Optimizing Cs2AgXCl6 (X=Bi, In) Double Perovskites for Light-Harvesting Devices

This research employed Density Functional Theory (DFT) calculations, utilizing both GGA-PBEsol and HSE06 approximations, to comprehensively investigate the impact of replacing Bi³⁺ with In³⁺ in Cs2AgXCl6 (X = Bi, In) double perovskites. The substitution resulted in a lattice contraction from 10.587Å to 10.249Å, increased the bulk modulus from 29.60GPa to 32.81GPa, and led to a transition from an indirect bandgap of 2.90eV in Cs2AgBiCl6 to a direct yet parity-forbidden bandgap of 3.204eV in Cs2AgInCl6. Our analysis confirms the stability of both materials through calculations of elastic constants, bulk modulus, phonon frequencies, and thermodynamic potentials, along with verification of phase stability using octahedral, tolerance, and new tolerance factors. Notably, Cs2AgBiCl6 exhibited a wider absorption range extending into the visible spectrum, while Cs2AgInCl6 excelled in high-energy photon absorption. Their distinct optical behaviors were further reflected in their refractive indices of 1.75 and 1.61 for Cs2AgBiCl6 and Cs2AgInCl6, respectively. These findings highlight the potential of Cs2AgBiCl6 for applications in solar energy harvesting due to its enhanced visible light absorption, while Cs2AgInCl6 shows promise for high-energy photon detection and related technologies. This study elucidates the structure-property relationships governing these double perovskites, paving the way for their rational design and optimization for diverse light-harvesting and energy conversion applications.