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

TL;DR

This study reveals that slow cation molecular dynamics in lead iodide perovskite FAPbI$_3$ correlates with longer charge carrier lifetimes, with molecular motion and crystal symmetry both influencing charge dynamics across different phases.

Contribution

The paper demonstrates the link between molecular reorientation dynamics and charge carrier lifetime in FAPbI$_3$, highlighting the role of phase-dependent molecular motion and crystal symmetry.

Findings

Longer charge carrier lifetime in low-temperature hexagonal phase.

Increased molecular motion above 50 K reduces charge lifetime.

High-temperature cubic phase shows promoted molecular motion but retains longer charge lifetime.

Abstract

Using muon spin relaxation ($\mu$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 $\approx50$ 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 ($\tau_{\rm c}\sim$10$^{-6}$ s). In contrast, a sharp increase in the FA molecular motion was found above $\approx50$ 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 $\tau_{\rm c}$ decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures ($>380/280$ K), while $\tau_{\rm c}$ is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine $\tau_{\rm c}$, 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.