Electric and magnetic field dependence of the conductivity in poly(3-hexylthiophene langmuir-blodgett films

The conductivity of undoped and NOPF₆-doped Langmuir-Blodgett thin films of poly(3-hexylthiophene) has been studied under strong electric (up to 60 kV/cm) and magnetic (up to 15 T) fields at low temperatures (down to 4.3 K). The electric-field dependence of the conductivity in doped films fits the theory of charging-energy-limited tunneling between highly conducting regions. In the electric-field-induced conduction mode, the magnetoresistance adopts a positive B² dependence which is argued to arise from the geometrical contribution corresponding to an extremely high and bias-dependent Hall mobility, over 10³ cm²/Vs below 40 K. However, the high Hall mobility was not measurable using a conventional four-point technique. The results can be reproduced in free-standing films of doped poly(3-hexylthiophene). Undoped Langmuir-Blodgett films show a linearly varying positive magnetoresistance the magnitude of which increases with decreasing temperature.