Even though the charge transport calculations roughly reproduce the temperature dependence of the experimental mobility, there is a factor of three difference in room temperature mobility between computation and experiment. We expect this is mostly due to the polycrystalline nature of the films, and possibly due to scattering by non-polar optical phonons, which are not considered in the AMSET workflow. It is plausible that extended intra-grain defects and amorphous inclusions are present in our sputtered films. These crystalline imperfections would increase the GBS term in the calculation, and indeed changing the effective crystallite size to 20 nm (Figure ) † drastically changes the contribution of GBS scattering to the mobility, and the room temperature value drops to 90 cm 2 V -1 s -1 . Additional ionised or neutral defects besides the dominant Cu vacancies are also expected in an off-stoichiometric film. We expect that epitaxial CaCuP films and CaCuP single crystals could display hole mobilities closer to the theoretical limit, as is often observed for many semiconductors.