Amorphous silicon

Amorphous silicon (a-Si or α-Si) is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, which offers some unique capabilities in a variety of electronics.

Description

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 anomalous electrical behavior.

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.

Amorphous silicon and carbon

Amorphous alloys of silicon and carbon (amorphous silicon carbide, also hydrogenated, a-Si_1-xC_x:H) are an interesting variant to this material. Introduction of carbon adds extra freedom to controlling the properties of the material. The film could also be made transparent to visible light.

Increasing concentrations of carbon in the alloy 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. 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.

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.

Applications

While a-Si suffers from lower electronic performance compared to c-Si, it is much more flexible in its applications. It may also produce savings on silicon material cost, as a-Si layers can be made thinner than c-Si.

One advantage is that a-Si can be deposited at very low temperatures, as low as 75 degrees Celsius. This allows for deposition on not only glass, but plastic as well, making it a candidate for a roll-to-roll processing technique. Once deposited, a-Si can be doped in a fashion similar to c-Si, to form p-type or n-type layers and ultimately to form electronic devices.

Another advantage is that a-Si can be deposited over large areas by PECVD. The design of the PECVD system has great impact on the production cost of such panel, therefore most equipment suppliers put their focus on the design of PECVD for higher throughout, that leads to lower manufacturing cost. ^[1]

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

a-Si has been used as a photovoltaic solar cell material for calculators for some time. Although they are lower performance than traditional c-Si solar cells, this is not important in calculators, which use very low power. a-Si's ability to be easily deposited during construction more than makes up for any downsides.

More recently, improvements in a-Si construction techniques have made them more attractive for large-area solar cell use as well. Here their lower inherent efficiency is made up, at least partially, by their thinness - higher efficiencies can be reached by stacking several thin-film cells on top of each other, each one tuned to work well at a specific frequency of light. This approach is not applicable to c-Si cells, which are thick as a result of their construction technique and are therefore largely opaque, blocking light from reaching other layers in a stack.

The main advantage of a-Si in large scale production is not efficiency, but cost. a-Si cells use approximately 1% of the silicon needed for typical c-Si cells, and the cost of the silicon is by far the largest factor in cell cost. However, the higher costs of manufacture due to the multi-layer construction have, to date, make a-Si unattractive except in roles where their thinness or flexibility are an advantage.

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. Uni-Solar produces a version of flexible backings, used in roll-on roofing products.

Microcrystalline and Micromorphous Silicon

Microcrystalline silicon (also called nanocrystalline silicon) is amorphous silicon, but also 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. Sharp produces cells using this system in order to more efficiently capture blue light, increasing the efficiency of the cells during the time where there is no direct sunlight falling on them. Protocrystalline silicon is often used to optimize the open circuit voltage of a-Si photovoltaics. ^[2]

Large-scale production

Xunlight Corporation^[3], which has received over $40 million of institutional investments, has completed the installation of its first 25 MW wide-web, roll-to-roll photovoltaic manufacturing equipment for the production of thin-film silicon PV modules.^[4] Anwell Technologies has also completed the installation of its first 40 MW a-Si thin film solar panel manufacturing facility in Henan with its in-house designed multi-substrate-multi-chamber PECVD equipment. ^[5]

See also

• Atomic layer deposition (ALD)
• Chemical mechanical planarization (CMP)
• Chemical vapor deposition (CVD)
• Crystalline silicon
• Ion implantation
• Nanoparticle
• Prometheus Institute for Sustainable Development
• Physical vapor deposition (PVD)
• Rapid thermal processing (RTP)
• Wafer wet cleaning

References

External links

• Amorphous Silicon Devices group at the University of Waterloo, Ontario, Canada
• Theory and Simulation at Ohio University, Athens Ohio
• National (U.S.) Renewable Energy Laboratory Information on Amorphous Silicon^{[dead link]}
• Xunlight Corporation - A company led by Dr. Xunming Deng that has developed a manufacturing technique for the large-scale production of thin film Si cells