. sputter depth profile is integrated through the bulk of the crystallised Se absorber layer (see Fig. ) to get a representative measurement of the extrinsic elements present in the bulk of the thin film. Quantification of impurity concentrations requires elemental standards, which were not present, and the results presented in Fig. should therefore be treated only as a qualitative demonstration of the presence of F and Cl in the absorber. O and, particularly, Te are also present, but only at the surface and interface. These are expected in higher concentrations than the halogens, given the use of an ultra-thin trigonal Te substrate to favour the trigonal Se allotrope and aid adhesion, and O from thermal annealing in air during crystallisation. The halogen impurities are verified to originate from the Te source material using ToF-SIMS (Fig. ). Moreover, the rich variety of naturallyoccurring isotopes of both Se ( 76 Se, Se, Se and 82 Se all >5 % natural abundance) and Te ( 125 Te, 126 Te, Te, 130 Te; >5 % abundance) makes the identification of Br ( 79 Br/ 81 Br) and I ( 127 I) difficult given the strong overlap of masses, and so they may also be present in significant amounts. Given these known impurities, we calculated the formation en-ergies of their corresponding point defects (substitutions and interstitials) in t-Se, including H (given its ubiquity and invisibility to ToF-SIMS) and the pnictogen family (given their position to the right of selenium on the periodic table, and thus potential acceptor dopant behaviour) as well as the first 4 rows of chalcogen and halogen groups. From the formation energy diagrams in Fig. , we observe consistent behaviour in energies and charge states within each periodic group. Within the same group, there are mostly rigid shifts of formation energies due to size effects, with larger anions tending to be lower energy as X Se substitutions, but higher energy as X i interstitials -as is typically the case. N is an outlier to these trends, being high energy in both cases and only stable in the neutral and singly-negative charge states. Each impurity shows similar electrical behaviour (charge state preferences) as both substitution and interstitial defects (X Se & X i ), due in part to the neutral oxidation state of the substitutional site (Se). Each heterovalent impurity species investigated (halogens, pnictogens and hydrogen; with preferred valence differing from Se by ±1) shows amphoteric behaviour, having stable +1 and -1 charge states -except for nitrogen. X Se +1 and X i +1 defects exhibit geometries similar to V Se +1/+2 (Figs. and), with bridging bonds between Se chains stabilising the positive charge states -behaviour which has been termed 'valence alternation' 136 ; Section S5.1. Homovalent impurities (chalcogens) adopt neutral charge states for most Fermi levels in the band gap, consistent with their covalent nature and dominance of neutral Se i .