In several papers I found that the optimized band gap for solar cells is close to 1.5 eV. This value corresponds to a wavelength of about 830 nm, in infrared. Is it due to the fact that we use more silicon or silicon-like devices ?
In the following picture the relations between band gap energy (x-axis), open circuit voltage Voc (upper y-axis on the right), short circuit current Isc (uper y-axis on the left) and efficiency (lower y-axis) are shown. It is a qualitativ picture I've made for my thesis from a AM1.5D spectrum. This picture is similar to the calculations done by Shockley and Queisser. Efficiency is defined as Voc*Isc*FF/ Input power. FF is slightly increasing with band gap energy.
how can we generate an electron (electricity) from an incoming photon? This happens when a photon goes into the cell and bounces into a valence electron that was part of the silicon molecular structure. Since valence electrons are in the outer orbit of the electron cloud, only a little bit of energy is needed to make it loose and allow it to travel freely. By leaving the organized structure of the silicon, the electron leaves a 'hole' behind. This is called the generation of an electron-hole pair. Once the electron is free, the electric field pulls it towards the P-side and the hole is pulled towards the N-side. All the free electrons forced to one side of the cell form the current. And the current combined with the voltage of the electric field gives Power.
Not all the photons can create an electron-hole pair. Only the photons that have enough energy to knock an electron out of its place can do that.Photons carry a tiny amount of energy (in electronvolts or: eV). The amount of energy that is required the knock an electron off a Silicon atom (Si) is 1.1eV. This is called the 'Bandgap'. Every type of solar cell has its own bandgap.
Bandgaps of Solar cells
Only photons with an energy higher than the bandgap energy, can knock off electrons and generate electricity. However, if a photon has 1.7 eV and falls onto a 1.1 eV cell, the excess energy (0.6 eV) will be lost in the form of heat. So there's a trade-off there: if you set the bandgap too high, you don't generate a lot of electrons (current) because few photons have so much energy. However, a bandgap too low will generate a lot of electrons, but most of the energy is lost in the form of heat.
Thank you for your answer, nevertheless I can say that the solar spectrum is optimum at a wavelength of 530 nm, according to AM1.5. This corresponds to a band gap of 2.33 eV and a yellow color, I think. It seems to be far from 1.5 eV. Could you explain me more ?
At low band gap open circuit voltage is low and at higher band gap short circuit current is low. Theoretical calculation shows that the efficiency for a single band gap semiconductor, is maximum 33% at a band gap 1.4 eV for AM1.5 solar spectrum. Hence it is mentioned that band gap for solar cells should be around 1.5 eV.
I think you also want to also check the water absorption lines in the atmosphere. There are some big dips in the solar spectrum that reaches earth.
@Joerg, you are absolutely right. I was thinking of single junction devices where the junction peak needs to be roughly in the middle of two water absoprtion lines. It is the case of InGaAs junctions in GaAs.
@Macho Anani: The efficiency limit for a solar cell with only one band gap material is called the Shockley-Queisser-limit. Shockley and Queisser have shown in a detailed balance calculation in 1961 that the efficiency for a blackbody spectrum is limited to about 30% for a band gap around 1.1 eV [1]. Latest detailed balance calculations e.g. [2] for the real sun spectrum show the limits as mentioned above.
1. Shockley, W. & Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 32, 510 (1961).
2. Miller, O. D.; Yablonovitch, E. & Kurtz, S. R. Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit IEEE Journal of Photovoltaics, 2012, 2, 303-311, [http://optoelectronics.eecs.berkeley.edu/ey2012ieeepv303.pdf]
On earth's surface, the photons incident on a solar cell will have a distribution of energy; this distribution is affected by the atmosphere or the Air Mass (AM). The optimal band gap for a solar cell is linked to the incident photon spectrum and will be different for Air Mass 0, Air Mass 1, Air Mass 2, etc. spectrum.
As Michael Haberle has pointed out, it is a trade off between the maximum conversion of absorbed photons (lowest band gap possible to absorb the most photons) and the cells open circuit voltage which is a function of band gap and charge carrier density (higher bandgap favorable for higher Voc). The Shockley-Queisser limit balances these two. Silicon is a nice material because it is earth abundant and manufacturing technologies are mature (purity, optical texturing, electrical contact, passivation etc...), and it is close to the 1.5 eV theoretical limit.
Note also that this optimal value is a function of the incident spectrum and that this calculation assumes a single p-n junction device.
For a detailed answer look into the Shockley-Queisser limit publications or textbooks which reference this.
In the following picture the relations between band gap energy (x-axis), open circuit voltage Voc (upper y-axis on the right), short circuit current Isc (uper y-axis on the left) and efficiency (lower y-axis) are shown. It is a qualitativ picture I've made for my thesis from a AM1.5D spectrum. This picture is similar to the calculations done by Shockley and Queisser. Efficiency is defined as Voc*Isc*FF/ Input power. FF is slightly increasing with band gap energy.
Could anyone specify bandgaps with respect to applications or vice versa, please? For example, a semiconductor within some specified lower and upper limit may give highest performance as a solar cell, or field effect transistor, etc....
This is the Shockley-Quiser limit. Tthe efficiency for a single junction solar cell is maximum (33% ) at a band gap of 1.4 eV for AM1.5 solar spectrum.
Dear Macho,
I know that i came very late to this forum but as the question is a basic one it is always open for answering. The conversion efficeincy of a solar cell Eta= Isc Voc FF/ INSOLATION,
the terms have their usual meaning.
As for the dependence of Eta on the absorber material bandgap it peaks at Eg= 1.35 eV for one junction solar c ells.
It is so that:
Isc is inversely proportional to Eg
Voc is proportional to Eg,
So there will an optimum value for Eg which is about 1.35 eV
For a detailed discussion on this effect please refer to the course:
Book A course on photo-voltaic array systems
Best wishes
Maybe I am late for this question. But I need to add my contibution.
Some authors give the efficiency of solar cell as the maximum power delivered by this solar cell divided by the isolation.
Dear Adama Ouedraogo,
what do you mean with "isolation" (insolation perhaps ?), and thank you.
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