Theory of electrical conduction in organic molecular crystals. III. Temperature-independent mobilities in naphthalene

Temperature-independent electron mobilities (TIEM) observed in two crystal axes (b and c′) in naphthalene are explained by an extension of a model which was proposed previously [H. Sumi, J. Chem. Phys. 70, 3775 (1979); 71, 3403 (1979)] to explain TIEM observed in anthracene only in a single crystal axis (c′). The electron mobility in the a direction, which decreases with increasing temperature, is postulated to arise from electron motion in a one-dimensional Bloch band along the a axis. TIEM in the b and c′ directions arises because electrons hop from one a-axis Bloch band to another because of a transfer integral induced by thermal rotational vibrations of molecules. An important feature in the model is that sharp peaks in the density-of-state function of the one-dimensional band are rounded by the broadening due to phonon scattering.