Exciton Polarons in Two-Dimensional Hybrid Metal-Halide Perovskites

Abstract

While polarons, charges bound to a lattice deformation induced by electron− phonon coupling, are primary photoexcitations in bulk metal-halide hybrid organic− inorganic perovskites (HOIPs), excitons, Coulomb-bound electron−hole pairs, are the stable quasi-particles in their two-dimensional (2D) analogues. However, are polaronic effects consequential for excitons in 2D-HOIPs? We argue that they are manifested intrinsically in the exciton spectral structure, which is composed of multiple nondegenerate resonances with constant interpeak energy spacing. We highlight population and dephasing dynamics that point to the apparently deterministic role of polaronic effects. We contend that an interplay of long-range and short-range exciton−lattice couplings gives rise to exciton polarons, which fundamentally establishes their effective mass and radius and, consequently, their quantum dynamics. Finally, we highlight opportunities for the community to develop the rigorous description of exciton polarons in 2D-HOIPs to advance their fundamental understanding as model systems for condensed-phase materials with strong lattice-mediated correlations.

Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

“…The intrinsic nature of the asymmetric PL with a low-energy tail can be attributed the polaronic effect in lead halide perovskites and we assign the LE emission to intrinsic STE emission. Carriers (or excitons) in lead halide perovskites can couple to lattice vibrations through both Fröhlich-like long-range interaction, due to their polar and ionic lattice, and Holstein-like short-range interaction, due to the soft and dynamic nature 7,12,16,25 . The interplay between them determines the excited-state energy landscape.…”

“…4), indicating weaker exciton-phonon coupling. The increasing exciton-phonon coupling from bulk to 3L and 2L perovskites is reasonable since excitons in thinner perovskites are more tightly confined in a more deformable and dynamic lattice, in favor of enhanced short-range exciton-phonon interaction 12,20,62 . This leads to an unique intermediate exciton-phonon coupling regime with momentarily trapped excitons in 2D perovskites, which is different from Fröhlich-like large polaron with dominating long-range interaction and Holstein-like STE with strong short-range interaction.…”

“…Si, GaAs), the soft and ionic lattice of lead halide perovskites exhibits highly anharmonic fluctuations 7,8 and is deformable under photoexcitation 9,10 . As a result, exciton-phonon coupling dresses excitons in 2D perovskite with significant polaronic character 11,12 , leading to complex excited-state energetic/configurational landscape [13][14][15] .…”

“…Depending on the nature of exciton (carrier)-phonon interaction, exciton (carrier) in lead halide perovskites can exist as exciton polaron (large polaron) which can move freely but dressed with phonon clouds through long-range Frohlich-like interaction, or self-trapped exciton (STE) (small polaron) through short-range Holstein-like interaction 12,16 . While free exciton (FE) polarons can move coherently and are characterized with relatively narrow and near-band edge emission 17 , STEs are tightly localized by lattice distortion and typically exhibit a broad emission with large Stokes shift 18 .…”

“…However, excitons in 2D RP perovskites remain poorly defined so far. The coexistence and complex interplay between long-and short-range interactions put 2D RP perovskites in a unique intermediate regime and impose significant challenges on understanding and describing their exciton behaviors 12 .…”