Electronic transport properties in conducting Langmuir-Blodgett films

Conducting Langmuir-Blodgett (LB) films made of a ternary compound N-docosylpyridinium-tetracyanoquinodimethane-iodine have been recently obtained by Ruaudel and co-workers (1985), after an in-situ oxidation of a precursor LB film. This paper describes a new well controlled method used to perform the iodine 'doping' in the precursor LB film. The chemical reactions involved in this process are followed by UV-visible spectroscopy. Direct current and microwave conductivities of samples obtained in that way have both been measured in the range 300-120 K and are correctly fitted by the relation sigma = sigma ₀ exp - Delta E/kT with two different activation energies: 0.15 eV and about 0.08 eV respectively. Thermoelectric power measurements have also been performed in the range 298-176 K and the Seebeck coefficient, the value of which is 26 mu V K^-1 at room temperature, exhibits a linear-like behavior against 1/T with a slope dS/d(1/T) approximately=11 mV. All these measurements suggest a semiconducting behavior and are tentatively analysed in terms of an intrinsic semiconductor model. However, as an important discrepancy about the mobility value appears, the authors suggest a presumable interpretation in terms of hopping model between localised states. But they conclude that more precise information about the inplane molecular arrangement and the carrier density is needed to provide a more coherent model for the transport properties in these conducting films.