Thermodynamic origin of instability in hybrid halide perovskites

Degradation of hybrid halide perovskites under the influence of environmental factors impairs future prospects of using these materials as absorbers in solar cells. First principle calculations can be used as a guideline in search of new materials, provided we can rely on their predictive capabilities. We show that the instability of perovskites can be captured using ab initio total energy calculations for reactants and products augmented with additional thermodynamic data to account for finite temperature effects. Calculations suggest that the instability of CH₃NH₃PbI₃ in moist environment is linked to the aqueous solubility of the CH₃NH₃I salt, thus making other perovskite materials with soluble decomposition products prone to degradation. Properties of NH₃OHPbI₃, NH₃NH₂PbI₃, PH₄PbI₃, SbH₄PbI₃, CsPbBr₃, and a new hypothetical SF₃PbI₃ perovskite are studied in the search for alternative solar cell absorber materials with enhanced chemical stability.