Overall, our results find the formation energy of antisite defects to be significantly larger than in CIGS and kesterite materials (e.g. Cu Zn de-Figure : Position of the Fermi level during annealing (red) and quenching (blue) under Cupoor (solid line) and Cu-rich (dashed line) synthesis conditions. fects has a formation energy of 0.4 eV in CZTS), due to larger valence and size differences between the cation species (i.e. Si IV (r = 0.26 Å) and Cu I (r = 0.6 Å) vs Sn IV (r = 0.55 Å) and Zn II (r = 0.6 Å) in CZTS, where Sn Zn are the killer defects). This would have the added benefit of reducing cation disorder in Cu 2 SiSe 3 which has been shown to lower the formation energy of defects in CZTS. All remaining defect species not discussed in detail have too high formation energies to form in sufficient concentrations to significantly affect device performance. Our study of the intrinsic defect in Cu 2 SiSe 3 shows that the antibonding VBM, often associated with defect-tolerance, indeed contributes to the formation of electrically benign defects, 51 borne out by the V Cu and V Se discussed earlier. This paints a promising picture for potential defect tolerance in this compound.