Electrode Edge Research Articles

En Route to Wide Area Emitting Organic Light‐Emitting Transistors for Intrinsic Drive‐Integrated Display Applications: A Comprehensive Review

AbstractOrganic light‐emitting transistors (OLET) evolved from the fusion of the switching functionality of field‐effect transistors (FET) with the light‐emitting characteristics of organic light‐emitting diode (OLED) that can simplify the active‐matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide‐area emission for display applications via three different topics, that is, narrow to wide‐area emission, vertical architecture, and impact of high‐κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non‐planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light‐emitting area. Transistors utilizing high‐κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide‐area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high‐κ ‐dielectrics, and device architectural engineering.

Influence of film morphology and crystallinity on charge carrier concentration-dependent hole mobility in hexamethyldisilazane treated Pentacene bottom contact devices

The effect of substrate temperature and deposition rate on the film morphology, crystallinity, and electronic properties of Pentacene transistors treated with hexamethyldisilazane (HMDS) is studied. The gate bias dependence of mobility is used to estimate the width of the density of states and thereby quantify the disorder of the highest occupied molecular orbital. A low deposition rate and the substrate held at room temperature are shown to be the optimal conditions for good mobility (0.20 cm2 V−1 s−1) and low electronic disorder (50 ± 10 meV). X-ray diffraction measurements are performed to quantify the ratio of the two crystalline phases (thin-film phase and bulk phase) present in the film. The crystalline phases, rather than grain size, plays a significant role in determining the charge carrier mobility. Film deposition with the substrate at room temperature leads to low electronic disorder as the film is composed of one crystalline phase (thin-film phase), while high substrate temperature makes the film increasingly polymorphic, leading to increased electronic disorder (up to 230 meV). A high deposition rate leads to poor morphology of Pentacene near the source/drain electrode edge, thereby leading to increased contact resistance and electronic disorder. Hence, a low growth rate at room temperature is required for HMDS treated substrates to induce good crystalline properties of the film in the channel region, which results in enhanced electronic properties of the transistors.

Location-Dependent Cobalt Deposition in Smartphone Cells upon Long-Term Fast-Charging Visualized by Synchrotron X-ray Fluorescence

Author(s): Besli, MM; Usubelli, C; Subbaraman, A; Pour Safaei, FR; Bone, S; Johnston, C; Schneider, G; Beauchaud, F; Ravi, N; Christensen, J; Doeff, MM; Metzger, M; Kuppan, S | Abstract: In this work, we investigate the transition-metal dissolution of the layered cathode material LiCoO2 upon repeated fast-charging of three smartphone batteries from different manufacturers using synchrotron micro X-ray fluorescence (μ-XRF). Using this spatially resolved technique, dissolution of Co and subsequent location-dependent deposition on the anode are observed. μ-XRF mapping of selected parts of the anode electrode sheets, such as electrode folds and edges of the jelly roll, reveals the difference in the way Co is deposited on specific regions of the anode electrode. While some folds show no depositions, edges of the anode show gradually accumulating Co depositions. Careful quantification of the dissolved Co reveals that the capacity loss scales with the amount of deposited Co on the anode, that is, total Co loss from within the cathode. Soft X-ray absorption spectroscopy of the Co depositions on the anode shows that Co is mainly deposited in a reduced 2+ state. While optimization of the fast-charging protocol mitigates Li plating on the anode, no significant difference in the amount of deposited Co can be observed between an optimized and a nonoptimized fast-charging algorithm.

Residual stress analysis of aluminum nitride piezoelectric micromachined ultrasonic transducers using Raman spectroscopy

In this study, the Raman biaxial stress coefficients KII and strain-free phonon frequencies ω0 have been determined for the E2 (low), E2 (high), and A1 (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E2 (high) mode of AlN is recommended for the residual stress analysis of AlN due to its high sensitivity and the largest signal-to-noise ratio among the studied modes. The E2 (high) Raman biaxial stress coefficient of −3.8 cm−1/GPa and strain-free phonon frequency of 656.68 cm−1 were then applied to perform both macroscopic and microscopic stress mappings. For macroscopic stress evaluation, the spatial variation of residual stress was measured across an AlN-on-Si wafer prepared by sputter deposition. A cross-wafer variation in residual stress of ∼150 MPa was observed regardless of the average stress state of the film. Microscopic stress evaluation was performed on AlN piezoelectric micromachined ultrasonic transducers (pMUTs) with submicrometer spatial resolution. These measurements were used to assess the effect of device fabrication on residual stress distribution in an individual pMUT and the effect of residual stress on the resonance frequency. At ∼20 μm directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of ∼100 MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film.

Revealing the Mechanism behind the Catastrophic Failure of n‐i‐p Type Perovskite Solar Cells under Operating Conditions and How to Suppress It

AbstractThe n‐i‐p type perovskite solar cells suffer unpredictable catastrophic failure under operation, which is a barrier for their commercialization. The fluorescence enhancement at Ag electrode edge and performance recovery after cutting the Ag electrode edge off prove that the shunting position is mainly located at the edge of device. Surface morphology and elemental analyses prove the corrosion of the Ag electrode and the diffusion of Ag+ ions on the edge for aged cells. Moreover, much condensed and larger Ag clusters are formed on the MoO3 layer. Such a contrast is also observed while comparing the central and the edge of the Ag/Spiro‐OMeTAD film. Hence, the catastrophic failure mechanism can be concluded as photon‐induced decomposition of the perovskite film and release reactive iodide species, which diffuse and react with the loose Ag clusters on the edge of the cell. The corrosion of the Ag electrode and the migration of Ag+ ions into Spiro‐OMeTAD and perovskite films lead to the forming of conducting filament that shunts the cell. The more condensed Ag cluster on the MoO3 surface as well as the blocking of holes within the Spiro‐OMeTAD/MoO3 interface successfully prevent the oxidation of Ag electrode and suppress the catastrophic failure.

Hot-carrier-induced device degradation in Schottky barrier ambipolar polysilicon transistor

This study experimentally investigated the hot-carrier-induced device degradations in Schottky barrier (SB) ambipolar polysilicon transistors and validated the degradation mechanism. The behavior of hot-carrier-induced device degradation in SB ambipolar transistors was different from that of a unipolar MOSFET. The phenomenon of hot-carrier generation at the source electrode, attributed to the existence of a high electric field across the reverse-biased source electrode in SB transistors and the competing degradation mechanism between the trapped charges at the gate oxide near the source edge and drain electrode, explained the experimental results. The degradation mechanism was justified by measuring the gate current and transfer characteristics in the forward and reverse modes before and after hot-carrier stress, respectively. The results can contribute to the non-volatile memory applications and aid in investigating the aging effects of multi-functional integrated circuits using reconfigurable field-effect transistors.

Surface Charge Accumulation on a Real Size Epoxy Insulator With Bouncing Metal Particle Under DC Voltage

Free metal particle can affect surface charge accumulation on epoxy insulator installed in gas-insulated transmission line (GIL) or gas-insulated switchgear (GIS), which results in reduction of surface flashover voltage thus threatening the safe operation of the power equipment. This article reports on the charge accumulation behavior on a real size epoxy insulator with bouncing metal particle under DC voltage. A coaxial electrode system was designed for the test under -30 kV, a metal particle-launching system was employed to inject the bouncing linear particle to the insulator surface. The charge distribution was measured by a Kelvin-type probe, and the influence of the particle on the charge accumulation was analyzed with the help of electric field simulation. The results showed that there were mainly two moving patterns of the bouncing particle: the charges were accumulated with speckle shape and the charged particle that hit the edge of electrode had more remarkable influence on the charge accumulation than other particles. It is suggested that the charge distribution feature is dependent upon the collision point and the trajectory of the bouncing particle.

Numerical verification of the two-spike-current behavior in the initial stage of plasma formation in a pulsed surface dielectric barrier discharge

Nanosecond pulsed surface dielectric barrier discharge (SDBD) is a hot topic in many fields, but the mechanisms of discharge evolution and micro-channel propagation are still not clearly understood. In this paper, a plasma fluid model of nanosecond pulsed SDBD is established, and the deductions of the two current spikes are verified and improved by simulation. The two current spikes correspond to the two stages of the discharge: the ionization wave propagation and the repeated re-ignition in the gap between the ionization wave and the dielectric surface. In the first stage, the ionization wave develops rapidly at first and propagates very slowly in the end, which produces the first current spike with a very short rise time and a tailing falling edge. The curve profile of the ionization wave velocity is very similar to that of the first current spike. A certain distance is maintained between the bottom of the ionization wave and the dielectric surface in this stage, which forms the gap for the next stage. In the second stage, the charged particle cloud induced by the quasi-uniform electric field in the middle of the gap and the new micro-channel from the edge of the high-voltage electrode propagate at first, and then merge together to establish a new discharge channel. The reverse electric field in the gap induced by the accumulated charges on the dielectric surface restricts the expansion velocity of the second discharge channel, which results in a longer rise time and falling time of the second current spike.

Communication-In Situ Differential Reflectance Spectroscopy Monitoring the Asynchronous Transient Response at the Edge and Center of the Disk Electrode

The asynchronous transient responses at the edge and center of a gold disk electrode are monitored by in situ differential reflectance spectroscopy (DRS). The DRS spectrum collected in 10 mM perchloric acid shows that the edge of the electrode achieves the equilibrium sooner than the center does in chronoamperometry, which establishes the foundation for a similar electrochemical protocol to remove the metallic Cu at the edge of a Cu/Au disk electrode while keeping these at the center intact. Both phenomena are caused by the larger resistance at the center than the edge of a disk electrode.

The formation mechanism of nonuniformity from 2D nonlocal particle-dynamics in capacitive RF discharges

Kinetic effects and nonlinear characteristics caused by multi-dimensional effects in capacitively coupled plasma (CCP) devices are reported using particle-in-cell (PIC) Monte-Carlo collision simulations. Many PIC simulations so far are limited to one-dimensional (1D) dynamics due to computational load, and the nonuniformity occurring in a direction parallel to the electrode cannot be reflected consequently. A two-dimensional (2D) PIC–MCC simulation based on a graphics processing unit is utilized to observe the variations in the spatial distribution that cannot be dealt with by 1D simulations. This simulation considers the pressure regime of the nonlocal electron kinetics. Contrary to the expectation that motions in the vertical direction to the electrode in a CCP dominate most of the particle dynamics, it was observed that electron heating in the horizontal direction around the electrode edge plays an important role. Accordingly, the change in the spatial distribution between the center of the CCP equipment and the peripheral region of the electrode edge has been analyzed by phase-resolved electron flux and power deposition to understand the principle of the formation of spatial nonuniformity.

Alignment Design in Consideration with Tertiary Current Distribution of Thick Electrodes

With the increasing demands of lithium-ion batteries in electric vehicles (EVs) and energy storage systems (ESSs), high energy density and extreme fast-charging (XFC) characteristics are required. The easiest way to design high-energy-density cells is to make thick electrodes by minimizing separators and current collectors and maximizing active compounds, but for thicker electrodes, the kinetic losses increased with C-rate, and cycle stability deteriorated.1 On the sidewall of an electrode, there exists a current difference due to the current distribution by the location. This effect gets maximized in case of the thick electrode because there is a non-uniformity of the kinetics through the electrode thickness direction due to the difference of ionic and electronic conductivity within the pores of the electrode. Besides, different alignment margin of cathode and anode causes another current distribution on the sidewall.In this study, pouch full cells with different alignment margin of LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode and graphite anode are considered to investigate the difference of the current distribution. N/P ratio is set under 1 to induce complete intercalation of lithium-ion from the cathode into graphite anode so that distinguish the capacity from lithium-ion intercalation and lithium plating onto the surface of an anode. Through the calculation of the current distribution of the edge of the thick electrode, the optimal alignment design of the anode is suggested. There exists the optimal alignment margin of anode and cathode with respect to the dimension of the electrode and the current density (C-rate) applied into the electrode.References Kuang, Yudi, et al. "Thick electrode batteries: principles, opportunities, and challenges." Advanced Energy Materials 33 (2019): 1901457.

A novel structure to enable low local electric field and high on-state current in GaN photoconductive semiconductor switches

The use of GaN photoconductive semiconductor switches (PCSSs) is plugged by the high local electric field at the electrode edges and the low on-state current in the sub-bandgap triggering mode. Therefore, a new high-power semi-insulating GaN PCSS structure, which is a quantum well coupled with a trench PCSS (QWTPCSS), is presented. The trench changes the electric field distribution. In the off state, the maximum electric field of the GaN QWTPCSS is 202.1 kV cm −1, which is 60.57% lower than the 512.6 kV cm −1 of the conventional structure. Moreover, the introduced AlGaN layer forms two-dimensional electron gases (2DEGs) in the conduction state; when triggered by a 532 nm laser, such 2DEGs contribute to the on-state current, which increases by 9.37% relative to that of the structure without AlGaN. These results demonstrate the potential of QWTPCSS to achieve high power.

Experimental investigation of the electromagnetic effect and improvement of the plasma radial uniformity in a large-area, very-high frequency capacitive argon discharge

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency (LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency (VHF, e.g. 60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band’ effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.

A CONCEPTUAL STUDY OF A LIQUID METAL ALLOY IN A DISK-SHAPED MAGNETOHYDRODYNAMICS CONVERSION SYSTEM

The use of solar-heated liquid metal in a magnetohydrodynamics (MHD) generator provides an alternative and direct conversion method for electric power generation. This prompted the present study to conduct a three-dimensional numerical analysis for a liquid Ga68In20Sn12 flow exposed to several uniform magnetic field intensities (Bo of 0.5 T, 1T and, 1.41 T) within a disk channel geometric boundary. The aim is to study the influence of the external magnetic fields on the generator performance and the fluid flow stability at a high Reynolds number (Re) and Hartmann number (Ha) using the Ansys Fluent software. The simulation results show that at Re of ≈ 2.44e6, the fluid velocity decreases inside the generator regardless of Bo. When Bo of 1T and 1.41T are applied, the velocity magnitude decreases and spreads within the disk channel and walls due to high Ha values (5874 and 8282). The fluid pressure increases from the nozzle pipe inlet to the disk channel and decreases towards the outlet. The induced current density in the radial direction, jx, increases within the disk channel and near the inner electrode edge as Bo increases. A significant observation is that the current densities obtained for Bo of 1T and 1.41T cases are higher than in other cases. The numerical analysis obtained in this study showed that the Bo of either 1T or 1.41T is needed to achieve the required flow stability, current density, and output powers.

Optical Momentum Alignment Effect in WSe2 Phototransistor

AbstractThe optical momentum alignment effect is demonstrated in WSe2 phototransistors . When the photon energy is above the A exciton energy, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, suggesting that electrons in the valence band of WSe2 prefer to absorb photons with the polarization direction perpendicular to their momentum direction. Further studies indicate that the anisotropic distribution of photo‐excited carriers is likely due to the pseudospin‐induced optical transition selection rules. If the photon energy is below the A exciton energy, the photocurrent signals are maximized when the incident light is polarized in the direction perpendicular to the electrode edge, which is mainly attributed to the polarized absorption of the plasmonic gold electrodes. Moreover, the photocurrent peak can be controlled by an electric field via the quantum confined Stark effect. This resonance peak can also be shifted by adjusting environmental temperatures due to the temperature‐dependent nature of the WSe2 band gap. These experimental studies shed light on the knowledge of photocurrent generation mechanisms, opening the door for engineering future anisotropic optoelectronics.

Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells.

Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li6PS5Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical 'pothole'-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs.

Electrically turning periodic structures in cholesteric layer with conical\u2013planar boundary conditions

Electro-optical cell based on the cholesteric liquid crystal is studied with unique combination of the boundary conditions: conical anchoring on the one substrate and planar anchoring on another one. Periodic structures in cholesteric layer and their transformation under applied electric field are considered by polarizing optical microscopy, the experimental findings are supported by the data of the calculations performed using the extended Frank elastic continuum approach. Such structures are the set of alternating over- and under-twisted defect lines whose azimuthal director angles differ by 180^circ. The U^+ and U^--defects of periodicity, which are the smooth transition between the defect lines, are observed at the edge of electrode area. The growth direction of defect lines forming a diffraction grating can be controlled by applying a voltage in the range of 0le , V le 1.3 V during the process. Resulting orientation and distance between the lines don’t change under voltage. However, at V>1.3 V U^+-defects move along the defect lines away from the electrode edges, and, finally, the grating lines collapse at the cell’s center. These results open a way for the use of such cholesteric material in applications with periodic defect structures where a periodicity, orientation, and configuration of defects should be adjusted.

Nonlinear harmonic excitations in collisional, asymmetrically-driven capacitive discharges

The standing wave effect, which may lead to center-high density profiles in high frequency capacitive discharges, can be enhanced by nonlinearly excited harmonics. In this work, a nonlinear transmission line model, which solves for the electromagnetic fields in the time domain, is coupled to a two-dimensional bulk plasma fluid model to study nonlinear effects in asymmetric cylindrical capacitive argon discharges. An analytical collisional or collisionless (ion) sheath model is used to determine the stochastic and ohmic sheath heating and the nonlinear dependence of sheath voltage on sheath charge. We first examine a base case of a 20 mTorr argon discharge driven with an electron power P e = 40 W at a frequency f = 60 MHz, using collisionless and collisional sheath models. For the collisionless sheath model, the nonlinearly excited harmonics near the series and spatial resonance frequencies significantly enhance the on-axis power deposition and lead to a sharp peak of electron density at the discharge center. The collisional sheath model gives a smaller sheath width, leading to lower series and spatial resonance frequencies and a smaller source voltage for the fixed electron power. As a result, lower harmonics with broader spatial profiles and decreased magnitude are excited, reducing the center-high plasma nonuniformity. Then, we examine the discharge in a pressure range of 20–100 mTorr at fixed P e = 40 W and f = 60 MHz, using the collisional sheath model. As pressure increases, the harmonics gradually damp out, and the enhancement of on-axis power deposition becomes less significant. At the same time, more power is localized near the powered electrode edge due to decreased skin depth and smaller energy diffusion. As a result, the density peak shifts from the radial center to the powered electrode edge.

Empirical relations for discharge current and momentum injection in dielectric barrier discharge plasma actuators

Dielectric barrier discharge (DBD) plasma actuators with an asymmetric, straight edge electrode configuration generate a wall-bounded jet without moving parts. Mechanistic description of the interaction between the Coulombic forces and fluid motion as a function of DBD parameters remains unclear. This paper presents an experimental investigation of DBD actuators, including electrical current associated with microdischarges, plasma volume and the wall jet momentum over a range of alternating current (AC) frequencies (0.5–2 kHz) and peak-to-peak voltages up to 19.5 kV. Discharge current is measured with a high temporal resolution, plasma volume is characterized optically and the momentum induced by the DBD wall jet is computed based on the axial velocities measured downstream of the actuator using a custom-built pitot tube. Discharge current analysis demonstrated asymmetry between the positive and negative semi-cycle; both currents yielded a power–law relationship with empirical fitting coefficients. Plasma length varies linearly and volume quadratically with voltage. Although plasma length reached an asymptotic value at a higher frequency, the plasma volume grows due to the increasing height of the ionization region. In a simple two-dimensional configuration, the DBD wall jet momentum shows near-linear dependency with discharge current in the range of voltages and frequencies considered in this work. The presented empirical model characterizes the DBD wall jet momentum and the discharge current based only on the AC inputs. With the estimation of plasma volume, the model can be applied for determining more realistic boundary conditions in numerical simulations.

Electromechanical Analysis of Red Blood Cell Under AC Electric Field

This article presents the numerical analysis of electromechanics of a red blood cell under electric field in a microchannel. We use 3-D computation to study the dielectrophoretic (DEP) characteristics of the cell subjected to electric field generated by parallel planar electrodes. The aim of this work is to investigate the variation in electric field frequency of the DEP force on the cell in different orientations and to clarify cell behavior during the transition between the positive and negative dielectrophoresis, which is important for on-chip cell manipulation. We use the boundary element method and apply the boundary condition of transmembrane potential on the cell membrane for calculation. The simulation results show that the DEP characteristics depend highly on the orientation and the position of the cell from electrode edge. The vertical and horizontal DEP forces change their directions at different values of field frequency. The re-orientation of red blood cells observed in the experiment is related to the calculated DEP behavior.