Computational and Experimental Investigation of Chiral and Achiral 2D Organic Lead Bromide Perovskites: Octahedral Distortions and Electronic and Optical Properties

TL;DR

This work addresses how octahedral distortions in 2D chiral and achiral lead bromide perovskites influence electronic and optical properties. It combines experimental synthesis and XRD with DFT-PBE+vdW and RPA optical calculations, plus a Python tool to quantify octahedral distortions via parameters such as $D_{tilt}$, $D_{in}$, $D_{out}$, and $ΔD_{tilt}$. Key findings show that van der Waals corrections are essential to accurately reproduce lattice geometry and distortion metrics, while DFT tends to overestimate tilt angles; notably, FMBA exhibits a very large $ΔD_{tilt} ≈ 14^ ext{o}$, tied to strong symmetry breaking and chiral emission. Polarized low-energy transitions are in-plane and Br p to Pb p in character, whereas excitonic features observed experimentally are not captured by the RPA approach, underscoring the need for beyond-RPA methods; the provided octahedral-distortion analysis tool links structure to function, with implications for optoelectronic and spintronic applications.

Abstract

We present a computational investigation, in conjunction with synthesis and experimental characterization, into the structural, electronic, and optical properties of layered 2D organic lead bromide perovskites. We contrast materials based on the chiral (R/S)-4-fluoro-$α$-methylbenzylammonium (R/S-FMBA), which have been shown to lead to bright room-temperature circularly polarized luminescence, with the similar achiral 4-fluorobenzylammonium (FBA). Using density functional theory (DFT) with van der Waals (vdW) corrections, we study relaxed structures (compared with X-ray diffraction, XRD) and optical absorption spectra (compared with experiments), as well as bandstructure and orbital character of transitions. We develop and provide a Python code to calculate octahedral distortions and compare DFT and XRD results, finding that vdW corrections are important for accuracy and that DFT overestimates octahedral tilt angles. (FMBA)$_2$PbBr$_4$ shows among the largest tilt angle differences (often termed $Δβ$) reported, $14^\circ$, indicating strong inversion symmetry-breaking which enables its chiral emission. The lowest-energy optical transitions involve the perovskite only and are polarized within the layer. This work furthers understanding of structure-property relations with applications to optoelectronics and spintronics.

Figures (7)

• Figure 1: (A--C) Organic ammonium cations, (D--F) single crystal structures of $(\mathrm{FBA})_2\mathrm{PbBr}_4$, $(\mathrm{R\text{-}FMBA})_2\mathrm{PbBr}_4$, and $(\mathrm{S\text{-}FMBA})_2\mathrm{PbBr}_4$, (G--I) Top-down view and (J--L) side view of the connected lead bromide octahedra with different bond angles and lengths from the crystal structures of $(\mathrm{FBA})_2\mathrm{PbBr}_4$, $(\mathrm{R\text{-}FMBA})_2\mathrm{PbBr}_4$, and $(\mathrm{S\text{-}FMBA})_2\mathrm{PbBr}_4$.
• Figure 2: A schematic of octahedral distortion parameters in a 2D perovskite: within an octahedron, deviations $\lambda_{\text{oct}}$ and $D$ of the bond lengths $d_i$, and deviation $\sigma$ of bond angles $\theta_i$; between neighboring octahedra, the tilt angle $\theta_{\text{tilt}}$ and its resolution into components in plane $\theta_{\text{in}}$ and out of plane $\theta_{\text{in}}$, with reference to the 2D perovskite layers smith2017structural.
• Figure 3: Comparison of XRD (solid) and DFT-relaxed (partially transparent) structures of (FBA)$_2$PbBr$_4$ (top) and (R-FMBA)$_2$PbBr$_4$ (bottom).
• Figure 4: Polarization-dependent optical absorption spectra, calculated with RPA, for (a) $(\mathrm{FBA})_2\mathrm{PbBr}_4$, (b) $(\mathrm{R\text{-}FMBA})_2\mathrm{PbBr}_4$, and (c) $(\mathrm{S\text{-}FMBA})_2\mathrm{PbBr}_4$ structures.
• Figure 5: (a) Computed absorption coefficients for (R/S-FMBA)$_2$PbBr$_4$, and (b) computed and measured absorption spectra for (S-FMBA)$_2$PbBr$_4$.