Amorphous silicon α - SiAmorphous silicon。A form of simple silicon.Brownish black or grayish black microcrystals.Silicon does not have complete diamond cells and its purity is not high.The melting point, density and hardness are also significantly lower than those of crystalline silicon.Chemical properties are more active than crystalline silicon.It can be prepared by reducing silicon tetrahalide with active metals (such as sodium, potassium, etc.) under heating, or reducing silicon dioxide with carbon and other reducing agents.Amorphous silicon films containing hydrogen are obtained by glow discharge deposition.
Amorphous silicon is a direct energy band semiconductor. There are many so-called "hanging bonds" in its structure, that is, there are no electrons bonding with the surrounding silicon atoms. These electrons can generate current under the action of an electric field without the help of phonons. Therefore, amorphous silicon can be made very thin and has the advantage of low production cost.
structure
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Amorphous silicon is basically in the form of regular tetrahedron, but deformation has produced many defects - suspension chains and voids.α - silicon characterized by short range order and long range disorder.Pure α - silicon cannot be used because of its high defect density.Hydrogen compensates the suspension chain, dopes and makes the pn junction[1]。
Amorphous silicon structure
Amorphous silicon structure
Technical advantages and disadvantages
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Advantages of amorphous silicon[2]
It can be cut freely, so it can make full use of synthetic products. Unlike crystal silicon, it can not be cut freely. When it is made into a device, a lot of material is ground down, which wastes a lot;Its manufacturing process is vapor deposition (1976, Spear method) - thermal decomposition of hydrogen chloride, which can be doped as needed, such as phosphine or boride. Because it is vapor deposition, the manufacturing process conditions are easy to be automatically controlled;It can also be made into very thin films, while crystalline silicon is at least hundreds of microns thick.This is because crystal 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 come from lattice vibration. If the crystal is too thin, too few phonons will be generated, and the photoelectric conversion rate will be too low.
Disadvantages of amorphous silicon
First, the service life is short. The so-called Staebler Wronski effect will occur under the continuous illumination of light, and the photoelectric conversion efficiency will drop to 25% of the original. This is essentially because there are too many defects in amorphous silicon, represented by dangling bonds, which leads to structural instability;
Second, its photoelectric conversion efficiency is far lower than that of crystalline silicon.The photoelectric conversion efficiency of crystalline silicon on the market today is 12%. The photoelectric conversion efficiency of crystalline silicon recently launched has been increased to 18%, and even 29% in the laboratory (contrast: the photoelectric conversion efficiency of green plant chloroplasts is less than 1%!). However, the photoelectric conversion efficiency of amorphous silicon has never exceeded 10%.
chemical property
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Double junction amorphous silicon solar panel
Chemical properties are more active than crystalline silicon.May be determined byActive metal(such as sodium, potassium, etc.) reduction under heatingSilicon tetrachloride, or withcarbonetc.reducing agentreductionsilicon dioxideMade.Characterized by short range order and long range disorderAlpha silicon。Pure α - silicon cannot be used because of its high defect density.
Scope of application
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Amorphous siliconSolar radiation peakNearbyLight absorption coefficientIt is one order of magnitude larger than crystalline silicon.Band gap width1.7 ～ 1.8eV, whilemobilityandMinority carrier lifetimeIt is much lower than crystal silicon and can be made intoAmorphous silicon field-effect transistor；be used forliquid crystalDisplay device, integrated a-Si inverter, integratedImage sensor, and bistableMultivibratorAsNonlinear device；The amorphous silicon film can be used to make various photosensitive, position sensitive, force sensitive, heat sensitive and other sensors;Made of amorphous silicon filmXerographic filmNot only will the copying speed be greatly improved, but also the image is clear and the service life is long;wait.At present, the application of amorphous silicon is developing rapidly. It is believed that more new devices will be produced in the near future.
Amorphous silicon solar cell
As a solar material, although it is a good battery material, its optical band gap is 1.7 eV, making the material itself 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 decline with the extension of the illumination time, that is, the so-called light induced decay S-W effect, which makes the battery performance unstable.The way to solve these problems is to prepare stacked solar cells, which are made by depositing one or more P-i-n sub cells on the prepared p, i, n single junction solar cells.The key problems for improving the conversion efficiency of the stacked solar cell and solving the instability of the single junction battery are: ① it sets up materials with different bandgap widths together, improving the spectral response range; ②The i-layer of the top battery is thin, and the electric field intensity generated by light changes little, so as to ensure the extraction of photogenerated carriers in the i-layer; ③The carrier produced by the bottom battery is about half of that of the single battery, and the photo induced decay effect is reduced; ④The sub cells of the stacked solar cell are connected in series.[3]
Preparation method
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fromAmorphous alloyIt is known that high cooling rate is required to obtain amorphous state, and the specific requirements for cooling rate depend on the material.Silicon requires a very high cooling rate, and amorphous state can not be obtained by liquid rapid quenching at present.In recent years, many technologies for vapor deposition of amorphous silicon films have been developed, including vacuum evaporation, glow discharge, sputtering andChemical vapor depositionEtc.The main raw materials commonly used aremonosilane （SiH4）、disilane （Si2H6）、Silicon tetrafluoride(SiF4), etc., with high purity requirements.The structure and properties of amorphous silicon films are closely related to the preparation processGlow discharge methodThe quality of amorphous silicon film prepared is the best, and the equipment is not complicated.
Schematic Diagram of Amorphous Silicon Prepared by Glow Discharge
Glow discharge method: the chemical vapor deposition is actually carried out with the help of plasma by using reaction gas to decompose in the plasma and deposit film on the substrate.Plasma is produced in vacuum system by high-frequency power supply.According to the way of applying electric field in the vacuum chamber, the glow discharge method can be divided into DC, high-frequency, microwave and glow discharge with additional magnetic field.In the glow discharge device, the growth process of amorphous silicon film is the process of silane decomposition in the plasma and deposition on the substrate.
Development history
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The rapid development of solid state physics and the subsequent invention of the transistor in the 1920s and 1930s of this century created the modern semiconductor industry, caused great changes in social life, and laid the foundation of the information age.Up to now, all electronic devices are made of crystal materials, of which single crystal silicon is the most important.Since 50 years ago, some people have tried to prepare amorphous silicon by evaporation, sputtering and other methods, hoping to obtain materials comparable to monocrystalline silicon.After more than 20 years of efforts, Spear of Dundee University in Britain successfully doped amorphous silicon into n or P type semiconductors and made n-p junctions in 1975 by using a new method called "glow discharge".Two years later, Carlson of RCA Company in the United States used the same method to produce amorphous silicon solar cells with an efficiency of more than 6.A new field of amorphous semiconductors has suddenly emerged in front of people[4]。
Amorphous silicon (a-Si ∶ H) is a new typeSemiconductor thin film materialsAs a new energy material and new electronic information material, it has made rapid development since it came out in the 1970s.Amorphous silicon solar cellIt is currently amorphousSilicon materialThe most widely used field is also an ideal material for solar cells. The photoelectric conversion efficiency has reached 13%. This solar cell will become a special pollution-free energy.In 1988, the total output of various solar cells in the world was 35.2 MW, of which amorphousSilicon solar cell13.9 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 consumption of raw materials, and lower price.
