Electrical conductivity and relaxation in poly(3-hexylthiophene)

Abstract

We studied the complex conductivity of regioregular poly(3-hexylthiophene) (P3HT) in the temperature range between 193--333 K ($\ensuremath{-}80\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ to $60\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$) and in the frequency range from the direct current (dc) to 12 GHz. The identified relaxation process was investigated by quasielastic neutron scattering (QENS). The dielectric loss peak extracted from complex conductivity corresponds to local molecular motions having an activation energy of about 9 kJ/mol, which agrees well with the QENS results. The molecular motions of the hexyl side groups in poly(3-hexylthiophene) contribute to this relaxation process in P3HT, which is coupled with a cooperative charge transport along the P3HT chains. In the cutoff frequency range, the real part of complex conductivity $({\ensuremath{\sigma}}^{\ensuremath{'}})$ gradually transitions to a frequency-independent conductivity $({\ensuremath{\sigma}}_{\ensuremath{\propto}}^{\ensuremath{'}})$, which is thermally activated. The activation energy of ${\ensuremath{\sigma}}^{\ensuremath{'}}$ at 50 MHz is about 80 meV. In comparison, the activation energy of the dc conductivity, ${\ensuremath{\sigma}}_{0}$, is larger, about 280 meV, while the value of ${\ensuremath{\sigma}}_{0}$, is many orders of magnitude smaller than ${\ensuremath{\sigma}}_{\ensuremath{\propto}}^{\ensuremath{'}}$ We conclude that the local relaxation of the hexyl side groups contribute to a topological disorder in the polymer structure. As a consequence the energy barriers of the charge transport increase and the conductivity decreases. At 190 K the conductivity decreases from the disorder-free ${\ensuremath{\sigma}}_{\ensuremath{\propto}}^{\ensuremath{'}}$ of approximately $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{S}/\text{m}$ to ${\ensuremath{\sigma}}_{0}$ of about $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{S}/\text{m}$.