PV Manufacturing

 

Some of the manufacturing processes and resources for photovoltaics are shared with other applications, especially electronic chips for computers, mobile phones and any other electronic device. This competition has caused a shortage in supply of crystalline cells.

 

Manufacturing Silicon

 

The raw material of most solar cells today is crystalline silicon. Luckily, silicon is one of the most widely available elements in the form of sand. Before silicon can be cut into thin wafers, however, it has to be purified, as otherwise the photoeffect will not be very efficient. Purity levels for solar cells do not have to be as high as in chip applications. Solar-grade purity is 99.999% (5N) as opposed to electronic-grade silicon purity of up to 99.9999999% (9N).

 

 

There are three main categories of manufacturing processes,

resulting in different purity levels:

 

Electronic-grade Silicon: 9N

There are three main steps to produce high-purity polycrystalline silicon.

• Coke reduction: Metallurgical-grade silicon with 98.5% purity is produced from quartz sand in an arc furnace at very high temperatures.

• Distillation: In a second step, the metallurgical grade silicon powder is disolved in hydrogen chloride and subsequently distilled to form a silane gas. In most instances, this is the trichlorosilane, but could be others.

• Siemens Process: In the so-called Siemens Process the polycrystalline silicon is grown at very high temperatures. It requires hydrogen and produces more hydrogen-chloride as a by-product.

 

Medium-grade Silicon: 6-7N

The big drawback of the standard process as above is that a Siemens reactor is very expensive and the Siemens process itself requires a lot of energy. A number of new proprietary processes reduce the energy consumption and the capital costs for silicon production, though they are still similar to the traditional Siemens process.

• Fluidized Bed Reactor (Renewable Energy Corporation): operates at much lower temperatures and does not produce by-products.

• Vapour to liquid deposition (Tokuyama): similar to Siemens, but faster extraction.

 

Upgraded Metallurgical-grade Silicon (UMG): > 5N

In an altogether different process, metallurgical-grade silicon is chemically refined. By blowing gasses through the silicon melt, the boron and phosphorous impurities are removed, followed by directional solidification. Companies like Timminco (now bankrupt), Arise or RSI Silicon all have their own proprietary processes. However, they all have in common that by avoiding high purification, manufacturing costs are reduced significantly.

 

Manufacturing Wafers

 

There are mainly three different silicon wafer types of different qualities:

 

• Monocrystalline wafer: Silicon with a single, continuous crystal structure is grown from a small seed crystal that is slowly pulled out of a polysilicon melt into a cylindrical shaped ingot (Czochralski process). The ingot is cut into wafers using a diamond saw. Silicon waste from the sawing process can be re-cycled into polysilicon.

• Polycrystalline wafer:Polycrystalline silicon consists of small grains of monocrystalline silicon. Cube-shaped ingots can be made directly by casting molten polysilicon, which are then cut into wafers similar to monocrystalline wafers.

• Silicon ribbons: This is a continuous process whereby thin ribbons or sheets of multicrystalline silicon are drawn from a polysilicon melt. The subsequent cutting into wafers does not produce waste, as the drawn sheets are already wafer-thin. Silicon ribbons require around 5g of silicon per Watt rather than 8g/W using crystalline wafers.

 

Manufacturing Cells and Modules

 

 

Crystalline cells are made from silicon wafers by cleaning and doping the wafer. In a separate manufacturing process, a number of cells are wired up to form a module. As such the manufacturing process of crystalline modules consists of four distinct processes: Polysilicon production, Ingot & Wafer manufacturing, cell manufacturing and module manufacturing.