Armin Wedel

High surface-charge stability of porous polytetrafluoroethylene electret films at room and elevated temperatures

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Porous PTFE space-charge electrets for piezoelectric applications

Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d/sub 33/ coefficients of up to approximately 600 pC/N or 600 pm/V. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.

InP/ZnSe/ZnS: A Novel Multishell System for InP Quantum Dots for Improved Luminescence Efficiency and Its application in a Light-Emitting Device

Indium phosphide (InP) quantum dots (QDs) are considered alternatives to Cd-containing QDs for application in light-emitting devices. The multishell coating with ZnSe/ZnS was shown to improve the photoluminescence quantum yield (QY) of InP QDs more strongly than the conventional ZnS shell coating. Structural proof for this system was provided by X-ray diffraction and transmission electron microscopy. QY values in the range of 50–70% along with peak widths of 45–50 nm can be routinely achieved, making the optical performance of InP/ZnSe/ZnS QDs comparable to that of Cd-based QDs. The fabrication of a working electroluminescent light-emitting device employing the reported material demonstrated the feasibility of the desired application.

New n-type rigid rod full aromatic poly(1,3,4-oxadiazole)s and their application in organic devices

Abstract By introducing linear or branched alkoxy groups as side chains into the backbone, it is possible to synthesize rigid rod fully aromatic poly(1,3,4-oxadiazole)s which are soluble in organic solvents. Two different synthetic routes are used. First, a classical polycondensation reaction is developed involving an aromatic diacid dichloride and a dihydrazide followed by a ring closure reaction. As another polymerization reaction, the Suzuki coupling reaction is applied to synthesize polymers with a well-defined structure. The charge transport and optical properties are investigated by cyclic voltammetry (CV), current–voltage measurements and both absorption and photoluminescence spectroscopy. According to the CV-measurements, the oxadiazoles possess electron transport properties. First results will be presented about the application of these polymers in diodes.

A New Class of Organosoluble Rigid-Rod, Fully Aromatic Poly(1,3,4-oxadiazole)s and Their Solid-State Properties, 2. Solid-State Properties

New symmetrical and unsymmetrical dialkoxy-substituted aromatic poly(1,3,4-oxadiazole)s are investigated by cyclovoltammetric measurements. The electrochemical redox behaviour of these materials is compared with the chemical structure. The reduction behaviour allows the estimation of the electron affinity, and shows that these polymers are suitable candidates for electron-transport materials in electronic devices. Surprisingly it is found from UV-vis and fluorescence spectroscopic measurements that the polyoxadiazole with linear alkoxy side chains (C16) shows a reversible thermochromism in solution as well as in the solid state. The changes in this temperature range are ascribed to a phase transition between aggregated chains, in solution as well as in the film, at room temperature and molecularly dissolved polymer chains at higher temperature. The other poly(1,3,4 oxadiazole)s with branched alkoxy side chains exhibit an improved solubility and they are completely soluble in odichlorobenzene at room temperature. The absorption and emission behaviour does not change in the temperature range from 25 °C to 100 °C. The fluorescence excitation studies of these poly(1,3,4-oxadiazole)s show a blue emission in solution and in the film. Due to these results the poly(1,3,4-oxadiazole)s are used as electron transport and blue emitting layers in electronic devices like diodes or polymer light emitting diodes (PLED).

Piezoelectric activity in a copolymer of acrylonitrile and methylacrylate

Results of investigations of piezoelectric properties in a copolymer of acrylonitrile and methylacrylate are summarized. A preliminary explanation is given of the observed phenomena on the basis of surface potential measurements. The maximum values of piezoelectric strain coefficient d/sub 31/ obtained after optimizing the poling conditions for stretched and unstretched films were around 3 and 0.85 pC/N, respectively. The time stability of piezoelectricity was found to be relatively low. A hysteresis loop of electric displacement versus electric field was observed and assumed to be a manifestation of a ferroelectriclike behavior. All the observations point to a dipolar nature of piezoelectricity where ferroelectric and frozen-in dipole orientations obviously superimpose. >

Electrochemical redox behavior of aromatic poly(1,3,4-oxadiazole)s and sulfur containing polymers and their use in LED's

Abstract New polymers with 1,3,4-oxadiazole and thianthrene units in the main chain were synthesized and the electrochemical doping processes of these polymers have been studied by cyclic voltammetric measurements. The investigated silicon containing poly-1,3,4-oxadiazole can be reversibly reduced electrochemically. For the polymer with thianthrene units the cyclovoltammogram shows a reversible Oxidation process. A single layer device with the thianthrene polymer was prepared by spin-coating technique.

Polymer multilayer systems for electronic applications

Abstract For polymer electronic applications, it is important to use multilayer structures with conjugated polymers. For this reason, modified polymer structures were synthesized. The cyclovoltammetric measurements allows us to estimate the HOMO and LUMO energy levels of the synthesized materials. This knowledge is necessary for the development of efficient components like polymer light emitting diodes, polymer diodes and transistors. The characterization of such devices by current–voltage and brightness–voltage measurements will be described.

Increased shell thickness in indium phosphide multishell quantum dots leading to efficiency and stability enhancement in light-emitting diodes

We report efficient indium phosphide (InP) quantum dot-based light-emitting diodes (QD-LEDs). The current efficiency and the device stability of QD-LEDs were enhanced by increasing the thickness of ZnS outer shell of InP/ZnSe/ZnS multishell QDs. Reversible luminance degradation was observed in operation of QD-LEDs, which was hypothesized to result from QD charging. QDs having thicker ZnS shell with strong confinement suppressed the luminescence quenching as well as QD charging. Our findings about the reversible QD charging and the developed performance by the thick ZnS outer shell would help to rationalize the luminance quenching issue in QD-LED operation.