Determination of the Urbach Energy Eu and the Optical Band Gap Eg in Submicron C60 and C70 Fullerene Films. Dependences of Eu and Eg of the Films on Their Thickness in the Range 20–5000 nm
DOI:
https://doi.org/10.15407/ujpe70.5.303Keywords:
film, absorption spectrum, approximation, Gaussian function, C60 and C70 fullerenes, Urbach energy, optical band gapAbstract
The long-wavelength spectral edge of the absorption coefficient α has been studied in detail within a spectral interval of 1.492–2.605 eV for C60 and C70 fullerene films with thicknesses ranging from 20 to 5000 nm. The values of the optical band gap Eg and the Urbach energy Eu in submicron C60 and C70 films are determined for the first time. It is found that the Eu-value decreases and the Eg -value increases, as the thickness of C60 films increases from 20 to 5000 nm, and the thickness of C70 films from 20 to 1000 nm. The highest, intermediate, and lowest Eg -values for C60 and C70 films are obtained using the Tauc, classical, and Cody methods, respectively. The average value of Eg, ⟨Eg⟩, coincides with the Eg -value obtained using the classical method. The average values of the parameter α0, ⟨α0⟩, within the exponential sections of the α(E) spectra are estimated; they turned out significantly larger for C70 films. The long-wavelength edge of the spectra α(E) is approximated by exponential dependences with the parameters ⟨α0⟩, ⟨Eg⟩, and Eu.
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