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Processing and Characterization of Efficient Thin-Film Polycrystalline-Silicon Solar Cells

AbstractEfficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could substantially lower the price of photovoltaic electricity. We recently showed that good solar cells can be made from pc-Si obtained by epitaxial thickening using thermal CVD of a seed layer made by aluminium-induced crystallization (AIC) of amorphous silicon. We already reported cells in substrate configuration with energy conversion efficiencies up to 8.0% for layers on ceramic alumina substrates. However, much higher efficiencies (η > 10%) are needed for this type of pc-Si solar cells to become cost-effective. To achieve these higher efficiencies, cells will probably have to be made in a superstrate configuration on transparent substrates and advanced light trapping will need to be applied. In this paper we report on our recent progress with pc-Si solar cells made on transparent glass-ceramic substrates.So far, our best pc-Si solar cells in substrate configuration on glass-ceramics showed an efficiency of 6.4%. By using plasma texturing to lower the front side reflection, we increased the current density of our cells by roughly 1 mA cm-2. The Jsc is much lower for cells on glass-ceramic than for cells on alumina. This is the result of the better diffuse back reflectance of alumina compared to glass. The Voc and fill factor are comparable for cells on both substrates.To make pc-Si solar cells on glass in superstrate configuration, we will use a-Si/c-Si rear junction emitters. As a first test of the feasibility of this approach, we measured the illuminated IV parameters of pc-Si cells made for the substrate configuration in superstrate configuration. In superstrate configuration, the current density of the cells is much lower than in substrate configuration due to the non-optimized cell design for superstrate illumination. The Voc is slightly smaller in superstrate configuration due to the lower current density.These results indicate that the glass-ceramic substrates are fully compatible to our poly-Si solar cell process. Furthermore, rear-junction poly-Si cells in superstrate configuration should lead to good cell results once the absorber layer thickness is optimized to the diffusion length of the material and light trapping features adapted to the superstrate configuration are applied.

Effects of gas diffusion backings on performance of low power methanol sensor

not Available.

Slow positron beam study of nitrogen implanted CZ-Si subjected to rapid thermal processing

In this experiment, nitrogen ions were implanted into CZ-silicon wafer at 100keV at room temperature with the fluence of 5×1015N2+/cm2, followed by rapid thermal processing (RTP) at different temperatures. The single detector Doppler broadening and coincidence Doppler broadening measurements on slow positron beam were carried out to characterize the defects in the as-implanted silicon and RTP-treated samples. It is found that both nitrogen-vacancy complexes (N-Vsi) and oxygen-vacancy complexes (O-Vsi) produced by nitrogen implantation diffuse back to the sample surface upon annealing. But the N-Vsi and the O-Vsi complete with each other and give a summed effect on positron annihilation characteristics. It is shown that the N-Vsi win out the O-Vsi in as-implanted sample and by RTP at 650°C, 750°C, which make the S-parameter increase; O-Vsi plays a dominant role after annealing above 850°C, which makes the S parameter decrease.

Miniaturized electrochemical methanol sensor without gas diffusion backings

Miniaturized electrochemical sensors are assembled that monitor the concentration of methanol fuel used in direct methanol fuel cells (DMFC). Reduction in the size of the sensor unit is made possible by eliminating both the gas diffusion backings and any type of housing needed for physical integrity of the unit. The small unit can be placed at any site of the fuel supply path so that it has direct contact with the fuel through mesh-type current collector attached to catalyst-coated membrane. It is demonstrated that the sensor, with 0.15 mg of electrocatalyst loaded on each electrode of 0.25 cm 2 surface area, is capable of monitoring the fuel in the concentration range from 0.5 to 3.0 M and in the temperature range from ambient one up to 80 °C, consuming electrical power as low as 1 mW.

Spontaneous formation of the B2 phase from a decagonal quasicrystal under reduced constraint

Since the discovery that vacancies can quasiperiodically order in a basic B2 structure to yield a one-dimensional quasiperiodicity, these phases were subjected to intense investigations for possible links with quasicrystal. The B2 phase was found to coexist extensively with the icosahedral quasicrystalline phase, particularly in Al–Cu–TM (TM = transition metal) systems where the two phase fields are often adjacent to each other. The two phase exhibit a well-defined orientation relationship. There exist several studies dealing with quasicrystalline to B2 transformation, particularly in decagonal quasicrystals Doblinger et al. has carefully explored different metastable states in Al–Co–Ni decagonal quasicrystals including nanodomained 1D quasicrystals and multiple twinned approximant phase. The decagonal to a cubic B2 ordered phase transformation can often be observed. The nanocrystals of the B2 phase could be observed on the surface of the decagonal quasicrystal. Zhang and Urban have studied the stability of the QC phase by employing electron irradiation in an electron microscope and observed the transformation of the decagonal quasicrystal (DQC) phase, in an Al–Cu–Co–Si alloy, first to a disordered phase [body centered cubic (BCC phase)] and then to an ordered B2 phase. Zurkirch et al. observed a decagonal to crystalline structural transformation occurring on a surface of Al–Co–Ni upon sputtering the surface. The annealing treatment was found to restores the decagonal structure. Shimoda et al. confirmed the above observation. A similar result has been reported recently by Fluckinger et al. who have reported more than one variant of the cubic crystal at the surface. These authors attributed the structural change to the change in the surface composition during sputtering. The annealing treatment allows the A1 atoms from the subsurface to diffuse back and restore the composition resulting in a reappearance of the decagonal quasicrystal. The formation of a BCC phase during ion milling has also been reported in Al–Cu–Co decagonal quasicrystals. Recently Muthy et al. has reported a quasicrystalline to $_\beta$ phase transformation in as-cast $A1_{65}Cu_{20}Fe_{15}, A1_{65}Cu_{20}Co_{15}$ and $A1_{72}Pd_{19:5}Mn_{8:5}$ alloys during high energy ball milling. They have correlated the phase change with a critical grain size of 20 nm.

Optical diagnostics for plasma-surface interaction in CF4∕Ar radio-frequency inductively coupled plasma during Si and SiO2 etching

Highly selective etching of SiO2 over Si is central to the manufacture of ultralarge scale integration devices; the process is generally one of reactive ion etching using polymerizing fluorocarbon chemistry. A number of species including electrons, ions, and radicals are generated by reactions in the gas phase and on the surface in the plasma process. A large number of highly reactive fluorine atoms, fluorocarbon radicals, and ions interact with the substrate and produce etch products. These etch products, primarily SiF4 and SiF2, diffuse back into the bulk plasma where they are dissociated and ionized by interactions with electrons, and the resultant products are transported and redeposited onto the substrate and/or wall surface. That is, the plasma structure may differ depending on whether the Si (or SiO2) surface has been exposed to etching or not. Hence, it is essential to investigate the spatiotemporal characteristics of the plasma structure during etching. In this study, measurements of plasma structure during Si or SiO2 etching in CF4∕Ar radio-frequency inductively coupled plasma (rf-ICP) were performed using computerized tomography of optical emission spectroscopy to investigate plasma-surface interactions. We focused on the characteristics of etch products, their daughter products, and the etchant in the gas phase during Si and SiO2 etching in CF4∕Ar rf-ICP and the disturbance of the plasma structure at high amplitudes of LF bias.

Barrier membranes made with lithographically printed flakes

We have used photolithography to make barrier membranes containing titanium flakes in a polydimethylsiloxane matrix. The flakes, which are shaped as ribbons, squares or hexagons, retard the diffusion of helium across the membranes. The retardation is due to the tortuous diffusion through the composite. The tortuosity can be due to diffusion from the bulk to a permeable gap between flakes or “necking”; to diffusion through the gaps between flakes; to diffusion back and forth between layers of flakes, or “wiggling”. The results for necking into a single layer of flakes do not agree with previously published theories and simulations. The results are not limited by diffusion through the gaps between flakes. The results for multiple layers of flakes are close to those predicted by earlier theories which consider wiggling. All the results provide a benchmark for further theoretical development.

Effect of dispersion methods of an unsupported Pt-Ru black anode catalyst on the power performance of a direct methanol fuel cell

The effect of the dispersion methods for an unsupported Pt-Ru (1:1) black anode catalyst on the power performance of a direct methanol fuel cell has been studied. The anode catalyst inks were fabricated by ultrasonicating or ball-milling the catalyst in two different types of dispersion solvent, namely deionized water (DI) and isopropyl alcohol (IPA). To fabricate an anode, the inks were sprayed onto the gas diffusion backing electrode, consisting of microporous layer (MPL) and carbon paper. Scanning electron microscopy (SEM) of the cross-sectional morphology of the anode catalyst layers revealed that the layers fabricated by ultrasonicating the catalyst in DI or IPA, ball-milling in DI and ball-milling in IPA consisted of granular-shaped, granular + flake-shaped and flake-shaped agglomerates, respectively. The anode catalyst layer fabricated by ball-milling in IPA exhibited the most porous structure and correspondingly represented the best power performance of 0.13 W cm −2 (0.45 A cm −2, 0.3 V) at 90 °C with 1 M aqueous methanol and atmospheric air. Control of the morphology and porosity in the catalyst layer by means of the catalyst dispersion method is suggested.

Brewster Angle Microscopy Study of Poly(ε-caprolactone) Crystal Growth in Langmuir Films at the Air/Water Interface

Surface pressure-induced crystallization of poly(epsilon-caprolactone) (PCL) from a metastable region of the surface pressure-area per monomer (Pi-A) isotherm in Langmuir monolayers at the air/water (A/W) interface has been captured in real time by Brewster angle microscopy (BAM). Morphological features of PCL crystals grown in Langmuir films during the compression process exhibit four fully developed faces and two distorted faces. During expansion of the crystallized film, polymer chains slowly detach from the crystalline domains and diffuse back into the monolayer as the crystals "melt". Typical diffusion-controlled morphologies are revealed by BAM during the melting process as the secondary dendrites melt away faster, that is, at a higher surface pressure than the principal axes. Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the polymer chain axes perpendicular to the substrate surface, while atomic force microscopy reveals a crystal thickness of approximately 7.6 nm.

Is phosphorus retention in autochthonous lake sediments controlled by oxygen or phosphorus?

Eutrophication of various lakes on the Swiss Plateau was targeted in the 1980s by reducing external nutrient loading and installing aeration/oxygenation systems. For five eutrophic lakes, three of which have been artificially oxygenated and aerated for almost 20 yr, phosphorus (P) balances were established using input, water‐column inventory, export via the outlet, and sediment core data. To separate the effects of aeration/oxygenation from the effects of P‐input reduction, two measures of P retention were determined—the ratio of net sedimentation to input and the ratio of net sedimentation to gross sedimentation. A comparison among the five lakes shows that, presently, P retention does not differ significantly between lakes with aeration/oxygenation and lakes with an anoxic hypolimnion, whereas retention at times of maximum P concentrations in the 1970s and 1980s was significantly lower compared with retention in today’s moderately eutrophic lakes. This is in agreement with the finding that net sedimentation in all lakes remained relatively constant between 280 and 950 kg km−2, independent of their P concentration. It is shown that constant net sedimentation is likely to prevail as long as P concentrations remain above a critical value of around 40 mg m−3. Constant net sedimentation indicates that permanent deposition of P is limited by the sediment's binding capacity. Oxic conditions in the hypolimnion do not necessarily increase this capacity because redox‐dependent P species may still be released within the anoxic bulk sediment and diffuse back into the water column. To characterize the P retention capacity of anoxic sediments, we suggest that the P burial rate be determined from dated sediment cores rather than from input/output balances.

Effect of cathode structure on planar free-breathing PEMFC

Abstract The effect of cathode structure on the performance of planar free-breathing fuel cell is studied. Three different types of gas diffusion backings and current collector plates on the cathode were used. The gas diffusion backings used were thick carbon sheet, titanium sinter, and carbon paper. Difference between current collector plates was the size of the openings between current collecting ribs. Results showed that with thicker gas diffusion backings the structure of the current collector had only some effect on the cell performance but with thin carbon paper the effect was significant. Very high power densities for free-breathing fuel cell were achieved with thin gas diffusion backing with the record of approximately 360 mW cm −2 . However, the cell was also vulnerable for flooding as there was liquid water observed on the cathode surface. Despite the liquid water saturation it seems that this kind of cell design may be suitable especially for portable applications.

Surface composition and structure of Co3O4(110) and the effect of impurity segregation

The Co3O4(110) single crystal surface has been characterized by low energy electron diffraction (LEED), Auger electron spectroscopy, and x-ray photoelectron spectroscopy (XPS). LEED analysis of the clean Co3O4(110) spinel surface shows a well-ordered pattern with sharp diffraction features. The XPS spectra are consistent with stoichiometric Co3O4 as determined by the concentration ratio of oxygen to cobalt (CO/CCo) and spectral peak shape. In particular, the cobalt 2p XPS spectra are characteristic of the spinel structure with Co3+ occupying octahedral sites and Co2+ in tetrahedral sites within the lattice. During prolonged heating at 630 K, bulk impurities of K, Ca, Na, and Cu segregated to the surface. Sodium desorbed from the surface as NaOH at 825 K, potassium and calcium were only removed by sputtering since no desorption from the surface was detected for temperatures up to 1000 K. Copper also disappeared upon heating above 700 K, most likely by desorbing although the possibility of diffusion back into the bulk could not be eliminated. The appearance of copper impurities correlated with Co3O4(110) surface reduction to CoO, and the surface could not be fully reoxidized even upon extended oxygen annealing as long as the copper impurity remained on the surface. Upon removal of the Cu from the near-surface region, the surface was easily reoxidized to Co3O4 by O2.

Flooding of Gas Diffusion Backing in PEFCs : Physical and Electrochemical Characterization

In polymer electrolyte fuel cells (PEFCs) gas diffusion backings (GDBs) have a significant effect on water management and cell performance. In this study, methods for characterizing GDB performance by fuel cell testing and ex situ measurements are presented. The performance of four different commercial GDB materials was tested and significant differences were found between the materials. While the performance and behavior are almost similar in the single-phase region, the flooding behavior of different GDBs in the two-phase region varies widely. The results show that using high clamping pressures increases cell flooding, but the increase varies from material to material. Increased flooding is caused by the combination of decreased porosity and a temperature difference between GDB and current collector. Furthermore, it was observed that the decrease in porosity due to cell compression and corresponding increase in mass-transfer resistance should be studied in the single-phase region, because flooding of the GDB easily becomes the dominating source of mass-transfer resistance. In addition, a literature review on GDB studies and characterization methods was carried out. The review revealed a lack of an established GDB testing regime and the absence of a relation between physical properties of the GDB and fuel cell performance. © 2004 The Electrochemical Society. All rights reserved.

Particle morphology as a control of permeation in polymer films obtained from MMA/nBA colloidal dispersions.

The combination of precision-controlled weight loss measurements and spectroscopic surface FT-IR analysis allowed us to identify unique behaviors of poly(methyl methacrylate) (p-MMA). When MMA and n-butyl acrylate (nBA) are polymerized into p-MMA and p-nBA homopolymer blends, MMA/nBA random copolymers, and p-MMA/p-nBA core-shell morphologies, a controlled mobility and stratification of low molecular weight components occurs in films formed from coalesced colloidal dispersions. Due to different affinities toward water, p-MMA and p-nBA are capable of releasing water at different rates, depending upon particle morphological features of initial dispersions. As coalescence progresses, water molecules are released from the high free volume p-nBA particles, whereas p-MMA retains water molecules for the longest time due to its hydrophilic nature. As a result, water losses at extended coalescence times are relatively small for p-MMA. MMA/nBA copolymer and p-MMA/p-nBA blends follow the same trends, although the magnitudes of changes are not as pronounced. The p-MMA/p-nBA core-shell behavior resembles that of p-nBA homopolymer, which is attributed to significantly lower content of the p-MMA component in particles. Annealing of coalesced colloidal films at elevated temperatures causes migration of SDOSS to the F-A interface, but for films containing primarily p-nBA, reverse diffusion back into the bulk is observed. These studies illustrate that the combination of different particle morphologies and temperatures leads to controllable permeation processes through polymeric films.

Characterization of the interface region during the agglomeration of silicon nanocrystals in silicon dioxide

Si nanocrystals embedded in thermally grown SiO2 have been annealed at temperatures between 400 and 900 °C in a variety of atmospheres. Positron annihilation spectroscopy has been employed to study changes in the interface regions between nanocrystalline Si (nc-Si) and SiO2 with the support of photoluminescence measurements. We find that nitrogen and oxygen are trapped in the voids around nc-Si at low annealing temperatures. High-temperature annealing during the formation of nc-Si causes hydrogen originally residing in the SiO2/substrate region to enter the SiO2 structure. Hydrogen diffuse back to the SiO2/substrate region on annealing in vacuum at 400 °C because no other impurities block its diffusion channels. At annealing temperatures above 700 °C, both nitrogen and oxygen react with nc-Si, resulting in a volume increase. This introduces stress in the SiO2 matrix, which is relaxed by the shrinkage of its intrinsic open volume. The present data suggest that nitrogen suppresses Si diffusion in SiO2, so that the agglomeration of nc-Si is slower during annealing in nitrogen than in oxygen or vacuum.

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Adoption of a sintered stainless steel fiber felt was evaluated as gas diffusion backing in air-breathing direct methanol fuel cell (DMFC). By using a sintered stainless steel fiber felt as an anodic gas diffusion backing, the peak power density of an air-breathing DMFC is 24 mW cm(-2), which is better than that of common carbon paper. A 30-h-life test indicates that the degraded performance of the air-breathing DMFC is primarily due to the water flooding of the cathode. Twelve unit cells with each has 6 cm(2) of active area are connected in series to supply the power to a mobile phone assisted by a constant voltage diode. The maximum power density of 26 mW cm(-2) was achieved in the stack, which is higher than that in single cell. The results show that the sintered stainless steel felt is a promising solution to gas diffusion backing in the air-breathing DMFC, especially in the anodic side because of its high electronical conductivity and hydrophilicity. (C) 2004 Elsevier B.V. All rights reserved.

Improvement of MEAs for Direct‐Methanol Fuel Cells by Tuned Layer Preparation and Coating Technology

AbstractLarge scale production of membrane electrode assemblies (MEAs) for fuel cells requires a fabrication technology, which is not only based on electrochemical phenomena but also takes into account aspects of materials processing and conversion. For the fabrication of MEAs for DMFC a production sequence consisting of wet ball‐milling, wet spray coating and calendering was developed at the ICVT Clausthal. This has a proven ability for the preparation of diffusion backings and catalyst layers with high reproducibility. To improve MEA performance, investigations concerning the influence of layer fabrication and composition are made with respect to the resulting morphology of the structures. Intermediates and MEAs are evaluated by conductivity measurements, determination of permeability values and current density‐voltage characteristics supported by SEM and optical microscopy.

Evaluation of planar free-breathing polymer electrolyte membrane fuel cell design

A planar cell design for free-breathing fuel cell is studied. The cathode side of the cell was directly open to ambient air in a way that oxygen needed by the fuel cell reaction was provided by diffusion through the cathode side gas diffusion backing, i.e. the cell had no cathode side flow channels. The aim of the study was to demonstrate the feasibility of the cell concept by experimentally evaluating its performance with polarisation and current transient measurements. The orientation of the cell did not significantly affect the performance of the cell and signs of flooding were not observed. The maximum power density achieved in the polarisation measurements was ∼124 mW cm−2. The current transient measurements revealed that with a small base load the cell is capable of producing momentarily current densities up to 500 mA cm−2. The maximum transient power density achieved was ∼194 mW cm−2 at transient current density of 450 mA cm−2. According to the results it seems that this kind of cell design is feasible for small-scale applications, such as portable computers.

On the question of MEA preparation for DMFCs

To get information for the development of a continuous MEA fabrication sequence various preparation strategies are investigated. Diffusion layer suspension is made by magnetic stirring, ultrasound mixing and by use of a ball-mill and the slurry is characterised by particle size measurements. The catalytic layers are applied either to the diffusion backing or directly onto the membrane. The latter method yields to improved MEA performance due to the higher utilisation of the catalytic material. Experiments concerning the influence of the anodic diffusion layer thickness and composition show a strong influence on MEA characteristic. For the anodic diffusion layer the influence of the hydrophobicity on methanol transport and CO2 removal is investigated by using PTFE or Nafion© as binder. The Nafion© bonded backing shows better mass transport properties for methanol but CO2 removal becomes crucial at high current densities. Sintering of the PTFE bonded structure improves layer properties for the management of liquid and gas transport. Best results are achieved with sintered PTFE bonded layers with a reduced layer thickness.