# Fundamental Properties of Low-Dimensional Perovskite-Related Light Absorbers: [(CH2)3NH2]3Sb2X9 (X = I, Br, Cl), Azetidinium Antimony Halides

## TL;DR

This paper studies the properties of low-dimensional antimony-based halide materials, focusing on their structure, band gaps, and phase transitions for potential use in optoelectronics.

## Contribution

The study introduces azetidinium as a novel A-site cation in A3Sb2X9 systems and reports on their structural and optical properties.

## Key findings

- (Az)3Sb2I9 forms a 0D dimer structure, while (Az)3Sb2Br9 and (Az)3Sb2Cl9 adopt a 2D corrugated layer structure.
- Band edges of 2.11 eV, 2.63 eV, and 3.26 eV are observed for (Az)3Sb2I9, (Az)3Sb2Br9, and (Az)3Sb2Cl9, respectively.
- Thin-film morphology and band edge shift are strongly influenced by the solvent used during synthesis.

## Abstract

Over the past decade,
many Sb-based organohalides have been synthesized
and analyzed due to their intriguing optical and electronic properties.
They form low-dimensional (0D, 1D, and 2D) metal halide frameworks
due to the 3+ oxidation state of antimony (Sb3+). The low-dimensional
perovskite derivatives commonly have good moisture stability which
makes them attractive for replacing the lead-containing halide perovskites.
We focus on the A3Sb2X9 compositions
since the band gap tunability of the compounds is noteworthy. Cs3Sb2I9 is known to form both 0D and 2D
octahedral networks at room temperature. The A-site in A3Sb2X9 is crucial in determining the configuration
of the metal halide framework. Within the hybrid A3Sb2X9 systems, the azetidinium cation (Az+, [(CH2)3NH2]+) has rarely
been studied as a potential A-site candidate. We confirm that the
polycrystalline powders and thin films of (Az)3Sb2I9 exhibit a 0D dimer structure, while (Az)3Sb2Br9 and (Az)3Sb2Cl9 adopt a 2D corrugated layer structure at the molecular level.
(Az)3Sb2I9 and (Az)3Sb2Cl9 exhibit phase transitions at low temperatures,
as shown by differential scanning calorimetry. Thin films grow with
a strongly preferred (00l) orientation perpendicular
to glass substrates. The Tauc plot analysis from diffuse reflectance
spectra exhibits a band edge at 2.11 eV in (Az)3Sb2I9, at 2.63 eV in (Az)3Sb2Br9, and at 3.26 eV in (Az)3Sb2Cl9. A weak excitonic band is identified in the absorption spectrum
of the (Az)3Sb2I9 thin film. Interestingly,
there is a severe morphological difference in the thin films, depending
on the solvent used, resulting in a band edge shift. The temperature
dependence of the (Az)3Sb2I9 thin-film
XRD data indicates the existence of an intermediate phase.