Anion vacancies as a source of persistent photoconductivity in II-VI and   chalcopyrite semiconductors
  Using first-principles electronic structure calculations we identify the anion vacancies in II-VI and chalcopyrite Cu-III-VI2 semiconductors as a class of intrinsic defects that can exhibit metastable behavior. Specifically, we predict persistent electron photoconductivity (n-type PPC) caused by the oxygen vacancy VO in n-ZnO, and persistent hole photoconductivity (p-type PPC) caused by the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2. We find that VSe in the chalcopyrite materials is amphoteric having two "negative-U" like transitions, i.e. a double-donor transition e(2+/0) close to the valence band and a double-acceptor transition e(0/2-) closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects (e.g., donors): type-alpha, which have a defect-localized-state (DLS) in the gap, and type-beta, which have a resonant DLS within the host bands (e.g., conduction band). In the latter case, the introduced carriers (e.g., electrons) relax to the band edge where they can occupy a perturbed-host-state (PHS). Type alpha is non-conducting, whereas type beta is conducting. We identify the neutral anion vacancy as type-alpha and the doubly positively charged vacancy as type-beta. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between alpha- and beta-type behavior, resulting in metastability and PPC. In CuInSe2, the metastable behavior of VSe is carried over to the (VSe-VCu) complex, which we identify as the physical origin of PPC observed experimentally. We explain previous puzzling experimental results in ZnO and CuInSe2 in the light of this model. 
S. Lany and A. Zunger