate that the conduction mechanism probably involves thermally assisted tunnelling rather than a simple Schottky diode mechanism. In contrast, the current at low forward bias (0.1-3 V) is exceptionally sensitive to the top electrode material, fabrication conditions, and operating history of the device. Anomalous behaviour is often seen, particularly with new devices and with Mg or In electrodes. With Al electrodes, particularly after a top electrode annealing step, the behaviour becomes much more stable and resistive, and logJ increases linearly with V-1/4 over 3 orders of magnitude of current. The overall behaviour at low bias is controlled by the barrier to holes at the top electrode, but the role of filamentary defects, dopants, and the insulating interfacial layer is also discussed. (C) 1999 Elsevier Science S.A. All rights reserved.