IV. Two-dimensional variable-range hopping in planar conjugated macromolecules

Electrical conductivity, thermopower and Hall effect measurements have been made on a series of polyacene quinone radical (PAQR) polymers as a function of temperature. The PAQR polymers, prepared by condensing pyrene with three different acidic compounds, are highly conjugated and show fairly high conductivities (in the range 10^-4-10^-3 Ω^-1 cm^-1 at room temperature). The thermo-power is positive and increases slightly with increasing temperature. The Hall coefficient, measured on one sample, is negative and decreases with temperature; the Hall mobility is less activated than the conductivity and is of the order of 3 x 10^-2 cm² V^-1 s^-1 at room temperature. In order to alter the electrical properties by doping, the PAQR polymers were treated with iodine. This results in an increase of the conductivity, a sign reversal of the thermopower and a lowering of the Hall mobility. The conductivity and thermopower data seem to be explained in their general features by the two-dimensional variable-range hopping model. The most compelling evidence is provided by the extremely good linearity of the log σT^3/2 plots versus T^1/3 over the measured temperature range (100-450 K), with very reasonable values of the parameters involved in this model. The two-dimensional system is identified with the large planar macromolecules which can be thought of as forming a structure of randomly packed flat ribbons.