The heart of a PV cell is the interface between two different types of semiconductor (called p-type and n-type). When a light photon with sufficient energy hits an atom in this region, it throws out an electron. The electron, now free to move, travels through the n-type semiconductor to metal contacts on the surface. The hole left by the absence of the electron travels in the opposite direction, through the p-type semiconductor. Once at the metal contact, the electron flows around an electrical circuit, doing work in the process, to meet up with a hole at the rear contact.
The energy required to move an electron from the semiconductor atom to a conducting state is a discrete amount. The energy of a photon of light is determined by its wavelength, with shorter wavelength photons having higher energy than those with longer wavelengths.
A photon with wavelength 1,100 nanometres (nm), corresponding to short wave infra-red light has just enough energy to promote an electron in a silicon atom, the most commonly used semiconductor material.
All photons with a longer wavelength than this have insufficient energy to promote the electron and either pass straight through the PV cell or are absorbed as heat. This part of the solar spectrum cannot be used by a silicon PV cell.
Photons with a shorter wavelength than 1,100nm have more energy than is required to promote the electron. The excess energy above that needed to move it into a conducting state is lost as heat.
These two factors combine to produce a theoretical upper limit to PV efficiency from a single junction of p-n semiconductor of around 31% - called the Shockley–Queisser limit.
One way to circumvent this limit is to produce a PV material made up of multiple layers, each layer tuned to a different wavelength. Efficiencies up to 44% have been achieved in the laboratory this way.
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