All calculations were performed using periodic DFT within the Vienna Ab Initio Simulation Package (VASP). This work uses the HSE06 hybrid DFT functional which combines 75% exchange and 100% of the correlation energy from the Perdew, Burke, and Ernzerhof (PBE) semi-local functional with 25% exact Hartree-Fock (HF) exchange at short ranges. HSE06 has been shown to accurately reproduce the electronic structure and properties of many small bandgap semiconductors, including Cu-multinary chalcogenide semiconductors. add more. A plane wave energy cutoff of 350 eV was used and k-points were sampled with a Γ centered 4 × 4 × 4 mesh for the 12-atom primitive cell. The total energy was converged to less than 1 meV atom -1 with these parameters, and a force tolerance 0.01 eV Å-1 was used during geometry relaxation. The plane wave cutoff was increased to 500 eV during structural relaxation to avoid errors arising from Pulay stress. A denser 6x6x6 k-point mesh was used for the calculation of the optical properties, and a larger number of unoccupied bands were included in the calculation (NBANDS increased to 100). Upper limits to the photovoltaic efficiency were determined from the HSE06 electronic and optical calculations, using both the 'Spectroscopic Limited Maximum Efficiency' (SLME) metric and the method of Blank et al. To calculate the electronic band alignment, the core level alignment method was used with surfaces generated by surfaxe. Defect calculations were performed in a 96atom supercell created by the expansion of the primitive unit cell with the matrix: