Industrial & Specialty Printing - July/August 2012 - 35

silicon for solar
Wim Zoomer Technical Language The solar industry is booming and annually growing at double digits. Although it is not the ideal material (energy conversion, production costs) for the conversion of solar energy, since its application in the space industry 1958, silicon has been the dominant material in the solar industry. Today, approximately 90% of solar cells are still based on crystalline silicon and about 10% use thin-film methods. The absorption properties of silicon, at a wavelength of 400-1500 nm makes it an important element in solar cell performance. Photovoltaic wafer manufacturing makes up the first step in the complete production process of solar systems. The limited elements of the process considered are: silicon and its purification to at least the required SoG (Solar Grade) level; and the manufacturing of silicon wafers. Let’s first focus on silicon’s characteristics, appearance and a selection of different purification techniques. sIlIcON PuRIfIcatION In 1824, the Swedish chemist Berzelius made amorphous silicon by a chemical reaction between potassium fluorosilicate and potassium. He named the brown solid silicium, derived from Latin silicis. Silicon is almost 28% by weight the second most abundant element, after oxygen, in the earth’s crust; however, it does not exist naturally in elemental form. Most silicon can be found as silicon dioxide. Silicon is a tetravalent metalloid (Figure 1) with symbol Si and atomic number 14. A metalloid has intermediate chemical and physical characteristics between metals and non-metals. The reflective greycolored crystalline element has a specific density of 2.33 g/cm3 and is has a melting point of 1414°C. Solid silicon does not react with water, oxygen, and most acids. It also does react with halogens and diluted alkaline solutions. Chemically considered pure silicon is not a good electrical conductor at all; it is a semiconductor. Like carbon, silicon donates or shares its four outer electrons offering opportunities to combine with other elements, such as boron, phosphorous, arsenic, and antimony. It is commonly tetrahedrally bonded to four neighboring silicon atoms. In crystalline format, the tetrahedral structure creates a crystal lattice. The more dopant in the crystal lattice, the better the silicon conducts electricity. Silicon appears in several forms: amorphous, monocrystalline, polycrystalline, and microcrystalline. Amorphous silicon is known as a-silicon. It does not have a long-range tetrahedral structure. Instead, its atoms are a part of a random network, which means that not all atoms are fourfold coordinated, whereas some atoms may have a dangling bond. Due to a lower electron performance compared to the crystalline version, a-Si is more flexible. Additionally, amorphous silicon can be deposited at lower temperatures compared to the crystalline version. This makes amorphous silicon an interesting product for reel-to-reel manufacturing of flexible thin-film photovoltaic solar cells for devices requiring low power. These cells achieve approximately 10% efficiency rates. Amorphous silicon photovoltaic cells use only a fraction of the silicon required to manufacture crystalline silicon photovoltaic cells. You can compensate for this low efficiency by stacking films on top of each other, provided that every cell is tuned to the specific wavelength of light. The base material for the electronic industry, monocrystalline silicon, is also called single-crystal silicon (sc-Si) or simply, mono-silicon. The crystal lattice of monocrystalline silicon is continuous and homogenous—practically without any grain boundaries. The grain size is over 100 mm. Monocrystalline silicon is usually made of extremely pure silicon and used to manufacture high-performance photovoltaic solar cells. Most monocrystalline silicon is manufactured using the Czochralski process. The float-zone process is an alternative manufacturing method. Ingots of 2 m length and approximately 300 mm in di-

PRINtINg MEtHODs
Figure 1 A silicon unit cell Figure 2 Metalloid silicon
July/august 2012 | 35

Industrial & Specialty Printing - July/August 2012

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