Naked Science Forum
Non Life Sciences => Technology => Topic started by: thedoc on 17/07/2015 08:50:01
-
Robert Browell asked the Naked Scientists:
Does the Solar PN junction gain electric power from the red end of the spectrum?
What do you think?
-
That depends on the design of the junction. In theory, a PV device could be engineered with any band gap. Any photon with at least enough energy to promote an electron from below to above the band gap will result in 1 bandgap*electron worth of energy (for instance a device with a 1.2 V band gap will harness 1.2 eV of energy from any photon with at least 1.2 eV, and the rest of the photon's energy goes to heat).
This presents a somewhat tricky optimization problem (https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit )--materials with a small band gap will harness a large percentage of photons, but only get a small amount of energy out of each of them, and a material with a large band gap will get more energy from each photon, but from a smaller percentage of the incoming photons. The actual optimization depends on the spectrum of incoming light and other sources of inefficiency in the device. Other methods involve using multiple materials with different band gaps such that multiple parts of the spectrum can be harnessed at more ideal efficiencies (a multijunction photovoltai cell https://en.wikipedia.org/wiki/Multijunction_photovoltaic_cell )