The study investigates the upper efficiency limit of antimony selenide (Sb₂Se₃) solar cells, a promising sustainable photovoltaic material. Despite significant progress, the conversion efficiency of Sb₂Se₃ has plateaued at around 10%. Using first-principles defect analysis, the authors identify vacancies as the primary active recombination centers, particularly Se vacancies ($V_{\text{Se}}$) and Sb vacancies ($V_{\text{Sb}}$). They predict an upper limit of 25% efficiency under optimal equilibrium conditions where charged vacancy concentrations are minimized. The detrimental effect of Se vacancies can be reduced by extrinsic oxygen passivation, which transforms deep levels associated with Se vacancies to shallow ones, enhancing the performance of Sb₂Se₃ solar cells. The study provides insights into the loss mechanisms associated with intrinsic point defects and offers strategies to optimize the performance of Sb₂Se₃ solar cells.The study investigates the upper efficiency limit of antimony selenide (Sb₂Se₃) solar cells, a promising sustainable photovoltaic material. Despite significant progress, the conversion efficiency of Sb₂Se₃ has plateaued at around 10%. Using first-principles defect analysis, the authors identify vacancies as the primary active recombination centers, particularly Se vacancies ($V_{\text{Se}}$) and Sb vacancies ($V_{\text{Sb}}$). They predict an upper limit of 25% efficiency under optimal equilibrium conditions where charged vacancy concentrations are minimized. The detrimental effect of Se vacancies can be reduced by extrinsic oxygen passivation, which transforms deep levels associated with Se vacancies to shallow ones, enhancing the performance of Sb₂Se₃ solar cells. The study provides insights into the loss mechanisms associated with intrinsic point defects and offers strategies to optimize the performance of Sb₂Se₃ solar cells.