Light-induced metastability in Cu(In,Ga)Se2 caused by VSe-VCu complexes
  Using first-principles total-energy calculations we identify the microscopic origin and the physical mechanism that leads to light-induced metastability and persistent photoconductivity in the photovoltaic material Cu(In,Ga)Se2. In the presence of photoexcited or electrically injected conduction band electrons, the complex (VSe-VCu) of a Se vacancy with a Cu vacancy is predicted to transform from a shallow electron trap into a deep hole trap upon the persistent capture of two electrons. This explains the experimental evidence that for every two holes persistently released to the valence band, one hole trap is generated. 
S. Lany, A. Zunger