Cu₂SiSe₃ as a promising solar absorber: harnessing cation dissimilarity to avoid killer antisites

Copper-chalcogenides are promising candidates for thin film photovoltaics due to their ideal electronic structure and potential for defect tolerance. To this end, we have theoretically investigated the optoelectronic properties of Cu₂SiSe₃, due to its simple ternary composition, and the favorable difference in charge and size between the cation species, limiting antisite defects and cation disorder. We find it to have an ideal, direct bandgap of 1.52 eV and a maximum efficiency of 30% for a 1.5 μm-thick film at the radiative limit. Using hybrid density functional theory, the formation energies of all intrinsic defects are calculated, revealing the p-type copper vacancy as the dominant defect species, which forms a perturbed host state. Overall, defect concentrations are predicted to be low and have limited impact on non-radiative recombination, as a consequence of the p - d coupling and antibonding character at the valence band maxima. Therefore, we propose that Cu₂SiSe₃ should be investigated further as a potential defect-tolerant photovoltaic absorber.