Space charge effect and the step voltages in metal/polyimide/rhodamine–dendorimer/polyimide/metal junctions

Abstract

An electrostatic space charge layer of nanometer thickness is formed at organic film/metal interfaces due to the displacement of electrons from the metal to the film. This space charge layer directly affects the current–voltage (I–V) characteristics of artificially layered rhoadamine–dendorimer thin films, which show a step structure similar to a Coulomb staircase. The capacitance between one rhodamine–dendorimer molecule and a metal electrode is calculated using a sphere-plane model, assuming that excess electronic charges are displaced from the plane metal electrode into the film. This capacitance is found to increase as the excess charge increases which leads to a change in the voltage steps in the I–V characteristics as observed in metal/polyimide/ rhodamine–dendorimer/polyimide/metal junctions. Such a voltage step is derived theoretically, and will explain the difference between Au/polyimide/rhodamine–dendorimer/ polyimide/Al and Al/polyimide/rhodamine–dendorimer/polyimide/Al junctions.