Figure ,c shows the intrinsic defect transition level diagrams for CaCuP under strongly p-type and weakly p-type growth conditions. The lowest energy defect species under the most p-type growth conditions (Figure ) is the copper vacancy (V Cu , red), with a formation energy of 0.32 eV when in the neutral charge state. This species is likely to contribute the majority of charge carriers. It is stable in the -1 charge state (when the defect has accepted an electron, generating a hole in the valence band) until around 0.07 eV below the valence band maximum, indicating degenerate behaviour. This is in agreement with the temperatureindependent behaviour of the experimentally measured Hall hole concentration. In fact, there are three other degenerate acceptor defects, namely V Ca (black), P i (interstitial, dark cyan), and Cu Ca (hot pink) which are likely to contribute to p-type conductivity. However, they are not as prevalent as V Cu , due to significantly higher formation energies. The P Cu defect (grey) does not charge compensate these three species, as it is only stable in the neutral charge state across all Fermi levels. While the best quality CaCuP films are achieved under Ca-poor conditions, it is not the V Ca defect that is primarily responsible for p-type conductivity. However, the hole density generated by the Ca vacancy in the -1 charge state is still delocalised throughout the Cu-P network (similar to Cu vacancy hole density in Figure ), as the disruption to the crystal structure is spatially separated from the conduction pathway. Similar behaviour is observed in La-doping of the ntype transparent conducting perovskite BaSnO 3 . The conduction pathways are comprised of SnO 6 octahedra, so doping on the Ba site (the A cation site in the ABO 3 perovskite structure) does not affect the mobility, and instead drives up conductivity by increasing charge carrier concentration. This suggests that cation doping on the Ca site could be an effective strategy for increasing the concentration of charge carriers in CaCuP without significant degradation of the mobility.