R. Jones et al., Conductivity mechanisms in poly(p-phenylene vinylene) light-emitting diodes at high and low bias, THIN SOL FI, 340(1-2), 1999, pp. 221-229
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 elect
roluminescence is seen) the DC current is dominated by hole injection at th
e bottom ITO electrode and is not very sensitive to the top electrode metal
or the fabrication conditions. It always increases exponentially with volt
age at 20 degrees C, but studies on operating voltages and apparent idealit
y factors as a function of thickness indicate that the conduction mechanism
probably involves thermally assisted tunnelling rather than a simple Schot
tky diode mechanism. In contrast, the current at low forward bias (0.1-3 V)
is exceptionally sensitive to the top electrode material, fabrication cond
itions, and operating history of the device. Anomalous behaviour is often s
een, particularly with new devices and with Mg or In electrodes. With Al el
ectrodes, 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 bia
s is controlled by the barrier to holes at the top electrode, but the role
of filamentary defects, dopants, and the insulating interfacial layer is al
so discussed. (C) 1999 Elsevier Science S.A. All rights reserved.