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

TL;DR

This study uses density functional theory to analyze the stability and mixing behavior of mixed lead-tin methyl-ammonium halide perovskites, revealing phase-dependent mixing tendencies influenced by temperature and composition.

Contribution

It provides a systematic thermodynamic analysis of mixed MA(Pb:Sn)X3 perovskites, highlighting the effects of halogen substitution and temperature on phase stability and mixing.

Findings

Sn substitution has weaker structural impact than halogen substitution.

MA(Pb:Sn)I3 mixtures are feasible at any temperature.

Low-temperature bromide and chloride phases favor clustering over mixing.

Abstract

Using density functional theory, we investigate systematically mixed MA(Pb:Sn)X3 perovskites, where MA 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 MA(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 MA(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 MA(Pb:Sn)Br3 and MA(Pb:Sn)Cl3 due to the entropy contribution to the Helmholtz free energy.