Hybrid organic/inorganic perovskites are very versatile materials in terms of composition. Variations within the Goldschmidt’s tolerance factor generally lead to fully miscible structure, which in some cases allow for a continuous composition tuning. In this work we show that two fully miscible phases still exhibit different surface adsorption enthalpies for different substrates, which can lead to microscopic phase separation.

The study focused on the interaction between mixed perovskites and Si/SiO₂ wafer surface partially covered by graphene. We considered two possible variations of the composition: exchange of the organic cation (Piperonylammonium, PipA⁺ and methylammonium, MA⁺) or the anion (iodide and bromide). The obtained films were characterized by photoluminescence spectra. Next, a periodically modulated surface was created using photolithography to prepare a patterned graphene surface on which perovskite mixtures were crystallized. The composition variation of the perovskite assembly was investigated by correlating the photoluminescence spectra with the pattern size. The experiments with cation-varied perovskite mixtures revealed that the structure rich in MA⁺ formed preferentially on SiO₂ while the structure rich in PipA⁺, due to aromatic nature, outcompeted MA+ on graphene. In case of anion varied perovskite mixtures, it is known that the ratio between iodine and bromine in MAPbBr_3-xI_x can be changed continuously (0≤x≤3) and the phases are fully miscible. When we deposited a mixture having 1:1 molar ratio between iodide and bromide perovskite precursors on large-area graphene on SiO₂, the photoluminescence spectra corresponded to the iodine rich phase on both surfaces with only a small amount of MAPbBr₃ observable exclusively on graphene. However, on graphene on SiO₂ sample patterned into narrow stripes, significantly different intensity of the bromide- vs. iodide-rich phase was observed on the two substrates. We have thoroughly investigated the pattern size influence and found out that the feature size of 50 μm is the threshold above which no phase separation can be observed. We speculate that this is the mean diffusion path limit of precursor species during the perovskite crystallization. As a result, phase-separation is observed at shorter distances while uniform mixed phase dominates at longer ones. The study discovered several observations that can spark new directions in microstructured systems involving mixed perovskites and modulated surfaces.