A note on mobility. -P-type charge transport properties were modelled using the AM-SET package, 30 an extended explanation of which can be found in the Supplementary Material. Under the assumption of free-carrier transport, K-doping to a hole concentration of 4.6 × 10 14 cm -3 will yield a maximum room temperature hole mobility of 5.78 cm 2 V -1 s -1 , while In-doping to a hole concentration of 8.6 × 10 15 cm -3 will result in a 1.21 cm 2 V -1 s -1 mobility limit. Meanwhile, the mobilities reported for K-doped BaSnO 3 by Kim et al range between 0.06 cm 2 V -1 s -1 to 0.30 cm 2 V -1 s -1 , 8 in reasonably good agreement with the simulated value. Both theory and experiment suggest therefore that p-type BaSnO 3 will not display the high hole mobility required for p-type transparent conducting applications (critical for reaching high field-effect mobilities in p-n junctions), due to the highly localised VBM states. On the contrary, a hole mobility of 30 cm 2 V -1 s -1 has been measured in the p-type oxide Ba 2 BiTaO 6 (which crystallises in a rhombohedral "double-perovskite" structure) due to the strong interaction of Bi 6s 2 states with O 2p states at the VBM. This highlights valence band engineering as a strategy for increasing dispersion and thus hole mobility, shifting the focus away from pure oxides and towards light anion doping, such as N-doped BaSnO 3 , 45 formally mixed-anion systems, or even fully non-oxide materials. total scattering are in grey dots, lavender dash-dots, magenta dashes and solid black, respectively. Plotted using ThermoParser. Figure shows the scattering rates that determine mobility at the maximum carrier concentrations for K-and In-doping. Considering first K-doping, at room temperature both ionised impurity scattering (IMP) and polar optical phonon (POP) scattering contribute significantly to the overall scattering rate (Figure ). The positive temperature dependence of POP scattering causes it to dominate at temperatures above 300 K, while the IMP rate remains fairly constant. The main contributing factor to the POP scattering at higher temperatures is the high-frequency O-dominated modes (as shown in the phonon density of states in the Supplementary Material, and as is typical in the n-type metal oxides), which could again be reduced by valence band engineering. Meanwhile, the increased number of charged defects that are present under In-doping (over 3 orders of magnitude compared to K-doping, as shown in Table ) result in a drastically increased IMP scattering rate (Figure ) that is responsible for the lower hole mobility of 1.22 cm 2 V -1 s -1 . These insights demonstrate the importance of not only dopability when searching for a p-type TCM, but also the ability of a material to potentially withstand a high density of charged impurities.