Electronic transport in monolayers of phthalocyanine polymers

We report on a study of the electrical-transport properties of monolayers of phthalocyaninepolysiloxane (PcPS) polymers, and evaluate their potential for use as molecular wires. Monolayers have been deposited with the Langmuir-Blodgett technique on top of Si/SiO2 substrates with interdigitated electrodes. Current-voltage curves have been measured as a function of temperature for samples with varying electrode distance and number of monolayers. In the undoped state, the conduction is well described by the space-charge-limited-current model. From the data we obtain material characteristics such as the density of trap states within the gap and an estimate of the charge-carrier mobility. It appears that the conductivity is too low to yield a measurable current through a single PcPS polymer. Chemical doping and a field effect have been investigated. Oxygen is effective in doping the PcPS layers, resulting in a two orders of magnitude increase of the conductivity. Iodine is not effective as a dopant. By application of a voltage on a back-gate, we observe a field-effect-induced increase of the conductivity by three orders of magnitude. The effect however decays rapidly in time.

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