Tunnel electron transport in metal/Langmuir-Blodgett film/metal systems

This paper presents a new method for the calculation of the tunnel current through a metal/multilayer of conjugated molecules/metal sandwich structure under low and moderate electric field. The electron transfer from one electrode to the other through the cascaded molecules is treated step by step by considering hopping integrals between neighbouring molecules. A rather simple computer calculation gives access to the tunnel current. This method has been applied to a practical system : Langmuir-Blodgett films made of a two-dimensional porphyrin polymer. The LUMO level in tetrapyridinoporphyrin bromide, which is the monomeric unit of the polymer, has been calculated and found to lie 2.8 eV below the vacuum level. From this value, and literature values for the other parameters, the tunnel current densities vs. voltage have been calculated for various film thickness (2 to 5 monomolecular layers). The tetrapyridinoporphyrin polymeric Langmuir-Blodgett film is suggested to check the model experimentally.