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

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-$\alpha$-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 $\Delta \beta$) 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.