Understanding halide segregation in metal halide perovskites through defect thermodynamics

TL;DR

This study uncovers the thermodynamic mechanisms driving halide segregation in mixed halide perovskites, emphasizing the influence of surface defect energies, A-site cation composition, and hole localization on segregation behavior.

Contribution

It introduces a defect thermodynamics framework to explain halide segregation, highlighting the role of A-site cations and defect formation energies in mixed halide perovskites.

Findings

Bromide ions prefer surface sites over bulk in perovskites.

Segregation tendency varies with A-site cation composition.

Hole localization near iodide accelerates segregation.

Abstract

Halide segregation in metal halide perovskites limits their bandgap tunability and hinders their adoption in tandem solar cells and light emitting diodes. Here, we reveal the thermodynamic driving force behind halide segregation in mixed halide (Br-I) perovskites. By performing first-principles calculations on slab models with varying bromide and iodide distributions, we demonstrate that bromide ions preferentially occupy surface sites over bulk sites. Our simulations show that the segregation tendency is higher in MAPb(Br$_x$I$_{1-x}$)$_3$ (MA=methylammonium) compared to FA$_{0.8}$Cs$_{0.2}$Pb(Br$_x$I$_{1-x}$)$_3$, highlighting the role of the A-site cation. To quantify this effect, we establish a descriptor for halide segregation: the difference in defect formation energies of Br antisite defects between the bulk and the surface, which confirms the role of the A-site cation at equimolar Br-I concentration. Furthermore, we identify the localization of photo-generated holes near iodide ions, which triggers their oxidation and accelerates the formation of iodide vacancies, thereby promoting segregation. Overall, this work establishes defect thermodynamics as a framework for understanding halide segregation and provides a structural basis for designing stable mixed halide perovskites.