Indirect-to-direct band gap transition and optical properties of Cs2BiAgX6 with mechanical strains: the density functional theory investigation

The last research of Cs2BiAgX6 (X = Cl, Br) exhibiting a desired band gap in the visible imply that the compounds maybe as a potential candidate for CH3NH3PbI3. However, both the band gaps of Cs2BiAgCl6 and Cs2BiAgBr6 are indirect, which means the optical properties will be limit. Hence, it is a critical issue that obtain a direct band gap to improve optical properties of Cs2BiAgX6 (X = Cl, Br) as a material for the perovskite solar cells. In this work, we optimized their structures, and calculated their electronic properties as well as optical properties of Cs2BiAgX6 to guarantee the following calculation, that is, inducing indirect-to-direct band gap transition by mechanical strains, which is correct and meaningful. In the calculation, we found that the band gap calculating by GGA + U with strains on (001) plane and in [001] direction can be direct. In addition, the band gap will be decreasing with the rising strains, which are able to make the properties of Cs2BiAgX6 (X = Cl, Br) improved distinctly. Our work not only uncover screening new materials for solar cells, but also further understand the inherent properties of Cs2BiAgX6 (X = Cl, Br).