Limits to Hole Mobility and Doping in Copper Iodide

Over one hundred years have passed since the discovery of the p-type transparent conducting material copper iodide, pre-dating the concept of the ``electron-hole'' itself. Supercentenarian status notwithstanding, little is understood about the charge transport mechanisms in \ce{CuI}. Herein, a variety of modelling techniques are used to investigated the charge transport properties of \ce{CuI}, and limitations to the hole mobility over experimentally achievable carrier concentrations are discussed. Poor dielectric response is responsible for extensive scattering from ionised impurities at degenerately doped carrier concentrations, while phonon scattering is found to dominate at lower carrier concentrations. A phonon-limited hole mobility of \SI{162}{\centi\meter\squared\per\volt\per\second} is predicted at room temperature. The simulated charge transport properties for \ce{CuI} are compared to existing experimental data and the implications for future device performance are discussed. In addition to charge transport calculations, the defect chemistry of \ce{CuI} is investigated with hybrid functionals, revealing that reasonably localised holes from the copper vacancy are the predominant source of charge carriers. The chalcogens S and Se are investigated as extrinsic dopants, where it is found that despite relatively low defect formation energies, they are unlikely to act as efficient electron acceptors due to strong localisation of holes and subsequent deep transition levels.