Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Angelica Simbula,Qingqian Wang,Fang Liu,Mercouri G Kanatzidis,Ioannis D Spanopoulos,Daniela Marongiu,Francesco Quochi,Matteo Lodde,Stefano Lai,Andrea Mura,Giovanni Bongiovanni,Guido Mula,Riccardo Pau,Valerio Sarritzu,Michele Saba,Alessandra Geddo Lehmann,Francesco Mattana

doi:10.1002/adom.202100295

Angelica Simbula, Qingqian Wang + Show 15 more

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https://doi.org/10.1002/adom.202100295

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Abstract

AbstractRapid advances in perovskite photovoltaics have produced efficient solar cells, with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A major reason for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature. On the other hand, the current understanding is that in 2D perovskites or at cryogenic temperatures insulating bound excitons form, which need to be split in solar cells and are not beneficial to photoconversion. Here, a tandem spectroscopy technique that combines ultrafast photoluminescence and differential transmission is applied to demonstrate a plasma of unbound charge carriers in chemical equilibrium with a minority phase of light‐emitting excitons, even in 2D perovskites and at cryogenic temperatures. The underlying photophysics is interpreted as formation of large polarons, charge carriers coupled to lattice deformations, in place of excitons. A conductive polaron plasma foresees novel mechanisms for LEDs and lasers, as well as a prominent role for 2D perovskites in photovoltaics.

Highlights

Rapid advances in perovskite photovoltaics have produced efficient solar cells, onance, the majority optical excitation in with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites
The optical properties of hybrid perovskites are typically studied with two ultrafast spectroscopy techniques, namely timeresolved photoluminescence (PL) and differential transmission (DT), known as transient absorption or pump-probe
Dressing charged carriers by lattice deformations brings two main consequences: the first one is that polarons are energetically stabilized with respect to free electrons and holes, meaning that they sit at lower energy and effectively reduce the exciton binding energy; the second one is the heavier mass of polarons than free carriers, which increases the density of states making them entropically favored with respect to excitons

Summary

Introduction

Rapid advances in perovskite photovoltaics have produced efficient solar cells, onance, the majority optical excitation in with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A tandem spectroscopy technique that combines ultrafast photoluminescence and differential transmission is applied to demonstrate a plasma of unbound charge carriers in chemical equilibrium with a minority phase of light-emitting excitons, even in 2D perovskites and at of the flexibility of the materials class, layered 2D HPs are obtained by inserting bulky organic cations into the formulation, leading to materials inherently more stable than their 3D counterparts against cryogenic temperatures. Sisting for several nanoseconds at temperatures significantly higher than the lattice one.[12,16,17,21,22,23,24,25,26] Large polarons may be the enabling microscopic mechanism for efficient solar cells, including innovative architectures that exploit photoconversion with hot carriers.[27,28]

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