Properties of Electronic Excited States of C60 Thin Films as Seen from Photoconductivity and Luminescence Behavior: Dependence on Excitation Wavelength and Temperature

Photoconductivity and its temperature dependence have been measured for both oxygen-exposed and nonexposed C60 thin films. A constant activation energy for different wavelengths, together with the insensitivity of luminescence emission spectrum to excitation energy, suggests a molecular excitonic nature of solid C60's electronic states. Luminescence excitation spectra show that emission efficiency is higher for the excitation of forbidden states than allowed states. A turning point at 250-260K has been found in a luminescence vs. temperature curve and is discussed in relation to the phase transition from fcc to sc.