Effect of CsCl Additive on the Morphological and Optoelectronic Properties of Formamidinium Lead Iodide Perovskite

The quality of perovskite films plays a crucial role in improving the optoelectronic properties and performance of perovskite solar cells (PSCs). Herein, high‐quality Cs x FA 1− x PbI 3 perovskite films with different compositions ( x = 0, 5, 10, and 15) are achieved by controlling the amount of cesium chloride (CsCl) in the respective FAPbI 3 precursor solution. The effects of CsCl addition on the morphological and optoelectronic properties of the resulting perovskite films and on the performance of the corresponding devices are systematically studied. Introduction of CsCl into FAPbI 3 shows a great potential to stabilize the α‐FAPbI 3 perovskite phase by forming Cs x FA 1− x PbI 3 films with improved morphology and carrier lifetimes. With an optimal 10 mol% CsCl additive, the average power conversion efficiency (PCE) is increased from 16.83 ± 0.30% for the reference FAPbI 3 ‐based PSCs to 18.87 ± 0.25% (with a steady‐state PCE of 18.89%). Moreover, the optimized device performance is more stable after 20 days than the controlled one under ≈40% humidity in air.