Publication date: 15th December 2025
Mixed-valence gold halide perovskites such as Cs₂AuᴵAuᴵᴵᴵX₆ (X = Cl, Br, I) have recently emerged as a compelling platform for next-generation photonic technologies, offering intrinsically lead-free compositions, strong electron–phonon interactions, and ultralow bandgaps. Landmark advances—from the stabilization of Au²⁺ in 3D halide frameworks [1] to the identification of an indirect polar valley limiting radiative efficiency [2]—have highlighted the unique mixed-valence physics of these systems.
In this talk, I will present our latest results on the synthesis, structural control, and optoelectronic properties of the Cs₂AuᴵAuᴵᴵᴵX₆ family. Building on recent low-temperature synthesis strategies [3], we combine single-crystal growth, thin-film deposition, and advanced optical spectroscopies to unravel the mechanisms governing both light absorption and emission in these ultralow-bandgap double perovskites.
Cryogenic and temperature-dependent measurements reveal a remarkably red-shifted self-trapped exciton (STE) luminescence, displaced by approximately 400 meV from the absorption edge. This STE emission arises from the strong lattice distortions and mixed-valence Auᴵ/Auᴵᴵᴵ configuration characteristic of Cs₂AuᴵAuᴵᴵᴵX₆. In parallel, we identify a pronounced ~100 meV Urbach tail—consistent with the indirect polar valley physics previously reported [2]—which provides an efficient nonradiative relaxation channel and plays a key role in quenching the radiative emission through dense phonon-assisted intragap states.
Together, these findings provide a unified picture of strong exciton–phonon coupling, disorder-induced band tailing, and polaron formation in gold-based halide perovskites, and define the fundamental limits and opportunities for deploying Cs₂AuᴵAuᴵᴵᴵX₆ materials in next-generation infrared detectors and light-emission devices.
Acknowledgment — This work is supported by the ANR project POETESSE (Projet-ANR-22-CE42-0027).
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