Effects of Charge Localization on the Orbital Energies of Bithiophene Clusters

Abstract

Standard and constrained density functional theory calculations were used to study the degree of charge localization in positively charged bithiophene clusters. Although polarization effects due to the crystalline environment are known, many charge-transport models in π-conjugated organic materials assume a highly localized picture of carriers due to the strong electron–phonon interaction. These first-principles calculations show that the positive charge delocalizes over at least eight molecules in one-dimensional herringbone stacks. For such one-dimensional clusters, positive charge is more likely to localize on "tilted" molecules than on "parallel" molecules because of polarization effects. For two-dimensional clusters, whereas polarization effects cancel due to symmetry, positive charge is shown to affect molecular sites anisotropically. Differences in computed HOMO energies between the localized and delocalized charges are ∼1–2 eV, about the same as the difference in energy computed between a singly charged and doubly charged stack. These results suggest that models for charge transport in organic semiconductors should be revised to account for significant charge delocalization and intermolecular interactions such as polarization.

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Affiliated Institutions

• University of Pittsburgh US

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