Sequential Cesium Incorporation for Highly Efficient Formamidinium-Cesium Perovskite Solar Cells

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 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 and highly efficient solar cells based on FA1-xCsxPbI3 films, with uniform composition distribution and low defect densities. In addition, our cesium-incorporated perovskites demonstrate much enhanced stability compared with FAPbI3, as a result of suppressed ionic migration due to reduced electron-phonon coupling.