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-e mitting devices (OLEDs) is the frequency dependent investigation of their c omplex resistance. We report impedance spectroscopy measurements of electro luminescent (EL) single-layer and double-layer systems. It is shown that th ere are important differences found in the corresponding equivalent electri cal circuits. In EL single-layer devices the electrical field decreases hom ogeneously over the entire layer thickness, resulting in an equivalent circ uit of one RC component. In contrast, for the EL two-layer devices two RC c omponents are necessary for the description. In both systems no indications of an insulating layer or a depletion region are found. During the measure ments of the impedances of operated EL single-layer and EL two-layer system s, both under inert gas and in air, an increase of the resistance is observ ed. However, there are no signs for an insulating layer covering the entire contact area, through which the transport of charge carriers has to occur.