29th August 2006 - 08:21 PM
1. What wavelengths (or range of wavelengths) of energy are absorbed by photovoltaic cells?
2. What about other wavelengths? I understand that low energy photons just pass through the cells. However, I've read conflicting info on higher energy photons - do they just force their way through without effect? Or, do they free one or more electrons on their way through, with the remainder of their energy unused?
I'm probably misusing some of the terminology, but hopefully my questions are still clear...
29th August 2006 - 10:09 PM
Your talking about diodes. Where photovoltaic diodes absorb a wavelength range from red to violet. You can have diodes that can absorb a wider or more specific range than that. Radio frequency are part of the EM band just as visible light is (its all photons of different eV). There are diodes that detect infrared only and they are your TV receiver that receives the signals from your remote. The remote has a light emitting diode that sends out infrared light. There are some wavelengths that can go uncapturable but I believe solutions are being worked on.
29th August 2006 - 10:31 PM
If the diode absorbs just wavelengths in the visible spectrum (680nm to 410nm), what then happens to something like ultraviolet light (and higher energy photons) when it strikes the diode? Does it just pass through?
31st August 2006 - 05:03 AM
All photons get absorbed when they collide with an atom and diodes are no different. From what I can understand a diode functions by absorbing photons and generating electromotive force by the increase in energy level. Its design is what determines this. Where in the case of materials with specific colors (lets say yellow) the atom will absorb photons of all types causing the atom increase in energy level where the atom will drop back down in energy level by releasing photons. In this particular case the photons released by the atom are yellow photons. So lets say a photon whose energy level is twice that of a yellow photon then when it gets absorbed by the atom the atom will first increase in energy level then decrease back down by releasing two yellow photons. In the case of diodes and their design when the metals are bombarded with photons certain photons will create a jump in energy level for the atoms to just the right amount that it generates electromotive force.
In the case of photovoltaic cells I've encountered some where if you place your hand over it than the device being run by it will lose power. For others you couldn't keep it from producing power no matter what you put over the solar cells. I believe this is the difference between the ranges of the diodes. Where in the first one the diodes ran off energy levels closer to the visible spectrum and so placing something over it blocks the light. The other will accept more than just the visible light spectrum where some electromagnetic radiation can pass through objects and so the device won't die out no matter what you put over the panel.
31st August 2006 - 03:49 PM
I have wondered this myself for a long time.
I was playing with my brothers light meter in 1967 and
I had just read a story by Robert Heinlein called "Let There Be Light" from 1940.
Later I read of a technology that has layers that are tuned to different wavelengths. Trying for more efficiency per surface area I guess. Exxon bought the patent on this one.
Broader spectrum absorption is a solar cell goal.
Making it more that just an expensive lab experiment is the hard part.
I often hear of proposed orbital collectors that will "beam" power to Earth via microwaves.
But I haven't checked on how this is actually converted to electricity.
On the solar cells that seem to work when obscured, you may find that there is a battery or capacitor inside somewhere.
31st August 2006 - 05:48 PM
That's interesting about the yellow light example. I was always taught that a particular color was "reflected" and the rest absorbed. But what you're saying makes sense - the color yellow is produced by the fact that the photon released during the decrease of energy is in the yellow wavelength. Thanks!
Regarding the photovoltaic cells, I think between your answers and a couple of other articles I've read, I have my answer. It sounds like higher energy photons do, in fact, knock the electrons free, but any additional energy from the photon cannot be capitalized on. Of course, with newer multi-gap solar panels, this has been improved upon to some extent, because the higher energy photons can be used to create higher voltage output, while the "normal" energy photons are still used by the lower band gap materials.
Would you happen to know what the percentages of different energies are that are produced by the sun? (i.e. Visible Light is 20%, Radio through Infrared 70%, Ultraviolet through Gamma Ray 10%) I know it's an odd question, but I was wondering and couldn't find the information anywhere.
31st August 2006 - 06:43 PM
Hmm. That one I don't know sorry.