Defect Tolerance via External Passivation in the Photocatalyst SrTiO₃:Al

The efficiency of solar-to-energy conversion in semiconductors is limited by charge carrier recombination, often via defect-induced gap states. Although some materials exhibit an intrinsic defect tolerance that avoids fast recombination channels, there are few examples among metal oxides. We investigate the water splitting photocatalyst SrTiO₃, where photocatalytic performance is enhanced by extrinsic Al doping. We propose that defect tolerance emerges through a passivation effect that effectively eliminates in-gap states and nonradiative recombination. First-principles defect calculations show that oxygen vacancies are the primary defect species in SrTiO₃ under oxygen-poor synthetic conditions, which provide in-gap states that are active for carrier capture. Al substitutions are preferred at Ti sites adjacent to the oxygen vacancy, forming [V_O-Al_Ti] defect complexes. As the oxygen vacancy in-gap state is derived from Ti 3d-Ti 3d interactions across the vacancy, substituting Ti with Al deactivates this interaction and eliminates the in-gap state. The absence of valence d orbitals in Al is key for in-gap state reduction, as supported by the consideration of other dopants such as Sc. Our study illustrates how an orbital-wise understanding of defect states can enable doping strategies to achieve defect tolerance in materials like SrTiO₃, paving the way for improved solar-to-energy conversion.