Polaronic Charge Carrier–Lattice Interactions in Lead Halide Perovskites

Abstract

Almost ten years after the renaissance of the popular perovskite‐type semiconductors based on lead salts with the general formula AMX3 (A=organic or inorganic cation; M=divalent metal; X=halide), many facets of photophysics continue to puzzle researchers. In this Minireview, light is shed on the low mobilities of charge carriers in lead halide perovskites with special focus on the lattice properties at non‐zero temperature. The polar and soft lattice leads to pronounced electron‐phonon coupling, limiting carrier mobility and retarding recombination. We propose that the proper picture of excited charge carriers at temperature ranges that are relevant for device operations is that of a polaron, with Fröhlich coupling constants between 1<α<3. Under the aspect of light‐emitting diode application, APbX3 perovskite show moderate second order (bimolecular) recombination rates and high third‐order (Auger) rate constants. It has become apparent that this is a direct consequence of the anisotropic polar A‐site cation in organic–inorganic hybrid perovskites and might be alleviated by replacing the organic moiety with an isotropic cation.

Quantifying Polaron Formation and Charge Carrier Cooling in Lead‐Iodide Perovskites

“…For lead‐halide perovskite, α has been calculated to lie between 1.1 and 2.7, establishing the polaron as intermediate‐to‐large, which is supported by the increased mobility at lower temperatures . In addition to the origin of long lifetimes of the photogenerated carriers, polarons have also been invoked to account for the discrepancy between predicted high mobility (>1000 cm 2 V −1 s −1 , due to small effective mass of charger carriers) and the measured lower (<100 cm 2 V −1 s −1 ) one . A number of spectroscopic methods have been used to observe polaron behavior, but the formation has so far only been reported for bromine‐based perovskite .…”

“…Also under intense debate are ferroelectric properties in perovskites, which could allow spatial separation of electrons and holes in the lattice . Finally, it has been proposed that polaron formation allows reduced scattering of charge carriers with phonons, other carriers, or charged defects, resulting in low recombination rates . The concept of the polaron has also been put forward to explain the moderate mobility of charge carriers in lead‐halide perovskites …”

Quantifying polaronic effects on charge-carrier scattering and mobility in lead--halide perovskites

“…A distinction is typically made between the selftrapped "small" polaron, and the itinerant "large" polaron; in the remainder of this paper, we use the general term "polaron" to refer to the latter species exclusively. Polaron formation has been suggested to play a central role in numerous elementary processes underlying charge carrier dynamics in LHPs [3,4,5], including exciton dissociation [6,7,8,9], hot carrier cooling [10,11,12,13,14,15], radiative and non-radiative recombination [16,17,18,19,20,21,22] and steady state mobilities [16,23,24].…”

Modelization of charge carriers mobilities in halide perovskites: Fröhlich scattering and quantum localization effects in a dynamic disorder regime

“…This coupling to LO modes could also lead to the formation of large polarons. Although the mass renormalization estimated to about 40 percent is moderate this polaronic effect is often considered in the literature [9][10][11] . The dipolar moment of Methylammonium (MA) has also been considered as a possible source of scattering even though recent calculations suggest that the scattering by the associated dipolar field has a limited effect on transport 12 .…”