ELECTRICAL-CONDUCTIVITY STUDIES OF UNDOPED SOLID FILMS OF C60/70

We report the electrical conductivity, sigma, and its temperature dependence from 400 to 160 K measured in 1.5-mu-m thick, sublimed films of C60/70. At room temperature, sigma almost-equal-to 10(-14) (OMEGA-cm)-1. Reversible structure in the temperature dependence, extending from almost-equal-to 300 to 270 K, may reflect reported phase transitions related to orientational order. Assuming solid C60/70 is an intrinsic semiconductor, the high temperature bandgap is about 1.9 eV and the observations are consistent with a transition to a semiconductor, below almost-equal-to 250 K, with a smaller bandgap. Higher carrier mobilities, due to the increased order below the transition, may also play a role.