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

An electrostatic space charge layer of nanometer thickness is formed at org anic film/metal interfaces due to the displacement of electrons from the me tal to the film. This space charge layer directly affects the current-volta ge (I-V) characteristics of artificially layered rhoadamine-dendorimer thin films, which show a step structure similar to a Coulomb staircase. The cap acitance between one rhodamine-dendorimer molecule and a metal electrode is calculated using a sphere-plane model, assuming that excess electronic cha rges are displaced from the plane metal electrode into the film. This capac itance 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 meta l/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-den dorimer/polyimide/Al junctions. (C) 2001 American Institute of Physics.