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I was wondering if we could use background radiation to produce electricity (even a tiny amount). However, I've read somewhere that solar panels absorbing infrared (and lower frequencies photons) wouldn't be very efficient, as they are limited by the laws of thermodynamics. The arguments states that the panel needs to be cooler than the temperature of the body emitting the photon.

How is this true? I cannot explain why it would be that way with the traditional band diagrams I've learned. As far as I understand it, the only limiting factor would be the band gap of the material that is used, and it needs to be low enough to capture low energy photons.

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    $\begingroup$ Well, if the panel is radiating more black body energy than it is absorbing, that is a problem. A related paper to read is IEEE Trans Electron Devices ED-31(5) 711 (1984). $\endgroup$
    – Jon Custer
    May 4 at 16:26

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I think I found an answer to my question: if the panel gets too hot, thermal vibrations (phonons) have enough energy to knock out electrons from lower to higher energy as well, and so the gap which is normally ensuring that the electrons knocked by photons flow only in one direction isn't effective anymore.

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