Studies of C60 thin films using surface photovoltage spectroscopy

The electronic structure of polycrystalline C60 thin films has been investigated using surface photovoltage spectroscopy (SPS) and conductivity measurements. The films show n-type semiconductivity with an activation energy of /2~ 0.8 eV as found from the temperature dependence of the conductivity at high temperatures. The electronic structure emerging from our SPS results comprises a 1.6 eV photoconduction gap, a mobility gap of about 2.25 eV and two gap states, a donor and an acceptor, at 0.35 and 0.8 eV, respectively, below the photoconduction edge. The proposed model positions the Fermi level between these two localised levels. Thus, the high-temperature conductivity is due to electrons excited across the photoconduction gap.