Hidden Polymorphism Of Fapbi(3) Discovered By Raman Spectroscopy

Formamidinium lead iodide (FAPbI(3)) 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 degrees C has also a downside, which is the instability of the photo-active form at room temperature (RT). Thus, the black alpha-phase transforms at RT with time into a yellow non-photo-active delta-phase. The black phase can be recovered by annealing of the yellow state. In this work, a polymorphism of the alpha-phase at room temperature was found: as-synthesized (alpha(i)), degraded (alpha(delta)) and thermally recovered (alpha(rec)). 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 alpha(i)-polymorph at ca. 110 degrees C. Above 110 degrees C, the FAPbI(3) structure has undoubtedly cubic Pm3m symmetry (high-temperature phase: alpha(HT)). Below that temperature, the alpha(i)-phase was suggested to have a distorted perovskite structure with Im3 symmetry. Thermally recovered FAPbI(3) (alpha(rec)) also demonstrated the structural transition to alpha(HT) at the same temperature (ca. 110 degrees C) during its heating. The understanding of hybrid perovskites may bring additional assets in the development of new and stable structures.