Exploration of the Compositional Space in Methylammonium Free Lead Halide Perovskites for High-Performance Solar Cells

The certified power conversion efficiency (PCE) of state-of-the-art organic-inorganic metal halide perovskite solar cells (PSCs) has surpassed 25%. The most widely studied methylammonium (MA) lead iodide undergoes a structural phase transition and degradation due to the volatile MA cation, detrimental to the long-term stability of the devices. Formamidinium (FA), used as the majority cation in most PSCs with a PCE of >20%, is thermally more stable and has a more optimal red-shifted bandgap. However, FA with the relatively large ionic radius induces titled PbI₆ octahedra in FAPbI₃, leading to the photoinactive yellow δ-phase at room temperature. Herein, iodide and FA in FAPbI₃ were gradually replaced by bromide and cesium (Cs) with smaller radius to understand their role on yellow δ-phase suppression and to systematically explore the compositional space via regulating the tolerance factor, revealing the crucial link between the crystal structure and optoelectronic properties of the mixed-cation (FA/Cs) lead mixed-halide (I/Br) perovskite. The FA/Cs-Pb-I/Br films were probed by advanced synchrotron X-ray characterization to understand halide phase segregation, which has been found to be detrimental for solar cell performance.