C. Häßler

Solar cells from upgraded metallurgical grade (UMG) and plasma-purified UMG multi-crystalline silicon substrates

High impurity concentrations do not allow the direct use of upgraded metallurgical grade (UMG) Si for PV production. A newly developed prototype inductive plasma-purification system andprocess allowedthe significant reduction of the elements B, C, O, P, Al, Ca, Fe and Ti, depending on the duration of the treatment. Based on this type of purification, it is shown that subsequent appropriate low-cost cell-processing yields homogeneously distributed energyconversion efficiencies throughout the cast ingots. Stabilisedcell efficiencies of up to 14.7% were already experimentally shown to be attainable on highly B-doped (ro0:1O cm) 102 cm 2 multi-crystalline Si substrates of high purity. On plasma-purifiedUMG p-type 0.1–0.2 O cm ingots, efficiencies of up to 12.38% are reached, to be compared with about 10.12% on the same material without prior plasma treatment. Some light-induced degradation is present on processedsamples, which is most likely linkedto the presence of metastable boron–oxygen complexes in the material, andresults in stabilisedefficiencies of, respectively, 12.19% and 10.00%. r 2002 Elsevier Science B.V. All rights reserved.

Silicon ingot casting: process development by numerical simulations

Abstract Multicrystalline silicon from ingot casting processes has reached a high market share in photovoltaic industry. One reason for this is the increase of the quality of multicrystalline wafers which leads to higher solar cell efficiency. This progress was supported by numerical simulations, namely for the SOLPIN ingot casting process which is used by the Deutsche Solar GmbH for the production of high-quality multicrystalline wafers. In this paper, we give some examples of numerical simulation results, showing the temperature distribution in furnaces and silicon ingots for various process conditions. The influence of lateral and vertical heat flux due to the shape of the liquid–solid phase boundary and the solidification velocity is demonstrated in numerical case studies. As an example for the potential of our process simulations we report about the reduction of the dislocation density in a multicrystalline ingot, which was predicted by simulations and verified by experimental results.

Formation and annihilation of oxygen donors in multicrystalline silicon for solar cells

Abstract The efficiencies of solar cells based on multicrystalline silicon (mc-Si) have reached 17% even employing high-throughput crystallization steps and industrial-relevant solar cell processes. The efficiency of multicrystalline solar cells is governed by crystal defects, impurities and the interaction of both. The number of crystal defects, such as dislocations and grain boundaries, crucially depends on the crystallization conditions, while, with regard to impurities, electrically active transition metals, such as iron, are well-known to seriously reduce the minority carrier lifetime. A similarly important role, however, is played by oxygen. Various oxygen or oxygen-containing defect centers showing strong recombination activity may form in monocrystalline silicon as well as in mc-Si. In mc-Si blocks the formation of so-called thermal donors and nitrogen-oxygen complexes can take place during the relatively slow cooling of the ingots. Thermal donors and nitrogen-oxygen complexes lead to reduced lifetimes especially in the edge regions of the ingot. Whereas this lifetime reduction is hardly efficiency-relevant as long as annealing steps above 600°C for several minutes are implemented in solar cell processing, another species of oxygen donor, the new donor, forms in the temperature range between 600 and 900°C that is frequently used for solar cell fabrication. For silicon with a high oxygen content such as the Bayer RGS (ribbon growth on substrate) material, the new donors seem to be the most efficiency-relevant defects which can only be prevented using well-adjusted temperature profiles during crystallization and solar cell processing. Whereas monocrystalline silicon can benefit from high oxygen content through internal gettering steps in microelectronic device processing, a substantial improvement of mc-Si for solar cells is achievable by lowering the oxygen content. Oxygen contents considerably below those of monocrystalline silicon are therefore state of the art for modern high-throughput production material fabricated by the block-casting technology.

Multicrystalline Silicon for Solar Cells: Process Development by Numerical Simulation

Continuously improving crystallization conditions and solar cell processes have lead to steadily increasing efficiencies of solar cells based on multicrystalline silicon. There is, however, still an efficiency gap between mono- and multicrystalline silicon amounting to 1–2 % (absolute) depending on the cell process used. Topographies of the local solar cell performance clearly reveal that the main contribution to this efficiency gap is due to recombination-active dislocations present in multicrystalline silicon. A further improvement of the efficiencies attainable with multicrystalline solar cells therefore is achievable by a reduction of the dislocation density. Dislocations originate from thermal stress that originates from temperature gradients inside a multicrystalline ingot during crystallization and cooling. In order to reduce this thermal stress and consequently the dislocation density we employ a numerical simulation routine, the so-called virtual crystallization furnace, for perfect control of the temperature distribution during the entire ingot fabrication process.

Resistivity topography: a grain boundary characterisation method

Abstract Solar cells made from multicrystalline silicon are the ideal basis for photovoltaic systems. The solar cell efficiencies are still limited by the crystal defects (dislocations, grain boundaries) and their electrical activity. As a fast and efficient assessment of the electrical activity of specific grain boundaries high resolution resistivity maps will be used, measured on a set of wafers as cut coming from the same block. Resistivity maps proved to be a precise and yet simple method to characterise grain boundary activity. While measurements in the bulk of the crystal are symmetric because of the isotropy of the material, measurements crossing rain boundaries are depending on the relative orientation of the boundary with respect to the measurement geometry. The large number of measurement points of a high resolution resistivity map allows a statistical treatment of the data to evaluate one quantitative value for the grain boundary electrical activity. Solar cells from those have been processed. In the regions with a higher electrical activity at grain boundaries, the open circuit voltage of the solar cell decreased. A specific evaluation of the electrical activity of grain boundaries on the basis of the thermionic model allowed the fine tuning of production parameters leading to a homogeneous quality of cast mc-Silicon wafers bearing the potential of approaching highest efficiencies in industrial production processes.