ABSTRACT: We studied the complex conductivity of regio-regular poly(3-hexylthiophene) (P3HT) in the temperature range between 193K to 333 K (-80 C to 60 C) and in the frequency range from the direct current (DC) to 12 GHz. The identified relaxation process was investigated by quasi-elastic 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-hexyl-thiophene) contribute to this relaxation process in P3HT, which is coupled with a cooperative charge transport along the P3HT chains. In the cut-off frequency range, the real part of complex conductivity (Sig) gradually transitions to a frequency independent conductivity (Sig_inf), which is thermally activated. The activation energy of Sig at 50 MHz is about 80 meV. In comparison, the activation energy of the direct current (DC) conductivity, Sig0, is larger, about 280 meV, while the value of Sig0, is many orders of magnitude smaller than Sig_inf 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 Sig_inf of approximately 5 ×10-4 S/m to Sig0 of about 1×10-9 S/m.
Citation: Physical Review B
Pub Type: Journals
AC conductivity, organic semiconductors, dielectric relaxation, quasielastic neutron scattering