Stable and High‐Efficiency Methylammonium‐Free Perovskite Solar Cells

Organic–inorganic metal halide perovskite solar cells (PSCs) have achieved certified power conversion efficiency (PCE) of 25.2% with complex compositional and bandgap engineering. However, the thermal instability of methylammonium (MA) cation can cause the degradation of the perovskite film, remaining a risk for the long‐term stability of the devices. Herein, a unique method is demonstrated to fabricate highly phase‐stable perovskite film without MA by introducing cesium chloride (CsCl) in the double cation (Cs, formamidinium) perovskite precursor. Moreover, due to the suboptimal bandgap of bromide (Br−), the amount of Br− is regulated, leading to high power conversion efficiency. As a result, MA‐free perovskite solar cells achieve remarkable long‐term stability and a PCE of 20.50%, which is one of the best results for MA‐free PSCs. Moreover, the unencapsulated device retains about 80% of the original efficiencies after a 1000 h aging study. These results provide a feasible approach to enhance solar cell stability and performance simultaneously, paving the way for commercializing PSCs. A highly phase‐stable perovskite film without the methylammonium cation is fabricated by introducing cesium chloride in the double cation Cs, formamidinium perovskite precursor, leading to high power conversion efficiency of 20.5% and remarkable long‐term stability. The unencapsulated perovskite solar cell retains about 80% of its initial efficiency after a 1000 h aging study, demonstrating a feasible approach to enhance solar cell efficiency and stability simultaneously.