All the theories of solar cell absorption (that I have seen) are based on the assumption of weakly absorbing material. I understand that we need such an assumption in order to obtain the density of states or the modes inside the cell volume. But, is not it simply more efficient to use a highly absorbing medium? or at least a medium of a similar real and imaginary parts to avoid high reflections.
What do you mean by weak and strong absorbing incident solar radiation.
From the principle point of view every material has its absorption curve which gives the absorption coefficient as a function of the wavelength. This curve is given for the different materials in the literature.
Because silicon is indirect bandgap it has relativity small absorption coefficient above the band gap energy. Its alpha becomes large at high photon energies.
GalliumArsenide as a direct band gap energy has large absorption coefficient to near its cut off wavelength. CdTe has also large absorption coeffcient.
It is so that the required thickness that is required to absorb the incident radiation is = 1/alpha so when alpha is smaller the the material thickness increases to absorb most of the incident solar radiation. Therefore we need Si thickness much larger than that of Ga As.
There is new materials such the organic semiconductors and perovskites that are very strong absorbers such that few 100 nano meters are required only to absorb the incident solar radiation.And this is the strength of such materials. The solar cells are termed thin solar cells structures.
I would like that you follow the link: https://www.researchgate.net/publication/323309527_Solar_cells_and_arrays_Principles_analysis_and_design
Best wishes
Well you want strong absorption but you also want to be able to extract the absorbed energy as electricity, not just convert it to heat. That's why you can't just use a lump of coal. So far the only method that works is to use a semiconductor with a bandgap that prevents conversion to heat for long enough to extract charge carriers from the device. Both direct and indirect gap materials are used, direct is better but silicon has so many other advantages that it's used despite it's indirect bandgap.
Hello Mohamed,
in addition to that, what Manuel said I have the following remarks:
* The portion of reflected energy depends on the refractive index. This makes for glass nearly 4%.
*The absorbed portion depends, as Manuel Schnabel said, from the band gap. If you want to absorbe the visible spectrum, silicon with 1,1eV is a good choice. If you would use Ga2O3 ( what I investigate), you could use it with nearly 5eV only for ultraviolett radiation.
*The most important question is the absorbing mechanism. To get electricity, the absorbed light must generate charges. Therefore, the solar cells "contain" a space charge area (due to an pn-junction or a Schottky barrier). Within this space charge region the generated electron-hole pairs are separated within the electric field. This charges you can collect for instance to charge an accumulator and so on.
Now it is important to tune the extension of the space charge region with the reciproce absorbtion coefficient. Lets assume, the coefficient is 1E+4 cm-1, the light is absorbed on a length scale of 1 µm. Now, the doping profile of the junction, e.g. the extension of the implanted electric field, must fit the region, where light is absorbed. Otherwise you produce only heat.
With Regards
R. Mitdank
Even for low absorption coefficient materials, if you calculate the amount absorbed in a realistic thin film, it is quite high, like 95%. If the material has a low overall absorption, say 80% or so, theory suggests that putting a reflector behind the cell should improve the situation (like the tapetum layer in the eyes of nocturnal animals). However when you try this in a practical device (I have with DSSC) the gain is barely worth the effort.On the other hand, the thicker the absorption layer, as your correspondents point out, it is more likely that any photelectron will be lost as heat before it can be captured and directed to an external circuit.
What do you mean by weak and strong absorbing incident solar radiation.
From the principle point of view every material has its absorption curve which gives the absorption coefficient as a function of the wavelength. This curve is given for the different materials in the literature.
Because silicon is indirect bandgap it has relativity small absorption coefficient above the band gap energy. Its alpha becomes large at high photon energies.
GalliumArsenide as a direct band gap energy has large absorption coefficient to near its cut off wavelength. CdTe has also large absorption coeffcient.
It is so that the required thickness that is required to absorb the incident radiation is = 1/alpha so when alpha is smaller the the material thickness increases to absorb most of the incident solar radiation. Therefore we need Si thickness much larger than that of Ga As.
There is new materials such the organic semiconductors and perovskites that are very strong absorbers such that few 100 nano meters are required only to absorb the incident solar radiation.And this is the strength of such materials. The solar cells are termed thin solar cells structures.
I would like that you follow the link: https://www.researchgate.net/publication/323309527_Solar_cells_and_arrays_Principles_analysis_and_design
Best wishes
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