Charge Transport Parameters of the Pentathienoacene Crystal

Pentathienoacene, the thiophene equivalent of pentacene, is one of the latest additions to the family of organic crystal semiconductors with a great potential for use in thin film transistors. By using density functional theory and gas-phase ultraviolet photoelectron spectroscopy, we investigate the microscopic charge transport parameters of the pentathienoacene crystal. We find that the valence band exhibits a stronger dispersion than those in the pentacene and rubrene single crystals with marked uniaxial characteristics within the molecular layer due to the presence of one-dimensional pi-stacks; a small hole effective mass is also found along the direction perpendicular to the molecular layers. In the conduction band, strong intermolecular sulfur-sulfur interactions give rise to a significant interstack electronic coupling whereas the intrastack dispersion is greatly reduced. The intramolecular vibronic coupling (reorganization energy) is stronger than that in pentacene but comparable to that in sexithiophene; it is larger for holes than for electrons, as a result of low-frequency modes induced by the sulfur atoms. The polarization energy is large, but its effect on the vibronic coupling remains small. Charge transport is discussed in the framework of both band and hopping models.