In the past G. Paasch has collaborated on articles with G. Gobsch and L. Sawtschenko. One of their most recent publications is Studies on thermally structurized polyacrylonitrile by the electrochemical impedance method. Which was published in journal Electrochimica Acta.

More information about G. Paasch research including statistics on their citations can be found on their Copernicus Academic profile page.

G. Paasch's Articles: (16)

Studies on thermally structurized polyacrylonitrile by the electrochemical impedance method

AbstractThe ac impedance of electrodes prepared by pressing powdered, thermally structurized polyacrylonitrile was measured in a solutin of 1 M LiClO4 in propylene carbonate. Samples of material structurized at a temperature of 700°C or higher have a conductivity of 10−1 Ω−1 cm−1 in the dry pressed state. Their Nyquist diagrams resemble a deformed semicircle at higher frequencies followed by a steeply rising branch; they are discussed in terms of the theory of porous electrodes and different scales of the pore sizes. Small pores with up to 3 nm diameter, responsible for specific surface areas up to about 1000 m2g−1, and the layered structure determine the low-frequency impedance and the excellent charge storage properties of the material.

Theoretical analyses of the shubnikov-de haas oscillation in InSb inversion layers

AbstractA calculation is presented of the subband and Landau energy levels in inversion layers adjacent to grain boundaries in InSb bicrystals. Within a rather simplified model the transverse magnetoresistance is calculated and compared with recent experiments. Some unexpected new features in the Shubnikov-de Haas oscillations are related to the occupation of several electric subbands.

Polymers: PolyacetyleneOn the current limiting mechanism in highly conductive polyacetylene

AbstractOur proposal of a novel barrier model for fluctuation-induced tunneling in highly conductive polyacetylene is shown to be in good agreement with recent TEM observations of single doped N - (CH)x-fibrils. This allows a more detailed discussion of the conduction mechanism. A revised version of the theory is presented.

Electrochemical studies and application of thermally structurized polyacrylonirile

AbstractThe electrochemical properties of thermally structurized polyacrylonitrile have been studied by means of cyclic voltammetry and by investigations of the charge storage process. Results of the influence of physical structure of the cathode material on the kinetics and specific capacity as well as the interaction of Li+ or C104− ions with the polymer are discussed. The charge/discharge behaviour of two charge storage devices has been studied.

Charge transport in organic light-emitting diodes: polarons or holes

AbstractElectroluminescence in organic light-emitting diodes (LEDs) based on poly(p-phenylenevinylene) (PPV) or related materials is usually attributed to the radiative decay of polar excitons. A consistent description also requires the understanding of the carrier transport through the active layer. We analyse d.c. conductivity measurements on PPV and show that the conventional semiconductor description can hardly be applied. A model for the transport is proposed, which is based on assuming the existence of a bipolaron ground state and hopping transport of thermally activated polarons. The resulting conductivity mechanism is an admixture of thermal activation and variable range hopping.

In situ electrochemical ESR investigations of the growth of one- and two-dimensional polypyrrole films

AbstractIn situ ESR-spectroscopic investigations of the electrochemical growth of polypyrrole (PPy) in 0.1 mol dm−3 toluene sulfonic acid solutions in acetonitrile are presented. The films were deposited potentiostatically and galvanostatically onto gold electrodes. ESR spectra were simultaneously recorded during electrodeposition processes at high and low current densities, respectively. Differences in the paramagnetic properties of the PPy films thus prepared have been observed depending on the applied current densities. Furthermore, the spin concentration in the PPy film is not proportional to the injected charge. Therefore, two different mechanisms of film formation occur depending on the applied current densities.

ArticleStructure-conductivity relation for polypyrrole with a two-dimensional microscopic structure

AbstractPolypyrrole (PPy) films with dominating two-dimensional microscopic structures show higher electrical conductivities and a weaker temperature dependence when compared to the corresponding one-dimensional material. For the temperature-dependent conductivity data, σ(T), a reasonable fit to an analytic expression is obtained for the fluctuation-induced tunneling mechanism. We use improved expressions for the Sheng formula to give a realistic connection between the fit parameters and the barrier characteristics which include the height of the barrier and the effective mass of the tunneling carriers. From these expressions, we determine the nominal length of the tunnel junctions to be 8.5 Å and the cross section to be 6 Å2. Based on these values we suggest a structure model to explain the parameters derived from the electrical conductivity. In this model, the polymer consists of islands with a two-dimensional (macrocyclic) structure. These islands of finite size are crosslinked by segments of one-dimensional PPy chains, the latter acting as tunnel junctions. Accordingly, the parameters obtained from our analysis of the temperature dependence correspond roughly to a chain length of two monomeric units of the crosslinking one-dimensional chain segments and to the cross section of the π-electron system, respectively.

Connection of the generalized Shuttleworth equation for the elastic spherical electrode with the Laplace formula and the Gibbs adsorption equation

AbstractThe surface strain of a solid is related to the surface free energy (interfacial superficial work) by the Shuttleworth equation. Its generalization for the case of a finite surface strain has been derived only recently by one of the present authors (B.M.G). Now consequences are derived for the case of an elastic spherical electrode. At first, it is shown that this generalized form is in accordance with the Laplace formula connecting the capillary pressure with the surface stress. Further, the generalized Shuttleworth equation leads to an additional term in the Gibbs adsorption equation, which is of first order in the elastic strain. Whereas this first order term may be negligible in the adsorption equation itself even for non-infinitesimal strain, it leads to a significant modification when considering second order derivatives of surface charge and surface stress which are directly accessible in experiments. A reformulation is presented for changing variables and the applicability to small particles is discussed.

Transfer matrix method for the electrochemical impedance of inhomogeneous porous electrodes and membranes

AbstractThe method presented here is based on the two-phase model of a porous system with two continuous subsystems, electrons in the porous material and ions in the pore electrolyte. Both are continuously interconnected via the pore surfaces e.g. by the double layer capacity and/or the charge transfer resistance. The equivalent circuit for this system is the transmission line model. The method applies to systems with parameters which are not constant across the layer. The layer is divided into a number of slabs and in each slab all parameters are replaced by their mean values. The potentials and the currents of two adjacent slabs are connected by a matrix, in the general case a 4×4 matrix. The potential propagation in the whole layer is determined by the product matrix. The impedance for both a layer coating a metallic current collector and a porous membrane embedded in the electrolyte (or the porous layer with electrolyte-filled pores in between two metallic current collectors) can be expressed by the elements of the product matrix. The matrix is reduced to a 2×2-form if one of the resistivities is negligible. In this case for a system of two homogeneous sublayers an analytical formulation is given. The method is applied to a system with an interconnection consisting of double layer capacity, charge transfer resistance and its hindrance by finite diffusion (applicable to polymers). Here the inhomogeneity gradients of the resistivities are considered. It is demonstrated that they can result in significant qualitative modifications of the impedance. This concerns especially the low frequency pseudo-capacitive behaviour which is transformed into a dependence resembling the well known empirical description by constant phase elements often used to interpolate experimental data.

Electron Density at Simple Metal Surfaces and Lattice Relaxation

From LEED crystallography it was concluded in the last years that at metal surfaces there exsist a relaxation of the top lattice plane perpendicular to the surface (contraction or dilatation). Different attempts for the calculation of the relaxation for simple metals are critically reviewed. A model calculation shows that reliable results can be obtained only if i) the electron density is calculated for the relaxed surface and ii) interaction of the ions with the electrons is calculated not with point ions but with more realistic approximations (pseudopotentials). The results indicate that a contraction of about 10% of the interlayer distance does not occur. This is contrary to interpretations of LEED experiments which should be reconsidered.

Approximate Calculation Schemes for Inversion Layers and Multi-Layer Structures

Publisher SummaryInversion layers are usually generated at the interface of a semiconductor with an insulator. A wellknown example is the silicon field effect transistor. Device modelling is usually done with a classical description. On the other hand detailed investigations of the complete subband structure of inversion layers are carried out by self-consistently solving the equivalent Schrödinger equation. A modified local density approximation is developed and discussed in the chapter. It yields for most interesting quantities of inversion layers results with practically the same accuracy as by solving the Schrödinger equation. But it is numerically as simple as the classical method. Due to the simplicity of this new method, new results are obtained. Among them only the dependence of the surface electric field from the band bending and the influence of the interface barrier on transport quantities are mentioned in the chapter.

Simulation and modeling of C–V curves of OLEDs with trap states for the holes

AbstractSimulations of dynamic capacitance–voltage (C–V) curves for organic light emitting diodes (OLED) with deep traps for the holes are presented. A systematic variation of parameters leads to clearly identifiable variations of the C–V curves. An equivalent circuit is deduced from the ac small-signal equations. In connection with the Nyquist representation of the dielectric function it is well suited to extract trap- and other material parameters from experiments.

The influence of bulk traps on the subthreshold characteristics of an organic field effect transistor

AbstractThe subthreshold characteristics of fabricated organic field effect transistors based on regioregular poly(3-dodecylthiophene) (P3DDT) as the active layer and poly-4-vinylphenol (P4VP) as the gate insulator have been investigated. The transistor turn-on occurs at a threshold voltage of around Vth=0 V. The (hole) mobility of 0.002–0.005 cm2/(V s) has been estimated from the linear region of the transfer characteristics. As usually observed for organic transistors, the inverse subthreshold slope is very high, in our case S≈7 V/dec. Furthermore, the subthreshold current depends on the drain voltage although the transistor is a long channel device. One possibility to explain these peculiarities are interface traps, as demonstrated recently by Scheinert et al. [J. Appl. Phys. 92 (2002) 330]. In this paper, the influence of bulk traps is shown. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained also by recharging of bulk traps. Therefore, other frequency and temperature dependent dynamic measurements have to be applied to distinguish between the different possible influences.

Simulation of organic light emitting diodes: influence of charges localized near the electrodes

AbstractIn spite of experimental evidence for the formation of charged layers near the electrodes of organic light emitting diodes (OLED), the influence of such layers on the OLED performance has not yet been clarified. This article presents a simulation study of this subject, utilizing the drift-diffusion model. In order to understand the principal mechanism of the influence of such layers, only monolayer devices with unintentional low p-doping are considered. In this case, positively charged layers near the anode or the cathode modify the current voltage characteristics for areal charges above some critical value. Effectively, such areal charges create an additional barrier of a magnitude which depends on the applied bias. A fixed positive areal charge near the anode decreases the current. However, due to the additional bias-dependent barrier, with increasing forward bias one has then a rather strong increase of the current resembling trap assisted space charge limited current. A fixed positive areal charge near the cathode leads to an increase of the build-in potential compared to the ideal thin-layer value which is given by the work function difference of the electrodes. The possibility of compensating the effect of the fixed positive charge with the help of a p-doped layer is discussed.

Plasmon dispersion in quasi-one-dimensional (TaSe4)2I and K0.3MoO3

AbstractWe present results of experimental and theoretical studies of the plasmon dispersion in the quasi-one-dimensional conductors (TaSe4)2I and K0.3MoO3. For both compounds, the dispersion relation, as measured by electron energy-loss spectroscopy in transmission, is quasilinear over a wide momentum range. Different theoretical approaches are discussed to explain the data, all based on a one-band model within the framework of the random-phase approximation. From this analysis, we find in contrast to photoemission measurements no hint for singular one-dimensional properties.

The influence of deep traps on transient current–voltage characteristics of organic light-emitting diodes

AbstractTransient current–voltage (I–V) characteristics of organic light-emitting diodes made from both conjugated polymers and low molecular-weight materials show hysteresis effects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine with Ca and indium–tin oxide as electrodes. To clarify the origin of this peculiarity numerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such as measuring conditions, trap characteristics, basic doping level, mobility and injection conditions. Based on the simulated potential and concentration profiles it is shown that the hysteresis of the I–V characteristics is caused by recharging of deep traps for holes. It occurs only if the reverse steady-state current is lower than the trap recharging current and if both currents have different bias dependencies. The origin for the large time needed for the traps to relax into the equilibrium state is clarified. In accordance with the high barrier for the holes at the cathode the calculated reverse current is much smaller than the measured one. Using a new analytical expression for the Schottky diode I–V characteristics for a low-doped thin film device, it is shown qualitatively that in real devices a leakage current should dominate for reverse bias.

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