Formation energy diagrams for the remaining chemical potential limits and a plot of the chemical potential space are provided in the Supporting Information Section 3 and 4. Most Cu-based absorbers are synthesized under Cu-poor conditions, with V Cu being the dominant defect species -although in the case of kesterites (e.g. CZTS), Cu Zn antisite defects dominate. Indeed for Cu 2 SiSe 3 , we find V Cu to have the lowest formation energy and find it to be a shallow acceptor state resulting in Cu 2 SiSe 3 being intrinsically p-type, with the hole carrier concentrations being of the same order of magnitude as the V Cu concentration (Table 2). At the experimental annealing temperature of 775 K, the high concentration of V Cu places the Fermi level at -0.02 eV (Cu-poor) and 0.06 eV (Cu-rich) relative to the VBM (Figure ).