First-principles investigations on the structural, optoelectronic, mechanical and transport properties of new stable lead-free double perovskites Cs2BB′I6 (B = Ag/Rb, B′ = Bi/Ga) halides

•Investigation of the structural, mechanical, electronic, transport and optical properties of lead-free Cs2BB′I6 (B = Ag/Rb, B′=Bi/Ga) double perovskites is done by employing first-principles calculation.•The optimization cures, negative formation energy and evaluation of the tolerance factor support the stability of 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 these compounds are useful for the application in optoelectronics.•The high Seebeck, ZT and electrical conductivity values suggests their availability for thermoelectric devices.