Exploring the optoelectronic attributes, thermoelectric and photocatalytic potential of double perovskites Cs2BB'H6 (B = Al, Na and B'=Tl, In): A DFT study

•First-principles investigation of the structural, phonon, elastic, electronic, thermoelectric, optical and photocatalytic properties of double perovskites Cs2BB'H6 (B = Al, Na and B'=Tl, In) is done.•The optimization cures, negative formation energy and positive phonon curves show the stability of the studied compounds.•The electronic band structure and density of states (DOS) reveals the semiconductor nature of both compound.•An optical band gap with maximum absorption reveals that this compound is also useful for the application in optoelectronics.•The thermoelectric response suggests their availability for thermoelectric devices.•Photocatalytic properties show the excellent response of both compounds for solar hydrogen synthesis.