Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

We detected a very low density of electronic states in the band gap of a nondoped pentacene thin film by using ultraviolet photoelectron spectroscopy with ultrahigh sensitivity and ultralow background. The gap states, which may originate from the highest occupied molecular orbital (HOMO) state in imperfect molecular packing regions, are distributed exponentially up to the Fermi level and control the Fermi level relative to the HOMO band.