Reaction kinetics of the thermal decomposition of MAPbI3 thin films

Abstract

Despite their outstanding optoelectronic properties, metal halide perovskites have not yet seen widespread use in commercial applications. This is mostly due to their lack of stability with respect to several external factors, e.g., humidity, heat, light, and oxygen. Even though extensive studies have been carried out over the last decade, a lot of questions regarding their thermal stability still remain, and various publications have put forth different approaches for measuring and quantifying the conditions of their decomposition. However, differences in the experimental setups and in the reported values make comparisons of the reported results challenging. We show an approach, where ${\mathrm{MAPbI}}_{3}$ thin films are thermally decomposed in high vacuum within a temperature range from $220{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, while monitoring changes in the crystal structure using an in situ x-ray diffraction setup. This process reveals the complete phase evolution of the thin films from ${\mathrm{MAPbI}}_{3}$ into ${\mathrm{PbI}}_{2}$. The time resolved data was then evaluated in view of the reaction kinetics using three different approaches. First, an Arrhenius fit was applied to $lnk$ over $1/T$, where the rate constant $k$ was determined by fitting a first order exponential decay onto the decreasing peak area of the most prominent diffraction peaks. Secondly, a model fitting approach was used, where the data was tested against a set of different reaction models. Lastly, a model free isoconversional approach was applied. By doing this, we succeed in characterizing the decomposition and determine the kinetic triplet, consisting of the activation energy $E$, the frequency factor $A$, and the reaction model $f(\ensuremath{\alpha})$. With the help of the kinetic triplet the decomposition reaction can be expressed in a physically meaningful way and allows us to predict the decomposition dynamics of ${\mathrm{MAPbI}}_{3}$ thin films for varying temperatures.