Solar cells have a band gap energy which corresponds to a specific frequency of light. Photons below that frequency don’t knock any electrons loose (they just m

Solar cells have a band gap energy which corresponds to a specific frequency of light. Photons below that frequency don’t knock any electrons loose (they just make heat) and photons above that frequency still only knock one electron loose (and the rest of the energy goes to heat). So a single band gap cell is limited to 20-something % max efficiency as a theoretical limit. The ability to re-emit heat at a specific frequency (that could be fed into a cell tuned for that frequency) would be really neat.