Unraveling the Formation Mechanism and Ferroelastic Behavior of MAPbI3 Perovskite Thin Films Prepared in the Presence of Chloride

Hybrid perovskites have attracted much attention as promising photovoltaic materials in the past few years. However, the fundamental understanding of their crystallization behavior lags far behind the pace of empirical solar cell efficiency improvement. Methylammonium iodide (MAPbI3) is a widely studied reference compound whose solar cell performance can be improved by chloride addition (e.g., in the form of PbCl2) during the thin-film preparation. Because of the large difference in the ionic radii of both halides, no mixed perovskites MAPbI3–x Cl x are formed and generally only minute amounts of chlorine can be detected in the final MAPbI3 thin films. Here, we demonstrate by means of a variety of complementary X-ray diffraction (XRD) techniques that, unexpectedly, the formation mechanism proceeds via an initial MAPbCl3 layer, which subsequently transforms to MAPbI3 in an anion exchange reaction during the thermal annealing step, completing the thin-film preparation. The perovskite lattice is highly strained along the process, much more than what is expected from the sole effect of the difference between the thermal expansion coefficients of the perovskite and the substrate. At room temperature, the existence of a double [hh0]/[00l] texture is explained by the ferroelastic character of the cubic/tetragonal transition of MAPbI3, which induces the formation of twins. The relative population of these domains is correlated to their strain level. Although strain is known to weaken the stability of the MAPbI3 phase, our results unambiguously show that it also favors the reproducibility of the thin-film microstructure. When used as active layers in solar cells, the dependence of the cell efficiency and stability on the annealing time is in striking accordance with the formation kinetics of MAPbI3, as revealed by the XRD measurements. Therefore, the understanding of the crystallization behavior achieved with the present approach, applicable also to other types of metal halide perovskites, allows for the rational optimization of the device performance and long-term stability.