Conjugation length dependence of the charge transport in oligothiophene single crystals

The intrinsic charge-transport mechanism in oligothiophene single crystals is investigated by temperature-dependent space-charge-limited current spectroscopy. The mobility for in-plane transport in these layered p-type semiconductors follows a power law behavior typical for coherent bandlike charge transport. A value as high as 80 cm²/V s is measured for the mobility in the high-temperature polymorph of α-sexithiophene at low temperatures. In contrast to that the charge transport perpendicular to the molecular planes can be described by incoherent hopping motion revealing the typical thermally activated temperature dependence. Consequently, the charge carrier can be characterized as partly delocalized between molecules of the molecular 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 interaction decreases with increasing conjugation length of the oligothiophene molecule leading to higher room temperature mobilities for α-octithiophene and α-sexithiophene than for α-quaterthiophene.