Accelerated Carrier Relaxation through Reduced Coulomb Screening in Two-Dimensional Halide Perovskite Nanoplatelets

Verena A Hintermayr,Jochen Feldmann,Lakshminarayana Polavarapu,Alexander S Urban

doi:10.1021/acsnano.8b05029

Abstract

For high-speed optoelectronic applications relying on fast relaxation or energy-transfer mechanisms, understanding of carrier relaxation and recombination dynamics is critical. Here, we compare the differences in photoexcited carrier dynamics in two-dimensional (2D) and quasi-three-dimensional (quasi-3D) colloidal methylammonium lead iodide perovskite nanoplatelets via differential transmission spectroscopy. We find that the cooling of excited electron–hole pairs by phonon emission progresses much faster and is intensity-independent in the 2D case. This is due to the low dielectric surrounding of the thin perovskite layers, for which the Fröhlich interaction is screened less efficiently leading to higher and less density-dependent carrier-phonon scattering rates. In addition, rapid dissipation of heat into the surrounding occurs due to the high surface-to-volume ratio. Furthermore, we observe a subpicosecond dissociation of resonantly excited 1s excitons in the quasi-3D case, an effect which is suppressed in the 2D nanoplatelets due to their large exciton binding energies. The results highlight the importance of the surrounding environment of the inorganic nanoplatelets on their relaxation dynamics. Moreover, this 2D material with relaxation times in the subpicosecond regime shows great potential for realizing devices such as photodetectors or all-optical switches operating at THz frequencies.

Highlights

We investigate charge carrier relaxation in 2D and quasi-three-dimensional methylammonium lead iodide (MAPI) NPls by means of femtosecond differential transmission spectroscopy (DTS) at room temperature
We have compared the relaxation and recombination dynamics of photoexcited electron−hole pairs in 2D and 3D MAPI NPls by means of transient DTS
For the 2D case, relaxation is significantly faster than for their 3D counterparts and is independent of the excitation fluence. We attribute this to a reduced screening of the Fröhlich interaction in the 2D system and its low dielectric surrounding resulting in enhanced and density-independent phonon emission as well as rapid dissipation of the energy from the phonon system to the surrounding medium

Summary

Introduction

Lead halide perovskite thin films have established themselves as an excellent material system for photovoltaic applications due to favorable absorption and charge transport properties.[1−4] With bandgaps tunable throughout the visible range via halide ion replacement[5,6] and quantum yields approaching unity, perovskite nanocrystals (NCs) exhibit strong potential for a variety of other optoelectronic applications.[7−10] While in both of these fields either bulk or only weakly confined perovskite materials have been employed, recently strongly confined perovskite NCs, such as two-dimensional (2D) nanoplatelets (NPls), with a monolayer-precise control of the thickness and resulting quantum size effects have emerged.[11−15] With extremely high exciton binding energies and transition energies tunable through quantum confinement, they demonstrate intriguing possibilities for light-emitting applications such as LEDs or lasers.[16−18] with excitons dominant at roomtemperature perovskite NPls are promising for excitonic device concepts. Our findings highlight the vast differences occurring in perovskite materials based on their dimensionality. Knowledge of these processes could prove critical to designing optoelectronic devices relying on fast relaxation as well as charge- or energy-transfer mechanisms

Methods

Results

Conclusion