Metastable alloying structures in MAPbI3−xClx 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 MAPbI3−xClx 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 MAPbI3−xClx (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 MAPbI3−xClx crystals are metastable and decompose over time. In addition, the photocurrent response measurement of MAPbI3−xClx 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. A more complete understanding of alloy formation in an exciting class of optoelectronic materials has been developed by researchers in China, which could help improve solar-cell efficiency. Lead halide perovskites are used in low-cost, lightweight and flexible electronic devices. The properties of the perovskite can be modified to suit a specific application by doping, exchanging some atoms in the crystal for others, but this can make the structure unstable. Xue Lu Wang, Ye-Feng Yao and colleagues from the East China Normal University, Shanghai, tracked atomic structure changes in perovskite methylammonium lead iodide as the iodine atoms were partially replaced by chlorine atoms. The team showed that the state of chloride in MAPbI3−xClx crystals is metastable and the crystal lattice can transform from a cubic to a tetragonal structure over time. The characteristics of I/Cl alloying structures in MAPbI3-xClx mixed-halide perovskites and their influences on the optoelectronic properties have been issues of long-standing controversy. In this work, by using on the time-dependent X-ray diffraction, 207Pb and 2H NMR, the authors investigated the I/Cl alloying structures and its long term stability in MAPbI3-xClx (x = 0.0 to 0.3) single crystals. It was found that Cl can substitute for I of the PbI3 inorganic lattice, leading to the tetragonal to cubic phase transition. Moreover, it revealed that the alloying structures of the MAPbI3-xClx crystals are metastable and may decompose over time.