Selenium vacancies present significantly different behaviour to the interstitials, being amphoteric and exhibiting three deep charge-state transition levels spread throughout the band gap as shown in Fig. . ¶ The location of these defect levels in the mid-gap region suggests they could be active for electron-hole recombination, potentially harming the efficiency of t-Se PV devices. The formation energy of the vacancy, though still relatively low at 1.5 eV, is significantly higher than the self-interstitial for most Fermi levels and so is expected to only present in relatively low concentrations. However, the actual defect concentrations in as-grown samples will depend on synthesis procedures, which may result in non-equilibrium defect populations due to kinetic trapping. Beyond the bulk of the material, a higher prevalence of vacancy-type defects is to be expected at grain boundaries and interfaces.