DFT-based (HSE06) investigation of band gap engineering: Optoelectronic, mechanical and thermodynamic insights in Cs₂AgBiY₆ (Y--Br, I) for photovoltaic applications

The study investigates the properties of double perovskites Cs2AgBiBr6 and Cs2AgBiI6, focusing on their structural, mechanical, dynamic, thermodynamic, and optic properties. These compounds have cubic structures with specific elements at different Wyckoff sites. The lattice constants are 11.043 & Aring; (11.854 & Aring;) for Cs2AgBiBr6 (Cs2AgBiI6) respectively, indicating an increase in lattice constant with iodine substitution. The elastic constants and bulk modulus confirm mechanical stability, and the anisotropy factor suggests greater anisotropy in the iodine-containing perovskite. Thermodynamic stability is confirmed by positive phonon frequencies and the convergence of thermodynamic potentials to zero at zero absolute temperature. The study uses HSE06 approximation for electronic band structures, showing an indirect band gap at zero pressure for both compounds, with Cs2AgBiBr6 (2.322 eV (HSE06), 1.305 eV (PBEsol)) having a larger gap than Cs2AgBiI6 (1.526 eV (HSE06), 0.795 eV (PBEsol)). The optical properties reveal the materials' behavior in the UV-visible range. Cs2AgBiI6 (1.494 eV) has a lower optical band gap than Cs2AgBiBr6 (2.213 eV), making it a promising candidate for solar cells. The refractive index and reflectivity indicate energy-dependent transparency. Optical loss analysis suggests that Cs2AgBiI6 is more suitable for solar cells due to minimal energy losses and significant absorption in the visible range.