Polar Indirect Valley as a Limiting Factor for Radiative Efficiency in Gold-Based Mixed-Valence Double Perovskites

Double perovskites have emerged as promising alternatives to lead halide perovskites, aiming to mitigate challenges related to toxicity and chemical instability. Among them, mixed-valence gold halides such as Cs₂Au⁺Au^{3 +}Cl₆, which contain only a single type of metal cation in two oxidation states, stand out due to their unique structural and electronic properties. These materials exhibit strong absorption in the near-infrared range, making them attractive candidates for optoelectronic applications such as photovoltaics. In this work, temperature-dependent optical spectroscopy techniques are employed to demonstrate that these compounds exhibit particularly strong polar electron-phonon coupling, which has a profound impact on their optoelectronic properties. In particular, this coupling induces a temperature-dependent absorption tail that progressively reshapes the absorption spectrum. It is shown that this tail leads to a forbidden band-egde recombination, which explains the reported difficulties in detecting a photoluminescence signal from this class of double perovskites.