Interlayer molecular diffusion and thermodynamic equilibrium in organic heterostructures on a metal electrode

In this paper, we investigate by photoemission the electronic properties of an organic/organic interface consisting in a strong electron acceptor hexaazatriphenylene-hexacarbonitrile (HATCN) deposited on a physisorbed N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (alpha-NPD) monolayer on Ag(111). At the first HATCN deposition steps (similar to 1 monolayer), the sample work function increases by 1.05 eV and the hole injection barrier (HIB) in the pre-adsorbed alpha-NPD monolayer is lowered by 0.65 eV. This results from HATCN diffusion to the silver surface through the alpha-NPD monolayer. Furthermore, this HATCN monolayer is proposed to form a compact chemisorbed monolayer, with a different structural arrangement than that observed on pristine Ag(111). In a second step, the additional deposited HATCN start growing on top of the alpha-NPD layer, and Fermi-level pinning, associated with the formation of HATCN negative polarons, is identified at the HATCN/alpha-NPD interface. Finally, HATCN is deposited on a alpha-NPD multilayer film. In this case, HATCN is also found to diffuse to the silver surface giving a similar HIB value in the alpha-NPD multilayer. This work demonstrates the importance of studying in detail the formation of organic heterostructures to understand the interplay between growth mode and electronic properties. (C) 2011 American Institute of Physics. [doi:10.1063/1.3662878]