Temperature dependence of the optical-absorption edge inC60thin films

The optical absorption edge of semiconductor ${\mathrm{C}}_{60}$ films is studied in a wide temperature range of 8.8–470 K. We find three temperature regimes for both the optical gap $E_o$ and the Urbach tail parameter $E_u:$ in the range $T<\mathrm{}150\mathrm{}\mathrm{K},$ they do not change; $150<T<\mathrm{}260\mathrm{}\mathrm{K},$ they change gradually; and $T>\mathrm{}260\mathrm{}\mathrm{K},$ they change rapidly. We also find that the subgap absorption increases with prolonged exposure of the film to air but the Urbach tail parameter $E_u$ is not affected. The results are discussed in terms of the relation of the electron density of states to the molecular orientational disorder and the structural phase transition.