Thermodynamic Stability and Structural Insights for CHNHPbSiI, CHNHPbGeI, and CHNHPbSnI Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations.

The recent reaching of 20% of conversion efficiency by solar cells based on metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide, CHNHPbI (MAPbI), has excited the scientific community devoted to the photovoltaic materials. However, the toxicity of Pb is a hindrance for large scale commercial of MHP and motivates the search of another congener eco-friendly metal. Here, we employed first-principles calculations via density functional theory combined with the generalized quasichemical approximation to investigate the structural, thermodynamic, and ordering properties of MAPbSiI, MAPbGeI, and MAPbSnI alloys as pseudo-cubic structures. The inclusion of a smaller second metal, as Si and Ge, strongly affects the structural properties, reducing the cavity volume occupied by the organic cation and limitating the free orientation under high temperature effects. Unstable and metaestable phases are observed at room temperature for MAPbSiI, whereas MAPbGeI is energetically favored for Pb-rich in ordered phases even at very low temperatures. Conversely, the high miscibility of Pb and Sn into MAPbSnI yields an alloy energetically favored as a pseudo-cubic random alloy with tunable properties at room temperature.