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Amorphous silicon
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Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. Silicon is a four-fold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystalline silicon this tetrahedral structure is continued over a large range, forming a well-ordered lattice (crystal). In amorphous silicon this long range order is not present and the atoms form a continuous random network.
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Encyclopedia
Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. Silicon is a four-fold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystalline silicon this tetrahedral structure is continued over a large range, forming a well-ordered lattice (crystal). In amorphous silicon this long range order is not present and the atoms form a continuous random network. Not all the atoms within amorphous silicon are four-fold coordinated. Due to the disordered nature of the material some atoms have a dangling bond. These dangling bonds are defects in the continuous random network, which cause electrical behaviour. If desired, the material can be passivated by hydrogen, which bonds to the dangling bonds and can reduce the dangling bond density by several orders of magnitude. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices. However, the hydrogen is unfortunately associated with light induced degradation of the material, termed the Staebler-Wronski Effect.
Ultraviolet
Amorphous silicon layer 812 absorbs UV radiation .
Production
One of the main advantages of amorphous, over crystalline silicon relies in its production technique, as thin films of it can be deposited over large areas by PECVD, as opposite to crystalline silicon (c-Si) wafers, which are sliced from bulk monocrystalline boules. It can be doped in a fashion similar to c-Si, to form p- or n-type layers and ultimately to form electronic devices.
a-Si can also be deposited at very low temperatures, as low as 75 degrees Celsius, which allows for deposition on not only glass, but plastic as well, making it a candidate for a roll-to-roll processing technique. The relatively lower electronic performance of low-temperature a-Si devices could be compensated by the cheaper production, for future, ultra-low-cost, high-volume applications (e.g. RFID tags).
Applications
Because of the production way used, amorphous silicon has become the material of choice for the active layer in thin-film transistors (TFTs), which are most widely used in large-area electronics applications, mainly for liquid-crystal displays (LCDs).
Solar cells
It is also used to produce large-area photovoltaic solar cells. This is a relatively new application, although the small solar cells used in some pocket calculators have been made with a-Si for many years.
It uses approximately 1% of the silicon needed for typical crystalline silicon cells.
Typically, amorphous silicon thin-film cells use a p-i-n structure. Typical panel structure includes front side glass, TCO, thin film silicon, back contact, polyvinyl butyral (PVB) and back side glass.
Producers include Signet Solar (on glass modules), that has begun volume production at its manufacturing facility in Mochau, Germany , that uses Applied Materials technology .
Other producers:
- ENN Solar Group (China) .
- Ersol Solar Energy
- Fuji Electric Systems
- Innovalight
- Kaneka (semi-translucent PV module)
- Mitsubishi Heavy Industries
- OptiSolar
- Sharp
- SUNGEN International Limited (A member of Anwell Group)
- NexPower Technology
- Sontor GmbH
- United Solar Ovonic
- Xunlight Corporation
- VHF Technologies
Some equipment suppliers provide turnkey amorphous silicon factories for producers:
Amorphous silicon and carbon
Amorphous alloys of silicon and carbon (amorphous silicon carbide, also hydrogenated, a-Si1-xCx:H) are an interesting variant to this material.
Introduction of carbon adds extra freedom to controlling the properties of the material.
Increasing concentrations of carbon in the alloy (x) widen the electronic gap between conduction and valence bands (also called "optical gap" and bandgap), in order to potentially increase the light efficiency of solar cells made with amorphous silicon carbide layers.
The film could also be made transparent to visible light.
On the other hand, the electronic properties as a semiconductor (mainly electron mobility), are badly affected by the increasing content of carbon in the alloy, due to the increased disorder in the atomic network. Bringing x to the opposite extreme (100%) we have amorphous carbon, or synthetic diamond-like films.
Several studies are found in the scientific literature, mainly investigating the effects of deposition parameters on electronic quality, but practical applications of amorphous silicon carbide in commercial devices are still lacking.
Microcrystalline and Micromorphous Silicon
Microcrystalline silicon (also called nanocristalline silicon) is also amorphous silicon, but contains small crystals. It absorbs a broader spectrum of light and is flexible.
Micromorphous silicon module technology combines two different types of silicon, amorphous and microcrystalline silicon, in a top and a bottom photovoltaic cell.
See also
External links
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