Mechanism of charge transport in discotic liquid crystals

Abstract

A comparative study is reported of transient photoconductivity and ac conductivity in the liquid-crystalline columnar hexagonal (${\mathit{D}}_{\mathit{h}}$) phase of pure and p-doped hexakis(n-hexyloxy)triphenylene. In both materials, charge-carrier transport is dispersive, and the measured carrier mobilities along the direction of the molecular columns are very similar, ${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The dispersion arises from the intrinsic liquidlike disorder in the face-to-face packing of the triphenylene rings within the columns. Charge transport is found to be highly anisotropic (${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\sim}${10}^{3}$). These results are discussed in terms of possible mechanisms for carrier transport in discotic liquid crystals.