Why are Si and GaAs preferred materials for solar cells ?

Explanation

The solar radiation spectrum received by us is shown in figure .
The maxima is near 1.5 eV . For photo- excitation , `h upsilon gt E_(g)` . Hence semiconductor with band gap `~ 1.5 eV` or lower is likely to give better solar conversion efficiency . Silicon has `E_(g) ~ 1.1 eV` while for GaAs it is `-1.53` eV . In fact , GaA is better (in spite of its higher band gap ) than Si because of its relatively higher absorption coefficient . if we choose materials like CdS or CdSe `(E_(g) ~ 2.4` eV) , we can use only the high energy component of the solar energy for photo-conversion and a significant part of energy will be of no use .
We do not use material like PbS (`E_(g) ~ 0.4 eV`) which satisfies the condition `hupsilon gt E_(g)` of `upsilon` maxima corresponding to the solar radiation spectra . If we do so , most of the solar radiation will be absorbed on the top-layer of solar cell and will not reach in or near the depletion region . For effective electron-hole separation . due to the junction field , we want the photo-generation to occur in the junction region only.