Monday, September 27, 2010

Quantum efficiency of Solar Cells

As described above, when a photon is absorbed by a solar cell it can produce a pair of free charge carriers, i.e. an electron-hole pair. One of the carriers (the minority carrier) may then be able to reach the p-n junction and contribute to the current produced by the solar cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without
being collected; this process is then called recombination since one electron and one hole recombine and thereby annihilate the associated free charge. The carriers that recombine do not contribute to the generation of electrical current.
Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions. External quantum efficiency (EQE) is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency (IQE) is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance (R) and the transmittance (T) of the solar cell by IQE = EQE / (1 − RT). Please note that for a thick bulk Si solar cell T is approximately zero and is therefore in practical cases often neglected.
Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the fraction of power that is converted by the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield valuable information about the quality of the semiconductor bulk and surfaces.

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