A theoretical study of stable direct band gap double perovskites X₂YIO₆ (X = K, Rb) for renewable energy applications

Double perovskite semiconductors have got extensive research interest for renewable energy applications because of their excellent optoelectronic and thermoelectric properties together with significant structural and chemical stability. However, most of the efficient perovskites are lead based and there is still a big issue of toxicity with these materials. In present study, we have investigated the two non toxic double perovskite X₂YIO₆ (X = K, Rb) materials for renewable energy applications via first principles calculations. The negative formation and Gibbs free energies confirm the thermodynamic stability while the real frequencies of phonons establish the dynamical stability of these double perovskites. The electronic, optical and thermoelectric properties were calculated via Tran and Blaha modified Becke Johnson potential. The band structure calculations show the direct band gap of 0.98 and 0.75 eV for K₂YIO₆ and R₂YIO₆, respectively. On the basis of calculated optical parameters like dielectric function, absorption coefficient and reflectivity, the X₂YIO₆ compounds could show better solar cells and other optoelectronic applications. The positive values of the Seebeck coefficient categorize both of these double perovskites as p-type semiconductors. Their high values of power factor of the order 10^–2 Wm⁻¹ K⁻² and figure of merit of the order 0.7 at 1000 K, manifest high power conversion efficiency of these materials at elevated temperatures.