Dieter Meissner

Photocurrent spectroscopy for the investigation of charge carrier generation and transport mechanisms in organic p/n-junction solar cells

The photovoltaic behavior in a perylene/phthalocyanine hetero-p/n-junction solar cell was investigated using intensity-dependent I/V-characteristics and short circuit photocurrent spectroscopy. It is concluded that the charge carrier generation occurs only in a very thin active region at the contact. By optimizing the light trapping, a maximum solar AM 1.5 efficiency of about 2% can be obtained. A further increase requires better material properties or new cell structures.

STM investigation of the growth structure of Cu-phthalocyanine films with submolecular resolution

In this study Cu-phthalocyanine (CuPc) films have been investigated by scanning tunneling microscopy to get quantitative insight into the morphology of the films from the micrometer range down to nanometer resolution to see the arrangement of the molecules. The molecules on the surface have been found to be arranged in a slipped stacking order rather than in the expected zig-zag orientation normally found for CuPc crystals. The cystal structure could be identified to be an α-polymorph. The STM image of the individual molecules on the surface is in good agreement with calculations of the charge density of the LUMO for an isolated molecule. A model for the growth of the crystals was derived from this study.

The use of a high temperature wind tunnel for MT-SOFC testing—Part I: detailed experimental temperature measurement of an MT-SOFC using an avant-garde high temperature wind tunnel and various measurement techniques

The purpose of the first part of this study was to compare four different temperature measuring methods. The application of these tools for possible temperature monitoring or calibration of monitors of microtubular solid oxide fuel cells (MT-SOFCs) is explored. It was found that a thermographic camera is very useful to visualize the temperature gradient on the outside of a cell, while an electrochemical impedance spectroscopy method was useful for estimating the core temperature of a test cell. A standard thermocouple was also used in combination with the previous two methods. Furthermore, an inexpensive laser guided thermometer was also tested for MT-SOFC temperature measurement. This initial study has opened up a range of questions not only about the effect of the experimental apparatus on the measurement results but also about the radial temperature distribution through a MT-SOFC in a working mode. Both these topics will be further investigated in part II of this study through a computational fluid dynamics study. This should provide additional interesting information about any differences between testing single cells and those within a bundle of cells. The discussed results are expected to be mainly temperature related, which should have direct consequences on power output and optimized gas inlet temperatures.

Operating Microtubular SOFCS With Hydrogen Chloride and Hydrogen Sulfide Containing Fuels and Synthetic Wood Gas

Solid oxide fuel cells are known to be able to handle a large variety of different fuels. Because of the greenhouse effect the use of carbon dioxide neutral gases or liquids are of special interest. In this context wood-gas has a big potential to be an alternative fuel for solid oxide fuel cells (SOFCs). The gas is generated by a fluidized bed steam gasifier and consists of various components such as 25 Vol % carbon monoxide, 20 Vol % carbon dioxide, 10 Vol % methane, 2.5 Vol % ethylene, 0.5 Vol % propylene, 2 Vol % nitrogen, and the rest hydrogen (values in dry state). The water concentration of the original pyrolysis gas is about 35 Vol %. Besides these main ingredients there are of course many impurities like dust, tars, ammonia, hydrogen sulphide, and hydrogen chloride present in the product gas. Especially the last two ones may lead to degeneration of the fuel cell anode and must therefore be almost totally removed before feeding the gas into the cell. In order to reduce energy losses, hot gas cleaning systems are favored. This, however, limits the possibility to reduce the impurity concentrations to very low levels. Therefore the aim of this work is to define the maximum acceptable output concentrations for the hydrogen chloride adsorber also in combination with hydrogen sulphide, since for a micro-tubular SOFC there are as yet hardly any data available. In order to determine the influence of the hydrogen chloride on the performance of the fuel cell, different concentrations of this impurity were fed to the cell. Here, also the flow rate was changed while the electrochemical output was determined. In addition it was analyzed if there were any effects when changing from pure hydrogen to the HCl containing fuel. This was investigated at 1123 K and 1173 K, which are the preferred working temperatures for our cells. Cooling down as well as heating up procedures were tested with cells between 1173 K and 573 K. In a second series of experiments, combinations of hydrogen chloride and hydrogen sulphide of variable concentrations were tested. As before, changing between pure hydrogen and the acid containing fuel at above given temperatures was analyzed by determining the cell performance. In parallel to the above experiments, synthetic wood gas was used for operating the microtubular fuel cell while monitoring the electrochemical output with time.

CZTS Monograin Powders and Thin Films

This paper reviews results of studies on different materials and technologies for polycrystalline solar cells conducted at Tallinn University of Technology. Structural properties and defect structure of kesterite CZTS compounds (Cu2ZnSnSe4, Cu2ZnSn(SSe)4) were studied. Influence of selenization parameters of a Zn-Cu-Sn stacked layer on the CZTS layer growth and on the morphology, distribution of elements was analyzed. All the results obtained have been used to optimize the technology of producing solar cell structures in different designs. Cu2ZnSnSe4 and, Cu2ZnSn(SSe)4 based monograin layer solar cells were developed.

Characterization of Fuel Cells and Fuel Cell Systems Using Three-Dimensional X-Ray Tomography

Three dimensional (3D) computer aided X-ray tomography (CT) has proven to be an extremely useful tool in developing our own as well as in examining commercially available solid oxide fuel cells. The results of 3D-CT measurements became very important for understanding the functionality of our first generation and improving the development of our second fuel cell generation. Also geometrical measurements, especially the roundness and the straightness of the tube, can be evaluated, both critical parameters when the stack is heated and mechanical stress has to be avoided. By using this technique the structure of the first generation cells proved to be of insufficient quality. Problems like the variation in thickness of the electrolyte tube as well as the homogeneity in thickness of the electrodes deposited can easily be detected by this nondestructive technique. Microscopic investigations of this problem of course provide equal results, but only after cutting the samples in many slices and many single measurements of different areas of the fuel cell. Using cells with inhomogeneous thickness of course results in drastic variations of the current densities along a single cell. Electrolyte layers that are too thick will result in power loss due to the increased resistance in the ionic conductivity of the electrolyte. If the electrolyte of an electrolyte supported cell is too thin, this can cause mechanical instability. Problems can also occur with the leak tightness of the fuel cell tube. Gas diffusion through the electrode layer can become a problem when the thickness of the electrode layer is too high. On the other hand, if the layers are too thin, the result can be a discontinuous layer, leading to a high electrical series resistance of the electrode. Besides determining the thickness variations also the porosity of the electrolyte needs careful attention. Larger cavities or shrink holes form insulating islands for the ion-stream and are therefore limiting the ionic conductivity. They are also diminishing the mechanical stability and provide problems for depositing a closed electrode film in electrode supported cells.

Scanning tunneling microscopy investigation of tricycloquinazoline liquid crystals on gold

Self-assembled monolayers (SAMs) of hexaalkylthioether derivatives of tricycloquinazoline (TCQ) on Au(111) and tungsten diselenide (WSe2) were investigated by scanning tunneling microscopy (STM). The Au(111) surfaces were found to be etched by the thioether containing solutions. Corroded surfaces which are similar to gold surfaces that were coated with SAMs of thiols or disulfides were revealed by STM. Atomic adsorption spectroscopy proved that an amount of gold that corresponds to ca. 30% of a monolayer was dissolved in the assembly solutions. On gold, the aromatic cores of the molecules were found to be in face-on orientation. The alkyl substituents were in most cases folded upwards and shielding the aromatic cores. Only after long immersion times were crystalline areas observed locally, in which the alkyl chains were lying flat on the gold surface. On WSe2 mono- and multilayers of TCQ molecules in face-on orientation could be imaged by STM. The observed columnar structures displayed long-range order. q 2000 Elsevier Science S.A. All rights reserved.

Problems of roughness measurements using STM

Abstract Scanning tunneling microscopy (STM) seems to be the ideal tool for measurements of surface roughness on a nanometer scale. However, especially in the nanometer size range the extension and form of the tunneling tip comes into play. As an example for the problems arising in investigations of the surface topography a study of indium tin oxide (ITO) films is presented. Roughness measurements are performed using various methods. Several examples of pictures are discussed in which only multiple images of the tunneling tip are seen. A convolution of the substrate and of the STM tip topography occurs. Roughness parameters determined on the basis of such images reflect also the form of the tunneling tip. Reliable data for the surface can be obtained only with very fine tips. All etching techniques tested turned out to be not very reliable. However, in a few cases very sharp tips have been produced by electrochemical etching techniques, leading to the first images of an ITO surface with an ordered domain structure.

Stability of Micro Tubular SOFCs Operated with Synthetic Wood Gases and Wood Gas Components

In this work maximum concentrations of the impurities hydrogen chloride and hydrogen sulfide in synthetic wood gas for operation of a solid oxide fuel cell (SOFC) at different working temperatures and flow conditions are reported. Additionally, an analysis of the influence of the impurity concentrations during the heating up and cooling down phases of the SOFC is given. The paper also deals with wood gas which is generated by a fluidized bed steam gasifier or made by air gasification. It was found that operation with up to 50 ppm hydrogen chloride in hydrogen was possible. Also stable cell performances could be observed with the combination of 10 ppm hydrogen chloride and 3 ppm hydrogen sulfide in the fuel. Wood gas from air gasification always lead to a stable performance but wood gas from steam gasifier sometimes caused power degradation.

Operating Micro-Tubular SOFCs Containing Nickel Based Anodes with Blends of Methane and Hydrogen

In this study different concentrations of methane in hydrogen have been fed into a microtubular SOFC while determining the cell performance over time. Also the effect of adding water to the gas as a reforming agent has been analyzed with respect to power output and cell stability. In addition to the characterization by measuring the power with time gas chromatography and impedance spectroscopy were used in order to obtain further information about the cell behavior. Post mortem electron microscopy and atomic force microscopy analysis has shown the influences of the blends on the anode microstructure after different test series. Also the influence of the working temperature and of the rate of fuel utilization on the cell performance was investigated. Depending on the working conditions stable cell performance could be achieved but also three different degradation mechanisms could be found.