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My understanding is that the ISS's solar panels are silicon and double sided to maximize bang-for-the-pound (average power per kilogram transported to orbit). See Are the ISS US Segment solar arrays double-sided? and the image there, and Why does the ISS not use the most efficient solar panels available? and associated images and answers.

But I don't understand the appearance of this panel in the photo below. I'm assuming the dark stuff is silicon, but there are a lot of silver/gray dots and for every four of those there is a larger interstitial red dot. And the whole think looks flexible as if it were a film rather than a crystal.

What is the structure of these devices? Are they made from amorphous flexible films of silicon, or lots of small little thin crystals of silicon on a flexible matrix of some kind? How does sunlight reach both sides - does it pass through a backing on one side?


below: Cropped and original image "Scott Kelly fixing a cooling pump during a spacewalk." from Gizomodo's Astronaut Scott Kelly on Liquid Salt, a Stinky Station, and Sleeping in Freefall. Image credit: NASA/Kjell Lindgren

expanded and cropped

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below: original image shown in reduced size. (right) click for full size.

original

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The cells themselves are not flexible -- indeed, they are fairly thin and delicate (I should know, I've broken quite a few of them myself).

The flexibility you see comes from the fact that the cells are individually welded and glued to a very thin, flexible blanket made from Kapton, silicone, and fiberglass.

I can't find a public datasheet for the solar cells (if I find one, I'll edit the post), but I can do my best to describe what you're seeing, with the help of this image I found online (rehosted from http://pages.erau.edu/~ericksol/courses/sp300/images/iss_cells.jpg).

enter image description here

The inset image on the lower right is a closeup of the back side of a single cell as seen through the blanket substrate.

As you can see, the four dots are "wrapthrough" connections that take the collector grid from the front face and feed it to a set of contacts on the back face. The ISS solar cells are a little bit unconventional in that rather than having the interconnect contacts on opposite edges of the cells, all of the contacts are on the back face of the cell. Those dots are actually silver-plated holes in the silicon cell, much like a via in a printed circuit board. Those holes are covered by the coverglass, which is a thin sheet of glass with UV-blocking and anti-reflective coatings that protect the exposed face of the cell.

The interstitial red dots you see are actually the blanket material behind the cells. The cells are shaped like squares with the corners lopped off, so the dots are actually where the missing corners of four adjacent cells come together. You can see this somewhat in the image, though the edges of the cell are obscured by the copper traces on the blanket layup.

Sunlight (when the back of the blanket faces the sun -- earth albedo light otherwise) reaches the back of the cell by nature of the fact that the blanket material is largely transparent, which is plainly apparent in the image. The orange color is due to the Kapton on the blanket. The back face of an ISS solar cell is actually green.

Side note, the numbers you see printed on the cell are the lot number (on top) and the grade number (on bottom). The cell depicted is on an engineering test article (hence the extra wiring for what appears to be thermocouples) and is not a flight-grade article, but the physical appearance is identical.

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  • $\begingroup$ I am in awe! OK I see that in the inset the back of one wafer (to me it looks more like a round wafer with four large "flats") connects to the front of the next one to the right. The holes (vias or wraparounds) could be chemically etched on a wafer basis, or the boule could have been drilled before slicing. I have no idea how the holes were electrically passivated so they don't short circuit, but diffusion engineers have all kinds of tricks. The details of the electrode patterns near the holes show they drew the pattern to collect as absolutely much as they could. Beautiful Engineering! $\endgroup$ – uhoh Sep 13 '17 at 15:11
  • $\begingroup$ ...and in the view in the inset, the back is considered the "front side". $\endgroup$ – uhoh Sep 13 '17 at 15:15
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    $\begingroup$ @uhoh Largely due to the design of the collector and contact arrangement, the cells are extremely robust to physical damage. I've shot a hole nearly a half inch in diameter through one and it still produced a surprising amount of power, despite the hole and all the cracking. $\endgroup$ – Tristan Sep 13 '17 at 15:44
  • $\begingroup$ There are flexible cells too, but they are made for use on earth only. They may be used on uneven surfaces, the deck of a sailboat for instance. Of course they may be bent in one direction only. The materials used would withstand the marine environment, but not the use in space. $\endgroup$ – Uwe Sep 14 '17 at 9:19
  • $\begingroup$ @Uwe They're also horrendously inefficient, last time I checked. $\endgroup$ – 0xDBFB7 Sep 14 '17 at 14:16

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