Gap States in Amorphous Selenium: What Are the Atomic Structures?

Substantial studies have been performed on optical absorption/photo‐conducting spectra and charge‐transport properties in amorphous Se, while interpretations of the results remain elusive. This work explores, after a brief review of notable outcomes, the atomic structures of defective and trapping sites with their densities, through ab initio molecular‐orbital calculations of small clusters such as H6SeH. The analyses suggest that the Urbach edge arises from fluctuating highest occupied molecular orbital (HOMO) levels, giving rise to the top of the valence band, which is caused mainly by disorder in the dihedral angle. And, several electron traps are produced by the lowest unoccupied molecular orbital (LUMO) states of intimate valence‐alternation‐pairs and neutral dangling bonds of Se (and impurity O atoms as well). Isolated charged defects C 1 − /C 3 + of Se are few, which scarcely affect electrical/optical properties.