Metastable alloying structures in MAPbI_3−xCl _x crystals

Chlorine incorporation engineering has been widely used in optoelectronic devices based on methylammonium lead iodide (MAPbI₃) perovskites. However, the characteristics of I/Cl alloying structures in MAPbI_3−xCl_x mixed-halide perovskites and their influences on the optoelectronic properties have been issues of a long-standing controversy. Here, we present a detailed study of the I/Cl alloying structures in MAPbI_3−xCl_x (x = 0.0 to 0.3) single crystals. We found that a small amount of Cl can substitute for the iodide of the PbI₃ inorganic lattice, leading to a phase transition from the tetragonal to cubic phase and anomalous cation dynamics evolution. Analyses based on time-dependent X-ray diffraction, ²⁰⁷Pb NMR, and ²H NMR indicate that the alloying structures of the MAPbI_3−xCl_x crystals are metastable and decompose over time. In addition, the photocurrent response measurement of MAPbI_3−xCl_x proved a close correlation between the alloying structures and photoelectric properties of the material. This work sheds light on the essential understanding of the I/Cl alloying structure and provides a plausible explanation for the controversy regarding the role of chloride ions in optoelectronic devices.