Two cluster centroids were chosen from each cluster; the compositional centroid and RGB centroid that is the most typical color degradation curve. XRD is analyzed from the non-aged cluster centroids (Figure ). All of these samples were dark and similar by the eye as fresh samples (Supplementary Figure ). Nevertheless, the XRD analysis shows distinct patterns between the clusters. Both cluster 2 centroids (Figures and) show distinctive features from clusters 0 and 1 attributed to their high δ-CsPbI3 content. δ-CsPbI3 is an orthorhombic perovskite phase that is yellow and non-photoactive, and in high content it may even result in the film appearing yellow. During aging tests, this yellow phase is also an accelerator for α-perovskite degradation, possibly by acting as nucleation centers for decomposition . In cluster 1 samples, more Cs are incorporated into the α-mixed-cation-perovskite, and excess PbI2 remains in the film. Higher Cs in perovskite crystal structure in cluster 1 samples is evident by the contraction of the cubic perovskite lattices, shown in the right-shifted (001) perovskite peak at around 14°. During aging, the incorporation of Cs into α-perovskite lattice is expected to improve perovskites' thermal stability, leading to a delay in the color transition between the black α-mixed-cation-perovskite and its decomposition product of yellow PbI2. We shall note that the stabilization effect from Cs incorporation is counterbalanced by the accelerated degradation due to the presence of excess PbI2.