Schottky barrier height at an organic/metal junction: A first-principles study of PTCDA/X (X = Al, Ag) contacts

First-principles calculations within the density-functional theory have been performed for a 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) molecule deposited on Al(111) and Ag(111) substrates, focusing on the structural and electronic properties. The relatively large interplanar distance between the PTCDA plane and the Al surface, along with the small adsorption energy, suggest the interaction to be pretty weak. Moreover, the analysis of Mulliken population combined with the density of states shows that the main interactions occur in the molecular anhydride end groups, whereas the perylene core is basically unaffected by the Al substrate. Very similar results are obtained for PTCDA deposited on the Ag(111) surface, the interaction being even weaker than with Al, as expected for the less reactive noble metal. As for the technologically important issue of the potential lineup, our results show that the PTCDA/Al contact has a rectifying character, with a p-type Schottky barrier height of about 1.5 eV. This same value is obtained for the PTCDA/Ag contact, irrespective of the interface geometry. This suggests that, irrespective of the underlying metal, the Fermi level is pinned at the same energy position with respect to the PTCDA highest occupied molecular orbital, in excellent qualitative agreement with experimental findings.