Excitons, Fine Structure Splitting, Spin and Chirality—The Exciting Optical Properties of 2D Halide Perovskites from Combined Symmetry Analysis and Green-Function Based Simulations

Two-dimensional (2D) layered metal-halide perovskites are solution-processed inorganic/organic quantum-well structures, holding great potential for opto-electronic applications. Initially considered as an ideal study-cases to investigate the optical properties of dimensionally confined systems, these compounds are currently rising as suitable alternatives to their parental 3D analogues, for technological applications.[1] Due to both quantum- and dielectric-confinement, 2D halide perovskites sustain stable excitons, with binding energy of few hundreds meV.[2] Several investigations pointed out the presence of several, closely spaced excitonic resonances, near the absorption onset, which influence the exciton dynamics.[3] In particular, symmetry analysis anticipates the existence of a dark-exciton, and three bright excitons, a pair with in-plane polarization and one with out-of-plane polarization.[4] However, contradicting results about the energy ordering of these resonances have been reported in the literature.[5–6] Furthermore, though disproved by later investigations,[7] previous claims of potential reversal ordering in 3D halide perovskite nanocrystals, due to the Rashba-effect,[8] suggested the possibility of resorting to chiral spacers, to modulate the dark-bright ordering.
Aiming to provide a theoretical support, to complement the experimental understanding of the exciton properties of 2D halide perovskites, we perform accurate excited state simulations of these compounds, resorting to the solution of the Bethe-Salpeter equation, from first-principles.[9] Interestingly, the results from these cutting-edge computational methods can be nicely interpreted on the basis of the expectations from symmetry-analysis and of simplified models from the theory of conventional semiconductors. The predicted ordering of the exciton fine structure well agrees with very recent magneto-absorption measurements for most common PEA₂PbI₄ (PEA=phenylethynylammonium) and BA₂PbI₄ (butylammonium) perovskites,[5] showing that the in-plane component is more stable than the out-of-plane one, further contributing to solve an ongoing debate in the literature.[5–6] In addition, our calculations shed light on the spin-properties of the excited states of 2D lead-halide perovskites, which are sometimes incorrectly described as pure spin-singlet and -triplet states. Namely, they show that the huge spin-orbit-coupling due to the presence of heavy atoms ultimately imparts the excitons of these hybrids with a mixed spin-singlet/-triplet character, a fact that should be carefully considered when discussing recent findings of perovskite-induced triplet-sensitization in organic molecules.[10-11] Our calculations also show that ferro-electric distortions may induce, indeed, a dark-bright reversal ordering but this happens only for sizable distortions. Further studies are needed to verify if the distortions reported in lattices incorporating (pure enantiomeric) chiral spacer actually drives to such ordering reversal.
The present set of knowledge will be useful to set a common reference frame for the discussion of the optical properties of 2D halide perovskites of various structure and composition.

[1] W. Li et al., Nat. Natotechnol, 2022, 17, 45.
[2] C. Katan, et al. Chemical Reviews, 2019, 119, 3140.
[3] H.-H. Fang, Adv. Funct. Mater. 2020, 30, 1907979.
[4] C. Quarti et al,. J. Phys: Mater., Mater. 2020, 3, 042001.
[5] K. Tanaka, et al., Jpn. J. Appl. Phys. 2005, 44, 5923.
[6] M. Dyksik, et al. Sci. Adv. 2021, 7, eabk0904.
[7] P. Tamarat, et al., Nature Commun., 2023, 14, 229.
[8] M. A. Becker, et al., Nature 2018, 553, 189.
[9] C. Quarti, et al. Adv. Opt. Mater. 2024, 12, 2202801.
[10] Z. A. VanOrman, ACS Energy Lett. 2021, 6, 3686.
[11] F. Ledée et al., Mater. Horiz., 2021, 8, 1547.