We describe a first open-access database of experimentally investigated hybrid organic-inorganic materials with two-dimensional (2D) perovskite-like crystal structure. The database includes 515 compounds, containing 180 different organic cations, 10 metals (Pb, Sn, Bi, Cd, Cu, Fe, Ge, Mn, Pd, Sb) and 3 halogens (I, Br, Cl) known so far and will be regularly updated. The database contains a geometrical and crystal chemical analysis of the structures, which are useful to reveal quantitative structure-property relationships for this class of compounds. We show that the penetration depth of spacer organic cation into the inorganic layer and M-X-M bond angles increase in the number of inorganic layers (n). The machine learning model is developed and trained on the database, for the prediction of a band gap with accuracy within 0.1 eV. Another machine learning model is trained for the prediction of atomic partial charges with accuracy within 0.01 e. We show that the predicted values of band gaps decrease with an increase of the n and with an increase of M-X-M angles for single-layered perovskites. In general, the proposed database and machine learning models are shown to be useful tools for the rational design of new 2D hybrid perovskite materials.

The chemical origin of solvents typically used for preparation of hybrid lead halide perovskitesdimethyl sulfoxide (DMSO), dimethylformamide (DMF), and γ-butyrolactone (GBL)strongly influences the process of perovskite crystallization because of the formation of intermediate adducts with different structures and morphology. The composition and crystal structures of the adducts depend on the coordination and binding ability of the solvents and the ratio of the precursors. New adducts of perovskite and GBL with either an unusual cluster structure, (MA) 8 (GBL) x [Pb 18 I 44 ], or an adduct, (MA) 2 (GBL) 2 Pb 3 I 8 , similar to those observed for DMF and DMSO are described for the first time. Complex equilibriums between chemical species existing in perovskite solutions are revealed by Raman spectroscopy. As a result, new features of the perovskite crystallization through intermediate adduct phases are discussed, and effective perovskite deposition pathways are suggested. ■ EXPERIMENTAL SECTIONMaterials and Methods. Methylammonium iodide (CH 3 NH 3 I = MAI, Dyesol), formamidinium iodide (CH 5 N 2 I = FAI, ≥98%, Dyesol), lead iodide (PbI 2 , 99%, Sigma-Aldrich), dimethyl sulfoxide

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Small cations such as guanidinium and cesium can act as templating cations to form low dimensional phases (2D, 1D, 0D) in the case of excess of organic halides. However, such phases with the widely used formamidinium (FA + ) cation have not been reported so far. In this study, we discovered two novel low dimensional phases with composition of FA2PbBr4 and investigated the prerequisites of their formation upon crystallization of FABr-excessive solutions of FAPbBr3. We found that both phases have the structure of (110) layered perovskite but is represented by two different polymorphs with "eclipsed" and "staggered" arrangement of adjacent layers. It was shown that FA2PbBr4 phases usually exist in a labile equilibrium with FAPbBr3 3D perovskite and can form composites with it. The optical properties of both polymorphs were comprehensively studied by means of absorption spectroscopy, diffuse reflection spectroscopy and photoluminescence spectroscopy. DFT calculations were applied to investigate the band structure of the FA2PbBr4 and to corroborate the conclusions on their optoelectronic properties. As a result, we found that FA2PbBr4 phases irradiated by UV can exhibit effective green photoluminescence due to a transfer of excitation energy to defective states or 3D perovskite inclusions.

A complete screening of compositions of crystallizing products of hybrid perovskites in the most popular dimethylformamide (DMF)/ dimethyl sulfoxide (DMSO) solvents is performed for various cations (FA + / MA + ) and anions (I − /Br − ). We found four new solvate phases of formamidinium hybrid perovskites, (FA) 2 Pb 3 I 8 •4DMF, FAPbI 3 •2DMF, (FA) 5 Pb 2 I 9 •0.5DMSO, and even the bromide solvate (FA) 2 PbBr 4 •DMSO. These compounds are observed for the first time, and their refined crystal structures showed large cells of unique types dependent on solvent and perovskite compositions. We also monitored crystallization pathways of multicompositional thin films and identified phases that are able to crystallize from mixed cation and anion solutions. Based on the obtained data, we performed a deep analysis of the structural peculiarities of all the solvate phases observed in the screened compositional space and discussed how the solution composition would predetermine the early stages of crystallization of target perovskite films.