The dominant mobility-limiting mechanism is ionised impurity scattering, showing a very weak temperature dependence. Grain boundary scattering also has a weak temperature dependence but a lower scattering rate, assuming a grain diameter L=50 nm based on scanning electron microscopy images of the films (Figure ) †. At around 250 K, polar optical phonon scattering begins to dominate over scattering from grain boundaries, due to its much stronger temperature dependence. Scattering from acoustic phonons has a slightly weaker temperature dependence, and the lowest contribution to the overall scattering rate. The combined effect of these four competing mechanisms is a moderate decrease of the total mobility with temperature, in agreement with experiment. The room-temperature mobility is predicted to be 113 cm 2 V -1 s -1 , with polar optical phonons becoming increasingly important at high temperatures. These results confirm that it is reasonable to describe the experimental mobility as the reciprocal sum of a temperature independent mobility (representing ionised impurity scattering and potentially grain boundary scattering) and a phonon-limited mobility which is temperature dependent. Since the calculations predict that phonon scattering is predominantly from polar optical phonons, we can replace the generic power law µ p (T ) ≡ AT γ with the analytically derived expression µ p (T ) ≡ B exp(E p /k B T ) -1, where E p is the energy of the optical phonon mode responsible for hole scattering. Fitting again the experimental mobility as µ -1 (T ) = µ -1 i + µ -1 p (T ) with the new expression for µ P (T ) yields E p =32 meV (black line, Figure ). This value is in good agreement with the effective optical phonon energy of CaCuP (∼ 27 meV) derived from our calculated phonon band structure following Ref. 47 (Figure ) †.