The remaining intrinsic defect species are the p-type iodine interstitial, (I int ), the n-type iodine vacancy (V I ) and the n-type copper interstitial (Cu int ). The iodine interstitial is an ultra-deep acceptor with prohibitively high formation energies under all growth conditions (2.5 eV in the best case), and is unlikely to play a major rôle in boosting p-type conductivity. The n-type defects are both deep donors, and under Cu-rich conditions are predicted to charge compensate the holes generated by the copper vacancy (where the blue and orange lines intersect in Figure ). However, as growth conditions are modulated towards a Cu-poor environment, the formation energies of both the iodine vacancy and copper interstitial are sufficiently raised such that they can no longer charge compensate.