Theory of small-polaron band conduction in ultrapure organic crystals

We present a novel theory of charge-carrier mobilities in organic molecular crystals of high purity. Our approach is based on Holstein's original concept of small-polaron bands but generalized with respect to the inclusion of nonlocal electron-phonon coupling. We derive an explicit expression for the mobilities as a function of temperature and, using ab-initio methods to obtain the material parameters, we demonstrate its predictive power by applying it to naphthalene. The results show a remarkably good agreement with experiments and provide new insight into the difference between electron and hole mobilities as well as their peculiar algebraic and anisotropic temperature dependences.