Two-dimensional lead-halide perovskites have attracted intense attention because of quantum confinement and reduced dielectric screening from the 2D layered structure. In addition to their novel electronic properties, they promise applications in light-emitting devices, photodetectors, and photovoltaics. The efficiency of organic and inorganic perovskites depends on the complex interplay between the electronic and lattice dynamics (large polaron formation) as a result of strong electron/exciton-phonon coupling due to the presence of soft and polar lattice with large anharmonicity. While large polaron formation in 3D perovskite structure has been studied in detail, analogous understanding is in 2D perovskites is necessary for understanding their novel physics as well as to discover rational design principles to improve device performance. In this study, we show that impulsively created coherent phonons in Dion−Jacobson (D-J) type 2D perovskites materials mediate the ultrafast hot carrier relaxation and exciton-polaron formation. These D-J type 2D perovskites have recently been reported to provide highly stable, robust, and rigid structure that supports targeted ligand modifications. We identify specific modes near 50cm<sup>-1</sup> which strongly couples to the electronic excitation at room temperature. Excitation wavelength dependence study suggests that these phonon modes strongly excite near the exciton resonance, suggesting a strong exciton-phonon coupling. Our results reveal that modes near 50cm<sup>-1</sup>, which are primarily the motions of the inorganic cage, drive the structural distortion and play a critical role in the exciton-polaron formation at room temperature.

Coherent phonon mediated exciton-polaron formation in rigid dion−Jacobson (D-J) type two-dimensional perovskites at room temperature

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