Ibaceta-Jaña, J.; Muydinov, R.; Rosado, P.; Vinoth Kumar, S.H.B.; Gunder, R.; Hoffmann, A.; Szyszka, B.; Wagner, M.R.: Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy. Physical Chemistry Chemical Physics 23 (2021), p. 9476-9482
10.1039/d1cp00102g
Open Access Version
Abstract:
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.