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

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.