Abstract
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 fields. The electron transfer from one electrode to the other through the cascaded molecules is treated step by step by considering hopping integrals between neighboring molecules. A rather simple computer calculation gives access to the tunnel current. A second approach is also given which consists in approximating the cascaded tunnel mechanism by a homogeneous rectangular insulating barrier, the height of which can be calculated simply from the basic parameters of the system. These two calculation methods have been applied to a practical system: Langmuir–Blodgett films made of a two-dimensional porphyrin polymer. The lowest unoccupied molecular orbital (LUMO) level in tetrapyridino–porphyrin bromide, which is the monomeric unit of the polymer, has been calculated and found at 2.8 eV below vacuum level. From this value and literature values for the other parameters, the tunnel current densities vs voltage have been calculated for various film thicknesses (2–5 monomolecular layers). The two calculation methods give quite close values for the tunnel current densities. The tetrapyridinoporphyrin polymeric Langmuir–Blodgett film is suggested to check the model experimentally, because once fabricated, the rigid polymeric film cannot undergo molecular reorganisation or evaporation and hence should contain few pinholes.
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