Decoupling engineering of formamidinium-cesium perovskites for efficient photovoltaics

Although pure formamidinium iodide perovskite (FAPbI₃) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI₃. However, state-of-the-art formamidinium-cesium (FA-Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI₃, limited by the different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible, highly efficient and stable solar cells based on FA_{1 -} Cs PbI₃ (x = 0.05-0.16) films with uniform composition distribution in the nanoscale and low defect densities. We also revealed a new stabilization mechanism for Cs doping to stabilize FAPbI₃, i.e. the incorporation of Cs into FAPbI₃ significantly reduces the electron-phonon coupling strength to suppress ionic migration, thereby improving the stability of FA-Cs-based devices.