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

The exceptional photovoltaic properties of hybrid organic-inorganicperovskites have attracted increasing interest in the past decades. Among these materials,FAPbI3 shows two structural phases: the high temperature perovskite delta-phase, with directbandgap close to the Shockley-Queisser limit, and the much less photoactive nonperovskite delta-phase, stable at ambient conditions. Although the presence of the delta-phase hasbeen usually regarded as a limitation for FAPbI3 optoelectronic applications, recentstudies have found that devices with increased stability and efficiency can be designed bymixing alfa- and delta-phases. This has brought out the need for a deeper understanding of thephysical properties of delta-FAPbI3. In this paper, we present an original high-pressure Ramanand photoluminescence study to address the effects of compression on the lattice andoptoelectronic response of the sample. Also, based on the previous findings on differenthybrid perovskites, our results for delta-FAPbI3 show that the cation configuration goes froma 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 regimetakes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state.Compared with delta- FAPbI3, we found that the hexagonal ?-phase is less affected by external compression, as both the first detectablestructural transition and the amorphous-like behavior occur at higher pressures.