Phonons and anisotropic mobility in a single crystal organic semiconductor

Intermolecular phonons are theorized to cause transient localization of charge carriers which is a proposed limiting factor in achieving band-like mobilities in organic semiconductors. In this study we probe the molecular motions associated with phonons in single crystal tetracene within the high-mobility plane correlate with the anisotropy of the field-effect mobility. Phonons measured at low energy (< 25 meV) have associated molecule and lattice distortions that are directionally oriented within the crystal. Calculated changes to the HOMO level show a disruption of orbital overlap between molecules along the high mobility direction for one large amplitude phonon. The measured anisotropic high-to-low mobility ratio in the ab-plane of single crystal tetracene is lower than published static calculations of anisotropy suggesting that transient localization has directionality.