Ionic Space Charge Polarization Research Articles

PEO‐PVP‐NaIO4 Ion‐Conducting Polymer Electrolyte: Inspection for Ionic Space Charge Polarization and Charge Trapping

Ion‐conductive solid polymer electrolytes composed from blends of poly(ethylene oxide) (PEO) and poly(vinylpyrrolidone) (PVP), as complexed with the ionic compound sodium periodate (NaIO4), are inspected for the presence of electric charge trapping (CT) and ionic space charge polarization (SCP) under static electric field. Thin films (110 μm‐thick) of these materials are produced at a ratio of the polymers PEO:PVP = 70:30 wt%, the concentration of NaIO4 is 5, 7.5, or 10 wt%. The electrical current at room temperature, as well as the charging/discharging in the films are studied as depending on applied voltage and time. At a detectable level, no SCP and CT processes in PEO‐PVP‐NaIO4 are evidenced, in contrast to identical experiments by PEO film under the same experimental conditions. The largely reduced SCP and CT are of importance for electrochemical applications of the considered ion‐conducting PEO‐PVP‐NaIO4 ion‐polymer coupled system.

Electrode effects in solid electrolyte capacitors

Thin films of ion-conducting polyethylene oxide (PEO) are prepared as a model system for solid polymer electrolytes. Plane-parallel capacitance structures of the type Al-PEO-Al (Metal-Insulator-Metal, MIM) with blocking electrodes are produced. In the short time range t⩽1s after the application of an external field, an ionic space-charge polarization with an exponential time dependence prevails. In the long time range the samples exhibit a Kohlrausch behavior of the polarization current, i.e., j∝t−α. This time dependence can be explained by image charges in the electrodes which attract the mobile ions. This attraction of ions is responsible for the static and the dynamic behavior of the system. The remanent polarization appearing in hysteresis measurements is an affirmation of the presence of these image charges. Computer simulations of a three-dimensional hopping model yield qualitatively the same results. Here the attraction of ions at the electrodes due to the image charges can be observed, as well as the experimentally determined t−α currents and the hysteresis of the polarization. This behavior cannot be simulated with a standard continuum model.

Photovoltaic properties of double layer devices consisting of titanium dioxide and porphyrin dispersed hole transporting material layer

The photovoltaic effect in the double layer device consisting of titanium dioxide (TiO 2) and tetraphenyl-porphirin (TPP) blended with hole transporting material (HTM), denoted as ITO/TiO 2/TPP+HTM/Au, has been investigated in this study. Short circuit photocurrent increases drastically by the usage of double-layered structure and fluorene based monomeric material as HTM. Photovoltaic properties are improved considerably using dopants and dyes with higher ionization potential energy than that of HTM. Furthermore, it was proposed that the quantum efficiency of the photovoltaic device is improved effectively by the treatment of ionic space charge polarization (poling) when supporting electrolyte was doped into organic layers. The enhancement of photocurrent in double layer device is explained in terms of the increase of electric field at the TiO 2/organic layer interface due to the accumulation of ionic space charges.

Dispersive ionic space charge relaxation in solid polymer electrolytes. II. Model and simulation

Dynamic Monte-Carlo simulations of ionic space charge relaxations are carried out using a three-dimensional model for thermally activated ion hopping in a multiwell energy structure. In this model a solid polymer electrolyte is embedded between two ideal blocking electrodes. The polymer is subdivided into 100×100×100 lattice cells. Positive ions (typically 1000) are distributed on the cells. To provide charge neutrality a negative background charge, constant in space and time, is introduced. The positive ions are able to hop between neighboring cells, surmounting energy barriers of distributed heights. The barrier heights consist of an intrinsic part due to the polymer structure, a part due to the Coulomb interaction of the ions, and a part due to the externally applied field. To calculate the interaction between the ions and the electrodes a method of images is used. Periodical boundary conditions are used for those lattice surfaces which are not in contact with the electrodes. The ionic space charge polarization process is simulated as dependent on the time, the sample thickness, the ion concentration, and the externally applied voltage. The polarization current after a step of the electric field shows dispersion due to distributed energy barrier heights in the short time range and a Kohlrausch behavior due to image charges in the long time range.

Structures and electrical properties of arachidic acid Langmuir–Blodgett films with cyanine dyes prepared by applying bias voltages

Arachidic acid (C20) Langmuir–Blodgett (LB) films with cyanine (Cy) dyes were fabricated by the diffusion–adsorption LB method with applying bias voltages Vd, and the structures and the electrical properties were investigated. From the absorption spectra, the amount and the orientation of the Cy dyes and the formation of the aggregates in the LB films were found to be controlled by Vd. The electrical properties were investigated from conduction current and thermally stimulated current (TSC) measurements. The current–voltage characteristics and transient current properties showed the ionic hopping conduction. From the transient current properties, the sample deposited under negative Vd seemed to include larger amounts of mobile ions and the mobile ions were supposed to be positive. The TSCs were also due to the ionic space charge polarization. The results in this work will be useful for the control of electrical properties in LB film devices.

Thermally stimulated currents and electrical conduction in self-assembled ultrathin films

Ultrathin films were formed from two types of non-conjugated polyelectrolytes, that is, polyallylamine (PAH) and polystyrensulfonate (PSS) using a self-assembly process and their electrical properties were investigated. Current versus voltage properties of the devices showed hopping conduction. The conduction was considered to be due to the migration of mobile ions in the sample, and that was also supported by temperature and humidity dependence of transient currents. Furthermore, thermally stimulated currents (TSCs) properties also suggested the ionic space charge polarization. Although the origin of mobile ions is not yet clear, the existence of sodium in the films was observed from the electron spectroscopy for the chemical analysis (ESCA). The humidity (i.e. water in the devices) also had a great influence on the electrical properties. The sodium ions and/or oxonium ions in the device are considered mobile ions.

Ionic Conduction and Photoisomerization in Azobenzene Langmuir-Blodgett Films Adsorbing Cyanine Dyes

Azobenzene (Az) Langmuir-Blodgett (LB) films adsorbing cyanine dyes (NK-3) were deposited and the electrical properties were investigated in detail. The current vs electric field characteristics suggested ionic hopping conduction, and the transient current properties showed current peaks due to migration of mobile ions. The thermally stimulated currents (TSCs) were also due to the ionic space charge polarization, that is, displacement of mobile ions over long distances in the LB films. These results indicated that the electrical properties of the Az LB devices are strongly dependent on the mobile ions in the devices. In addition, the TSC and the short-circuit photocurrent properties due to the dyes were also changed by isomerization of the Az molecules. The results showed that the ionic space charge polarization and the aggregates of the dyes were affected by the isomerization.

Thermally Stimulated Measurements in Solid Dielectrics

Thermally stimulated measurements for discriminating charge species and polarity of charges in solid dielectrics are described in the present paper. Thermally stimulated current characteristics due to dipolar polarization, charge trapping and ionic space charge polarization are clearly different from one another. The three charge species mentioned above can be discriminated by thermally stimulated currents. Further total electronic space charge and its mean spatial depth from the electrode due to injection can be measured by thermally stimulated current. Polarity of trapped charge (electron hole) and mobile ions can be discriminated by temperature gradient thermally stimulated surface potential measurement where temperature gradient in sample films is formed in heating.

Thermally Stimulated Current of Tsuga (Tsuga sieboldii CARR) in Cellulose I

Thermally stimulated current (TSC) of tsuga (Tsuga sieboldii CARR) in Cellulose I has been measured for the fiber and tangential directions in the temperature range from −20-°C to 200°C. Three peaks A, B, and C for the fiber direction are observed at about 30°C, about 85°C and about 160°C, respectively. The peaks A and B appear for the tangential direction. The TSC currents Imax of the peaks A and B are proportional to the thickness of the sample, and increase with the polarizing field in the relation of the sine-hyperbolic equation. The peaks A and B are considered to be due to ionic space-charge polarization caused by the migration of mobile ions between neighbour-sites and next neighbour-sites of the long period with hopping.

架橋ポリエチレンのイオン空間電荷分極による残留電圧

架橋ポリエチレンのイオン空間電荷分極による残留電圧

Residual voltage due to ionic space charge polarization in cross‐linking polyethylene

Residual voltage due to ionic space charge polarization in cross‐linking polyethylene

Numerical analysis of thermally stimulated current due to ionic space charge polarization

Numerical analysis of thermally stimulated current due to ionic space charge polarization

Numerical hopping method analysis of ionic space charge polarization characteristics at ac large signal application

Numerical hopping method analysis of ionic space charge polarization characteristics at ac large signal application

Stability of heterocharges in poly(methyl methacrylate) thermoelectrets. II. Effect of γ‐irradiation on thermally stimulated current spectra

AbstractThermally stimulated currents (TSC) of γ‐irradiated poly(methyl methacrylate) thermoelectrets have been investigated in the high‐temperature range. The two main peaks of the TSC spectrum, appearing, respectively, at about 100 (α) and 120°C (ρ), show marked differences in stability under irradiation: while the α peak is little affected by doses up to 107 rad, the ρ peak vanishes following doses of 105 to 106 rad. On the other hand, a study of the rate of decrease of the ρ peak as a function of irradiation conditions reveals the important role of the surrounding atmosphere and the existence of a flux effect, probably related to irradiation‐accelerated diffusion of gas into the polymer. All these facts confirm the respective attributions of α and ρ peaks to a dipolar volume polarization and to an ionic space charge polarization, showing that the TSC study of irradiated thermoelectrets can be a useful method for identification of polarization processes in polymers.

イオン空間電荷分極による熱刺激電流

イオン空間電荷分極による熱刺激電流

Dielectric relaxation in thermally grown SiO<inf>2</inf>films

An extensive investigation of the dielectric properties of thermally grown silicon dioxide films was performed in the temperature range from 400°C to 525°C. Principally, the variation of dissipation factor with frequency was observed at various applied peak fields and oxide thicknesses. In the temperature and frequency domain investigated, a large peak in the dissipation factor occurred. This peak corresponded to a massive capacitance dispersion with accompanying peaking of the loss factor at low frequencies. The phenomenon was apparently an ionic space-charge polarization. Theoretical developments based on this model were verified by experiment. The activation energy for the relaxation time was 10.1 ± 0.4 kcal/mole. Examination of the oxide thickness dependence indicated that the carriers were not uniformly distributed initially, but a fixed number was present at a particular temperature and this number was independent of thickness. Further investigation led to the conclusion that a reaction between the metal electrode and the SiO 2 produced positively charged oxygen vacancies which migrated through the oxide under the influence of an electric field. A mechanism for the production and migration of these vacancies is proposed which complies with the low activation energy as well as the observed temperature and electrode dependence.