Photo-excited charge carrier lifetime enhanced by slow cation molecular dynamics in lead iodide perovskite FAPbI3

Using muon spin relaxation measurements on formamidinium lead iodide [FAPbI(3), where FA denotes HC(NH2)(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 approximate to 50 K over the time scale of 10(-6) s in the low-temperature (LT) hexagonal (Hex-LT, <160 K) phase, which has a relatively longer photo-excited charge carrier lifetime (tau(c) similar to 10(-6) s). In contrast, a sharp increase in the FA molecular motion was found above approximate to 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 tc decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures (>380/280 K), while tau(c) is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine tc, 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.