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Bartel T.,Calisolar GmbH | Gibaja F.,Calisolar GmbH | Forster M.,Apollon Solar
Energy Procedia | Year: 2014

Low-cost photovoltaic grade feedstock using multiple dopants - mainly Boron and Phosphorous but also Gallium and trace amounts of Aluminum - is currently establishing itself on the market. In this paper, the measurement of dissolved iron by lifetime spectroscopy introduced on the incumbent Boron-only doped Silicon is adapted to materials with several acceptors. A detailed theoretical description is given and pitfalls are identified. Since precise knowledge of the relative acceptor concentrations is necessary for a correct iron determination, a novel method for measuring acceptor concentration fractions in Silicon is proposed. It uses lifetime spectra analysis to determine the crossover point position. Using this simple method, it is possible to fully analytically characterize compensated silicon doped with two acceptor types. © 2014 Published by Elsevier Ltd.

Lauer K.,CiS Research Institute for Micro Sensors and Photovoltaics | Moller C.,CiS Research Institute for Micro Sensors and Photovoltaics | Moller C.,TU Ilmenau | Bartel T.,Calisolar GmbH | Kirscht F.,Calisolar GmbH
Energy Procedia | Year: 2014

Low-temperature FTIR spectroscopy is further developed to be applicable to measure the aluminum concentration in solar-grade silicon in concentrations up to 4 × 1016 atoms/cm3. Absorption spectra of multicrystalline silicon samples doped with varying aluminum content are measured at 10 K and correlated to the dopant density obtained by four point probe resistivity measurements. Calibration factors for absorption peaks of unpaired substitutional aluminum at 443, 472, 516+524 and 867 cm-1 as well as for a Fano anti-resonance at 962 cm-1 are reported. © 2014 Published by Elsevier Ltd.

Bartel T.,Calisolar GmbH | Gibaja F.,Calisolar GmbH | Graf O.,Calisolar GmbH | Gross D.,Calisolar GmbH | And 6 more authors.
Applied Physics Letters | Year: 2013

The pairing dynamics of interstitial iron and dopants in silicon co-doped with phosphorous and several acceptor types are presented. The classical picture of iron-acceptor pairing dynamics is expanded to include the thermalization of iron between different dopants. The thermalization is quantitatively described using Boltzmann statistics and different iron-acceptor binding energies. The proper understanding of the pairing dynamics of iron in co-doped silicon will provide additional information on the electronic properties of iron-acceptor pairs and may become an analytical method to quantify and differentiate acceptors in co-doped silicon. © 2013 AIP Publishing LLC.

Lauer K.,CiS Research Institute for Micro Sensors and Photovoltaics | Moller C.,CiS Research Institute for Micro Sensors and Photovoltaics | Moller C.,TU Ilmenau | Schulze D.,TU Ilmenau | And 2 more authors.
Physica Status Solidi - Rapid Research Letters | Year: 2013

The photoluminescence spectroscopy method for determining the concentration of shallow acceptors or donors in silicon is extended for the case of aluminum. A calibration function of the photoluminescence intensity ratio of the aluminum bound exciton AlTO(BE) and the free exciton ITO(FE) is reported in the aluminum concentration range of 1015-1017 atoms/cm3 and in the temperature range of 15 K < T < 27 K. It is described as AlTO(BE)/ITO( FE) = 10-15.1 × cAl0.84 × e5.9meV/kT. Dependence of the PL intensity ratio AlTO(BE)/ITO(FE) on the aluminum concentration. The line represents the calibration function. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Langkau S.,University of Leipzig | Wagner G.,University of Leipzig | Kloess G.,University of Leipzig | Heuer M.,Calisolar GmbH
Physica Status Solidi (A) Applications and Materials Science | Year: 2010

The present article provides evidence that Fe impurity atoms in silicon can be gathered by NiSi 2 precipitates at temperatures near RT via solid-state diffusion. Mixtures of silicon and silicide grains, resulting from annealing at 800-900 °C, were analysed. High supersaturation (about 10 19-10 20 atoms/cm 3) of metal impurities in Si was achieved locally by incorporation of nickel and iron atoms from silicide grains into silicon grains during ion milling for TEM specimen preparation. Consequently, particles with local densities of 10 13-10 14 precipitates/cm 3 and average volumes of 10 -18-10 -16 cm 3 formed via diffusion and precipitation. Precipitates with an irregular octahedra shape and platelet-like habit were found at dislocations. Less frequently, precipitates with a regular octahedra shape were observed in undisturbed matrix. HRTEM images and SAD patterns demonstrate that all precipitates have NiSi 2 structure. Ni contents were detected for all precipitates by EDX analysis, but accumulated amounts of Fe could only be proven for some precipitates at dislocations. Quantitative EDXanalysis revealed Fe/(Fe\+Ni) ratios between 16 and 30 at% for these ternary precipitates. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Breitenstein O.,Max Planck Institute of Microstructure Physics | Bauer J.,Max Planck Institute of Microstructure Physics | Bauer J.,Calisolar GmbH | Bothe K.,Institute for Solar Energy Research Hamelin | And 9 more authors.
Journal of Applied Physics | Year: 2011

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between -4 and -9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between -9 and -13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond -13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here. © 2011 American Institute of Physics.

Moller C.,CiS Research Institute for Micro Sensors and Photovoltaics | Moller C.,TU Ilmenau | Bartel T.,Calisolar GmbH | Gibaja F.,Calisolar GmbH | Lauer K.,CiS Research Institute for Micro Sensors and Photovoltaics
Journal of Applied Physics | Year: 2014

Iron-boron (FeB) pairing is observed in the n-type region of a boron and phosphorus co-doped silicon sample which is unexpected from the FeB pair model of Kimerling and Benton. To explain the experimental data, the existing FeB pair model is extended by taking into account the electronic capture and emission rates at the interstitial iron (Fei) trap level as a function of the charge carrier densities. According to this model, the charge state of the Fei may be charged in n-type making FeB association possible. Further, FeB pair formation during illumination in p-type silicon is investigated. This permits the determination of the charge carrier density dependent FeB dissociation rate and in consequence allows to determine the acceptor concentration in the co-doped n-type silicon by lifetime measurement. © 2014 AIP Publishing LLC.

Rudolph P.,Leibniz Institute for Crystal Growth | Czupalla M.,Leibniz Institute for Crystal Growth | Lux B.,Leibniz Institute for Crystal Growth | Kirscht F.,Calisolar GmbH | And 3 more authors.
Journal of Crystal Growth | Year: 2011

Conventionally grown Czochralski (Cz) silicon crystals for photovoltaic (PV) application have an unfavourable cylindrical shape leading to essential material loss during the wafer cutting process. Additionally, the typical high oxygen concentration promotes solar cell degradation. In this paper a new pulling technology for growth of silicon crystals with both quadratic cross section and relatively low as-grown oxygen content is presented. A dynamic-magnetic-field-assisted Cz growth of facetted crystals is reported. At [0 0 1]-oriented growth in very low radial temperature gradient holding steady by a special traveling magnetic field (TMF) the growing crystal body becomes self-profiling by four {1 1 0} facets parallel to the pulling direction. To keep down costs the KRISTMAG̃® principle was used whereupon the Lorentz field and heat are simultaneously generated within a graphite heater design supplied by alternating (AC) multiphase current of various frequencies and phase shifts. The first experimental results show single crystalline Si crystals with reproducible square cross sections up to 91×91 mm 2 including rounded corners. Until now TMF frequencies of f=180 and 300 Hz and a phase shift of φ=90° were applied. For high-purified material an average facet undercooling of ΔT≈2 K has been deduced from the observed rectangular side plane widths. According to high-resolution transmission electron microscopy (HRTEM) the four macroscopically flat faces are microscopically composed of {1 1 0} sub-facets and {1 1 1} macrosteps. Etch pit densities (EPD) between 0 and 104 cm-2 were ascertained. Due to the magnetically induced high-speed melt flow toroid around the growing crystal a relatively low and homogeneously distributed oxygen concentration can be achieved. A minimum value of 7.5×1017 cm-3 was measured in high-purity as-grown crystals at a TMF frequency of f=300 Hz. © 2010 Published by Elsevier B.V.

Heuer M.,Calisolar GmbH
Semiconductors and Semimetals | Year: 2013

The chapter intends to give a summary on the production of metallurgical grade silicon and the status of research and development on solar silicon, which is obtained by the metallurgical route. In a first section, the carbothermic reduction of SiO2 in the submerged arc furnace is explained briefly.Metallurgical processes to refine silicon for photovoltaics are explained and summarized in the second section. Details for the following approaches are given:. -Acid leaching-Slag treatment of the silicon melt-Vacuum degassing of the silicon melt-Purification of liquid silicon using gases or water vapor-Plasma treatment of the silicon melt-Segregation during solidification-Refining silicon from Si-Al melt solutions-Particle removal from liquid siliconFinally, the characteristics of the obtained solar silicon are discussed and summarized. © 2013 Elsevier Inc.

Johnston S.,National Renewable Energy Laboratory | Guthrey H.,National Renewable Energy Laboratory | Yan F.,Applied Materials | Zaunbrecher K.,National Renewable Energy Laboratory | And 4 more authors.
IEEE Journal of Photovoltaics | Year: 2014

A set of neighboring multicrystalline silicon wafers has been processed through different steps of solar cell manufacturing and then images were collected for characterization. The imaging techniques include band-to-band photoluminescence (PL), defect-band or subbandgap PL (subPL), and dark lock-in thermography (DLIT). Defect regions can be tracked from as-cut wafers throughout processing to the finished cells. The finished cell's defect regions detected by band-to-band PL imaging correlate well to diffusion length and quantum efficiency maps. The most detrimental defect regions, type A, also correlate well to reverse-bias breakdown areas as shown in DLIT images. These type A defect regions appear dark in band-to-band PL images, and have subPL emissions. The subPL of type A defects shows strong correlations to poor cell performance and high reverse breakdown at the starting wafer steps (as-cut and textured), but the subPL becomes relatively weak after antireflection coating (ARC) and on the finished cell. Type B defects are regions that have lower defect density but still show detrimental cell performance. After ARC, type B defects emit more intense subPL than type A regions; consequently, type B subPL also shows better correlation to cell performance at the starting wafer steps rather than at the ARC process step and in the finished cell. © 2011-2012 IEEE.

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