Conduction of Metal–Thin Organic Film–Metal Junctions at Low Bias

Abstract

A model for the low bias electric conductance of junctions, consisting of a thin organic film (TOF) positioned between two metallic electrodes (M), has been developed. In contrast with other theoretical studies, the proposed model relies on the energy band picture of M–TOF–M systems. Theoretical analysis of the band-like transport has shown that the electronic flow between metallic electrodes can exist in M–TOF–M junctions only if injected charge carriers are able to overcome the potential barrier with the thickness-dependent height. Such an obstacle to the motion of injected charges in the TOF conduction band arises due to the bending of this band caused by carriers localized in structural traps. Two regimes of the zero bias conductance of M–TOF–M junctions have been studied theoretically for situations, where charges overcome the thickness-dependent barrier undergoing either thermally activated or tunneling transitions. Analytical expressions derived for the zero bias conduction in these two regimes enable us to specify key physical parameters controlling charge transport across the film and provide results consistent with observations. On the basis of our findings, we infer that thermally activated and tunneling conductances can be distinguished by temperature and thickness dependencies. Theoretical results obtained for the electric conductance of M–TOF–M systems in the tunneling regime are compared with those obtained for assemblies in which TOF has been replaced by a single molecule. Distinctions between transport properties of these two systems and their similarities resulting from the present model are discussed.