What is a solar panel's frequency range (i.e. from THz to THz)? Is there a way to capture energy that exceeds that frequency range, either more towards IR or UV? If so, you could produce energy from sound, considering its frequency is 20-20,000Hz.

  • $\begingroup$ I don't think that would work with sound even if the panel worked on those frequencies, since light is electromagnetic waves while sound are mechanical waves. $\endgroup$ – Frotaur Jan 1 '17 at 21:22
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    $\begingroup$ To produce electric energy from sound, we use microphones ;-) $\endgroup$ – hdhondt Jan 1 '17 at 21:22
  • $\begingroup$ The photon has to be above the band gap. Much higher and most of the energy is wasted. $\endgroup$ – Jon Custer Jan 1 '17 at 21:25

First off, sound is a mechanical form of energy (energy carried by molecules oscillating in air), whereas light is electromagnetic in nature (energy carried by photons of light).

In wavelength of light, the solar spectrum peaks at about 500 nm (600 THz), and the distribution extends from 300–2500 nm (1,000–120 THz). There is very little solar radiation outside that range. (The solar spectrum can be approximated by a black body at 6000 K.)

A solar cell produces power by electrons absorbing photons from light at a particular frequency to a higher energy state, as described by the photovoltaic effect. Only semiconductors can accomplish this, because there is a range of energies (determined by its band gap) that electrons are quantum mechanically not allowed to have, which significantly slows them from spontaneously "falling back down" in energy.

If a band gap becomes small enough (on the order of kbT/q, = ~6 THz), and at room temperature, electrons would constantly be able to jump into the higher-energy state by random thermal fluctuations alone. At that point, effectively corralling all those electrons becomes difficult; to generate current you need some mechanism to separate electrons and holes, which would be probably difficult to do in such a system.

Also, lower frequency means less energy (they're directly proportional), so practically speaking, energy in the tens to thousands of Hz range (i.e., radio waves) carries far less energy compared to light and UV/IR. It is possible to generate energy from radio waves (I believe by using electron "filters" in antenna-like devices), but again the amount of energy is much less. For such a device to be effective, the intensity of such radiation would need to be very high.


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