PHOTOCONDUCTIVITY OF THIN-FILM FULLERENES - EFFECT OF OXYGEN AND THERMAL ANNEALING

The photoconductive response of both oxygen-free and oxygenated thin fullerene films is compared and contrasted. Exposure to oxygen results in a significant decrease in the magnitude of the photoconductivity. The qualitative features of the spectral dependence of the photoconductivity in oxygen-free and oxygenated films are the same, however, and differences in the temperature dependence are small. It is concluded that the influence of oxygen is not the source of subgap responses and anomalous features in the temperature dependence. In addition, the features of the temperature dependence are not directly related to the structural phase transition, Upon heat treatment of the oxygen-free films under vacuum, both the photoconductive response at subgap photon energies, and the anomalous features in the temperature dependence are substantially reduced. The treatment is interpreted as a thermal annealing process. In oxygenated films heat treatment results in an overall increase in the photoconductive response by an order of magnitude, a result of a partial outheating of the absorbed oxygen.