Redox mediator-stabilized wide-bandgap perovskites for monolithic perovskite-organic tandem solar cells

Halide segregation critically limits the stability of mixed-halide perovskite solar cells under device operational conditions. There is a strong indication that halide oxidation is the primary driving force behind halide de-mixing. To alleviate this problem, we develop a series of multifunctional redox mediators based on anthraquinone that selectively reduce iodine and oxidize metallic Pb⁰, while simultaneously passivating defects through tailored cationic substitution. These effects enable wide-bandgap perovskite solar cells to achieve a power conversion efficiency of 19.58% and a high open-circuit voltage of 1.35 V for 1.81-eV PSCs. The device retains 95% of its initial efficiency after operating at its maximum power point for 500 h. Most notably, by integrating the perovskite device into the monolithic perovskite-organic tandem solar cell as a wide-bandgap subcell, we report an efficiency of 25.22% (certified 24.27%) with impressive long-term operational stability (T₉₀ > 500 h).