Negative impedance of organic light emitting diodes in AC electrical response

Abstract

In previous papers, we introduced a novel analysis technique of the impedance spectroscopy of organic light emitting diodes to elucidate their layer-to-layer carrier dynamics and proposed a novel carrier transport model. In this article, we will discuss the carrier dynamics in the voltage region where the layer-to-layer dynamics cannot be distinguished. There appear several specific characteristic features in frequency- and voltage-dependence of complex impedance, the negative impedance, and the diffusion impedance. In the negative bias voltage region, where neither carrier injection nor accumulation occurs, conductance shows ω1/2 frequency dependence. On the other hand, at the higher voltage region where all the semicircles in Cole-Cole plot coalesce, there appear two types of negative impedance, which have ω1/2 and a Debye-like frequency dependence. We consider that the ω1/2 dependences are attributed to the diffusion impedance (Warburg impedance) coupled with and without the interfacial charge transfer reaction. The Debye-like negative impedance is attributed to the carrier trap at the interface of the Alq3 layer.