Gap states in Pentacene Thin Film Induced by Inert Gas Exposure

We studied gas-exposure effects on pentacene (Pn) films on SiO₂ and Au(111) substrates by ultrahigh sensitivity photoelectron spectroscopy, which can detect the density of states of ~10¹⁶ states eV^-1 cm^-3 comparable to electrical measurements. The results show the striking effects for Pn/SiO₂: exposure to inert gas (N₂ and Ar) 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-exposure effect is observed for Pn/Au(111). 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 crystal grain boundaries.