Study Charge Injection Research Articles

The effect of chemistry and thermal fluctuations on charge injection barriers at aluminum/polyolefin interfaces

Charge injection at metal/polymer interfaces is a critical process in many technological devices, including high voltage capacitors and cables in which polyolefin materials, such as polyethylene (PE) and polypropylene (PP), are often used as insulation materials. We use simulations based on density-functional theory to study charge injection at aluminum/PE and aluminum/PP interfaces. Specifically, we investigate the influence of incorporating a variety of polar chemical impurities at the PE and PP chain ends on electron and hole injection barriers. Crucially, we account for the effect of thermal disorder by considering ensembles of thousands of interface structures obtained from ab initio molecular dynamics trajectories at 373 K. We show that the mean injection barrier can change by up to 1.1 eV for Al/PE and 0.6 eV for Al/PP, as compared to the pristine case, depending on which chemical impurity is introduced. We also show that the spread of injection barriers from thermal fluctuations also depends strongly on the chemistry of the impurity. The observed trends can be understood with a simple model based on thermal fluctuations of the dipole moment density associated with the chemical impurity at the interface. We further verify this model by considering larger interface models with lower impurity densities. Our results demonstrate that small chemical modifications, which may arise from oxidation, for example, have a significant influence on charge injection barriers in metal/polyolefin interfaces.

Research on Insulation State of 10kV XLPPE Fault Cable Based on Qt Technology

The cross-linked polyethylene cable will undergo thermal aging during service, which will seriously affect the service life of the cable. The current integration method can analyze the charge injection and conduction parameters from the dynamic change rate of the charge over time, and then analyze the insulation state of the polymer. Aiming at the field 10KV fault breakdown cable, this paper uses the current integration method to compare the dynamic change characteristics of the charge of the faulty cable and the new cable, and uses the ratio of Q(t) at t=600s and t=4s to study Charge injection and accumulation situation. Through analysis, it is found that the fault breakdown of the cable, the maximum charge accumulation under high voltage is 9000 Q/nC, the charge change rate is about 2, and the dielectric constant is 3.1, which is much greater than that of the new cable, indicating that the breakdown of the three-phase cable is mainly caused by the cable. It is caused by aging and eventually leads to insulation deterioration and breakdown.

First-principle investigation of the charge injection barriers of polyethylene and polytetrafluoroethylene oligomers

Experimental research has shown that much less charge injection occurs in polytetrafluoroethylene (PTFE) compared to polyethylene (PE). To clarify the mechanisms of charge injection from metals into polymer insulators, we comparatively studied charge injection in PE and PTFE oligomers using first-principles calculations. Two different models were studied: chemisorption (bonding) and physisorption (nonbonding). The results show that the electron injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface only in the case of chemisorption. The larger electron injection barrier of the metal/PTFE oligomer interface is mainly affected by the positive vacuum level shift of the metal/PTFE interface induced by electron transfer from the metal to PTFE along the chemical bonds. In the case of physisorption, the hole injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface. This is attributed to the larger ionization potential of PTFE compared to PE. The calculated results reasonably explain the experimental phenomena. The agreement between the experimental and calculated results verifies the rationality of our calculation models. The models used herein can likely be applied in other metal/polymer interfacial systems with acceptable accuracy.

Quantitative analysis of charge transport in intrinsic and doped organic semiconductors combining steady-state and frequency-domain data

Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordinary two-plateau capacitance–frequency curve of these hole-only devices, which originates from charges that flow into the intrinsic layer at zero applied offset voltage. We demonstrate that the observation of this behaviour is a direct indication for ideal charge injection properties and the observed capacitance–frequency feature is linked to the charge carrier mobility in the intrinsic layer. The extracted material parameters may directly be used to simulate and optimize full OLED devices employing the investigated hole-injection and -transport materials.

Engineering charge injection and charge transport for high performance PbSe nanocrystal thin film devices and circuits.

We study charge injection and transport in PbSe nanocrystal thin films. By engineering the contact metallurgy and nanocrystal ligand exchange chemistry and surface passivation, we demonstrate partial Fermi-level pinning at the metal-nanocrystal interface and an insulator-to-metal transition with increased coupling and doping, allowing us to design high conductivity and mobility PbSe nanocrystal films. We construct complementary nanocrystal circuits from n-type and p-type transistors realized from a single nanocrystal material by selecting the contact metallurgy.

Electric field induced transition from electron—only to hole-only conduction in polymer—fullerene metal-insulator-metal devices

We studied charge injection and transport mechanisms in blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM), by analysing dark, temperature dependent current-voltage characteristics of the P3HT:PCBM blend thin films sandwiched between aluminium electrodes in a metal/insulator/metal (MIM) configuration. We show that by applying appropriate electric fields, it is possible to switch from electron-only conduction to hole-only conduction in a single Al/P3HT:PCBM/Al device and present a general method of interpreting Fowler-Nordheim plots of MIM devices with pronounced non-linear characteristics by dividing them into several regions based on physical origins. We affirm that electrons can be selectively transported through the lowest unoccupied molecular orbital of PCBM at low applied voltages and low temperatures; and alternatively holes can be transported through the highest occupied molecular orbital of P3HT at higher applied voltages and high temperature, within a single device.

Study of Charge Diffusion at the Carbon Nanotube−SiO2Interface by Electrostatic Force Microscopy

Hysteresis behavior is observed in the transfer characteristic of most carbon-nanotube-based field effect transistors, and charges trapped at the carbon nanotube−dielectric interface are believed to be the cause. We have studied charge injection and dissipation around the interface of carbon nanotubes and SiO2 at different temperatures using an electrostatic force microscope. Numerical simulations were performed to extract the charge diffusion coefficients on the SiO2 surface under ambient conditions at different temperatures, and a critical temperature of ∼150 °C is observed. The activation energy of charge diffusion changes from ∼0.43 to ∼0.98 eV above this temperature, which is attributed to the change of surface chemistry. A more accurate model taking into consideration the electrostatic interaction among charges is used subsequently, and the fitting results are significantly improved. It is noted that the two models lead to similar activation energies.

Real versus Measured Surface Potentials in Scanning Kelvin Probe Microscopy

Noncontact potentiometry or scanning Kelvin probe microscopy (SKPM) is a widely used technique to study charge injection and transport in (in)organic devices by measuring a laterally resolved local potential. This technique suffers from the significant drawback that experimentally obtained curves do not generally reflect the true potential profile in the device due to nonlocal coupling between the probing tip and the device. In this work, we quantitatively explain the experimental SKPM response and by doing so directly link theoretical device models to real observables. In particular, the model quantitatively explains the effects of the tip-sample distance and the dependence on the orientation of the probing tip with respect to the device.

Charge injection in study of ferroelectric film properties

A model of a capacitor with a two-layer insulator has been considered. The current density exponentially depends on voltage in one of layers and is negligible in the other layer. The dependences of current on the time of voltage action and the charge-voltage characteristics of the two-layer capacitor have been calculated. The results obtained can be used to detect and study charge injection during polarization of ferroelectrics.

Controlling Charge Injection in Organic Field-Effect Transistors Using Self-Assembled Monolayers

We have studied charge injection across the metal/organic semiconductor interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect transistors, with Au source and drain electrodes modified by self-assembled monolayers (SAMs) prior to active polymer deposition. By using the SAM to engineer the effective Au work function, we markedly affect the charge injection process. We systematically examine the contact resistivity and intrinsic channel mobility and show that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated Au electrodes.

Probing Electronic Properties of dendritic Ruthenium Complex bound to Single Walled Carbon Nanotubes

ABSTRACTWe have functionalized single walled carbon nanotubes (SWNTs) using ruthenium centered organometallic supramolecular dendritic complexes. These dendrimers have multiple binding sites with chemically specified chirality. Most importantly, they are mechanically rigid. We have fabricated field effect transistors (FET) using these functionalized SWNTs. Devices were processed with standard optical lithography and high resolution e-beam lithography. We are using FET response of these devices to study charge injection into the nanotubes and the resultant effect on the tube's transport properties. Organometallic based molecular adsorbents onto the nanotubes effect the transistor response. These functionalized tubes change their majority carriers from holes to electrons in these FETs. We believe this is due to charge transfer from the metal center through the ligand and finally onto the nanotube. This results in the potential for optically altering the carrier density, and therefore the transport properties of the nanotubes.

New methodology for surface potential decay measurements: application to study charge injection dynamics on polypropylene films

Our work shows, through measurements on polypropylene films, the validity of surface potential measurements as a technique for dielectric properties analysis. We propose a representation of the decay and potential return curves that separates different components of dielectric response, by using tdv/dt vs log t plots. Using this transformation, and varying some experimental conditions we have identified different contributions to the potential decay. From our experimental observations we suggest a qualitative interpretation of the initial seconds of potential decay for negative corona charged samples that uses a model controlled by surface trap emission and charge exchanges between corona discharge and the insulator surface.

Charge injection and transport in films of CdSe nanocrystals

We have studied charge injection and charge transport in thin disordered films of CdSe nanocrystals between metal electrodes. Current–voltage characteristics of these devices are investigated as a function of electrode material, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanoparticles. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. High work function materials such as gold and indium-tin oxide are found to be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current–voltage characteristics exhibit a history and time dependence which is characteristic of persistent photoconductivity, with current at constant bias decaying with time according to a stretched exponential form. We propose a model based on space-charge limited current dominated by mobile electrons which slowly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K. We use the data and simulations to estimate the electron mobilities to be in the range of ∼10−4–10−6 cm2 V−1 s−1 and the trap densities to be approximately 2×1016 cm−3.

Charge injection and transport in poly( p-phenylene vinylene) light emitting diodes

We have measured the temperature dependence of electroluminescence in poly(p-phenylene vinylene) (PPV) devices in order to study charge injection across the electrode/polymer interfaces. The devices used were made with an indium tin oxide (ITO) hole injecting contact, and either an aluminium, magnesium, or calcium electron injecting contact. The IV curves show two distinct features. There is a thermally activated process at low fields, followed by field emission at high fields, when light is seen. The thermally activated process is frozen out below ∼200K, and has an activation energy of ∼0.3eV – 0.4eV. These data suggest that the field emission is that of holes across the ITO/PPV interface. The thermally activated process may be space charge limited conduction, in which case the activation energy measured is that of the charge mobility in the polymer.