Confinement-Driven Exciton Behavior in 2D Halide Perovskites from Dielectric-Dependent Hybrid Methods

TL;DR

This study combines experimental and theoretical methods to explore how dielectric anisotropy influences excitonic behavior in 2D halide perovskites, revealing key insights for optoelectronic device design.

Contribution

It introduces a refined hybrid computational approach to accurately model dielectric screening and excitonic properties in layered perovskites, linking anisotropic dielectric response to exciton binding energies.

Findings

Exciton binding energy decreases from 0.26 eV to 0.21 eV as layer number increases.

In-plane dielectric constant predominantly influences optical transitions.

Calculated absorption spectra match experimental data within 0.02 eV.

Abstract

Understanding how dielectric anisotropy governs excitonic behavior in two-dimensional (2D) halide perovskites is critical for predicting and engineering their optoelectronic properties. In this work, we investigate Cs(n+1)PbnBr3n+1 nanoplatelets (n = 2-5) experimentally and theoretically and show that the interplay between dielectric confinement and anisotropic screening critically determines both their electronic structure and excitonic landscape. To incorporate the dielectric screening effects, the Coulomb kernel in the Fock exchange term is refined using a model dielectric function together with a model Bethe-Salpeter Equation approach. The exciton binding energies show a monotonic decrease from 0.26 eV to 0.21 eV from n = 2 to 5, with 20 meV decrease per layer up to n = 4, and thereafter less change. The relatively small change per layer is a consequence of the strong spatial localization of excitons. By analyzing directionally resolved dielectric tensors, we demonstrate that the in-plane dielectric constant predominantly dictates optical transitions and is close to converging to the bulk value already at n = 5, while the out-of-plane dielectric response reflects the confined nature of excitonic wave functions as expected. Our calculated absorption spectra capture experimental results within 0.02 eV throughout the confinement regime (n = 2-5). The effects of lattice dynamics on the dimensionally dependent dielectric response and subsequent exciton screening occurring on longer time-scales than the optical response are also analyzed, important for analysis and interpretation of exciton lifetime, diffusion, and band alignments. The results establish a clear correlation between dielectric anisotropy, electronic structure, and exciton binding energy at different timescales in layered perovskites, providing essential insight for the design of 2D optoelectronic materials and devices.