Tailoring the Optoelectronic and Thermoelectric Properties of Lead‐Free Double Perovskites K ₂ NaBiZ ₆ (Z = Br, I) for Renewable Energy Applications: A First‐Principles Density Functional Theory Investigation

Lead‐free double perovskites are gaining significant attention due to their potential in a range of advanced technologies, including solid‐state lighting, thermoelectric systems, and photovoltaic applications. This work presents a comprehensive investigation of the physical properties of the inorganic double perovskites K 2 NaBiZ 6 (Z = Br, I) based on density functional theory and semiclassical transport calculations. The study examines the mechanical, electronic, optical, and transport behaviors of K 2 NaBiBr 6 and K 2 NaBiI 6 to evaluate their potential for use in optoelectronic devices and solar energy applications. The exchange–correlation effects were modeled using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional. The computed indirect bandgap values for K 2 NaBiBr 6 were ≈2.99 and 2.90 eV and for K 2 NaBiI 6 were 2.15 eV and 1.95 with PBE‐GGA and PBE + SOC, respectively. Mechanical assessments confirmed the stability of the materials, indicating their potential suitability for experimental synthesis. Optical behavior was evaluated through key parameters including the dielectric constant, absorption coefficient, refractive index, and reflectivity. Strong absorption peaks observed in the visible spectrum suggest these compounds are promising candidates for optoelectronic applications. The transport characteristics of the materials were analyzed by evaluating parameters such as electrical and thermal conductivity, Seebeck coefficient, and the dimensionless figure of merit (ZT). The estimated ZT values across the temperature range of 200–1200 K were found to be 0.69 for the bromide‐based compound and 0.72 for the iodide‐based compound. To investigate thermal behavior, properties such as heat capacity ( C v ), entropy ( S ), and Debye temperature ( θ D ) were determined using the quasiharmonic Debye model. Moreover, the transport data suggest that K 2 NaBiZ 6 (Z = Br, I) behaves as a p‐type semiconductor, highlighting its potential in thermoelectric applications.