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The electrical conduction of regioregular poly(3-hexylthiophene) (RR-P3HT) and regiorandom poly(3-hexylthiophene) (RRa-P3HT) doped with iodine is studied as a function of doping level. The electrical conductivity shows the thermally-activated behavior at low doping levels below 10% for RR-P3HT and 18% for RRa-P3HT. At high doping levels higher than 10% for RR-P3HT and 20% for RRa-P3HT, the electrical conductivity shows the three-dimensional variable-range hopping. The change in the conduction mechanism is explained qualitatively in terms of the growth of the density of states at the Fermi level and the dimensional crossover of the conduction path. The difference in the doping levels where the change occurs in RR-P3HT and RRa-P3HT is understood in terms of the carrier localization length, that is, the spatial extent of polaronic excitations in these polymers.

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