Ab Initio Thermodynamic Study of PbI$_2$ and CH$_3$NH$_3$PbI$_3$ Surfaces in Reaction with CH$_3$NH$_2$ Gas for Perovskite Solar Cells

TL;DR

This study uses ab initio thermodynamics to analyze surface reactions of PbI$_2$ and MAPbI$_3$ in perovskite solar cell synthesis, revealing key factors affecting stability and formation pathways.

Contribution

It provides a detailed thermodynamic analysis of surface phases and reaction mechanisms for perovskite precursor conversion, incorporating defect and additive effects.

Findings

Adsorption of CH$_3$NH$_2$ is enhanced by dissociation of additives like HI or NH$_4$I.

Stable surface terminations are MA-terminated (110) and MAI-terminated (100) at specific pH levels.

The work offers fundamental insights into solid-gas reactions for high-quality perovskite synthesis.

Abstract

Hybrid organic-inorganic halide perovskites for photovoltaics have attracted research interest due to their unique material properties, but suffered from poor material stability. In this work, we investigated the surface phase diagrams of PbI$_2$ and cubic CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) for a better understanding of precursor effect on perovskite synthesis via solid-gas reaction, by density functional theory calculations combined with thermodynamics. Using the devised slab models of PbI$_2$(001) and MAPbI$_3$(100), (110) and (111) surfaces with perfect and various vacant defect terminations, we calculated their formation energies and adsorption energies of CH$_3$NH$_2$ molecule on the PbI$_2$(001) surfaces under different synthesis conditions of temperature, pressure and pH via chemical potentials of species. Our calculations revealed that the adsorption can be facilitated by including HI or NH$_4$I molecule by dissociation of this additive and formation of CH$_3$NH$_3^+$ cation or NH$_3$-CH$_3$NH$_3^+$ complex, which is beneficial for conversion of PbI$_2$ to MAPbI$_3$ via solid-gas reaction. Furthermore, we found that among different perovskite MAPbI$_3$ surfaces, the MA-terminated (110) and MAI-terminated (100) surfaces are placed on the thermodynamically stable region of chemical potentials at pH values of 1 and 6, being agreed well with the experimental findings. We believe this work gives a fundamental understanding of solid-gas reaction for high-crystallinity perovskite synthesis towards perovskite solar cells with improved stability.