Applying strong external electric field to thiophene-based oligomers: A promising approach to upgrade semiconducting performance.

A key parameter dictating the rate of charge transfer (CT) is reorganization energy ( λ), an energy associated with geometry changes during hole/electron transfer. We show that 'ironing' the inter-ring dihedral angles of oligothiophenes via proper substitutions or insertions (e.g., -OR, -F or -C≡C-), decreases the λ and thus promotes CT according to Marcus equation. Our results demonstrate, to attain a smaller λ, extending oligomer length is only significant if the flattened backbone structure is realized. Of great interest is that external electric fields, which are ubiquitous in electronic devices yet commonly overlooked in the computation of λ, can have a significantly greater impact than conventional substitutions. It is important to emphasize, the responses of λ to external fields is system-dependent. Compared to fused-ring conjugated systems, single-bond connected thiophenes are more sensitive to external fields. F_x lowers the λ (552 meV) of quaterthiophene by almost 80% at the intensity of 1 V/Å, down to a value (125 meV) which is even lower than that of pentacene (154 meV) and rubrene (219 meV) at the same level of theory. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]