K₂AlInZ₆ (Z = F, Cl, Br) Double Perovskites: Potential Candidates for Optoelectronic and Photovoltaic Devices

In this research, K₂AlInZ₆ (Z = F, Cl, Br) are double perovskite compounds with unique and complementary characteristics, rendering them exceptionally appropriate for many modern technological applications. This paper provides a thorough examination of the structural, electronic, elastic, mechanical, optical, and thermodynamic features of K₂AlInZ₆ (Z = F, Cl, Br) double perovskites by first-principles calculations based on density functional theory (DFT). The structural characteristics, encompassing lattice parameters and formation energies, validate the stability of these materials, which exhibit a cubic configuration with the Fm-3m space group. The electronic band structure calculations with the modified Becke-Johnson exchange potential indicate indirect band gaps for K₂AlInZ₆ (Z = F, Cl, Br), with band gaps of 3.73, 2.88, and 2.41 eV for K₂AlInF₆, K₂AlInCl₆, and K₂AlInBr₆, respectively, rendering them viable candidates for optoelectronic applications. The estimated elastic constants, bulk modulus, and shear modulus demonstrate mechanical stability, indicating their suitability for durable and flexible devices. The optical characteristics, including dielectric functions and absorption spectra, exhibit considerable absorption in the ultraviolet range, indicating their potential use in photovoltaic systems. Furthermore, the thermodynamic characteristics are examined by assessing formation energy and Debye temperature. The negative formation energies of these materials signify their strong thermodynamic stability, whereas the Debye temperature analysis elucidates their lattice vibrations and heat capacity, further substantiating their stability and applicability in diverse energy technologies. At 800 K, K₂AlInF₆, K₂AlInCl₆, and K₂AlInBr₆ show Seebeck coefficients of ~150, ~160, and ~135 μV/K, respectively, with κ_e/τ rising to ~4.0–4.75 × 10¹⁴ W/mKs. ZT values peak at ~0.69, ~0.68, and ~0.58, indicating strong thermoelectric potential at high temperatures.