Modeling of Electronic Mobilities in Halide Perovskites: Adiabatic Quantum Localization Scenario.

The transport properties of MAPbI3 are analyzed within a tight-binding model. We find a strong Fröhlich interaction of electron and holes with the electrostatic potential induced by the longitudinal optical phonon modes. This potential induces a strong scattering and limits the electronic mobilities at room temperature to about 200 cm^{2}/V s. With additional extrinsic disorder, a large fraction of the electrons and holes are localized, but they can diffuse by following nearly adiabatically the evolution of the electrostatic potential. This process of diffusion, at a rate which is given by the lattice dynamics, contributes to the unique electronic properties of this material.

Non-Hermitian quantum mechanics and exceptional points in molecular electronics.

In non-Hermitian (NH) quantum mechanics, Hamiltonians are studied whose eigenvalues are not necessarily real since the condition of hermiticity is not imposed. Certain symmetries of NH operators can ensure that some or all of the eigenvalues are real and thus suitable for the description of physical systems whose energies are always real. While the mathematics of NH quantum mechanics is well developed, applications of the theory to real quantum systems are scarce, and no closed system is known whose Hamiltonian is NH. Here, we consider the elementary textbook example of a NH Hamiltonian matrix, and we show how it naturally emerges as a simplifying concept in the modeling of molecular electronic devices. We analyze the consequences of non-Hermiticity and exceptional points in the spectrum of NH operators for the molecular conductance and the spectral density of simple models for molecules on surfaces.

Impact of electron-phonon coupling on the quantum yield of photovoltaic devices.

In describing the charge carriers' separation mechanism in the organic solar cell, providing a method, which considers the impact of all parameters of interest on the same footing within an inexpensive numerical effort, could play an essential role. We use here a simple tight-binding model to describe the dissociation of the charge carriers and investigate their dependence on the physical parameters of the system. We demonstrate that the quantum yield of the cell is subtly controlled by the collective action of the Coulomb interaction of the electron-hole pair, electron-phonon coupling, and the geminate recombination of the charge carriers. This approach should help us understand the performance of organic solar cells and optimize their efficiency.

Two-Dimensional Electronic Transport in Rubrene: The Impact of Inter-Chain Coupling.

Organic semi-conductors have unique electronic properties and are important systems both at the fundamental level and also for their applications in electronic devices. In this article we focus on the particular case of rubrene which has one of the best electronic transport properties for application purposes. We show that this system can be well simulated by simple tight-binding systems representing one-dimensional (1D) chains that are weakly coupled to their neighboring chains in the same plane. This makes in principle this rubrene system somehow intermediate between 1D and isotropic 2D models. We analyse in detail the dc-transport and terahertz conductivity in the 1D and in the anisotropic 2D models. The transient localisation scenario allows us to reproduce satisfactorily some basics results such as mobility anisotropy and orders of magnitude as well as ac-conductivity in the terahertz range. This model shows in particular that even a weak inter-chain coupling is able to improve notably the propagation along the chains. This suggest also that a strong inter-chain coupling is important to get organic semi-conductors with the best possible transport properties for applicative purposes.

Impact of offset energies on the yield of interfacial charge separation in molecular photocells.

We display that the process of charge carriers' separation at molecular photocells is a complex phenomenon that is controlled by the cumulative action of Coulomb interaction for electron-hole pairs and LUMO-LUMO offset energies. By applying quantum scattering theory and the Lippmann-Schwinger equation, we provide a comprehensive framework of the device operation in which the operating molecular photocell is described by a wave function. We find that depending on the magnitude of offset energies, the electron-hole interaction can normally decrease or abnormally increase the charge separation yield. To rationalize the results, we benefit from the spectral information provided in the model. Our analysis helps to optimize molecular photocells operation.

The impact of long-range electron-hole interaction on the charge separation yield of molecular photocells.

We discuss the effects of charge carrier interaction and recombination on the operation of molecular photocells. Molecular photocells are devices where the energy conversion process takes place in a single molecular donor-acceptor complex attached to electrodes. Our investigation is based on the quantum scattering theory, in particular on the Lippmann-Schwinger equation; this minimizes the complexity of the problem while providing useful and non-trivial insight into the mechanism governing photocell operation. In this study, both exciton pair creation and dissociation are treated in the energy domain, and therefore there is access to detailed spectral information, which can be used as a framework to interpret the charge separation yield. We demonstrate that the charge carrier separation is a complex process that is affected by different parameters, such as the strength of the electron-hole interaction and the non-radiative recombination rate. Our analysis helps to optimize the charge separation process and the energy transfer in organic solar cells and in molecular photocells.

Simple model of a coherent molecular photocell.

Electron transport in molecular electronic devices is often dominated by a coherent mechanism in which the wave function extends from the left contact over the molecule to the right contact. If the device is exposed to light, photon absorption in the molecule might occur, turning the device into a molecular photocell. The photon absorption promotes an electron to higher energy levels and thus modifies the electron transmission probability through the device. A model for such a molecular photocell is presented that minimizes the complexity of the problem while providing a non-trivial description of the device mechanism. In particular, the role of the molecule in the photocell is investigated. It is described within the Hückel method and the source-sink potential approach [F. Goyer, M. Ernzerhof, and M. Zhuang, J. Chem. Phys. 126, 144104 (2007)] is used to eliminate the contacts in favor of complex-valued potentials. Furthermore, the photons are explicitly incorporated into the model through a second-quantized field. This facilitates the description of the photon absorption process with a stationary state calculation, where eigenvalues and eigenvectors are determined. The model developed is applied to various generic molecular photocells.

Modeling of molecular photocells: Application to two-level photovoltaic system with electron-hole interaction.

We present a novel simple model to describe molecular photocells where the energy conversion process takes place by a single molecular donor-acceptor complex attached to electrodes. By applying quantum scattering theory, an open quantum system method, the coherent molecular photocell is described by a wave function. We analyze photon absorption, energy conversion, and quantum yield of a molecular photocell by considering the effects of electron-hole interaction and non-radiative recombination. We model the exciton creation, dissociation, and subsequent effects on quantum yield in the energy domain. We find that depending on the photocell structure, the electron-hole interaction can normally decrease or abnormally increase the cell efficiency. The proposed model helps to understand the mechanisms of molecular photocells, and it can be used to optimize their yield.

Conductivity of graphene with resonant and nonresonant adsorbates.

We propose a unified description of transport in graphene with adsorbates that fully takes into account localization effects and loss of electronic coherence due to inelastic processes. We focus in particular on the role of the scattering properties of the adsorbates and analyze in detail cases with resonant or nonresonant scattering. For both models, we identify several regimes of conduction, depending on the value of the Fermi energy. Sufficiently far from the Dirac energy and at sufficiently small concentrations, the semiclassical theory can be a good approximation. Near the Dirac energy, we identify different quantum regimes, where the conductivity presents universal behaviors.

Influence of the electron-vibration coupling on the interfacial charge transfer in organic solar cells: a simple quantum model.

In this paper, we analyze the influence of the electron-vibration interaction on the charge transfer process at the donor-acceptor interface in an organic solar cell. We present an essentially exact numerical analysis for a minimal model with only one vibration mode which is coupled to the charge transfer state. We show that the charge transfer state can be hot or cold depending on the parameters and in particular on the value of the energy of the lowest unoccupied molecular orbital on the donor side. We analyze also different regimes where electron-hole attraction or hybridization effects at the interface can modify the quantum yield of the transfer. We discuss also briefly the possible effects of the other vibration modes that are coupled to the charge either on the donor side or on the acceptor side.

Quantum localization and electronic transport in covalently functionalized carbon nanotubes.

Carbon nanotubes are of central importance for applications in nano-electronics thanks to their exceptional transport properties. They can be used as sensors, for example in biological applications, provided that they are functionalized to detect specific molecules. Due to their one-dimensional geometry the carbon nanotubes are very sensitive to the phenomenon of Anderson localization and it is therefore essential to know how the functionalization modifies their conduction properties and if they remain good conductors. Here we present a study of the quantum localization induced by functionalization in metallic single walled carbon nanotubes (SWCNT) with circumferences up to 15 nm. We consider resonant and non-resonant adsorbates that represent two types of covalently functionalized groups with strong and moderate scattering properties. The present study provides a detailed analysis of the localization behaviour and shows that the localization length can decrease down to 20-50 nm at concentrations of about 1 percent of adsorbates. On this basis we discuss the possible electronic transport mechanisms which can be either metallic like or insulating like with variable range hopping.

Inhomogeneous dynamical mean-field theory of the small polaron problem.

We present an inhomogeneous dynamical mean field theory (I-DMFT) that is suitable to investigate electron-lattice interactions in non-translationally invariant and/or inhomogeneous systems. The presented approach, whose only assumption is that of a local, site-dependent self-energy, recovers both the exact solution of an electron for a generic random tight-binding Hamiltonian in the non-interacting limit and the DMFT solution for the small polaron problem in translationally invariant systems. To illustrate its full capabilities, we use I-DMFT to study the effects of defects embedded on a two-dimensional surface. The computed maps of the local density of states reveal Friedel oscillations, whose periodicity is determined by the polaron mass. This can be of direct relevance for the interpretation of scanning-tunneling microscopy experiments on systems with sizable electron-lattice interactions. Overall, the easy numerical implementation of the method, yet full self-consistency, allows one to study problems in real-space that were previously difficult to access.

Mobility gap and quantum transport in a functionalized graphene bilayer.

In a Bernal graphene bilayer, carbon atoms belong to two inequivalent sublattices A and B, with atoms that are coupled to the other layer by [Formula: see text] bonds belonging to sublattice A and the other atoms belonging to sublattice B. We analyze the density of states and the conductivity of Bernal graphene bilayers when atoms of sublattice A or B only are randomly functionalized. We find that for a selective functionalization on sublattice B only, a mobility gap of the order of 0.5 eV is formed close to the Dirac energy at concentration of adatoms [Formula: see text]. In addition, at some other energies conductivity presents anomalous behaviors. We show that these properties are related to the bipartite structure of the graphene layer.

Theory for STM images of resonances in the near-field regime: application to adsorbates and local defects on graphene.

We analyze the STM current through electronic resonances on a substrate as a function of tip-surface distance. We show that when the tip approaches the surface a maximum of the density of states of the electronic resonance at some energy can lead to a dip of the STM signal dI/dV. This phenomenon is a Fano effect resulting from the coupling of the electronic states of the STM tip with the resonance. In graphene such resonances can be produced by local defects or adsorbates and we analyze the cases of top and hollow configurations of adsorbates.

Coherent electronic transport through graphene constrictions: subwavelength regime and optical analogy.

Nanoelectronic devices smaller than the electron wavelength can be achieved in graphene with current lithography techniques. Here we show that the electronic quantum transport of graphene subwavelength nanodevices presents deep analogies with subwavelength optics. We introduce the concept of electronic diffraction barrier to represent the effect of constrictions and the rich transport phenomena of a variety of nanodevices. Results are presented for Bethe and Kirchhoff diffraction in graphene slits and Fabry-Perot interference oscillations in nanoribbons. The same concept applies to graphene quantum dots and gives new insight into recent experiments in these systems.

Quenching of the quantum Hall effect in multilayered epitaxial graphene: the role of undoped planes.

We propose a mechanism for the quenching of the Shubnikov-de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. Experimental data show that the scattering time of the conduction electron is magnetic field dependent and of the order of the cyclotron orbit period, i.e., it can be much smaller than the zero field scattering time. Our scenario involves the extraordinary graphene n=0 Landau level of the uncharged layers which is pinned at the Fermi level. We find that the coupling between this n=0 Landau level and the conducting states of the doped plane leads to a scattering mechanism having the right magnitude to explain the experimental data.

Quantum transport of slow charge carriers in quasicrystals and correlated systems.

We show that the semiclassical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wave function. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metalliclike to an insulatinglike regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy.

Electronic confinement and coherence in patterned epitaxial graphene.

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.

Mesoscopic transport in chemically doped carbon nanotubes.

Electronic quantum transport is investigated in boron- and nitrogen-doped carbon nanotubes using tight-binding methods correlated to ab initio calculations. The present technique accurately accounts for both effects of dopants, namely, charge transfer and elastic scattering. Generic transport properties such as conduction mechanisms, mean-free paths, and conductance scalings are derived for various concentration of randomly distributed boron and nitrogen dopants. Our calculations allow direct comparison with experiments and demonstrate that a small amount of dopants (<0.5%) can drastically modify the electronic transport properties of the tube, which is certainly a key effect feature for envisioning nanoelectronics.