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Apr 28, 2020

ACS Spring 2020 National Meeting & Expo

Using vibrational spectroscopy to assess the influence of defects on the local structures of low dimension hybrid perovskites

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2D Perovskites

Raman Spectroscopy

Non-covalent Interactions

Anharmonic Effects

Phonons

Abstract

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54

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Abstract

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Keywords

2D Perovskites

Raman Spectroscopy

Non-covalent Interactions

Anharmonic Effects

Phonons

Abstract

With the possibilities of obtaining efficient broadband emission in low dimension hybrid perovskites towards application in lighting being pursued, the need to understand the mechanism involved in this emission is key. We employed temperature and power-dependent photoluminescence spectroscopy in combination with THz spectroscopy to assign broad mid-gap emission in butylammonium spaced lead iodide perovskite (BA2PbI4) to the presence of metastable charged defects. We develop a sum rule correlating changes in the oscillator strengths of lattice vibrations to observable changes in measured vibrational spectra. Using this sum rule, we propose changes of THz light absorption in the lattice region of BA2PbI4 upon sample annealing stem from thermal ejection of charged defects from the bulk of the sample. To further assess the origin of subgap light emission from hexylammonium spaced lead iodide perovskite (HA2PbI4), we use temperature dependent Raman spectroscopy and density functional theory calculations to probe the influence of charged defects on the vibrational spectra of the molecular subunits of this material. In the presence of charged defects, there should exist local electric fields that could imprint themselves on vibrational peaks according to the Stark effect. While we observe the Raman peaks due to bending motions of the ammonium group of the examined molecular cation do shift, this shift appears in the vibrational spectra for both pristine and defective samples. Based on these results we propose similar shifts of the molecular vibrational spectra should be interpreted with caution and not necessarily attributed to effects such as the vibrational Stark shift due to the presence of local electric fields from charged defects. In addition, we model the temperature dependent shifts and line shapes of the observed peaks to understand interactions between the inorganic layer and molecular cations for analogue materials. The line shape analysis shows the shifts in Raman peaks do not depend on the identity of the organic cation. Establishing the strength of these interactions may aid in the deterministic design of broadband light emission from low dimension hybrid perovskite materials.

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All rights reserved.