Controlling the energy level alignment at the ubiquitous interfaces in modern organic light emitting diodes, i.e., organic/electrode and organic/organic, is mandatory for achieving highest performance. While for some interfaces the understanding has matured over the past years – often with the help of photoelectron spectroscopy investigations, a lack of material-overarching and general models seems to persist. In this context, it is interesting to note that photoelectron experiments reported by different groups often returned a different level alignment for a given interface, which certainly should be unsettling for device engineers. It turns out that Fermi-level pinning and its consequences for charge density re-distribution across a device stack is an overarching mechanism that should always be considered. For intrinsic organic heterojunctions of materials with moderate acceptor/donor character the electrostatic potential across the interface changes only marginally – if at all. This situation, however, can be significantly altered when at least one of the two semiconductors is Fermi-level pinned by the "effective work function" of the other one, which is established by the contact to the electrode. Consequently, device engineering has to fully take into account the effect of adding the electrodes to a device stack, otherwise correlations between assumed electronic structure and device performance remain uncertain.
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 <i>first-principles</i> 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.