DARK CONDUCTIVITY AND PHOTOCONDUCTIVITY IN SOLID FILMS OF C-70, C-60,AND KXC70

We report the dark conductivity and photoconductivity (sigma and DELTA sigma) in sublimed films of C70, C60, and KxC70. In comparison with C6 0, the maximum quantum efficiency Of C70 in air (approximately 5 X 10( -3)) is two orders of magnitude higher. The frequency-response range o f the photoconductivity of C70 is wider than that of C60 as well. The disorder and charge localization are important for the charge transpor t in C70, reflected in a bimolecularlike recombination and a large tem perature dependence of the dark conductivity. The Fowler plot suggests that the energy gap Of C70 is smaller (1.5 eV) than that of C60 (1.7 eV). The temperature dependence of DELTAsigma, for C70 has a local pea k between 260 and 270 K, which is possibly related to the freezing of the orientational disorder in C70 below 270 K. The photoconductivity i n KxC70 is extremely small, probably due to the charge delocalization which increases the recombination rate.