Long-range Coherent Charge Transport in Physisorbed Molecular Layers*

The charge transport in molecular ensembles is intrinsically related to a series of carrier-molecule quantum interactions which result in variety device concepts, such as organic transistors, organic light-emitting diodes, and sensors [1]. At the nanoscale, such interactions are described as either direct tunneling for the 1-10 nm range or activated hopping for longer distances. This work describes the first experimental evidence of long-range (> 10 nm) coherent tunneling occurring for physisorbed small-molecule semiconductors-based vertical junctions. By investigating the charge transport in copper-phthalocyanine (CuPc) thin films, we have identified the sequential tunneling as the long-range charge transport mechanism [2]. Such a sequential coherent process is not only different from the direct tunneling, but intrinsically distinct from the hopping conduction. Consequently, a gradual transition from direct tunneling to sequential tunneling, within the 10-22 nm barrier width, to hopping conduction can be verified. Our results contribute to bridging the gap between molecular and organic electronics.
[1] A. Vilan et al, Chem. Rev. 117 (2017), 4248–4286
[2] L. Merces, R. F. de Oliveira, D. H. S. de Camargo, C. C. Bof Bufon , J. Phys. Chem. C 121, (2017), 16673.