Photovoltaic cells operate by converting light energy, photons, into electrical charge, electrons. The efficieny of photovoltaic cells is defined to be the amount of work done per photon. Solar cells are able to achieve a maximum efficiency of 33%. In other words, a best case scenario cell would absorb 33% of the photons that the cell receives. The 33% limiting efficiency follows from various assumptions:
- one electron hole pair is generated by each absorbed photon
- electrons and holes relax to form populations in thermal equilibrium with the lattice
- all the light is being absorbed by a junction with a single band gap
High efficiency cells generally have band gaps close to the photon's energy. Failure to obtain the energy of photons with energy less than that of the band gap accounts for a large loss of the sun's energy. Carriers generated with photon energy much greater than band gap energy results in thermal dissipation of kinetic energy; this loss of kinetic energy accounts for another large loss of the sun's energy.
This section will focus on the various theoretical and practical methods of increasing efficiency