Internal Potential Distribution Research Articles

Insight into the Impact of Electron Drift Trajectory on Charge Collection in Silicon Drift Detector

The internal electric field distribution is one key design consideration, which affects the charge collection efficiency in silicon drift detectors (SDDs). The internal electrostatic potential distributions along SDD front and back surfaces, which are determined by the applied voltages at cathode electrodes, define the final internal field distribution. Front-back bias coupling leads to the complexity of electrode structure design and voltage tuning. Device simulation is performed to investigate the performance of SDDs with varied bias voltages. When the cathode bias is −40 V with the first ring bias of −15 V and the outermost ring bias of −80 V, the detector is biased with a uniform electric field distribution, favorable electron drift trajectories. The simulation results provide new insight into the influence of internal electric field and electron drift trajectories on the charge collection efficiency. According to the analysis of simulation results, a 2000 × 2000 μm area concentric silicon drift detector was designed and fabricated. The electrical characteristics of the designed detectors were studied to show the validity of the proposed device design methodology. The internal electric field distribution and electron drift trajectories can be tuned to improve the charge collection efficiency.

Eliminating the Imbalanced Mobility Bottlenecks via Reshaping Internal Potential Distribution in Organic Photovoltaics.

The imbalanced carrier mobility remains a bottleneck for performance breakthrough in even those organic solar cells (OSCs) with recorded power conversion efficiencies (PCEs). Herein, a counter electrode doping strategy is proposed to reshape the internal potential distribution, which targets to extract the low mobility carriers at far end. Device simulations reveal that the key of this strategy is to partially dope the active layer with a certain depth, therefore it strengthens the electric field for low mobility carriers near counter electrode region while avoids zeroing the electric field near collection electrode region. Taking advantage of these, PCE enhancements are obtained from 15.4% to 16.2% and from 16.9% to 18.0%, respectively, via cathode p-doping and anode n-doping. Extending its application from opaque to semitransparent devices, the PCE of dilute cell rises from 10.5% to 12.1%, with a high light utilization efficiency (LUE) of 3.5%. The findings provide practical solutions to the core device physical problem in OSCs.

Research on discharge characteristics of GLA with surface pollution/icing

In operation of the AC power system, when the surface of gapped line arresters (GLA) is polluted, moist and iced, its internal electric field and potential distribution will be distorted. It will lead to the internal partial discharge and the significantly decrease of the power frequency discharge voltage, which seriously threatens the safety of the power system. Therefore, the influence on the body-gap voltage ratio of two typical structures of GLA under clean, polluted and iced external surface state need to be studied. And the finite-element method was used to calculate the electric field and potential distribution under different surface state. Based on the experiment of discharge characteristics of GLA under different conditions in artificial climate chamber, the electric field and potential distribution under the polluted and iced surface, the body-gap voltage ratio and the discharge characteristics of GLA were obtained, which was of great significance for the safety and stability of power system and the design of GLA.

A robust single device MOSFET series resistance extraction method considering horizontal-field-dependent mobility

A simple MOSFET series resistance extraction method using multiple drain current versus gate voltage curves of a single device is proposed, where mobility modulation by a horizontal electric field (i.e., weak velocity saturation) is taken into account. The method is validated using TCAD, where series resistance determined from internal potential distributions was used as a reliable reference. Measurement results were also obtained which further support the validity of the method.

Study on Structure Type and Operation Condition of UHVDC Wall Bushing

When the ±800kV UHVDC capacity is increased to 10000MW, the transmission current is 5kA and 6.25kA, which will put forward new requirements for the key technology of UHVDC through wall bushing: in UHV level, the key of through wall bushing lies in the research of basic technology of voltage sharing and field sharing. In the case of large current carrying capacity, the key lies in the basic technology research of current sharing and heat sharing. When the DC wall bushing, converter bushing and outgoing line device operate at the rated voltage, the central conductor is heated due to the carrying large DC current. Therefore, the temperature field distribution inside and at the tail of the bushing core is uneven, and the resistivity is the function of temperature, resulting in the change of resistivity with temperature and the change of potential and electric field distribution with resistivity during operation. Therefore, under the above high current operation conditions, in-depth key technology research is required for the design of bushing current carrying device, calculation of internal potential and electric field distribution, voltage sharing, field sharing, current sharing and heat sharing technology. The combined action of electrical and thermal stress will significantly affect the internal electric field distribution of converter transformer core and accelerate the aging rate of core composite insulation material, which introduces new research topic for the key technology of UHVDC converter bushing. The electric field and voltage distribution of UHVDC bushing are theoretically analyzed, the insulation structure is optimized from the aspects of material and structure, and the manufacturing process and the test technology are deeply and systematically studied.

Dynamically visualizing battery reactions by operando Kelvin probe force microscopy

Energy storage devices using electrochemical reactions have become an integral part of our daily lives, and further improvement of their performance is highly demanded. An important task for this purpose is to thoroughly understand the electrochemical processes governing their chemistry. Here we develop a method based on Kelvin probe force microscopy that enables dynamic visualization of changes in the internal potential distribution in an operating electrochemical device and use it to characterize an all-solid-state lithium ion battery. Observation of the cathode composite regions during a cyclic voltammetry operation reveals differences between the behavior of local electrochemical reactions in the charge and discharge processes. Based on careful inspection of the results, we show that the difference arises from a change in the state of an electronic conductive path network in the composite electrode. Our method provides new insights into the local electrochemical reactions during electrochemical operation of devices.

A generalised model for electro-osmotic flow in porous media

Purpose This study aims at developing a comprehensive model for the analysis of electro-osmotic flow (EOF) through a fluid-saturated porous medium. To fully understand and exploit a number of applications, such a model for EOF through porous media is essential. Design/methodology/approach The proposed model is based on a generalised set of governing equations used for modelling flow through fluid saturated porous media. These equations are modified to incorporate appropriate modifications to represent electro-osmosis (EO). The model is solved through the finite element method (FEM). The validity of the proposed numerical model is demonstrated by comparing the numerical results of internal potential and velocity distribution with corresponding analytical expressions. The model introduced is also used to carry out a sensitivity analysis of the main parameters that control EOF. Findings The analysis carried out confirms that EO in free channels without porous obstruction is effective only at small scales, as largely discussed in the available literature. Using porous media makes EO independent of the channel scale. Indeed, as the channel size increases, the presence of the charged porous medium is essential to induce fluid flow. Moreover, results demonstrate that flow is significantly affected by the characteristics of the porous medium, such as particle size, and by the zeta potential acting on the charged surfaces. Originality/value To the best of the authors’ knowledge, a comprehensive FEM model, based on the generalised equations to simulate EOF in porous media, is proposed here for the first time.

Electric Field Analysis of 35kV Line Arrester Under Different Environmental Conditions

In actual operations, the surface insulation strength of metal oxide line arrester can be thus decreased due to inevitably dirt and icing, and the flashover voltage can be significantly reduced, which threatens the operation safety of the entire transmission line. Therefore, in this paper, the internal and external electric field and potential distribution of a 35kV line arrester under operating voltage are studied quantificationally. ANSYS and COMSOL finite-element software was used to calculate the field and potential distribution of the arresters under clean, polluted and icing condition. Based on the analysis of the electric field distribution, the main influence factors of the electric field and potential distribution as well as the equivalent calculation method under different environmental conditions of the arrester are obtained, which can give help to the design and application of line arresters under different environmental conditions.

(Invited) Operando Characterization of Energy Conversion and Storage Devices Using Scanning Probe Microscopy

Energy conversion/storage devices, such as solar cells (SCs) and lithium ion batteries (LIBs), have been actively researched along with the growing interest in energy and environment issues. In order to further improve the performance of these devices, it is essential to have a deeper understanding of their working principles. To achieve this, the characterization of their physical and chemical properties under device operation conditions, so-called operando measurement, is thought to be highly effective. For example, in the case of SCs, important information related with photovoltaic conversion processes can be obtained by measuring the electrical potential, electronic states, and charge distribution in the vicinity of the excitation center under light irradiation conditions. In the case of LIBs, characterization of electrical potential and lithium ion distribution in the electrodes and electrode-electrolyte interfaces during charging and discharging processes is expected to provide strong insight into the origin of interface resistance and mechanism of cell degradation. Up to date, X-ray and electron spectroscopy techniques have led the field of operando measurement. However, the spatial resolution of those techniques is insufficient to characterize next generation devices that actively utilize nanostructure. Thus, in our group, as an operando measurement technique to evaluate the local properties at nanoscale and even down to the atomic scale, we have been working on developing scanning probe microscopy (SPM) systems that operate under various environments (under light irradiation, voltage application, inert atmospheric conditions). In this talk, we will introduce recent our works related to the characterization of perovskite SCs and all-solid-state LIBs by operando nano-scale profiling of electrical potential distribution using Kelvin probe force microscopy (KPFM). Perovskite solar cells (PSCs) have attracted great research interest owing to their advantages in high power conversion efficiency and low-cost fabrication. Although the power conversion efficiency of PSCs is very high (record is 22.7 %), the detailed photovoltaic conversion processes has not well understood yet compared with the conventional inorganic SCs. In this work, we used KPFM to measure internal electrical potential distribution of PSCs during device operation. We observed that the change of electrical potential induced by illumination occurred strongly at the interface between the perovskite film and charge transport layers, which suggests that charge separation mainly occurred at those interfaces, not in the perovskite film. Also, we found that the position of charge separation varied depending on the composition of perovskite materials and the device structure. These findings will be helpful for understanding the fundamental mechanism within PSCs and achieving high device efficiency. All-solid-state (ASS) LIBs are a promising candidate for next-generation energy storage devices. However, solid electrolytes for the current ASS-LIBs have various disadvantages, such as low power densities caused by high ionic resistivity at the interfaces between active materials and solid electrolytes. The high ionic resistivity has been attributed to the Li depleted layer or defects at the interfacial layer. For the fundamental understanding of the origin of interfacial resistivity, novel in situ techniques for measuring the distribution of the internal potential and/or Li ion concentration of LIB cells are strongly required. In this work, we combined Ar ion milling under non-atmospheric conditions with cross-sectional KPFM for direct imaging of the internal electrical potential distribution of the ASS-LIBs. We succeeded in the direct visualization of the change in the potential distribution of the cathode composite electrode arising from the battery charging (electrochemical reaction). The results obtained provided several insights into the battery operation, such as the behavior of Li ions and inhomogeneity of electrochemical reactions in the electrode.

Heat and fluid flow in electro-osmotically driven systems

A numerical investigation of heat and fluid flow in electro-osmotically driven systems is presented, by considering plain channels and channels packed with charged solid particles. The results show that the introduction of charged solid particles a_ects the internal potential distribution, fluid flow and temperature distribution in the channel. Under the analysed conditions, the effect on heat transfer is confined to the centre of the channel. This topic needs to be further investigated since it is of interest in practical applications.

Internal potential mapping of charged solid-state-lithium ion batteries using in situ Kelvin probe force microscopy.

Solid-state-lithium ion batteries (SS-LIBs) are a promising candidate for next-generation energy storage devices. Novel methods for characterizing electrochemical reactions occurring during battery operation at the nanoscale are highly required for understanding the fundamental working principle and improving the performance of the devices. In this work, we combined Ar ion milling under non-atmospheric conditions with in situ cross-sectional Kelvin probe force microscopy (KPFM) for direct imaging of the internal electrical potential distribution of the SS-LIBs. We succeeded in the direct visualization of the change in the potential distribution of a cathode composite electrode (a mixture of the active materials, solid electrolytes, and conductive additives) arising from battery charging (electrochemical reaction). The observed results provided several insights into battery operation, such as the behavior of Li ions and inhomogeneity of electrochemical reactions in the electrode. Our method paves the way to characterize the fundamental aspects of SS-LIBs for the improvement of device performance, including the evaluation of the distribution of the Li ion depleted regions, visualization of the conductive paths, and analysis of the cause of degradation.

Extending the dynamic range of fully depleted pnCCDs

pnCCDs are a special type of charge coupled devices (CCD) which were originally developed for applications in X-ray astronomy. At X-ray Free Electron Lasers (XFEL) pnCCDs are used as imaging X-ray spectrometers due to their outstanding characteristics like high readout speed, high and homogeneous quantum efficiency, low readout noise, radiation hardness and high pixel charge handling capacity. They can be used both as single-photon counting detectors for X-ray spectroscopy and as integrating detectors for X-ray imaging with count rates up to 104 photons of 1 keV per pixel and frame. However, extremely high photon intensities can result in pixel saturation and charge spilling into neighboring pixels. Because of this charge blooming effect, spatial information is reduced. Based on our research concerning the internal potential distribution we can enhance the pixel full well capacity even more and improve the quality of the image. This paper describes the influence of the operation voltages and space charge distribution of the pnCCD on the electric potential profile by using 2D numerical device simulations. Experimental results with signal injection from an optical laser confirm the simulation models.

Evaluation of internal potential distribution and carrier extraction properties of organic solar cells through Kelvin probe and time-of-flight measurements

The carrier extraction property of a prototypical small molecule organic solar cell (OSC) composed of copper phthalocyanine (CuPc), C60, and bathocuproine (BCP) was studied on the basis of the internal potential distribution and carrier dynamics in the device. The internal potential distribution in the OSC structure at the interfaces and in the bulk region was determined by the Kelvin probe method. Significant potential gradients were found in the CuPc film on indium tin oxide and in the C60 film on CuPc, consistent with charge transfer through the contacts. Moreover, surface potential of the BCP layer grew linearly with increasing film thickness with a slope of ca. 35 mV/nm (giant surface potential: GSP), which indicated spontaneous orientation polarization in the film. The potential gradient and GSP significantly changed the built-in potential of the device. Current–voltage and modified time-of-flight measurements revealed that the BCP layer worked as an electron injection and extraction layer despite the wide energy gap. These results were discussed based on the contributions of GSP and the gap states in the BCP layer.

Ion optical effects in a low pressure rf plasma

Ion optical effects in low pressure gas discharges are introduced as a novel input into low pressure plasma technology. They are based on appropriate geometrical plasma confinements which enable a control of the shape of internal density and potential distributions and, hence, the ion motion in the plasma bulk. Such effects are exemplified for an electron cyclotron wave resonance plasma in Ar at 1–5 × 10−3 millibars. The geometry of the plasma chamber is modified by a conical and a cylindrical insert. Computer simulations display spherical plasma density contours to be formed around the conical confinement. This effects an increase of the ratio of the ion currents into the conical and the cylindrical inserts which depends on the fourth power of the plasma electron temperature. A quantitative understanding of this behavior is presented. As another essential result, the shape of the internal plasma contours is found to be independent of the pressure controlled plasma parameters.

Biased internal potential distributions in a bulk-heterojunction organic solar cell incorporated with a TiOx interlayer

External-biased potential distributions of a polymer bulk-heterojunction (BHJ) solar cell, incorporated with electron/hole transporting layers, were directly obseved through a cross-sectional Kelvin probe force microscopy. The bulk electric field of BHJ was found to be nearly field-free even under reverse biases, and the field-free region was probed to expand with the incorporation of TiOx electron transporting layer; as a result, inducing a decrease of quasi-Fermi level splitting region in obtaining a high fill factor in the TiOx-interlayered junction photodiodes.

Simulation of space charge limited organic non volatile memory elements

ABSTRACTWe present a model for organic bistable devices (OBDs) embedded with metallic nanoparticles. In particular, two device architectures have been studied: a single layer device with metallic nanoparticles dispersed in a organic material matrix and a three layer device where two organic material regions are separated by a layer of heavy packed nanoparticles. The model describes the different behavior, the internal charge and potential distributions in the ON-OFF states. The OFF state is represented by charged nanoparticles forming a space charge layer which limits the current. The ON state occurs with neutral nanoparticles.

Direct observation of internal potential distributions in a bulk heterojunction solar cell

Cross-sectional potential distributions of a polymer bulk heterojunction (BHJ) solar cell, consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester blends, were directly observed by using Kelvin probe force microscopy. It was found that the bulk electric field of BHJ is nearly field-free compared with that of classical metal-insulator-metal model, whereas the electric field of BHJ device is confined to the cathode interface. The cathode-interfaced rectification junction is studied to originate from the formation of bilayered p-n junction in the BHJ.

Effects of the Shapes of GaN/AlN Quantum Dots on the Internal Electric Potential Distribution

Effects of the Shapes of GaN/AlN Quantum Dots on the Internal Electric Potential Distribution

Effect of the Volume of Truncated Quantum Dots on the Internal Electric Potential Distribution

Effect of the Volume of Truncated Quantum Dots on the Internal Electric Potential Distribution

Internal potential distribution in organic light emitting diodes measured by dc bridge

Internal potential distribution of organic light-emitting diodes (OLEDs) is an essential problem. By using dc bridge to eliminate errors due to high resistance of the devices at low bias, the potential distribution has been accurately measured for both double-layer and single-layer OLEDs. It is found that the electric field inside the device is not uniform, and the potential distribution changes with external bias. This phenomenon could be the effect of space charge originating from the unequal injections of holes and electrons, which is confirmed by the results of the device with modified work function of the anode.