Dark- and photo-conductivity measurements on films of fullerene C60

Thin-film samples have been prepared by evaporating a commercial fullerene C60 powder from a tungsten crucible, at a pressure of 10-5 Pa on to quartz or Corning glass substrates. Samples were equipped with coplanar Cr-Au electrodes evaporated onto the surface of the film. Dark-conductivity measurements in the range 300-600 K evidence the existence of hysteresis phenomena. Thermal annealing influences photoconductivity and changes the power coefficient in the power law illumination-intensity dependence of photoconductivity. The influence of oxygen absorbed in the film matrix on the electrical properties and on the density of states is discussed. A tentative for explaining the photoconductivity-light-intensity dependence has been done assuming a continuous density-of-state distribution in the energy gap of the semiconductor