Tunneling and hopping conduction in Langmuir-Blodgett thin films of poly(3-hexylthiophene)

The electrical conductivity and field-effect mobility of undoped and ${\mathrm{NOPF}}_6$-doped Langmuir-Blodgett thin films of poly(3-hexylthiophene) have been measured as a function of temperature, electric field, and film thickness. These properties are compared with those of free-standing films. In undoped samples the conductivity is best described by variable-range hopping, whereas in the doped and dedoped states the model of charging-energy-limited tunneling between conducting islands is applicable. The data indicate metallic charge carrier densities within the conducting islands in the highly doped state. At electric fields close to the dielectric breakdown, a possible onset to Fowler-Nordheim tunneling is observed. As the samples become dedoped, the electrical properties indicate an increase in the average size of conducting islands, which are suggested to consist of polymer backbones separated by the alkyl side chains of neighboring molecules.