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For DFT calculations, this work was granted access to the HPC resources of TGCC/CINES/IDRIS under the allocation 2020-A0090907682 made by GENCI; International audience; Metal-halide perovskites have recently demonstrated great potential for a wide variety of optoelectronic applications, with their layered subfamily offering improved stability as compared to their 3D analogs. Among the layered compounds, subclass of terminated compounds are currently dominating the field, while subclass is comparably much less explored. Here we report on the synthesis of (FA) 3 (HEA) 2 Pb 3 I 11 obtained by room-temperature liquid-gas diffusion involving the intermediate-size cation hydroxyethylammonium (HEA +), formamidinium (FA +) and PbI 2 dissolved in an acidic solution. This multilayered hybrid perovskite is a rare example of a m= 3 member of the -oriented series (A') 2 (A) m B m X 3m+2. Structural characterization based on X-ray diffraction and NMR investigations reveals that the small size cation FA + is located both in the inner layer of the perovskite sheet, but also in the interlayer space, while the intermediate-size cation HEA + is situated in the outer cavities of the perovskite sheet. (FA) 3 (HEA) 2 Pb 3 I 11 exhibits a broad absorption band in the visible region (400-650 nm) leading to an optical band gap of 1.96 eV. Electronic structure calculations confirm the direct nature of the band gap and evidence sizeable inter layer electronic coupling related to short I…I distances. These features are reminiscent of those observed for -oriented Pb 3 I 10 analogs that have shown photovoltaic efficiencies over 18%. These findings should prompt further investigations for the design of other -oriented multilayered (m= 3, 4,…) metal halide perovskites.

Computational investigations were conducted thanks to HPC resources provided by [TGCC/CINES/IDRIS] under the allocation 2020-A0010907682 made by GENCI.; International audience; Halide perovskites and related nanostructures show complex structural interplay between well-defined long-range structural order, associated to their corner-shared MX6 organization, and short-range dynamic disorder, due to the soft nature of the lattice, libration motion and conformational freedom of the organic component. The structural characterization of these materials reveals therefore challenging, as it calls for an appropriate description of both short-range and long-range structural features, then requiring a smart combination of different structural investigation techniques. In this frame, solid state Nuclear Magnetic Resonance (ss-NMR) emerges as unique technique to complement long-range description routinely addressed via X-ray diffraction, to probe the material structure at different scales[1].We perform ss-NMR characterization of halide perovskites adopting synergic experimental and theoretical tools. The ss-NMR of 3D methylammonium lead halide perovskites is investigated, first, via periodic Density Functional Theory (DFT) simulations, with good match between the theoretical results for 1H and 13C nuclei and the experimental literature data. Theoretical/experimental agreement is comparably less accurate for 207Pb nuclei, due to lack of Spin-Orbit-Coupling (SOC) in our computational set-up, but still provides correct trends with respect to the PbX6 octahedra halide composition, hence making the NMR signature of 207Pb the ideal marker for characterizing halide segregation down to the atomic scale.[2] Further investigations involve a recently synthesized terminated, m=3 halide perovskite. Again, ssNMR of 207Pb nicely distinguishes between the lead atoms in the inner and outer layers.[3] Further efforts in the directions of incorporation of SOC in the computational set-up and better framing of thermal narrowing is needed but the present results highlight the potential of joint theoretical/experimental NMR simulations in the field of halide perovskites for energy conversion applications.[1] Dahlman C. J., Kubicki D. J., Reddy G. N. M., Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy, J. Mater. Chem. A, 2021,9, 19206[2] Quarti C., Furet E., Katan C., DFT simulations as valuable tool to support NMR characterization of halide perovskites: the case of pure and mixed halide perovskites, Helv. Chim. Acta 2021, 104, e2000231.[3] M. Ben Haj Salah, N. Mercier, M. Allain, A. Leblanc, J. Dittmer, C. Botta, C. Quarti, C. Katan, Synthesis and characterization of (FA)3(HEA)2Pb3I11: a rare example of -oriented multilayered halide perovskite, accepted in Chemistry of Materials.

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ACS Award in the Chemistry of Materials (2020): Symposium Honoring David Brian Mitzi; International audience; Layered halide perovskites are a class of semiconductors produced via chemical approaches andshowing unique opto-electronic properties. Pioneered by Prof. David Mitzi in the 1990s, these materials have anatural quantum-well electronic structure (the band-gap of the inorganic frame is embedded into that of theorganic spacers) and feature positive transport properties, effective visible-light absorption, narrow excitonicemission, etc.The full exploitation of these perovskites cannot go without deep understanding of their native electronic andoptical properties, as accessible, via complex experimental set-ups and computationally expensive ab-initiocalculations. However, interpretation of experimental and theoretical results can greatly benefit from basic“pencil-and-paper” symmetry analysis. Based on group-theory, we discuss how the change in thedimensionality influences the corresponding electronic and optical properties. In the direct band gap quantumwellscenario, we highlight how the breaking of the chemical connectivity influences the electronic structure, interms of hybridization of the composing atoms (see Figure a). We also discuss the corresponding evolution ofthe optical properties, with focus on the lowest energy exciton fine structure. The present analysis will be thenextended to the case of defective-halide perovskite frames, a class of systems recently reported in theliterature.As an alternative scenario, substitution of commonly used saturated organic spacers with molecules featuringextended π-conjugation breaks the quantum-well picture. Indeed, the band-gap closing due to extended π-system can result in the (de)stabilization of the frontier orbitals of the spacer, compared to those of theinorganic frame, leading to the formation of a type II heterojunction at the organic/inorganic interface (seeFigure b). We discuss the related charge/energy transfer processes on the basis of recently reportedPbX (X=Cl,Br) layered perovskite frame incorporating tetrazine derivative as spacer. Featuring resonancesboth for the single particle states and excitonic states, these systems allow to discuss several diversionchannels for the relaxation of carriers generated in the perovskite, in the detail.

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International audience; Abstract Body: Currently, many different perovskite -with corner-sharing octahedra- as well as non-perovskite metal-halide solids are synthetized worldwide entailing the need for in-depth understanding of their structure/property relationships. In this regard, combining the huge accumulated knowledge over the last decades, on halide but also oxide perovskites, with modern atomic scale modeling as well as symmetry analysis has proved useful. Among others, new compositions such as A'2An-1MnX3n+1 (where A and A' are cations, X is halide and M is metal) afford layered structures with a controlled number (n, currently ? 7) of octahedra in the perovskite layer. Those correspond to innate heterostructures that offer an ideal platform for fundamental understanding such as effect of quantum or dielectric confinement.[1] Efforts have also been successful in building new perovskite structures with cations surpassing the Goldschmidt tolerance factor. For instance, this has been recently demonstrated for layered perovskites with the use of ethylammonium, isopropylammonium or dimethylammonium as perovskitizer, having a perovskite sheet retaining its continuous corner-sharing connectivity.[2] On another note, new three dimensional networks have been achieved by mixing larger cations (e.g., hydroxyethylammonium) with the standard ones (e.g. methylammonium, formamidinium) leading to the formation of voids in the perovskite lattice, either disordered (Hollow perovskites) or with structural signature of a periodic pattern (Deficient perovskites).[3] In this talk, I will discuss some of our recent theoretical results paying attention on newly discovered halide perovskite phases and opportunities to further engineer their properties in connection with their use in devices. References including references therein: [1] C. Katan, N. Mercier, J. Even, Quantum and dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors, Chem. Rev. 119, 3140 (2019); [2] Y. Fu, X. Jiang, X. Li, B. Traore, I. Spanopoulos, C. Katan, J. Even, M. G. Kanatzidis, E. Harel, Cation Engineering in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites with Mixed Large A-Site Cations in the Cages, J. Am. Chem. Soc. 142, 4008 (2020); X. Li, Y. Fu, L. Pedesseau, P. Guo, S. Cuthriell, I. Hadar, J. Even, C. Katan, C. C. Stoumpos, R. D. Schaller, E. Harel, M. G. Kanatzidis, Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening, J. Am. Chem. Soc. (2020) DOI: 10.1021/jacs.0c03860; [3] A. Leblanc, N. Mercier, M. Allain, J. Dittmer, T. Pauporté, V. Fernandez, F. Boucher, M. Kepenekian, C. Katan, Enhanced Stability and Band Gap Tuning of ?-[HC(NH2)2]PbI3 Hybrid, ACS Appl. Mater. Interfaces, 11, 20743 (2019); C. Zheng, O. Rubel, M. Kepenekian, X. Rocquefelte, C. Katan, Electronic properties of Pb-I deficient lead halide perovskites, J. Chem. Phys. 151, 234704 (2019); C. Quarti, C. Katan and J. Even, Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective, under revision.

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J.E. acknowledges financial support from the Institute Universitaire de France.; International audience; Hybrid halide perovskites are now superstar materials leading the field of low-cost thin film photovoltaics technologies. Following the surge for more efficient and stable 3D bulk alloys, multilayered halide perovskites and colloidal perovskite nanostructures appeared in 2016 as viable alternative solutions to this challenge, largely exceeding the original proof of concept made in 2009 and 2014, respectively. This triggered renewed interest in lower-dimensional hybrid halide perovskites and at the same time increasingly more numerous and differentiated applications. The present paper is a review of the past and present literature on both colloidal nanostructures and multilayered compounds, emphasizing that availability of accurate structural information is of dramatic importance to reach a fair understanding of quantum and dielectric confinement effects. Layered halide perovskites occupy a special place in the history of halide perovskites, with a large number of seminal papers in the 1980s and 1990s. In recent years, the rationalization of structure–properties relationship has greatly benefited from new theoretical approaches dedicated to their electronic structures and optoelectronic properties, as well as a growing number of contributions based on modern experimental techniques. This is a necessary step to provide in-depth tools to decipher their extensive chemical engineering possibilities which surpass the ones of their 3D bulk counterparts. Comparisons to classical semiconductor nanostructures and 2D van der Waals heterostructures are also stressed. Since 2015, colloidal nanostructures have undergone a quick development for applications based on light emission. Although intensively studied in the last two years by various spectroscopy techniques, the description of quantum and dielectric confinement effects on their optoelectronic properties is still in its infancy.