Solar panels only produce energy from a small section of the total wavelength spectrum of light. I believe some panels have a mirrored back to pass light through the solar panel a second time. Much of the light can never be converted because it is at the wrong wavelength.

Does anyone know if attempts have been made to coat the top of the reflective back surface with materials that would shift the wavelength of light to to the region which can be converted by the solar panel? So that the second pass through has more bang in it?

Since I haven't seen anything like that on the market then perhaps its not possible or too expensive for any gain.

Please do share your thoughts.


  • $\begingroup$ Photons above the bandgap energy may be absorbed, with increasing path length helping. Upconverting photons that are below the bandgap energy is a difficult (and not very efficient) process. $\endgroup$
    – Jon Custer
    Aug 9, 2022 at 14:44
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    $\begingroup$ You may wish to look into "multi-junction" solar cells, which involve passing the light through multiple materials with different band gaps in order to convert a wider range of photon wavelengths into electrical energy. At present, their increased complexity leads to higher cost, but they can be useful in situations where weight-to-power ratio is a bigger concern than cost (e.g. aerospace). $\endgroup$ Aug 9, 2022 at 14:54

1 Answer 1


Shortest answer: TINSTAAFL

Short answer: That will only work to down-convert wavelengths too short to be absorbed, and even that requires expensive materials and processing. Up-conversion is even more expensive and typically uses more energy than you'd get back.

Longer Stuff:
there are optical detectors/solar cells with mirrored backside just so that photons in the absorption band which aren't captured first time pass thru the absorption region again. On a cost/benefit basis this tends to get you a lot more electrical power than any up or down converting mechanisms. You need to analyze the spectral photon density incident on the solar panel and compare that with spectral quantum efficiency curves as well as putative wavelength converters to see where the "sweet point" would be.
Generally, at least at present, it's cheaper and easier to use a "solar collector" to focus a large area of sunlight on something you just heat up (water, e.g.) and use to drive a generator, than to play photon conversion games.

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    $\begingroup$ Another alternative that will work: multi-junction solar cells, with 2 or more layers tuned to different photon energies, $\endgroup$ Aug 9, 2022 at 15:22

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