Cu-Doped Cs3Sb2Cl9Nanocrystals: Revisiting the Low Bandgap of Cs2CuSbCl6Double Perovskites

Lead-free halide double perovskite Cs2CuSbCl6nanocrystals 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 Cs2CuSbCl6can 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 Cs3Sb2Cl9lattice, which do not affect the wide bandgap. Ab initio calculations suggest that Cs2CuSbCl6is thermodynamically unstable with respect to decomposition to Cs3Sb2Cl9, in line with the low reproducibility of Cs2CuSbCl6observed in its synthesis.