High-Pressure Behavior of δ-Phase of Formamidinium Lead Iodide by Optical Spectroscopies

The exceptional photovoltaic properties of hybrid organic–inorganic perovskites have attracted increasing interest in the past decades. Among these materials, FAPbI3 shows two structural phases: the high temperature perovskite α-phase, with direct bandgap close to the Shockley–Queisser limit, and the much less photoactive non-perovskite δ-phase, stable at ambient conditions. Although the presence of the δ-phase has been usually regarded as a limitation for FAPbI3 optoelectronic applications, recent studies have found that devices with increased stability and efficiency can be designed by mixing α- and δ-phases. This has brought out the need for a deeper understanding of the physical properties of δ-FAPbI3. In this paper, we present an original high-pressure Raman and photoluminescence study to address the effects of compression on the lattice and optoelectronic response of the sample. Also, based on the previous findings on different hybrid perovskites, our results for δ-FAPbI3 show that the cation configuration goes from a dynamically disordered regime at ambient conditions to a statically ordered phase at ∼1.5 GPa. On further increasing pressure, above 7 GPa, a statically disordered regime takes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state. Compared with α- FAPbI3, we found that the hexagonal δ-phase is less affected by external compression, as both the first detectable structural transition and the amorphous-like behavior occur at higher pressures.