The transition levels of all three M XIII Sn substitutions are quite deep, resulting in trapped hole polarons (as shown for In Sn in Figure ). Analogous to the case of the M I dopants, the oxygen vacancy is expected to charge compensate the holes generated by the p-type substitutions, as highlighted by points D and E in Figure . However, self-compensation is not predicted to be an issue for the M XIII species, due to the higher formation energies and charge states of the interstitial species, compared to the M I series. Furthermore, n-type substitution onto the Ba site requires prohibitively high formation energies (nearly 5 eV for In Ba and several eV higher for the should not be possible to achieve metallic-like p-type conductivity in BaSnO 3 , as shown in Table . For K-doping, the SCFL is found at 1.40 eV, while for In-doping it drops down to 1.13 eV -the ideal position of the SCFL for a degenerate p-type semiconductor is within species in BaSnO 3 at 1050 K, calculated using the py-sc-fermi package. Despite the reasonably high concentration of p-type substitutional defects (particularly in the case of In-doping), hole carrier densities are limited to 4.6 × 10 14 cm -3 and 8.6 × 10 15 cm -3 for K-and In-doping, respectively (equivalent to rather poor conductivities of 4.26 × 10 -4 S cm -1 and 1.68 × 10 -3 S cm -1 ). This due to the low energy oxygen vacancy which acts as a major charge compensating centre. To boost the hole conductivity further, it is imperative that oxygen deficiency is minimised during growth, which could be achieved by using non-equilibrium growth techniques such as pulsed laser deposition (PLD) and molecular beam epitaxy (MBE). Unfortunately, these techniques do not scale well and are extraordinarily expensive, severely limiting the commercialisability of p-type BaSnO 3 grown in this fashion.