In this paper we explore the effects of off‐diagonal disorder on electronic energy transfer (EET) in an impurity band of an isotopically‐mixed, organic solid at low temperatures. We have considered the localization of an elementary excitation in a system characterized by both diagonal disorder, originating from inhomogeneous broadeningWof the site‐excitation energies, and of off‐diagonal disorder arising from the energetic spread sgr; of the transfer integrals. We have utilized an exact expression for the self‐energy of a disordered system where both the site‐excitation energies and the transfer integrals are characterized by a Lorentzian distribution, together with the localization function method of Liciardello and Economou to establish the localization condition in the center of the impurity band. Model calculations were performed for a Bethe lattice and for the Hubbard density of states, demonstrating the enhancement of delocalization due to off‐diagonal disorder, whereupon the Anderson transition (AT) will be exhibited at higher values ofWthan in the original Anderson model (OAM), when sgr;=0. For large values of the ratioW/sgr;≳12 the effects of off‐diagonal disorder are negligible. Numerical calculations of sgr; were performed for a random distribution of impurities, whileWwas roughly estimated for recent spectroscopic measurements. These data, together with the results of the model calculations for a Bethe lattice, established the existence of the critical impurity concentration ? for EET in the impurity band. Off‐diagonal disorder results in the lowering of ? relative to the OAM; however, the effect of diagonal disorder is dominant in determining the termination of EET in the impurity band.
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