Observations about the universe, life, Lausanne and me

Thursday, April 2, 2009

Solar cells hate the sun

I've been following a course in "Solar photovoltaics and energy systems" here at the EPFL - which provides a very interesting overview about all the different solar technologies out there, from ye olde silicon monocrystalline cell to the fancy new quantum dot or dye-sensitised solar cells.

Today our prof told us (with a completely straight face):

"People always worry about what happens when you put a solar cell out into the sun."

And they do. Most types of solar cells work perfectly fine in the dark, i.e. they produce no current at all, exactly what you would predict, but put them into the sun and they'll be up to all kinds of shenanigans. They'll degrade, they'll heat up and be much less efficient than predicted, etc. etc. Really, if you want your solar cells to last, keep them in a cool, dark place!

 The problem is of course that the semiconductor of a classical cell will heat up, which will change the characteristics and kill efficiency. UV will do all sorts of evil to your cell too.

Dye-sensitised solar cells (invented by Prof. Grätzel here at the EPFL!) don't suffer from heat-inefficiencies, and offer the additional advantage of working in low-light conditions as well (Both feats are due to the fundamentally different approach and quantum physics this is actually kinda cool if you are into this kind of stuff). Unfortunately, they degrade under continuous operation, and since they utilise a liquid electrolyte, they suffer a more destructive reaction to temperature extremes: The electrolyte will either freeze (and crack the cell) or expand (and crack the cell). Still, they might get that under control, or maybe we can switch to a solid electrolyte, as reported by Prof. Grätzel in Nature Materials last year. Although then you loose the advantage of flexible cells...


  1. Despite the limitations, solar cells have been the unsung workhorse of space endeavors. In space, they're exposed to more radiation, more light, more thermal extremes, and atomic oxygen, yet we have successful utilized them on nearly every spacecraft we've ever built ( a few crew transport modules being the most notable exceptions).

    Of course, space solar arrays get considerably more direct light and most are less efficient than what's been developed the past decade or so, but I just thought we should give credit where credit was due.

    Dyes that are subject to freezing (and subsequent cracking of cells) are probably not going to be space-friendly.

  2. Ah, but you space guys get to use the good stuff - money is no object in space, more important to keep weight down.

    So you can get away with using the hideously expensive and toxic gallium arsenide multijunction cells, which are up to 40% efficiency in the lab, something that is not an option on earth...

    I am not really knocking solar cells - they will be the future of energy production one way or another.

  3. Money is no object! Ha, that's a good one!

    However, for space, buying something that won't work is false economy. It's so expensive to put things in orbit, it's better to spend more for something that can go the distance than less for something that won't last the mission.

    I'm continually amazed at the spacecraft we've made that have doubled, tripled, increased tenfold their expected useful lives. That, my friend, is good engineering.

  4. Well, that is more or less what I meant when I wrote "Money is no object": Since it is still so beastly expensive to put stuff into orbit (I have this figure of 50 k$/kg in my head, but no idea if it still holds), it makes sense to use the best and most compact technology you have (and the most reliable of course, but once you are sure your solar cells will stand up to the radiation and heat, you have that pretty much covered), which will save weight for the cool stuff, i.e. scientific instruments.

    And you are damn right about the amazing engineering, I am flabbergastet that the Mars Rovers are still rolling! They have surpassed their original primary mission time by what, 20 times now? And are you not still in contact with Voyager 1 & 2?