Low-temperature transport in high-mobility polycrystalline pentacene field-effect transistors

The charge transport in high-mobility polycrystalline pentacene field-effect transistors is investigated in the temperature range from 1.7 to 40 K for carrier concentrations ranging from ${10}^{11}$ to $5\times {10}^{12}{\mathrm{cm}}^{\mathrm{-}2}.$ For hole densities below $6\times {10}^{11}{\mathrm{cm}}^{\mathrm{-}2}$ the conductance in the channel is thermally activated and can be described in the framework of an Anderson localization in two dimensions. The charge carriers are localized in the band tails. At low temperatures the transport mechanism crosses over to Mott-type variable-range hopping and finally to Efros-Shklovskii-type hopping due to the presence of a Coulomb gap. Above the critical concentration of $6\times {10}^{11}{\mathrm{cm}}^{\mathrm{-}2}$ the conductance is more or less temperature independent. However, effects of weak localization are observed below 10 K.