Charge Trap Depth Research Articles

Bio-based epoxy resin demonstrating high breakdown strength and low dielectric loss via intrinsic molecular charge traps construction

In the ultra-high voltage, ultra-high capacity and high frequency energy systems, epoxy-based dielectric materials demonstrating high breakdown strength and low dielectric loss are urgently needed. This work reports a strategy to modulate the charge trap depth in bio-based epoxy dielectric materials by tailoring the local molecular chain structures, ultimately leading to the significant suppression of high-field conductivity and dielectric loss. With the introduction of octa (dimethylsiloxy)octasilsiloxane (POSS) as the intrinsic molecular charge traps, our synthesized epoxy dielectric material exhibits a high breakdown strength of Eb-DC = 81.5 kV/mm, 37 % higher than the traditional bisphenol A epoxy resin (DGEBA), and low dielectric loss of tan δ = 0.0022, 45 % lower than that DGEBA. Furthermore, simulation results give the structure-property relationships, guiding the molecular design. Specifically, the dielectric properties are positively correlated with the LUMO energy difference and charge separation index of the polymer molecular chains. This work provides a promising pathway to enhance the dielectric properties of polymers by building intrinsic molecular charge traps, which is prospective for practical electronics and electrical power systems.

Methyl and carbon–carbon double bond in anhydride molecules modulated surface charge trap depth of epoxy materials

Epoxy insulators in gas insulated switchgear and gas insulated transmission lines tend to accumulate surface charges, leading to insulation flashover. Improving the surface trap characteristics of epoxy materials, which can accelerate the surface charge dissipation of epoxy insulators, is a promising method to improve the surface insulation performance. The surface trap characteristics of epoxy materials are strongly influenced by the chemical groups in the acid anhydride molecules. In this work, by quantum chemical calculations and isothermal surface potential decay tests, taking six organic anhydrides that differ only in the methyl and carbon–carbon double bonds, we find the modulation laws of methyl and carbon–carbon double bonds on the charge trap depth within and between molecular chains. The regulation mechanism is revealed from the microscopic perspectives of electron energy structure and electron cloud offset. The changes of surface charge trap depth of epoxy materials are primarily attributed to the changes in the spatial distribution of the electron cloud density between and on the valence bonds caused by the interaction between the electron-donating methyl group and the electron-absorbing carbon–carbon double bond.

Direct measurement of charge trap depth in polymer nanocomposites

Polymer nanocomposites (PNCs) exhibit excellent electrical properties owing to charge trapping provided by nanofillers. However, the role of nanofillers in trap formation at the microscopic level is poorly understood. In this study, we propose a method to determine the charge trap depth of nanofillers in PNCs using x-ray photoelectron spectroscopy (XPS) measurements and first-principles calculations. The low-density polyethylene (PE)/TiO2 nanocomposite is selected as the measurement target as it was previously reported the charges are trapped by TiO2 loading to PE. We observe TiO2 can serve as a trap for holes, and the trap depth is determined to be 0.9 eV. Furthermore, the computed charge trap depth calculated by G 0 W 0 calculation, which reproduce the experimental band gap, is comparable to the XPS result, which strongly supports the validity of our method. In addition, owing to the quantitative evaluation of the electronic structure, it was shown that the charge trap depth of the nanofiller can be controlled by tuning the surface dipole with surface modification of the nanofiller. The approach proposed in this study to determine the charge trap depth of nanofillers provides the prospect of designing PNCs with desirable properties from the atomic or molecular level.

Remarkable modification of transferring characteristics for both positive and negative charges in XLPE-PS composite

Insulating materials with excellent performance have been one of the key factors that keep the safe operation of high-voltage direct-current (HVDC) cable systems, and investigating their charge trap distribution characteristics is of great significance to improve electrical properties, such as reducing conductivity, suppressing space charge accumulation, and elevating breakdown strength. In this paper, surface potential decay processes of low-density polyethylene (LDPE), low-density polyethylene/polystyrene (LDPE/PS), crosslinked polyethylene-polystyrene (XLPE-PS) and crosslinked polyethylene (XLPE) were investigated, and migration properties of positive charges and negative charges as well as related charge trap characteristics were analyzed. The results demonstrate that for positive charges and negative charges, the formation of featured structure in XLPE-PS composites reduces carrier mobility and deepens charge trap depth but does not change the width of charge trap energy distribution, which remains 0.27eV. Compared with XLPE, XLPE-PS shows a more unified trap distribution under positive and negative polarity, which may be attributed to the featured structure. The findings are helpful to clarify the relationship between multi-phase structure and electrical insulation performance of XLPE, and to provide a reference for the design of insulating materials for extruded HVDC cables.

High Charge Transfer from C60 Pyrrolidine Tris-Acid to SnO2 Electron Transport Layer Directly Observed by ESR Spectroscopy

Charge transfer characteristics of perovskite solar cells (PSCs) are effectively modified using fullerene-passivated SnO2 electron transport layers (ETLs), which are used as electron injection components from the perovskite layer in PSCs. Characterizing the charge accumulation states of decisive scientific evidence is highly expected. Therefore, to investigate the charge accumulation states over time from a microscopic viewpoint, this study presents electron spin resonance (ESR) spectroscopy of two fullerenes layered with SnO2. The advantages of a C60 pyrrolidine tris-acid (CPTA) over [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) were demonstrated using their high performance in terms of the number of accumulated charges, location of charge accumulation, and comparison of the charge trap depth. In terms of the number of accumulated charges, a decrease in electrons on CPTA and an increase in the electrons on PC61BM were directly observed in their layered films with SnO2 under light irradiation, which indicates that the CPTA can be sufficient to transfer photocarriers under light irradiation, while PC61BM remains saturated during the transfer of photocarriers leading to the accumulation of charges. In terms of the location of charge accumulation, the electrons on CPTA are accumulated at bulk areas, whereas those on PC61BM are accumulated at the interface between PC61BM and SnO2. The interfacial charge accumulation will demonstrate a negative effect on the device’s performance. In terms of the comparison of the charge trap depth, the number of accumulated electrons on CPTA decreased rapidly, whereas those on PC61BM showed almost no change under dark conditions after light irradiation, indicating that the electrons on CPTA were trapped at a shallower depth than those on PC61BM. The direct observation of charge accumulation states on fullerenes of the fullerene-passivated SnO2 ETLs under light irradiation over time will be essential for optimizing the cell structures and enhancing cell power conversion efficiency and stability.

Charge trap depth prediction of grafted polypropylene system using machine learning

Carbon neutrality is a state of dynamic equilibrium in carbon dioxide emissions. In insulation material aspect, polypropylene (PP) with grafted side chain segments possesses recyclability and may have better electrical property under high temperature compared with traditional norecyclable insulation insulation material like cross-linked polyethylene, which will reduce the carbon emission and help carbon neutrality processing. However, focused on the ideal grafted PP chain, the bandgap () parameter is no longer suitable for describing the electrical properties of this system for the small grafting ratio. The induced grafted monomer might generate new band energy level and reduce the bandgap but with improvement on the electrical properties, which is not consistent with the positive correlation of the bandgap and the electrical properties. Alternatively, the trap depth, as one of the important electrical performance parameters, can reflect its influence on conductivity and breakdown. In this paper, the trap depth of the grafted PP chain is predicted by machine learning method with recursive feature elimination-based fingerprints. The paper analyzes the influence of different component groups and the ratioes of PP monomers on the charge trap depth. The grafted monomers with strong positive correlations on the electron/hole trap depth are given, and the electrical conductivity of PP-g-VK polymer has been tested for validation from the side. This paper provides a predictable parameter for the analysis of grafted systems and accelerates future predictions of performance parameters such as conductivity and breakdown strength.

Crosslinking-modulated direct-current conductivity of XLPE-PS composite via charge trap characteristics

Low direct-current (DC) conductivity is one of the most desired characteristics for crosslinked polyethylene (XLPE) as a high-voltage DC cable insulation material. In this Letter, a correlation between the DC conductivity and cross-linking characteristics of XLPE-polystyrene (PS) composites at 50 °C was studied. Experimental results show that by adjusting the cross-linking structure, different trap distribution characteristics for XLPE-PS composites were realized. With the increase in the cross-linking agent content, DC conductivities of XLPE-PS composites tend to decrease, and the introduced average trap depth increases correspondingly. An increase of 0.07 eV for average charge trap depth in XLPE-PS composites could be acquired in the test range. It is considered that the increase in the average trap depth reduces the carrier mobility, contributing to the reduction of DC conductivity for XLPE-PS composites. Thus, the DC conductivity and average trap depth of XLPE composites show a strong relevance. The results suggest that the interaction between PS particles and the matrix introduced by cross-linking plays a dominant role in determining the charge conduction for XLPE-PS composites.

Surface discharge of bulk materials investigated from change of charge trap characteristics of polyimide single molecular chain

Surface discharge is one of the reasons for insulation failure. Polyimide (PI) is used in gas-solid insulation of high-frequency electric power equipment. Therefore, based on density functional theory, the effects of single molecular chain structure, energy level, density of states, electrostatic potential, excited state and other micro parameters under external electric field on trap formation and surface discharge of both PI and polar- group- OH<sup>– </sup>affected PI are discussed from the atomic and molecular level. The results show that the structure of PI is crimped and the dipole moment increases under external electric field, which is easy to accumulate charges to form space charge center, especially after the introduction of polar group OH<sup>–</sup>. In the PI molecules, hole traps are formed in the benzene ring region, and electron traps are formed in the imide ring region. The number of electron trap energy levels is large, in which the space charge trap depth gradually deepens with the increase of external electric field. After the introduction of polar group OH<sup>–</sup>, the excitation energy of PI molecules decreases, which makes the electrons inside the molecules excited easily. The spatial separation of electrons and holes decreases with the increase of electric field, which is conducive to the recombination of holes and electrons to emit photons.

Investigation on Electrical, Thermal, and Mechanical Properties of Silicone Rubber ATH Nanocomposites

In the present study, investigations have been carried out by choosing the nano-sized ATH (30 – 50 nm) particles as filler content in silicone rubber to acquire a deep insight to understand the use of nano ATH fillers in silicone rubber on its performance, to use it as insulation structure as an outdoor insulator. Variation in the surface condition of silicone rubber nanocomposites due to corona aging and the recovery characteristics analysis carried out through contact angle measurement. Nano ATH added in silicone rubber, up to 3 wt% of filler, shows increased dielectric constant with less loss factor. Water droplet-initiated discharges on silicone rubber nano ATH composites under DC voltage measured using UHF technique and the damage caused zone analyzed through AFM studies and by surface potential measurement, which indicates higher surface roughness and increased charge trap depth. It is observed that negative DC voltage has higher CIV followed by positive DC and AC voltage. Laser Induced breakdown studies (LIBS) show a direct correlation between plasma temperature and material surface hardness. Also, on laser shining on the surface of the specimen, its temperature was measured on the backside of the specimen through thermal imaging confirming increased diffusivity of temperature. Laser flash analysis shows a direct correlation between filler content in base polymer and thermal conductivity and diffusivity. Higher storage modulus and activation energies measured from the dynamic mechanical analysis indicate the significance of prepared composites as an excellent mechanical support structure for the power system network.

Mode of action of the third component in ternary organic photovoltaic blend PBDB-T/ITIC:PC70BM revealed by EPR spectroscopy

Light-induced EPR (LEPR) and out-of-phase electron spin echo (ESE) techniques were applied to trace the difference of charge generation and recombination processes between highly efficient ternary OPV blend PBDB-T/ITIC:PC70BM and the corresponding binary blends PBDB-T/ITIC and PBDB-T/PC70BM. The average charge trap depth measured from LEPR decay traces decreases in the series PBDB-T/ITIC, PBDB-T/PC70BM, PBDB-T/ITIC:PC70BM. As it is evident from the out-of-phase ESE, the initial electron-hole distance within the charge-transfer state upon its formation increases in this series. The former effect facilitates charge transport, in accordance with increase of the electron mobility for the ternary blend revealed previously. The latter effect leads to reduced geminate recombination. Both effects are suggested to improve the power conversion efficiency of the ternary blends. Presumably, they originate from modification of the PBDB-T/ITIC blend morphology by addition of small amount of PC70BM.

An evaluation method of observable charge trap depth for the pea method and its complementarity with the Q(t) method

An evaluation method of observable charge trap depth for the pea method and its complementarity with the Q(t) method

Achieving tunable memory performance from nonvolatile to volatile by altering the trap depth of charge trapping sites in functional imides containing carbazole moieties

In this work, two functional imides were designed and synthesized to elucidate the influence of charge trap depth on the memory behavior. Two dianhydrides, 3,3′,4,4'-diphenylsulfonetetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride were selected as the electron acceptor considering their functional moieties of different charge-trapping depth and electron-withdrawing ability. Electrical characterization indicated that the memory devices based on the two imides exhibited nonvolatile write once read many times (WORM) memory behavior and volatile static random access memory (SRAM) behavior. Mechanisms associated with the distinct memory effect were demonstrated based on the molecular simulation. Analysis results indicated that charge trapping process is responsible for the electrical bistability and the different depth of charge-trapping sites in the two imides accounts for the distinct memory behavior of corresponding memory devices. Meanwhile, the two memory devices both show excellent long term operation stability. This paper not only reports two novel memory materials but also provided some guiding principle to the design of organic based memory devices.

Thermally stimulated depolarization current and dielectric spectroscopy used to study dipolar relaxations and trap level distribution in PMMA polymer

In this work, thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS) techniques were performed in complementarities to characterize the dielectric relaxation processes in poly(methylmethacrylate) (PMMA) polymer. Three relaxations were highlighted. Based on correlation between TSDC and DRS data, they were attributed respectively to dipolar β and α relaxations as well as interfacial ρ relaxation. New experimental procedure of fractional polarization was used to investigate space charge trap depth distribution within the band-gap of PMMA. Then the individual peaks were fitted to Bucci–Fieschi model, to evaluate the relaxation parameters. The obtained values were discussed in correlation to those evaluated from DRS data fitted to Havriliak–Negami and Arrhenius models. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energies were evaluated from the fit to the Bucci–Fuschi model. Similarly other parameters such as relaxation time and charge release are evaluated.

Effect of fluorination on the surface electrical properties of epoxy resin insulation

Epoxy samples were surface fluorinated in a laboratory vessel using a F2/N2 gas mixture to suppress surface charge accumulation. Attenuated total reflection infrared analyses indicate that the fluorination led to substantial variations in chemical composition and structure of the sample surface layer. Measurement results of surface properties indicate that surface conductivity and wettability or polarity were dramatically increased by the fluorination. A very likely decrease in charge trap depth and the adsorbed water on the surface in air are responsible for the high surface conductivity. As a result, charge cannot accumulate on the fluorinated surface even at room temperature, rapidly transporting along the surface. Surface charging current measurements further show a much larger steady state current flowing along the fluorinated surface, suggesting much lower dynamic surface potential or charge density during charging, compared with those for the original surface.