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.

The main process steps in solar cell production are the preparation of the p/n junction by doping and the metallization or 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.

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.

For 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.

The last step of the manufacturing of crystalline solar cells is the deposition of the anti-reflective coating. In most cases silicon nitride is deposited by PECVD-technology.

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