Stability and optoelectronic property of lead-free halide double perovskite Cs2 B′BiI6 (B′ = Li, Na and K)*

Although lead-based perovskite solar cells have achieved more than 25% power conversion efficiency, the toxicity of lead and instability are still urgent problems faced in industrial application. Lead-free halide double perovskite (DP) materials are promising candidates to resolve these issues. Based on the density functional theory, we explore the geometric stability, thermodynamic stability, mechanical stability, electronic structures, and optical properties of the Cs2B'BiI6 (B' = Li, Na and K) DP materials. By analyzing the tolerance factor and octahedral factor, we find the geometric stabilities of Cs2NaBiI6 and Cs2KBiI6 DPs are better than Cs2LiBiI6. By calculating the total energy, formation energy and decomposition energy, we propose that the most favorable structure of Cs2B'BiI6 is the orthorhombic phase, and Cs2LiBiI6 is less stable relative to the other two counterparts from an energetic viewpoint. Mechanical stability evaluations reveal that the orthorhombic Cs2LiBiI6 material is less stable relative to the isostructural Cs2NaBiI6 and Cs2KBiI6 DPs. The mechanical property calculations indicate that the Cs2B'BiI6 DPs possess good ductility, which can be used as flexible materials. Electronic structures and optical property calculations show that the orthorhombic Cs2B'BiI6 DPs have suitable band gap values, weaker exciton binding energies, and excellent optical absorption performance in the visible-light range. Based on the above comprehensive assessments, we can conclude that the orthorhombic Cs2NaBiI6 and Cs2KBiI6 DPs with good stability are promising candidates for solar cell applications.