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 Gamma 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 10(11) W/msK(2) and ZT value of 0.3. The optical properties found consist of high absorption coefficients (about 10(6) cm(-1) ), low reflectivity (around 1-15 %), and high optical conductivity (approximately 10(15) sec(-1) ).