Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

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 (α_i), degraded (α_δ) and thermally recovered (α_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 α_i-polymorph at ca. 110 °C. Above 110 °C, the FAPbI₃ structure has undoubtedly cubic Pmm symmetry (high-temperature phase: α_HT). Below that temperature, the α_i-phase was suggested to have a distorted perovskite structure with Im symmetry. Thermally recovered FAPbI₃ (α_rec) also demonstrated the structural transition to α_HT 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.