Description Of Charge Transport Research Articles

Modeling of ambipolar charge transport in all-organic ferroelectric field-effect transistor

A model describing charge transport in disordered ambipolar all-organic-ferroelectric field-effect transistors (AoFeFETs) is proposed by examining the interaction between semiconductor charge and ferroelectric polarization. A density dependent variable-range hopping charge transport in ambipolar organic semiconductors and an analytical ferroelectric-polarization Miller-Lue expression were adopted in this model. An effective potential comprising the threshold voltage and channel potential V ch , has been used to consider the possible effects due to the nonuniform field distribution along the device-channel. The results show it can be used to model the transfer and output characteristic in AoFeFETs via a single formulation. An excellent agreement between theory and experiment is observed over a wide range of biasing regimes.

Impact of exact exchange in the description of the electronic structure of organic charge-transfer molecular crystals

We evaluate the impact that the amount of nonlocal Hartree-Fock (%HF) exchange included in a hybrid density functional has on the microscopic parameters (transfer integrals, band gaps, bandwidths, and effective masses) describing charge transport in high-mobility organic crystals. We consider both crystals based on a single molecule, such as pentacene, and crystals based on mixed-stack charge-transfer systems, such as dibenzo-TTF--TCNQ. In the pentacene crystal, the band gap decreases and the effective masses increase linearly with an increase in the amount of %HF exchange. In contrast, in the charge-transfer crystals, while the band gap increases linearly, the effective masses vary only slightly with an increase in %HF exchange. We show that the superexchange nature of the electronic couplings in charge-transfer systems is responsible for this peculiar evolution of the effective masses. We compare the density functional theory results with results obtained within the ${G}_{0}{W}_{0}$ approximation as a way of benchmarking the optimal amount of %HF exchange needed in a given functional.

Materials Meets Concepts in Molecule‐Based Electronics

In this contribution, molecular materials are highlighted as an important topic in the diverse field of condensed matter physics, with focus on their particular electronic and transport properties. A better understanding of their performance in various applications and devices demands for an extension of basic theoretical approaches to describe charge transport in molecular materials, including the accurate description of electron–phonon coupling. Starting with the simplest case of a molecular junction and moving on to larger aggregates of bulk organic semiconductors, charge‐transport regimes from ballistic motion to incoherent hopping, which are frequently encountered in molecular systems under respective conditions, are discussed. Transport features of specific materials are described through ab initio material parameters whose determination is addressed.

Nanoscale Orientation Effects on Carrier Transport in a Low-Band-Gap Polymer

We show that due to a substrate-induced orientation of the local morphology of thin supported conjugated polymer films of poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexan-1-one)thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-C), the out-of-plane hole mobility, μ, increased with increasing film thickness h, becoming independent of h for h > 110 nm. The temperature and electric field dependencies of μ are in agreement with predictions of the well-known Gaussian Disorder Model (GDM), developed to describe charge transport in materials possessing positional and energetic disorder. Room temperature studies reveal a negative dependence of μ on the electric field E, with a strength quantified by the parameter β. The largest magnitude of β was measured in the thinnest films, indicative of the strongest dependence of μ on the electric field E; β decreased monotonically with increasing film thickness. The thickness dependencies of μ and β manifest an increase of the average anisotropy of the films with decreasing film thickness, corroborated by UV–vis spectroscopy and spectroscopic ellipsometry measurements.

Excited state and charge dynamics of hybrid organic/inorganic heterojunctions. I. Theory

The different cohesive forces that bond organic (i.e. excitonic) and inorganic semiconductors lead to widely disparate dielectric constants, charge mobilities, and other fundamental optoelectronic properties that make junctions between these materials interesting for numerous practical applications. Yet, there are no detailed theories addressing charge and energy transport across interfaces between these hybrid systems. Here, we develop a comprehensive physical model describing charge transport and photocurrent generation based on first-principles charge and excited state dynamics at the organic/inorganic heterojunction. We consider interfaces that are trap-free, as well as those with an exponential distribution of trap states. We find that the hybrid charge-transfer state resulting from photon absorption near the junction that subsequently migrates to the heterointerface is often unstable at room temperature, leading to its rapid dissociation into free charges that are collected at the device contacts. In the companion Paper II [A. Panda et al., Phys. Rev. B 90, 045303 (2014)], we apply our theories to understanding the optical and electronic properties of archetype organic/inorganic heterojunction diodes. Our analysis provides insights for developing high performance optoelectronic devices whose properties are otherwise inaccessible to either conventional excitonic or inorganic semiconductor junctions.

Mapping orientational order of charge-probed domains in a semiconducting polymer.

Structure-property relationships are of fundamental importance to develop quantitative models describing charge transport in organic semiconductor based electronic devices, which are among the best candidates for future portable and lightweight electronic applications. While microstructural investigations, such as those based on X-rays, electron microscopy, or polarized optical probes, provide necessary information for the rationalization of transport in macromolecular solids, a general model predicting how charge accommodates within structural maps is not yet available. Therefore, techniques capable of directly monitoring how charge is distributed when injected into a polymer film and how it correlates to structural domains can help fill this gap. Supported by density functional theory calculations, here we show that polarized charge modulation microscopy (p-CMM) can unambiguously and selectively map the orientational order of the only conjugated segments that are probed by mobile charge in the few nanometer thick accumulation layer of a high-mobility polymer-based field-effect transistor . Depending on the specific solvent-induced microstructure within the accumulation layer, we show that p-CMM can image charge-probed domains that extend from submicrometer to tens of micrometers size, with markedly different degrees of alignment. Wider and more ordered p-CMM domains are associated with improved carrier mobility, as extracted from device characteristics. This observation evidences the unprecedented opportunity to correlate, directly in a working device, electronic properties with structural information on those conjugated segments involved in charge transport at the buried semiconductor-dielectric interface of a field-effect device.

Computational Evaluation of Optoelectronic Properties for Organic/Carbon Materials

CONSPECTUS: Organic optoelectronic materials are used in a variety of devices, including light-emitting diodes, field-effect transistors, photovoltaics, thermoelectrics, spintronics, and chemico- and biosensors. The processes that determine the intrinsic optoelectronic properties occur either in the photoexcited states or within the electron-pumped charged species, and computations that predict these optical and electrical properties would help researchers design new materials. In this Account, we describe recent advances in related density functional theory (DFT) methods and present case studies that examine the efficiency of light emission, carrier mobility, and thermoelectric figures of merit by calculation of the electron-vibration couplings. First we present a unified vibrational correlation function formalism to evaluate the excited-state radiative decay rate constant kr, the nonradiative decay rate constant knr, the intersystem crossing rate constant kISC, and the optical spectra. The molecular parameters that appear in the formalism, such as the electronic excited-state energy, vibrational modes, and vibronic couplings, require extensive DFT calculations. We used experiments for anthracene at both low and ambient temperatures to benchmark the calculated photophysical parameters. In the framework of Fermi's golden rule, we incorporated the non-adiabatic coupling and the spin-orbit coupling to evaluate the phosphorescence efficiency and emission spectrum. Both of these are in good agreement with experimental results for anthracene and iridium compounds. Band electron scattering and relaxation processes within Boltzmann theory can describe charge transport in two-dimensional carbon materials and closely packed organic solids. For simplicity, we considered only the acoustic phonon scattering as modeled by the deformation potential approximation coupled with extensive DFT calculations for band structures. We then related the carrier mobility to the band-edge shift associated with the lattice dilation of longitudinal waves. The calculated relaxation time was in good agreement with experimental data for the graphene sheet, which supports the methodology. We then found that the intrinsic electron mobility for a 6,6,12-graphyne sheet can be even larger than that of graphene. We extended this approach to investigate the thermoelectric transport of electrons in metal phthalocyanines, which showed reasonable Seebeck coefficients when compared with experiments. For the thermal lattice transport, we employed nonequilibrium molecular dynamics simulations. Combining both electron transport and lattice thermal conductivity, we can evaluate the thermoelectric figure of merit.

Theoretical description of charge transport in disordered organic semiconductors (Phys. Status Solidi B 3/2014)

Theoretical description of charge transport in disordered organic semiconductors (Phys. Status Solidi B 3/2014)

A simulation framework for modeling charge transport and degradation in high-k stacks

In this paper we present a comprehensive physical model that describes charge transport and degradation phenomena in high-k stacks. The physical mechanisms are modeled using a novel material-relate...

A Density Functional Study of Furofuran Polymers as Potential Materials for Polymer Solar Cells

The structural, electronic, and optical properties of poly(3-hexylthiophene) (P3HT) have been comprehensively studied by density functional theory (DFT) to rationalize the experimentally observed properties. Rather, we employed periodic boundary conditions (PBC) method to simulate the polymer block, and calculated effective charge mass from the band structure calculation for describing charge transport properties. The simulated results of P3HT are consistent with the experimental results in band gaps, absorption spectra, and effective charge mass. Based on the same calculated methods as P3HT, a series of polymers have been designed on the basis of the two types of building blocks, furofurans and furofurans substituted with cyano (CN) groups, to investigate suitable polymers toward polymer solar cell (PSC) materials. The calculated results reveal that the polymers substituted with CN groups have good structural stability, low-lying FMO energy levels, wide absorption spectra, and smaller effective masses, which are due to their good rigidity and conjugation in comparison with P3HT. Besides, the insertion of CN groups improves the performance of PSC. Synthetically, the designed polymers PFF1 and PFF2 are the champion candidates toward PSC relative to P3HT.

Transport mechanisms of electrons and holes in dielectric films

Electron and hole transport mechanisms in amorphous silicon oxide, silicon nitride, and aluminum oxide, dielectric materials of high relevance to silicon device technology, are reviewed. It is established that the widely accepted Frenkel model provides a formal description of transport in trap-containing insulators, but nonphysical model parameters must be introduced in order to obtain quantitative agreement. It is shown that the multiphonon model of trap ionization consistently describes charge transport in insulators with traps.

Effect of charged line defects on conductivity in graphene: Numerical Kubo and analytical Boltzmann approaches

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically-vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.

Regularity of the solution and well-posedness of a mixed problem for an elliptic system with quadratic nonlinearity in gradients

A boundary value problem for an elliptic system of equations is studied that arises in the analysis of a new hydrodynamic model describing charge transport in a planar semiconductor MESFET (metal semiconductor field effect transistor). The problem has a number of features, specifically, the equations of the system involve squared components of the gradients of the unknown functions; the boundary conditions are of a mixed character, i.e., Dirichlet and Neumann conditions are set on different portions of the boundary; and the boundary of the domain is a nonsmooth curve, namely, a rectangle. Under a certain optimal condition, the C 1,α-regularity of a weakened solution of the problem is justified and its existence is proved, while its uniqueness is shown under additional constraints. The results are used to justify the stabilization method as applied to finding approximate stationary solutions of the hydrodynamic model.

A Hydrodynamic Model for Covalent Semiconductors with Applications to GaN and SiC

In this paper we present a hydrodynamical model which, in principle, is able to describe charge transport in a generic compound semiconductor. The model makes use of an analytic approximation for the conduction bands. Energy dispersion relationships in the neighbors (valleys) of the lowest minima are, in fact, taken to be spherical, nonparabolic. The model considers the main scattering mechanisms in polar semiconductors, that is the acoustic, polar optical, intervalley non-polar optical phonon interactions and the ionized impurity scattering. Simulations are shown for the cases of bulk GaN and SiC.

A hydrodynamical model for holes in silicon semiconductors

PurposeThis paper intends to present a hydrodynamical model which describes the hole motion in silicon and couples holes and electrons.Design/methodology/approachThe model is based on the moment method and the closure of the system of moment equations is obtained by using the maximum entropy principle (hereafter MEP). The heavy, light and split‐off valence bands are considered. The first two are described by taking into account their warped shape, while for the split‐off band a parabolic approximation is used.FindingsThe model for holes is coupled with an analogous one for electrons, so obtaining a complete description of charge transport in silicon. Numerical simulations are performed both for bulk silicon and a p‐n junction.Research limitations/implicationsThe model uses a linear approximation of the maximum entropy distribution in order to close the system of moment equations. Furthermore, the non‐parabolicity of the heavy and light bands is neglected. This implies an approximation on the high field results. This issue is under current investigation.Practical implicationsThe paper improves the previous hydrodynamical models on holes and furnishes a complete model which couples electrons and holes. It can be useful in simulations of bipolar devices.Originality/valueThe results of the paper are new since a better approximation of the band structure is used and a description of both electron and hole behavior is present, therefore the results are of a certain relevance for the theory of charge transport in semiconductors.

An Implicit-Explicit Runge-Kutta scheme for an extended hydrodynamic model describing charge transport in silicon semiconductors.

Many applications involve hyperbolic systems of conservation laws with source terms. Solving numerically such systems is often challenging expecially when the source terms are stiff. We present here implicit-explicit (IMEX) Runge-Kutta schemes which are widely used, especially in conjunction with spectral methods, for the time integration of spatially discretized partial differential equations (PDEs) of diffusion-convection type. The explicit part is treated by a strong-stability preserving (SSP) scheme, the implicit part is treated with a diagonally implicit Runge-Kutta (DIRK). These schemes avoid the onset of spuri- ous numerical oscillations arising near discontinuities of the solution. Here, an extended hydrodynamic model describing charge transport in semiconductors is considered, and applications to bulk silicon are presented.

A renormalization approach to describe charge transport in quasiperiodic dangling backbone ladder (DBL)-DNA molecules

We study the charge transport properties of a dangling backbone ladder (DBL)-DNA molecule focusing on a quasiperiodic arrangement of its constituent nucleotides forming a Rudin–Shapiro (RS) and Fibonacci (FB) Poly (CG) sequences, as well as a natural DNA sequence (Ch22) for the sake of comparison. Making use of a one-step renormalization process, the DBL-DNA molecule is modeled in terms of a one-dimensional tight-binding Hamiltonian to investigate its transmissivity and current–voltage (I–V) profiles. Beyond the semiconductor I–V characteristics, a striking similarity between the electronic transport properties of the RS quasiperiodic structure and the natural DNA sequence was found.

ChemInform Abstract: Charge Transport in High Mobility Molecular Semiconductors: Classical Models and New Theories

AbstractReview: 53 refs.

Analysis of the oscillating photocarrier grating technique

In this paper we present a complete theoretical analysis of the oscillating photocarrier grating (OPG) method, starting from the generalized equations that describe charge transport and recombination under oscillating grating illumination conditions. The solution of these equations allows us to implement a calculation reproducing the experimental OPG curves. We study both experimentally and from our calculations the dependence of the OPG curves on different external parameters, such as the applied electric field, grating period and illumination intensity. We find that the response of the sample is linked to a characteristic time of the material, which could be the dielectric relaxation time or the small signal lifetime depending on the regime at which the experiment is performed. Therefore, the OPG technique provides a simple method to estimate these parameters. In addition, we demonstrate that the small signal lifetime provides information on the density of states of the material.

Charge transport in high mobility molecular semiconductors: classical models and new theories

The theories developed since the fifties to describe charge transport in molecular crystals proved to be inadequate for the most promising classes of high mobility molecular semiconductors identified in the recent years, including for example pentacene and rubrene. After reviewing at an elementary level the classical theories, which still provide the language for the understanding of charge transport in these systems, this tutorial review outlines the recent experimental and computational evidence that prompted the development of new theories of charge transport in molecular crystals. A critical discussion will illustrate how very rarely it is possible to assume a charge hopping mechanism for high mobility organic crystals at any temperature. Recent models based on the effect of non-local electron-phonon coupling, dynamic disorder, coexistence of localized and delocalized states are reviewed. Additionally, a few more recent avenues of theoretical investigation, including the study of defect states, are discussed.