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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

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. Source

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

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. Source

Heuer M.,Calisolar GmbH
Semiconductors and Semimetals

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. Source

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

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. Source

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

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. Source

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