Once again, no potential impurity species are expected to contribute significantly to carrier doping in t-Se, as each extrinsic defect is either low-energy and electrically-neutral, or higherenergy and amphoteric (i.e. self-compensated). In fact, several of these impurities are low-energy amphoteric species with e(+1/ 1) transition levels near the mid-gap, which will act to compensate both p and n-type doping, including H interstitials, halogen substitutions and pnictogen substitutions. The most ptype conditions are obtained for F doping due to the lower position of the self-compensated F Se e(+1/ 1) level at 0.42 eV above the VBM, giving a maximum hole concentration ⇠10 12 cm 3 as shown in Fig. . However, this still does not explain the apparent p-type doping / acceptor density of 10 14 -10 16 cm 3 in as-grown polycrystalline Se samples. Looking at the experimental literature, we note that both Nijland and Plessner 139 reported that addition of halogens (Br & I) improved the conductivity of polycrystalline t-Se, but that this was not due to changes in carrier concentrations (in agreement with our predictions) but rather increased carrier mobilities by reducing the resistivities of grain boundaries and remnant crystalline-amorphous interfaces in the samples. Notably, Todorov et al. reported a much lower hole concentration of 3 ⇥ 10 12 cm 3 in polycrystalline t-Se using AC Hall measurements. However, Nielsen et al. recently expanded on these findings through carrier-resolved photo-Hall and temperature-dependent Hall measurements, revealing that the observed carrier densities at room temperature in the dark are significantly affected by carrier freeze-out and depletion through surface and/or interface defects. These findings suggest that the true acceptor density is on the order of N A ⇠ 10 16 cm 3 138 .