The most important element that is used for the production of a solar cell is silicon.
Silicon is the eight most common elements in the universe by mass, but very rarely occurs as the pure free element in nature. It is more widely distributed in dusts, sands, planetoids and planets in various forms silicon dioxide (silica) or silicates. In Earth’s crust, silicon is the second most abundant element after oxygen, making up 27.7 % of the crust by mass.
Silicon has many industrial uses. It’s the principal component of most semiconductor devices, most importantly integrated circuits or microchips. Silicon is widely used in semiconductors because it remains a semiconductor at higher temperatures than the semiconductor germanium and because its native oxide is easily grown in a furnace and forms a good semiconductor/dielectric interface.
In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics.
Silicon is also a constituent of silicones, a class-name for various synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often confused with silicon itself.
Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals. It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatoms.
For the production of a solar cell, silicon needs not as pure as in the semiconductor branch. Silicon mainly exists in the compound silica (SiO2). Silicon may be one of the most common elements on earth but is costs a lot of energy to reach the pure form of silicon (Si).
In the present production process silica reacts with carbon at a temperature of 1000 degrees Celsius. Then silicon is produced with a purity of 98 %. Hereafter additional treatments are necessary to reach the final target.
Solid silica can be directly converted in pure silicon by means of electrolysis in a salt solution at low temperature. That kind of silicon is easily pulverizing in grains of some micrometers thick. Maybe this property gives additional opportunities in the future for the photovoltaic industry.
There are different kinds of silicon, generally classified into three groups:
1. Monocrystalline silicon
2. Polycrystalline silicon
3. Amorphous silicon
Monocrystalline silicon (single-crystal Si, mono-Si or crystalline Si = c-Si) is often produced according the Czochralski-process. A little bar of pure silicon is put in a rotating vessel (furnace) with molten impure silicon. The little pure silicon bar is slowly pulled upward. During this process, molten silicon sticks to the bar and adopts the crystal direction of the pure silicon. After this growth process a big round silicon bar exists with a diameter of about 4.5 inches and are 0.3 mm thick. This bar is one big single crystal of pure silicon.
Next the silicon cylinder is cut in thin slices, the so called wafers. These wafers have a circle shape. That’s the reason why the surface of a solar panel isn’t fully covered with silicon material. The corners of a c-Si cell are always ineffective.
Polycrystalline silicon (poly-Si or mc-Si) is made by casting molten silicon in square shaped moulds. The big blocks of silicon are carefully cooled down to the solid state.
The square shaped blocks are cut to thin slices with wire saws for further use.
This production process is cheaper as described before.
The look of the slice surface is not as nice as the mono crystalline silicon surface.
The surface of a poly crystalline silicon wafer is smudged with dark patches in different colours. This is caused by different directions of the multi crystals in the material.
The efficiency of the poly cells is lower compared with mono cells, however big manufacturers vote for poly cells because the production cost of poly cells are far less.
Amorphous silicon (a-Si) is used in extremely thin layers of silicon which is often obtained in an electrochemical process. These very tiny layers are rather transparent and can be used as an advanced coating for various carriers such as curtains, glass windows, textile, etc.
They are suitable for special applications and sophisticated architectural designs.
Sometimes they are called slivers.
Another method of making slivers is the production by pealing off thin layers of molten silicon. These slivers have a amorphous structure. Again slivers have still a lower efficiency factor as normal poly-Si cells. But improvements are still going yet. Slivers are cheap to make and there is a minimum waste of expensive material. Slivers are so extremely thin, that they become a transparent look. They can be use at both sides. The Australian National University produces sliver cells.