Exciton properties: learning from a decade of measurements on halide perovskites and transition metal dichalcogenides

TL;DR

This review analyzes various measurement techniques for exciton binding energies in halide perovskites and transition metal dichalcogenides, aiming to reconcile conflicting data and establish more reliable values for these key parameters.

Contribution

It critically compares measurement methods for exciton binding energies and consolidates data to resolve discrepancies in reported values for key 2D materials.

Findings

Agrees on ~350-450 meV for TMDC monolayers on SiO2 and vacuum

Establishes ~150-200 meV for hBN-encapsulated TMDC monolayers

Finds ~200-300 meV for common lead-iodide 2D halide perovskites

Abstract

The exciton binding energy ($E_b$) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of $E_b$ challenge theoreticians to refine models, are a driving force in advancing the understanding a material system, and lead to efficient device design. At their worst, inaccurate $E_b$ measurements lead theoreticians astray, sew confusion within the research community, and hinder device improvements by leading to poor designs. This review article seeks to highlight the pros and cons of different measurement techniques used to determine $E_b$, namely, temperature-dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting $E_b$ values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in attempt to reconcile the variance between different measurement techniques. By considering the published data en masse, we argue the experiments with the clearest indicators are in agreement on the following values: ~350 - 450 meV for TMDC monolayers between SiO$_2$ and vacuum, ~150 - 200 meV for hBN-encapsulated TMDC monolayers, ~200 - 300 meV for common lead-iodide 2D HPs, and ~10 meV for methylammonium lead iodide.