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Issue 15, 2021
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Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

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Formamidinium lead iodide (FAPbI3) can be used in its cubic, black form as a light absorber material in single-junction solar cells. It has a band-gap (1.5 eV) close to the maximum of the Shockley–Queisser limit, and reveals a high absorption coefficient. Its high thermal stability up to 320 °C has also a downside, which is the instability of the photo-active form at room temperature (RT). Thus, the black α-phase transforms at RT with time into a yellow non-photo-active δ-phase. The black phase can be recovered by annealing of the yellow state. In this work, a polymorphism of the α-phase at room temperature was found: as-synthesized (αi), degraded (αδ) and thermally recovered (αrec). They differ in the Raman spectra and PL signal, but not in the XRD patterns. Using temperature-dependent Raman spectroscopy, we identified a structural change in the αi-polymorph at ca. 110 °C. Above 110 °C, the FAPbI3 structure has undoubtedly cubic Pm[3 with combining macron]m symmetry (high-temperature phase: αHT). Below that temperature, the αi-phase was suggested to have a distorted perovskite structure with Im[3 with combining macron] symmetry. Thermally recovered FAPbI3rec) also demonstrated the structural transition to αHT at the same temperature (ca. 110 °C) during its heating. The understanding of hybrid perovskites may bring additional assets in the development of new and stable structures.

Graphical abstract: Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

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Supplementary files

Article information

08 Jan 2021
01 Mar 2021
First published
12 Apr 2021

Phys. Chem. Chem. Phys., 2021,23, 9476-9482
Article type

Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

J. Ibaceta-Jaña, R. Muydinov, P. Rosado, S. H. B. Vinoth Kumar, R. Gunder, A. Hoffmann, B. Szyszka and M. R. Wagner, Phys. Chem. Chem. Phys., 2021, 23, 9476
DOI: 10.1039/D1CP00102G

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