Thermodynamic stability of mixed Pb:Sn methyl-ammonium halide perovskites

• Published in: Physica Status Solidi. B: Basic Solid State Physics 2016-10, Vol.253 (10), p.1907-1915
• Main Authors: Korshunova, Ksenia, Winterfeld, Lars, Beenken, Wichard J. D., Runge, Erich
• Format: Article
• Language: English
• Subjects: ab initio calculations; Chlorides; Entropy; halides; Halogens; Mathematical models; mixing; Pb:Sn; Perovskites; Phases; solar cells; Stability; thermodynamic stability; Thermodynamics
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201600136

Using density functional theory, we investigate systematically mixed M−0.35exA0.35ex(Pb:Sn)X3 perovskites, where M−0.35exA0.35ex is CH3NH3+, and X is Cl, Br, or I. Ab initio calculations of the orthorhombic, tetragonal, and cubic perovskite phases show that the substitution of lead by tin has a much weaker influence on both structure and cohesive energies than the substitution of the halogen. The thermodynamic stability of the M−0.35exA0.35ex(Pb:Sn)X3 mixtures at finite, non‐zero temperatures is studied within the regular solution model. We predict that it will be possible to create M−0.35exA0.35ex(Pb:Sn)I3 mixtures at any temperature. Our results imply that mixing is unlikely for the low‐temperature phase of bromide and chloride compounds, where instead local clusters are more likely to form. We further predict that in the high‐temperature cubic phase, Pb and Sn compounds will mix for both M−0.35exA0.35ex(Pb:Sn)Br3 and M−0.35exA0.35ex(Pb:Sn)Cl3 due to the entropy contribution to the Helmholtz free energy.