PHOTOCONDUCTIVITY IN SOLID FILMS OF C60/70

We report photoeffects in the visible and near-infrared spectrum in sublimed films of C60/70. The peak photoefficiency (photocarrier per absorbed photon) is almost-equal-to 10(-4). Since for wavelengths > 7000 angstrom, absorption in C60/70 films is very weak, carrier photoinjection from the electrodes into the conduction states of C60/70 must be considered. Evidence, based on the energy level structure of C60 films, is discussed which suggests this may not be the source of the photocurrents. Alternatively, the observed threshold may be the photoconductive edge associated with the reported weak absorption due to forbidden transitions in the lowest direct bandgap, which recent calculations place at 1.5 eV.