Local Energy Landscape Drives Long-Range Exciton Diffusion in Two-Dimensional Halide Perovskite Semiconductors.

Alan Baldwin,Krzysztof Galkowski,Géraud Delport,Samuel D Stranks,Rosemonde Chahbazian,Kian Ping Loh,Kai Leng

doi:10.1021/acs.jpclett.1c00823

Abstract

Halide perovskites are versatile semiconductors with applications including photovoltaics and light-emitting devices, having modular optoelectronic properties realizable through composition and dimensionality tuning. Layered Ruddlesden–Popper perovskites are particularly interesting due to their unique 2D character and charge carrier dynamics. However, long-range energy transport through exciton diffusion in these materials is not understood or realized. Here, local time-resolved luminescence mapping techniques are employed to visualize exciton transport in exfoliated flakes of the BA2MAn–1PbnI3n+1 perovskite family. Two distinct transport regimes are uncovered, depending on the temperature range. Above 100 K, diffusion is mediated by thermally activated hopping processes between localized states. At lower temperatures, a nonuniform energy landscape emerges in which transport is dominated by downhill energy transfer to lower-energy states, leading to long-range transport over hundreds of nanometers. Efficient, long-range, and switchable downhill transfer offers exciting possibilities for controlled directional long-range transport in these 2D materials for new applications.

Highlights

Halide perovskites are versatile semiconductors with applications including photovoltaics and light-emitting devices, having modular optoelectronic properties realizable through composition and dimensionality tuning
The two-dimensionality of each of the layers coupled with the low dielectric constant of the spacer molecules results in charge dynamics which are dominated by strongly bound excitons.[5,10−12] The remarkable photophysical properties of these Ruddlesden−Popper perovskites (RPPs) have been utilized in various domains such as lasers[13] and LEDs14 and in efficient, stable perovskite solar cells.[15]
The exciton binding energy in such n = 4 perovskites is on the order of hundreds of millielectron volts,[10] implying that the carriers form a majority of excitons under all but one of the experimental conditions used in our study. (See SI Section V and Figure S2 for details.) For temperatures in the range of 300−100 K, we observe diffusive transport in which exciton−phonon coupling plays a crucial role, resulting in a thermally activated hopping process through localized states, which are responsible for the nonradiative decay of these excitons

Summary

Introduction

Halide perovskites are versatile semiconductors with applications including photovoltaics and light-emitting devices, having modular optoelectronic properties realizable through composition and dimensionality tuning. (See SI Section V and Figure S2 for details.) For temperatures in the range of 300−100 K, we observe diffusive transport in which exciton−phonon coupling plays a crucial role, resulting in a thermally activated hopping process through localized states, which are responsible for the nonradiative decay of these excitons.

Results

Conclusion