arXiv subjects

Claudine Katan

Publications and source records attributed to Claudine Katan.

12 recordsLinked to original sources

arXiv2025

Roadmap for electronic structure, anharmonicity, and electron-phonon calculations in locally disordered inorganic and hybrid halide perovskites

The role of data in modern materials science becomes more valuable and accurate when effects such as electron-phonon coupling and anharmonicity are included, providing a more realistic representation of finite-temperature material behavior. Furthermore, positional polymorphism, characterized by correlated local atomic disorder usually not reported by standard diffraction techniques, is a critical yet underexplored factor in understanding the electronic structure and transport properties of energy-efficient materials, like halide perovskites. In this manuscript, we present a first-principles methodology for locally disordered (polymorphous) cubic inorganic and hybrid halide perovskites, rooted in the special displacement method, that offers a systematic and alternative approach to molecular dynamics for exploring finite-temperature properties. By enabling a unified and efficient treatment of anharmonic lattice dynamics, electron-phonon coupling, and positional polymorphism, our approach generates essential data to predict temperature-dependent phonon properties, free energies, band gaps, and effective masses. Designed with a high-throughput spirit, this framework has been applied across a range of inorganic and hybrid halide perovskites: CsPbI3, CsPbBr3, CsSnI3, CsPbCl3, MAPbI3, MAPbBr3, MASnI3, MAPbCl3, FAPbI3, FAPbBr3, FASnI3, and FAPbCl3. We provide a comprehensive comparison between theoretical and experimental results and we systematically uncover trends and insights into their electronic and thermal behavior. For all compounds, we demonstrate strong and consistent correlations between local structural disorder, band gap openings, and effective mass enhancements.

cond-mat.mtrl-sci↗

arXiv2025

Electron-phonon couplings in polymorphous crystals

Positional polymorphism in solids refers to locally disordered unit cells that, on average, reproduce the high-symmetry structures observed in diffraction experiments. Standard theories of electron-phonon interactions fail to describe the temperature-dependent electronic structure of such polymorphous systems. Hybrid halide perovskites are a prime example, where configurational entropy from both polymorphism and molecular disorder plays a central role. Here we generalize the special displacement method to polymorphous crystals, providing an efficient ab initio framework for electron-phonon couplings without resorting to molecular dynamics. We resolve long-standing discrepancies in hybrid halide perovskite physics, including temperature-dependent anharmonic phonons and band gaps. Our approach provides a practical route to link local disorder, configurational entropy, and electron-phonon interactions, with applicability across diverse material classes, from optoelectronics and ferroelectrics to thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2025

Empirical approaches to Fr\"ohlich excitonic polarons in polar semiconductors

The excitonic correlation between electron and hole and these carriers' interactions with underlying crystal lattice govern the opto-electronic response of semiconductors. The latter is dominated in ionic crystals by the Fr\"{o}hlich interaction corresponding to the long-range electric field generated by polar phonons. Theoretical description of complex interplay of these two effects, excitonic and polaronic, has been a formidable challenge. The present paper reviews the physics of Fr\"ohlich excitonic polarons from the point of view of empirical approaches and supplements it with a few original developments. At first, we review the models for excitonic polarons in ionic semiconductors built analogously to Lee-Low-Pines (LLP) model for free polarons, and later extended by Pollman and B\"uttner (PB). These models have been applied in past to the case of weakly interacting polarons (e.g. GaAs). We consider their applicability to ionic solids such as TlCl or 3D lead halide perovskites, where electron-hole correlations are relatively stronger. In these compounds, electrons and holes have almost equal effective masses, which allows us to derive a new analytical expressions of PB effective interaction potential. The refined Kane approach to PB's model is shown to (i) bridge the regime between weakly interacting polarons and excitonic polarons with strong electron-hole correlations and (ii) recover the LLP model for free polarons in the limit of vanishing correlation. Various developments carried out in this paper also include extension of Kane and PB's semi-empirical models to incorporate Fr\"ohlich-like interaction with multiple polar phonons, essential for reconciliation of experimental observations in multi-atom systems. In the end, we discuss the relation of the empirical approaches with ab initio methods and provide an outlook of their application to lower-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2025

Two-dimensional perovskites with maximum symmetry enable exciton diffusion length exceeding 2 micrometers

Realizing semiconductors with high symmetry of their crystallographic structures has been a virtue of inorganic materials and has resulted in novel physical behaviors. In contrast, hybrid (organic and inorganic) crystals such as two-dimensional metal halide perovskites exhibit much lower crystal symmetry due to in-plane or out of plane octahedral distortions. Despite their amazing ability for photoinduced light emission at room temperature, the Achilles' heel of this attractive class of 2D materials for optoelectronics remains the poor control and lack of performance for charge carrier transport. Inspired by the tremendous charge carrier properties of the 3D cubic perovskite phase of FAPbI3 and combining the use of the appropriate cage cation, the spacer molecule and the temperature and rate of crystallization, we report a new series of FA-based layered two-dimensional perovskites that exhibits the highest theoretically predicted symmetry with a tetragonal P4/mmm space group, resulting in no octahedral distortion in both in-plane and out-of-plane directions. These 2D perovskites present the shortest interlayer distances (4 angstrom), which results in systematically lower bandgaps (1.7 to 1.8 eV). Finally, the absence of octahedral distortions, results in an exciton diffusion length of 2.5 μm, and a diffusivity of 4.4 cm2s-1, both of which are an order of magnitude larger compared to previously reported 2D perovskites and on par with monolayer transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2025

Three-Dimensional to Layered Halide Perovskites: A Parameter-Free Hybrid Functional Method for Predicting Electronic Band Gaps

Accurate prediction of electronic band gaps in halide perovskites using density functional theory is crucial for optoelectronic applications. Standard hybrid functionals like HSE and PBE0 are becoming computationally accessible, yet can fail at predicting the band gaps for three-dimensional (3D) and/or layered halide perovskite. This study evaluates the doubly screened dielectric-dependent hybrid (DSH) functional for predicting band gaps of Pb- and Sn-based inorganic and hybrid 3D halide perovskites, as well as layered hybrid perovskites. The DSH functional employs material-dependent mixing parameters derived from macroscopic dielectric constants, and accurately predicts band gaps for 3D perovskites only if structural local disorder is taken into account. For layered hybrid perovskites, the DSH functional based on average dielectric constants tends to overestimate the band gaps. To improve predictions, we propose using the calculated dielectric constant of the respective 3D perovskites to define the DSH screening. This method is then applied to Pb- and Sn-based layered halide perovskites with various organic spacers and multilayered structures, such as $BA_2MA_{n-1}Pb_{n}I_{3n-1}$ with n =1, 2, 3, resulting in improved precision. The HSE functional systematically underestimates band gaps in layered perovskites due to the missing non-local long-range dielectric screening. On the other hand, the PBE0 is in good agreement with the experimental values, in particular for the layered iodide perovskites. The computational framework introduced here provides an efficient parameter-free \textit{ab initio} methodology suitable for predicting the electronic properties of 3D, layered halide perovskites and their heterostructures, towards modelling materials for advanced optoelectronic devices

cond-mat.mtrl-sci↗

arXiv2025

Flexible and Efficient Semi-Empirical DFTB Parameters for Electronic Structure Prediction of 3D, 2D Iodide Perovskites and Heterostructures

Density Functional Tight-Binding (DFTB), an approximative approach derived from Density Functional Theory (DFT), has the potential to pave the way for simulations of large periodic or non-periodic systems. We have specifically tailored DFTB parameters to enhance the accuracy of electronic band gap calculations in both 3D and 2D lead-iodide perovskites, at a significantly reduced computational cost relative to state-of-the-art ab initio calculations. Our electronic DFTB parameters allow computing not only the band gap but also effective masses of perovskite materials with reasonable accuracy compared to existing experimental data and state-of-the-art DFT calculations. The electronic band structures of vacancy-ordered and, lead- and iodide- deficient perovskites are also explored. Additionally, we demonstrate the efficiency of DFTB in computing electronic band alignments in perovskite heterostructures. The DFTB-based approach is anticipated to be beneficial for studying large-scale systems such as heterostructures and nanocrystals.

cond-mat.mtrl-sci↗

arXiv2025

Overcoming the surface paradox: Buried perovskite quantum dots in wide-bandgap perovskite thin films

Colloidal perovskite quantum dots (PQDs) are an exciting platform for on-demand quantum, and classical optoelectronic and photonic devices. However, their potential success is limited by the extreme sensitivity and low stability arising from their weak intrinsic lattice bond energy and complex surface chemistry. Here we report a novel platform of buried perovskite quantum dots (b-PQDs) in a three-dimensional perovskite thin-film, fabricated using one-step, flash annealing, which overcomes surface related instabilities in colloidal perovskite dots. The b-PQDs demonstrate ultrabright and stable single-dot emission, with resolution-limited linewidths below 130 μeV, photon-antibunching (g^2(0)=0.1), no blinking, suppressed spectral diffusion, and high photon count rates of 10^4/s, consistent with unity quantum yield. The ultrasharp linewidth resolves exciton fine-structures (dark and triplet excitons) and their dynamics under a magnetic field. Additionally, b-PQDs can be electrically driven to emit single photons with 1 meV linewidth and photon-antibunching (g^2(0)=0.4). These results pave the way for on-chip, low-cost single-photon sources for next generation quantum optical communication and sensing.

physics.optics↗

arXiv2022

Exciton ground state fine structure and excited states landscape in layered halide perovskites from combined BSE simulations and symmetry analysis

Layered halide perovskites are solution-processed natural heterostructures where quantum and dielectric confinement effects down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Achieving a detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with current on-going debate related to the energy order of the four components of the most stable exciton. To provide theoretical feedback and solve among contrasting literature reports, we perform here ab-initio solution of the Bethe Salpeter Equation (BSE), with detailed interpretation of the spectroscopic observables based on symmetry-analysis. We confirm the Edark < Ein plane < Eout-of-plane fine-structure assignment, as from recent magneto-absorption experiments. We further suggest that polar distortions may lead to stabilization of the in-plane component and ultimately end-up in a bright lowest exciton component. Also, we discuss the exciton landscape over a broad energy range and clarify the exciton spin-character, when large spin-orbit coupling is in play, to rationalize the potential of halide perovskites as triplet sensitizers in combination with organic dyes. In addition to contributing to the current understanding of the exciton properties of layered halide perovskites, the work further evidence the in-depth knowledge gained by combining advanced ab-initio simulations and group theory.

cond-mat.mtrl-sci↗

arXiv2022

Fine Structure of Excitons in Vacancy Ordered Halide Double Perovskites

Vacancy ordered halide double perovskites (VODP) have been widely explored throughout the past few years as promising lead-free alternatives for optoelectronic applications. Yet, the atomic-scale mechanisms that underlie their optical properties remain elusive. In this work, a throughout investigation of the excitonic properties of key members within the VODP family is presented. We employ ab-initio calculations and unveil critical details regarding the role of electron-hole interactions in the electronic and optical properties of VODP. The materials family is sampled based on the electronic configuration of the tetravalent metal at the center of the octahedron. Hence, groups with a valence comprised of s, p and d closed-shells are represented by the known materials Cs$_{2}$SnX$_{6}$, Cs$_{2}$TeX$_{6}$ and Cs$_{2}$ZrX$_{6}$ (with X=Br, I), respectively. The electronic structure is investigated within the G$_{0}$W$_{0}$ method, while the Bethe-Salpeter equation is solved to account for electron-hole interactions that play a crucial role in the optical properties of the family. A detailed symmetry analysis unravels the fine structure of excitons for all compounds. The exciton binding energy, excitonic wavefunctions and the dark-bright splitting are also reported for each material. It is shown that these quantities can be tuned over a wide range, form Wannier to Frenkel-type excitons, through for example substitutional engineering. In particular, Te-based materials, which share the electronic valency of corner-sharing Pb halide perovskites, are predicted to have exciton binding energies of above 1 eV and a dark-bright splitting of the excitons reaching over 100 meV. Our findings provide a fundamental understanding of the optical properties of the entire family of VODP materials and highlight how these are not in fact suitable Pb-free alternatives to traditional halide perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Direct visualization of ultrafast lattice ordering triggered by an electron-hole plasma in 2D perovskites

Direct visualization of ultrafast coupling between charge carriers and lattice degrees of freedom in photo-excited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behavior of materials under extreme non-equilibrium conditions. Here, by monitoring the evolution of the wave-vector resolved ultrafast electron diffraction intensity following above-bandgap photo-excitation, we obtain a direct visual of the structural dynamics in monocrystalline 2D perovskites. Analysis reveals a surprising, light-induced ultrafast lattice ordering resulting from a strong interaction between hot-carriers and the perovskite lattice, which induces an in-plane octahedra rotation, towards a more symmetric phase. Correlated ultrafast spectroscopy performed at the same carrier density as ultrafast electron diffraction reveals that the creation of a hot and dense electron-hole plasma triggers lattice ordering at short timescales by modulating the crystal cohesive energy. Finally, we show that the interaction between the carrier gas and the lattice can be altered by tailoring the rigidity of the 2D perovskite by choosing the appropriate organic spacer layer.

cond-mat.mtrl-sci↗

arXiv2019

Electronic properties of Pb-I deficient lead halide perovskites

The electronic structure evolution of deficient halide perovskites with a general formula $(A,A')_{1+x}M_{1-x}X_{3-x}$ was investigated using the density functional theory. The focus is placed on characterization of changes in the band gap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties of the defect corresponding to a $M\!-\!X$ vacancy filled with an $A'$ cation. This defect "repels" electrons and holes producing no trap states and, in moderate quantities ($x\le0.1$), does not hinder charge transport properties of the material. This behavior is rationalized using a confinement model and provides an additional insight to the defect tolerance of halide perovskites.

cond-mat.mtrl-sci↗

arXiv2018

Influence of Disorder and Anharmonic Fluctuations on the Dynamical Rashba Effect in Purely Inorganic Lead-Halide Perovskites

Doping organic metal-halide perovskites with cesium could be the best solution to stabilize highly-efficient perovskite solar cells. The understanding of the respective roles of the organic molecule, on one hand, and the inorganic lattice, on the other, is thus crucial in order to be able to optimize the physical properties of the mixed-cation structures. In particular, the study of the recombination mechanisms is thought to be one of the key challenges towards full comprehension of their working principles. Using molecular dynamics and frozen phonons, we evidence sub-picosecond anharmonic fluctuations in the fully inorganic $CsPbI_3$ perovskite. We reveal the effect of these fluctuations, combined with spin-orbit coupling, on the electronic band structure, evidencing a dynamical Rashba effect. Our study show that under certain conditions space disorder can quench the Rashba effect. As for time disorder, we evidence a dynamical Rashba effect which is similar to what was found for $MAPbI_3$ and which is still sizable despite temperature disorder, the large investigated supercell, and the absence of the organic cations' motion. We show that the spin texture associated to the Rashba splitting cannot be deemed responsible for a consistent reduction of recombination rates, although the spin mismatch between valence and conduction band increases with the ferroelectric distortion causing the Rashba splitting.

cond-mat.mtrl-sci↗