Determination of the Urbach energy <i>Eu</i> and the optical band gap <i>Eg</i> of submicron C60 and C70 fullerene films

The spectra of long-wavelength absorption edge in the range of 1.62–2.08 eV of C60 and C70 films with a thickness of 20–900 nm were studied in detail. The Eg and Eu values of these films were determined for the first time. It was established that the Eu of submicron films C70 and C60 decreases if their thickness increases. The Eg values of these films were obtained by Tauc, classical, and Cody methods. The average ⟨Eg⟩ value coincides with Eg for the classical method. The average values of absorption coefficient ⟨α0⟩ were estimated, which are significantly larger for C70 films. This is due to the stronger absorption of C70 compared to that of C60. Absorption spectra of these films were approximated by exponential lines with parameters ⟨α0⟩, ⟨Eg⟩, and Eu.