Model Of Charge Carrier Transport Research Articles

Dependence of Faraday Efficiency on Operation Conditions and Cell Properties for Proton Ceramic Electrolysis Cells

Proton-conducting solid oxide electrolysis cells (p-SOECs) have attracted much attention due to their low operating temperature and low degradation rate compared with conventional oxygen-ion conducting solid oxide electrolysis cells (o-SOEC). However, p-SOECs suffer from relatively low Faradaic efficiency due to the electronic leakage of the electrolyte. Using an electrolyte charge carrier transport model, we quantified the dependence of Faraday efficiency on the electrolysis operation conditions. Our model describes the transport of charge carriers in the electrolyte when the polarization resistance can not be neglected during cell operations. By accounting for the overpotentials at the interface of electrode and electrolyte in the model, we found that the Faraday efficiency decreases with the increasing current densities at electrolysis mode for both BZY20 and BCZYYb. Our results provide significant insights into the development of highly efficient p-SOECs.

General Model for Charge Carriers Transport in Electrolyte‐Gated Transistors

AbstractA charge carrier transport model able to describe the typical modes of operation and some non‐ideal ones from electrolyte‐gated field effect transistors and organic electrochemical transistors (OECTs) is proposed. The analysis include the effect of 2D or 3D percolation transport (PT) and the influence of a shallow exponential trap distribution on the transport. Under these considerations, a non‐constant accumulation layer thickness along the channel can be formed, resulting in a non‐constant effective mobility. The accumulation thickness can depict 2D or 3D PT or even a transition between them. This transition can produce a non‐ideal profile between the linear and saturation regimes in the output curve, region in which a lump appears. Other analyzed phenomenon is the non‐linear behavior for low drain voltage in the output curve, even considering an ohmic contact. This phenomenon is attributed to the traps distribution profile into the semiconductor and the thin accumulation layer thickness close to the injection contact. The conditions when the linear field effect mobility is higher or lower than the saturation one is also analyzed. Finally, electrolyte‐gated organic field effect transistors and OECT experimental data are successfully fitted with this model showing its versatility.

Investigation and modeling of trap states in ambipolar organic field-effect transistor

This work presents a compact analytical model for charge carrier transport in an ambipolar OFET (a-OFET) based on trap-limited conduction (TLC). The model accounts for tail traps in the band gap of the organic semiconductor (OSC) in order to analyse the transistor behaviour in above threshold regime. For the sub-threshold operation of ambipolar OFET, both deep and interface trap states are accounted for, thus accurately describing the transistor behaviour comprising of drift and diffusion elements respectively. A detailed description of surface potential and field-effect mobility for both the electron and hole carriers, in consequence to these trap states has been presented. Individual current components, obtained as a result of modeling tail, deep and interface trap states are combined using the hyperbolic tangent transition function and are in good agreement with the available experimental data. • This work presents modeling of ambipolar OFET using trap-limited conduction (TLC) approach. • Presented work includes modeling of ambipolar current in both above and below threshold regime, by considering tail, deep and interface acceptor and donor-like states in organic semiconductor. • Subthreshold modeling includes both drift and diffusion components. • Dependence of the mobility of electrons and holes on the characteristic temperatures of the distributions of both tail and deep acceptor and donor-like states has been studied. • The model accurately describes the transfer characteristics of ambipolar OFET.

Contributions of Polymer Chain Length, Aggregation and Crystallinity Degrees in a Model of Charge Carrier Transport in Ultrathin Polymer Films

Herein we report a systematic study on the quantitative relationships between molecular structure, crystallinity, aggregation, and charge carrier transport in films of poly(3-hexylthiophene-co-thiophenes) (P3HTTs) with thickness ranging from 5.7 to 8.0 nm. The P3HTTs contained 10, 20, or 30 mol % of thiophene monomer units. Our UV–Vis, X-ray diffraction, and atomic force microscopy (AFM) studies indicated that the variable thiophene content in the P3HTTs as well as the solution aging prior to film casting permitted a control over aggregation density (Xa), crystallinity degree (Xc), crystal fragmentation, and overall film morphology. Increasing the thiophene content in P3HTT caused a linear increase in the Xa and a linear drop in the Xc as well as crystal sizes. Aging the solution prior to casting resulted in a change of film morphology from granular to fibrillar and a noticeable increase in both Xa and Xc. The hole mobility determined from field-effect transistor characteristics grew nonlinearly with increasing thiophene content in P3HTT and was boosted by the aggregation-induced increase in Xa or Xc. For the 7.5 nm-thin P3HTT film (30 mol % of thiophene), the μh was as high as 0.017 cm2 × V–1 × s–1. In order to explain the observed μh variation, we have employed a model of charge transport through a series circuit, where crystals, aggregates, and the amorphous phase were considered units with different resistance to charge carrier transport. A systematic analysis of the experimental dataset revealed a quantitative correlation of the μh with the polymer end-to-end distance, interaggregate distance, and crystallinity index. Also, the correlation expressed with a single equation derived in this work enabled estimation of the minimum mobility of the amorphous phase and maximum mobility of the crystal phase in the quasi-two-dimensional films. Our original findings suggest that in quantitative considerations of charge transport in the ultrathin polymer films, the morphology should be considered secondary to molar mass and phase composition.

Influence of DBD treated nano‐Al2O3 on thermal and electrical insulation properties of epoxy nanocomposites at high temperatures

AbstractThis paper presents the effect of addition of dielectric barrier discharge (DBD) plasma‐treated nano‐Al2O3 particles on electrical treeing characteristics of epoxy resin (EP) at 30, 60, and 90°C, respectively. The electrical treeing and simultaneous partial discharge measurements, surface potential decay, scanning electron microscope, Fourier transform infrared spectrograms and differential scanning calorimetry are adopted to analyze the influence of plasma‐treated nano‐Al2O3 particles on the different insulation properties of EP and its composites. The results show that the high temperature adversely affects the electrical tree initiation, growth rate, and discharge activities of the EP nanocomposites. The DBD‐treated nano‐Al2O3 particles increase the electrical tree initiation voltage of the EP nanocomposites compared to the nanocomposite with the untreated nano‐Al2O3 particles. In contrast, the electrical tree growth rate decreases in the EP nanocomposite with the DBD treated nano‐Al2O3 particles at high temperatures. It is found that DBD treated nanoparticles affect the trap density and energy level of the composites by generating new chemical bonds, and then change their electrical properties. A charge carrier transport model is proposed to explain the obtained experimental results.

An Enhanced Charge Carrier Separation in a Heterojunction Solar Cell with a Metal Oxide

A model of charge carrier transport in heterojunction solar cells composed of a solar light‐absorbing semiconductor and a wide‐bandgap semiconducting metal oxide is proposed. It describes an electric field in the semiconductor originating from the difference in the electron work functions between the two contacting materials, an enhanced hole separation by this field, and subsequent trap‐assisted tunneling of holes through the semiconducting oxide. The model predicts a dramatic influence of the donor concentration in the semiconductor, trap parameters in the oxide, and ideality factor of the semiconductor/oxide heterojunction on the performance of the cell. The efficiency of the solar energy harvesting by MoO x /n‐Si solar cells is calculated to reach 23% of the donor concentration in Si of 1018 cm−3 at a limited hole current density. It is predicted to be reduced to 16−18% when donor concentration is in the range of 1015−1016 cm−3. The numerical predictions agree well with experimental data, at least for donor concentrations in Si below 1018 cm−3, confirming suitability of the model for heterojunction solar cells composed of other semiconductors and semiconducting metal oxides.

The oxygen partial pressure in solid oxide electrolysis cells with multilayer electrolytes

A number of degradation mechanisms have been observed during the long-term operation of solid oxide electrolysis cells (SOEC). Using an electrolyte charge carrier transport model and a diffuse interface treatment for a multilayer electrolytes, we quantify the oxygen potentials across the electrolyte and thereby provide insights into these degradation mechanisms. Our model describes the transport of charge carriers in the electrolyte when the oxygen partial pressure is extremely low by accounting for the spatial variation of the concentration of oxygen vacancies in the electrolyte which is closely related to the degradation of the SOEC near the interface of hydrogen electrode and electrolyte. Moreover, we identify four quantities that characterize the distribution of oxygen partial pressure in the electrolyte, which are directly related to the degradation mechanisms in the electrolyte as well, and give analytical estimates for them. These analytical expressions provide guidance on the parameters that need to be controlled to suppress the degradation observed in the electrolyte.

Numerical simulation of multilayer organic light-emitting diodes with host–guest emissive layer: the role of defect states

In this study, a comprehensive model for the numerical simulation of electrical and optical characteristics of multilayer organic light-emitting devices (OLED) is presented. The model consists of three parts: charge carrier transport model, exciton model, and a post-processing part for calculating luminance. The defect states in the organic layers are modeled and are introduced into the equations. For demonstrating the rule of the defect states in such devices, a multilayer OLED is considered consisting of a host–guest emissive layer sandwiched between electron and hole transport layers. The current density and luminance results of the analyzed device are compared with the experimental data. We have found that by considering the defect states, there is good accordance between the simulation results and the experimental data.

Controlling the color with different excitations in a dysprosium-activated apatite oxynitride phosphor

Among the f-f transition of lanthanide ions, trivalent dysprosium (Dy3+) activated luminescence materials always exhibit polychromatic emission with different ratios of blue to yellow emission (B/Y ratio), which originates from the two main transitions 4F9/2 to 6H15/2 and 6H13/2, respectively. Researchers have found the B/Y ratio is dependent on the host structure. Meanwhile, Dy3+-activated luminescence materials have various applications under different excitations. However, the relationship between them (excitation vs emission color) has not been studied yet, which restricts the development of fundamental research and applications. In this study, we reported an excitation-controlled color variation material, the apatite oxynitride phosphor Y5Si3O12N: Dy3+ and investigated the color-reversal phenomenon by switching excitation sources (from UV to VUV or cathode ray) in the material. This phenomenon is unraveled by a defect-assisted charge carrier transport model that the defect could change the chemical surrounding of Dy3+ and result in the blue emission dominant instead of the initial yellow emission. The mechanism is evidenced by the crystal structure analysis, luminescence properties dissect, defects state parse, etc. This study also provides new insights to understand and modulate Dy3+-activated luminescence materials.

Beyond 30% Conversion Efficiency in Silicon Solar Cells: A Numerical Demonstration

We demonstrate through precise numerical simulations the possibility of flexible, thin-film solar cells, consisting of crystalline silicon, to achieve power conversion efficiency of 31%. Our optimized photonic crystal architecture consists of a 15 μm thick cell patterned with inverted micro-pyramids with lattice spacing comparable to the wavelength of near-infrared light, enabling strong wave-interference based light trapping and absorption. Unlike previous photonic crystal designs, photogenerated charge carrier flow is guided to a grid of interdigitated back contacts with optimized geometry to minimize Auger recombination losses due to lateral current flow. Front and back surface fields provided by optimized Gaussian doping profiles are shown to play a vital role in enhancing surface passivation. We carefully delineate the drop in power conversion efficiency when surface recombination velocities exceed 100 cm/s and the doping profiles deviate from prescribed values. These results are obtained by exact numerical simulation of Maxwell’s wave equations for light propagation throughout the cell architecture and a state-of-the-art model for charge carrier transport and Auger recombination.

Geobacter sulfurreducens pili support ohmic electronic conduction in aqueous solution.

The bacterium Geobacter sulfurreducens is a model biological catalyst in microbial electrochemical devices. G. sulfurreducens forms electrically conductive, electrode-associated biofilms, but the biological structures mediating electrical conduction from cells to the electrodes are a matter of debate. Bacteria in these communities produce a network of fiber-like Type IV pili, which have been proposed to act either as inherent, protein-based electronic conductors, or as electronically inert scaffolds for cytochromes mediating long-range charge transport. Previous studies have examined pilus conduction mechanisms under vacuum and in dry conditions, but their conduction mechanism under physiologically relevant conditions has yet to be characterized. In this work, we isolate G. sulfurreducens pili, and compare the electronic conduction mechanism of both live biofilms and purified pili networks under dry and aqueous conditions. Solid-state I-V characteristics indicate that electronic transport in films of purified pili is representative of conduction in a fiber percolation network. Electrochemical gating measurements in a bipotentiostat device configuration confirm previous results suggesting redox currents dominate live biofilm conduction. Purified pili films, however, exhibit non-redox electronic conduction under aqueous, buffered conditions, and their conductivity increases with decreasing temperature. These findings show that isolated pili possess inherent, non-redox-mediated conductivity consistent with a metallic-like model of charge carrier transport. The results demonstrate an experimental platform for studying electronic transport in biomaterials and suggest that pili serve as an exemplary model for designing bioelectronic interfaces.

Universality of charge-carrier transport in molecularly doped polymers

The charge-carrier transport model based on the multiple-trapping quasi-band theory with the Gaussian or exponential energy distributions of traps for the two-layer structure of the molecularly doped polymer sample is used to explain the experimentally observed constancy of the flat plateau on time-of-flight curves in a wide electric-field range. The constant shape of the time-of-flight curve under the condition of nonequilibrium transport is observed for the exponential, rather than Gaussian, energy trap distribution. This observation may be used to distinguish between these trap distributions. Finding the true Poole–Frenkel constant requires that the nonequilibrium transport in molecularly doped polymers be taken into account during treatment of the data on the field dependence of mobility.

A charge carrier transport model for donor-acceptor blend layers

Highly efficient organic solar cells typically comprise donor-acceptor blend layers facilitating effective splitting of excitons. However, the charge carrier mobility in the blends can be substantially smaller than in neat materials, hampering the device performance. Currently, available mobility models do not describe the transport in blend layers entirely. Here, we investigate hole transport in a model blend system consisting of the small molecule donor zinc phthalocyanine (ZnPc) and the acceptor fullerene C60 in different mixing ratios. The blend layer is sandwiched between p-doped organic injection layers, which prevent minority charge carrier injection and enable exploiting diffusion currents for the characterization of exponential tail states from a thickness variation of the blend layer using numerical drift-diffusion simulations. Trap-assisted recombination must be considered to correctly model the conductivity behavior of the devices, which are influenced by local electron currents in the active layer, even though the active layer is sandwiched in between p-doped contacts. We find that the density of deep tail states is largest in the devices with 1:1 mixing ratio (Et = 0.14 eV, Nt = 1.2 × 1018 cm−3) directing towards lattice disorder as the transport limiting process. A combined field and charge carrier density dependent mobility model are developed for this blend layer.

A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells.

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials. Here we perform a systematic study on PbS nanocrystal-based diodes using temperature-dependent current–voltage characterization and thermal admittance spectroscopy to develop a model for charge transport that is applicable to different nanocrystal-solids and device architectures. Our analysis confirms that charge transport occurs in states that derive from the quantum-confined electronic levels of the individual nanocrystals and is governed by diffusion-controlled trap-assisted recombination. The current is limited not by the Schottky effect, but by Fermi-level pinning because of trap states that is independent of the electrode–nanocrystal interface. Our model successfully explains the non-trivial trends in charge transport as a function of nanocrystal size and the origins of the trade-offs facing the optimization of nanocrystal-based solar cells. We use the insights from our charge transport model to formulate design guidelines for engineering higher-performance nanocrystal-based devices.

A Geant4 based framework for pixel detector simulation

The output from a hybrid pixel detector depends on the interaction of the radiation with the sensor material, the transport of the resulting charge in the sensor, the pulse processing in the readout circuit and processing of the resulting signal. In order to understand the full behaviour of the device and to predict the performance of future devices it is important to have a framework that can simulate the entire process in the detector system. Geant4 is a Monte Carlo based toolkit for simulation of particle interaction with matter which is developed and actively used for CERN experiments and detector development [1]. By extending the Monte Carlo code in Geant4 with a charge carrier transport model of the sensor material and basic amplifier functionality as well as read out logic, a simulation of the complete detector system is possible.The MEDIPIX is a state of the art hybrid pixel detector that allows bonding of a wide range of sensor materials [2,3]. Simulation models have been developed and tested for different chips from the MEDIPIX family. The simulation is defined using configuration files to set the geometry, sensor material properties, number of pixels, pixel pitch and chip properties. Source properties as well as filters and objects in the beam can be added for different experimental set-ups. The interaction of radiation with the sensor is taken into account in the transport of the charge carriers in the sensor material and a current induced in the pixel electrode that triggers an amplifier response. Simulation results have been verified with X-ray fluorescence and radioactive sources using MEDIPIX family chips. In this paper we present the developed simulation framework and first results.

Incompressible Type Limit Analysis of a Hydrodynamic Model for Charge-Carrier Transport

This paper is concerned with the rigorous analysis of the zero electron mass limit of the full Navier--Stokes--Poisson. This system has been introduced in the literature by Anile and Pennisi (see [Phys. Rev. B, 46 (1992), pp. 13186--13193]) in order to describe a hydrodynamic model for charge-carrier transport in semiconductor devices. The purpose of this paper is to prove rigorously zero electron mass limit in the framework of general ill-prepared initial data. In this situation the velocity field and the electronic fields develop fast oscillations in time. The main idea we will use in this paper is a combination of formal asymptotic expansion and rigorous uniform estimates on the error terms. Finally we prove the strong convergence of the full Navier--Stokes--Poisson system toward the incompressible Navier--Stokes equations.

Monte Carlo simulation based on dynamic disorder model in organic semiconductors: From coherent to incoherent transport

The dynamic disorder model for charge carrier transport in organic semiconductors has been extensively studied in recent years. Although it is successful on determining the value of bandlike mobility in the organic crystalline materials, the incoherent hopping, the typical transport characteristic in amorphous molecular semiconductors, cannot be described. In this work, the decoherence process is taken into account via a phenomenological parameter, say, decoherence time, and the projective and Monte Carlo method are applied for this model to determine the waiting time and thus the diffusion coefficient. It is obtained that the type of transport is changed from coherent to incoherent with a sufficiently short decoherence time, which indicates the essential role of decoherence time in determining the type of transport in organics. We have also discussed the spatial extent of carriers for different decoherence time, and the transition from delocalization (carrier resides in about 10 molecules) to localization is observed. Based on the experimental results of spatial extent, we estimate that the decoherence time in pentacene has the order of 1 ps. Furthermore, the dependence of diffusion coefficient on decoherence time is also investigated, and corresponding experiments are discussed.

Effect of thermal oxidation on charge carrier transport in nanostructured silicon

The effect of short-time annealing in air on the conductivity of porous silicon in two different directions (parallel and orthogonal to the sample surface) is studied. The samples under study are produced by electrochemical etching of single-crystal silicon wafers with (100)-oriented surfaces. It is found that thermal oxidation of the resulting porous silicon layers has different effects on the conductivity measured normally and parallel to the sample surface. For the oxidized samples, the conductivity in the direction orthogonal to the surface is noticeably higher than the conductivity in the direction parallel to the surface. The results are interpreted in the context of the model of charge carrier transport with consideration for potential barriers at the boundaries of silicon nanocrystals.

Quadrupolar glass as a model for charge carrier transport in nonpolar organic materials

We have applied our two recent results [depending on its helicity photon carries a quantum flux of ± Φ 0 = ± hc/e and the quantized magnetic fluxes through the electronic orbits of the Dirac hydrogen atom are given by: Φ (n,l,mj) = (n-l-mj)Φ0) ] to the 1s-2p and 2p-3d excitonic transitions in nanostructures. It is shown that the flux changes for the non-zero matrix elements in the 1s-2p and 2p-3d excitonic transitions is either ± Φ0 or zero. The present result supports the previous results stated above. It is also shown that spin flip is possible in the 1s-2p and 2p-3d excitonic transitions.

Quadrupolar glass as a model for charge carrier transport in nonpolar organic materials

AbstractMonte Carlo simulation of the charge carrier transport in disordered nonpolar organic materials has been carried out. As a suitable model we considered the model of quadrupolar glass. A general formula for the temperature and field dependence of the mobility was suggested. A comparison with experimental data has been carried out.