Charge Transport in Oligothiophene Single Crystals

The charge transport in alpha-quaterthiophene and alpha-hexathiophene single crystals was studied by space charge limited current spectroscopy. Record high mobilities of up to 10 cm2/Vs were observed at low temperatures. The strongly anisotropic charge transport in these materials is explained by the molecular packing giving rise to a strong pi-pi* overlap in the molecular planes due to short sulphur-carbon contacts, but almost no molecular overlap along the ends of the molecules. Therefore, the charge carrier can be described as a two-dimensionally extended polaron. The transport mechanism is governed by two-dimensional band-like motion within the planes and a phonon-assisted, thermally activated hopping process perpendicular to them. To determine the influence of molecular packing on the charge transport two different single crystalline modifications have been investigated in order.