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Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

Ibaceta-Jaña, Josefa; Muydinov, Ruslan; Rosado, Pamela; Vinoth Kumar, Sri Hari Bharath; Gunder, Rene; Hoffmann, Axel; Szyszka, Bernd; Wagner, Markus. R.

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 FAPbI3 (αrec) 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.
Published in: Physical Chemistry, Chemical Physics, 10.1039/D1CP00102G, Royal Society of Chemistry (RSC)