Metastable alloying structures in MAPbI3-xClx crystals

Chlorine incorporation engineering has been widely used in optoelectronic devices based on methylammonium lead iodide (MAPbI(3)) perovskites. However, the characteristics of I/Cl alloying structures in MAPbI(3-x)Cl(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-x)Cl(x)(x = 0.0 to 0.3) single crystals. We found that a small amount of Cl can substitute for the iodide of the PbI(3)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,Pb-207 NMR, and(2)H NMR indicate that the alloying structures of the MAPbI(3-x)Cl(x)crystals are metastable and decompose over time. In addition, the photocurrent response measurement of MAPbI(3-x)Cl(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.