Electrical Charge Transport Research Articles

Carbon black/graphene-modified aluminum foil cathode current collectors for lithium ion batteries with enhanced electrochemical performances

Though aluminum foils are widely applied as cathode current collectors for lithium ion batteries, they still face many challenges, such as the limited contact area, weak adhesion with electrode materials and localized corrosion by electrolytes during long-term cycling, which will lead to the degradation of electrochemical performances. Here, graphene-modified aluminum foils and carbon black/graphene modified aluminum foils are prepared as the current collectors for lithium iron phosphate lithium ion batteries by a facile solution coating method. We find that the batteries fabricated with carbon black/graphene-modified aluminum foils exhibit largely improved electrochemical performances in rate capability, internal resistance and long-term cycling capacity retention, compared with the bare aluminum and graphene-modified aluminum foils. This work reveals the coatings of graphene nanosheets not only can increase the contact area and enhance the adhesion between electrode materials and current collectors, which is beneficial for their electrical contact and charge transport, but also can suppress the aluminum foil corrosion during long-term cycling. Meanwhile, the excessive coverage of graphene nanosheets will lower the battery performances due to the interlayer contact increment of graphene nanosheets. In addition, carbon black combines graphene to form a co-conductive network effectively compensating the low interlayer electrical conductivity of graphene nanosheets.

Evaluation of Electrical Carrier Transport Mechanism in Plasma Polymerized n-Butyl Methacrylate Thin Films

Aluminium/poly(n-butyl methacrylate)/aluminium pattern thin films of different thickness were fabricated through plasma polymerization technique and their DC electrical characterization was conducted with a view of better understanding of the electrical charge transport mechanism. The current density—voltage and current density—film thickness characteristics of fabricated thin films of different thickness indicated that the dominant conduction mechanism in the higher voltage region is space charge limited conduction. The estimated carrier mobility, trap density and free carrier density decreased with the increase of thickness of the prepared thin films.

Electrical Properties of Cerium and Yttrium Co-substituted Strontium Nanohexaferrites

Ce and Y substituted SrFe12O19 (x = 0.4 and 0.5) nanohexaferrites have been synthesised by sol–gel auto-combustion. The pure hexagonal SrFe12O19 phase was confirmed by XRD, FE-SEM and HR-TEM. The ac conductivity and dielectric properties of Ce and Y substituted Sr:Fe12O19 nanohexaferrites were studied by using complex impedance technique. Dielectric properties such as dielectric constant, dielectric loss, dielectric tangent loss, as well as ac conductivity are measured at various bias potentials in a varying frequency range from 10 Hz to 10 MHz. The frequency dependency of the ac conductivity could be evaluated by a power exponent law at higher frequencies, which is a characteristic mechanism for electrical charge transport by tunnelling processes with which it is more dispersive at lower frequencies. The conduction mechanisms were also investigated at various dc bias potentials, mainly being an indicative of hopping type conduction. So, the dielectric dispersion behaviour can be well elucidated in terms of Maxwell–Wagner polarization in accordance with the Koop’s phenomenological theory. Utilizing an electrical equivalent circuit model, impedance studies were carried out in a specific frequency domain to characterize all the contributions of the dielectric response between the grains and grain boundaries to the dielectric parameters.

Numerical modeling of solid acid fuel cell performance with CsH2PO4- AAM (anodic alumina membrane) composite electrolyte

In the present study, a full three dimensional, isothermal, and steady state model is proposed for a planar solid acid fuel cell (SAFC) that uses CsH2PO4-AAM (anodic alumina membrane) composite as an electrolyte. A fuel cell is used for the generation of electricity and consists of anode, electrolyte, and cathode. In this model, mass, momentum, electrical charge transport and electrochemistry equations were used to describe the fuel cell regions (flow channel, gas diffusion layer, and catalyst layer). By using finite element method in COMSOL MULTIPHYSICS (V. 3.4) software, the equations of the model were solved numerically. Finally, the results of this model were compared with the experimental data reported in the literatures (polarization graph) to validate the model. The results show a good agreement with the reported experimental data. The advantage of this model is the prediction of SAFC behavior in different operational and geometrical conditions, and the results would be useful for establishing guidelines for SAFCs’ design and optimization.

Vacuum birefringence and the Schwinger effect in (3+1) de Sitter

In de Sitter space, the current induced by an electric field in vacuum is known to feature certain peculiarities, such as infrared hyperconductivity for light bosons in weak electric fields. Moreover, negative conductivity has been claimed to occur for light bosons in moderate electric fields, and for fermions of any mass in electric fields below a certain threshold. Furthemore, in the limit of large mass and weak electric field, the current contains terms which are not exponentially suppressed, contrary to the semiclassical intuition. Here we explain these behaviors, showing that most of the reported negative conductivity is spurious. First, we show that the terms which are not exponentially suppressed follow precisely from the local Euler-Heisenberg Lagrangian (suitably generalized to curved space). Thus, such terms are unrelated to pair creation or to the transport of electric charge. Rather, they correspond to non-linearities of the electric field (responsible in particular for vacuum birefringence). The remaining contributions are exponentially suppressed and correspond to the creation of Schwinger pairs. Second, we argue that for light carriers the negative term in the regularized current does not correspond to a negative conductivity, but to the logarithmic running of the electric coupling constant, up to the high energy Hubble scale. We conclude that none of the above mentioned negative contributions can cause an instability such as the spontaneous growth of an electric field in de Sitter, at least within the weak coupling regime. Third, we provide a heuristic derivation of infrared hyperconductivity, which clarifies its possible role in magnetogenesis scenarios.

Electric transport in organic system with planar DBP/F16ZnPc junction on the basis of direct current and small signal admittance spectra analysis

The objective of this work was to determine electric transport in the organic device based on a planar junction of electron donor and electron acceptor materials, namely ITO/MoO3/DBP/F16ZnPc/BCP/Ag. The analysis reported herein was based on direct current-voltage measurements and small-signal admittance spectra in the dark and under illumination. Such analysis may provide information on potential barriers, parasitic resistances and presence of space charge affecting the electric current flow within the device. Therefore, this approach could be applied for determination of physical processes related to electric charge transport within multilayer structures, such as photovoltaic cells or photodetectors.In the case of the investigated system, the parallel parasitic resistance, the resistance of electrodes, and the geometric capacitance of 10 MΩ, 55 Ω, and 1.6 nF respectively were found. It was also shown that the direct current flowing from ITO to Ag was limited by charge carrier injection from electrodes, while in the case of current flowing from Ag to ITO no essential barriers at electrodes were noticed.

Organic photovoltaic cell analysis through quantum efficiency and scanning tunneling microscopy of the donor/blend as an active film

In this work is reported a comparison of the film morphology, film molecular ordering and X-ray diffraction pattern between two of the most common and efficient donor polymers used in organic photovoltaic (OPV) cells: PTB7 and PTB7-Th. These comparisons indicate that PTB7-Th film chains are somewhat thicker and less spaced than those for PTB7; also, PTB7-Th films have a slightly better organized structure and higher co-planarity, which could provide a possible better electrical charge transport. On the order hand, an analysis of the external/internal quantum efficiency (EQE/IQE) of OPVs, based on PTB7-Th, as a function of the active layer thickness ranging from 40 to 165 nm was carried out. It was used the bulk heterojunction architecture to fabricate OPVs cells under the configuration glass/ITO/PEDOT:PSS/PTB7-Th:PC71BM/PFN/FM (Field’s Metal: eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn by weight that melts at 62 °C). IQE spectra were determined by using the active layer absorption calculated through the transfer matrix method (TMM). Our results show a significant reduction of IQE when increasing the active layer thickness above 120 nm. IQE decreases, and consequently EQE and PCE, mainly due to the reduction in charge carriers collection probability. On the reversed side, when the active layer is very thin (< 70 nm), there exists also a decrease in the IQE values. A comparison between the experimental measurements and theoretical simulations (by TMM) is discussed in order to have better understanding of the OPVs performance.

Thickness-Dependent DC Electrical Breakdown of Polyimide Modulated by Charge Transport and Molecular Displacement.

Polyimide has excellent electrical, thermal, and mechanical properties and is widely used as a dielectric material in electrical equipment and electronic devices. However, the influencing mechanism of sample thickness on electrical breakdown of polyimide has not been very clear until now. The direct current (DC) electrical breakdown properties of polyimide as a function of thickness were investigated by experiments and simulations of space charge modulated electrical breakdown (SCEB) model and charge transport and molecular displacement modulated (CTMD) model. The experimental results show that the electrical breakdown field decreases with an increase in the sample thickness in the form of an inverse power function, and the inverse power index is 0.324. Trap properties and carrier mobility were also measured for the simulations. Both the simulation results obtained by the SCEB model and the CTMD model have the inverse power forms of breakdown field as a function of thickness with the power indexes of 0.030 and 0.339. The outputs of the CTMD model were closer to the experiments. This indicates that the displacement of a molecular chain with occupied deep traps enlarging the free volume might be a main factor causing the DC electrical breakdown field of polyimide varying with sample thickness.

Transport properties in Sb-doped SnO2 thin films: Effect of UV illumination and temperature dependence

Films of Sb-doped SnO2 were deposited by dip-coating technique. The morphology analysis showed that the microstructure of the samples exhibits many shaped nano-sized crystallites homogenously dispersed on the surfaces. The roughness varies with Sb-content, implying that doping has an important effect on the surface morphology of the films. Raman and FTIR-ATR investigations revealed that the films exhibit a vibration mode at 564 cm−1 ascribed to the amorphous phase overlapping the crystallites. Thus, the selection rule (k=0) is relaxed because of disorder and reduced crystallites size to nanoscale by Sb-doping, which may explain the broadening and shift of the lattice modes. The vibration of sub-bridging oxygen atoms seems to be significantly affected by oxygen vacancies and Sb-derived surface mode has been identified at 578 cm−1. The optical band gap is red-shift due to the introduction of localized states in the SnO2 bulk associated to Sb-doping and oxygen defects. The refractive index varies due the enhanced grain boundaries effect and surface disorder induced by Sb incorporation in the crystal lattice. The photoconductivity (σac) shows a dispersion pattern with two regions, corresponding to extrinsic and intrinsic conductivities. Under UV-illumination, the electrical charge transport mechanism changes from multi-hopping to single-hopping at higher temperatures.

Galvanomagnetic methods of Curie temperature determination in (Ga,Mn)As

We critically discuss various experimental methods to determine Curie temperature TC of (Ga,Mn)As thin layers or other conducting magnetic materials by means of electric charge transport measurements. They all base on the influence of sample magnetization on the magnetoresistivity tensor ρ̂ and are an alternative to the method based upon an analysis of the temperature derivative of the sample resistance (Novák et al., 2008). These methods can be applied even when standard SQUID magnetometers are difficult or impossible to use – for example for extremely small samples or in the case of experiments performed at very specific physical conditions, e.g. at high hydrostatic pressure inside the clamp cell. We show that the use of the so called Arrott plot prepared with the use of high magnetic field isotherms ρxx(H0),ρxy(H0) (H0 – external magnetic field) may lead to substantial (of the order of 10 K) divergence of the obtained TC values depending on the assumptions which are necessary to make in this case and depending on the direction of a magnetic anisotropy easy axis. We also propose a number of ways how to obtain, basing on low magnetic field isotherms ρxx(H0),ρxy(H0), clear and characteristic features which are closely related to the ferromagnetic–paramagnetic phase transition.

Physical modeling of polymer-electrolyte membrane fuel cells: Understanding water management and impedance spectra

A transient 2D physical continuum-level model for analyzing polymer electrolyte membrane fuel cell (PEMFC) performance is developed and implemented into the new numerical framework NEOPARD-X. The model incorporates non-isothermal, compositional multiphase flow in both electrodes coupled to transport of water, protons and dissolved gaseous species in the polymer electrolyte membrane (PEM). Ionic and electrical charge transport is considered and a detailed model for the oxygen reduction reaction (ORR) combined with models for platinum oxide formation and oxygen transport in the ionomer thin-films of the catalyst layers (CLs) is applied. The model is validated by performance curves and impedance spectroscopic experiments, performed under various operating conditions, with a single set of parameters and used to study water management in co- and counter-flow operation. Based on electrochemical impedance spectra (EIS) simulations, the physical processes which govern the PEMFC performance are analyzed in detail. It is concluded that the contribution of diffusion through the porous electrodes to the overall cell impedance is minor, but concentration gradients along the channel have a strong impact. Inductive phenomena at low frequencies are identified from physics-based modeling. Induction is caused by humidity dependent ionomer properties and platinum oxide formation on the catalyst surface.

Tunable Optical and Electrical Transport Properties of Size- and Temperature-Controlled Polymorph MoS2 Nanocrystals

The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail. The metallic 1T phase NCs were prepared by the Li+ intercalation–deintercalation exfoliation techniques followed by prolonged sonication. The effect of ex situ thermal annealing on MoS2 polymorphs and their transformation from the 1T to 2H phase has been extensively monitored by the X-ray photoelectron, Raman, and optical absorption spectroscopy techniques. Electrical conductivity measurements have also been carried out to probe the phase transition of the synthesized NCs. The temperature-dependent (10–350 K) electrical charge transport properties of variable-sized NCs have been investigated to probe the scaling of conductivity and activation energy with size, which are yet to be reported experimentally. The charge transport mechanisms through the NC assembly for different temperature regions have been modeled and it is observed that the electron trans...

Swelling kinetics and electrical charge transport in PEDOT:PSS thin films exposed to water vapor

We report the swelling kinetics and evolution of the electrical charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films subjected to water vapor. Polymer films swell by the diffusion of water vapor and are found to undergo structural relaxations. Upon exposure to water vapor, primarily the hygroscopic PSS shell, which surrounds the conducting PEDOT-rich cores, takes up water vapor and subsequently swells. We found that the degree of swelling largely depends on the PEDOT to PSS ratio. Swelling driven microscopic rearrangement of the conducting PEDOT-rich cores in the PSS matrix strongly influences the electrical charge transport of the polymer film. Swelling induced increase as well as decrease of electrical resistance are observed in polymer films having different PEDOT to PSS ratio. This anomalous charge transport behavior in PEDOT:PSS films is reconciled by taking into account the contrasting swelling behavior of the PSS and the conducting PEDOT-rich cores leading to spatial segregation of PSS in films with PSS as a minority phase and by a net increase in mean separation between conducting PEDOT-rich cores for films having abundance of PSS.

Electrical Transport Signature of the Magnetic Fluctuation-Structure Relation in α-RuCl3 Nanoflakes.

The small gap semiconductor α-RuCl3 has emerged as a promising candidate for quantum spin liquid materials. Thus far, Raman spectroscopy, neutron scattering, and magnetization measurements have provided valuable hints for collective spin behavior in α-RuCl3 bulk crystals. However, the goal of implementing α-RuCl3 into spintronic devices would strongly benefit from the possibility of electrically probing these phenomena. To address this, we first investigated nanoflakes of α-RuCl3 by Raman spectroscopy and observed similar behavior as in the case of the bulk material, including the signatures of possible fractionalized excitations. In complementary experiments, we investigated the electrical charge transport properties of individual α-RuCl3 nanoflakes in the temperature range between 120 and 290 K. The observed temperature-dependent electrical resistivity is consistent with variable range hopping behavior and exhibits a transition at about 180 K, close to the onset temperature observed in our Raman measurements. In conjunction with the established relation between structure and magnetism in the bulk, we interpret this transition to coincide with the emergence of fractionalized excitations due to the Kitaev interactions in the nanoflakes. Compared to the bulk samples, the transition temperature of the underlying structural change is larger in the nanoflakes. This difference is tentatively attributed to the dimensionality of the nanoflakes as well as the formation of stacking faults during mechanical exfoliation. The demonstrated devices open up novel perspectives toward manipulating the Kitaev-phase in α-RuCl3 via electrical means.

Terahertz magnonics: Feasibility of using terahertz magnons for information processing

An immediate need of information technology is designing fast, small and low-loss devices. One of the ways to design such devices is using the bosonic quasiparticles, such as magnons, for information transfer/processing. This is the main idea behind the field of magnonics. When a magnon propagates through a magnetic medium, no electrical charge transport is involved and therefore no energy losses, creating Joule heating, occur. This is the most important advantage of using magnons for information transfer. Moreover the mutual conversion between magnons and the other carriers e.g. electrons, photons and plasmons shall open new opportunities to realize tunable multifunctional devices. Magnons cover a very wide range of frequency, from sub-gigahertz up to a few hundreds of terahertz. The magnon frequency has an important impact on the performance of magnon-based devices (the larger the excitation frequency, the faster the magnons). This means that the use of high-frequency (terahertz) magnons would provide a great opportunity for the design of ultrafast devices. However, up to now the focus in magnonics has been on the low-frequency gigahertz magnons. Here we discuss the feasibility of using terahertz magnons for application in magnonic devices. We shall bring the concept of terahertz magnonics into discussion. We discuss how the recently discovered phenomena in the field of terahertz magnons may inspire ideas for designing new magnonic devices. We further introduce methods to tune the fundamental properties of terahertz magnons, e.g. their eigenfrequency and lifetime.

Charge splitters and charge transport junctions based on guanine quadruplexes.

Self-assembling circuit elements, such as current splitters or combiners at the molecular scale, require the design of building blocks with three or more terminals. A promising material for such building blocks is DNA, wherein multiple strands can self-assemble into multi-ended junctions, and nucleobase stacks can transport charge over long distances. However, nucleobase stacking is often disrupted at junction points, hindering electric charge transport between the two terminals of the junction. Here, we show that a guanine-quadruplex (G4) motif can be used as a connector element for a multi-ended DNA junction. By attaching specific terminal groups to the motif, we demonstrate that charges can enter the structure from one terminal at one end of a three-way G4 motif, and can exit from one of two terminals at the other end with minimal carrier transport attenuation. Moreover, we study four-way G4 junction structures by performing theoretical calculations to assist in the design and optimization of these connectors.

Investigating the influence of electrode Miller indices alteration on electronic transport in thiophene-based molecular junctions.

Electrical charge transport through thiophene-dithiol-based molecular wires attached to gold electrodes with three different types of crystallographic orientations (<1,1,1>, <1,1,0> and <1,0,1 >) was investigated. Electron transport in the systems under consideration was evaluated systematically by analyzing current values, transmission spectrum, projected device density of states and zero bias orbital analysis utilizing density functional theory in conjunction with non-equilibrium Green's function. Investigations proved that tuning of conductance in nano-molecular junctions is possible through different electrode orientations. As the HOMO-LUMO gap in the <1,1,0 > oriented thiophene dithiol junction is drastically less than that of the other configurations under consideration, the <1,1,0 > configuration exhibited superior constructive conductance in comparison to other junction orientations. This provided us with ideas for designing pioneering hetero-cyclic nano-scale electronics devices. Also, <1,1,0 > has been found to show negative differential conductance behavior above +2.6V and below -2.6V, and hence has potential applications in oscillating and switching circuits.

Aerosols and seismo-ionosphere coupling: A review

The role of atmosphere aerosols in the global electric circuit, particularly during earthquakes preparation periods, is discussed in this review paper. Aerosols participate in production and transport of electric charges as well as in clouds formation. Satellite imagery shows increased aerosol optical depth over the tectonic faults and formation of the anomalous clouds aligned with the faults shortly before the earthquake shocks. At the same time variations of the ionospheric electric field and total electron content (TEC) are observed. We assume that the vertical electric current is generated over the fault due to the separation and vertical transport of charges with different masses and polarities. This charges the ionosphere positively relative to the Earth in the same way as the thunderstorm currents do. The resulting electric field in the ionosphere drives F2-layer plasma via the electromagnetic [E→×B→] drift and decreases or increases electron density depending on the configuration of the electric field, thus, creating observed negative or positive TEC disturbances. The important role of the electric dynamo effect in these processes is underlined.

Reinforcing the Built‐In Field for Efficient Charge Collection in Polymer Solar Cells

AbstractThe collection efficiency of photogenerated charges in polymer solar cells (PSCs) is strongly influenced by the built‐in field (Ein) that develops across the photoactive materials. Here, by investigating the Ein‐development regimes in PSCs by introducing two types of interlayers, electric dipole layers (EDLs) and charge transport layers (CTLs), the device architecture is optimized to result in a larger Ein. By incorporating a pair of EDLs on both sides of the photoactive layer, the Ein is modulated by shifting the vacuum energy at each metal–semiconductor interface, providing a larger Ein than that in conventional PSCs using typical CTLs, such as metal oxides and/or conducting polymers. These devices using paired EDLs exhibit an average PCE of 9.8%, which far surpasses the average PCE of ≈8.5% for paired CTLs.

A new organic compound of 2-(2,2-diphenylethenyl)anthracene (DPEA) showing simultaneous electrical charge transport property and AIE optical characteristics

DPEA shows good charge carrier mobility and AIE efficient luminescence.