Increase In Open Voltage Research Articles

Coupled Electrochemical-Thermo-Stress Analysis for Methanol-Fueled Solid Oxide Fuel Cells

A three-dimensional model of a methanol-fueled solid oxide fuel cell (SOFC) that sufficiently considers the rib structure to understand the effects of operating conditions on a methanol-fueled SOFC is developed. The model considers the coupling of electrochemical reactions, mass transport, heat transfer, and thermal stress. The overall performance of the methanol-fueled SOFC is investigated under the influence of operating voltage, steam-to-carbon ratio, and porosity. The results indicate that as the operating voltage increases, the overall anode switches from exothermic to endothermic, and the stress of the anode first decreases and then increases. As the steam-to-carbon ratio increases, the overall temperature of the anode decreases. However, when the steam-to-carbon ratio is less than 1, the temperature at the inlet is lower than the ambient temperature. Meanwhile, the first principal stress on the anode increases as the steam-to-carbon ratio increases. Increasing the porosity reduces the length of the three-phase boundary, thereby decreasing the current density and the overall temperature and thermal stress on the anode. This study revealed the effects of operating conditions on the methanol-fueled SOFC, especially on the rib, and contribute to controlling the operant conditions for SOFC fueled by a mixture of steam and methanol.

Origin of operating voltage increase in deep UV light-emitting diodes with ITO/Al reflector

In an attempt to further elucidate the operating voltage increase in a vertical UV-C LED with p-electrode composed of transparent conducting Sn-doped indium oxide (ITO)/Al reflector, the interface formation between ITO and Al thin film was studied by using scanning transmission electron microscopy in combination with electron energy loss spectroscopy. It was confirmed that the oxidized layer was formed at the interface of the ITO/Al electrode in accordance with the thermal annealing. It was found that not only the thickness of oxide formation grew with the increased annealing temperature, the content of oxygen also increased. Moreover, it was also ascertained that the prolonged annealing time at high temperature induced the indium diffusion into the Al-oxide layer.

Coupling Effect of Electron Irradiation and Operating Voltage on the Deep Dielectric Charging Characteristics of Solar Array Drive Assembly

A deep dielectric charging model of a solar array drive assembly (SADA) under the coupling effect of electron irradiation and operating voltage is established based on the bipolar charge transport process. The distribution of space charge, potential and electric field in SADA are simulated and analyzed under three different operating conditions, i.e., flux model for internal charging (FLUMIC) spectrum electron irradiation acting alone; operating voltage acting alone; electron irradiation and operating voltage coupled. The effects of electron flux enhancement, electron direction, and operating voltage on charging properties are studied. Based on these investigations, discharge risk and location are pointed out. In the case of electrons incident from the end surface acting alone, much more space charge will accumulate than that from the columnar surface, and the maximum electric field will be above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> V/m when the flux enhancement exceeds 100. When the operating voltage acts alone, the maximum electric field will not exceed 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> V/m as long as the operating voltage is below 8772 V. Under typical spectrum, the maximum electric field under coupling effect increases linearly as the operating voltage increases. When the electron flux enhancement increases to 100 and 1000, the electron irradiation dominates the deep charging characteristic. This provides theoretical guidance for the methods of discharge containment in SADAs.

"Positive Incentive" Approach To Enhance the Operational Stability of Quantum Dot-Based Light-Emitting Diodes.

Balanced charge injection promises high efficiency of quantum dot-based light-emitting diodes (QD-LEDs). The most widely used approach to realize charge injection balance impedes the injection rate of the dominant charge carrier with energetic barriers. However, these approaches often accompany unwanted outcomes (e.g., the increase in operation voltage) that sacrifice the operational stability of devices. Herein, a "positive incentive" approach is proposed to enhance the efficiency and the operational stability of QD-LEDs. Specifically, the supply of hole, an inferior carrier than its counterpart, is facilitated by adopting a thin fullerene (C60) interlayer at the interface between the hole injection layer (MoOX) and hole transport layer (4,4'-bis(9-carbazolyl)-1,1'-biphenyl). The C60 interlayer boosts the hole current by eliminating the universal energy barrier, lowers the operation voltage of QD-LEDs, and enhances the charge balance in the QD emissive layer within the working device. Consequently, QD-LEDs benefitting from the adoption of the C60 interlayer exhibit significantly enhanced device efficiency and operation stability. Grounded on the quantitative assessment of the charge injection imbalance within the QD emissive layer, the impact of electrical parameters of QD-LEDs on their optoelectronic performance and operational stability is also discussed.

Amorphous Indium-Gallium-Zinc-Oxide Transistor Used Ion Gel Dielectric for Low Driving Voltage and Easy Processing

Transistors using organic materials are attracting huge attention in the market today due to their low cost and possibility of solution process. Moreover, since the organic materials used in transistors are intrinsically flexible, they are believed to be essential elements for the realization of flexible devices such as stretchable displays, rollable displays, and wearable devices. In most cases, however, because of the low dielectric constant of organic materials, the operating voltage of the transistor is very high when the organic materials are used as the gate dielectric layer. When the operating voltage of the transistor is high, two serious problems can occur. First, the high operating voltage can cause increased power consumption of the transistor. It is a fatal drawback that reduces the applicability to wireless device applications because in the case of wireless electronic equipment, lifetime of battery is limited. Also, as the operating voltage increases, additional gate driver integrated circuits are required to control the driving voltage. This leads to the increased process complexity, and reduces the price competitiveness of the device. Therefore, it can be said that the possibility of mass production and functionality of transistors using organic materials will increase by solving these two problems. As a result, various studies have been focused on solving these problems by reducing the thickness of dielectric or choosing unique organic material with high dielectric constant [2] [3]. In order to solve above problems, we proposed the transistors based on ion gel dielectric, which has high capacitance. Ion gel refers to the substance in which ionic liquid is trapped in the polymer matrix [1]. When the voltage is applied to the electrode, the ion gel forms an electric double layer (EDL) at the interface with the electrode. Since EDL forms a capacitor with thickness of several nanometers, the ion gel has a very high capacitance above 1μF/cm2 [1]. From the capacitance-frequency curve measurement of Metal-Ion gel insulator-Metal capacitor (MIM), it was confirmed that the capacitance of ion gel increased as the amount of ionic liquid in ion gel increased. In addition, MIM showed the capacitance around 1μF/cm2 even if the ionic liquid content was very small. In the case of the transistor structure, the transistor was easy to fabricate because the gate electrode was deposited on the same substrate as the source-drain electrode. The active layer had the width of 20μm and the length of 160μm. The gate leakage current of the transistor increased with increasing the amount of ionic liquid in the ion gel. By optimizing the amount of ionic liquid to control gate leakage current, the ionic liquid was chosen 3% relative to the polymer in terms of mass and the gate leakage current was reduced to several tens of picoampere. The transfer characteristic represented that turn-on-voltage was near -1V and on-off ratio was 3.93·104 when the gate voltage was swept from -4V to 4V. This result shows that the transistor can operate with well-behaved electrical property even at small driving voltage near 4V. The transistor made by simple fabrication with ion gel dielectric showed outstanding electrical characteristics with low driving voltage around 4V and this result will pave the way for the next generation of flexible devices. [1] Wang, H., Wang, Z., Yang, J., Xu, C., Zhang, Q., & Peng, Z. (2018). Ionic Gels and Their Applications in Stretchable Electronics. Macromolecular rapid communications, 39(16), 1800246. [2] Yoon, M. H., Yan, H., Facchetti, A., & Marks, T. J. (2005). Low-voltage organic field-effect transistors and inverters enabled by ultrathin cross-linked polymers as gate dielectrics. Journal of the American Chemical Society, 127(29), 10388-10395. [3] Hung, C. C., Wu, H. C., Chiu, Y. C., Tung, S. H., & Chen, W. C. (2016). Crosslinkable high dielectric constant polymer dielectrics for low voltage organic field‐effect transistor memory devices. Journal of Polymer Science Part A: Polymer Chemistry, 54(19), 3224-3236. This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609) Figure 1

Effect of Electron Irradiation and Operating Voltage on the Deep Dielectric Charging Characteristics of Polyimide

Space Solar Power Station (SSPS) will operate in geosynchronous orbit. Its operating environment is rather complicated, including space plasma, energetic particles radiation, extreme temperature, and cosmic rays. To reduce power loss, SSPS has to apply high-voltage (HV) power generation and transmission technique. Deep dielectric charging will be a major problem in the design of insulation structure in SSPS. In this paper, the hopping conductivity of polyimide was studied. Hopping distance and hopping barrier height were obtained from surface potential decay analysis. Second, a deep dielectric charging model of polyimide under FLUMIC spectrum electron irradiation was established, in which the hopping conductivity parameters were applied. The effects of electron flux enhancement and operating voltage on the charging characteristics were discussed in four cases, i.e., A) suspended-HV; B) HV-suspended; C) grounded-HV; and D) HV-grounded. When the electron flux enhancement increases, the influence on case A is much larger than the other three cases. When the operating voltage increases, the influence on cases A and B is small, whereas the influence on cases C and D increases fast. Thus, when designing the SSPS, the effect of operating voltage and electron flux enhancement on the insulation system must be carefully considered.

Electron Affinity Control of Amorphous Oxide Semiconductors and Its Applicability to Organic Electronics

AbstractEnergy level matching is a key factor for realizing efficient optoelectronics, such as organic light‐emitting diodes (OLEDs). Transition metal oxides (TMOs) are widely used as hole‐injection layers (HILs) because of their large electron affinities, corresponding to conduction band minimum levels (ECBM). However, the poor electrical properties and chemical instabilities of TMOs restrict device performance and fabrication processes. Conversely, amorphous oxide semiconductors (AOSs) exhibit superior electrical properties and chemical stability but cannot be applied to HILs because of their small electron affinities. In this study, a novel method is proposed to tune the electron affinity of conventional AOSs and an example material design of AOSs for HILs. The electron affinity deepening phenomenon that is attributable to the higher oxidation states of transition metal cations achieves a very deep ECBM of 5.7 eV for the proposed amorphous In–Mo–O (a‐IMO). Furthermore, an a‐IMO exhibits a high mobility of ≈1 cm2 V−1 s−1 and high chemical stability against various solvents. As a result, it is demonstrated that a very thick a‐IMO layer is applicable to OLEDs as an HIL with no operating‐voltage increase. This study provides a new approach for efficient organic optoelectronics by tuning the electron affinity of AOSs.

Phenomena at Porous Carbon Electrode/Saturated Electrolyte Interface in Electrochemical Capacitors

Development of electrochemical capacitors focuses today on the improvement of their energy density (or specific energy) and cyclability. The energy density might be improved by capacitance enhancement and operating voltage increase. To some extent, the relation between capacitance and voltage and their impact on the energy output reflects an interfacial character of charge storage mechanisms in capacitors; capacitance is intimately linked with the electrode material whereas the operating voltage is governed by the electrolyte applied. Of course, they cannot be considered separately, since the final performance is always a combination of various factors (e.g., electrode porosity – electrolyte viscosity, wettability, etc.). In case of capacitance-related issues, it appears that activated carbons with their well-developed surface area and suitable porosity, recently reached the limit of purely capacitive storage and further progress in this field require another insight into the electrostatic charge accumulation. That approach enforces intensive research on the novel architectures of the electrode materials. On the other hand, the electrolyte-related issues create an interesting pathway for investigations aiming at maximum voltage increase and energy density enhancement. Undoubtedly, organic media (based on acetonitrile or propylene carbonate) and ionic liquids are the optimal electrolytic solutions in terms of electrochemical stability. However, their impact on the environment and user safety is quite often questioned. Water-based electrolytes seem to be an interesting alternative, but the major objection against their commercialization concerns their low electrochemical stability governed by water decomposition voltage. Although the neutral electrolytes might demonstrate a significant decomposition overpotential once applied to porous carbon electrodes and thus provide the operating voltage up to 1.8 V, the performance of water-based capacitors is still not satisfactory enough for a broad application. Since the significant disadvantage of water-based solutions is attributed to the solvent decomposition, in our recent study, we decided to decrease the amount of water in aqueous electrolytes to the minimal level. Following the approach presented by the Belanger group, we have formulated the series of electrolytes with water: salt ratio up to 50% (either by mass, volume or molarity). Since the primary study has been done on LiTFSI salt, in our study, we have focused on the conventional inorganic salts, based on the well-known anions like NO3 - or SO4 2-. It appeared that these formulations demonstrated various correlations with concentration, conductivity and decomposition potentials. Moreover, their viscoelastic properties perfectly correlated with electrochemical performance, mostly with charge propagation. Surprisingly, the role of cation appeared to be crucial, essentially for capacitance values. The paper will discuss the correlation between the pore size distribution of carbon electrodes, electrolyte viscosity, and capacitor performance. Elucidation of the energy/power characteristics as well as cyclability at various temperatures will give a novel insight into ‘water’-based electrolytes. Spectroelectrochemical measurements will support the physicochemical data (e.g., operando Raman spectroscopy), unraveling (or trying to unravel) the mysteries at carbon electrode/highly concentrated electrolyte interface. Acknowledgements European Research Council is acknowledged for financial support within the Starting Grant project (GA 759603) under European Unions’ Horizon 2020 research and innovation programme.

Ultra thin silicon wafer slicing using wire-EDM for solar cell application

The ever increasing demand of silicon solar cells in PV industry calls for minimizing the material loses (kerf) during Si wafer slicing. The currently employed abrasive slicing methods are capable of slicing ~350μm thick wafers. Recent research efforts have put forward wire-EDM as a potential method. This work presents an extensive experimentation to understand the parametric effects that give ultra thin wafer while minimizing the kerf-loss and maximizing the slicing rate. Ultra thin wafers of size 130–150μm were fabricated using wire-EDM while controlling the input energy by avoiding wire or wafer breakage. The kerf-loss was reduced by ~50% (121μm) while maintaining a high slicing rate of 1.05mm/min. A typical wafer and associated kerf profiles showed a wider thickness at the entry and exit than the middle of the wafer. An increase in open voltage and a decrease in servo voltage increase the slicing rate in frontal direction and cause a decrease in slicing rate in the lateral direction, consequently decreasing the kerf-loss.

Surface Roughness Research of Piezoelectric Self-adaptive Micro-EDM

Based on inverse piezoelectric effect, a new micro-EDM method called piezoelectric self-adaptive micro-EDM was proposed in this paper. The tool structure and working principle of this new method is simple and it is different from the conventional micro-EDM system. A piezoelectric actuator with high resolution and high response frequency was employed as the micro driven mechanism in this system. Because of the special structure, this new technology could regulate the discharge gap depending on discharge condition and even could realize self-elimination of short circuits during machining process.Working principle and processing of the new method was introduced, and the distinct machining characteristics were analyzed in this paper. Based on orthogonal experimental design and S/N analysis method, the effects of machining parameters such as open voltage, capacitance, current-limiting resistance, spindle rotational speed and initial feed speed on surface roughness (SR) were researched. The experimental results show that the open voltage and capacitance were the main influence factors. With the increase of open voltage and capacitance, the SR rapidly increased.

Fabrication of Micro/Nanoelectrode Using Focused-ion-Beam Chemical Vapor Deposition, and its Application to Micro-ECDM

Micro electro-chemical discharge machining (micro-ECDM) is a key technology in the precision manufacturing. The electrode, which is the key element for achieving a high-precision ECDM, attracts the interests of many researchers. However, regarding the fabrication techniques of the electrode, the achievable precisions are still resting on micrometer scale, the processes are not simple, or electrodes with high complex three-dimensional (3D) shapes are difficult to be obtained. Basing on such a background, in this work, focused-ion-beam chemical vapor deposition (FIB-CVD) which is an effective tool in fabricating 3D micro/nanostructure for achieving various micro/nanoelectromechanical devices (MEMS/NEMS), was first time introduced to fabricate micro/nanoelectrode and furthermore ECDM processes using FIB-CVD-fabricated electrodes could be attempted. Differing to the conventional metallic electrodes, micro/nanoelectrodes in the material of amorphous platinum (Pt) were obtained. Three electrodes with the same diameter 10μm and height 28μm were fabricated on the polished tip surfaces of stainless needles, and consequently applied to micro-ECDM with varied low open voltages 10, 12.5 and 15V. Microholes were fabricated on the 304 austenitic stainless steel. The hole diameter and the unilateral discharge gap were found to be almost linearly increasing with the increase of open voltage. In case of using the open voltage 10V, the unilateral discharge gap was found to be 6.6μm. Moreover, fabrications of pillar, horn, corner, array types of 3D nanoelectrodes were demonstrated. These results indicate the potentialities of 3D micro-ECDM using complex 3D micro/nanoelectrode, and a deeper study into the science of ECDM/EDM phenomenon at submicron scale.

Computational analysis of IR-SOFC: Thermodynamic, electrochemical process and flow configuration dependency

Fuel cell is the most promising candidate for future energy system as it is energy efficient and utilizes a renewable fuel. However, there still is need for upgradation and a fully paired computational fluid dynamics (CFD) approach (based on the finite volume method). A 3D simulation of planar Solid Oxide Fuel Cell (SOFC) having internal reforming has been carried out by examining the geometry of a single cell which comprises of a fuel channel, air channel, cathode, anode, and electrolyte layers. Cell parameters such as temperature, current density, concentration of fuel species, electro chemical processes and voltage are analyzed for both co-flow and counter-flow configurations. The obtained finite volume results were validated with experimental and numerical results showing good agreement within acceptable limits. An increase in inlet temperature resulted in significant increase in cell operating voltage as well as cell power density and the cell efficiency increases by 12–13%. It is also observed that the co-flow configuration showed more uniform current density and temperature distributions as well as smaller temperature gradients. Therefore, co-flow configuration is superior as it experiences low thermal stresses in PEN structure and enhanced cell life.

Characteristics of induced discharge on a polymer insulator surface under electro-wetting conditions

In this paper, laboratory experiments were conducted to investigate the precipitation characteristics and induced surface discharge on polymer insulator specimens under electro-wetting conditions simulated by artificial fog-wetting conditions at different operating voltages. Based on image processing methods, quantitative analysis was carried out on precipitation parameters to obtain the deposited amount, relative distribution area, statistics of droplet size distribution and distributed droplet box dimension in relation with the duration of precipitations and operating voltages. Also, the onset voltage of surface discharge and flashover voltage were measured for the purpose of establishing the influence of the obtained precipitation parameters on the surface discharge. It was found that the precipitation process is in a dynamic balance between fog deposition and droplet evaporation, and is significantly affected by the type and level of operating voltage. Operating voltage increase results in a relatively lower deposit amount and distributing area of water droplets. There are further reduced when the precipitating duration is increased at higher voltage level. The smallsize ( 2 mm) droplets increase with increasing AC operating voltage, but do not appear significantly under high ±DC operating voltage. When precipitation duration is increased, box dimension of droplet distribution under nonand low- operating voltage shows tendency to increase, but tendency to decrease under high operating voltage. Finally, a relationship between the onset voltage of surface discharge, the flashover voltage and the droplet distribution parameters was established, showing that the increasing precipitation regularity of water droplets on the polymer insulator specimen surface can induce more likely occurrence of surface discharge, even flashover.

Overcoming the “retention vs. voltage” trade-off in nonvolatile organic memory: Ag nanoparticles covered with dipolar self-assembled monolayers as robust charge storage nodes

Organic non-volatile memory devices with significantly enhanced retention are explored with C60 thin-film transistors containing silver nanoparticles (Ag-NPs) within gate dielectrics as charge storage nodes. Dipolar self-assembled monolayers covering Ag-NPs effectively prevent stored charges from being lost by providing an additional energy threshold for back-tunneling process. This enables long retention even with ultrathin tunneling dielectric layers, providing a simple means to realize long retention without causing an excessive increase in operation voltage.

The operating voltage behavior of green fluorescent organic light emitting diode with blue common layer structure during laser imaging process

The blue common layer (BCL) approach has become very useful methodology to reduce the process steps which may increase the production yield of active matrix organic light emitting diode (AMOLED) fabrication. From this approach, we can reduce one step (from 5 to 4) for fine metal masking (FMM) method as a current mass-production technology to form a common thick-microcavity structure. Moreover, we can reduce the patterning steps from 5 to 2 if we use a laser induced thermal imaging (LITI) technology for the same structure. Nevertheless, we still prefer to apply the FMM technology for the mass production because there are lots of problematic issues on LITI process such as an operating voltage increase (by 0.5–1.0V), efficiency drop (by ∼10%), shorter lifetime, etc. Here, we report about the fundamental causes of these problems during LITI processes.

Thermal stability of N-polar n-type Ohmic contact for GaN-based light emitting diode on Si substrate

Thermal stability of N-polar n-type Ohmic contact for GaN light emitting diode (LED) on Si substrate was investigated. Al/Ti/Au were deposited as the contacts on the N-polar n-type GaN with and without AlN buffer layer on the surface, respectively, and both contacts exhibited Ohmic behaviors. The samples with AlN showed excellent Ohmic contact thermal stability when annealed below 700 °C, while the samples without AlN experienced serious degradation on electrical properties after being annealed in the temperature range of 250–600 °C. After the process of aging at 30 mA (155 A/cm 2) and room temperature for 1000 h, operating voltage increase less than 0.05 V for LEDs with AlN but more than 0.45 V for LEDs without AlN. Therefore, we conclude that the existence of AlN buffer layer is a key of forming high stable Ohmic contact for GaN-based vertical structure LED on Si substrate.

Influence of molecular weight of PEO on performance of quasi-solid-state dye-sensitized solar cells

Quasi-solid-state dye-sensitized solar cells (DSCs) were prepared using gel electrolyte gelated by poly(ethylene-oxide) (PEO) with different molecular weight. With the increase in PEO molecular weight, the short circuit current densities (Jsc) increase significantly, which is related to the increase in I− diffusion due to the increase in free volume of gel electrolyte. However, only a slight increase in open voltage (Voc) is observed, which is explained by the decrease in dark current arising from the reduction of triiodide (I3−) by conduction band electrons.

Optimization of assembly clamping pressure on performance of proton-exchange membrane fuel cells

The compression induced by the assembly of proton-exchange membrane (PEM) fuel cells causes partial deformation of the gas-diffusion layers (GDLs) and affects the characteristics of the porous media and, consequently, influences the performance of PEM fuel cells. The objective of the present study is: (1) to develop a three-dimensional model to investigate the effect of assembly clamping pressure on the GDL properties and thus on the performance of PEM fuel cells, and (2) to determine the optimum clamping pressures when the cell is operated under different operating voltages. The optimum clamping pressures under different operating voltages are explored by using a global searching method, namely, the simultaneous perturbation stochastic algorithm (SPSA) method. The simulation results indicate that a clamping pressure of 1 or 1.5 MPa improves the fuel cell performance when the cell is operated under high operating voltages, and causes the cell performance to decrease when it is operated under low-voltage conditions. The optimum clamping pressures increase when the operating voltage increases.

Single-Event Effects on Combinational Logic Circuits Operating at Ultra-Low Power

The minimum laser energy required to cause sustained harmonic oscillations in a 201-stage ring oscillator varies little with increasing power supply voltage when operating in the subthreshold region. These small changes in threshold laser energy in the subthreshold region suggest that the cross-section curves as a function of power supply voltage remain relatively constant. Simulations show that the minimum pulsewidth required to generate higher-order oscillations decreases as operating voltage increases. Single-event transients are wider when circuits are operating in the subthreshold region than when operating at the nominal power supply voltages. Narrower single-event transients result for strikes on PMOS transistors as compared to those for strikes on NMOS devices in the subthreshold region; the opposite is observed when circuits are operating at the nominal power supply voltages.

Optical Output Power Enhancement of Gallium Nitride Light-Emitting Diodes with Microlens Array on p-GaN Layer and Wave-Patterned Sidewalls by Gas-Assisted Focused Ion Beam Etching

By using iodine gas-assisted focused ion beam etching, a gallium nitride light-emitting diode with wave-patterned sidewalls and a microlens array on a p-GaN layer was successfully fabricated without an additional lithography process. The gallium nitride light-emitting diode after gas-assisted focused ion beam etching on the sidewalls and p-GaN layer shows 24% enhancement from the sidewalls and 53% from the top surface in optical output power with an operating voltage increase of 0.06 V at 20 mA.