Tapio Saukkonen

Dye Solar Cells on ITO-PET Substrate with TiO2 Recombination Blocking Layers

Atomic-layer-deposited TiO 2 recombination blocking layers were prepared on indium tin oxide-poly(ethylene terephthalate) (ITO-PET) photoelectrode substrates for dye solar cells and were examined using several electrochemical methods. The blocking layers increased the open-circuit voltage at low light intensities. At high light intensities, a decrease in the fill factor (FF) due to the additional resistance of the current transport through the layer was more significant than the positive effect by the reduced recombination. The decrease in the FF was reduced by a thermal treatment that made the blocking layer more conductive due to a structural change from an amorphous to a crystalline form. Therefore, thinner blocking layers of this type are required for plastic cells prepared at low temperature than for conventional glass dye solar cells made with temperature processing.

Stability of Dye Solar Cells with Photoelectrode on Metal Substrates

In this study, the stability of dye solar cells DSCs with different kinds of metals as the photoelectrode substrate is studied.Stainless steels StSs, Inconel, and titanium substrates were tested to find stable substrate options. Photovoltaic characterization,electrochemical impedance spectroscopy EIS, scanning electron microscopy, and substrate polarization measurements were usedin the characterization. DSCs based on different grades of StS suffered from rapid degradation of efficiency within few hours inlight soaking. Good stability was demonstrated with DSCs with Inconel and Ti photoelectrode substrates. The Inconel substrateshave a thick passive oxide layer, which is likely related to good stability. However, according to the EIS analysis, the oxide layerof Inconel substrates increased resistive losses, which caused a lower fill factor and photovoltaic efficiency compared to theTi-based cells.© 2010 The Electrochemical Society. DOI: 10.1149/1.3374645 All rights reserved.Manuscript submitted December 17, 2009; revised manuscript received February 19, 2010. Published April 21, 2010.

A durable SWCNT/PET polymer foil based metal free counter electrode for flexible dye-sensitized solar cells

An ITO free, highly conductive PET foil is fabricated by depositing aqueous single-walled carbon nanotube (SWCNT) ink that exhibits remarkable durability when exposed to severe mechanical stability tests. Excellent adhesion of the SWCNT film on PET was obtained by aging the ink overnight at 50 °C before deposition. A counter electrode for a dye-sensitized solar cell was fabricated by electro-polymerizing the PEDOT polymer over the SWCNT film which gave 7% solar cell efficiency and low (0.4 Ω cm2) charge transfer resistance.

High performance low temperature carbon composite catalysts for flexible dye sensitized solar cells

Roll-to-roll manufacturing of dye sensitized solar cells (DSSCs) requires efficient and low cost materials that adhere well on the flexible substrates used. In this regard, different low temperature carbon composite counter electrode (CE) catalyst ink formulations for flexible DSSCs were developed that can be simply and quickly coated on plastic substrates and dried below 150 °C. The CEs were investigated in terms of photovoltaic performance in DSSCs by current-voltage measurements, mechanical adhesion properties by bending and tape tests, electro-catalytic performance by electrochemical impedance spectroscopy and microstructure by electron microscopy. In the bending and tape tests, PEDOT-carbon composite catalyst layers exhibited higher elasticity and better adhesion on all the studied substrates (ITO-PET and ITO-PEN plastic, and FTO-glass), compared to a binder free carbon composite and a TiO2 binder enriched carbon composite, and showed lower charge transfer resistance (1.5-3 Ω cm(2)) than the traditional thermally platinized CE (5 Ω cm(2)), demonstrating better catalytic performance for the tri-iodide reduction reaction. Also the TiO2 binder enriched carbon composite showed good catalytic characteristics and relatively good adhesion on ITO-PET, but on ITO-PEN its adhesion was poor. A DSSC with the TiO2 binder enriched catalyst layer reached 85% of the solar energy conversion efficiency of the reference DSSC based on the traditional thermally platinized CE. Based on the aforementioned characteristics, these carbon composites are promising candidates for replacing the platinum catalyst in a high volume roll-to-roll manufacturing process of DSSCs.

Banding in copper friction stir weld

Abstract Formation and microstructure of banding in a copper friction stir weld was studied using scanning electron microscopy, electron backscatter diffraction, and nanoindentation. Unlike in aluminium, banding only appears in ‘cold’ friction stir welding (FSW) welds of copper. Banding is formed by layers of small and large grains. It was noticed that higher local misorientations and small grain size correlate in the welds. Owing to differences in the restoration processes of copper and aluminium, texture changes are not visible in copper even though they are in aluminium. Texture of copper FSW welds is weak and independent of the grain structure of the base material, because in the steady state dynamic recrystallisation, the restoration process of copper, the grain structure is mainly dependent of the processing parameters. The hardness differences between the small and large grains are small.

Role of Nonmetallic Inclusions in Hydrogen Embrittlement of High-Strength Carbon Steels with Different Microalloying

High-strength carbon steels of 1200 MPa strength level with different microalloying were tensile tested at constant extension rate and constant load under continuous electrochemical hydrogen charging. The results show that hydrogen markedly reduces elongation and time to fracture of all the studied steels. Fractography of the steels shows that nonmetallic inclusions (NMIs) play the major role in crack initiation in hydrogen-charged specimens. The role of NMIs in the hydrogen-induced fracture of steels is discussed.

Hot Cracking Susceptibility of Ni-Base Alloy Dissimilar Metal Welds

Hot cracking susceptibility of Ni-base alloy dissimilar metal welds in nuclear power plant (NPP) applications was studied by preparing mock-ups of various relevant dissimilar metal welds of the NPPs. The motivation for the study was the need for repair welding of Alloy 182 welds of safe ends in BWR plants and possible welding problems with new Alloy 152/52 weld metals for new reactors. Weldability of the Ni-base alloys (Alloys 182/82 and Alloys 152/52) was evaluated based on their composition and weld metal microstructures. Susceptibility to hot cracking was examined by Varestraint tests. The studied dissimilar metal welds showed clear segregation of Nb, Si, P and Mn to the last liquid to solidify at dendrite boundaries. Hot cracking occurred along the dendrite boundaries and the susceptibility to hot cracking in the weld mock-up samples was observed to follow the order: Alloy 152 > Alloy 52 > Alloy 182 > Alloy 82, while in pure (undiluted) weld metal hot cracking tests the susceptibility followed the order: Alloy 182 ≥ Alloy 152 > Alloy 52 ≥ Alloy 82. The differences are thought to be related to dilution effects in mock-up welds because Fe, Si, and C enhance the eutectic phases and expand the solidification temperature range.

Effect of Thermal Aging on SCC, Material Properties and Fracture Toughness of Stainless Steel Weld Metals

An experimental program has been conducted in order to understand how the spinodal decomposition may affect material properties changes in Type 316L BWR pipe weld metals. The program includeed Charpy-V, tensile, SCC crack growth and in-situ fracture toughness testing as a function of aging time and temperature. In this paper we report results of fracture toughness, SCC crack growth rate and fracture morphology studies of Type 316L stainless steel weld metals under simulated BWR conditions, consisting of 288°C, high purity water containing 300 ppb dissolved oxygen (defined for purposes of this paper as “In-Situ”). SCC crack growth results show an approximately 2X increase in crack growth rate over that of the unaged material. In-situ fracture toughness measurements indicate that environmental exposure can result in a reduction of toughness by up to 40% over the corresponding at-temperature air values. Detailed analysis of the results strongly suggest that spinodal decomposition is responsible for the degradation in properties measured ex-environment. Analysis of the results also strongly suggests that the in-situ properties degradation is the result of hydrogen absorbed by the material during exposure to the high temperature aqueous environment.

Plastic strain and residual stress distributions in an AISI 304 stainless steel BWR pipe weld

In AISI 304 stainless steel pipe welds weld shrinkage causes large variations in residual plastic strain in different parts of the weld metal and heat-affected zone (HAZ). The amount of strain was analyzed by EBSD quantitatively by comparing the intra-grain misorientations to the calibration curve. Highest degrees of plastic strain (10...20%) were detected in the HAZ close to the root area of a prototypical BWR plant weld. Strain in the weld metal varies in the different directions of solidification, being high in the weld bead boundaries and near the fusion lines. Preliminary studies of the effects of mechanical and elastic anisotropy of the weld metal microstructure on the grain size level were performed by EBSD and nanoindentation. The residual stress distribution in the same weld cross-section was determined by a contour method. The residual strain and stress distributions are superimposed and EAC susceptibility of various areas of the pipe weld is evaluated and discussed.

Hydrogen effects on fracture of high-strength steels with different micro-alloying

Abstract Constant load tests of high-strength carbon steels with different micro-alloying using strengths in the range of 1000–1400 MPa were performed at ambient temperature under continuous electrochemical hydrogen charging. Hydrogen markedly affects delayed fracture of all the studied steels. Fractography of the studied steels shows that fracture mechanism depends on the chemical composition of the studied steels and hydrogen-induced cracking exhibits intergranular or transgranular character occurring often in the form of hydrogen flakes. The size and chemical composition of non-metallic inclusions are analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Hydrogen-induced cracking initiates at TiN/TiC particles in steels with Ti alloying. Crack paths are studied with electron backscatter diffraction mapping to analyze crack initiation and growth. The thermal desorption spectroscopy method is used to analyze the distribution of hydrogen in the trapping sites. The mechanisms of hydrogen effects on fracture of high-strength steels are discussed.