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In organic electronics, device performance and lifetime depend crucially on the properties of both the active materials and their interfaces. Unfortunately, the progress which has been achieved in materials design and manufacturing has not been matched by an equal improvement in interface design. Perhaps the main obstacle lies in the difficulties associated with translating the detailed, but very case-specific, knowledge that exists about interfacial properties into design criteria for applications. It is therefore of utmost importance to understand which materials parameters and fundamental physical and chemical processes are important for the various different kinds of interfaces encountered in organic electronics. Here, we focus on a specific class of interfaces that is very common in a large range of applications, namely weakly interacting systems of pi-conjugated molecules (or polymers) on metallic contacts. By choosing a unique model system for decoupled molecular pi-orbitals and metal continuum states, tetrakis(dimethylamino)ethylene (TDAE) on Au, we present direct experimental and theoretical evidence that the interface energetics for such systems are governed by the exchange of an integer amount of electrons, as opposed to partial charge transfer in hybridized systems, in strong analogy with single electron devices in molecular electronics. These results help to explain several recently reported studies of weakly interacting interfaces, in which such a mechanism was proposed but never observed. |