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.

Intensive development of solution deposition methods for perovskite solar cell fabrication requires rational optimization of existing methods and search for new effective solvents of different origin involved in key stages of the techniques. We propose a novel approach for rational solvent selection predetermining the character of their interaction with lead halide perovskites in terms of the analysis of donor numbers, dipole moments, and Hansen hydrogen bonding parameters. Experimental proof of the proposed approach provided a cutting-edge categorization of a wide range of possible solvents into four groups: inert, weak, strong, and AX-selective solvents. The resultant approach is a useful tool for perovskite solar cell fabrication.

The unprecedented structural flexibility and diversity of inorganic frameworks of layered hybrid halide perovskites (LHHPs) rise up a wide range of useful optoelectronic properties thus predetermining the extraordinary high interest to this family of materials. Nevertheless, the influence of different types of distortions of their inorganic framework on key physical properties such as band gap has not yet been quantitatively identified. We provided a systematic study of the relationships between LHHPs' band gaps and six main structural descriptors of inorganic framework, including interlayer distances (d int ), in-plane and out-of-plane distortion angles in layers of octahedra (θ in , θ out ), layer shift factor (LSF), axial and equatorial Pb-I bond distances (d ax , d eq ). Using the set on the selected structural distortions we realized the inverse materials design based on multi-step DFT and machine learning approach to search LHHPs with target values of the band gap. The analysis of calculated descriptors -band gap dependences for the wide range of generated model structures of (100) single-layered LHHPs results in the following descending order of their importance: d int > θ in > d ax > LSF min > θ out > d eq > LSF max , and also implies a strong interaction value for some pairs of structural descriptors. Moreover, we found that the structures with completely different distortions of inorganic framework can have similar band gap, as illustrated by a number of both experimental and model structures.

Ascending interest of the scientific community in layered hybrid halide perovskites (LHHPs) as materials for innovative photovoltaic and optoelectronic applications led to unprecedented expansion of this family of compounds, reaching now several hundred refined structures. Despite the unique structural diversity of LHHPs, traditional approaches of describing their structures, such as dividing into Dion-Jacobson (DJ) or Ruddlesden−Popper (RP) phases for most structures are ambiguous and unquantifiable. Here, we introduced a quantitative layer shift factor (LSF) for a univocal classification and quantitative comparison of the structures. We also developed an algorithm for automatic calculation of the LSF for such structures. We demonstrate the application of the proposed approach for an analysis of correlations between the LSF and band gap to reveal "structure-property" relationships. Our study gives a simple and useful approach to classify of either the layered perovskite-like structures or other layered compounds composed of layers of vertexconnected octahedra as a structural unit.