Polaron transport in triphenylene-based discotic liquid crystals

We report on the investigation of charge carrier transport in the columnar phase of the triphenylene-based discotic liquid crystal, hexapentyloxytriphenylene, by the time-of-flight technique. The hole mobility was found to be temperature and electric field dependent with a maximum value of 2$\times $10$^{-3}$ cm$^{2}$/Vs. Its temperature dependence is described by a T$^{-n}$ power law, with an electric field dependent n varying from 2.5 to 4.5 corresponding to electric field values from 5$\times $10$^{4}$ V/cm to 5$\times $10$^{3}$ V/cm, respectively. The drift velocity of charge carriers is a linear function of the electric field for small fields below 5$\times $10$^{4 }$V/cm and tends to saturation at strong fields. These results are interpreted in the framework of correlated polaron motion as described by the non-adiabatic low-temperature limit of Holstein's small polaron theory. The applicability of the Holstein polaron model to triphenylene-based materials showing thermally activated or temperature-independent carrier mobilities will be discussed.