A theoretical view on self-assembled monolayers in organic electronic devices

Self-assembled monolayers (SAMs) of covalently bound organic molecules are rapidly becoming an integral part of organic electronic devices. There, SAMs are employed to tune the work function of the inorganic electrodes in order to adjust the barriers for charge-carrier injection into the active organic layer and thus minimize undesired onset voltages. Moreover, in the context of molecular electronics, the SAM itself can carry device functionality down to a few or even a single molecule. In the present contribution, we review recent theoretical work on SAMs of prototype π-conjugated molecules on noble metals and present new data on additional systems. Based on first-principles calculations, we establish a comprehensive microscopic picture of the interface energetics in these systems, which crucially impact the performance of the specific device configuration the SAM is used in. Particular emphasis is put on the modification of the substrate work function upon SAM formation, the alignment of the molecular levels with the electrode Fermi energy, and the connection between these two quantities. The impact of strong acceptor substitutions is studied with the goal of lowering the energy barrier for the injection of holes from a metallic electrode into the subsequently deposited active layer of an organic electronic device.