Understanding the Instability of the Halide Perovskite CsPbI3 Through Temperature-Dependent Structural Analysis
Daniel Straus a, Shu Guo a, Milinda Abeykoon b, Robert Cava a
a Department of Chemistry, Princeton University, Princeton, NJ 08544 USA
b National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973 USA
Proceedings of Atomic-level characterization of hybrid perovskites (HPATOM)
Online, Spain, 2021 January 26th - 28th
Organizers: Dominik Kubicki and Amita Ummadisingu
Oral, Daniel Straus, presentation 020
Publication date: 14th January 2021

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all-inorganic perovskite CsPbI3 to be unstable at room temperature remain mysterious especially since many tolerance factor-based approaches predict CsPbI3 should be stable as a perovskite. We use a solid-state method to synthesize single crystals of perovskite-phase CsPbI3 that are kinetically stable at room temperature, allowing us to characterize its bulk properties and rationalize its thermodynamic instability. Electronically, CsPbI3 does not behave like a conventional semiconductor because its optical absorption and joint density-of-states is greatest near the band edge and decreases beyond the band gap for at least 1.9 eV. Structurally, single-crystal X-ray diffraction measurements reveal that while Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number. This finding along with other structural parameters suggests that Cs rattles in its coordination polyhedron. Pair distribution function measurements reveal that on the length scale of the unit cell, the Pb-I octahedra concurrently become greatly distorted, with one of the I-Pb-I angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of Cs-I contacts, and high degree of octahedral distortion cause the instability of perovskite-phase CsPbI3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI3-based solar cells.

The synthesis of the compound and analysis of the diffraction data was supported by the Gordon and Betty Moore Foundation as part of the EPiQS initiative under Award No. GBMF-4412. The single crystal X-ray diffraction data collection was supported as part of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award No. DE-SC0019331. This research used resources of the National Synchrotron Light Source II, a DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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