We report on investigations of V2O5 thin film cathodes prepared by pulsed laser deposition and the phase transformations which occur during electrochemical cycling. Our experimental results on PLD-grown, textured V2O5 crystalline films concur with reports in the literature that there is a voltage threshold above which, cycling appears to be completely reversible and below which, cycling appears to be irreversible. Crystalline films discharged beyond the threshold to 2.0 V vs. Li exhibited an immediate and continuous fade in capacity as well as a ∼90% decrease in XRD peak intensity and a similar decrease in Raman signal intensity in as few as 10 cycles. We have made co-phase material by both electrochemically discharging virgin, crystalline V2O5 and by further discharging previously cycled films which showed irreversible structural changes.

The photovoltaic (PV) industry has recently undergone a period of oversupply and plunging prices which has imposed a harsh reevaluation of criteria for viability. The background history leading up to the current PV technology situation is presented and discussed in terms of the PV learning curve (PV module selling price as a function of amount produced). The effects of the past and recent learning curve fluctuations and their technological causes on various PV material crystal growth approaches are described. Some crystal growth research and development (R&D) needs for future viability are discussed.

The lab-scale process described here forms near-spherical, mm-sized Si shot via solidification from the melt in a short vertical height. Liquid silicon is pressure ejected through quartz or high-density graphite nozzles to form a laminar stream of liquid, which breaks up by the Rayleigh instability. A circulating water bath located 10–20cm below the nozzle cools the droplets and moves the solid shot away from the impact region. Si shot in the size range 1–4mm has been produced.

Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

We present a comprehensive summary of our observations of metal-rich particles in multicrystalline silicon (mc-Si) solar cell materials from multiple vendors, including directionally-solidified ingot-grown, sheet, and ribbon, as well as multicrystalline float zone materials contaminated during growth. In each material, the elemental nature, chemical states, and distributions of metal-rich particles are assessed by synchrotron-based analytical x-ray microprobe techniques. Certain universal physical principles appear to govern the behavior of metals in nearly all materials: (a) Two types of metal-rich particles can be observed (metal silicide nanoprecipitates and metal-rich inclusions up to tens of microns in size, frequently oxidized), (b) spatial distributions of individual elements strongly depend on their solubility and diffusivity, and (c) strong interactions exist between metals and certain types of structural defects. Differences in the distribution and elemental nature of metal contamination between different mc-Si materials can largely be explained by variations in crystal growth parameters, structural defect types, and contamination sources. Copyright (C) 2006 John Wiley & Sons, Ltd.

In this study, synchrotron-based x-ray absorption microspectroscopy (μ-XAS) is applied to identify the chemical states of copper-rich clusters within a variety of silicon materials, including as-grown cast multicrystalline silicon solar cell material with high oxygen concentration and other silicon materials with varying degrees of oxygen concentration and copper contamination pathways. In all samples, copper silicide (Cu3Si) is the only phase of copper identified. It is noted from thermodynamic considerations that unlike certain metal species, copper tends to form a silicide and not an oxidized compound because of the strong silicon–oxygen bonding energy; consequently the likelihood of encountering an oxidized copper particle in silicon is small, in agreement with experimental data. In light of these results, the effectiveness of aluminum gettering for the removal of copper from bulk silicon is quantified via x-ray fluorescence microscopy, and a segregation coefficient is determined from experimental data to be at least (1–2)×103. Additionally, μ-XAS data directly demonstrate that the segregation mechanism of Cu in Al is the higher solubility of Cu in the liquid phase. In light of these results, possible limitations for the complete removal of Cu from bulk mc-Si are discussed.

Synchrotron-based, spectrally resolved x-ray beam-induced current (SR-XBIC) is introduced as a technique to locally measure the minority carrier diffusion length in semiconductor devices. Equivalence with well-established diffusion length measurement techniques is demonstrated. The strength of SR-XBIC is that it can be combined in situ with other synchrotron-based analytical techniques, such as x-ray fluorescence microscopy (μ-XRF) and x-ray absorption microspectroscopy (μ-XAS), yielding information about the distribution, elemental composition, chemical nature, and effect on minority carrier diffusion length of individual transition metal species in multicrystalline silicon. SR-XBIC, μ-XRF, and μ-XAS measurements were performed on intentionally contaminated multicrystalline silicon, revealing a strong correlation between local concentrations of copper and nickel silicide precipitates and a decrease of minority carrier diffusion length. In addition, the reduction of minority carrier diffusion length due to submicron-sized Cu3Si and NiSi2 precipitates could be decoupled from the influence of homogeneously distributed nanoprecipitates and point defects.

Synchrotron-based microprobe investigations were conducted to study the effect of heat treatments and cooling rates on the distribution and chemical state of metal-rich precipitates in multicrystalline silicon. A brief summary of these results is presented herein; complete reports will be published elsewhere. The effect of temperature on the dissolution of metal-silicide precipitates during rapid thermal processing has been investigated, revealing that higher temperatures can lead to the dissolution of metal silicide precipitates correlated with decreases in cell performance. The effect of modifying cooling rates on the distributions of metals has also been investigated, indicating that while fast cools lead to widespread nucleation, slow cools can lead to the formation of significantly larger clusters and decreased intragranular recombination activity.

The garnet-structured compound Ca2NaMg2V3O12 is amenable to growth in the laboratory by synthetic contact metamorphism using a solution of V2O5 and Na2SiO3 in the approximate ratio 4.3:1 acting on dolomite marble “rock.” Cooling a 320g crucible-charge at −2.6°Cmin−1 from a soak temperature of 1020°C yielded drusy clusters of crystals exhibiting characteristic {211} and {110} garnet faces, with individual crystals as large as 5mm. X-ray powder diffraction confirmed a cubic garnet structure with lattice constant=12.427Å. The crystals had a specific gravity of 3.43gcm−3, refractive index >1.8, and Mohs hardness about 4.

We report on a solid-source B doping method for float-zone growth that uses nontoxic pyrolytic boron nitride as the source material and has the flexibility to allow step changes in doping concentration during the growth process. Doping concentrations of 6×1016cm−3 or less (⩾0.3Ω-cm resistivity) can be achieved in meter-long crystals without exceeding the solubility limit for N. Shorter crystals can be doped more heavily. The time required to transition from undoped (5000Ω-cm) growth to a 0.2Ω-cm uniform doping level is about 3min. This corresponds to less than 1cm of growth at 0.3cmmin−1. Doping uniformity is within 9% or better axially and 2% radially. We present and experimentally substantiate model equations for dopant incorporation from a pyrolytic BN solid-source, for B dopant dissipation when the source is removed, and for nitrogen co-dopant incorporation.

Nitrogen doped Czochralski (N-CZ) and Float Zone (N-FZ) silicon were measured by high resolution synchrotron Fourier Transform IR spectroscopy (HR-FTIR). The chemical complexes were analyzed in specific regions with known extended defects, i.e., denuded or precipitated regions of annealed N-CZ Si wafers, in N-FZ Si with ring defects and on ''N-Skin'' region. The absorption lines were assigned to chemical complexes previously studied by first principles calculations. In annealed N-CZ Si wafers, a strong correlation was observed between the absorption line intensity depth variations and the defect distributions revealed by an Oxygen Precipitate Profiler (OPP), and oxygen and nitrogen SIMS profiles. Transformation of chemical complexes from one type to another was observed. A defect band, visible as an OPP peak at the denuded zone-bulk interface was found to be related to vacancy defect enhancement of oxygen precipitation via production of mobile N2. For the as grown N-FZ, the radial dependency of IR absorption line intensity is correlated to x-ray topography contrast.

the chemical state and distribution of Cu-rich clusters were determined in four different silicon-based materials with varying contamination pathways and degrees of oxygen concentration, including as-grown multicrystalline silicon. In all four samples, Cu3Si was the only chemical state observed. Cu3Si clusters were observed at structural defects within all four materials; XBIC measurements revealed that the presence of Cu3Si corresponds to increased recombination activity. Oxidized Cu compounds are not likely to form in silicon. The +1 eV edge shift in the -XAS absorption spectrum of Cu3Si relative to Cu metal is believed to be an indication of a degree of covalent bonding between Cu atoms and their silicon neighbors.

We have developed efficient p-type crystalline-Si-based solar cells using a fast (1 min) and low-substrate temperature (<220/spl deg/C) hot-wire CVD technique to deposit n-type Si thin films to form n-p junction. We achieved 13.3% energy conversion efficiency with V/sub oc/ of 0.58 V, FF of 0.773, and J/sub sc/ of 29.86 mA/cm/sup 2/ under AM 1.5 for a 1-cm/sup 2/ solar cell, which is the highest reported efficiency to date for a single-junction device using the hot-wire CVD technique. Transmission electron microscopy reveals Epitaxial growth of Si at the junction. Quantum efficiency measurement on this solar cell shows over 90% response in the region between 540 and 780 nm, but a relatively weak blue and near infrared response. The solar cells show no degradation after 1000 hours of standard light soaking.