Our results above show that while halogen impurities are present and may contribute low levels of hole doping, neither intrinsic nor extrinsic point defects can explain the reported doping concentrations in t-Se PV devices. Se is a soft, highly-deformable solid as discussed above and evidenced by the low formation energies of competing allotropes (Table ), low bulk modulus and low defect formation energies. Coupled with its ability to catenate and bond in a variety of chain and ring-like structures, this indicates the potential inclusion of non-crystalline regions in t-Se films, comprised of over-and under-coordinated Se atoms in inter-chain bridging bonds, stacking faults, ring-like formations and/or amorphous clusters. Indeed, Nijland and others have reported polycrystalline t-Se samples to consist of 'wellconducting crystals embedded in badly conducting layers of more or less amorphous selenium', while Lu et al. reported improved crystallinity (and optoelectronic performance) with their criticalmelting annealing treatment. We further note that all charged intrinsic defect species in t-Se involved the formation of bridging bonds between Se chains, which are expected to occur at any grain boundaries or extended defects, indicating the potential doping activity of these regions. Thus it is certainly possible that significant concentrations of extended defects and/or noncrystalline regions may be present in the as-grown t-Se films and contribute to doping, by forming or attracting charged defects. This possibility would also be exacerbated by the typical growth process of t-Se; where amorphous Se is first prepared and then annealed just below the melting temperature (often on a trigonal-Te substrate) to yield the trigonal phase, which is likely to give increased defect concentrations and potentially incomplete removal of amorphous phases. Indeed, Plessner found carrier concentrations in selenium films to be highly sensitive to the annealing and crystallisation conditions, with greater crystal quality yielding much lower hole concentrations, in agreement with these conclusions. Other possible origins of unexpected doping measurements include complex defect formation or surface effects due to unpassivated Se chain terminations in thin films.