Polarons in Halide Perovskites: A Perspective

Abstract

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron–phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Polaron and Spin Dynamics in Organic–Inorganic Lead Halide Perovskite Nanocrystals

“…[ 10,27 ] The spin degrees of freedom of charge carriers in these novel photonic quantum materials with strong spin–orbit coupling (SOC) and the polaron‐associated symmetry breaking offer new quantum pathways for manipulating spin via light alone without any external electric or magnetic field. [ 56–60 ] We studied the spin interactions in FAPbI 3 and Cs 0.01 FA 0.99 Pb(Br 0.11 I 0.89 ) 3 NCs by using a right‐circularly polarized fs resonant pump (σ + ) and white light probe pulses (σ + /σ − ) of duration two orders of magnitude shorter than the carrier lifetime (details in Experimental Section). σ + pump pulse carries an angular momentum +1, which excite carriers in |+⟩ spin state.…”

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.…”

“…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 .…”

Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double‐Perovskite Single Crystals through Alkali‐Metal Substitution

“…The high mobility and long recombination lifetime in combination with the so‐called “giant” electron–phonon coupling reported for Cs , [ 9 ] and polar nature of the metal halide framework and its deformability suggests the presence of polarons in our system, as recently debated in halide perovskites. [ 38–40 ] In contrast to 2D halide perovskites in which small polarons form as a result of lattice confinement, [ 41 ] in 3D perovskites, the extent of lattice deformation as a result of electron–phonon interaction is limited by the topological constraints (keeping the electronic energy and the lattice elastic energy balanced). Hence, it has been proposed that large polarons may form spanning over a few lattice sites.…”