Amorphous silicon

Amorphous silicon (a-Si) is the non- crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto a variety of flexible substrates, such as glass, metal and plastic. Amorphous silicon cells generally feature low efficiency.

As a second-generation thin-film solar cell technology, amorphous silicon was once expected to become a major contributor in the fast-growing worldwide photovoltaic market, but has since lost its significance due to strong competition from conventional crystalline silicon cells and other thin-film technologies such as CdTe and CIGS. Amorphous silicon is a preferred material for the thin film transistor (TFT) elements of liquid crystal displays (LCDs) and for x-ray imagers.

Amorphous silicon differs from other allotropic variations, such as monocrystalline silicon—a single crystal, and polycrystalline silicon, that consists of small grains, also known as crystallites.

Description

Silicon is a fourfold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystalline silicon (c-Si) this tetrahedral structure continues over a large range, thus forming a well-ordered crystal lattice.

In amorphous silicon this long range order is not present. Rather, the atoms form a continuous random network. Moreover, not all the atoms within amorphous silicon are fourfold coordinated. Due to the disordered nature of the material some atoms have a dangling bond. Physically, these dangling bonds represent defects in the continuous random network and may cause anomalous electrical behavior.

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 such as solar photovoltaic cells, particularly in the protocrystalline growth regime. However, hydrogenation is 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. Introduction of carbon atoms adds extra degrees of freedom for control of the properties of the material. The film could also be made transparent to visible light.

Increasing the concentration of carbon in the alloy widens the electronic gap between conduction and valence bands (also called "optical gap" and bandgap). This increases 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 adversely affected by the increasing content of carbon in the alloy, presumably 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.

Properties

The density of amorphous Si has been calculated as 4.90×10²² atom/cm³ (2.285 g/cm³) at 300 K. This was done using thin (5 micron) strips of amorphous silicon. This density is 1.8±0.1% less dense than crystalline Si at 300 K. Silicon is one of the few elements that expands upon cooling and has a lower density as a solid than as a liquid.

Hydrogenated amorphous silicon

Unhydrogenated a-Si has a very high defect density which leads to undesirable semiconductor properties such as poor photoconductivity and prevents doping which is critical to engineering semiconductor properties. By introducing hydrogen during the fabrication of amorphous silicon, photoconductivity is significantly improved and doping is made possible. Hydrogenated amorphous silicon, a-Si:H, was first fabricated in 1969 by Chittick, Alexander and Sterling by deposition using a silane gas (SiH₄) precursor. The resulting material showed a lower defect density and increased conductivity due to impurities. Interest in a-Si:H came when (in 1975), LeComber and Spear discovered the ability for substitutional doping of a-Si:H using phosphine (n-type) or diborane (p-type). The role of hydrogen in reducing defects was verified by Paul's group at Harvard who found a hydrogen concentration of about 10 atomic % through IR vibration, which for Si-H bonds has a frequency of about 2000 cm⁻¹. Starting in the 1970s, a-Si:H was developed in solar cells by David E. Carlson and C. R. Wronski at RCA Laboratories. Conversion efficiency steadily climbed to about 13.6% in 2015.

Deposition processes

Applications

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

One further advantage is that a-Si can be deposited at very low temperatures, e.g., as low as 75 degrees Celsius. This allows 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 throughput, that leads to lower manufacturing cost particularly when the silane is recycled.

Arrays of small (under 1 mm by 1 mm) a-Si photodiodes on glass are used as visible-light image sensors in some flat panel detectors for fluoroscopy and radiography.

Photovoltaics

Hydrogenated amorphous silicon (a-Si:H) has been used as a photovoltaic solar cell

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