Electrical conductivity in Langmuir-Blodgett films of porphyrins: in-plane and through-the-film studies

Abstract

We have prepared Langmuir-Blodgett (LB) multilayers of several metal complexes of mesoporphyrin IX dimethyl ester and investigated their dark conductivity normal to the film plane (on aluminized glass substrates with a gold top electrode) and parallel to the film plane (on quartz substrates with two interdigitating gold top electrodes). The silver(II) complex is amongst the most conductive of LB film materials yet measured in the film plane (resistivity, 4 × 10 4 Ω m) and the conductance is anisotropic. The dependence of the current on film thickness, voltage, temperature and frequency has been extensively investigated in both directions and reveals that these multilayers have some unusual conductivity properties which are quite different from those normally reported for conventional insulating multilayers. Conduction through the film is barrier limited with some evidence that the barrier is in the first few monolayers of the film. Conduction along the film is bulk limited but does not scale with thickness, the thinnest films ( n ⩽ 5 layers) being anomalously resistive. The conduction in both directions is thermally activated, with the same activation energy (0.43–0.48 eV). The results are discussed in terms of two possible models: one model involves structural defects in the films and the other model assumes that a depletion layer occurs in the film near an interface as in a conventional semiconductor.