What Is Amorphous Silicon?

amorphous silicon -Si is also called amorphous silicon. A form of elemental silicon. Brown-black or gray-black microcrystals. Silicon does not have a complete diamond cell, and its purity is not high. Melting point, density and hardness are also significantly lower than crystalline silicon. Chemically more active than crystalline silicon. It can be produced by reducing silicon tetrahalide under active metals (such as sodium, potassium, etc.) under heating, or reducing silica with reducing agents such as carbon. An amorphous silicon thin film containing hydrogen was obtained by glow discharge vapor deposition.

Chinese name: Amorphous silicon
Foreign name: amorphous silicon -Si
Advantages: Can be freely cropped

Missing point: Short life
Nature: Lively
main application: Solar battery

Introduction to Amorphous Silicon

Amorphous silicon is a direct band semiconductor. There are many so-called "dangling bonds" in its structure, that is, there are no electrons that bond with surrounding silicon atoms. These electrons can generate electric current under the action of an electric field. The help of phonons is needed, so amorphous silicon can be made very thin and has the advantages of low production cost.

Amorphous silicon structure

Amorphous silicon is basically in the form of a regular tetrahedron, but its deformation has caused many defects-suspension chains and voids. The structure is characterized by short-range ordered and long-range disordered -silicon. Pure -silicon cannot be used due to its high defect density. Hydrogen compensates the dangling chains in it, doping and making pn junctions ^[1] .

Advantages and disadvantages of amorphous silicon technology

Advantages of amorphous silicon ^[2]

It can be cut freely, so it can make full use of the synthesized product. Unlike crystalline silicon, which cannot be cut freely, the material is ground a lot of debris when it is made into a device, which is a waste. Its production process is vapor deposition (1976, Spear method)- Hydrogen hydride is thermally decomposed, and can be doped according to needs during decomposition. For example, doped with phosphine or boron hydride. Because it is vapor deposition, the manufacturing process conditions are easy to be automatically controlled; it can also be made into very thin and thin films, and The crystalline silicon must be at least a few hundred microns thick. This is because crystalline silicon is an indirect energy band semiconductor. Photons alone cannot excite electrons into the conduction band to generate current, but rely on the help of so-called phonons, which are derived from lattice vibrations. The crystal is made too thin, too few phonons are produced, and the photoelectric conversion rate is too low.

Disadvantages of amorphous silicon

The first is short life. The so-called Staebler-Wronski effect will occur under the continuous irradiation of light, and the photoelectric conversion efficiency will drop to 25%. This is essentially the existence of too many defects represented by dangling bonds in amorphous silicon. Resulting in structural instability;

Second, its photoelectric conversion efficiency is much lower than that of crystalline silicon. The photoelectric conversion efficiency of crystalline silicon on the market today is 12%, and the photoelectric conversion efficiency of recently released crystalline silicon has increased to 18%, and in the laboratory, it can even reach 29% (comparison: the photoelectric conversion efficiency of chloroplasts of green plants Less than 1%!), But the photoelectric conversion efficiency of amorphous silicon has never exceeded 10%.

Amorphous silicon chemical properties

Chemically more active than crystalline silicon. By active metals (such as sodium, potassium, etc.)

Double junction amorphous silicon solar panel It is prepared by reducing silicon tetrachloride under heating, or reducing silica with a reducing agent such as carbon . The structure is characterized by short-range ordered and long-range disordered -silicon. Pure -silicon cannot be used due to its high defect density.

Amorphous silicon applications

The optical absorption coefficient of amorphous silicon near the peak of solar radiation is an order of magnitude greater than that of crystalline silicon. Forbidden band width 1.7 1.8eV, while mobility and minority carrier lifetime are much lower than crystalline silicon, can be made into amorphous silicon field effect transistor; used for liquid crystal display device, integrated a-Si inverter, integrated image sensor , And bistable multivibrators as non-linear devices; using amorphous silicon film can be made into a variety of light-sensitive, position-sensitive, force-sensitive, thermal and other sensors; using amorphous silicon film for electrostatic photocopying Film, not only the copy rate will be greatly increased, but also the image is clear, long life; etc. At present, the application of amorphous silicon is developing rapidly. It is believed that in the near future, there will be more new devices.

Amorphous silicon solar cell

As a solar material, although it is a good battery material, its optical band gap is 1.7eV, which makes the material insensitive to the long-wave region of the solar radiation spectrum, which limits the conversion efficiency of amorphous silicon solar cells. . In addition, its photoelectric efficiency will decay with the extension of the light time, the so-called photo-induced decay S-W effect, making the battery performance unstable. The way to solve these problems is to prepare a laminated solar cell, which is made by depositing one or more Pin sub-cells on the prepared p, i, n single-junction solar cells. The key problems of stacked solar cells to improve the conversion efficiency and solve the instability of single-junction cells are: it brings together materials of different forbidden band widths to increase the spectral response range; the i-layer of the top cell is thinner The intensity of the electric field produced by the light does not change much, ensuring that the photo-generated carriers in the i-layer are extracted; the carriers generated by the bottom cell are about half of the single cell, and the photoinduced decay effect is reduced; The batteries are connected in series. ^[3]

Preparation method of amorphous silicon

It is known from the preparation of amorphous alloys that to obtain an amorphous state, a high cooling rate is required, and the specific requirements for the cooling rate depend on the material. Silicon requires a very high cooling rate, and the rapid quenching method of liquid cannot currently obtain an amorphous state. In recent years, many techniques have been developed for vapor deposition of amorphous silicon films, including vacuum evaporation, glow discharge, sputtering, and chemical vapor deposition. The main raw materials generally used are monosilane (SiH4), disilane (Si2H6), silicon tetrafluoride (SiF4), etc., which have high purity requirements. The structure and properties of the amorphous silicon film are closely related to the preparation process. At present, it is believed that the quality of the amorphous silicon film prepared by the glow discharge method is the best, and the equipment is not complicated.

Glow discharge method: The reaction gas is used to decompose in the plasma to deposit a thin film on the substrate, which is actually chemical vapor deposition with the help of plasma. The plasma is generated in a vacuum system by a high-frequency power source. According to the method of applying an electric field in a vacuum chamber, the glow discharge method can be divided into a direct current, a high frequency method, a microwave method, and a glow discharge with an additional magnetic field. In a glow discharge device, the growth process of an amorphous silicon film is a process in which silane is decomposed in a plasma and deposited on a substrate.

Amorphous silicon development history

The rapid development of solid physics in the 1920s and 1930s and the subsequent invention of the transistor created the modern semiconductor industry, caused great changes in social life, and laid the foundation for the information age. So far, all electronic devices are made of crystalline materials, of which single crystal silicon is the most important. Since fifty years, some people have tried to prepare amorphous silicon by evaporation, sputtering, and other methods, and expect to obtain materials comparable to single crystal silicon. After more than 20 years of hard work, Spear of Dundee University in the UK successfully doped amorphous silicon into n or P-type semiconductors in 1975 using a new method called "glow discharge" and produced np Knot. Two years later, Carlson of the American RCA company used the same method to produce more than 6 amorphous silicon solar cells. A new field of amorphous semiconductors was suddenly shown in front of people ^[4] .

Amorphous silicon (a-Si: H) is an emerging semiconductor thin film material. As a new energy material and a new electronic information material, it has achieved rapid development since its introduction in the 1970s. Amorphous silicon solar cell is currently the most widely used field of amorphous silicon material, and is also an ideal material for solar cells. The photoelectric conversion efficiency has reached 13%. This solar cell will become a special energy source without pollution. In 1988, the total output of various solar cells in the world was 35.2 MW, of which amorphous silicon solar cells were 13.9 MW, ranking first, accounting for about 40% of the total output. Compared with crystalline silicon solar cells, it has the advantages of relatively simple preparation process, less raw material consumption, and cheaper prices.