Dear Maya,

The optical absorption coefficient of the semicondcutor material normally does not peak as it begins from almost zero value at frequencies smaller than the cut off frequency corresponding to the energy gap, then it increases reaching to maximum and saturates at this value in the ultraviolet.The energy gap indicates how much incident solar radiation may be absorbed, but does not tell how much of the generated electron hole pairs will be collected to outer electrodes.

So, does also the complex dielectric consent or imaginary refractive index.

Accordingly, there are some properties that a material must possess for the photo voltaic effect:

- Suitable bandgap that near the optimum bandgap of 1.35 eV,

- High absorption coefficient to save the the thickness of the material.

- High collection efficeincy of the photogenerated electron hole pairs which requires high lifetime of the mobile carriers in the material

- High mobility of the charge carrier to reduce the ohmic losses.

Best wishes

@Khaoula Hidouri:

Thank you very much for your time and revert.. :-)

Well, what you say is completely fine.. and I agree with that..

however, my question here is very precise..

I want to know if any material, consisting a band gap value, that lie in solar spectrum range... I mean around the peak portion..

like say around 1.5 ev?? would it be sufficient for a material to get it be used as a photovoltaic???

or any other factor is to be considered?? regarding band gap energy only??

Hopefully here we are only talking about semiconductors, and not metals or insulators. If yes, then a semiconductor can be used for photovoltaic application if the bandgap falls somewhere within the solar spectrum. However, the power conversion efficiency of solar cell increases if the semiconductor is a "direct bandgap" semiconductor, in which it is easier to transfer the energy of photons to create electron-hole pairs. Also, you need to consider the "absorption coefficient" of the semiconductor you are making solar cell of.

In conclusion, in my opinion, the main criteria for selecting a semiconductor for solar cell application is its bandgap. However, there are several secondary issues that you need to consider when selecting a semiconductor for making solar cells.

References to read:

https://en.wikipedia.org/wiki/Direct_and_indirect_band_gaps

http://www.pveducation.org/pvcdrom/absorption-coefficient

@Amirhossein Ghods:

Thank you very much..

yes, here is the point, my confusion came from..

Actually, most of the literature dealing with photovoltaic and semiconductors, elucidate the photovoltaic application of any material just on the basis of its band gap value..

if more.. by the nature (direct or indirect as you said )

but while dealing with my compounds, I came to know that although I do have a favorable range of band gap values.. still, plotting their imaginary part of dielectric function do not show any peak in that region..

what is the conclusion then???

and what may be the possible reasons???

Dear Maya,

The optical absorption coefficient of the semicondcutor material normally does not peak as it begins from almost zero value at frequencies smaller than the cut off frequency corresponding to the energy gap, then it increases reaching to maximum and saturates at this value in the ultraviolet.The energy gap indicates how much incident solar radiation may be absorbed, but does not tell how much of the generated electron hole pairs will be collected to outer electrodes.

So, does also the complex dielectric consent or imaginary refractive index.

Accordingly, there are some properties that a material must possess for the photo voltaic effect:

- Suitable bandgap that near the optimum bandgap of 1.35 eV,

- High absorption coefficient to save the the thickness of the material.

- High collection efficeincy of the photogenerated electron hole pairs which requires high lifetime of the mobile carriers in the material

- High mobility of the charge carrier to reduce the ohmic losses.

Best wishes

Dear Abdelhalim

about the High mobility issue:

to reduce the efficiency degradation caused by series resistance.

there is an other possible way exept the low doping profile that reduce harshely the current value as you know (the tradeoff mobility-doping)

@ Mr. Zekry:

Thank you so much for such insightful, clear and to the point revert.

Although still a little bit confused I am.. As you said, absorption coefficient or the imaginary part of the dielectric function does not show any peak near the band gap region. Fine...

While explaining any such plot, we emphasize that the energies (or quanta possessing that value) corresponding to the peaks are mostly absorbed, since the intensity or the absorption comes out to be the maximum at such points. So, what you mean is that those quanta, possessing the energy value equal to the band gap are not absorbed at all.. (or not maximum at least)

Then what's the significance of that value!!??? (band gap I mean)

.

Moreover, we have a standard Solar spectrum, having maximum intensity in the range around 1 to 3eV.. and I do have that band gap but still no absorption!.. then what's the deal??? I'm having peaks after 3eV, for which solar spectrum itself doesn't have much photons!!...

Thanks.. :-)

Dear Maya,

welcome,

The optical absorption occurs when an incident photon has a sufficient energy to transfer an electron from the valence band to the conduction band. So, only photons which have energy equal or greater than the energy gap will be absorbed. The other condition of the absorption is that the absorbed electron is transferred from an occupied energy level in the valence band to an empty level in the conduction band, then the absorption efficient will increase as the energy of the photon becomes larger than the Eg the energy gap because the availability of the more empty electronic states in the conduction band.

The other point is that it is known that the absorption efficient is proportional to the imaginary part of the refractive index. So, it is expected that the behavior of the two is similar.

I want that you read the attached leaf to elaborate this point: https://www.overleaf.com/latex/.../optical-absorption...semiconductors/zdrkvrnfpnnb....

I would like to bring your attention that the peak of the incident solar radiation is at the green photo energy of about 2.2 eV, while the conversion efficeincy of single junction solar cell peaks at a lower energy of 1.35 eV

It is clear that a material with and gap higher than the 1,35 will absorb less incident solar radiation.

When you you have a material with a band gap of about 3eV, then your material will start absorbing the incident solar radiation from that photon energy and up, then it will absorb a fraction much less than that at silicon for example with Eg =1.1 eV. In spite of that the material can show photovoltaic effect if it could be made to collect the generated electro hole pairs due to photo absorption. Here is the challenge. Such wide gap materials are used as a window layer in heterojunction solar cells. In the sense that it passes all the radiation below the the 3 eV to an absorber material with lower bandgap such as CdTe.

Up till now i did not tell you why such widegap materials may not be efficeint. The problem is that the photons will be absorbed in a very thin layer near the surface of the material where they may recombine very rapidly because of the wealth of the recombination centers near the surface. So in order to collect such electron hole pairs one must passivate the surface and make the junction near the surface such that the built in electric field extracts the electrons and holes before they recombine.

I would like that you read the chapter:Chapter Solar cells and arrays: Principles, analysis and design

After reading this chapter you will get an insight in the solar cell materials and solar cells.

Best wishes

Dear Hichem,

I agree with you. To decrease the series resistance one has to increase the doping of the layers, however increasing the doping leads to a decrease in the mobility. However in my first post i just spoke from requirements on the material to be suitable for the photovoltaic effect.

Best wishes

Thank you very much Mr. Zekry.. :-)

So, As I grasp the significance of band gap from your explanation is that.. (correct me please If needed)

It's just a parameter showing the threshold value of energy that makes inter band transition (VB to CB specifically) possible.. Even after having that value, that's gonna happen or not.. no clue..

I mean, any transition gonna happen or not would be decided on the basis of the absorption spectra only.., no??

So, It may happen that I do have a band gap of 1.35eV but not getting any significant absorption,, and for another material, while the band gap energy is say 2.5 eV but still in that small region, I may have good absorption possibilities.. ???? Isn't it??

.

And what references you provided (Overleaf).. I'm not able to open that and getting error 404..

looking forward for your revert..

thanks (Regards)

Dear Maya,

welcome,

After reading the paper titled: Optical Absorption of Semiconductors , authored by: Chong Hui Yong and then read my posts you will get much information to understand the the role of the bandgap and the energy band structure in optical absorption as a function of the wave length lambda.

Best wishes