Theory of electrical conduction in organic molecular crystals. II. Characteristic effects of electric field and defect scattering on temperature-independent mobilities

Abstract

The model proposed by the present author [J. Chem. Phys. 70, 3775 (1979)] explained successfully the almost-temperature-independent electron mobilities in the c′ direction of anthracene and napthalene in the high temperature region. The present work extends the model to reproduce observed varieties in the temperature dependence of the c′ mobilities below ambout 130 °K under experimentally realistic conditions of high electric fields and nonvanishing defect scattering of electrons. It is ascribed to defect-scattering effects that the observed temperature dependence of the electron mobility along the (a,b) plane of anthracene becomes gradual towards low temperatures. It is predicted that for kBT ≲h/ω2 (the energy of rotational vibations which induce an additional transfer integral of electron in the c′ direction) the drift velocity in the c′ direction should become saturated with electric field, F, when the consequent energy difference, eFa, between neighboring (a,b) planes exceeds h/ω2. It is also predicted that for kBT≲h/ω1 (the Debye cut-off energy of acoustic lattice vibrations which scatter electrons within a single (a,b) plane) the c′ drift velocity should show marked deviation from a simple linear relationship with the electric field even when eFa is smaller than h/ω2 and here it should be sensitive to defect scattering as by impurities. Numerical calculation was performed for h/ω1?50 cm−1 and h/ω2?125 cm−1, where eFa=h/ω2 occurs at a field strength about 2×105 V/cm.