Editing Quantum efficiency (section)

==QE of solar cells==
[[Image:Solarcellige-en.svg|thumb|right|400px|A graph showing variation of internal quantum efficiency, external quantum efficiency, and reflectance with wavelength of a crystalline silicon solar cell.]]

A [[solar cell]]'s [[Quantum efficiency of a solar cell|quantum efficiency]] value indicates the amount of current that the cell will produce when irradiated by photons of a particular wavelength. If the cell's quantum efficiency is [[integral|integrated]] over the whole [[sunlight|solar electromagnetic spectrum]], one can evaluate the amount of current that the cell will produce when exposed to sunlight. The ratio between this energy-production value and the highest possible energy-production value for the cell (i.e., if the QE were 100% over the whole spectrum) gives the cell's overall [[solar cell efficiency|energy conversion efficiency]] value. Note that in the event of [[multiple exciton generation]] (MEG), quantum efficiencies of greater than 100% may be achieved since the incident photons have more than twice the [[band gap]] energy and can create two or more electron-hole pairs per incident photon.

===Types===
Two types of quantum efficiency of a solar cell are often considered:

*'''External quantum efficiency (EQE)''' is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy ''shining on the solar cell from outside'' (incident photons).
*'''Internal quantum efficiency (IQE)''' is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that shine on the solar cell from outside ''and'' are absorbed by the cell.

The IQE is always larger than the EQE in the visible spectrum. A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons, most likely due to poor carrier collection efficiency. To measure the IQE, one first measures the EQE of the solar device, then measures its transmission and reflection, and combines these data to infer the IQE.
<math display="block"> \text{EQE} = \frac{\text{electrons/sec}}{\text{photons/sec}}= \frac{\text{(current)}/\text{(charge of one electron)}}{(\text{total power of photons})/(\text{energy of one photon})}</math>
<math display="block"> \text{IQE} = 
\frac{\text{electrons/sec}}{\text{absorbed photons/sec}}= 
\frac{\text{EQE}}{\text{1-Reflection-Transmission}}
</math>

The external quantum efficiency therefore depends on both the [[Absorption (electromagnetic radiation)|absorption]] of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A "good" material avoids charge recombination. Charge recombination causes a drop in the external quantum efficiency.

The ideal quantum efficiency graph has a [[Boxcar function|square shape]], where the QE value is fairly constant across the entire spectrum of wavelengths measured. However, the QE for most solar cells is reduced because of the effects of recombination, where charge carriers are not able to move into an external circuit. The same mechanisms that affect the collection probability also affect the QE. For example, modifying the front surface can affect carriers generated near the surface. Highly doped front surface layers can also cause 'free carrier absorption' which reduces QE in the longer wavelengths.<ref>{{Cite journal| last1=Baker-Finch|first1=Simeon C.|last2=McIntosh|first2=Keith R.|last3=Yan|first3=Di|last4=Fong|first4=Kean Chern|last5=Kho|first5=Teng C.|date=2014-08-13| title=Near-infrared free carrier absorption in heavily doped silicon|url=https://aip.scitation.org/doi/10.1063/1.4893176|journal=Journal of Applied Physics| volume=116|issue=6|pages=063106|doi=10.1063/1.4893176|bibcode=2014JAP...116f3106B |hdl=1885/16116|issn=0021-8979|hdl-access=free}}</ref> And because high-energy (blue) light is absorbed very close to the surface, considerable recombination at the front surface will affect the "blue" portion of the QE. Similarly, lower energy (green) light is absorbed in the bulk of a solar cell, and a low diffusion length will affect the collection probability from the solar cell bulk, reducing the QE in the green portion of the spectrum. Generally, solar cells on the market today do not produce much electricity from [[ultraviolet]] and [[infrared]] light (<400&nbsp;nm and >1100&nbsp;nm wavelengths, respectively); these wavelengths of light are either filtered out or are absorbed by the cell, thus heating the cell. That heat is wasted energy, and could damage the cell.<ref>[http://www.autobloggreen.com/2007/08/21/silicon-nanoparticle-film-can-increase-solar-cell-performance/ Silicon nanoparticle film can increase solar cell performance]</ref>