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 (H2O) molecules with perovskite materials is a key way to explore their stabilities. In this study, we evaluate the geometric stabilities and thermodynamic stabilities of lead-free double perovskite materials Cs2B'BiCl6 (B' = Li, Na, K), and further explore the effect of air and H2O molecules on the stability and degradation of Cs2B'BiCl6. 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 attack Cs2LiBiCl6 and Cs2NaBiCl6 because of positive insertion energies, while N2 and O2 can easily insert Cs2KBiCl6 due to negative insertion energies. In contrast, with respect to other gas molecules, the H2O molecules can easily insert into the lattice of all Cs2B'BiCl6 materials with negative insertion energies. Furthermore, the insertion of H2O molecules causes structural deformations of the double perovskites and 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 Cs2B'BiCl6 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. Additionally, Cs2KBiCl6 shows much worse stability, which is responsible for the scarcely experimental reports on its synthesis and characterization.

Keywords: Lead-free double perovskite, First-principles study, Stability, Degradation