The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the KxPTCDA/Ag(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (ACS Nano>2016, 10, 2365–2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal–organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra ofK2PTCDA/Ag(111) and their counterintuitive similarity tothose of neutral PTCDA layers. They also clearly describe the transitionfrom a metallic character of the (pristine) adsorbed PTCDA layer onAg(111) to a semiconducting state upon doping, which is the oppositeof the effect (degenerate) doping usually has on semiconducting materials.All experimental and theoretical efforts also unanimously reveal areduced electronic coupling between the adsorbate and the substrate,which goes hand in hand with an increasing adsorption distance ofthe PTCDA molecules caused by a bending of their carboxylic oxygensaway from the substrate and toward the potassium atoms.
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