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 delta-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(I0.94Br0.06)(3) perovskite, an inverted planar device with Cs(0.17)FA(0.83)Pb(I0.94Br0.06)(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(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 FAPbI(3) PSCs.