The neutral chalcogen interstitials insert in the Se chains similar to the split-interstitial Se i 0 geometry discussed above, while the neutral substitutions incur minimal bond distortions around the X Se site, helping to explain the low formation energies. Te substitutions are extremely low energy (E f (Te Se )) = 67 meV), explaining the ready uptake of Te in Se films, particularly when grown on trigonal Te substrates as discussed above. However, the tight in-chain binding of the chalcogen substitutions and interstitials means their diffusibility is likely low, while that of the negatively-charged halogen interstitials -which are essentially free ions between the Se chains -is likely high. This rationalises the impurity distributions measured by ToF-SIMS (Fig. ), with Te and O present in high concentrations but only at the absorber interfaces, while halogens (which originate from the Te substrate; Fig. ) are present throughout the film -potentially accumulating at grain boundaries or interfaces. While the formation energies of all extrinsic point defects here are still relatively low (<2 eV, except for nitrogen), substitutional defects are found to be lower energy than interstitials in most cases.