Trap Density Energy Research Articles

Research and Analysis on Enhancement of Surface Flashover Performance of Epoxy Resin Based on Dielectric Barrier Discharge Plasma Fluorination Modification.

To conquer the challenges of charge accumulation and surface flashover in epoxy resin under direct current (DC) electric fields, numerous efforts have been made to research dielectric barrier discharge (DBD) plasma treatments using CF4/Ar as the medium gas, which has proven effective in improving surface flashover voltage. However, despite being an efficient plasma etching medium, SF6/Ar has remained largely unexplored. In this work, we constructed a DBD plasma device with an SF6/Ar gas medium and explored the influence of processing times and gas flow rates on the morphology and surface flashover voltage of epoxy resin. The surface morphology observed by SEM indicates that the degree of plasma etching intensifies with processing time and gas flow rate, and the quantitative characterization of AFM indicates a maximum roughness of 144 nm after 3 min of treatment. Flashover test results show that at 2 min of processing time, the surface flashover voltage reached a maximum of 19.02 kV/mm, which is 25.49% higher than that of the untreated sample and previously reported works. In addition to the effect of surface roughness, charge trap distribution shows that fluorinated groups help to deepen the trap energy levels and density. The optimal modification was achieved at a gas flow rate of 3.5 slm coupled with 2 min of processing time. Furthermore, density functional theory (DFT) calculations reveal that fluorination introduces additional electron traps (0.29 eV) and hole traps (0.38 eV), enhancing the capture of charge carriers and suppressing surface flashover.

Electrical conduction behavior in ferroelectric polymer-based composites incorporating metal halide perovskite

Halide perovskites have established their intriguing performances in optoelectronics and been expanded to energy harvesting application when incorporated into polymers due to their polarization related properties. Yet, fundamental understanding of the electrical conduction behavior of such composites has seldomly been addressed. Herein, composite films employing CH3NH3PbBr3 (MAPB) nanoparticles and ferroelectric polymer poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] have been investigated upon their conduction behaviors. Both electric field and temperature-dependent currents have been thoroughly analyzed based on hopping and space charge-limited current mechanisms in combination with trap energy and trap density calculation from thermal stimulated depolarization currents. The conduction mechanism of MAPB/P(VDF-TrFE) composite has been established within the frame of band diagram. Our studies, thus, are essential for developing high-performance optoelectronics and energy harvesters based on halide perovskites/polymer composites.

Guest Editorial: Polymer electrets and ferroelectrets

Guest Editorial: Polymer electrets and ferroelectrets

Fluorination Treatment and Nano-Alumina Concentration on the Direct Current Breakdown Performance & Trap Levels of Epoxy/Alumina Nanocomposite for a Sustainable Power System

Epoxy resin is extensively used in gas insulated switches as a renewable energy coating due to its exceptional insulation, mechanical characteristics, and environmental friendliness. The higher resistivity of the epoxy resin causes numerous surface charges to accumulate on the surface of the epoxy resin as a result of carrier injection due to the high DC electric field, which may cause insulation failure of the power transmission system. In this study, various concentrations of epoxy resins blended with nano-alumina (nano-Al2O3) at 0 wt%, 1 wt%, 3 wt%, and 5 wt% were created. Afterwards, the epoxy resin and Al2O3 nanocomposites were fluorinated by utilizing a combination of F2 and N2 with a ratio of 20% F2 at 0.05 MPa while maintaining the temperature at 40 °C. In order to improve dispersion, nano- Al2O3 was treated with a silane coupling agent called γ-aminopropyltriethoxysilane (KH550). Additionally, infrared spectroscopy based on the Fourier transform was used to investigate the structure of chemical bonds. Furthermore, the changes in the molecular chains were verified by the FTIR spectra. The DC breakdown strength of epoxy resin\Al2O3 nano-composites showed that breakdown strength significantly improved after gas-phase fluorination. Moreover, 1 wt% nano- Al2O3 showed a higher breakdown strength. The fluorinated layer had a charge-suppressing effect, reducing the charge injected into the polymer matrix of the epoxy-resin matrix and increasing its DC breakdown capability. Thermally stimulated current (TSC) measurements indicate that epoxy resin’s trap energy and trap density are altered by nano- Al2O3 incorporation and fluorination treatment (gas-phase). It was also observed that introducing nano- Al2O3 at a lower concentration (e.g., 1 wt%) can hinder the growth of space charge in the polymer matrix of the epoxy resin, thus enhancing the deep traps’ energy. Furthermore, a fluorination layer containing a strong polarization of C-F bonding would seize the charge injection from electrodes, thus decreasing the conductivity and suppressing the charge injection.

High temperature energy storage and release properties of polyimide nanocomposites simulated by considering charge trapping effects

AbstractDielectric energy storage capacitors with excellent high temperature resistance are essential in fields such as aerospace and pulse power. However, common high‐temperature resistant polymers such as polyimide (PI) and polyether sulfone have low energy storage densities and energy efficiencies at high temperature, which are greatly limited in practical applications. The polymer nanocomposites prepared by doping modification can regulate the charge injection and transport process, and improve the high‐temperature energy storage performance. However, the quantitative relationship between charge injection and charge trapping and the energy storage performance of linear polymer nanocomposites still needs further study. An energy storage and release model considering the charge trapping effects is constructed by the authors. We simulate the high‐temperature energy storage properties of polyimide nanocomposite dielectrics (PI PNCs) with different charge injection barriers and trap parameters at 150°C. A triangular voltage is applied to the electrodes at both sides of the PI PNCs, the electric displacement‐electric field loop is simulated, and the discharged energy densities and energy efficiencies are calculated. The simulation results are consistent with the experimental results. Increasing the charge injection barrier, deep trap energy and deep trap density can effectively reduce the charge injection and the carrier mobility, thereby improving the discharged energy densities and energy efficiencies of dielectric capacitors. In the case of low charge injection barrier (1.3 eV), with the increase of deep trap energy (0.7–1.5 eV) and deep trap density (1 × 1021–1 × 1025 m−3), the discharged energy density changes from 0.20 to 1.44 Jcm−3, the energy efficiency changes from 9.0% to 99.9%, and the high‐temperature energy storage performance improves significantly. This research provides theoretical and model support for the improvement of the high‐temperature energy storage performance of nanocomposites.

Electrical transport properties of TiO2/MAPbI3 and SnO2/MAPbI3 heterojunction interfaces under high pressure.

The electrical transport properties of SnO2(TiO2)/MAPbI3 (MA = CH3NH3 +) heterojunction interfaces are investigated from ambient pressure to 20 GPa, and the transport properties are calculated by physical parameters such as trap energy density, binding energy, and charge transfer driving force and defect. Based on the partial density of states (PDOS) of the SnO2/MAPbI3 heterojunction interface MAI-termination and PbI2-termination, greater charge transfer driving force and higher binding energy are observed, obviously showing the SnO2-based heterojunction is more stable. The SnO2/MAPbI3 heterojunction interface possesses stronger electrical transport ability and is less prone to capture electrons compared with the TiO2/MAPbI3 heterojunction interface. The differential charge density spectrum shows that the density is lower in the trap energy level of SnO2/MAPbI3, whilst the effect of the charge transfer defect is weaker owing to the trap energy level only existing in SnO2. The SnO2/MAPbI3 heterostructure interface is less prone to capture electrons. The greater electron concentration difference is attributed to oxygen vacancy (Vo0) in the SnO-like environment, resulting in superior electron transport ability compared with the TiO-like environment.

Effect of microcapsules doping on DC flashover and trap level of self-healing epoxy resin composites

Minor physical defects can decrease the dielectric efficiency of epoxy resin and severely threaten the electrical device’s dependability. Self-healing may be considered an effective method to preserve the electrical and mechanical properties of the epoxy resin. Besides, self-healing significantly decreases the influence of small physical destruction on the power system. In this research, epoxy resin incorporated with various concentration of microcapsules was prepared. The polyurea formaldehyde (PUF) microcapsules were treated using a silane coupling agent (KH550) to enhance dispersion. Moreover, the self-healing performance of the epoxy resin incorporated with microcapsules was analyzed by FTIR, SEM, and DC Flashover in air and vacuum. In addition, trap energy and trap density were obtained from measurements of surface potential decay (ISPD). The SEM results indicate that in comparison with pure epoxy resin, the microcapsule/epoxy resin composite has an improved self-healing performance. The DC Flashover of epoxy resin in air and vacuum was found relatively higher after incorporation of microcapsules than pure epoxy resin. The distribution of trap energy and trap density measured by surface potential decay measurements confirms that 5 wt% have deeper traps and the highest trap energy levels.

Filler concentration effect on breakdown strength and trap level of epoxy resin–Al2O3 nanocomposites

In this work, epoxy resin samples incorporated with nano-Al2O3 of various concentrations 0, 1, 3 and 5 wt% were prepared. Surface treatment of nano-Al2O3 was performed by the saline coupling agent of γ-aminopropyltriethoxysilane (KH550) to restrict the aggregation. The thermal gravimetric analysis (TGA) of the epoxy resin/Al2O3 nanocomposites was observed at different filler concentrations. TGA analysis indicated that incorporation of nano-Al2O3 increases the decomposition temperature as compared to pure epoxy resin. Moreover, 3 wt% filler concentration showed increased decomposition temperature because nanoparticles form close connections between molecules due to which thermal stability improved at higher temperatures. Atomic force microscopy was employed for the surface morphology of epoxy resin/Al2O3 nanocomposites which has indicated that surface roughness of epoxy resin/Al2O3 nanocomposites increased slightly with the filler concentration. The distribution of trap energy and trap density obtained by isothermal surface potential decay measurements revealed that 1 wt% filler concentration results in decrease in surface potential decay rate. In addition, significant increase in deep trap energy level and trap density was obvious at 1 wt% filler concentration. AC and DC breakdown strength of the samples was measured to evaluate the effect of filler concentrations on the electrical characteristics of epoxy resin. The results showed that breakdown strength of the epoxy resin/Al2O3 nanocomposites increases with the nano-Al2O3 concentration. Moreover, increase in trap energy results in an increase in the breakdown strength of the nanoparticles.

Effect of micro and nano-size boron nitride and silicon carbide on thermal properties and partial discharge resistance of silicone elastomer composite

This study introduces silicone elastomers with micro and nano-sized boron nitride (BN) and silicon carbide (SiC) particles with various doping levels to improve thermal properties and partial discharge resistance. The effect of micro and nano-fillers on thermal conductivity, coefficient of thermal expansion (CTE), and thermal stability are investigated. Polarization and depolarization current and partial discharge are measured to investigate the non-linear conductivity, trap energy density distribution, and partial discharge resistance. Experimental results show higher thermal conductivity, lower CTE, and better thermal stability than the original silicone elastomer. Large size fillers dominate the thermal conductivity when the doping level is low, whereas the composite microstructure plays a significant role in the thermal conductivity when the doping level is high. The combination of different filler type and size has less effect on thermal stability and CTE compared to the effect of the doping level. According to the depolarization current, the composite's trap depth is generally shallower than that of the original silicone elastomer. With increased doping level, the shallow trap density increases, providing hopping sites for carriers, whereas the deep trap density decreases, reducing the number of trapped charges. The partial discharge inception voltage is higher than that of the original silicone elastomer, and it increases with the doping level, which may be because the electrostatic field generated by diffused charges reduces the local electric field near the high voltage electrode.

Improvement of electrical and material properties of epoxy resin/ aluminum nitride nanocomposites for packaging materials

This paper presents the influence of Aluminum Nitride (AlN) nanoparticles on the electrical and material properties of epoxy resin (EP). The EP/AlN nanocomposites with different concentrations of nano-AlN fillers are prepared. The dispersion of the nano-AlN particles in the composites is analyzed by a field emission scanning electron microscope (FESEM). The electrical properties are investigated by the space charge and DC conductivity measurements, whereas the material properties are studied by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that the homo-charge accumulation appears near both electrodes during the polarization, but there are limited negative charges left near both electrodes in the depolarization for the pure EP sample. There is no space charge accumulation in the 1 wt% and 2 wt% EP/AlN nanocomposites. The electric field distortion of the pure EP sample is 20%. Moreover, the electric field distortion initially decreases with the increase of the nano-AlN content, but it increases for the 2 wt% nano-AlN sample. Temperature has a dominant influence on the DC conductivity of the EP/AlN nanocomposites comparing to the pure EP. However, the DC conductivity of the nanocomposites becomes stable at high temperatures. It is also found that the weight loss of the samples decreases with the addition of the nano-AlN and the 1 wt% nano-AlN sample has the highest glass transition temperature. It is elucidated that the high apparent mobility and activation energy facilitate the space charge transport and suppressing the space charge accumulation. Furthermore, the nano-AlN filler can increase the trap level and trap energy density of the deep traps in the sample. The dielectric loss of the EP at high frequency is reduced with the content of 1 wt% nano-AlN. Furthermore, the addition of the nano-AlN can improve the thermal stability of the EP. The 1 wt% nano-AlN sample has the superior electrical insulation and material performance amongst the tested materials.

Three-Dimensional Mechanistic Modeling of Gate Leakage Current in High- κ MOSFETs

Gate leakage current is an issue for the reliability of modern high-$\ensuremath{\kappa}$ MOSFETs. Although various physical models describing both direct tunneling and trap-assisted contribution of leakage current have been presented in the literature, many of them treats traps in the dielectric as a continuum distribution in energy and position, and trap-to-trap transport of electrons has so far been mostly neglected or not treated three dimensionally. In this work, we present a mechanistic model for calculation of gate leakage current in high-$\ensuremath{\kappa}$ MOSFET multilayer stacks based on multiphonon trap-assisted tunneling theory, taking into account the intrinsic three-dimensional (3D) discreteness of traps in the dielectric. Our model can to a good approximation reproduce the experimental results at different dielectric thicknesses, gate voltages, temperatures, and different gate materials. We find that in realistic devices, the 3D trap-to-trap transport of electrons contributes a non-negligible part to the gate leakage current. This contribution is more pronounced at low-voltage device operations, which is important for low-power applications. We calculate the intrinsic fluctuation of gate leakage current due to positional and energetic disorder of traps in the dielectric, and conclude that positional disorder is more important than energetic disorder for realistic material parameters. The calculated gate leakage current depends sensitively on temperature, trap energy, and trap density. We provide a computationally efficient 3D master equation approach that enables 3D mechanistic simulation of ${10}^{3}$ traps on the order of minutes on a standard desktop computer.

Effect of thermal ageing on charge dynamics and material properties of 320 kV HVDC XLPE

This paper reports the effect of thermal ageing on the space charge characteristics, DC conductivities and trap energy distributions of 320 kV HVDC XLPE. Specimens were prepared by a heating press method in the laboratory. The specimens were placed in a thermal oven and thermally aged at 130 °C up to 30 days. Space charges were measured by a pulsed electro-acoustic setup. The results showed that the reference samples had limited hetero charge accumulations near both electrodes in the samples. The homo-charge accumulation was appeared with an increase of the aged time. The space charge accumulation in the samples was transferred from hetero-charge accumulation to homo-charge accumulation with an increase of the ageing time. The electric field distribution in the sample gradually increased with the appearance of the homo-charge accumulation, whereas the homo charges evidently distorted the electric field in the samples with the increasing ageing time. The DC conductivities of the aged samples consistently increased with the increase of the measured electric field. The obtained results of apparent mobility of the charge carriers, the activation energy and the trap energy density illustrated that thermal ageing could facilitate the homo-charge accumulation in the sample. The dominant effect of thermal ageing on the sample was elucidated by using various analytical techniques. It was found that the thermal ageing would affect the material properties and facilitate the increase of the trap energy density and the space charge accumulations in the samples.

Understanding the dielectric and mechanical properties of self‐passivated Al–epoxy nanocomposites

Epoxy nano passivated aluminium composites with optimised size and filler contents were fabricated. Variation in contact angle and surface roughness is insignificant with increasing filler into nanocomposites but it showed a drastic reduction on corona ageing. Water droplet initiated corona inception voltage (CIV) is high under the negative DC voltage followed with positive DC and AC voltages. The bandwidth of ultra-high frequency signal generated due to water droplet initiated corona discharge lies in the range of 0.5–1.2 GHz. Surface potential measurements have shown that the decay in the potential was fast initially, and it became slower and sluggish subsequently. The trap energy density versus trap depth plot exhibits shallow traps and deep traps at around 0.8 and 0.87 eV, respectively. Permittivity, conductivity and loss factor have increased with an increase in the filler content in nanocomposites. The bulk resistance and capacitance of samples were determined for obtaining the equivalent parallel RC circuit model. Incorporation of nanofillers increases the glass transition temperature and reduces the tan δ with increasing frequencies as evident from dynamic mechanical analysis studies. A direct correlation is observed between the plasma temperature measured through laser-induced breakdown spectroscopy spectra and hardness of the material.

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

This paper gives a detailed introduction of a simultaneous measurement system that can obtain the space charge distribution and the thermally stimulated current (TSC) of the same sample over the temperature range 213–373 K. This measurement system adopts a 4-electrode structure and a time sharing strategy to avoid the mutual interference between the space charge measurement and the current measurement. In addition, a space charge recovery method is proposed to obtain the real space charge distribution from the original data under varying temperature situation. Furthermore, a simultaneous measurement is performed on a cross-linked polyethylene (XLPE) sample. The experimental results show that the positive charges accumulate and dominate in the bulk during polarization and the cooling stages. During the thermal stimulation stage, the accumulated charges decay after ∼330 K, and the thermally stimulated current increases significantly after ∼330 K. The analysis results imply that the release of trapped charges may be the main cause of the thermally stimulated current, and most of the charges are trapped in traps more than 1 eV.

Polyethylene blends with/without graphene for potential recyclable HVDC cable insulation

This paper presents investigations on a usage of graphene as a voltage stabilizer for polyethylene blends in the potential recyclable high voltage direct current (HVDC) cable insulation. Thermal and electrical properties of the blends with/without graphene were evaluated by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), electrical treeing tests, DC conductivity and space charge measurements. Trap energy distributions in the blends were estimated using the measured space charge decay data. The obtained results show that additive-like amounts of graphene can improve the thermal stability of the polyethylene blends. The influence of the graphene on the tree inception voltage (TIV) of the polyethylene blends is not significant, whereas the low density polyethylene/high density polyethylene (LDPE/HDPE) with/without graphene demonstrates the highest TIV comparing to the LDPE, LDPE/polypropylene, and their corresponding graphene added materials. It is found that the DC conductivity of the polyethylene blends with/without graphene increases consistently with the increase of the temperature. The conductivity of the LDPE/HDPE decreases apparently with the inclusion of graphene. It has been shown that space charge accumulations can be suppressed by the addition of graphene in the polyethylene blends. It is postulated that the high trap level and trap energy density of the polyethylene blends with graphene are closely connected to the suppression of the space charge accumulation in the blends.

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

Effect of Radiation on Interface Traps of SOI NMOSFETs by the Direct-Current Current-Voltage Technique

This paper investigates the effect of total ionizing dose radiation on back-gate interface traps in SOI NMOSFETs. The concentration and energy distribution of interface traps at Si/SiO 2 back-gate interface of SOI NMOSFETs during irradiation are studied by the direct-current current-voltage technique. When transistors are subjected to radiation, DCIV bulk current increases. The calculated results suggest that the interface trap density increases and its equivalent energy level is far away from the midgap with irradiation dose increasing, which can be explained by the energy distribution of interface traps. The interface trap energy density D IT (E IT ) as a function of the energy level E IT has been obtained by the least square optimization and shows the typical “U-shape”distribution. In detail, the rising humps at equivalent energy level in D IT (E IT ) curves are due to Si-H bonds that are broken down after irradiation, which corresponds to the increasing trap density. Moreover, it is found that the energy level of interface traps is redistributed after irradiation. The peak of humps in D IT (E IT ) curves occurs at the farther energy level with the increase of dose, which is similar to the equivalent energy level. It might arise from the shallow energy level of interface traps induced by radiation.

Beyond 11% efficient Cu2ZnSn(Se,S)4 thin film solar cells by cadmium alloying

Cd-alloying CZTSSe (sulfur about 1%) film was prepared by selenizing CZTS precursor with CdS on the top. The XRD and Raman spectra indicated that Cd alloyed into CZTSSe lattice and the calculated Cd/(Cd+Zn) ratio was 0.13. Some small grains with increased Sn/Cu ratio existed near back contact. Grain growth enhanced after Cd alloying, resulting in a more homogenous Sn/Cu ratio along thickness direction. SIMS profile of solar cell confirmed Cd incorporation in bulk while also a higher Cd content near absorber surface. Corresponding to microstructure, electrical properties were also modified by Cd alloying. In the absorber bulk, the trap energy levels and density of Cd-alloying sample were 38meV and 1.47 ×1016cm−3, while 118meV and 6.98 ×1015cm−3 for reference sample. The dominant recombination was in bulk, instead of at interface as in reference sample. Back contact was improved owing to a smaller series resistance and a smaller rise of Rs at low temperature. J-V curve observed an improvement of Voc and Jsc. The EQE curve indicated sharper absorption edges and band gap reducing from 1.00eV to 0.95eV. As a consequence, a highest efficiency of 11.2% for CZTSSe solar cells was achieved.

Photoresponse Performance Evaluation of ZnO UV Photodetector Based on Noise Analysis

The ZnO film photosensitive chips prepared by sputtering method were post-treated by air-anneal at different temperatures and Ar or air-plasma treatments for different time. Then, photosensitive chips were packaged into UV photodetectors with a standard TO-5 package to meet the sensitivity, selectivity, and stability standards. Noise analysis was used as an efficient tool to evaluate the quality and reliability of ZnO UV photodetectors, and the overall performance evaluation based on 1/f noise physical model is made to build the relationship between photoresponse and surface defect state density. The 1/f noise comes from defect-related carriers’ number fluctuation, which are trap and detrap processes of free electrons from oxygen vacancies at the surface of ZnO. This can be analyzed semi-quantitatively by the surface trap energy density obtained by voltage power spectral density. For the photosensitive chip treated by 300° air-anneal, we can obtain the largest responsivity 6.7 A/W, the smallest noise equivalent power $2.7 \times 10^{-9}$ W, and the largest specific detectivity $9\ast 10^{10}$ Jones at 300 nm under the bias of 5 V. The excellent photoresponse performance is also indicated by large linear dynamic range and fast response and recovery speeds.

Influence of oil aging on the space charge dynamics of oil‐immersed paper insulation under a DC electric field

The formation of space charge in oil/paper insulation system can lead to material degradation in the region of high electric field. Therefore, it is important to understand the factors that affect space charge formation in oil/paper insulation system. In this paper, the effect of oil aging on space charge dynamics in oil/paper insulation system is investigated using the pulsed electroacoustic (PEA) technique under a DC electric field at room temperature. The ultraviolet/visible (UV/vis) spectrum of the oil shifts to longer wavelengths, and the oil acidity increases as the aging time increases. It has been found that the oil property has a significant effect on the space charge distribution in the oil/paper insulation sample. The more the deterioration of the oil, the larger amount of negative and positive charges accumulated in the bulk of the oil/paper insulation sample. The total amount of slow moving charges, the surface trap energy density, and the electric field distortion of the oil/paper insulation sample increased with the degree of deterioration due to oil aging. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.