Bandgap Engineering of Two-Dimensional Double Perovskite Cs₄AgBiBr₈/WSe₂ Heterostructure from Indirect Bandgap to Direct Bandgap by Introducing Se Vacancy

Heterostructures based on layered materials are considered next-generation photocatalysts due to their unique mechanical, physical, and chemical properties. In this work, we conducted a systematic first-principles study on the structure, stability, and electronic properties of a 2D monolayer WSe₂/Cs₄AgBiBr₈ heterostructure. We found that the heterostructure is not only a type-II heterostructure with a high optical absorption coefficient, but also shows better optoelectronic properties, changing from an indirect bandgap semiconductor (about 1.70 eV) to a direct bandgap semiconductor (about 1.23 eV) by introducing an appropriate Se vacancy. Moreover, we investigated the stability of the heterostructure with Se atomic vacancy in different positions and found that the heterostructure was more stable when the Se vacancy is near the vertical direction of the upper Br atoms from the 2D double perovskite layer. The insightful understanding of WSe₂/Cs₄AgBiBr₈ heterostructure and the defect engineering will offer useful strategies to design superior layered photodetectors.