We find that Ga Zn and Al Zn are low energy donors, with formation energies of 0.45 eV and 0.58 eV in their neutral charge states, respectively. In both cases, the dopant interstitial defects are rather high in energy (around 5 eV at the CBM), and are charge compensated by their respective dopantsubstitutions onto the Sb site. F O anion substitutions have higher formation energies of 1.38 eV and 1.46 eV for the inequivalent oxygen sites, while the F interstitials do not donate electrons to the conduction band. Crucially, we find that the native p-type defects, V Zn and Zn Sb are too high in energy to charge compensate Ga Zn , Al Zn and F O . We complete the same SCFL analysis as before for each case, and find that Ga emerges as the superior dopant with a predicted room temperature charge carrier concentration of 3.4 × 10 19 cm -3 and a SCFL of 3.69 eV (0.15 eV above the CBM), thereby predicting degenerate conductivity. For Al and F, the predicted charge carrier concentrations are 3.2 × 10 19 cm -3 and 7.2 × 10 18 cm -3 , with the SCFL sitting above the CBM in both cases. Experimentally, we record carrier concentrations of 8.9 × 10 19 cm -3 , 2.0 × 10 20 cm -3 and 2.4 × 10 20 cm -3 in Ga-doped single crystals. The presence of adventitious H is the most likely origin of discrepancy, but again we find qualitative agreement with the SCFL analysis. Conductivity rises with increasing carrier concentration by several orders of magnitude in the single crystals to 526 S cm -1 , 1230 S cm -1 and 1890 S cm -1 respectively, and a similar trend is observed in the doped powders (conductivity plotted in Figure ), competitive with established TCOs.