TEMPERATURE-DEPENDENCE OF THE OPTICAL-ABSORPTION EDGE IN C-60 THIN-FILMS

The optical absorption edge of semiconductor C-60 films is studied in a wide temperature range of 8.8-470 K. We find three temperature regim es for both the optical gap E-o and the Urbach tail parameter E-u: in the range T<150 K, they do not change; 150<T<260 K, they change gradua lly; and T>260 K, they change rapidly. We also find that the subgap ab sorption increases with prolonged exposure of the film to air but the Urbach tail parameter E-u is not affected. The results are discussed i n terms of the relation of the electron density of states to the molec ular orientational disorder and the structural phase transition. [SO16 3-1829(98)05939-6].