Dynamic Disorder Dominates Delocalization, Transport, and Recombination in Halide Perovskites

We investigate the origins of exceptionally long charge-carrier lifetimes of halide perovskites by using temperature-dependent and time-resolved mid-infrared spectroscopy. We report direct observations of large polarons through their distinct mid-infrared electronic transitions and quantitatively correlate their formation with enhancement of electron-phonon coupling that quenches photoluminescence. The temperature-dependent study reveals a branching between free-carrier and large-polaron states in association with thermally induced dynamic disorder of the anharmonic inorganic framework, which is examined through local vibrational modes of the perovskite films. This dynamic disorder localizes charge carriers and enhances electron-phonon coupling, which both quenches photoluminescence and promotes large-polaron formation. Fortunately, such localization extends the charge recombination lifetime by nearly an order of magnitude. This work highlights the interplay between charge-carrier mobility and lifetime and suggests design rules about how cation and anion substitution can be used to tune the structural dynamics that underpin their relationship.