Tuning the hole injection barrier height at organic/metal interfaces with (sub-) monolayers of electron acceptor molecules

We demonstrate that the insertion of an electron acceptor molecular (sub-) monolayer at organic/metal interfaces enables systematic tuning of the hole injection barrier height (Δh). Ultraviolet photoemission spectroscopy results show that Δh for α-sexithiophene thin films deposited on Ag(111) can be reduced from 1.3to1.0eV by introducing a chemisorbed monolayer of 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) on the metal surface. Fine-tuning of Δh within the interval of 0.3eV is achieved by using TCAQ coverages in the submonolayer range.