The Manufacturing and Science behind Solar Photovoltaic Cells

Solar photovoltaic cells are made out of a semiconductor material, which is a special type of material that conducts electricity at certain conditions. In the case of solar photovoltaic cells, the condition is dependent on the different amounts of wavelength of light that it accepts, which it then converts into electricity.

There are several types of materials that can be classified as semiconductors but the most common one used in the photovoltaics module industry is silicon. This is primarily because Silicon is the second most abundant element in the earth’s crust and because of the existing infrastructure developed by the semiconductor industry and their accumulated experience of using this material.

To a lot of industry experts this means in the long run silicon based solar photovoltaic modules should be the most affordable method of generating renewable electricity from the sun.

This leads to the next point and probably the question you are thinking about. What types of silicon photovoltaic cells are out there?

There are several types of solar cells but the general categories are:

1)  Monocrystalline
 

Chemical Definition: This type of Silicon has an ordered crystal structure, with each atom ideally lying in a pre-ordained position and exhibits predictable and uniform behaviour.
 

What this means: This type of Silicon goes through several cycles of slow and energy intensive filtration and separation processes and thus is the most expensive type of silicon.
 

Interesting point: These cells are usually created in a circular shape or a ‘square-without-corners’. This is because, when they are grown from an ingot, the only way to create high purity crystal structures is to extruded the molten liquid and gravity does the rest with respect to creating a cylindrical block out of which the smaller cells are cut. Usually manufacturers will leave cells in a circular shape however due to advances in recycling, the cells are being chopped into squares-without-corners to maximize the packing density of the modules.

2)  Multicrystalline/Polycrystalline
 

Chemical Definition: This type of silicon contains several regions of crystalline silicon kept together through covalent bonding and separated by ‘grain boundaries.’
 

What this means: This type of silicon goes through a lower number of cycles of the energy intensive filtration and separation processes that monocrystalline cells do and therefore are a less expensive material for manufacturers to use.
 

Interesting point: These cells are usually grown in a square shape. This is because the molten liquid in the (square) ingots does not to be extruded or go through another process to produce the big block of silicon out of which the little cells are cut. (grain boundaries can have an interesting effect on efficiency and this shall be covered in a later article)

3) Amorphous
 

Chemical Definition: This type of silicon has an even less regular arrangement of atoms, which result in dangling bonds and several gaps where recombination can take place.
 

What this means: This type of Silicon can be grown in any shape or size and can be produced, in theory, very cheaply.
 

Interesting point: These were the first type of solar energy cells that were used in the application of consumer products such as the watch, calculator and other non-critical outdoor applications and given their low cost they were adopted by other larger scale applications in the US.

 

Micro/Nanocrystalline or better known as Thin Film Solar Energy Panels are also one category of photovoltaic cells. This concept is an extension of the idea behind Amorphous silicon however instead of using silicon the industry in this case uses other elements, the most efficient of which is Gallium-Arsenide Thin-Film. These types of solar cells require less raw material which would imply that the cost of raw materials should be lower however, due to the high handling and processing costs of such thin cells, combined with the higher efficiency it makes it really difficult to judge whether its worth paying for. Nonetheless there are applications for these cells as well such as solar cars.