A Ferrocene Capping Layer Enabling Highly Efficient and Stable Sn–Pb Mixed Perovskite Solar Cells

The Sn–Pb mixed perovskite with a narrow bandgap is essential for the construction of all‐perovskite tandem solar cells. However, the oxidation of Sn 2+ to Sn 4+ makes the fabrication of Sn–Pb perovskite solar cells (PSCs) challenging. Herein, a ferrocene (Fc) capping layer is introduced on the surface of Sn–Pb mixed perovskite to maintain a reducing environment. This is due to a strong electron supply capacity from the π ‐orbitals in cyclopentadiene in Fc, which effectively suppresses the oxidation of Sn 2+ , reduces the defect density, and leads to an improvement in carrier lifetime. Therefore, the Fc capping layer significantly improves the power conversion efficiency (PCE) of single‐junction Sn–Pb PSCs from 17.49% to 21.02%. It maintains 87% of its initial PCE after light exposure at 1 sun condition in N 2 atmosphere for 288 h, demonstrating good long‐term stability. The strategy outlined herein offers a straightforward and efficient approach to enhance the stability and PCE of narrow‐bandgap Sn–Pb mixed PSCs.