Franz-Keldysh and Stark Effects in Two-Dimensional Metal Halide Perovskites

TL;DR

This study uses electroabsorption spectroscopy and established exciton theory to precisely measure fundamental properties of 2D metal halide perovskites, resolving previous inconsistencies and enabling better material tuning.

Contribution

It demonstrates a high-precision method to determine exciton binding energy and related properties in 2D MHPs using electroabsorption analysis based on the Wannier exciton model.

Findings

Accurate measurement of exciton binding energy with 2% uncertainty.

Deep understanding of Stark and Franz-Keldysh effects in 2D MHPs.

Determination of exciton Bohr radius, dipole moment, and polarizability.

Abstract

As the field of metal halide perovskites (MHP) matures, state-of-the-art techniques to measure basic properties such as the band gap and exciton binding energy continue to produce inconsistent values. This issue is persistent even for 2D MHPs wherein the large separation between exciton and continuum states should make such measurements more straightforward. In this study, we revert to the established theory of a 2D Wannier exciton in a uniform electric field to analyze the electroabsorption response of an archetypal 2D MHP system, phenethylammonium lead iodide (PEA2PbI4). The high level of agreement between the electroabsorption simulation and measurement allows for a deepened understanding of the exciton's redshift according to the quadratic Stark effect and the continuum wavefunction leaking according to the Franz-Keldysh effect. We find the field-dependency of each of these effects to be rich with information, yielding measurements of the exciton's Bohr radius, transition dipole moment, polarizability, and reduced effective mass. Most importantly, the exciton binding energy is unambiguously determined with 2% uncertainty. The high precision of these new measurement methods opens the opportunity for future studies to accurately determine the influence of chemical and environmental factors on the optoelectronic properties of MHPs and thereby increase the tunability of this important class of materials.