Unusual thickness dependence of exciton characteristics in 2D perovskite quantum wells

TL;DR

This study reveals that optical resonances in 2D perovskite quantum wells are due to tightly bound excitons with high reduced mass, and their properties vary with layer thickness, impacting optoelectronic device design.

Contribution

It provides the first clear evidence that optical resonances in these materials are excitonic and details how exciton properties scale with quantum well thickness.

Findings

Optical resonances are from tightly bound excitons.

Exciton reduced mass is unexpectedly high (0.20 m0).

Exciton binding energy varies from 470 meV to 125 meV with thickness.

Abstract

Understanding the nature and energy distribution of optical resonances is of central importance in low-dimensional materials$^{1-4}$ and its knowledge is critical for designing efficient optoelectronic devices. Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A$_2$A'$_{n-1}$M$_n$X$_{3n+1}$, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free-carriers) and the exciton reduced mass, and their scaling with quantum well thickness remains unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with unexpectedly high exciton reduced mass (0.20 m0) and binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5. Our work demonstrates the dominant role of Coulomb interactions in 2D solution-processed quantum wells and presents unique opportunities for next-generation optoelectronic and photonic devices.