First-principles study on the stability and degradation of the lead-free double perovskite materials Cs2B'BiCl6 (B' = Li, Na, K) under natural environment

Abstract

The stability of perovskite solar cell materials in the natural environment is crucial to their commercial application. Understanding the detailed interaction mechanism between the air or water molecules (H2O) and the perovskite materials is a key way to explore their stabilities. In this study, we evaluate the geometric stabilities and thermodynamic stabilities of the lead-free Cs2B'BiCl6 (B' = Li, Na, K) double perovskite materials, and further explore the effect of air and H2O molecules on the stability and degradation of the Cs2B'BiCl6 double perovskite materials. The calculated results indicate that the pristine Cs2B'BiCl6 materials possess good geometric stabilities and thermodynamic stabilities. Under natural environment, the N2, O2, and CO2 gas molecules hardly penetrate into Cs2LiBiCl6 and Cs2NaBiCl6 because of positive absorption energies, while N2 and O2 can easily penetrate into Cs2KBiCl6 due to negative absorption energies. In contrast, the H2O molecules can easily penetrate into all Cs2B'BiCl6 double perovskite materials with negative absorption energies. Furthermore, the penetration of H2O molecules causes structural deformations of the double perovskites and the resultant rotation or rupture of some octahedra. Further simulations of H2O molecules adsorption on the surface of the double perovskites reveal that the adsorption energies of H2O molecules at all adsorption sites are negative, indicating that the Cs2B'BiCl6 double perovskite materials are prone to absorb H2O molecules in a humid environment. Therefore, the H2O molecules are the main factor affecting the stability and degradation of all Cs2B'BiCl6 double perovskite materials. Additionally, Cs2KBiCl6 shows much worse stability, which is responsible for the scarcely experimental reports on its synthesis and characterization.