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 Cs(0.1)FA(0.9)PbI(3-x)Br(x). 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 Cs(0.1)FA(0.9)PbI(2.55)Br(0.45) have good potential for the use in optoelectronic devices.