Time-dependent Current Response Research Articles

Switching Response in Organic Electrochemical Transistors by Ionic Diffusion and Electronic Transport.

The switching response in organic electrochemical transistors (OECT) is a basic effect in which a transient current occurs in response to a voltage perturbation. This phenomenon has an important impact on different aspects of the application of OECT, such as the equilibration times, the hysteresis dependence on scan rates, and the synaptic properties for neuromorphic applications. Here we establish a model that unites vertical ion diffusion and horizontal electronic transport for the analysis of the time-dependent current response of OECTs. We use a combination of tools consisting of a physical analytical model; advanced 2D drift-diffusion simulation; and the experimental measurement of a poly(3-hexylthiophene) (P3HT) OECT. We show the reduction of the general model to simple time-dependent equations for the average ionic/hole concentration inside the organic film, which produces a Bernards-Malliaras conservation equation coupled with a diffusion equation. We provide a basic classification of the transient response to a voltage pulse, and the correspondent hysteresis effects of the transfer curves. The shape of transients is basically related to the main control phenomenon, either the vertical diffusion of ions during doping and dedoping, or the equilibration of electronic current along the channel length.

Nonlinear electron transport in miniband superlattice driven by dual terahertz fields and a transverse magnetic field

Time-dependent electron current response of GaAs-based miniband superlattice under dual ac electric fields and a magnetic field is studied using balance equation approach. The space charge-induced self-consistent electric field is taken into account in the model. The miniband superlattice operates in the diffusive regime without electric field domain formation. Electron current displays very complicated oscillating behavior with the influence of external fields. The effect of dissipation on nonlinear electron transport is carefully studied based on Poincaré bifurcation diagram and power spectrum. The exhibition of complicate nonlinear oscillation in superlattice is attributed to the nonlinearity induced by self-consistent field and interaction between external radiation and internal cooperative oscillating mode relative to Bloch oscillation and cyclotron oscillation.

Convolutive modeling of cyclic voltammetry, AC-voltammetry, sine wave voltammetry and impedance spectroscopy with interfacial CPE behaviour and uncompensated ohmic resistances: A Unified Theory

The constant phase element (CPE) is a well-established circuit component for the frequency domain simulation of electrochemical reactions at disperse and heterogeneous electrodes. Computing the time-dependent current response of voltammetric experiments with interfacial CPE behaviour is, however, not a straightforward task, owing to the lack of analytical time domain solutions of the convoluted capacitive current. In this paper, a universal framework for the convolutive treatment of cyclic voltammetry (CV), alternating current cyclic voltammetry (ACCV), (large) sine wave voltammetry (LSWV) and electrochemical impedance spectroscopy (EIS) in presence of interfacial CPEs and uncompensated Ohmic resistances is presented. A combination of numerical and analytical inverse Laplace transformation techniques allows the accurate assessment of the non-ideal capacitive behaviour, which can be used in turn for classical convolution algorithms. The convolutive approach bears the advantage that different electrode geometries with any kind of spatial boundary conditions as well as first order homogeneous chemical reactions, coupled to the charge transfer step, can be implemented readily. This offers an exceptionally large degree of flexibility in the simulations for the electrochemical system at hand.

Modeling Current Transients in a Reciprocal Motion Tribocorrosion Experiment

Tribocorrosion of passivating metals is a dynamic phenomenon causing degradation of materials by a combination of mechanical wear and electrochemical dissolution. The mechanical action typically produces a local removal of particles, metal as well as oxides, resulting in an exposure of the metal surface and is followed by a repassivation process, of which the time-dependent current response is a direct measure. Kinetics for interface-limited film growth were used to find an analytical expression for the current transients, including the conductivity of the electrolyte as well as the contribution from the confined geometry within the mechanical contact. The solution gave a good experimental fit to a series of experiments with a range of electrolyte conductivities, and also to correlate well with values obtained using electrochemical impedance spectroscopy. Parameters from the rubbing experiment were used to calculate curves that compared well with passivation transients recorded on a bare metal surface for the same electrolyte series. The evaluation procedure made it possible to assess the relative resistance contributions from the growing passive film, the confined mechanical contact geometry, and the electrolyte.

Lock-on physics in semi-insulating GaAs: Combination of trap-to-band impact ionization, moving electric fields and photon recycling

The time-dependent photoconductive current response of semi-insulating GaAs is probed based on one-dimensional simulations, with a focus on the lock-on phenomenon. Our results capture most of the experimental observations. It is shown that trap-to-band impact ionization fuels local field enhancements, and photon recycling also plays an important role in pushing the device towards lock-on above a 3.5 kV/cm threshold field. The results compare well with actual data in terms of the magnitudes, the rise times, and the oscillatory behavior seen at higher currents. Moving multiple domains are predicted, and the response shown depended on the location of the photoexcitation spot relative to the electrodes.

Growth of hierarchical strontium incorporated cadmium sulphide for possible application in optical and electronic devices

In this work, we have incorporated strontium (Sr) into cadmium sulphide (CdS) complex to explore its effect on optical and electrical properties. In this regard Sr composited CdS and a bared CdS were prepared by hydrothermal technique. X-ray diffraction pattern of the materials were analyzed to obtain their crystalline structure, size and internal lattice strain. The compositional details were obtained from SEM-EDAX study. The optical properties of the films of synthesized materials were analyzed. The dependency of refractive index on incident photon energy was studied in detail for each material. The dispersion energy related parameters, dielectric relaxation constant and optical conductivity followed by dispersion of the refractive index were determined and discussed by employing the single electronic oscillator model. From wavelength dependent refractive index data, the plasma frequency, ratio of free charge carrier concentration to effective mass, lattice dielectric constant and high frequency dielectric constant were calculated to determine the discrepancy of the compounds. The third order optical susceptibility was analyzed to check their potential ability of performance in optical limiter and optical switching device application. The electrical characterization was performed by measuring the current–voltage characteristics of structure ITO/CdS:Sr/Al and ITO/CdS/Al. The rectifying I–V showed improved On/Off ratio under dark (25.03 and 19.98) and light (35.51 and 25.09) for ITO/CdS:Sr/Al compared to ITO/CdS/Al. The dark and photoconductivity of the film were also determined for both the films. The time dependent current response was analyzed at constant bias voltage ±1 V.

Separating the Faradaic and Non-Faradaic Charge Storage Mechanisms in Electrochemical Capacitors Using Step Potential Electrochemical Spectroscopy

Electrochemical capacitors are valuable energy storage devices due to their high capacitance and high specific power. Electrochemical capacitors have a unique performance characteristics due to their ability to charge and discharge much faster than batteries, but with longer discharge times than conventional capacitors. However, these devices are currently limited by their low specific energy. In order to increase the potential applications of electrochemical capacitors, their performance needs to be optimised for their application. Electrochemical capacitor performance is largely influenced by the electrode material, as it determines both the nature and magnitude of the charge storage processes occurring within the electrode, such as double layer capacitance (non-faradaic) and redox reactions (faradaic; pseudo-capacitance). Understanding the mechanism by which an electrode material stores charge is fundamental to the improvement of electrochemical capacitors. However, conventional methods for evaluating performance, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. This is particularly important in pseudo-capacitors, which have both faradaic and non-faradaic processes contributing to charge storage, and the separation of these processes is crucial to understanding their performance. In this work, step potential electrochemical spectroscopy (SPECS) has been applied to electrochemical capacitors as a performance analysis method to determine the charge storage contributions from different processes. The SPECS experiment involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time-dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. SPECS has been used to examine the performance of a range of electrode-electrolyte conditions and has been successful in differentiating how the relative contributions to capacitance vary depending on the electrode material. The most commonly used electrode materials (activated carbon, manganese dioxide and ruthenium dioxide) have been examined. These materials were chosen because they each exhibit different charge storage mechanisms which can be differentiated by the SPECS method.

Parameters Affecting I-V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer.

Current-voltage (I-V) characteristics of CH3NH3PbI3 perovskite solar cells are studied using a time-dependent current response with stepwise sweeping of the bias voltage. Compared with the crystalline Si solar cell showing time-independent current at a given bias voltage, the perovskite solar cells exhibit time-dependent current response. The current increases with time and becomes steady at forward scan from short-circuit to open-circuit, whereas it is decayed and saturated with time at reverse scan from open-circuit to short-circuit. Time-dependent current response eventually leads to I-V hysteresis depending on the scan direction and the scan rate. Crystal size of CH3NH3PbI3 and the mesoporous TiO2 (mp-TiO2) film are found to influence I-V hysteresis, where the I-V hysteresis is alleviated as crystal size increases and in the presence of mp-TiO2. The capacitance observed at low frequency (0.1 to 1 Hz), associated with dipole polarization, tends to diminish as size of perovskite and mp-TiO2 layer thickness increases, which suggests that the origin of hysteresis correlates to the capacitive characteristic of CH3NH3PbI3 and the degree of hysteresis depends strongly on perovskite crystal size and mesoporous TiO2 layer.

Wave-packet representation of leads for efficient simulations of time-dependent electronic transport

We present theoretical foundations and numerical demonstration of an efficient method for performing time-dependent many-electron simulations for electronic transport. The method employs the concept of stroboscopic wavepacket basis for the description of electrons' dynamics in the semi-infinite leads.The rest of the system can be treated using common propagation schemes for finite electronic systems. We use the implementation of our method to study the time-dependent current response in armchair graphene nano-ribbons (AGNR) with sizes up to 800 atoms described within tight-binding approximation. The character of the time-dependent current is studied for different magnitudes of the bias voltage, variable width and length of AGNRs, different positions of the current measurement, and for full and reduced coupling of the AGNRs to the electrodes.

Time-dependent chronoamperometric response of dual inlaid disk electrodes

Dual-disk electrodes allow for novel electrochemical investigations, particularly when used in generator–collector mode. However analytical understanding of how the current response of each individual electrode is affected by the presence of the neighbouring electrode is limited. In this article, we derive an approximate analytical solution for the time-dependent chronoamperometric current response of a pair of electrodes, each subjected to a different initial potential step. The solution is valid when the centre-to-centre distance between the disks is much larger than the disk radii, and for times such that the diffusion layers generated at each electrode have started to interact. It accommodates quasi-reversible reactions and unequal diffusion coefficients of the oxidant and the reductant. The generator–collector system is a specific case, and we deduce simple expressions for the time-dependent current and steady-state collection efficiencies at each electrode. The analytical solution facilitates analysis of shielding and feedback effects, and shows the explicit dependence of the currents on the underlying electrochemical parameters.

Comparison of simulations to experiment for a detailed analysis of space-charge-limited transient current measurements in organic semiconductors

Space-charge-limited current transients (also referred as time resolved dark injection) is an attractive technique for mobility measurements in low mobility materials, particularly the organic semiconductors. Transients are generally analyzed in terms of the Many-Rakavy theory, which is an approximate analytical solution of the time-dependent drift-diffusion problem after application of a voltage step. In this contribution, we perform full time-dependent drift-diffusion simulation and compare simulated and experimental transients measured on a sample of triaryl-amine based electroactive dendrimer (experimental conditions: $\ensuremath{\mu}\ensuremath{\approx}{10}^{\ensuremath{-}5}$ cm${}^{2}/\text{(Vs)}$, $L=300$ nm, $E<{10}^{5}$ V/cm). We have found that the Many-Rakavy theory is indeed valid for estimating the mobility value, but it fails to predict quantitatively the time-dependent current response. In order to obtain a good agreement in between simulation and experiment, trapping and quasi-ohmic contact models were needed to be taken into account. In the case of the studied electroactive dendrimer, the experimental results were apparently consistent with the constant mobility Many-Rakavy theory, but with this model, a large uncertainty of $20%$ was found for the mobility value. We show that this uncertainty can be significantly reduced to $10%$ if a field-dependent mobility is taken into account in the framework of the extended Gaussian disorder model. Finally, we demonstrate that this fitting procedure between simulated and experimental transient responses also permits to unambiguously provide the values of the contact barrier, the trap concentration, the trap depth in addition to that of the mobility of carriers.

Bias voltage dependent electrochemical impedance spectroscopy of p- and n-type silicon substrates

For a comprehensive interpretation of the electrochemical processes occurring at a semiconductor/electrolyte interface, an accurate value of the flatband potential versus the reference electrode is needed together with the current/voltage response. The present work is mainly devoted to the analysis of impedance diagrams recorded with n- and p-type silicon electrodes in the dark, in a pure diluted HF solution under different bias values, the potential range being chosen so as the dc current is maintained at a low value, e.g. less than 5 μA cm −2, and even less than 10 nA cm −2 in the case of p-type Si under cathodic polarization. Two series of impedance diagrams were recorded. Firstly, the bias value was settled in the potential range where the depletion layer was generated within the semiconductor substrate. In this condition, the high resistance of the space charge region and the resulting low value of the dc current permitted to obtain well-defined semicircles leading to the derivation of both R and C values equivalent to the depletion layer charge distribution. The method based on the analysis of the voltage dependent R-value was compared to the usual Mott–Schottky plot treatment, and proved to be efficient for the determination of a reliable value of the flatband potential versus the reference electrode. In the second series, the analysis of the impedance diagrams was focused on the range of potentials corresponding to the onset of an accumulation layer within the semiconductor. In the case of p-Si, this condition corresponds to the rapid anodic etching of the silicon substrate. At a particular value of the bias, an important induction loop was observed in addition to the usual capacitive behaviour. A Fourier transform treatment demonstrated that the impedance components were interrelated to the time dependent current response under constant bias potential. Then, simultaneous experiments based on chronoamperometry measurements suggested that the electrochemical processes involved in the reaction of Si substrate with HF solution was correlated to a two-step charge transfer mechanism. This interpretation is valid for both the inductive loop obtained in electrochemical impedance spectroscopy and the rise of current versus time observed in chronoamperometry.

Time-dependent partition-free approach in resonant tunneling systems

An extended Keldysh formalism, well suited to properly take into account the initial correlations, is used in order to deal with the time-dependent current response of a resonant tunneling system. We use a partition-free approach by Cini in which the whole system is in equilibrium before an external bias is switched on. No fictitious partitions are used. Despite a more involved formulation, this partition-free approach has many appealing features being much closer to what is experimentally done. In particular, besides the steady-state responses one can also calculate physical dynamical responses. In the noninteracting case we clarify under what circumstances a steady-state current develops and compare our result with the one obtained in the partitioned scheme. We prove a theorem of asymptotic equivalence between the two schemes for arbitrary time-dependent disturbances. We also show that the steady-state current is independent of the history of the external perturbation (memory-loss theorem). In the so-called wide-band limit an analytic result for the time-dependent current is obtained. In the interacting case we work out the lesser Green function in terms of the self-energy and we recover a well-known result in the long-time limit. In order to overcome the complications arising from a self-energy which is nonlocal in time we propose an exact nonequilibrium Green-function approach based on time-dependent density-functional theory. The equations are no more difficult than an ordinary mean-field treatment. We show how the scattering-state scheme by Lang follows from our formulation. An exact formula for the steady-state current of an arbitrary interacting resonant tunneling system is obtained. As an example the time-dependent current response is calculated in the random-phase approximation.

Time response modelling in submicron avalanche photodiodes

Conventional models of the time-dependent current response to an impulse of injected carriers in an avalanche photodiode assume that carriers travel always at their saturated drift velocities. A Monte Carlo model is used to show that, while this assumption is reasonable for a 1.0 µm avalanche region in GaAs, it significantly underestimates the avalanche speed in a submicron device at high electric fields. The disparity is identified as due to an enhancement in the velocities of carriers to ionisation, defined as the path length to ionisation divided by the time to traverse it. Using the mean values of these velocities in the conventional model produces good agreement with the Monte Carlo results. Comparison with a local ionisation model also suggests that thin APDs will operate more quickly than expected.

Simulated current response in avalanche photodiodes

The time dependent current response to an impulse of injected carriers is calculated for an avalanche photodiode using Monte Carlo simulation. For low electric fields and long avalanche regions the results agree with the conventional model, which assumes that carriers travel always with their saturated drift velocities. However, while diffusion remains unimportant, for high fields and short avalanche regions, the conventional model underestimates the device speed. Monte Carlo simulations show that the mean downstream average velocity of ionizing carriers is significantly enhanced at high electric fields and agreement is restored if we allow for this effect in the conventional model.

Voltage vs time dependent current response of MOSFET humidity gas sensor

Current response of a MOSFET humidity gas sensors is investigated, depending on the voltage applied (constant or pulsed) in a temperature interval of 10–50°C. A model, based on the variation Δϕ of the p–n junction potential barrier, is used for the explanation of the results. Δϕ is found to be strongly dependent on the mode of operation, on the pause/pulse duration and on the temperature in the range of relative humidity 10–90%.

Theory and Experiment for the Substrate Generation/Tip Collection Mode of the Scanning Electrochemical Microscope: Application as an Approach for Measuring the Diffusion Coefficient Ratio of a Redox Couple

A numerical model for the substrate generation/tip collection (SG/TC) mode of the scanning electrochemical microscope (SECM), with amperometric electrodes, is developed and examined experimentally. Under the conditions of interest, this mode involves the diffusion-limited electrolysis of a solution species, R, to produce O, at a macroscopic substrate (generator) electrode. As O diffuses away, a fraction of the species is intercepted by a tip (collector) ultramicroelectrode (UME), positioned above the substrate, where the reverse reaction occurs, leading to the formation and diffusional feedback of R to the substrate. The time-dependent tip current response is shown to be sensitive to the ratio of diffusion coefficients of the O/R couple. Moreover, when the long-time (steady-state) tip current, determined under these conditions, is combined with measurements of the diffusion-limited current for the electrolysis of R at a tip positioned far from the substrate, differences in the diffusion coefficients of th...

Scanning electrochemical microscopy Kinetics of chemical reactions following electron-transfer measured with the substrate-generation–tip-collection mode

The substrate-generation–tip-collection (SG–TC) mode of the scanning electrochemical microscope (SECM) is used as a new approach to investigate the kinetics of EC processes. Under the conditions of interest, a species O is generated at a macroscopic substrate (generator) electrode, with potential-step control, through the diffusion-limited electrolysis of a solution species R (E step). As O diffuses away from the generator, it undergoes a first order chemical reaction in solution (C step). A fraction of O is collected by electrolysis back to R at an externally biased ultramicroelectrode (UME), positioned directly over the substrate. This promotes the diffusional feedback of R to the substrate. Theory for the problem, relating the time-dependent tip current response to the rate constant for the C step and the tip–substrate electrode separation is developed numerically. Results of the calculations illustrate how the characteristic features of the tip current transients: peak current, peak time and post-half-peak time, depend on the kinetics of the C step and the inter-electrode separation. It is shown that both the kinetics and tip–substrate separation can be determined independently from a single transient by simply measuring the peak current and peak time. The theoretical results are validated experimentally through model studies of the oxidative deamination of N,N,-dimethyl-p-phenylenediamine (DMPPD) in aqueous solution at high pH. The effective second-order rate constant for the deamination step is in excellent agreement with values measured by alternative methods.

Current suppression and harmonic generation by intense terahertz fields in semiconductor superlattices

The time-dependent current response of superlattice miniband conduction to a strong submillimeter wave electric field of varying frequency and strength is investigated at lattice temperature T=77 and 300 K by means of the three-dimensional balance-equation method with accurate microscopic treatment of impurity and phonon scatterings. The terahertz-radiation-induced current suppression and the generated third harmonic power as functions of the input power of the high-frequency field predicted theoretically are in reasonable agreement with recent experimental findings in heavily doped GaAs superlattices.

Time-dependent current response of semiconductor multi-barriers

The transient current response of n-doped GaAs—AlGaAs multiple barrier systems is studied within the time-dependent Schrödinger equation. Open boundary conditions are designed to minimize spurious reflections at the simulation boundaries. A local time-dependent Hartree-exchange-correlation-potential is used to self-consistently account for the inter-carrier Coulomb interaction. Application to the resonant-tunneling double barrier reveals current oscillations which are due to charging and discharging of the well. Current oscillations are also obtained in the transient for multi-barrier systems