The lowest formation energy n-type defect is Cu i . There are four distinct interstitial sites, with the lowest energy site having the deepest (+/0) donor level, 0.17 eV below the CBM. As a result, Cu i may trap charge carriers, however the impact on device performance is expected to be limited due to slow hole capture. V Se is an ultra-deep donor, with transitions levels lying below the VBM -aided by the high energy of the VBM due to Cu d -Se p repulsion, as observed in Cu 2 ZnSnSe 4 (CZTSe). While this defect forms in relatively high concentra-tions under Cu-rich conditions, due to being electronically inactive it should not contribute to non-radiative recombination. The Se Si antisite defect also has ultra-deep donor states in the valence band, however the defect is additionally found to be stable in the -1 and -2 charge states deep in the band gap. Notably, symmetry-breaking was required to identify the ground-state structures of both V Se and Se Si (Figure and). To stabilize the negative charge states of Se Si the defect undergoes a significant structural relaxation. The Se Si defect moves away from the tetrahedrally bonded cation site and forms a trimer with two of the neighboring Se, similar to the reconstructions observed in Sb 2 Se 3 . As a result, this defect site could act as a non-radiative recombination center, however to quantitatively determine its impact, this would require the calculation of its carrier capture cross-section. The remaining defect states with a formation energy below 1.5 eV, which will form in significant concentrations depending on the growth conditions, are the Cu Si antisite and V Si . These defects behave similarly due to the ShakeNBreak 47,48 method finding that the lowest energy configuration of a V Si defect is actually a Cu Si + V Cu complex. This rearrangement occurs due to the high Si IV oxidation state, resulting in it being unfavorable to leave the site unoccupied and the low energy of formation of a V Cu leading to a mobile cation. Thus, both Cu Si and V Si defects have similar deep-lying levels. However, the carrier capture cross-sections are expected to be very small due to weak structural relaxation between defect charge states, with the bonds surrounding the defect site distorting by less than 0.02 Å upon charge capture/emission. The lack of distortion between charge states can be explained by considering the orbital make-up of the valence band and Cu Si . As Cu and Si are tetrahedrallycoordinated by Se, this gives rise to the same Cu d -Se p antibonding interaction as found at the VBM. Thus in the fully-ionised charge state (Cu Si ), the occupied anti-bonding defect levels lie above the VBM due to Coulomb repulsion, resulting in deep transition levels. Consequently, it is favorable to remove electrons and depopulate the anti-bonding levels, placing holes in the Cu d -Se p states (Supporting Information Figure ). The reduction in charge lowers the energy of the defect states, resulting in them being more VBM-like and making q = 0, -1 and -2 the stable charge states for Cu Si .