Due to high absorption in the solar cell material,,the photon gets absorbed very close to the surface ,where recombination can b significant.In order to avoid front surface recombination, a window layer of large band gap is used.
Large bandgap of window layer is used to avoid front surafce recombination..
With a band gap in around 2.41 eV, CdS will allow the light in the region 400nm-500nm into the absorber layer.CdS has got good optical transmittance, good junction quality, optimum lattice constant and lower band offset with CIGS. This makes it to give highly efficient solar cell.
Dear Rekha Bai ,
welcome!
The widegap emitter layer in solar cells called also window layer passes the short wave length photons the underlying narrow band absorber without recombination at the surface of the narrow band absorber if the narrow band absorber contacts the metal directly making an ohmic contact. This has been hinted by the colleagues above.
There is another important reason which is that it suppresses the back injection to the emitter region increasing the dark current. This will lead to decrease the conversion efficiency.
Best wishes
Wider band gap is necessary to avoid recombination at the surface of the window layer so that light trapping capability of the cell increases.
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