Hidden polymorphism of FAPbI 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 °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 FAPbI 3 structure has undoubtedly cubic Pm 3&cmb.macr; m symmetry (high-temperature phase: α HT ). Below that temperature, the α i -phase was suggested to have a distorted perovskite structure with Im 3&cmb.macr; symmetry. Thermally recovered FAPbI 3 (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. Formamidinium lead iodide (FAPbI 3 ) can exhibit polymorphism at ambient conductions. Three different structural configurations and their thermally activated phase transitions are identified by temperature dependent micro-Raman spectroscopy.