In my simulation research work for nano structure based solar cells, for a particular wavelength range I have been getting quantum efficiency more than 100%, but till date not able to find out the reason behind the cause, that's why I need to know whether technically it is possible or not?
A photon (with energy h) can move an electron from valance band to conduction band (the energy difference is equal to optical band gap Eg). If the energy of incident photon is larger than the band gap then excess energy has been converted into heat. It is the main cause of low efficiency of solar cell. If the energy of incident photon is equal to the band gap and if the carrier is able to move from valance band to conduction band by this radiation then efficiency will be 100 percent, if the electric field of depletion layer is been able to flow those carrier to the outer circuit. Above 100 percent efficiency is impossible.
Hi,
You need to clarify your question a bit: internal or external QE?
In standard solar cells (1st and 2nd gen.) the above answers are correct, and the maximum "electrons-out" you'll get per photon absorbed is one, therefore max QE is 100%. However in 3rd generation SC, the excess photon energy (above band gap) is used to excite an electron to e.g. mid-gap state. If you could get those electrons out, you'd get >100% IQE.
Hope this helps
Yes it is possible, it is called multiple exciton generation in quantum dots solar cells and the cell itself is made by NREL. maybe a quick search you can find it. Off course this is new concept in photovoltatics. The simplified explanation is it happens when a photon with energy more than twice of the band gap absorbed by a semiconductor and result in generation of two electron-hole pair. So in this case, the generated current is twice the photon absorbed.
Dear Mr. Soheyl Mortazavi,
Thank you for your well describe. However, you have said " The simplified explanation is it happens when a photon with energy more than twice of the band gap absorbed by a semiconductor and result in generation of two electron-hole pair. So in this case, the generated current is twice the photon absorbed". This explanation does not support more than 100 percent efficiency of a solar cell is impossible. Rather than this explanation reveal that more than 100 percent efficiency is impossible because you have said " when a photon with energy more than twice of the band gap".
Dear Dr. Md. Muhibbullah
The question here is "Quantum efficiency" which is "the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell." off course for efficiency of solar cell, maximum theoretical efficiency is around 86.5% if I am not mistaken and maximum efficiency achieved is around 56% according NREL chart.
In short: *Quantum* efficiency can be higher than 100%, but *power* efficiency must remain lower than what predicted by Carnot's principle. In other words: One photon can generate more than one electrons, but the total energy of these electons will be lower than that of the incident photon.
In any application
efficiency = A/B
Some times A can be greater than B
However, output can not be more than input
The solar cell efficiency (output electrical power divided by incoming solar irradiance) can not be larger than 100 %, whereas the quantum yield (number of output photo-electrons divided by number of incoming photons) may be above 100 %. For example, see the investigations carried out by Semonin et al. (Semonin, O.E.; Luther, J.M.; Choi, S.; Chen, H.; Gao, J.; Nozik, A.J.; Beard, M.C. Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science 2011, 334, 1530-1533), where they have reported photocurrent quantum efficiencies exceeding 100 % in a quantum dot solar cell, being enabled by multiple exciton generation (MEG). The MEG process may occur in semiconductor nanocrystals or quantum dots where absorption of a photon with at least twice the bandgap energy creates two or more electron-hole pairs.
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