Physics geeking

[danaeris]

Will widening or thinning the energy gap in a semiconductor make it a more or less efficient solar cell? That is, is part of making solar cells more efficient the quest for a semiconductor with a larger or smaller energy gap?

Or, how could one make a solar cell more efficient? So far the only method I've heard is adding a fresnel lens to capture a larger amount of sunlight.

Comments

[polkamadness.livejournal.com]

I'm far from an expert, but I believe the size of the energy gap determines which frequencies of light are converted into electricity.

Try a Google search on solar cells and nanotechnology; I remember an article in science News about how nanotechnology might result in more efficient solar cells.

- Mark

[auros]

Nanotech used to print solar cells.

This was brought up in comments on my most recent entry on alt-energy.

[jadia.livejournal.com]

So, people can correct me if I'm wrong:

If the energy of the photons coming from the sun is smaller than the band gap of the material, it will be unable to move carriers from the valence band to the conduction band (which causes a current to run). If it is larger, it will be an inefficient process. The main problem right now is that we have no materials of the correct band gap.

I googled for semiconductor band gaps and got a page that suggested that they ranged from < 1 to 3 eV. A photon has hv of energy. h = 6.6262e-34 J s. 1 eV = 1.602e-19 J. So the bandgap is say, 1.6e-19 J. Ok. That means that the right frequency for that is 1.6e-19/(6.6e-34) = 2.42e14. Higher frequencies will work but inefficently; lower frequencies just won't work at all. What's the frequencies in sunlight? 500 nm wavelength (I think that's green?) is 6e14. Sunlight is also UV which is even higher frequencies than that.

I think there was a recent result in Nature (last year?) where some group made this material that was a lot better for sunlight than anything that had previously existed. Unfortunately, I don't remember more than that.

(Also, it's not just a question of finding the right material with the right bandgap; that material has to interface well with what we mostly use right now, which I think is silicon. And be easy to integrate and fabricate.)

[auros]

Broad-Spectrum Solar Cells.

Incidentally, these are mostly made from gallium arsenide (mixed with varying amounts of indium nitride), which is being explored as an alternative to silicon for several purposes

This particular article does appear to address the "band gap" question. I don't pretend to completely understand it, though. I'm mostly interested in the practical / applied side, and how that interfaces with gov't policy. *g*

[qedrakmar.livejournal.com]

Well, look at it this way...

Start with realizing that the solar cell is basically an LED used backwards, the same way that a microphone is a speaker used backwards. I mean, this is the photoelectric effect, but with a conduction band in the way, so you're replacing your work function with the band gap energy.

If you decrease the band gap, your necessary energy per electron is decreased, meaning waves with lower energy transmission can stimulate current, but does that mean that the higher energy waves can't? We know that there can't be optical transmission in your range, if you want to get the energy from it, so, what is that range? What it comes down to would be tuning the gap to the highest wave density, based on the wave energy, which would be based on the spectrum of what you're throwing at it. If you're working with solar cells, you might want to react to a 200nm wave, assuming you're getting a lot of UV, or you might be trying to pick something mid-spectrum, around 500nm, hoping that no matter what, you'll have something in the power curve's range. (I should say, I don't remember what the corrolation between wave transmission based on frequency and the band gap actually is)

So, to sum up quick, larger or smaller is not relevant, compared to knowing the spectral breakdown of your energy source. I know that didn't really answer your question, but I was on a roll...