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

Abstract

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 (MAPbI 3 ) is a widely studied reference compound whose solar cell performance can be improved by chloride addition (e.g., in the form of PbCl 2 ) during the thin-film preparation. Because of the large difference in the ionic radii of both halides, no mixed perovskites MAPbI 3−x Cl x are formed and generally only minute amounts of chlorine can be detected in the final MAPbI 3 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 MAPbCl 3 layer, which subsequently transforms to MAPbI 3 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 MAPbI 3 , 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 MAPbI 3 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 MAPbI 3 , 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.

In-Situ Nano-Auger Probe of Chloride-Ions during CH3NH3PbI3−xClx Perovskite Formation

“…This small fraction of inherent chlorine content is expected to be one of reasons for large grain growth and improved diffusion coefficient of mixed halide MAPbI 3−x Cl x compared to pure MAPbI 3 composition. In support of previous reports [ 31 , 32 ], the in-situ experiment results presented here further suggest that the presence of chlorine precursors in MAPbI 3 thin film formation may play an important role in grain growth, which is closely relevant to engineering morphology and electrical properties that are desirable for photovoltaic devices.…”

Ferroelastic Hysteresis in Thin Films of Methylammonium Lead Iodide

“…This preference for the nOP orientation of the (220) plane is consistent with what is typically observed for MAPbI3 films. 31 In the nIP pattern, the peak near 3.16 Å was considerably more intense than in the nOP direction, and the (211) peak was much weaker, indicating less scattering from the (220) peak in the nIP direction. We therefore assigned the nIP peak near 3.16 Å to be the (004) plane that corresponds to crystallites with (220) planes oriented nOP.…”

“…[16][17][18][19] This phenomenon is common in a variety of materials, such as zirconia 20 or oxide and fluoride perovskites, 21,22 and has been increasingly investigated in halide perovskites. [23][24][25][26][27][28][29][30][31][32] At elevated temperatures many APbX3 materials have higher symmetry, but as the material cools the R4 + phonon mode condenses, inducing permanent out-of-phase tilting of the BX6 octahedra and a spontaneous strain in the material. [33][34][35][36][37] The strain magnitude is lowered by transition to a lower symmetry phase, which in MAPbI3 is the cubic Pm-3m to tetragonal I4/mcm transition.…”

“…Such domains have previously been observed in MAPbI3. 24,25,[29][30][31] Compositional derivatives of MAPbI3 that are thought to be cubic on the bulk scale (e.g. (Cs,FA,MA)Pb(Br,I)3) also exhibit twinning on the nanoscale.…”

Metal‐Halide Perovskite Crystallization Kinetics: A Review of Experimental and Theoretical Studies

“…Overall, the ability of Cl − to modify perovskite film quality has been widely recognized, [136] but the way in which Cl − modulates www.advancedsciencenews.com perovskite properties is not fully understood. Further exploration of the crystallization kinetics of Cl − -incorporated perovskites is required.…”