The reason behind the choice of a certain thickness of the absorber layer is the result of a compromise bwtween minimization to reduce costs and keep a proper mechanical robustness of the device on one side, and the need to assure a thickness large enaugh to allow interaxtion (that is absorption) with the incoming light.
Apart from the obvious need for using a material that can absorbe a large intensity of the incoming light, by just tuning the gap energy to the lower limit of the Vis-nearIR spectrum of solar light, we must remind that incoming wavelengths must be contained within the path of the same light through the layer. That is, the layer thickness must be larger of the above said lambda limit (maximum lambda of the spectrum to be absorbed). Therefore, if I want to absorb all the light with E>=1.5 eV, (that is with lambda
The thickness of the absorber layer is determined by both the carrier mobilities of the semiconductor as well as the absorption coefficient. Semiconductors with high optical absorption coefficients allow devices to be fabricated at very low thicknesses. If the mobility is low, the device thickness must be optimised to ensure that the carriers generated throughout the absorbing layer can be efficiently extracted.
IMNSHO, the absorber bandgap and the thickness of the absorber layer are related very indirectly.
The main criterium for choosing thickness is a tradeoff between absorption and energy and/or charge transfer efficiencies. Absorbance 0.3 means that about half of light is absorbed. Absorbance 2 corresponds to a nearly 3 times thicker film and means that almost all light (99%) is absorbed. Each absorbed photon generates a hole and an electron (with the probability called quantum yield). Now, the hole and electron need to reach their corresponding electrodes. And the thicker absorber layer, the bigger is distance to travel, probability of the pair recombination, and serial resistance.
The absorbance and absorber thickness are related by volume per unit absorbing cross section, and not by the wavelength. Of course, there exist laws describing how dipole (or other) absorption depends on wavelength, but those are important only for really long ranges.
And one more point. For thin films (thicknesses in nanometer range), it is possible that the absorber is in a place of a node of a standing wave, created by some reflection. You can find publications about a special transparent spacer used to move the absorbing layer closer to an antinode to achieve higher cell efficiency. I do not think that this is applicable in your case.
Correction. Asorbances 0.3 and 2 correspond to thicknesses ratio about 6 times. Sorry... Late, not completed change.
Dear Adnan Hosen ,
This question needs elaboration. Adding to the answers of the respected colleagues, it is so that the minimum thickness of a thin film absorber is equal to the inverse of the absorption coefficient, alpha, at the largest wave length of the incident radiation that can be absorbed.
So, the film thickness => 1/ alpha (lambda largest)
lambda largest = hc/ Eg,
h is the Planck consatnt,
c is the speed of light in free space,
and Eg is the energy gap.
For more information please follow the book chapter:
Chapter Solar cells and arrays: Principles, analysis and design
Best wishes
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