“If you want to make a solar cell with unmodified cesium lead iodide, it’s going to be very hard to work around this and stabilize this material,” said Straus. However, unlike methylammonium lead iodide, the perovskite phase of cesium lead iodide is metastable at room temperature. To correct this problem, researchers have attempted to replace the organic cation with inorganic cesium, which is significantly less volatile. While this “remarkable” efficiency drives interest, methylammonium lead iodide suffers from instability problems thought to originate from the volatile nature of the organic cation. “Remarkable” efficienciesĬurrently, the dominant halide perovskite in solar energy conversion applications is based on methylammonium lead iodide, an organic-inorganic hybrid material that has been incorporated into solar cells with certified efficiencies of 25.2% this rivals the efficiency of commercial silicon solar cells. "Finding an explanation for a problem that so many people in the research community are interested in is great, and our collaboration with Brookhaven has been beyond fantastic,” said Robert Cava, the Russell Wellman Moore Professor of Chemistry, an expert in synthesis and structure-property characterization. The room-temperature metastability of CsPbI 3 has long been a known factor, but it had not previously been explained. (A unit cell is the smallest repeating unit in a crystal.) It is on this local level that the high degree of octahedral distortion became obvious, said Straus. At Brookhaven, the X-ray pair distribution function allowed researchers to determine the behavior of the structure on the length scale of the unit cell. In the research, the single-crystal measurements characterized the average structure of the material. In addition, the low number of cesium-iodine contacts within the structure and the high degree of local octahedral distortion also contribute to the instability. Along with other structural parameters, this suggests evidence of the rattling behavior of cesium within its iodine coordination polyhedron.
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Daniel Straus, postdoctoral research associate in the Cava Group.ĭaniel Straus, a postdoctoral research associate in the Cava Group and lead author on the paper, explained that while cesium occupies a single site within the structure at temperatures below 150 K, it “splits” into two sites above 175 K.
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The research, “Understanding the Instability of the Halide Perovskite CsPbI 3 through Temperature-Dependent Structural Analysis,” was published in the journal Advanced Materials. (Bottom) Cs-I distances for the dominant Cs site CsA and secondary site CsB with (upper right) histogram of distances. (Upper left) Room temperature Cs electron density from single crystal X-ray diffraction measurements showing significant elongation, a signature of rattling. X-ray diffraction yields a clear experimental signature of this movement. Using single crystal X-ray diffraction performed at Princeton University and X-ray pair distribution function measurements performed at the Brookhaven National Laboratory, Princeton Department of Chemistry researchers detected that the source of thermodynamic instability in the halide perovskite cesium lead iodide (CsPbI 3) is the inorganic cesium atom and its “rattling” behavior within the crystal structure. Researchers in the Cava Group have demystified the reasons for instability in an inorganic perovskite that has attracted wide attention for its potential in creating highly efficient solar cells. Lab Resolves Origin Of Perovskite Instability
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Lab Resolves Origin Of Perovskite Instability.