Intra- and Interlayer Excitonic Fine Structure of the Two-Dimensional Perovskite (PEA)$_2$PbI$_4$

TL;DR

This study combines polarization-resolved photoluminescence and first-principles calculations to clarify the intralayer and interlayer excitonic fine structures in (PEA)$_2$PbI$_4$, resolving spectral feature controversies.

Contribution

It provides a detailed excitonic fine structure analysis of (PEA)$_2$PbI$_4$, emphasizing the role of crystal symmetry and distortions without invoking Rashba or polaron effects.

Findings

Identified intralayer excitonic fine structure governed by crystal symmetry.

Detected interlayer excitons approximately 45 meV above bright intralayer states.

Distortion-induced mixing enhances interlayer optical activity, explaining their visibility.

Abstract

Two-dimensional halide perovskites host strongly bound excitons whose fine structure controls polarization selection rules and radiative recombination, yet several spectral features in (PEA)$_2$PbI$_4$ remain controversially assigned. Here, polarization-resolved low-temperature photoluminescence combined with first-principles G$_0$W$_0$+BSE calculations resolves both the intralayer and interlayer excitonic fine structure of this prototypical n=1 Ruddlesden-Popper perovskite. The low-energy multiplet is consistently described as a purely excitonic intralayer fine structure governed by crystal symmetry, octahedral distortions, and the two-layer unit cell, without invoking Rashba or exciton-polaron mechanisms as the primary origin. A weaker doublet ~45 meV above the bright intralayer states is identified as interlayer excitons from its agreement with the calculated interlayer manifold in energy and splitting. Although the static calculations underestimate their oscillator strength and do not reproduce the observed orthogonal polarizations, distortion-induced mixing with bright intralayer excitons strongly enhances interlayer optical activity and provides a plausible explanation for their visibility. Our results establish interlayer excitons in (PEA)$_2$PbI$_4$ and refine the excitonic description of fine structure in two-dimensional perovskites.