Diverging Expressions of Anharmonicity in Halide Perovskites

Adi Cohen; Thomas M Brenner; Johan Klarbring; Rituraj Sharma; Douglas H Fabini; Roman Korobko; Pabitra K Nayak; Olle Hellman; Omer Yaffe

doi:10.1002/adma.202107932

Diverging Expressions of Anharmonicity in Halide Perovskites

Adi Cohen, Thomas M Brenner, Johan Klarbring, Rituraj Sharma, Douglas H Fabini, Roman Korobko, Pabitra K Nayak, Olle Hellman, Omer Yaffe

Department of Chemical and Biological Physics

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Abstract

Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs2AgBiBr6, has been recently studied as a lead-free alternative for optoelectronic applications. However, it does not exhibit the excellent photovoltaic activity of the lead-based halide perovskites. Therefore, to explore the correlation between the anharmonic structural dynamics and optoelectronic properties in lead-based halide perovskites, the structural dynamics of Cs2AgBiBr6 are investigated and are compared to its lead-based analog, CsPbBr3. Using temperature-dependent Raman measurements, it is found that both materials are indeed strongly anharmonic. Nonetheless, the expression of their anharmonic behavior is markedly different. Cs2AgBiBr6 has well-defined normal modes throughout the measured temperature range, while CsPbBr3 exhibits a complete breakdown of the normal-mode picture above 200 K. It is suggested that the breakdown of the normal-mode picture implies that the average crystal structure may not be a proper starting point to understand the electronic properties of the crystal. In addition to our main findings, an unreported phase of Cs2AgBiBr6 is also discovered below approximate to 37 K.

Original language	English
Article number	2107932
Number of pages	7
Journal	Advanced Materials
Volume	34
Issue number	14
Early online date	25 Jan 2022
DOIs	https://doi.org/10.1002/adma.202107932
Publication status	Published - 7 Apr 2022

Funding

O.Y. acknowledges funding from European Research Concil (850041 — ANHARMONIC). D.H.F. gratefully acknowledges financial support from the Alexander von Humboldt Foundation. D.H.F. thanks Reinhard Kremer and Gisele Siegle for performing the specific heat measurement. P.N. acknowledges the support from the Department of Atomic Energy, Government of India, under Project Identification No. RTI 4007 and SERB India core research grant (CRG/2020/003877). O.H. acknowledges support from the Swedish Research Council (VR) program 2020‐04630. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC), partially funded by the Swedish Research Council through grant agreement no. 2018‐05973. J.K. acknowledges support from the Swedish Research Council (VR) program 2021‐00486. The authors acknowledge Yevgeny Rakite for giving us a CsPbBr crystal for the Raman measurements. 3

All Science Journal Classification (ASJC) codes

General Materials Science
Mechanics of Materials
Mechanical Engineering

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title = "Diverging Expressions of Anharmonicity in Halide Perovskites",

abstract = "Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs2AgBiBr6, has been recently studied as a lead-free alternative for optoelectronic applications. However, it does not exhibit the excellent photovoltaic activity of the lead-based halide perovskites. Therefore, to explore the correlation between the anharmonic structural dynamics and optoelectronic properties in lead-based halide perovskites, the structural dynamics of Cs2AgBiBr6 are investigated and are compared to its lead-based analog, CsPbBr3. Using temperature-dependent Raman measurements, it is found that both materials are indeed strongly anharmonic. Nonetheless, the expression of their anharmonic behavior is markedly different. Cs2AgBiBr6 has well-defined normal modes throughout the measured temperature range, while CsPbBr3 exhibits a complete breakdown of the normal-mode picture above 200 K. It is suggested that the breakdown of the normal-mode picture implies that the average crystal structure may not be a proper starting point to understand the electronic properties of the crystal. In addition to our main findings, an unreported phase of Cs2AgBiBr6 is also discovered below approximate to 37 K.",

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Diverging Expressions of Anharmonicity in Halide Perovskites

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