A comprehensive and unified picture of energy level alignment at interfaces with organic semiconductors

Abstract

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.