Stable Cesium Formamidinium Lead Halide Perovskites: A Comparison of Photophysics and Phase Purity in Thin Films and Single Crystals

The stability of the active layer is an underinvestigated aspect of metal halide perovskite solar cells. Furthermore, the few articles on the subject are typically focused on thin films, which are complicated by the presence of defects and grain boundaries. Herein, a different approach is taken: a perovskite composition that is known to be stable in single crystal form is used, and its (photo‐)physical properties are studied in the form of spin‐coated thin films. The perovskites are lead‐based with cesium and formamidinium as the A‐site cations and iodide and bromide as the halide anions, with the formula Cs0.1FA0.9PbI3−xBrx. These compounds show high potential in terms of stability in single crystal form and closely resemble the compounds that have successfully been used in highly efficient perovskite–silicon tandem solar cells. It is found that a small difference in bromine content (x = 0.45 vs 0.6) has a significant impact in terms of the phase purity and charge carrier lifetimes, and conclude that the thin films of Cs0.1FA0.9PbI2.55Br0.45 have good potential for the use in optoelectronic devices. Stability is an underdeveloped aspect of metal halide perovskite solar cells. Herein, a structurally stable single crystal perovskite is selected to facilitate the step to thin films. Slightly increasing the bromine content in films of cesium formamidinium (FA) lead halide perovskite Cs0.1FA0.9PbI3−xBrx makes a big difference in purity: the film with more bromine has detrimental non‐perovskite impurities.