4 ms·

So is two the most possible? Is it possible to activate three electrons with a single photon?

sp332 7y ago
Yeah but you'd have to start with very energetic photons. The article says that you mint get two elections when a green or blue photon hits, so to get three even theoretically you'd need to use ultraviolet optics photons. And there just aren't enough of those in sunlight you make a noticeable difference in efficiency.
- perlgeek 7y ago
  Let me try to do the math here, please correct me if I'm wrong. The band gap of Si in solar cells is about 1.1 ev. To excite three electrons, a photon thus needs 3.3ev, or about 370nm. The visible spectrum starts around 380nm or 390nm, depending on which source you believe. So the triple photon frequency would be very near UV. According to this spectrum [0], there's a sharp drop in solar power around the 370nm. It seems that with the 1.1ev band gap, exciting three electrons doesn't seem to be worth it. But, if you could reduce it just a bit (by doping, most likely), it might be worth considering. Of course, reducing the band gap would likely lead to much worse performance overall, so one would need a multi layer approach, which sounds pretty complicated. And then you need to pray that your glass cover doesn't absorb the UV light, which seems to be a pretty close call[1]. [0]: https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe... [1]: https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media/File:Soda-lime_glass,_typical_transmission_spectrum_(2_mm_thickness).svg https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media...
  - pixl97 7y ago
    All that said, in space based operations your UV isnt going to be filtered and may allow you to generate more power.
aurizon 7y ago
It may be possible, if an intermediate storage mechanism can be found to add the energy 2 or more which is then tapped to initiate extra electron(s). I understand that some natural process do this and with research a useable mechanism may be found?
whatshisface 7y ago
You can activate as many electrons as you want, in fact photomultipliers can detect single photons by setting them up to cause cascades of countless electrons. Granted, number of electrons isn't what matters.
- perlgeek 7y ago
  I think that's a bit beside the point. A photomultiplier has a pre-charged state where a photon triggers an avalanche -- not really relevant for photovoltaic applications. The question was more if you can lift three electrons trough the band gap, triggered by a single photon, which would increase the solar cell's output current without dropping the voltage.
  - selimthegrim 7y ago
    A singlet splits into two opposite spin triplets. You need a higher spin exciton to split to get more than two.
selimthegrim 7y ago
I don't think that three is possible, and I told Baldo as much back in 2013 at APS March Meeting (hence my downvoted comment below). He didn't really care to listen to my explanation why because he'd already gotten a paper out of the experiment.

Yeah but you'd have to start with very energetic photons. The article says that you mint get two elections when a green or blue photon hits, so to get three even theoretically you'd need to use ultraviolet optics photons. And there just aren't enough of those in sunlight you make a noticeable difference in efficiency.

perlgeek 7y ago
Let me try to do the math here, please correct me if I'm wrong. The band gap of Si in solar cells is about 1.1 ev. To excite three electrons, a photon thus needs 3.3ev, or about 370nm. The visible spectrum starts around 380nm or 390nm, depending on which source you believe. So the triple photon frequency would be very near UV. According to this spectrum [0], there's a sharp drop in solar power around the 370nm. It seems that with the 1.1ev band gap, exciting three electrons doesn't seem to be worth it. But, if you could reduce it just a bit (by doping, most likely), it might be worth considering. Of course, reducing the band gap would likely lead to much worse performance overall, so one would need a multi layer approach, which sounds pretty complicated. And then you need to pray that your glass cover doesn't absorb the UV light, which seems to be a pretty close call[1]. [0]: https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe... [1]: https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media/File:Soda-lime_glass,_typical_transmission_spectrum_(2_mm_thickness).svg https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media...
- pixl97 7y ago
  All that said, in space based operations your UV isnt going to be filtered and may allow you to generate more power.

aurizon 7y ago

It may be possible, if an intermediate storage mechanism can be found to add the energy 2 or more which is then tapped to initiate extra electron(s). I understand that some natural process do this and with research a useable mechanism may be found?

whatshisface 7y ago

You can activate as many electrons as you want, in fact photomultipliers can detect single photons by setting them up to cause cascades of countless electrons. Granted, number of electrons isn't what matters.

perlgeek 7y ago
I think that's a bit beside the point. A photomultiplier has a pre-charged state where a photon triggers an avalanche -- not really relevant for photovoltaic applications. The question was more if you can lift three electrons trough the band gap, triggered by a single photon, which would increase the solar cell's output current without dropping the voltage.
- selimthegrim 7y ago
  A singlet splits into two opposite spin triplets. You need a higher spin exciton to split to get more than two.

selimthegrim 7y ago

I don't think that three is possible, and I told Baldo as much back in 2013 at APS March Meeting (hence my downvoted comment below). He didn't really care to listen to my explanation why because he'd already gotten a paper out of the experiment.