Lifetimes of Core-Excited Organic Adsorbates from First Principles

The development of efficient organic electronic devices depends substantially on the electronic coupling of the molecules at interfaces and on their arrangement at the nanometer length scale. An effective molecule-substrate interaction is mandatory for solar cells in which electrons should be collected before recombination. Core electron spectroscopies are usually the most suitable experimental method to access fast electron transfer but the perturbation they induce on the electronic system requires further theoretical work. By first principles simulations we study the resonant electron transfer lifetime from the excited state of an organic adsorbate to a semiconductor surface, namely isonicotinic acid on rutile TiO2(110). The molecule-substrate interaction is described using density functional theory, while the effect of a truly semi-infinite substrate is described within the Green’s function formalism. Our method applies techniques already employed in conductivity calculations. Excitonic effects due to the presence of core-excited atoms in the molecule are shown to be instrumental to understand the electron transfer times measured using the so-called core-hole-clock technique. In particular, we find that the charge injection from the LUMO is quenched, since this state lies within the substrate band gap. We compute the resonant charge transfer times from LUMO+1 and LUMO+2, and systematically investigate the dependence of the elastic lifetimes of these states on the alignment among adsorbate and substrate states. G. Fratesi, C. Motta, M.I. Trioni, G.P. Brivio and D. Sánchez-Portal: Resonant Lifetime of Core-Excited Organic Adsorbates from First Principles, J. Phys. Chem. C 118, 8775-8782 (2014), doi: 10.1021/jp500520k