Mixed Valence Perovskite Cs₂Au₂I₆: A Potential Material for Thin‐Film Pb‐Free Photovoltaic Cells with Ultrahigh Efficiency

Abstract New light is shed on the previously known perovskite material, Cs₂Au₂I₆, as a potential active material for high‐efficiency thin‐film Pb‐free photovoltaic cells. First‐principles calculations demonstrate that Cs₂Au₂I₆ has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂Au₂I₆ a double‐perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂Au₂I₆ has a direct‐band‐gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state‐of‐the‐art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂Au₂I₆ will serve as the active material in highly efficient, nontoxic, and thin‐film perovskite solar cells in the very near future.