Defects and band positions in the p-type transparent conductor CuI

While high-performance n-type transparent conductive materials (TCMs) have existed for decades, heavy p-type doping of wide band-gap materials has proven much more challenging. The simple cubic compound CuI was recently rediscovered for this application and is currently the p-type TCM with the highest figure of merit. However, the native defects responsible for p-type conductivity in CuI, as well as compensating defects limiting the maximum achievable doping levels, have not yet been identified experimentally. Furthermore, there is disagreement in the literature on the work function and absolute band positions of CuI relative to vacuum. In this experimental study, we employ temperature- and intensity-dependent photoluminescence to draw new conclusions on the defect landscape of CuI. We then employ a combined photoemission spectroscopy-Kelvin probe system to show that the measured band positions depend critically on surface phenomena. Finally, we demonstrate that terahertz spectroscopy is an ideal tool for characterizing the electrical properties of CuI reliably and non-destructively.