Voltage Scan Rate Research Articles

Comprehensive Performance Calibration Guidance for Perovskites and Other Emerging Solar Cells

AbstractEmerging photovoltaic (PV) technologies (e.g., organic, perovskite, and solution processed quantum dot) have attracted remarkable attention with the rapid growth of their efficiencies, and their transition toward commercialization. Accurate and reliable efficiency measurements of these PV technologies are crucial, yet much more complicated than for conventional PV technologies due to the former's pronounced dynamic responses to changes in measurement conditions (e.g., current–voltage (I–V) scan rate and preconditioning) and their susceptibility to degradation. Adjustments to the measurement procedures are therefore necessary so that a reproducible “steady state” is reached during measurement. Furthermore, given the small size of many emerging cells, inappropriate device area definition and solar simulator setup can lead to measurement errors. Here, comprehensive efficiency calibration guidance is offered for emerging solar cells, including area measurement; spectral irradiance translation to standard test conditions; and steady‐state electrical performance. The necessity of reporting steady‐state efficiency is justified with a statistical performance comparison between conventional and steady‐state I–V scans over hundreds of cells the group has received globally for efficiency certifications. The procedures described here do not require specialized measurement instrumentation; what matters most are changes to the measurement protocols. These described changes aim to enable better comparisons between reported efficiencies.

Thermal contact resistance evaluation of a thermoelectric system by means of three I-V curves

To achieve a suitable performance in a thermoelectric (TE) device it is important to minimize the thermal contact resistances between the device external surfaces and the heat exchangers of the system (heat source and heat sink). Despite its relevance, there are not many methods available for the evaluation of the thermal contact resistance, and the existing ones typically employ complex setups. Here, we present a new method to determine the thermal contact resistance of a TE device thermally contacted to a heat sink and a heat source. The method is based on performing three current-voltage (I-V) curves at different system conditions under a small temperature difference. First, an I-V curve with a high voltage scan rate, which avoids the variation of the initial temperature difference, provides the ohmic resistance. A second I-V curve performed with a constant input heat power (or the device suspended) provides the TE resistance. Finally, a third I-V curve with a constant temperature difference between the heat exchangers allows obtaining the thermal contact resistance. Using this method, a thermal contact resistivity value of 3.57 × 10−4 m2KW−1 was obtained for a commercial Bi-Te TE module contacted with a heat source and a heat sink using thermal grease as thermal contact interface material, which is in good agreement with reported values. The new method is highly advantageous, since neither involves complex setups nor requires the measurement of heat fluxes. Moreover, it measures directly under operating conditions for small temperature differences.

Artificial Sub-60 Millivolts/Decade Switching in a Metal-Insulator-Metal-Insulator-Semiconductor Transistor without a Ferroelectric Component.

Negative capacitance field-effect transistors (NC-FETs) have attracted wide interest as promising candidates for steep-slope devices, and sub-60 millivolts/decade (mV/decade) switching has been demonstrated in NC-FETs with various device structures and material systems. However, the detailed mechanisms of the observed steep-slope switching in some of these experiments are under intense debate. Here we show that sub-60 mV/decade switching can be observed in a WS2 transistor with a metal-insulator-metal-insulator-semiconductor (MIMIS) structure without any ferroelectric component. This structure resembles an NC-FET with internal gate, except that the ferroelectric layer is replaced by a leaky dielectric layer. Through simulations of the charging dynamics during the device characterization using a resistor-capacitor network model, we show that the observed steep-slope switching in our "ferroelectric-free" transistors can be attributed to the internal gate voltage response to the chosen varying gate voltage scan rates. We further show that a constant gate voltage scan rate can also lead to transient sub-60 mV/decade switching in an MIMIS structure with voltage-dependent internal gate capacitance. Our results indicate that the observation of sub-60 mV/decade switching alone is not sufficient evidence for the successful demonstration of a true steep-slope switching device and that experimentalists need to critically assess their measurement setups to avoid measurement-related artifacts.

Variables of the Analytical Electrochemical Data Acquisition for Boron Subphthalocyanines

AbstractThe electrochemical behavior of boron subphthalocyanines (BsubPcs) has been investigated using cyclic voltammetry in the presence of various solvents, internal standards, supporting electrolytes, working electrodes, and sweep voltage scan rates. We have focused on halogenated BsubPcs (Cl−Cl6BsubPc, Cl−Cl12BsubPc, F−F6BsubPc, F−F12BsubPc) and a non‐halogenated baseline (Cl−BsubPc). Halogenated BsubPcs are of interest to the field due to their promising advances as organic electronic materials for applications based on redox or electron transfer processes. We had pre‐established a standard operating procedure (SOP) for electrochemical data acquisition, but it was timely to consider alternative variables, their impact on the electrochemical data and re‐establish an alternative SOP. We observed modest shifts (up to 49 mV) of the BsubPc redox potentials when changing the internal standard, working electrode and/or the electrolyte concentration. In scan rate range between 20 and 250 mV s−1, the peak (ir)reversibility for F−F6BsubPc and F−F12BsubPc remained unchanged and the electron transfers at the surface electrode remained diffusion‐controlled.

The self-assembled, atomically defined, flexible and highly tunable bilayered Au/L-cysteine/Cu(II/I) junctions capable of voltage-gated coherent multiple electron/hole exchange

Contemporary 2D spintronics (spin-based electronics) is a highly interdisciplinary field with numerous elaborated branches, mostly focusing on atomically thin, layered nano-junctions functionalized within ‘dry’ or ‘wet’ cells/cubicles/circuits. The charge carriers’ spin-implicated aspects emerge throughout, albeit the most nanotechnologically promising issue (implying the information and energy transfer/storage aspects) among them, is perhaps the uniquely complex yet robust and rather universal phenomenon of a hybrid inter- and intra-layer Bose–Einstein-like (BE) condensation. However, this issue is still not sufficiently explored, especially, in the framework of the ‘wet’ spintronic domain. Thus, searching for new types of bilayer junctions, and testing of charge/spin allocation and flow within respective nano-devices, is a primary task of current 2D spintronics. In this paper we report on the novel effort towards an extension of the voltage-gated ‘wet’ 2D spintronics enabled through the self-assembling of bilayered Au/L-cysteine/Cu(II/I) junctions, and their rigorous, yet preliminary current-voltage testing towards the hidden collective spin-related manifestations. Our experimental efforts led to a cascade of rare, uniquely combined observations encompassing the temperature induced, directly visible (irreversibly shape-shifting) single-stage transformation of a CV signal (the natural signature of a voltage-gated interlayer Faradaic process). The ultra-thin shape of the resulting CV signal (unavoidably emerging under certain ‘standard’ conditions), turned to be readily explainable by the Laviron’s general statistical formalism, as due to a multi-charge exchange process with the number of transferred electrons/holes ranging within 4 to 16 (per single elementary act) or even out of this range, being extra tunable via the experimental variables. Furthermore, cathodic and anodic peaks of the ‘new’ signal are moderately separated from each other and have nearly similar shapes. Additional experiments with a variation of the voltage scan rate, demonstrated the exceptional, very regular decaying of a number of simultaneously transferred electrons/holes (extracted from the peak-shape analysis) on the voltage scan rate; although the former parameters shows some fluctuational scatter in time, and/or from sample to sample. The subsequent multi-theory-based analysis of a whole scope of obtained voltammetric data, allowed for a preliminary conjecturing of the formation of a hybrid BE-like dipolar superfluid encompassing electron/hole-hosting clusters emerging within the bilayer junction. The specific electron/hole ratio within the layers is switchable (gated) by the interlayer potential (voltage) bias. The clusters’ dimensions, charge distribution and dynamic exchange are reasonably fluctuative and essentially tunable through the applied potential (i.e. the voltage-gating). New experiments are on their way, revealing unlimited future promises of our current endeavor.

High-rate supercapacitor using magnetically aligned graphene

Graphene has been widely used as an electrode material for supercapacitors. However, parallelly-stacked graphene layers often result in inefficient ion diffusion and electron transfers that usually reduce the rate capability of a supercapacitor. In this study, reduced graphene oxide (rGO) and poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) composite films were prepared by a solvent evaporation method using PEDOT:PSS as the binder to fix aligned graphene for its good conductivity and strong π-π stacking interactions with the graphene sheets. Analyses using scanning electron microscopy (SEM), nitrogen adsorption-desorption, and small-angle X-ray scattering show that the graphene sheets were well aligned when a magnetic field was applied, though they were oriented randomly without the magnetic field. As a capacitor electrode material, the aligned rGO shows a specific capacitance of 169 F g−1 with a capacitance retention of about 70% at a current density of 50 A g−1 and its cyclic voltammetry (CV) loops maintained a rectangular shape at a voltage scan rate of 2 V s−1. The aligned rGO electrode can help break through the limitations of traditional supercapacitors and increase significantly their charge/discharge rate and power density.

Ultrahigh‐Rate Supercapacitor Based on Carbon Nano‐Onion/Graphene Hybrid Structure toward Compact Alternating Current Filter

AbstractTraditional filtering capacitors suffer from low volumetric energy density (Ev), which hinders their integration in miniaturized electronics and their high‐frequency response ability to alternating current (AC). Here, a supercapacitor based on a carbon nano‐onion (CNO)‐graphene hybrid structure concurrently possessing high Ev and ultrahigh rate capability is reported for AC filtering. The hybrid structure is synthesized on etched nickel foil using preformed monodispersed nanocrystals through a chemical vapor deposition method. The supercapacitor device shows capacitive behavior independent of voltage scan rate up to 5000 V s−1 in both aqueous and organic electrolytes, which represents a record high among the reported filtering capacitors to date. It also exhibits a high Ev of 14.9 F V2 cm−3 at 120 Hz and 28.8 mWh cm−3 at a current density of 0.25 mA cm−2. Moreover, the supercapacitor is capable of filtering AC with different waveforms at high frequencies into direct current. The hybrid structure holds promise for applications in compact filtering units. The ultrafast ion transportation performance is enabled by the highly positive surface curvature of CNOs, hierarchical interconnection of CNO particles, and the covalent interfacial bonding between CNOs and graphene.

Sputter deposited crystalline V2O5, WO3 and WO3/V2O5 multi-layers for optical and electrochemical applications

Crystalline V2O5, WO3, WO3/V2O5 (bi-layer) and WO3/V2O5/WO3/V2O5 (multi-layer) thin film samples were deposited on fused silica and fluorinated tin oxide coated microscopy glass substrates by reactive dc magnetron sputtering. Structure-property correlation studies were carried out by X-ray diffraction, Raman and X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy, atomic force microscopy, UV–vis spectroscopy and cyclic voltammetry techniques. V2O5 and WO3 layers have orthorhombic and monoclinic crystal structures respectively and their phase formation was confirmed by XPS. Cyclic voltammetry studies were used to determine the electrochemical properties. H+ diffusion coefficient decreases with increase in the voltage scan rate in all samples except for the bi-layer. At each scan rate the highest diffusion coefficient was achieved for multi-layer sample and the values were 7.4 × 10−14 cm2s−1, 2.4 × 10−14 cm2s−1, 1.4 × 10−14 cm2s−1 at scan rates of 10 mVs−1, 100 mVs−1 and 500 mVs−1, respectively. The multi-layer sample has large areal capacitance of 1165 mFcm−2. The study shows that WO3/V2O5/WO3/V2O5 multi-layer sample has significantly enhanced electrochemical properties as compared to the single layer V2O5 and WO3 films.

Space-charge-limited electron and hole currents in hybrid organic-inorganic perovskites

Hybrid organic-inorganic perovskites are promising materials for the application in solar cells and light-emitting diodes. However, the basic current-voltage behavior for electrons and holes is still poorly understood in these semiconductors due to their mixed electronic-ionic character. Here, we present the analysis of space-charge-limited electron and hole currents in the archetypical perovskite methyl ammonium lead iodide (MAPbI3). We demonstrate that the frequency dependence of the permittivity plays a crucial role in the analysis of space-charge-limited currents and their dependence on voltage scan rate and temperature. Using a mixed electronic-ionic device model based on experimentally determined parameters, the current-voltage characteristics of single-carrier devices are accurately reproduced. Our results reveal that in our solution processed MAPbI3 thin films transport of electrons dominates over holes. Furthermore, we show that the direction of the hysteresis in the current-voltage characteristics provides a fingerprint for the sign of the dominant moving ionic species.

Investigation of hysteresis in hole transport layer free metal halide perovskites cells under dark conditions

Recent research is a testimony to the fact that perovskite material based solar cells are most efficient since they exhibit high power conversion efficiency and low cost of fabrication. Various perovskite materials display hysteresis in their current-voltage characteristic which accounts for memory behaviour. In this paper, we demonstrate efficient non-volatile memory devices based on hybrid organic-inorganic perovskite (CH3NH3PbI3) as a resistive switching layer on a Glass/Indium Tin Oxide (ITO) substrate. Our perovskite solar cells have been developed over the fully solution processed electron transport layer (ETL) which is a combination of SnO2 and mesoporous (m)-TiO2 scaffold layers. Hysteresis behaviour was observed in the current-voltage analysis achieving high ratio of ON & OFF current under dark and ambient conditions. Proposed perovskite-based Glass/ITO/SnO2/m-TiO2/CH3NH3PbI3/Au device has a hole transport layer (HTL) free structure, which is mainly responsible for a large ratio of ON & OFF current. The presence of voids in the scaffold m-TiO2 layer are also accountable for increasing electron/hole path length which escalates the recombination rate at the surface of the ETL/perovskite interface resulting in large hysteresis in the I–V curve. This memristor device operates at a low energy due to SnO2 layer’s higher electron mobility and wide energy band gap. Our experimental results also show the dependency of voltage scan range & rate of scanning on the hysteresis behaviour in dark conditions. This memristive behaviour of the proposed device depicts drift in hysteresis loop with respect to the number of cycles, which would have a significant impact in neuromorphic applications. Moreover, due to the identical fabrication process of the proposed perovskite-based memristor device and perovskite solar cells, this device could be integrated inside a photovoltaic array to work as a power-on-chip device, where generation and computation could be possible on the same substrate for memory and neuromorphic applications.

Novel diverse-structured h-WO3 nanoflake arrays as electrode materials for high performance supercapacitors

Hexagonal tungsten trioxide (h-WO3) nano-materials can be widely used in many fields of optics, electrics and chemistry. In this work, three types of h-WO3 (single crystal, polycrystal and hierarchical) nanoflake arrays (WNFs) are synthesized through a simple hydrothermal method. The morphologies, crystalline degree and exposed faces of these WNFs can be controlled with simple adjustments of reaction parameters. The relationships between the crystal structures and supercapacitive properties of these WNFs are systematically investigated and the results show that: the single crystal WNFs with a fast electron transmitting channel exhibits a high rate performance at high voltage scan rate, while the polycrystal and hierarchical WNFs with more oxygen vacancies and ion storage spaces have relatively high specific capacitances at low scan rate. These WNFs are used to fabricate supercapacitors, which show specific energy density as high as 88.2 W h kg−1 at a power density of 400 W kg−1. It is concluded that the electrode materials should have single structure facilitating the electrons transfer at current collector side and a porous structure for ion storage at the electrolyte side.

Исследование плотности интерфейсных состояний на границах раздела диэлектрик / In-=SUB=-0.52-=/SUB=-Al-=SUB=-0.48-=/SUB=-As

The capacitance-voltage (С-V) characteristics of Au/Al2O3/In0.52Al0.48As and Au/SiO2/In0.52Al0.48As metal-insulator-semiconductor (MIS) structures were studied. It was found that the measurement of the C-V characteristics of MIS structures based on InAlAs by the fragment method, in contrast to the standard technique at a constant bias voltage scan rate, significantly reduces the hysteresis effect and allows to obtain stationary curves. It was shown that the fast interface states density calculated by the Terman method from such C-V characteristics varies slightly along the InAlAs band gap and amounts to (3-6) 1011 eV-1cm-2 and (1-3)·1011 eV-1cm-2 for MIS structures with Al2O3 and SiO2, respectively.

Oriented Graphenes from Plasma-Reformed Coconut Oil for Supercapacitor Electrodes.

The utilization of vertical graphene nanosheet (VGN) electrodes for energy storage in supercapacitors has long been desired yet remains challenging, mostly because of insufficient control of nanosheet stacking, density, surface functionality, and reactivity. Here, we report a single-step, scalable, and environment-friendly plasma-assisted process for the fabrication of densely packed yet accessible surfaces of forested VGNs (F-VGNs) using coconut oil as precursor. The morphology of F-VGNs could be controlled from a continuous thick structure to a hierarchical, cauliflower-like structure that was accessible by the electrolyte ions. The surface of individual F-VGNs was slightly oxygenated, while their interior remained oxygen-free. The fabricated thick (>10 μm) F-VGN electrodes presented specific capacitance up to 312 F/g at a voltage scan rate of 10 mV/s and 148 F/g at 500 mV/s with >99% retention after 1000 cycles. This versatile approach suggests realistic opportunities for further improvements, potentially leading to the integration of F-VGN electrodes in next-generation energy storage devices.

Modeling and simulation of a LaCoO3 Nanofibers /CNT electrode for supercapacitor application

The supercapacitors are expected to represent the future of energy storage unit equipment due to their good performance on both the energy density and the power density. This paper is the first presentation of the mathematical form of the supercapacitor, which consists of LaCoO3 nanofibers electrodes and carbon nanotube electrode (CNT) as well as the electrical and concentration channels, the dynamic model used to determine the impact of the thickness of CNT electrode on the performance of supercapacitors in COMSOL Multiphysics. The results showed that when changing the voltage scan rate from 0.001, 0.01, 0.1 and 1.0 V/s, current and voltage increased. But the time to change the current density is reduced by a voltage scan rate of 0.001 V/s. The range of voltage changes from -0.4 to 0.4 V in the period of 1, 500 s and when adding the position of the length, the concentration will gradually decrease. The present model is conclusively verified to be effective and successfully provides a methodology to optimize the cell size in various applications.

Effect of varying the gate voltage scan rate in a MoS2/ferroelectric polymer field effect transistor

A ferroelectric FET using MoS2 and PVDF-TrFE was fabricated, and the effects of varying the gate voltage scan rate from 200 mV/s to 4 mV/s investigated. Charge mobility, sub-threshold voltage swing and the memory window width depended on the scan rate. Prior to switching on, a negative trans-conductance was observed for all scan rates, followed by a rapid increase in the channel current to the on state. A model based on nucleation of ferroelectric domains and unrestricted domain growth was used to explain these results. By lowering the scan rate, the performance of polymer based FE-FET’s can be improved.

Preface to the special issue on electrochemical energy storage discussed at the NZEE conference 2018 in Czech Republic

This paper describes fabrication and characterization of a hybrid nanostructure comprised of multi-walled carbon nanotubes (MWCNTs) on reduced graphene oxide (r-GO) via layer by layer (LBL) assembly. This structure was prepared by vacuum filtration and heat-treated at 500 °C. The morphology of the sample was investigated by field emission electron microscopy (FE-SEM). The crystalline structure and chemical composition were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Cyclic voltammetry (CV) curve of MWCNTs/r-GO hybrid film appeared close to that of an ideal double layer capacitor, in quasi-rectangular shape. The current density gradually increased with increasing voltage scanning rate, exceeding 20 A/g at a scan rate of 500 mV/s. Accordingly, the specific capacitance decreased with the increase of the voltage scanning rate as expected. Electrochemical performance of this hybrid film indicates that it can further improve supercapacitor electrodes.

SILAR-synthesized CdO thin films for improved supercapacitive, photocatalytic and LPG-sensing performance

In the presented work, the influence of cationic precursor molarity on supercapacitive, photocatalytic and LPG-sensing performance of CdO thin films synthesized by successive ionic layer adsorption and reaction method has been investigated. An attempt has been made to correlate the observed properties with structure and morphology of the deposited films. Structural characterization using XRD indicates an increase in crystallite size from ~ 12.3 to ~ 25.6 nm with increasing molarity of Cd2+ precursor solution from 0.1 M to 0.3 M, beyond which there is a reverse tendency. Band gap energy decreases from 2.22 to 2.04 eV with increasing molarity of Cd2+ precursor solution from 0.1 M to 0.3 M. For further increase of molarity to 0.4 M, the band gap energy increases to 2.08 eV. Thin film CdO electrode deposited from 0.3 M cadmium nitrate solution shows maximum specific capacitance of 228 Fg−1 at a voltage scan rate of 2 mVs−1 measured from cyclic voltammetry (CV) measurements. Charging–discharging measurements show maximum specific capacitance of 220 Fg−1 at a current density of 1 Ag−1. Capacitance retentivity of 96.5% after 5000 cycles was exhibited by the above said thin film electrode. Films deposited from 0.3 M Cd2+ content precursor solution also show improved photocatalytic activity as well as gas-sensing property in presence of LPG. The highest degradation of methyl orange dye of 76.2% under exposure to visible light of 200 W energy and maximum sensitivity of 78% in presence of 1000 ppm LPG at an operating temperature of 260 °C were observed.

MWCNTs/r-GO hybrid films fabricated by layer by layer assembly for supercapacitor electrodes

Abstract This paper describes fabrication and characterization of a hybrid nanostructure comprised of multi-walled carbon nanotubes (MWCNTs) on reduced graphene oxide (r-GO) via layer by layer (LBL) assembly. This structure was prepared by vacuum filtration and heat-treated at 500 °C. The morphology of the sample was investigated by field emission electron microscopy (FE-SEM). The crystalline structure and chemical composition were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Cyclic voltammetry (CV) curve of MWCNTs/r-GO hybrid film appeared close to that of an ideal double layer capacitor, in quasi-rectangular shape. The current density gradually increased with increasing voltage scanning rate, exceeding 20 A/g at a scan rate of 500 mV/s. Accordingly, the specific capacitance decreased with the increase of the voltage scanning rate as expected. Electrochemical performance of this hybrid film indicates that it can further improve supercapacitor electrodes.

Nanoparticle electrocatalysis: Unscrambling illusory inhibition and catalysis

Voltammetry is ubiquitously used in the characterisation of new materials for electrocatalysis. It often sets at least the starting point of any analysis aiming to explore whether electrodes decorated with nano- or other particles feature advanced catalytic behaviour over their bare equivalents. Such analysis bears a substantial and entirely unexpected level of complexity as the mass transport inside the porous layer formed by the particles has a dramatic impact on the measurement. It is shown even for entirely inactive, non-conducting, and non-adsorbing particles, the voltammetry of any electrochemically reversible reaction necessarily varies both between seeming catalysis and apparent irreversible kinetics merely depending on the voltage scan rate. This effect arises from the superposition of thin layer effects and locally enhanced diffusive fluxes inside the porous material with the latter being reminiscent of convergent diffusion and leading to apparent electrochemically-irreversible reactions. While the relevance of thin layer effects in porous materials has been realised before, the seeming irreversible reactions and the understanding of them as result of the internal structure of the modifying layer are, to our knowledge, an entirely new finding. The presented results are counter-intuitive and yet essential for all voltammetric analysis of catalysis at nanostructured interfaces and at porous electrode materials.

Adsorption processes coupled with mass transport at macro-electrodes: New insights from simulation

A general simulation model is developed for voltammetry of species adsorbed from solution involving coupled mass transport and adsorption which distinguishes the relative contributions from the adsorption kinetics, the mass transport and electron transfer kinetics of the reactants and products. The model assumes only the adsorbed species are electrochemically active. To provide meaningful insights, we focus on three limiting cases in which the fate of the formed product differs. In Case One, the product remains on the surface and does not desorb; in Case Two, the product desorbs slowly into solution; in Case Three, the product desorbs rapidly into solution. In each case, the effect of independent parameters such as the adsorption rate constants, the electron transfer kinetics and the voltage scan rate are investigated. New insights from this simulation include a steady state voltage-current response for a macro-electrode when rate determining step becomes ‘bottlenecked’ by the slow desorption of the product. Furthermore, when the rate of adsorption and desorption of the reaction and product are fast compared to the mass transport and are not rate determining, the simulated current for the redox process via the adsorption pathway can be ca. 2.3 times higher to that when electron transfer occurs via the solution phase.