Equipment for Solar Cell Production.
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Equipment for Solar Cell Production

A typical representative of a silicon solar cell consists of a photoactive p/n junction formed on the surface, a front ohmic contact stripe and fingers, a back ohmic contact that covers the entire back surface, and an antireflection coating on the front surface.

For silicon solar cell production either poly-crystalline or mono-crystalline material is used. Poly-crystalline silicon for photo-voltaic applications is normally produced by casting methods while mono-crystalline silicon is prepared in a Czochralski growing process.

The poly-crystalline or mono-crystalline ingots are cut to wafers. Poly-crystalline material is mostly cut to square wafers with a side length of 125mm or 150mm while mono-crystalline material is used to produce round wafers of 100 - 150mm of diameter. Sometimes square material with rounded edges is prepared from round wafers (125mm side length) in order to get a denser packing of the solar cells in the solar module.

solar cell

The main process steps in solar cell production are the preparation of the p/n junction by doping and the contacting of the photovoltaic cell. Beside that, further deposition processes are used to establish an antireflection coating or to improve the solar cell setup.

The single side polished or mirror-etched wafers that are used for photovoltaic application have to undergo a doping process first in order to create the photo-active p/n junction. This is in most cases a n+ doping with phosphorous. The doping is either done by the deposition of a doping glass and following diffusion in a conveyor furnace or in a tube furnace, using Phosporousoxychloride POCl3. The doping methode, using doping glass is simple and can be done in a continuous process in a conveyor furnace. However this methode requires two process steps more compared to the POCl3-doping process, because the doping glass has to be deposited and removed. In case that the POCl3-doping is used, in the past horizontal furnaces have been selected in most cases for cost reasons and because of the low demands to this process.

conveyer furnace   horizontal furnace

Koyo Thermo Systems furnace model 206 is designed for this application and for wafers up to 150mm in diameter and fulfills all requirements of this process. The LGO heating elements that are used in this furnace have a very low thermal mass and can reduce therefore the process time. They can also save energy and costs for the doping process. All normal sizes of solar wafers can be processed in this type of furnace. Liquid POCl3 is supplied in a bubbler. Nitrogen passes at a well defined temperature through the liquid and is transporting the dopant. Typical doping temperatures are 800 - 900C.

vertical furnaceFor higher demands to the homogeneity of the doping profile or to the automation level, vertical furnaces are available. The smallest version of a Koyo Thermo Systems vertical furnace can be very well used especially for the use in research and development of solar cells. The furnace model VF1000 is designed as a mini-batch furnace, has a manual loading and is therefore very flexible regarding sample sizes. This vertical furnace is equipped with a cost saving LGO heating element. The process performance equals the big production versions of vertical furnaces for IC production. The price of this unit is similar to the price of a horizontal tube in a horizontal furnace model 206.

For mass production, Koyo Thermo Systems developed a special vertical furnace, which gives better process results compared to a horizontal one, but still does not increase much the production costs for solar cells. This twin furnace loads 3-4 boats in one vertical tube and has therefore a capacity of 600-800 solar cells. This is the same capacity that you get on a horizontal furnace. Automation is easier to do on a vertical furnace.

Contacting of the photo cells is done by screen printing of metallic thick film pastes. For the backside contacting mostly aluminum paste is used. The finger contacts of the front side consist often of silver. The thick film pastes are then fired in a conveyor furnace. The meshbelt furnace Koyo Thermo Systems model 47-MT has a proven performance for this process. The firing can be done under exclusion of oxygen or even using a reducing atmosphere (forming gas). Automatic loading and unloading is available as an option.

conveyer furnace

After the testing of the solar cells, they are combined to solar modules in a continuous soldering procedure. This can be done in a Koyo Thermo Systems soldering furnace. Meshbelt transport is used in this soldering furnace. Temperature is 260 °C. The furnace has a downstream gas flow and can be run under nitrogen atmosphere or using a reducing atmosphere (forming gas). Full consideration has been taken on safety mechanisms and for easy handling of the equipment.

soldering furnace

The last step in solar panel production is then the packing between glass plates and testing.


Plasma Enhanced Chemical Vapor Deposition, PECVD

In semiconductor technology three methods are used for the deposition of layers on semiconductor wafers:
APCVD. Atmospheric Pressure Chemical Vapor Deposition requires rather high temperatures and is used only for very few applications like the formation of epitaxial silicon.
LPCVD. Low Pressure Chemical Vapor Deposition is widely used for the deposition of silicon oxide, nitride and poly-silicon. The process is performed in tube furnaces and requires also rather high temperatures.
PECVD. Plasma Enhanced Chemical Vapor Deposition is mainly used for the deposition of dielectric films and passivation films like silicon oxide or nitride or ONO layers at low temperature. It can be also used for SiC layers of poly-Silicon deposition. The necessary energy for the chemical reaction is not introduced by heating the whole reaction chamber but just by heated gas or plasma. It is the best method, if dopant diffusion has to be kept low, wafers have to be treated, which are sensible to high temperature or have been aluminium metallized already. The thermal budget of the treated wafers stays low with PECVD.
Using an RF generator, the plasma is formed in the reaction chamber. It contains reactive ions and radicals. The growth of the deposit starts easily because of the activation and cleaning of the surface by the more of less intense bombarding with ions from the plasma. You get good adhesion and high growth rates. The properties of the coated layers can be better influenced with PECVD than in simply thermal deposition technique, because more process parameters can be varied. Important are the adjustment of adhesion, compressive and tensile stress causing warpage, hydrogen content and density, etchability, etch rate and selectivity in etching, step coverage as well as stoichiometry (consistence) and cleanliness of the deposited layers, which can be measured by the refractive index. The maximum thickness of the deposit and the best uniformity of the coating is also dependent of the PECVD process parameters. Some film properties can be modified also subsequently.

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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