Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites

Abstract

In metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling. Our results can be well described by the polaronic-exciton picture where electron and hole interactions are no longer described by a Coulomb potential. Furthermore, we show experimental evidence that the carrier-phonon interaction leads to the enhancement of the carrier’s effective mass. Notably, our measurements reveal a pronounced temperature dependence of the carrier’s effective mass, which we attribute to a band structure renormalization induced by the population of low-energy phonon modes. This interpretation finds support in our first-principles calculations.

Electronic Structure of Biexcitons in Metal Halide Perovskite Nanoplatelets

“…The latter contains not only Pb but also large cations (typically Cs, methylammonium or formamidinium), which results in a strong X-phonon (polaron) coupling. − Several nontrivial effects have been associated with polaronic coupling in halide perovskites. These include exciton self-trapping, − reduced mass renormalization, , complex spectral structure, ,− enhanced intra-X and inter-X interaction.…”

“…Exciton binding energies and recombination rates in metal halide perovskites (bulk and nanocrystals) have been successfully interpreted by means of simpleyet intuitiveeffective mass models. ,,, A more demanding case is that of quasi-2D NPLs, where polaronic effects coexist with pronounced quantum and dielectric confinement. To deal with this system, we recently developed an effective mass model combining all three physical factors .…”

“…The second term is a short-range (Yukawa) one, which starts prevailing at distances r ≲ l i , where the ionic contribution to dielectric screening is lost (ϵ ∞ dominates). Equation constitutes a simple model of X-polaron interactions, which has been shown to improve estimates of X binding energies in different semiconductors, − including lead halide perovskites. ,,, …”