Charge transport in organic crystals: interplay of band transport, hopping and electron-phonon scattering

We present an ab initio description of charge transport in organic semiconductors based on a recently developed theory that goes beyond small-polaron and/or narrow-band models. The mobility expression is evaluated with parameters from density functional theory, and application to naphthalene crystals demonstrates substantial progress in the simulated temperature dependence and relative magnitudes for all transport directions. The scattering by phonons is described in a manner that goes beyond the Holstein model for small polarons and, consequently, significantly improves the temperature dependence and anisotropy of carrier mobility with respect to the previous narrow-band theory. The contributions of coherent and incoherent scattering processes are analyzed. Special emphasis is placed on the origin and understanding of the transition from band transport at low temperatures to hopping at high temperatures, both of which are fully included in the theory. Additionally, in contrast to earlier narrow-band theories, the unphysical divergence at zero temperatures is removed.