STUDIES OF C-60 THIN-FILMS USING SURFACE PHOTOVOLTAGE SPECTROSCOPY

The electronic structure of polycrystalline C-60 thin films has been i nvestigated using surface photovoltage spectroscopy (SPS) and conducti vity measurements. The films show n-type semiconductivity with an acti vation energy of approximate to 0.8 eV as found from the temperature d ependence of the conductivity at high temperatures. The electronic str ucture emerging from our SPS results comprises a 1.6 eV photoconductio n 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 photoconduct ion edge. The proposed model positions the Fermi level between these t wo localised levels. Thus, the high-temperature conductivity is due to electrons excited across the photoconduction gap.