Eq uivalent-circuit analysis of organic light-emitting diodes by using the frequency-dependent response of an ITO/Alq(3)/Al device

Abstract

We used the frequency-dependent response of an ITO/Alq(3)(60 nm)/Al device at two different bias voltages to perform an equivalent-circuit analysis of organic light-emitting diodes. The complex impedance Z of the device was measured in the frequency range of 40 Hz similar to 1 MHz. A Cole-Cole plot showed that there were two dielectric relaxations at a bias below the turn-on voltage and one relaxation at a bias above the turn-on voltage. We were able to interpret the frequency-dependent response in terms of the equivalent-circuit model of a contact resistance R-s in series with a parallel combination of a resistance RP and a capacitance C-p. We obtained a contact resistance R-s of around 90 Omega, mainly from the ITO anode.