Impacts of Electrostatic Screening and Interfacial Charge Transfer on Molecular Donor-Acceptor Heterojunctions Studied by Scanning Probe Microscopy/Spectroscopy

A common need in organic electronics is the thorough understanding and control of the electronic structures and interfacial properties of molecular thin films on inorganic substrates. However, when donor (D) and acceptor (A) molecular heterojunctions are concerned, the interfacial charge transfer effects can compete with intermolecular charge transfer, which, along with the electrostatic effects from the substrate, exerts a significant substrate perturbation on the heterojunction. Employing archetypal D and A organic molecules and a variety of different supporting substrates, we show that the presence of the substrate, even a weakly interacting one, can still significantly perturb the intrinsic properties of the D-A heterojunction via interfacial charge transfer. Furthermore, substrate electrostatic screening can be modulated by interfacial charge transfer. This study highlights the impacts of the substrate electrostatic environment and the interfacial coupling on molecular electronic structures, an essential aspect in the applications of organic and molecular electronic devices.