Band structure engineering to improve the optical and thermoelectric properties of Rb2AgXBr6 (X=Al, In, Ga) for energy applications within DFT framework

In the current study, we have employed density functional theory to evaluate thermoelectric, optical and electronic properties of rubidium bromide based double perovskites. It has been found that all three compounds i.e. Rb₂AgAlBr₆, Rb₂AgGaBr₆ and Rb₂AgInBr₆ have a direct band gap and the band gap appears at Γ symmetry point. The band gap of Rb₂AgAlBr₆ = 0.92 eV and of Rb₂AgInBr₆ = 0.29 eV whereas for Rb₂AgGaBr₆ has shown bands overlapping. The merged band gap of Ga-substituted material imparts in it excellent conductivity which makes it a potential candidate for application in conducting materials. Among all Rb₂AgAlBr₆ composition showed efficient TE properties with power factor of around 3.0 x 10¹¹ W/msK² and ZT value of 0.3. The optical properties found consist of high absorption coefficients (about 10⁶ cm⁻¹), low reflectivity (around 1–15 %), and high optical conductivity (approximately 10¹⁵ sec⁻¹).