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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

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

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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).

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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
    Commented Sep 13, 2017 at 15:11
  • $\begingroup$ ...and in the view in the inset, the back is considered the "front side". $\endgroup$
    – uhoh
    Commented Sep 13, 2017 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
    Commented Sep 13, 2017 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
    Commented Sep 14, 2017 at 9:19
  • $\begingroup$ @Uwe They're also horrendously inefficient, last time I checked. $\endgroup$
    – 0xDBFB7
    Commented Sep 14, 2017 at 14:16
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It took a lot of searching but the model is Spectrolab K6700B Wrapthru https://solarmuseum.org/cells/k6700b-wrapthru/ I have additional photos on my website. Sadly I only have a partial cell but it goes to show how thin / fragile like you were guessing they are.

Part of the reason they used standard silicon solar cells at the time is they have the most research into making them safe for space since cosmic radiation will degrade most solar cells quite quickly without the proper protection and design.

I was able to find a document from NASA that contains a lot more info about the panels. Now I just need to find something like this for the Hubble Space Telescope panels. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050239010.pdf

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  • $\begingroup$ Welcome to Space! Nice answer! $\endgroup$
    – DrSheldon
    Commented Jul 17, 2019 at 2:37
  • $\begingroup$ Very interesting, thank you! In case you are interested, I have another ISS solar panel question that could use a more specific answer: How are the orientations of the ISS' eight independent solar arrays optimized? $\endgroup$
    – uhoh
    Commented Jul 17, 2019 at 13:03
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    $\begingroup$ Excellent find! Interesting fact: the collector grid on the front of that horribly abused cell suggests it's an older model of the K6700B. Newer models only had three rings around each contact instead of six. $\endgroup$
    – Tristan
    Commented Oct 8, 2019 at 22:15
  • $\begingroup$ I updated the post with both versions of the cell. Tristan, do you know what the number slots were for? $\endgroup$
    – karl_duluth
    Commented May 10, 2020 at 18:58
  • $\begingroup$ @karl_duluth I mentioned it up in my answer above, but the 2197 in the one picture you posted is the manufacturing lot number. The other spot would have a grade rating that runs from 1 to (I think) 15 and represents how much power the completed cell was measured to produce. $\endgroup$
    – Tristan
    Commented Jul 28, 2020 at 3:13

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