Author Topic: Tabbing solar cells  (Read 3715 times)

Kavan

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Tabbing solar cells
« on: April 03, 2013, 03:53:18 PM »
Hello Forum,

a question about tabbing solar cells for series connection:

1) the top part of the cell is the one that gets exposed to light and has two wide buses and many thin finger connections.
Why do we tab only the two vertical busses and not the fingers?

2) we tab the top busses along their whole lengths and then we tab the wire to the bottom of the other solar cell to make the series connection. What is important is that we use the tabbing wire through the whole length of the bus, correct?
So, by itself, the bus is not conductive?

thanks,
kavan

SeanB

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Re: Tabbing solar cells
« Reply #1 on: April 04, 2013, 12:23:37 AM »
The top metal is very thin, and is going to have a high resistance. Thus for the lowest loss you need to have multiple connections to it, so you attach a thicker wire to the bus at many points. The manufacturers cannot make the top metal thicker as it then will tend to peel off the cell.

Kavan

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Re: Tabbing solar cells
« Reply #2 on: April 04, 2013, 07:49:57 AM »
Thanks Sean!

Good and clear answer.

The metallic contacts on top of the cell are a weak point. Too much contact and we shadow the cell. To little contacts and we have too much resistance.....

Are there cells that have the contacts underneath the top surface and give maximum exposure to the sunlight?

thanks,
Kavan

Kavan

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Re: Tabbing solar cells
« Reply #3 on: April 04, 2013, 07:52:08 AM »
Also, on the back of the cell, we don't see a bus going along the full length of the cell.
We see instead just 2 or 3 white spots at different distance from each other.

Are those the conductive spots? Why not have a full bus on the back too?


SeanB

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Re: Tabbing solar cells
« Reply #4 on: April 04, 2013, 11:30:58 AM »
The top of the cell is where the junction is, just a few atoms under the surface. The cells are coated with a very thin ITO coating to collect this current, and the grid is there to reduce resistance, as the ITO is conductive, but not a good conductor. There is a trade off between collector that reduces cell resistance, and size of cell, as the current collectors reduce the cell area. The back of the cell has a much higher doping level, and thus is quite low resistance silicon, but this highly doped silicon is not good at being a solar cell, but is good at conducting electricity. You cannot put this on the front. As well the back will have a full metal layer on it, just a few spots where they put something you can solder to.

Kavan

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Re: Tabbing solar cells
« Reply #5 on: April 10, 2013, 11:06:20 AM »
Hello Sean,

have a look at this link:

http://www.actec.dk/Schott-Solar/pdf/efg-100x100.pdf

the back of the solar cell on the top left has some small white conductive spots...why? Why don't we see a full busbar in the back of the cell?

thanks,
Kavan

SeanB

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Re: Tabbing solar cells
« Reply #6 on: April 11, 2013, 12:52:28 AM »
Back is coated with a conductive aluminium layer that covers the whole area, the spots are a layer of extra metal that is probably a nickel layer  and then a copper overflash that is tinned, that are sputtered on to provide a good connection to solder or weld to, the rest being coated with an oxide layer for protection. You can see the whole back is a metal layer except right at the edges, as it is not exposed to light it is a full cover.