Have we understood the optical absorption edge in chalcogenide glasses?

We consider relationships between Tauc and mobility gaps in simple chalcogenide glasses such as As2S3 and Se. Photoconduction spectra and Urbach-edge characteristics of a glass and optical absorption edges of the corresponding crystal suggest that the Tauc gap (~ 2.5 eV in As2S3 at 0 K) is smaller than the mobility gap (~ 2.8 eV), which is consistent with an idea that the former is determined by photoelectronic transitions from localized to extended states. Tellurides such as Ge2Sb2Te5 are exceptional, in which the Tauc gap in amorphous states is greater than the optical gap in crystalline phases.