# Cu-Doped Cs3Sb2Cl9 Nanocrystals: Revisiting the Low Bandgap of Cs2CuSbCl6 Double Perovskites

## TL;DR

Researchers found that a material previously thought to have a low bandgap is actually a different compound, which changes how we understand its properties.

## Contribution

The study identifies Cs3Sb2Cl9:Cu as the correct material, challenging prior assumptions about Cs2CuSbCl6's bandgap and stability.

## Key findings

- Optical absorption and X-ray diffraction data were better explained by Cs3Sb2Cl9:Cu rather than Cs2CuSbCl6.
- Ab initio calculations show Cs2CuSbCl6 is thermodynamically unstable and decomposes into Cs3Sb2Cl9.
- Low-energy absorption is due to localized electronic transitions at copper dopants in Cs3Sb2Cl9.

## Abstract

Lead-free halide double perovskite Cs2CuSbCl6 nanocrystals have recently been reported to have a low bandgap
of
1.66 eV. In this work, we show that the optical absorption spectra
and X-ray diffraction patterns previously attributed to Cs2CuSbCl6 can rather be explained with Cs3Sb2Cl9:Cu: X-ray absorption spectroscopy identifies
[CuCl3]− trigonal pyramids, with Cu2+ possibly replacing two Cs+ sites. The broad low-energy
optical absorption is then assigned to localized electronic transitions
at copper dopants within the Cs3Sb2Cl9 lattice, which do not affect the wide bandgap. Ab initio calculations
suggest that Cs2CuSbCl6 is thermodynamically
unstable with respect to decomposition to Cs3Sb2Cl9, in line with the low reproducibility of Cs2CuSbCl6 observed in its synthesis.