The Effect of Cs/FA Ratio on the Long‐Term Stability of Mixed Cation Perovskite Solar Cells

Formamidinium lead iodide (FAPbI3) is ideal for highly efficient and operationally stable perovskite solar cells (PSC). However, a primary challenge for FAPbI3 PSC is to suppress the phase transition from the photoactive black phase into the yellow nonperovskite δ‐phase. The preparation of Cs‐containing mixed FAPbI3 perovskite by cation stoichiometric engineering is demonstrated and the influence of the Cs/FA ratio on its phase stability and device performance is discussed. By exploring the optimal ratio of Cs and FA cations in Cs x FA1−x Pb(I0.94Br0.06)3 perovskite, an inverted planar device with Cs0.17FA0.83Pb(I0.94Br0.06)3 composition shows the best power conversion efficiency (PCE) of 16.5% in an active area of 1.08 cm2. More importantly, the Cs0.17FA0.83Pb(I0.94Br0.06)3 perovskite photoactive layer showed remarkable long‐term stability, maintaining 88.1% of its initial efficiency for 1128 h in the presence of moisture and oxygen and without any encapsulation. The excellent long‐term stability is found to originate from the appropriate tolerance factor and low thermodynamic decomposition energy, which underpins the strong potential for the commercialization of Cs‐containing mixed FAPbI3 PSCs. Power conversion efficiency of the Cs x FA1−x Pb(I0.94Br0.06)3‐based perovskite solar cells (PSCs) as a function of time and a cross‐sectional transmission electron microscopy (TEM) image to demonstrate stable interface of K5B active layer‐based PSC after 1128 h.