Why are monocrystalline cells cut hexagonal?

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Art
Art Solar Expert Posts: 32
Hi Guys,
As the thread title...
Why are they always cut this way?
Cheers, Art.

Comments

  • DeltaFox
    DeltaFox Solar Expert Posts: 27
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    Re: Why are monocrystalline cells cut hexagonal?

    I think they do that just so they fit in the box better. Thar's what I was told.
  • GreenPowerManiac
    GreenPowerManiac Solar Expert Posts: 453 ✭✭✭
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    Re: Why are monocrystalline cells cut hexagonal?

    There could be numerous reasons why. Patents, Copyrights, all shapes and sizes, or simply to identify certain types. Cutting the corners makes it look appealing and interesting. Might sacrifice a small amount of power, barely noticeable.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,491 admin
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    Re: Why are monocrystalline cells cut hexagonal?

    I assume you mean a modified octagon shape?

    Many silicon "blanks" start out as round bars that are sliced into wafers (at least in the olden days). To get "square" cells, they have to trim the cells to get them into other shapes.

    My guess is that there is a trade off between cost to throw away material vs cost of manufacturing. Also a very square corner can be fragile and result in higher reject rates--so some vendors may be slicing corners for that reason too..

    Evergreen Solar has a process where they grow the wafers as long ribbons and just slicing them for length. Much less waste because and the cells are rectangular by default.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
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    Re: Why are monocrystalline cells cut hexagonal?

    Monocrystaline are processed like integrated circuits, only much simplier in number of steps.

    The substrate is grown from a seed into a ingot of 4", 6", 8" and 12" diameter. These 'log' is sawed into thin wafers. A flat notch is placed on one side to signify grain orientation for I.C. processing. I have not seen 8" and 12" wafers used for solar cells.

    I come from I.C. processing and latest technology process requires very perfect wafers. It is highly likely that a lot of monocrystaline solar cells come from wafers that are rejects from semiconductor industry.

    As to your specific question. Four sides are cut off to provide a more square shaped cell. This does waste some of the cell material but is better for solar collection surface area. It is a compromise between waste and surface area for solar collection. As long as it meets or beats the next best competition (polycrystaline) then unnecessary to go to actual square area cut. You have probably seen some panels with 'half moon' wafer cuts used. These waste no wafer area but have more non-solar cell area on the PV panel, so net lower efficiency per square meter of panel.
  • Art
    Art Solar Expert Posts: 32
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    Re: Why are monocrystalline cells cut hexagonal?

    Thanks. Just wondered :)
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
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    Re: Why are monocrystalline cells cut hexagonal?

    Monocrystaline cells are sliced from highly pure cylinders called boules pulled slowly from molten silicon in order to get a close to perfect crystal. They clip the corners to save as much of the wafer as possible leaving the octagon shape.
    Polycrystaline cells are sliced from less pure rectangular blocks cast quickly from molten silicon. Much faster and cheaper but leaves a lot of crystal boundaries impeding the electron mobility and as such are less efficient. Almost no wafer is lost because they start out square.
    There is also the ribbon technique used by Evergreen where the silicon is basicly extruded as a thin ribbon and doesn't even need slicing, but is a little less efficient still.