First-principles study on the electronic and optical properties of strain-tuned mixed-halide double perovskites Cs2TiI6−xBrx

The properties of Cs 2 TiI 6 − x Br x using density functional theory were investigated. With the increase of Br doping, the structural stability is improved. Then the pressure is introduced to change the energy gap of Cs 2 TiI 6 − x Br x . Meanwhile, the optical absorption coefficient changes obviously with both pressure and Br doping. The major absorption region narrows and the peak value becomes higher with the increasing of pressure. While the peak value starts with a small decrease and then a rise with Br doping. Normalized the absorption rate and compared with the wavelength in sunlight, we get the optical coincidence index (OCI). These results demonstrate that under the influence of doping, both Cs 2 TiI 2 Br 4 and Cs 2 TiI 3 Br 3 are suitable materials for solar cells. When the pressure increases, the stability of Cs 2 TiI 6 − x Br x is enhanced, while the optical absorption is attenuated. Based on suitable absorption coefficient and strong OCI, Cs 2 TiI 2 Br 4 and Cs 2 TiIBr 5 are ideal light harvesting materials. • With density functional theory (DFT) to study properties of double perovskite Cs 2 TiI 6 − x Br x • Optical coincidence index (OCI) was calculated by normalizing the absorption rate and comparing with the wavelength in sunlight. • Influenced by doping, Cs 2 TiI 3 Br 3 and Cs 2 TiI 2 Br 4 are suitable photoelectric material. • Under high pressure, Cs 2 TiIBr 5 has more potential in solar cells