Turning our focus to the impact of intrinsic defects on chargecarrier populations, we can see that there are no charged defects which are low energy for all Fermi levels in the band gap in t-Se (i.e. no low energy acceptors near the VBM or donors near the CBM), as discussed above. Thus, Se i and V Se on their own do not contribute significantly to doping in t-Se, as shown in the thermodynamics analyses in Fig. , where calculated equilibrium hole concentrations in as-grown Se films do not reach 10 10 cm 3 even up to the melting temperature (T = 493 K). We note that the typical synthesis procedure for t-Se, involving crystallisation from the amorphous phase, may induce higher defect concentrations than equilibrium predictions due to kinetic trapping. However, the approximate orders of magnitude are still expected to be reliable estimations, especially given the proximity to the melting temperature and low expected migration barriers (discussed later) which should favour rapid equilibration. Intrinsic defects may increase the concentration of charge carriers (doping) or decrease them through charge compensation. This balance can be described in terms of 'doping windows', which are related to the formation energy of the lowest energy charge-compensating species at the corresponding band edge (i.e. the lowest energy intrinsic donor/acceptor at the valence/conduction band). For t-Se, we have no n-type doping window, with the formation of the dominant intrinsic acceptor becoming spontaneous (E f (V Se -2 ) < 0) at Fermi levels just below the CBM, while we have an extremely large p-type doping window of 1.29 eV, set by the formation energy of V Se +1 at the VBM. The presence of this large p-type doping window can be ascribed to the limited number of inequivalent intrinsic point defects in this single-element system, the covalent bonding (and resultant low dielectric constant which disfavours charged defects), and relatively high energy cost of breaking the Se chains. Similar behaviour is found in other covalent semiconductors such as Si and Ge. Consequently, while intrinsic point defects do not directly contribute to doping in t-Se and n-type doping is mostly prevented by the low energy of V Se -2 , extrinsic p-type doping should be accessible.