Low-temperature electrical anisotropy of self-assembled organic films

We have examined the low-temperature characteristics of the charge transport in self-assembled organic films with thickness varying from 5 to 60 deposited bilayers of polypyrrole and polythiophene acetic acid. When the temperature is progressively decreased, independent measurements of the longitudinal and transversal components of the resistivity tensor of these thin films reveal that while the conductivity along the surface directions has the expected semiconducting profile, the charge transport in the transversal direction exhibits a positive temperature coefficient (PTC). For the samples with a small number of deposited bilayers we observe that at very low temperatures the PTC characteristics disappear and a tunnelinglike current begins to dominate the transversal charge transport.