Non-Coherent Charge Transport in Donor–Acceptor Systems: A Self-Consistent Description of the Intramolecular Charge Flow

Abstract

We have investigated the role that molecular orbitals (MOs) play in the electron transport through a single donor–acceptor molecule submitted to an external voltage applied through two metallic electrodes. Considering the weak coupling limit, in which level-broadening effects are negligible, we investigate the microscopic processes associated to the charge flow through the molecule by examining how the individual molecular levels actually respond to the external electric field. By taking into account the active role played by the MOs in the charge transport between the two electrodes, we have shown that an important contribution may arise in the situation of a field-induced “avoided-crossing” between neighboring energy levels, especially if the corresponding MOs are localized in different regions of the molecule. Conduction channels can be opened or closed as the result of “avoided-crossing” situations in which the spatial localization of the MOs considered changes between the acceptor and the donor opposite sides of the molecule. Our results indicate that the charge transfer between the electrodes is mainly dominated by noncoherent mechanisms involving hole transport through the uppermost occupied molecular orbitals. We suggest that these field-induced changes in the molecular environment may play a key role in the overall transport process and should be considered whenever actual measurements are being performed in single molecules.