Low-temperature synthesis of mixed valence gold halide perovskites and exploration of their photoluminescence properties

In recent years, the photovoltaic community has shown a growing interest in lead-free halides perovskites (HaPs), i.e., ABX3 where B a Pb, A = monovalent cation and X = halide, as candidates to solve some of the issues inherent to their lead-based cousins. The gold HaP family (A2Au2X6, with mono- and tri-valent Au) is one such example and has been getting increasing attention from theoretical and experimental points of view. In particular, because of the mixed valence character of the gold species, the compounds are double perovskites, A2AuIAuIIIX6. We report a simple synthesis route to obtain inorganic gold HaP (Cs2AuIAuIIIX6, with X = I, Br, Cl) powders at low temperatures, and present thermodynamic constants associated with these materials. We confirm the structure of the compounds by XRD and Raman spectroscopy in accordance with the mixed valence character of the Au species. Additional chemical analyses using XPS and SEM/EDX confirm the stoichiometry of the compounds, though surface iodine deficiency was observed for Cs2AuIAuIIIX6. These results further elucidate the potential of these materials for optoelectronic applications. We report the photoluminescence (PL) spectra for this family of materials to demonstrate their potential photo-activity, with bandgaps in the range of 1.4 eV to 1 eV. Hence, our results open the door to dedicated studies of gold halide perovskites towards possible future integration of these materials in optoelectronics, such as photovoltaic (PV) applications.