Theory of electrical conduction in organic molecular crystals: Temperature-independent mobilities

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Abstract
Almost‐temperature‐independent mobility (ATIM) has been observed above about 100 K for electrons in anthracene, naphthalene, and As2S3. The conduction‐band structure of all these crystals is almost two‐ dimensional with a very narrow bandwidth in one (c′) direction, and it is in this c′ direction that the ATIM has been observed. The ATIM is ascribed to diffusive electronhopping induced by lattice distortions in the course of lattice vibrations. It is crucial to the mechanism proposed that electron motion is coherent in the two‐dimensional Bloch‐band states along the (a,b) plane perpendicular to the c′ direction. The steep increase of the mobility below about 100 K observed in naphthalene is ascribed to the increasing contribution of the mobility component determined by the electron transfer of the rigid lattice at low temperatures. The mobility along the (a,b) plane which shows the bandlike temperature dependence in these crystals is, on the other hand, determined only by electron transfer of the rigid lattice at all temperatures. The electron mobility in the c’ direction and that along (a,b) plane are both calculated with a unified treatment based on the linear response theory. The observed magnitude, temperature dependence, and anisotropy of the mobility are all reproduced consistently by adopting reasonable values for the energy bandwidths of electron and the electron–phonon coupling energies.