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This is a very simple question, but I haven't found the answer yet. It seems to me that monocrystalline panels are always black (though I have seen some dark blue ones), and polycrystalline panels are bright with lots of patches of different shades of blue. However, I've seen some panels that still seem to have the patches but they are very dark overall. Is there a proper way to tell what kind of panel one has just by the looks? is the presence of shades/patches of blue a good indicator of a panel's polycrystalline structure no matter how dark they are? or for that matter, are monocrystalline panels always black?

Thanks in advance.

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  • \$\begingroup\$ I believe the blue color is a coating to make the cell absorb the daylight better. \$\endgroup\$ – jippie Oct 14 '13 at 19:29
  • \$\begingroup\$ Looking at the price tag probably helps; monocrystalline is more difficult to make and usually more efficient. \$\endgroup\$ – Nick T Oct 14 '13 at 21:01
  • \$\begingroup\$ Yep, I knew about it. Unfortunately I don't have the price info available. \$\endgroup\$ – Antonio Azuara Oct 15 '13 at 5:31
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You can't trust the "blackness" which is determined by the cells BBAR (Broad Band Anti-Reflective) coatings. If both cells had ideal coatings there would be no light reflected back and therefore you couldn't tell from this if it was Mono or Poly. However, no AR coating is ideal, and in fact are far from it, so the fact that you have multiple different crystal orientations with be very evident. Mono crystalline cells will be very boringly uniform, you might see broader more smooth transitions in a mono crystalline cell but that will be from surface layer (like SiO2) non-uniformity. 100's of nm thickness differences are noticeable by the human eye. Poly will have abrupt transitions in the patterns.

On Edit: It is possible to have what are known as Slips and dislocations in a mono-crystaline wafer that would show up as a "grain-like" boundary. It won't affect performance of the Si as such and won't be as mottled as Poly-Silicon but my statement above as to uniformity isn't an absolute.

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  • \$\begingroup\$ Thanks! your message is actually quite useful. I was wondering about the blackness because in some panels I have I can see "crystals", however they are really faint and it makes me doubt whether it's mono or poly. Of course, the logical thing to do is to trust the manufacturer, but in this case I'm having my doubts (they claim mono and I see faint, dark blue crystal-like shapes). \$\endgroup\$ – Antonio Azuara Oct 15 '13 at 5:34
  • \$\begingroup\$ It is possible to have "boundaries" in the cell - while technically not a grain boundary they might be noticeable. I'll edit my post. \$\endgroup\$ – placeholder Oct 15 '13 at 19:06
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Are the internal crystal boundaries not always visible in polycrystalline cells?

enter image description here

image from M0UKD

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  • \$\begingroup\$ I think those internal boundaries is what Antonio Azuara is referring to by "lots of patches of different shades of blue". You're actually looking at the many (=poly) different crystals there. \$\endgroup\$ – jippie Oct 14 '13 at 19:31
  • \$\begingroup\$ Yeah, I meant that, sorry for not using the correct terminology. \$\endgroup\$ – Antonio Azuara Oct 15 '13 at 5:32
  • \$\begingroup\$ @Antonio: Those "patches" are the individual crystals, hence poly-crystalline = many crystals. \$\endgroup\$ – RedGrittyBrick Oct 15 '13 at 8:33
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Another way is to look at the shape of the cell. Usually monocrystalline cells have a rounded shape with cut corners whereas the polycrystalline cells are square. This is due to the fabrication process.

The monocrystalline are created in an ingot with a cylindrical shape (by the Czochralski process). To have the minimum waste when producing the cells, the ingot is cut as cuboid with rounded corners. On the other hand, the polycrystalline cells are made by casting the silicon in a mould. This mould can have a perfect square or rectangular shape.

left: polycrystalline — right: monocrystalline

Some references and further differences between these 2 types of solar cells:

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