# Solar panel frequency range

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.

• 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. Commented Jan 1, 2017 at 21:22
• To produce electric energy from sound, we use microphones ;-) Commented Jan 1, 2017 at 21:22
• The photon has to be above the band gap. Much higher and most of the energy is wasted. Commented Jan 1, 2017 at 21:25
• When you 'produce energy', are you interested in electricity generation or is the aim of your question "to capture energy" (storage)? Commented Jun 16, 2022 at 6:59

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

In terms of the wavelength of its light, the solar spectrum peaks at about 500 nm (600 THz), and the distribution extends from 300–2500 nm (1.00–120 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 $$k_{B}T/q=\,\sim$$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: $$E=h\nu$$), 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.

Currently the best way to generate electricity from sound is a piezoelectric transducer. You can find these in some microphones. Your garden-variety piezo buzzer (such as the little black cylindrical "speakers" in desktop computers) is capable of generating current from sound, though it's generally used to produce sound from current.

I'm not certain, but I believe piezoelectric transducers are generally optimized for a particular frequency range (based on certain parameters such as the diameter and thickness of the element), so you'd want to find one that is tuned for the predominant sound frequency you expect your system to encounter. However, most of these generate microamps or maybe milliamps, with the voltage being dependent on the size of the transducer and the loudness of the sound -- most sounds will likely generate 3-5V at best (such as clapping your hands hard near it).

This is just an idea to get you pointed in the right direction, hopefully others will be able to contribute more; I welcome suggestions that could improve this answer.

• Yes, piezoelectric crystals can be designed to transduce different frequencies by changing their material & structure/shape. Commented Jun 16, 2022 at 14:37
• Note also that the power carried by "everyday" sound waves is pretty small compared to that carried by "everyday" light waves. If I've done my math right, the intensity of a 120-decibel sound wave (which is painful to human hearing) is about 1 watt per square meter. Sunlight, on the other hand, carries about 1000 watts per square meter. Commented Jun 16, 2022 at 15:13

The question seems to be if solar panels could generate electricity from frequencies other than visible light.

From tests conducted at Suffolk, "different wavelengths have different energy levels, wavelengths that are too short will pass right through a solar cell, but wavelengths that are too long will not have enough energy to ‘excite’ the electrons in the solar cell to produce energy", so best production was "between yellow and red" or "wavelength of anywhere between 600 nm to 700 nm".

This paper outlines different types of energy and some ways to convert them to electricity eg.

1. photovoltaic cells to convert visible light
2. Inductors/Antennae to convert radio waves
3. Air vibrations eg. sound can be converted with piezoelectrics or electromechanics

Solar cells can be designed to capture other frequencies of light eg. UV

Some handy terminology:
photoelectric effect - a phenomenon by which electrons are ejected from a conducting material when light shines on it
photovoltaic effect - takes place at the boundary of two semiconducting plates, not on a single conducting plate.

The frequencies and efficiencies of the incident rays that will create a photovoltaic effect is determined by the materials making the solar cell junctions and their bandgap or work function (threshold energy to knock an electron out of its orbit).

Radiation with longer wavelengths would lack the energy to produce electricity from a solar cell. Very short wavelength photons would either just pass through the cell or send electrons clear out of the conduction band or the material.

Maximum wavelength for a silicon based junction is about 1100nm.

Research is underway for efficiencies higher than 31% in solar cells with materials based on perovskite crystals & organic polymer (like pentacene), that can convert higher frequencies of light to electricity.

In another research, inorganic semiconductor (lead selenide or cadmium selenide) nanocrystals coated with anthracene converted infra red light to higher frequencies like yellow or even UV with organic compound rubrene or diphenylanthracene. Conducting polymers are also being trialled in infrared plastic solar cells.

Since lower frequency photons have lesser energy, they need materials with higher conductivity and less resistance to generate electricity, as in conducting polymers for infra red in the above paragraph. Fibreglass and copper conductors have been shown to convert microwaves to electricity, and going even lower, we all know metal inductors & antennae convert radio frequencies to electricity.

Radio waves are the lowest energy photon, so you wouldn't have EM waves at sound frequencies. Sound, however is a feature of material space (unlike a vacuum for EM waves) relying on vibrations of adjacent matter eg. in a solid or fluid, and have much lower energy. Like any other forms of energy, these can also be harvested, stored and converted to electricity eg. with microphones.

In a twist however, high-frequency acoustic waves (around 100 GHz to 10 THz) can be converted to light with piezoelectric thin films made of gallium nitride and aluminum nitride. These "T-rays" could be a safer alternative to X-rays that currently dominate medicine in subsurface tissue detection, or then fed to a solar cell to convert to electricity (since at these high frequencies, piezoelectrics would convert the sound waves to light instead of electricity).