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Silicon wafers are produced from this purified silicon for use in solar cells. Monocrystalline silicon

To obtain these wafers, pure granulated silicon has to be converted to a solid material. The classic and still widespread method of obtaining monocrystalline silicon is the Czochralski process (Figure 3.3). This consists of drawing cylindrical bars from molten silicon.

The granulated polycrystalline silicon is melted in a crucible with a doping material, for example, boron to obtain a basic p-type material. (For the principle of doping, see Section 2.3.3.) During this process, a seed of monocrystalline silicon is placed in the precise orientation required and the crystal is made to grow, the

temperature being controlled very accurately (Figure 3.4). In this way, crystals 1–2 m long and up to 30 cm in diameter are grown with the same orientation as the seed inserted.

Another process (the ‘zone fusion process’), similar to the Czochralski process, consists of pulling more rapidly and then melting a zone of the rod by an electro- magnetic bobbin to enable it to crystallise regularly from the seed.

Finally, to obtain silicon wafers of around 150–300mm, rods are sliced with a

wire saw. Pulling Seed Rod Molten silicon Heating elements Crucible

Figure 3.3 Czochralski process

A steel wire of around 0.2 mm diameter coated with an abrasive compound, for example, silicon carbide, slices through the silicon at high speed. This procedure enables about a hundred wafers to be sliced at once, the wire being wound round the rod many times.

The whole process has a fairly low efficiency (15–25%) and uses a fair amount of energy.

Polycrystalline silicon

By the mid-1970s, it was understood which elements were detrimental to the effi- ciency of solar cells and how a cheap silicon crystal of ‘solar’ quality could be manufactured. The result was the development of solar quality polycrystalline silicon that appears as the juxtaposition of small monocrystalline crystals of dif- ferent orientations and sizes ranging from millimetre to centimetre. To manufacture this material, waste from the pulling of monocrystals, or purified metallurgical

silicon, is re-melted in a square crucible, at a temperature close to 1500C, in a

controlled atmosphere. Several thermal and chemical processes are used at this stage to ‘push’ the main impurities to the edge of the crucible, forming a crust, which is eliminated after solidification. The correct method of cooling is essential and determines the size of the crystals and the distribution of remaining impurities, which are mainly concentrated at the edges of the crystals, called grain boundaries. It is even possible to orient these grains parallel to the surface to improve the diffusion of electrical charges in the cells (see, for example, the Polix process developed by the Photowatt company). The ingot obtained in this way is then cut

into square rods (12.5 12.5 or 15.6  15.6 cm), which are then sliced into wafers

with a wire saw, like the rods of crystalline silicon. This process is economical: the wafers are directly produced in squares, the material efficiency is good and the ‘filling’ of the PV module is denser.

Silicon ribbon

To completely eliminate the sawing stage, which uses a lot of energy and results in considerable material losses, many methods have been tried since the end of the 1980s to produce PV panels directly from molten silicon, processes described under the generic term of silicon ribbon. The molten silicon is drawn directly in the form of a flat or tubular ribbon. The main problems in the process arise in the difficulty of finding a suitable support for the ribbon, how to remove the heat that arises and the treatment of the edges.

The EFG (Edge-defined Film-fed Growth) ribbon technology consists of drawing an octagonal tube from a bath of molten silicon up to 6 m long, the ends of which are subsequently sliced by laser to form wafers of today’s standard size of

156 156 mm.2The mechanical behaviour of wafers obtained by this method is a

critical parameter because the laser slicing makes the edges of the cells fragile. The degree of crystallisation depends on the speed of the pulling, and a slow speed can lead to silicon ribbon that is virtually monocrystalline with an efficiency of 15–16%. With another method, the SR (String Ribbon) technique, developed in the United 2

States, a single ribbon is pulled from a bath of silicon, supported on either side by high temperature wires (or strings). This simpler method results in better pro- ductivity, especially because the speed of pulling can be higher. The promoters of this impressive technique claim that material efficiency is doubled in comparison

with traditional slicing,3but its detractors maintain that it is too limited in terms of

wafer sizes.

Other methods consist of producing the ribbon on a backing, which is subse- quently removed, according to the technique called RGS (Ribbon Growth on Sub-

strate), in which the material is grown on a moving substrate.

Similarly, the technique called CDS (Crystallisation on Dipped Substrate) consists of dipping substrates in a bath of molten silicon. The promoters of this recent technique claim that it is the only method compatible with future mass production because it combines larger wafer size and high productivity.

However, it should be said that with the market as it is in 2009, with the price of silicon having fallen considerably, these technologies have lost part of their attraction, their main aim being to reduce the costs of the raw material when they were developed during the period of silicon shortage in 2003–04.