Defect Tolerance via External Passivation in the Photocatalyst SrTiO: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 SrTiO, 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 SrTiO 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 [-Al] 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 SrTiO, paving the way for improved solar-to-energy conversion.