Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites

Abstract

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency ( 50 cm −1 ) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

“…At low temperature, exciton state splitting is observed, which is caused by the electron-hole exchange interaction. [25,26] It is therefore crucial to understand elementary photoexcitations in these layered materials. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.…”

“…[8,22] Moreover, 2D perovskites can exhibit a variety of multiexciton species, including biexcitons and trions. [25,26] It is therefore crucial to understand elementary photoexcitations in these layered materials. In addition, recent experiments reveal an important role of electron-phonon couplings on the exciton dynamics in 2D lead-iodide perovskite, suggesting a complex scenario for carrier relaxation and exciton formation.…”

Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

“…For example, in analogy to ferroelectric materials, the high frequency LO mode at ω LO ∼4 THz [32], both optically dark and momentum forbidden, has been discussed to provide the macroscopic motion of charged lattice and internal electric field to mediate the large polaron formation. Previous studies have gained important insights into e-phonon interaction and polaronic couping in perovskites using Raman scattering [13] and coherent phonon pumping [14][15][16][17]. However, the high frequency LO modes ∼4 THz thus far is much less observed by ultrafast optical studies, which reveals mostly lower frequency modes [16,17].…”

Mixed Spacer Cation Stabilization of Blue‐Emitting n = 2 Ruddlesden–Popper Organic–Inorganic Halide Perovskite Films

“…Finally, as shown in Figure c, smaller e–ph coupling was found for n = 2 BA–PEA mixed RPP, compared to pure BA and PEA, which may contribute to the emission enhancement and/or improved stability of photoluminescence (PL) and electroluminescence. As shown in Figure S1 (Supporting Information), e–ph coupling is due to low‐frequency vibrations of inorganic frame (<200 cm −1 ) and coupled vibrations of organic cations.…”