arXiv subjects

Mikael Kepenekian

Publications and source records attributed to Mikael Kepenekian.

4 recordsLinked to original sources

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↗

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↗

arXiv2019

Phase Transition Induced Carrier Mass Enhancement in 2D Ruddlesden-Popper Perovskites

There is a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence between different tuning parameters hinders our fundamental understanding of their properties. In this work we attempt to address this issue for (C$_n$H$_{2n+1}$NH$_3$)$_2$PbI$_4$ (with n=4,6,8,10,12) using optical spectroscopy in high magnetic fields up to 67T. Our experimental results, supported by DFT calculations, clearly demonstrate that the exciton reduced mass increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies show that the effective mass, which is an essential parameter for optoelectronic device operation, can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. Moreover, we show that the complex absorption features visible in absorption/transmission spectra track each other in magnetic field providing strong evidence for the phonon related nature of the observed side bands.

cond-mat.mes-hall↗