Conjugation length dependence of the charge transport in oligothiophene single crystals - art. no. 035209

The intrinsic charge-transport mechanism in oligothiophene single crystals is investigated by temperature-dependent space-charge-limited current spect roscopy. The mobility for in-plane transport in these layered p-type semico nductors follows a power law behavior typical for coherent bandlike charge transport. A value as high as 80 cm(2)/V a is measured far the mobility in the high-temperature polymorph of alpha -sexithiophene at low temperatures. in contrast to that the charge transport perpendicular to the molecular pl anes can be described by incoherent hopping motion revealing the typical th ermally activated temperature dependence. Consequently, the charge carrier can be characterized as partly delocalized between molecules of the molecul ar layer but localized within this molecular plane. The effective hole mass of the polaronic charge carrier increases with increasing temperature due to electron-phonon interaction. The strength of this electron-phonon intera ction decreases with increasing conjugation length of the oligothiophene mo lecule leading to higher room temperature mobilities for alpha -octithiophe ne and alpha -sexithiophene than for alpha -quaterthiophene.