Hello Everyone!
My question is about deciding the applicability of any material on the basis of its dielectric function [Im(epsilon)] and absorption coefficient. first of all, do these both have the same physical significance or not?? And should these lie in the optimum range of solar spectrum to get utilized for solar applications???
My confusion arises after looking at the alpha and Im(eps) curve of Silicon, both are covering negligible part of solar spectrum (peaks lie in 250-350nm hardly) and still this is the only compound used commercially in solar panels and other applications!!!!
So, Am I retrieving wrong interpretations from the curves or something else??.
Thank you..
https://www.sciencedirect.com/search?qs=Photovoltaic%20Applicability%20on%20the%20basis%20of%20alpha&show=25&sortBy=relevance
Dear Maya,
The absorption coefficient is related to the extinction coefficient k by the relation:
alpha= 4pi k/ lambda
https://www.pveducation.org/pvcdrom/absorption-coefficient ,
So ,one can assess the application of the material as a photo absorber in the solar cell according to either alpha or k.
As alpha increases the material will be stronger absorber in the sense that the required thickness of the material to absorb the incident radiation is equal to 1/alpha. So, your observation is is okay as it is based on experimental results. It is so that silicon is most common used material for solar cells in spite of its relativity low alpha t at the wavelength rage greater and near its cut off wavelength of 1.1 um. But it is so the lower absorption coefficient dictated the use of thick silicon wafers about 300um to absorb such longer wavelengths. Material like GaAs because of rapid peaking , only needs few micrometer to absorb the longest wave radiation greater than its cut off wave length.
There is a study of the optimum semiconductor material as a solar cell. It is found that the material having energy gap of 1.35 eV is the material of the highest conversion efficeincy of solar radiation.
Best wishes
Hello Mr. Zekry
Thank you very much for the revert.
As you said, my observation is okay as it is based on the experimental results. Sorry, but I dint get it. I mean which exp results are concerned here?? I got the concept of thick material if the absorption coefficient is low. but this is my question after all. Why..?? I mean if the material is not good enough to absorb the desired radiation, then why so much preference to the same? Just because of its availability?? or something else?
And doesn't it mean that absorption properties do not matter that way. if its low we can compensate with the thickness!!!
And...
" There is a study of the optimum semiconductor material as a solar cell. It is found that the material having energy gap of 1.35 eV is the material of the highest conversion efficeincy of solar radiation. "
Can you please provide me the reference for this...
Kindly consider if possible.
Thanks :-)
The thickness of the absorber is based in the fact that the incident photon flux is absorbed according to an exponential decaying intensity such that
I(x) = I(0) exp - alpha x.
As a convention it is agreed that the penetration depth of the optical rays is x= 1/alpha where the intensity decreases to 1/e from that of the surface.
The more suitable measure for assessing the material is its bandgap and energy band structure where it is direct or indirect bandgap.
This because of these two facts:
The material begins to absorb light for lambda>= cut off wave length
The absorption mechanism will be direct for direct bandgap and photoassisted in case of indirect bandgap. This leads to observed difference of alpha in silicon (indirect) and GaAs(direct).
The band gap plays another role, it is one of the major factors defining the open circuit voltage. So, there will be an optimum value for it.
The book is semiconductor physics by S M Sze.
Best wishes
Dear Maya,
welcome,
we recently published a chapter in a book about the solar cells and arrays discussing the issue of selecting solar cell materials. For more information please see the link: Chapter Solar Cells and Arrays
After reading the chapter please tell me your opinion about it.
Best wishes
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