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The calculation of the electron–phonon coupling from first principles is computationally very challenging and remains mostly out of reach for systems with a large number of atoms. Semi-empirical methods, like density functional tight binding (DFTB), provide a framework for obtaining quantitative results at moderate computational costs. Herein, we present a new method based on the DFTB approach for computing electron–phonon couplings and relaxation times. It interfaces with phonopy for vibrational modes and dftb+ to calculate transport properties. We derive the electron–phonon coupling within a non-orthogonal tight-binding framework and apply them to graphene as a test case.

Thermal properties of 2H-TaS2 are investigated using first-principles calculations and the Boltzmann transport equation. Electronic thermal conductivity and four-phonon scattering, which were typically overlooked, are found to be non-negligible.

The study of MAPbI3 phase transitions based on temperature-dependent optical spectroscopy has recently gained a huge attention. Photoluminescence (PL) investigations of the tetragonal–orthorhombic transition suggest that tetragonal nanodomains are present below the transition temperature and signatures associated with tetragonal segregations are observed. We have studied the impact of phase nanosegregation across the orthorhombic–tetragonal phase transition of MAPbI3 on the system’s properties employing a tight binding (TB) approach. The particle swarm optimization has been used to obtain a consistent set of TB parameters, where the target properties of the system have been derived by first-principles calculations. The theoretical results have been compared with the measured PL spectra for a temperature range going from 10 to 100 K. Our model effectively captures the carriers’ localization phenomenon induced by the presence of residual tetragonal nanodomains and demonstrates that the assumption of phase nanosegregation can explain the low-energy features in the PL spectra of MAPbI3.

Very recent photoluminescence studies, investigating the tetragonal-to-orthorhombic phase transition of MAPbI 3 , demonstrated the presence of residual tetragonal phase far below the transition temperature, yielding spectral signatures from quantum confined tetragonal domains. We present a theoretical model of the coexistence of tetragonal and orthorhombic MAPbI 3 , based on tight binding simulations. The tight binding parameters are derived by particle swarm optimization and the band-offset between the two crystals is obtained by first-principles calculations. The impact of the tetragonal domain dimension on the optical properties of MAPbI 3 is discussed.

Using an 8-band k ⋅ p model it is demonstrated through the combination of strain and piezoelectricity that increasing the InN quantum-well thickness of a GaN-InN-GaN device changes the InN material from a positive bandgap semiconductor to a topological insulator (negative bandgap). Moderate strain tuning of a four monolayer InN layer for a GaN-InN-GaN device reveals a giant (one order of magnitude) tuning of current–voltage characteristics. It is verified that piezoelectricity plays an important role in controlling electron transport through the InN layer.

The tight binding (TB) approach represents a good trade-off between accuracy and computational burden. For this reason, it is widely used for device simulations. However, a proper description of a physical system by means of TB requires an accurate parameterization of the Hamiltonian matrix elements (HME), that is usually done by fitting over suitable properties that can be measured or computed with first-principles approaches. We show that the particle swarm optimization algorithm is a powerful tool for the parameterization of the TB HME, using the density functional theory band dispersions of bulk reference materials as a target. We discuss the results obtained for bulk MAPbI 3 perovskite in its high temperature cubic phase.

Statistical fluctuations in the alloy composition on the atomic scale can have important effects on electronic and optical properties of bulk materials and devices. In particular, carrier localization induced by alloy disorder has been a much discussed topic during the last decade with regard to III-nitride light emitting diodes (LEDs). Much experimental and theoretical work has been dedicated to the study of the effects of alloy disorder on carrier localization and finally on the efficiency and transport properties in such devices. Modeling approaches range from empirical analytical models down to atomistic ab initio ones, each with its advantages and disadvantages. In this tutorial, we discuss the simulation of alloy fluctuations in nitride quantum well LEDs by combining continuum device models and an atomistic empirical tight binding model, which provides a suitable compromise between atomic precision and computational effort.

A comparison between k → · p → and tight-binding methods for the analysis of InAs/GaAs quantum dot bandstructures is presented based on a fully coupled computation of electromechanical effects. Electromechanical effects are addressed using a continuum elastic model for the k → · p → method and a pre-conditioned Valence Force Field algorithm for the tight-binding atomistic calculations. The Valence Force Field method allows the direct identification of the impact of internal strain. Results to ensure model parameter consistency between the two methods are also given by comparing bulk and unstrained quantum-well dispersion relations. The quantum dot size dependence of the bandstructure is investigated based on the models including electromechanical fields. Additionally, the effect of the electromechanical fields is studied for a specific dot size by comparing results with and without electromechanical fields. Good agreement is found for the confined energy levels but model differences show up in the symmetry of probability densities mainly due to the underlying crystal structure details taken into account by the tight-binding method but lacking in the k → · p → formalism. The latter follows from not including bulk inversion-asymmetry effects in the k → · p → method. Inclusion of piezoelectric field effects in the k → · p → method, however, restores the correct symmetry in the k → · p → model (in agreement with the tight-binding symmetry). Results are also given for oscillator strengths where both quantitative and qualitative differences are found in the comparison of k → · p → and tight-binding models.

While graphene grain boundaries (GBs) are well characterized experimentally, their influence on transport properties is less understood. As revealed here, phononic thermal transport is vulnerable to GBs even when they are ultra‐narrow and aligned along the temperature gradient direction. Non‐equilibrium molecular dynamics simulations uncover large reductions in the phononic thermal conductivity (κ p) along linear GBs comprising periodically repeating pentagon‐heptagon dislocations. Green's function calculations and spectral energy density analysis indicate that the origin of the κ p reduction is hidden in the periodic GB strain field, which behaves as a reflective diffraction grating with either diffuse or specular phonon reflections, and represents a source of anharmonic phonon–phonon scattering. The non‐monotonic dependence with dislocation density of κ p uncovered here is unaccounted for by the classical Klemens theory. It can help identify GB structures that can best preserve the integrity of the phononic transport., Contrary to conventional wisdom, phononic transport in graphene is vulnerable to grain boundaries comprising 5‐7 defects aligned along the thermal gradient. Atomistic simulations uncover a dual role of the strain field as a reflective phononic diffraction grating, showing either specular (left) or diffuse (right) phonon reflections (depending on the 5‐7 density), and as a source of anharmonic phonon–phonon scattering.

Compositional variation in (In,Ga)N alloys is a controversial topic, still debated in the literature for its influence on the performance of light-emitting diodes. This study accounts for the presence of nanometer-scale indium clustering in an LED, and describes its impact on the optical properties of the device. Clustering induces substantial redshift and broadening of the emission spectrum; furthermore, the temperature dependence of the radiative coefficient derived for the nonuniform structures is in good agreement with experiments that show a trend opposite that expected from standard theoretical considerations.

Solar cells based on metal halide Perovskites have gained a key role in the field of photovoltaics due to their high efficiencies and low production costs. Still, there is considerable effort invested in tuning the perovskite crystal morphology and interface properties, in order to further improve the device performance. Among the solutions proposed so far, MXenes have recently turned into focus for their possibility of being incorporated within the perovskite and carrier transport layers, resulting in an important improvement of the cell efficiency. In this work, we present device simulations of Perovskite/MXene solar cells, where modeling of the interface energy alignments has been based on measurement data and ab initio calculations.

During the last decade a number of both theoretical and experimental studies have shown the importance and the possible effects of random alloy fluctuations in InGaN. Interesting results have been obtained in particular with atomistic simulation models. Based on experimental evidence, most theoretical studies so far concentrated on a uniform random alloy, i.e. where the probability of finding an indium instead of a gallium atom is spatially constant. In this work, we calculated the density of states, the spontaneous emission spectrum and the radiative coefficient for InGaN/GaN single quantum wells and for bulk InGaN in presence of alloy non-uniformity, using an empirical tight binding approach. We considered an indium concentration of 20%, and 10 nm large supercells. The non-uniform indium distribution has been obtained by distributing a certain percentage of all indium atoms with uniform probability, and the rest with a probability that depends on the number of indium atoms already present locally. This allows to produce structures ranging from random alloy up to strong clustering. We find that non-uniformity reduces the band gap and the peak energy of the optical emission spectrum. Moreover, increasing degree of clustering decreases the average value of the ground state transition matrix element, which can be explained by the carriers’ spatial localization, combined with quantum confined Stark effect in quantum wells. The radiative coefficient on the other hand is not substantially influenced by light non-uniformity, while it increases for stronger degree of clustering, compatible with a transition to a quantum dot system.

To improve the efficiency of perovskite solar cells, careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (MXene Ti3C2Tx) with various termination groups (Tx) to tune the work function (WF) of the perovskite absorber and the TiO2 electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and density functional theory calculations show that the addition of Ti3C2Tx to halide perovskite and TiO2 layers permits the tuning of the materials’ WFs without affecting other electronic properties. Moreover, the dipole induced by the Ti3C2Tx at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified perovskite solar cells, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without MXene. Addition of MXenes in the halide perovskite film, in the electron transport layer and at the interface between these layers is shown to enhance the efficiency of and reduce hysteresis in perovskite solar cells.

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets, otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green’s functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effects of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions, in order to enhance the thermal transport by suppressing the interfacial thermal resistances. This is attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

Quantum decoherence plays an important role in the charge transport characteristics of molecular wires at room temperature. In this paper we propose a generalization of an electron–phonon dephasing model to non orthogonal LCAO basis. We implemented the model in combination with a density functional-based tight binding (DFTB) theory framework and utilized it to model charge transport characteristics of an anthraquinone (AQ) based molecular wire. We demonstrate a modulation of Quantum Interference (QI) effects compatible with experiments and confirm the robustness of QI signatures with respect to dephasing. An analysis of the spatial localization of the dephasing process reveals that both the QI and the dephasing process are localized in the AQ region, hence justifying the general robustness of the transmission temperature dependence in different AQ-based systems.

Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.

In this work we present a theoretical study of the effects of nanometer-scale non-uniformities in InGaN alloys on their optical properties. We show by empirical tight-binding calculations that, compared to a random alloy, deviations from uniformity lead to an increasing scattering of both the optical matrix elements and the emission energy. While the extracted B-parameter of radiative recombination appears to be relatively insensitive to small non-uniformities, at larger degree of In clustering we can demonstrate the transition to quantum dot like behaviour. Our results show that alloy non-uniformity allows to reproduce both the measured spectral width of electroluminescence spectra and temperature behaviour of the radiative recombination parameter.

The efficient transport of charge within the bulk of active molecular materials is one of the main factors affecting the efficiency and performance of organic electronic devices. In amorphous molecular aggregates, the observed effective mobility of charge carriers is usually considered as resulting from the convolution of the manifold of intermolecular configurations. In this picture, individual molecules are considered as spherically-symmetric scattering points for charge hopping. Yet, the details of the molecular structure and the topology of the electronic states involved in the charge transport mechanism affect dramatically the intermolecular electronic coupling even in amorphous materials. In this work, we link the morphology of aggregates, in terms of intermolecular configurations, as obtained from atomistic molecular dynamics, to the distribution of diabatic electronic couplings and charge transfer energies, computed by constrained density functional theory simulations. In particular, we focus on aggregates of an organometallic system with multidentate ligands, the iridium complex fac-tris(1,3-diphenyl-benzimidazolin-2-ylidene-C, C20) iridium(III) (DPBIC), commonly used in OLEDs as host transporter and emitter. Despite the quasi-spherical symmetry of the molecule, our simulations suggest a strong correlation between intermolecular orientation and electronic coupling, indicating a strong impact of the mutual orientation of molecules on charge transport in bulk molecular materials.

In recent years, much interest has been attracted by multiscale approaches in the simulation of electronic devices. In this work, we present an overview of the project TiberCAD, a software tool for design and simulation of electronic and optoelectronic nanostructured devices. Examples of applications will be provided where the combination of continuous models and models with atomistic resolution are beneficial for the correct description of device physical behavior.

The correlation between nanoscale morphology and charge injection rates at the interface between an organic semiconductor layer and a transparent metal oxide electrode was investigated by integrating molecular dynamics simulations with electronic structure calculations. The simulation approach proposed has been applied to the analysis of the hole injection mechanism at the interface between an amorphous layer of tris[(3-phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]Ir (DPBIC), a hole transport and emitter molecule, and the surface of indium tin oxide (ITO), a material commonly used as anode in OLEDs. The link between interface morphology and charge injection was investigated by implementing a two-step, top-down simulation approach. Namely, nanoscale molecular aggregation phenomena at the organic/electrode interface were first assessed by molecular dynamics simulations, mimicking different processing conditions, and followed by density functional theory calculations of the electronic coupling between molecular levels and the manifold of electrode states involved in the charge injection process. The correlation between structural parameters and electronic coupling suggests a significant role of specific molecule/electrode configurations on charge transport processes at the interface, resulting in a broad distribution of charge injection rates, and highlights the link between molecular structure, nanoscale aggregation and processing in the realization of heterointerfaces for efficient charge injection in organic electronic devices.

This work analyzes in detail the effect of ferroelectric polarization patterns in methylammonium lead iodide (MAPbI3) thin-films on the J-V characteristics of the corresponding solar cells. The simulations are based on a finite-element discretization of the drift-diffusion equations and take into account the polarization pattern experimentally derived from piezoresponse force micrographs. Based on the knowledge of the crystalline structure, symmetry considerations and electrical simulations, we discuss models for the polarization orientation pattern and magnitude of the ferroelectric domains. We conclude that the in-plane polarization vectors have 45° orientation towards the domain walls and form herring-bone structures. The presence of ordered ferroelectric domains, even with a weak characteristic polarization magnitude enhances the power conversion efficiencies and are mandatory to reproduce the experimental J-V characteristics.

In this work, we concentrate on the graphics processing unit (GPU) implementation of three different methods that are common among peers in the electronic computational domain. We calculate the energy eigenstates of GaN/AlGaN quantum dots on GPU using the tight-binding approach with a $$ sp^3d^5s^* $$sp3d5s? + spin-orbit parametrization for structures ranging from 8039 atoms to 351,600 atoms corresponding to a Hamiltonian matrix size of around 160,780---7,032,000. We perform an analysis for timing, memory occupancy and convergence on a multi-GPU workstation and a high performance computing (HPC) cluster. We also present comparisons between the multi-GPU system having 4 Nvidia Kepler graphic cards and a HPC cluster where the algorithms are benchmarked on up to 256 CPU cores.

In this work we present how 3-dimensional ferroelectric domains observed in MAPbIs affect the performance of perovskite solar cells. We simulated a 3D system with dimensions of 500 nm × 500 nm × 300 nm, considering domains with 100 nm side length which have been assigned random orientations of the polarization field. Calculations are performed with the simulator TiberCAD using a 3D drift-diffusion model, that simulates the light absorption and charge carrier transport, including the local polarization. We show that the presence of polarization domains have a strong impact on the charge carrier separation, leading to a reduction of electron-hole recombination and current pathways formation at interfaces.

International audience; We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current–voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

We present a novel kinetic Monte Carlo version for the atomistic valence force fields algorithm in order to model a self-assembled quantum dot growth process. We show our atomistic model is both computationally favorable and capture more details compared to traditional kinetic Monte Carlo models based on continuum elastic models. We anticipate the model will be useful to experimentalists in understanding better the growth dynamics of quantum dot systems.

Novel two-dimensional (2D) materials show unusual physical properties which combined with strain engineering open up the possibility of new potential device applications in nanoelectronics. In particular, transport properties have been found to be very sensitive to applied strain. In the present work, using a density-functional based tight-binding (DFTB) method in combination with Green's function (GF) approaches, we address the effect of strain engineering of the transport setup (contact-device(scattering)-contact regions) on the electron and phonon transport properties of two-dimensional materials, focusing on hexagonal boron-nitride (hBN), phosphorene, and MoS2 monolayers. Considering unstretched contact regions, we show that the electronic bandgap displays an anomalous behavior and the thermal conductance continuously decreases after increasing the strain level in the scattering region. However, when the whole system (contact and device regions) is homogeneously strained, the bandgap for hBN and MoS2 monolayers decreases, while for phosphorene it first increases and then tends to zero with larger strain levels. Additionally, the thermal conductance shows specific strain dependence for each of the studied 2D materials. These effects can be tuned by modifying the strain level in the stretched contact regions.

We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding (DFTB) theory. This is of utmost importance for modeling ultra-thin and extremely thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with the available experimental data. Comparison with calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9-nm silicon-on-insulator.

In this article we highlight the necessity of atomistic based, fully quantum mechanical simulation approaches for modern electronic devices and their coupling with classical models. We review different ways of such couplings and provide application examples.

Cross-conjugated molecules are known to exhibit destructive quantum interference, a property that has recently received considerable attention in single-molecule electronics. Destructive quantum interference can be understood as an antiresonance in the elastic transmission near the Fermi energy and leading to suppressed levels of elastic current. In most theoretical studies, only the elastic contributions to the current are taken into account. In this paper, we study the inelastic contributions to the current in cross-conjugated molecules and find that while the inelastic contribution to the current is larger than for molecules without interference, the overall behavior of the molecule is still dominated by the quantum interference feature. Second, an ongoing challenge for single molecule electronics is understanding and controlling the local geometry at the molecule-surface interface. With this in mind, we investigate a spectroscopic method capable of providing insight into these junctions for cross-conjugated molecules: inelastic electron tunneling spectroscopy (IETS). IETS has the advantage that the molecule interface is probed directly by the tunneling current. Previously, it has been thought that overtones are not observable in IETS. Here, overtones are predicted to be strong and, in some cases, the dominant spectroscopic features. We study the origin of the overtones and find that the interference features in these molecules are the key ingredient. The interference feature is a property of the transmission channels of the π system only, and consequently, in the vicinity of the interference feature, the transmission channels of the σ system and the π system become equally transmissive. This allows for scattering between the different transmission channels, which serves as a pathway to bypass the interference feature. A simple model calculation is able to reproduce the results obtained from atomistic calculations, and we use this to interpret these findings.

Two-dimensional semiconductor materials with puckered structure offer a novel playground to implement nanoscale thermoelectric, electronic, and optoelectronic devices with improved functionality. Using a combination of approaches to compute the electronic and phonon band structures with Green's function based transport techniques, we address the thermoelectric performance of phosphorene, arsenene, and SnS monolayers. In particular, we study the influence of anisotropy in the electronic and phononic transport properties and its impact on the thermoelectric figure of merit ZT. Our results show no strong electronic anisotropy, but a strong thermal one, the effect being most pronounced in the.case of SnS monolayers. This material also displays the largest figure of merit at room temperature for both transport directions, zigzag (ZT similar to 0.95) and armchair (ZT similar to 1.6), thus hinting at the high potential of these new material's in thermoelectric applications.

This report constitutes the main document of Deliverable 1.6 "Model of injection rates" of the Work Package WP1 of the H2020 project "MOSTOPHOS". The aim of this work is to parametrize the rate of charge injection from metal electrodes into organic materials, constituting the injection layer of the OLED stack.

Following the experimental evidences of filament forming in organic thin film memories, we developed a semiclassical drift-diffusion model of electrical conductivity in the filament. We show that the global behavior of a memory device and the total current can be accounted for by fully-formed and well-connected filaments. We investigated and ruled out the eventual influence of coherent quantum tunneling in disconnected filaments. It is also shown how a heating model of the filament can be used to check if assumptions on the number of filaments and their radii are physically plausible.