Out-of-plane cations homogenise perovskite composition for solar cells.

Perovskite solar cells (PSCs) composing the formula of FA 1-x Cs x PbI 3 provide an attractive ption for integrating high efficiency, durable stability and compatibility with upscale fabrication. Despite the Cs cation incorporation potentially enabling a perfect perovskite lattice 1,2 , the compositional inhomogeneity caused by A-site cation segregation is likely detrimental to the photovoltaic performance of solar cells 3,4 . Here, we visualized the out of-plane compositional inhomogeneity along the vertical direction across the perovskite films and identified the underlying reasons and the potential impacts for devices. We devised a strategy using 1-(Phenylsulfonyl)pyrrole (PSP) to homogenize the cations components in the perovskite films. The resultant p-i-n devices yielded a certified steady state PCE of 25.2% and durable stability.