Photo-excited charge carrier lifetime enhanced by slow cation molecular dynamics in lead iodide perovskite FAPbI${}_3$

Abstract

Using muon spin relaxation ($\hat{I}¼$SR) measurements on formamidinium lead iodide [FAPbI${}_3$, where FA denotes HC(NH${}_2{)}_2$] we show that, among the five structurally distinct phases of FAPbI${}_3$ exhibited through two different temperature hysteresis, the reorientation motion of FA molecules is quasi-static below $\approx 50$ K over the time scale of 10${}^{-6}$ s in the low-temperature (LT) hexagonal (Hex-LT, $<160$ K) phase which has relatively longer photo-excited charge carrier lifetime ($\ddot{I}{„}_{\mathrm{c}}\sim$10${}^{-6}$ s). In contrast, a sharp increase in the FA molecular motion was found above $\approx 50$ K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, $<140$ K), the high-temperature (HT) hexagonal phase (Hex-HT, 160-380 K), and the HT-tetragonal phase (Tet-HT, 140-280 K) where $\ddot{I}{„}_{\mathrm{c}}$ decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures ($>380/280$ K), while $\ddot{I}{„}_{\mathrm{c}}$ is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine $\ddot{I}{„}_{\mathrm{c}}$, one related to the local reorientation of cationic molecules that is not unencumbered by phonons, and the other to the high symmetry of the bulk crystal structure.

7 pages, 4 figures