PHOTOCONDUCTION IN FULLERENE FILMS

We report results from a detailed study of photoconduction in C60 film s. At low temperatures the photoaction spectra show vibrational coupli ng to the first electronic transition, indicating that the generation of mobile carriers in the solid involves molecular excitation schemes. The lowest energy transition is found to occur at photon energies of approximately 1.8 eV Below this energy, only weak and field-dependent photoresponse is observed. We attribute this to a tail of localized st ates related to defects in the film. The edge between localized and ex tended states approximately coincides with the first vibronic peak. Fr om this peak we find the photoconduction edge of the film to be 1.81+/ -0.01 eV at room temperature in the face-centered-cubic phase and 1.85 +/-0.01 eV in the simple cubic phase below approximately 200 K.