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

Formamidinium lead iodide (FAPbI 3 ) is ideal for highly efficient and operationally stable perovskite solar cells (PSC). However, a primary challenge for FAPbI 3 PSC is to suppress the phase transition from the photoactive black phase into the yellow nonperovskite δ‐phase. The preparation of Cs‐containing mixed FAPbI 3 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 FA 1− x Pb(I 0.94 Br 0.06 ) 3 perovskite, an inverted planar device with Cs 0.17 FA 0.83 Pb(I 0.94 Br 0.06 ) 3 composition shows the best power conversion efficiency (PCE) of 16.5% in an active area of 1.08 cm 2 . More importantly, the Cs 0.17 FA 0.83 Pb(I 0.94 Br 0.06 ) 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 FAPbI 3 PSCs.