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I talked to a professor about solar panels and their efficiency. It seemed that the main reason solar panels aren't that efficient is because it can only accept a single energy input size. Anything below is ignored and anything above only gives the energy of that single limit.

That is, say the limit is 1W. A photon with 0.999W energy wouldn't give any voltage. A photon with 10W energy would give the same voltage as a 1W photon.

Does this mean I can get very high efficiency from a light source with a specific frequency? For example a solar-powered calculator in a room with only sodium lights. How high?

How does the efficiency change depending on the amount of light or the temperature of the solar cell? How about an extremely small amount of light, like 1nW, close to 0K?

This could be terribly wrong as there was quite a language barrier between us. I want to learn, so please point out any misstakes.

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In theory, you could get almost $100\%$ efficiency from a solar cell exposed to light with the photon energy just above the band gap. Each absorbed photon generates an electron of almost the same energy. The problem is that there are only so many band gaps available, so you have to find a light source at the correct wavelength to match one. The intensity of the light doesn't change the efficiency, just how much power you can get out of the cell. Over a wide range, if the intensity doubles, you can generate twice as much power.

As an aside, the energy of a photon is measured in energy units: eV, ergs, joules. Watts are power, or energy/time.

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  • $\begingroup$ And what about a really small energy input? Aren't solar cells insulators when they don't recieve enough light? $\endgroup$ – Filip Haglund Mar 2 '14 at 15:11
  • $\begingroup$ If the intensity gets small enough, there are probably losses that stop the cell from producing power, but I don't know. $\endgroup$ – Ross Millikan Mar 2 '14 at 15:36

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