science fair solar cell

[deleted-770381]

Hi. I am working on a science fair project examining the effect of different color light filters on the output of a solar cell. My research tells me that the wavelength of light hitting the cell shouldn't matter as long as it is below around 1,100 nm, the band-gap for silicon, but I am finding that it does seem to matter. The filters that block red light are causing the greatest reduction in current, and the ones blocking blue light are causing the least. Some of this may have to do with differences in the total amount of light transmitted by the various filters, but I'm thinking it also may be because there are more red photons in sunlight and that it is the number of photons being blocked that maters, not their wavelength. When I look at a graph of the sunlight radiation spectrum, I see that red light accounts for about as much solar energy as blue light, even though one red photon has about half the energy of a blue photon. So am I correct to assume that there are about twice as many red photons in sunlight as blue photons? If I divide irradiance (W/m2/nm) by photon energy (eV), will that give me a valid way to compare how many photons are present at different wavelengths in the sunlight spectrum? Would that be photons per square meter or something else? Thank you!

[LeungWilley]

Hi alexandranb,
You made some excellent observation! Yes, it make sense that the number of photon passing through the filter would have an impact on the output of the solar cell. I think the statement that said "wavelength of light hitting the cell shouldn't matter as long as it is below around 1,100 nm" is trying to say that the silicon will still generate electricity as long as the wavelength is below 1,100 nm. Stated another way, when light shines on a solar cell, the light (photon) may be reflected, absorbed, or pass through the cell. Since only the absorbed portion of light is converted into electrical energy, the number of photon that's not blocked by the filter will determine how much possible energy is available to be converted.

Now onto your second point, wavelength tells you how much energy a particular photon have (Blue light = 2.76 eV, Red Light = 1.8eV) so it does matter what you are blocking. (Also, red light have about 65% of blue light's energy)

Onto your next point, please take a look at this link https://www.advancedaquarist.com/2013/12/lighting for the composition of sunlight in figure 3. Depending how / what wavelength your filter is blocking out, you should have a good estimate of the remaining energy that's available for the solar cell to convert.

I hope that helps. Please post again if you have any other questions.
Good Luck with your experiment!
Willey