High-sensitivity ultraviolet photoemission spectroscopy technique for direct detection of gap states in organic thin films

We developed ultrahigh sensitivity, low-background ultraviolet photoemission spectroscopy (UPS) technique which does not introduce detectable radiation damages into organic materials. The UPS allows to detect density of states of the order of ∼1016 states eV−1 cm−3 even for radiation-sensitive organic films, this results being comparable to electrical measurements of charge trapping centers. In this review we introduce the method of ultrahigh sensitivity photoemission measurement and we present some results on the energy distribution of gap states in pentacene (Pn) films deposited on SiO2 and Au(1 1 1) substrate. For Pn/SiO2 thin film the results show that exposure to inert gas (N2 and Ar) atmosphere produces a sharp rise in gap states from 1016 to 1018 states eV−1 cm−3 and pushes the Fermi level closer to the valence band (0.15-0.17 eV), as does exposure to O2 (0.20 eV), while no such gas-induced effects are observed for Pn/Au(1 1 1) system. The results demonstrate that these gap states originate from small imperfections in the Pn packing structure, which are induced by gas penetration into the film through the Pn crystal grain boundaries. Similar results were obtained for CuPc/F16CuPc thin films, a prototypical example of donor/acceptor interface for photovoltaic application.