Disorder/order phase transition in C60 thin films studied by surface photovoltage spectroscopy

The electronic properties of C60 thin films have been studied using surface photovoltage (SPV) spectroscopy at 120–300 K. Temperature variations of the SPV spectra are correlated with temperature-dependent x-ray diffraction patterns of the same samples, which indicate the first-order phase transition at Tc=250 K. Absolute values of both the band-to-band and band (or band tail) -to-band tail SPV signals are shown to exhibit a clear minimum at Tc=250 K in contrast with the well-known increasing background of the SPV intensity with decreasing temperature. The energy positions of the thresholds of the band-to-band and band (or band tail) -to-band tail regions in the SPV spectra also exhibit nonmonotonic behavior with a mirror symmetry and distinct extrema near Tc. On the basis of the presented results, we discuss possible reasons behind the effect of rotational and orientational states of C60 molecules on the electronic structure of the C60 fullerite and charge carrier transport in C60 thin films.