Investigation of the electronic properties of organic light-emitting devices by impedance spectroscopy

A key for the understanding of the electronic properties of organic light-emitting devices (OLEDs) is the frequency dependent investigation of their complex resistance. We report impedance spectroscopy measurements of electroluminescent (EL) single-layer and double-layer systems. It is shown that there are important differences found in the corresponding equivalent electrical circuits. In EL single-layer devices the electrical field decreases homogeneously over the entire layer thickness, resulting in an equivalent circuit of one RC component. In contrast, for the EL two-layer devices two RC components are necessary for the description. In both systems no indications of an insulating layer or a depletion region are found. During the measurements of the impedances of operated EL single-layer and EL two-layer systems, both under inert gas and in air, an increase of the resistance is observed. However, there are no signs for an insulating layer covering the entire contact area, through which the transport of charge carriers has to occur.