For most Fermi level positions within the band gap, neutral interstitials Se i 0 are found to be the lowest energy native defect, with a formation energy of 0.82 eV. As shown in Fig. , Se i 0 adopts a split-interstitial type geometry -as is common for chalcogen/oxygen interstitials, where the additional Se inserts itself in one of the chains and displaces nearby atoms Moreover, the absence of any dangling bonds or excess charge for this species, with no in-gap single-particle states as shown in the eigenvalue plots in Fig. , renders the electrically-inactive neutral state quite stable for Se i . Consequently, the -2 charge state is only stabilised at Fermi levels close to the conduction band, while Se i -1 is never the ground-state charge state (Fig. ). § As such, there are no defect levels corresponding to a single charge transition (i.e. q ± 1) for Se i within the band gap -a requirement for efficient charge-carrier recombination, and so it is expected to be recombination-inactive in optoelectronic devices.