Temperature dependence of optical bandgap in C60 polycrystalline films

Temperature-dependent optical transmission is performed on C60 polycrystalline films of various thicknesses with incident photon energies from 1.46 to 2.07 eV. Remarkable absorption peaks together with the dependence of peak intensity on film thickness are observed and explained in terms of an energy terms diagram that accounts for the gap evolution from energy levels (related to free C60 molecules) to energy bands (attributed to molecular C60 solid). The interplay of strong intramolecular chemical bonds and weak intermolecular van der Waals bonds is also discussed. Then, the temperature dependence of bandgap Eg(T) is deduced from the transmission spectra for thick polycrystalline C60 films. From a numerical fit of Eg(T), the fitting parameters associated with electron–phonon interaction are obtained for this molecular C60 solid and are compared with those for other inorganic semiconducting solids.