Suitable point defect chemistry is crucial to the operation of transparent conducting materials. For a p-type system, it is necessary to have low formation energy acceptor defects at Fermi levels close to, or ideally within, the VBM. As the separation between defect state and band edge decreases, more holes can be generated at the VBM through thermal excitation of an electron to the defect (with energy ∼ 𝑘 𝐵 𝑇 ), increasing conductivity. It is also desirable that such p-type defects generate delocalised charge density, enabling a high charge carrier mobility with no energetic barrierthe formation of localised charge, or hole polarons, necessitates an activation energy 𝐸 𝐴 (with an Arrhenius-like temperature dependency) for charge transport, which significantly reduces optoelectronic performance (consider the polaron-limited mobility in CuAlO 2 of 3 cm 2 V -1 s -1 ). To this end, both the intrinsic defect chemistry of CuI and the effects of chalcogen doping (S, Se) were studied using hybrid functionals. Transition level diagrams under a variety of Cu-I chemical potentials (Cu-rich, mid-point, and Cu-poor) are shown in Figure , while tabulated formation energies of defects and competing phases, along with extended methodology, can be found in the Supple-mentary Information. The copper vacancy is the predominant defect in CuI, with a formation energy below 1 eV regardless of growth conditions. It has a transition level from the 0∕1-charge state approximately at 0.24 eV from the band edge, indicative of a deep defect. A significant disruption to the local crystal structure upon vacancy formation supports this behaviour, with one nearby Cu atom relaxing towards the vacancy by 0.09 Å. The hole generated at the VBM by the formation of a copper vacancy is found to localise on this displaced atom, shown in Figure . However, analysis of the Fröhlich polaron constant for CuI (𝛼 = 0.44 for PBEsol and 1.55 for PBE0) indicates that such a polaron is likely to be spread over several unit cells (supported by observation of several metastable defect geometries within 1 meV of the ground state with less localised hole density, see Figure ) so should still result in reasonable charge carrier mobility, and crucially indicates that the band carrier model assumed for the charge transport calculations is still valid. Under Cu-poor conditions, charge carrier concentrations of up to 2 × 10 19 cm -3 are predicted solely due to the increased concentration of copper vacancies, indicating that this species is the key factor in controlling p-type conductivity in CuI.