In-situ evaluation of the carrier mobility using Mott-Schottky-equivalent analysis in organic light emitting diodes

An equation describing the carrier accumulation in organic heterojunction devices is presented. The equation is the counterpart of the Mott-Schottky equation in semiconductor device physics, where the insulating depletion layer is replaced by the conductive space charge layer. This equation allows us to evaluate the mobility and the carrier density of the organic materials simultaneously from the analysis of the impedance spectrum using diode configurations. We tested 77 hole transporting materials developed in our company under fixed standard six layer structures. The mobilities are compared with those evaluated by the transit time measurement using impedance spectroscopy of hole only devices. The evaluated mobilities by the in-situ method have better correlation with the driving voltages than those by the conventional method based on the transit time effect.