Controlled formation of charge depletion zones by molecular doping in organic pin-diodes and its description by the Mott-Schottky relation

Abstract

Molecular doping of organic semiconductors is a key technology for highly efficient organic light-emitting diodes. Nevertheless, the underlying fundamental mechanisms are under discussion. This is because of the complex situation of structural disorder and strong polaronic coupling in such systems. We provide for the first time a systematic study of the formation of charge depletion zones in organic pin-diodes comprising molecular doped hole and electron transport layers. Impedance spectroscopy is employed to study the capacitance of these depletion zones. In particular, we show that the voltage dependent capacitance function obeys the Mott-Schottky relation concerning the influence of doping and the effect of an additional depletion zone given by the intrinsic interlayer. From temperature dependent measurements of the depletion capacitance, we can deduce the amount of active dopant states, their activation energy, and the spatial field distribution within the junction. The measured activation energy of the dopant states and the overall value of active dopant molecules suggest a highly efficient doping process, where dopant molecules act as shallow acceptor states.