eli5: Do solar panels only work with sunlight, or can they be tuned to capture energy from other wavelengths outside the visible spectrum?
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So first things first: sunlight includes light outside the visible spectrum. The sun is a[ blackbody radiator](https://en.wikipedia.org/wiki/Black-body_radiation). What we call sunlight is a a whole spectrum of wavelengths all the way from radio waves to gamma rays. The amount of light emitted in each part of the spectrum is determined by [Planck’s Law](https://en.wikipedia.org/wiki/Planck%27s_law), which is a function of the temperature of the sun. The concept of visible light is only relevant to us as humans, because it is simply the range of wavelengths we are capable of perceiving with our eyes. It is no coincidence that the sun shines most intensely in the visible spectrum; humans evolved to see the wavelengths most prevalent in their environment.
As for solar panels, the answer is yes, and to a degree they already are. The way a solar panel works is that a junction of two semiconductor crystals is created. The material properties of the crystals are different from each other, and this creates an interesting phenomenon in how they react to light. An incident photon can collide with an electron in one of the crystals, and bump it into the other. This turns one crystal slightly positively charged (due to missing an electron) and the other slightly negatively charged (due to an extra electron). Where you have two different charges, electricity will flow, and voila, you have a solar panel.
The two crystals have what’s called a “bandgap,” which can be thought of as the energy difference between them. An electron has to be boosted by at least this much energy in order to make the jump, and will release this much energy when it flows back as electricity. In practice this means that an electron will only jump if it is hit by a photon of at least this much energy. This creates an interesting optimization problem:
* Set the bandgap low, and more of your photons will induce a jump, but you get less energy per-jump, even from high-energy photons.
* Set the bandgap high, and you get more energy per-jump, but fewer of the photons will induce a jump.
So in one case you are losing energy by failing to capture low-energy photons, and in the other you are losing energy by only capturing a small percent of the energy in the high-energy photons (regardless of photon energy, you only capture the value of the bandgap per photon). There is a sweet spot where the amount of energy you can collect is maximized, and that’s the bandgap that we target for solar panels. These panels are already capturing photons above the visual spectrum, they’re just not wringing as much energy out of them as they could, because they have their bandgap set to ensure they capture the bulk of light in the visible spectrum as well. There are such things as “multijunction” solar panels which have multiple bandgaps stacked one on top of the other, and the high-energy photons are captured by the high-bandgap junctions, and so on.
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