Systematic analysis of lead-free halide K₂SnX₆ (X = Cl, Br, I) double perovskites for solar cell applications

Perovskites possessing lead have gained immense consideration recently owing to their unique optoelectronic properties. Thus, they are considered highly suitable materials for solar power and harvesting applications. However, the instability of perovskites in air and moisture, along with the toxicity of lead, has limited their use in developing practical devices. In this work, detailed first-principles research was carried out to discover the basic structural, electronic, optical, and thermoelectric properties of cubic lead-free double perovskites K₂SnX₆ (X = Cl, Br, I). All structures exhibited good mechanical stability as they satisfied the Born criteria. The values of their Poisson’s (v) and Pugh’s ratios (B₀/G) exceeded the critical numbers of 0.26 and 1.75, respectively, revealing their ductile nature. The bandgap calculations for the structures were accomplished using the generalized gradient approximation (GGA), which revealed that these perovskites exhibited direct band gaps except K₂SnI₆, having metallic characteristics. The bandgaps were also calculated by adding the modified Becke–Johnson potential (TB-mBJ). Moreover, computed refractive indices for K₂SnBr₆ and K₂SnI₆ revealed excellent luminescent properties in the UV region. The figure of merit (ZT) for K₂SnCl₆ and K₂SnBr₆ approached 1, whereas its value was around 0.568 for K₂SnI₆ at room temperature. The conclusions of this study provide sufficient evidence that these perovskite structures K₂SnX₆ (X = Cl, Br, I) show immense potential for upcoming energy conversion and solar cell-based technologies.