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International audience; In recent years, layered heterostructures of halide perovskites have emerged as promising optoelectronic systems due to their tunability and to the presence of strong excitonic effects [1], with potential applications for efficient light emission, spintronics, single-photon emission and photonic logic. This last technology is being developped by the H2020 POLLOC (polariton logic) consortium using exciton-polariton devices. From the theoretical point of view, first-principles studies of exciton physics in layered halide pervoskites are possible [2] but severely constrained by system size. To go beyond this limitation, symmetry-based semi-empirical tight-binding models have provided an efficient description of the electronic structure and optical properties of bulk halide perovskites [3-5]. In this talk, we will discuss such models and their extension to the case of layered perovskites, a stepping stone towards a robust description of excitons in these compounds within the tight-binding framework [6,7].[1] J.-C. Blancon et al., Nature Comm. 9, 2254 (2018) [2] C. Quarti et al., Adv. Optical Mater. 2023, 2202801[3] S. Boyer-Richard et al., J. Phys. Chem. Lett. 7, 3833 (2016)[4] A. Marronnier et al., ACS Nano 12, 3477 (2018)[5] Z. Wei et al., Nature Comm. 10, 5342 (2019) [6] R. Benchamekh et al., Phys. Rev. B 91, 045118 (2015)[7] Y. Cho et al., Phys. Chem. Lett. 10, 6189 (2019)This work was supported by the M-ERANET grant PHANTASTIC Call 2021.

International audience; In the past decade, hybrid halide perovskites have generated tremendous interest as optoelectronic materials, particularly for photovoltaïc applications due to impressive conversion efficiency values reaching over 25% in only a few years. Their large-scale industrial application, however, depends on subsequent improvements of their stability and tunability that can be achieved in both halide solid solutions (in which I, Br and Cl anions are mixed on a disordered sublattice) and two-dimensional halide perovskites (in which charge carriers are confined within inorganic atomic layers separated by organic spacers) [1,2]. Thus, it is crucial to gain an understanding of the electronic properties and carrier transport in these systems. In this talk, I will describe how a model tight-binding Hamiltonian can be extended from the archetypal CH3NH3PbI3 bulk perovskite [3-5] to mixed halide perovskites and two-dimensional perovskites using a combination of symmetry considerations and DFT calculations. This will pave the way towards a robust theoretical treatment of halide clustering effects in the former, and towards a proper description of the excitonic fine structure in the latter.[1] V Diez-Cabanes et al., Adv. Optical Mater 2001832 (2021).[2] S. Sidhik et al., Science 377, 1425 (2022)[3] S. Boyer-Richard et al., J. Phys. Chem. Lett. 7, 3833 (2016)[4] A. Marronnier et al., ACS Nano 12, 3477 (2018)[5] Z. Wei et al., Nature Communications 10, 5342 (2019)This work was supported by the M-ERANET grant PHANTASTIC Call 2021.

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.

National audience; The presentation will focus on the interplay between structural, interfacial and optoelectronic properties in 2D multilayered halide perovskites, either in Ruddlesden Popper (RP) or Dion Jacobson (DJ) structure. At present, perovskite materials are mixed with each other in complex alloys and heterostructures, including 2D/3D compositions, combined with additives or protecting layers to improve their stability and/or defectiveness as well as assembled with carrier selective layers, other materials and components to turn them into devices. Considering the target applications, ambient conditions predominate with additional issues related, for example, to the intrinsic dynamics in action in the MHP structure that may not be well captured using static averaged structures.Based on a set of customized examples, this talk will summarize recent theoretical results obtained by our group that aimed at answering questions such as: i) How do these engineering strategies impact the work function and absolute valence band energy of the MHP material? ii) Is it correlated with surface/interface dipoles? iii) Can one quantitatively assess energy level alignments of MHP semiconductors based on first-principles electronic structure calculations? How can one get rid of the stochastic dynamics of organic cations and further include cation and anion alloying while computing materials properties? iv) Is it possible to extract semi-empirical formula for molecular passivation at surface or transitivity rules for interface dipoles?Special attention will be paid on newly discovered phases and compositions and opportunities to further engineer MHP properties in connection with their use in devices under working conditions. We will see that in practical cases the funneling of electronic carriers to the 2D crystal edges, important for ionization of electron-hole pairs in 2D RP phases, is also at work in bromide perovskites. Continuous and ultrafast photodoping will be further discussed, demonstrating their strong impact on the structural and carrier transport properties of DJ perovskites

International audience; The considerable attention paid to metal halide perovskites (MHP) in recent years led to several scientific and technological advances. This in turn raised important questions that currently challenge the limits of available theoretical techniques and atomistic models. At present, perovskite materials are mixed with each other in complex alloys and heterostructures, including 2D/3D compositions, combined with additives or protecting layers to improve their stability and/or defectiveness as well as assembled with carrier selective layers, other materials and components to turn them into devices. Considering the target applications, ambient conditions predominate with additional issues related, for example, to the intrinsic dynamics in action in the MHP structure that may not be well captured using static averaged structures.Based on a set of customized examples, this talk will summarize recent theoretical results obtained by our group that aimed at answering questions such as: i) How do these engineering strategies impact the work function and absolute valence band energy of the MHP material? ii) Is it correlated with surface/interface dipoles? iii) Can one quantitatively assess energy level alignments of MHP semiconductors based on first-principles electronic structure calculations? How can one get rid of the stochastic dynamics of organic cations and further include cation and anion alloying while computing materials properties? iv) Is it possible to extract semi-empirical formula for molecular passivation at surface or transitivity rules for interface dipoles? Special attention will be paid on newly discovered phases and compositions and opportunities to further engineer MHP properties in connection with their use in devices under working conditions.*This research received funding from the European Union’s Horizon 2020 program, through and Innovation Action under the grant agreement No. 861985 (PeroCUBE). J.E. acknowledges the Institut Universitaire de France.

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.

International audience

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.

International audience

International audience; Halide perovskites are a complex class of semiconductors whose structure features two apparently contradictory properties: long-range order associated to the regular crystalline structure,[1] and short-range dynamic disorder, related to the soft nature of the lattice and the libration motion of the organic cations.[2] While the former feature is routinely addressed via X-ray diffraction techniques, the latter requires more local “probes”, with solid state Nuclear Magnetic Resonance (NMR) being an ideal method-of-choice, in light of its sensitivity to local chemical composition and environment, as demonstrated in the recent literature [3-6]. However, the complex line-shapes of solid state NMR measurements, as due to the inherent anisotropic response of this technique, make their interpretation usually a delicate task, hence calling for additional supportive tools.Here we show how periodic DFT simulations can ease the interpretation of the NMR spectroscopic features, providing a complementary perspective to experiments. Targeting nuclei with spin-quantum number I=1/2 for 3D methylammonium lead iodide, bromide and mixed halide systems, we demonstrate the reliability of DFT in reproducing experimental signatures of 13C and 1H light-atoms, with prospects in supporting the characterization of dimensionally confined or hollow perovskite architectures, where the organic component has more important structural/templating role. Quantitative prediction of 207Pb nuclei is more challenging but the different response from pure phase iodide and bromide lead perovskites is well reproduced by DFT simulations. This motivated us to investigate on mixed halide compositions, confirming the sensitivity of NMR to the halide composition,[6-7] and segregation, already at the scale of the individual PbX6 octahedron.[8] Our results open the prospect for joint theoretical-experimental investigations of halide perovskite materials, that combine solid state NMR and periodic DFT simulations. [1] Weller, M. T.; Weber, O. J.; Frost, J. M.; Walsh, A. Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α‐[HC(NH2)2]PbI3, at 298 K, J. Phys. Chem. Lett. 2015, 6, 3209-3212.[2] Quarti, C.; Mosconi, E.; De Angelis, F. Structural and electronic properties of organo-halide hybrid perovskites from ab initio molecular dynamics, Phys. Chem. Chem. Phys., 2015, 17, 9394-9409. [3] Piveteau, L.; Morad, V.; Kovalenko, M. V. Solid-State NMR and NQR spectroscopy of lead-halide perovskite materials, J. Am. Chem. Soc. 2020, 142, 19413-19437.[4] Kubicki, D. J.; Prochowicz, D.; Hofstetter, A.; Zakeeruddin, S. M.; Grätzel, M.; Emsley, L.; Phase Segregation in Cs‐, Rb- and K‐Doped Mixed-Cation (MA)x(FA)1−xPbI3 Hybrid Perovskites from Solid-State NMR’, J. Am. Chem. Soc. 2017, 139, 14173–14180.[5] Milic, J. V.; Im, J.-H.; Kubicki, D. J.; Ummadisingu, A.; Seo, J.-Y.; Li, Y. Ruiz-Preciado, M. A.; Ibrahim Bar, M.; Zakeeruddin, S. M.; Emsley, L.; Grätzel, M. Supramolecular Engineering for Formamidinium-Based Layered 2D Perovskite Solar Cells: Structural Complexity and Dynamics Revealed by Solid-State NMR Spectroscopy, Adv. Energy Mater. 2019, 9, 1900284.[6] Rosales, B. A.; Men, L.; Cady, S.; Hanrahan, M. P.; Rossini, A. J.; Vela, J. Persistent Dopants and Phase Segregation in Organolead Mixed- Halide Perovskites, Chem. Mater. 2016, 28, 6848–6859.[7] Roiland, C.; Trippé-Allard, G.; Jemli, K.; Alonso, B.; Ameline, J.-C.; Gautier, R.; Bataille, T.; Le Pollès, L.; Deleporte, E.; Even, J.; Katan, C. Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites, Phys. Chem. Chem. Phys. 2016, 18, 27133–27142.[8] 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, submitted under invitation to Helvetica Chimica ActaAcknowledgements This work has been supported by Agence Nationale de la Recherche, project ANR-18-CE05-0026 (MORELESS).

International audience

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.