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 ≈ 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.