Advances in multi-phase FAPbI3 perovskite: another perspective on photo-inactive δ-phase

Abstract Halide perovskites have attracted great interest as active layers in optoelectronic devices. Among perovskites with diverse compositions, α-FAPbI 3 is of utmost importance with great optoelectronic properties and a decent bandgap of 1.48 eV. However, the α-phase suffers an irreversible transition to the photo-inactive δ-phase, whereas the δ-phase is usually regarded as useless phase with poor optoelectronic properties. Therefore, it is commonly accepted that the thermodynamic stable δ-FAPbI 3 greatly limits the application of FAPbI 3 . Every coin has two sides, although the δ-phase is difficult to apply as photoelectrical active layers, it is possible to combine δ-FAPbI 3 with α-FAPbI 3 to realize functional applications. Firstly, this review analyzes the cause of the contrasting properties between α- and δ-FAPbI 3 , where the stronger electron−phonon coupling in 1D hexagonal δ-FAPbI 3 restricts its internal carrier and phonon transport. Secondly, the factors affecting the phase transitions and strategies to control phase transition between α- and δ-FAPbI 3 are presented. Finally, some functional applications of δ-FAPbI 3 in combination with α-FAPbI 3 are given according to previous reports. By and large, we hope to introduce δ-FAPbI 3 from another perspective and give some insights into its unique properties, hopefully providing new strategies for the subsequent advances to FAPbI 3 .